May 13

What Is Brushless Motors Best Performance Criteria

Last week I ran through the various linkage and servo setupemp_n4250_950kv procedures, ensuring that appropriate movement and travel adjustments were correct. This post will look at the way I determine what is brushless motors best performance for my model and how is brushless motors best performance obtained.

With any model I build the performance I need from my motor very much depends on what is an rc plane designed for. Trainers, WW1 Biplanes and Vintage types require much less power and performance than Aerobatic, Warplane and 3D types. So I have to decide the appropriate power output needed for this model.

What Power Do I Need?

The Hawklett is a sport aerobatic design so it needs a good margin of surplus power to fly through vertical manoeuvres. There are several charts available online that suggest suitable power levels  for different types of plane. The one I favour is here:

70-90 watts/lb. Trainer and slow flying aerobatic models.

90-110 watts/lb. Sport aerobatic and fast flying scale models.

110-130 watts/lb. advanced aerobatic and high speed models.

130-150 watts/lb. Lightly loaded 3D models and ducted fans.

150-200+ watts/lb. Unlimited performance 3D models.

 From this chart it is fairly obvious that my power requirement falls somewhere in the range of 90 to 130 watts per pound of fully loaded airplane. I have weighed my model with everything on board and it comes out at 5.2lbs (for our European friends that amounts to 2.36Kg). Taking a mid range value of 110 watts per pound (242 watts/Kg) I calculate that I need:

5.2 lbs x 110 watts = 572 Watts (2.36Kg x 242 = 572 Watts)

If you remember back to some of my earlier posts and if you’ve checked out my website,, you will know that power in Watts = Volts x Amps. For a motor of the size and kv rating I am using with a plane of this weight, the best Lipo options are either 3 cell (11.1 Volts nominal) or 4 cell (14.8 Volts nominal). If I were to use a 3 cell Lipo then I would need to pull more amps to attain the 572 watts than if I use a 4 cell Lipo.

Amps = watts/volts:    572/11.1 = 51.5 Amps whereas 572/14.8 = 38.65 Amps

The motor I am using has a continuous current rating of 50 amps so if I used a 3 cell Lipo I would risk the motor overheating and burning the coils out. On the other hand a 4 cell Lipo will provide the power without ever pushing the current to this maximum rating.

RC Motor KV Rating

The next important consideration is the kv rating of my motor. What is motor kv rating going to tell me? It is going to tell me a great deal. Rc motor kv rating influences the parameters of the propeller I will use.

My motor is an EMP 4250 – 950kv which, for its size, is a fairly fast spinning motor. The 950kv figure tells me that at full throttle it will be rotating at around 13,500 rpm ( 950 x 14 =.13,300).  This means that I will have to be careful not to over prop it so that I take the risk of pulling too much current and overheating the motor itself, ESC or Lipo or possibly all three!

You will notice that I have dropped the running voltage at full throttle to 14 volts. This is because the greater the load on the Lipo, the lower the actual voltage available will drop to. You will see this in my test results for the three propellers I tried.

Propeller Size v Power Output

Digital WattmeterWhilst testing the various propellers I needed to record values for Voltage from the Lipo, Current drawn in Amps and the resulting Power in Watts. The best way to do this in one test session was by using probably the most important tool in my electric flyers kit, my Watt Meter.

If you already fly electric or intend to do so, I strongly suggest you invest in a Watt Meter. It will save you so much time and expense avoiding damage to your motors, ESCs and Lipos.

Here are the results of the tests I ran with three sizes of propeller:-

Propeller Size & Make              Voltage              Current         Watts 

      APC 9 x 6                                13.9                       38A                  572 

       EMP 9 x 6                               13.8                        36A                  538 

         EMP 10 x 6                             13.6                        48A                  671   

These just happen to be the three sizes I had available to make a start with. It just so happens that the third one, the EMP 10 x 6, gives me what I need, plus some, without pushing the motor into the danger area. 671 watts/5.2lbs = 129 watts per pound (283.8 watts per kg). Most of my flying will be at around 1/2 to 2/3 throttle settings so the full current draw will not be called for regularly.

 the APC 9 x 6 propeller would be totally adequate but the overall performance would not be quite so good. So long as I don’t push the power train into the danger area, I prefer to have the extra power available. I can always throttle back if I don’t want to fly so fast.

In due course I propose to try a 9 x 7 or possibly a 10 x 5 just to see what difference there is between their performances but for the moment I will be test flying my plane with the 10 x 6.

As a matter of interest, this motor is capable of producing 1100 watts using up to a 7 cell Lipo so I am not expecting it to feel to much strain from my usage.

The Finished Product

I’m sure you would like to see a photograph of the model in all her glory so here she is. I have to say I’m quite pleased with the end result. Will she fly as well as she looks? That remains to be seen.

Finished Hawklett

Hopefully by this time next week I will have committed her to the wide blue yonder and be able to report that all went well. We should know what is brushless motors performance compared to the previous version I built with an old HP40 two stroke glow engine up front some years ago.

Watch this space, see you next time.



May 6

What Is An RC Plane Setup About?

All Present & Correct

My last post took us through the various installations needed to complete the radio and power train. Now that all of the essential components are installed, (I have to say it looks pretty busy in there)  I need to go through a full setup procedure to make the model ready for its first trial flights.

So what is an rc plane setup all about? Well really its just a matter of checking and making sure all of the active functions are correctly connected and operating as they should. Let us go through some of these checks together.

Arranging Aileron Differential

I have decided to use this arrangement as in the past I have found it beneficial in certain circumstance. If you visit my previous post entitled Differential on Ailerons for Radio Control Aeroplanes you will learn why this feature makes life easier when flying models that suffer from ‘Adverse Yaw’.Servo Offset Aileron Diff

Now, I’m not suggesting that the Hawklett is one such model but I have found that a little aileron differential helps the roll function of most planes.

Here in this diagram the differential is arranged by setting the servo output arm a few degrees forward of the vertical. As the arm moves in a circular arc, the forward linear movement is less than the rearward movement.

When this is transferred to the control surface horn the difference in linear movement causes the upward deflection to be greater than the downward deflection (As > Bs therefore Ah > Bh). Hence we have ‘Aileron Differential’.

Now I know that I could have arranged this with the two wing mounted aileron servos connected to two receiver sockets (aileron & auxiliary) along with a mixing facility but I prefer to keep the spare sixth channel for another function if needed. Consequently I find this mechanical option equally as effective.

Single Servo Rudder & Steering Control

Rudder & Steering Servo

As you can see from this diagram, there are two ways to arrange the pushrods to achieve control of both Rudder and Steering from a single servo.

In the first instance the steering horn on the nose leg is connected on the same side as the servo output arm connection. To have the rudder turn in the same sense as the wheel the closed loop cables have to cross over in front of the fin inside the fuselage.

In case two the steering pushrod crosses over from one side to the other so that the wheel turns in the opposite direction. but the closed loop cables do not cross over. Alternatively the ball link connector to the wheel could be moved over to the other side of the servo output arm whilst leaving the steering leg linkage as in the first diagram.

Separate Elevator Control Arrangement

I thought that it was worth spending a little time on an explanation of this arrangement. I personally have never seen this done before on any commercially available model kits or plans. Having said this, it does work very well and could be applied to other projects where rear fuselage space is at a premium.

Elevator Pushrod & Drive Yoke assembly

The basis of this arrangement is the clever little yoke that connects the two halves of the elevators together. It comprises a wire ‘U’ with a ‘Z’ bend soldered to the bottom of the ‘U’. This fits into a standard clevis pin hole in the horizontal leg of the bell crank. The upright leg of the bell crank is connected to the pushrod that runs the full length of the rear fuselage to where the servos are located under the cockpit.

On each end of the Yoke arms are ball link sockets that mate with the balls attached to ply extensions on the trailing edge of the elevators.

In use this arrangement gives very positive and easily adjusted elevator movement.  As there are at least three points at which the movement ratios can be adjusted (servo & bell crank x 2) the range of movement is adjustable to the Nth degree. The all important consideration is that all linkages are a good fit into the drive components to eliminate unwanted slop.

The final ball link connections on the elevators give fine adjustment to ensure the elevator halves are exactly in alignment on each side.

Setting Up The Retracts

The important thing to ensure when installing and operating mechanical retracts is:

a) That the travel of the pushrod(s) do not exceed the operating range of the servo(s).

b) That the travel is sufficient to locate the pushrod and actuator in the locked leg position.

Retract leg up

Retract leg down

In the two diagrams above you can see that the pushrod link/activator moves horizontally sliding a bar in the activation slot from one end to the other. This in turn pivots the leg mounting block, shown mainly in dashed lines, so that the leg travels in a 90 degree arc from the retracted position to the down position.

I have shown the pin that sits in the activation slot short of the maximum travel. This prevents the servo from being put under continuous load. Having said that, I have checked that the amount of travel is sufficient to ensure that the position of this pin at full travel locks the leg in the required position.

The main adjustment is made via selection of the correct pushrod connecting hole in the servo output arm. With some radio gear that does not have a ‘travel adjust’ programme, this is the only method available. Most modern digital gear has this facility so final adjustment can be made by reducing or increasing the travel slightly at each end. I have set mine at 95% at each extremity.

The drawings here show the nose leg where the mounting plate is hidden inside the fuselage. On the wing units the leg extends from the opposite side of the block so that the mounting plate sits flat against the wing surface.

Final Testing

At the beginning of this post I asked the question; “what is an rc plane setup about”? I hope that this has been answered for you in the above explanations.Digital Wattmeter

Now that all of the active controls are setup the next task is to do some motor run tests using different propellers and monitoring the current draw and logging the static power readings. This is where that invaluable piece of kit, the Power Meter or Watt Meter comes into its own.

I very strongly suggest that if you intend to go down the electric flight route, you should invest in one of these meters. It will save you not only a lot of time but also the money you could lose in burnt out motors, ESCs and blown Lipos. It will also help you select the best propeller for your plane.

I’ll catch you next week with some test results. Don’t forget to mention my website if you know anyone who’s just getting started. Also, if you want to follow the full series of my Hawklett build posts, the first is here.



April 29

Week Seven – RC Plane Servo Setup

Full Hawklett LiveryHaving completed the covering I have to say, I am very pleased with the final appearance. I decided to pick out the main join lines between the Red and White with a black trim line and I think this works well. Here is a photo of the finished colour scheme standing proudly on its extended retracts.

The next job is to install the rest of the radio control gear, motor drive components and then carry out a complete rc plane servo setup.  I intend to drive the motor via a 60 amp ESC with separate UBEC circuit and a four cell Lipo of around 3700 to 4000 maH.


Installing The Radio Control Receiver

I use Spektrum DX6i radio control gear and I have Receiver & Satelite Identifiedexperimented using ‘Orange’ 6 channel receivers both with and without satellites. I have found these to be totally reliable and at a considerable cost saving over the original Spektrum receivers.

I am installing one such receiver with satellite in this model. This entails locating the main receiver in a convenient place to accommodate the bunch of servo cables and for ease of access for connecting and disconnecting the servos.

The latest six channel Orange receiver available here does not require a satellite.

There also has to be clearance for the short aerial that protrudes from the back of the receiver. This needs to be as far away from other signal carrying wires as possible.

The satellite receiver also has to be secured on a surface where the aerial is positioned at right angles to the main receiver aerial. Again this needs to be as far away from signal carrying wires as possible.

In order to protect the satellite receiver I wrap it in electrical insulation tape, without obscuring the LED that glows red when fully bound to the transmitter, so that it can then be secured to the chosen location using a spot of CA glue.

Aileron Control Surfaces to Servo Linkages

Being a frugal sort of guy I try to make my own components Hand Made Aileron Hornswherever possible. I know that moulded nylon control horns are relatively cheap but most involve a mounting plate and matching retaining plate that I find rather bulky and an unnecessary embellishment on the aileron surface.

The photo on the right shows the ones I have made for this model. These are cut from a scrap sheet of 1.5mm fibreglass PCB board.

These are let into slots cut into the aileron so that the pushrod connecting holes are directly in line with the hinge centres and secured using slow setting two part epoxy. Inset Aileron Control Horn

The two small holes in the lower stem ensure a good key for the adhesive. Once installed they look quite neat and there are no disruptions to the upper aileron surfaces. The sketch here on the right shows how they are installed into the Aileron.

Note how the linkage holes line up with the hinge point. This is very important.

Installing The ESC and UBEC

This model has a fairly narrow fuselage so fitting both theESC & UBEC Fixed Locations ESC and Lipo into the space available is quite a challenge. The swept back wing of this model necessitates the balance point being just in front of the point where the wing wheel legs exit the wing surface.

To get the balance point in the right place requires the Lipo battery to be as far back in the compartment as possible. There is just enough space in front of it to fit the ESC  and UBEC (Universal Battery Elimination Circuit) right up against the side wall.

I prefer to use ESCs with a remote UBEC. I find that they are more reliable as there is no heat transfer from the ESC to the UBEC circuit. This keeps things more stable with less likelihood of heat induced failure.

Lipo Location & Access

The photo here on the right shows how the Lipo fits in closeESC, UBEC & Lipo Located proximity to the ESC and UBEC and, as mentioned previously, needs to be in the most rearward position possible.

The Lipo is removed by sliding it forward and upward toward the front of the fuselage which in the photo is on the left. A ‘Velcro’ strap will retain it in position.

The forward base of this compartment (see photo above) is removable for access to the nose leg retract installation. Slots will be cut in this base to facilitate the installation of the retaining strap.

Receiver & Servo Wiring

There are six servos in this model for the following controls:Receiver & Servo Wiring

Ailerons – 2

Rudder – 1

Elevator – 1

Retracts – 2

Four of these are located in close proximity to the receiver so it is important for the leads to be organised and kept tidy. I use nylon ‘tie wraps’ to keep them neat.

The two aileron servos are connected through a ‘Y’ lead located within the wing. This then connects to a short extension lead that remains permanently connected to the aileron socket on the receiver. The ‘Y’ lead and the short extension are connected before fixing the wings to the fuselage.

A further ‘Y’ lead is required to connect together the fuselage mounted retract servo for the nose leg and the wing mounted retract servo for the two main legs. this then connects to the ‘Gear’ socket on the receiver. The connection from the receiver to the nose leg servo remain permanently attached whilst the link to the wing servo is disconnected each time the wings are removed.

Final Setup

So far, all testing for servo and control surface movement has been done using my trusty servo checker. The next  step is to bind the receiver to the transmitter and check all control throws and make any necessary adjustments. This will then complete the rc plane servo setup necessary before committing  her to flight.

Once this is done I will be testing the motor static thrust using different propellers. The total weight of the model with flight battery installed is approximately 5.5lbs (2.5kg). I am aiming for a maximum power to weight ratio of 110watts for every one pound of model weight.

Using a four cell Lipo means that with an operating voltage of around 14 Volts, I will require a maximum current of 43 Amps. (Current = watts x weight/volts)

110 Watts x 5.5 lbs/14 Volts= 43.2 Amps

Based on this calculation I will be trying various propellers to find one that provides this power and current draw. The motor has a kV  (revs per volt) rating of 900 so with a 14 volt input the maximum rpm will be close to 12600. This is quite high so the appropriate propeller will be a fairly small diameter and medium pitch.

Watch this space for the final details next time. If you have missed my previous build blog posts, the first one is here.

I hope you don’t mind me reminding you to visit my main website, especially if you are new to the hobby.






April 23

Step 6 – How I Cover Balsa Wood RC Planes

At the very end of last weeks post I featured a photo of the HawklettHawklett Colour Scheme in the bare wood state apart from the Anti-Glare panel and the motor access panel. In this post I propose to take you through the process of how I cover Balsa Wood RC Planes.

This is perhaps one of the most rewarding tasks as by the end the model has turned from a dull bare wood airframe to a glowing colourful thing of beauty (if done well!). The job requires the use of some basic tools that I will go through with you before I get started.

Essential Tools for Covering

The images here on the right show the basis of my adaptation of the  original RAF trainer version of the Hawk jet on which this model is based.

I am using a heat shrink polyester material to achieve this scheme. This type of covering is available from various companies. Most reliable model shops and suppliers will stock it.

Being a heat sensitive product an appropriate heat source isProlux Heat Shrink Iron required to adhere it to the surface of the balsa wood. There are two options one of which is necessary whilst the other is desirable but not essential.

The options are either a dedicated modellers electric heat iron with temperature control or a domestic electric clothes iron. The former is the best option as it can be used to access more confined spaces and is specifically designed for the job. Having said this, it can be done with the domestic iron but not so easily.

In addition I use modelling pins, a soft cloth pad, scissors and a scalpel with new blade. To aid accurate cutting I also use a 1M straight edge, a short ruler and a plastic transparent set square.

When using a blade and straight edge to achieve a true straight line cut in the polyester material it is necessary to have a good cutting base. I use a compressed fibre board with cork laminate attached to the top side. The cork takes cutting from the blade and is self sealing as the blade passes over it. A good alternative is a sheet of plate glass which does not blunt the cutting edge of the blade.

Cutting Heat Shrink Polyester Material

Covering films are supplied in widths that vary from 26ins (0.66M) to 1M. It can usually be purchased in multiples of 3ft or 1M lengths either in pre-packed sheets or cut from the  roll.

Having ordered and received delivery of the two colours I had decided to use, I planned out the best way to cut the necessary panels to cover all top, side and bottom surfaces of the fuselage and fin, the panels required for the tops and bottoms of the wings, stabilizer, ailerons and elevators.

This is quite an exercise in logistics but if done well can save a considerable amount of wastage. Having said this, it is necessary to allow a good margin around all panels to enable them to be gripped and stretched gently over the surfaces before pinning in place.

Preparing To Cover The Surfaces

My preferred method is to cover the largest areas first followedTesting Temperature Of Heat Shrink Iron by the smaller areas and individual parts. When using multiple colours it is advisable to apply the light colours first followed by the darker ones.

The first thing to do is to clean the surface using a ‘tack’ cloth. This ensures that there are no bits on the surface to show through the finished covering. After removing the film that protects the adhesive, I lay the material over the surface to be covered and gently stretch it into position. I hold it in place using modelling pins located at strategic positions.

Whilst doing this the shrinking iron has been heating up ready to start attaching the covering to the surface. I test the temperature by placing a very small piece of material, adhesive side up, on the shoe of the iron and if it starts to curl up and wrinkle the temperature is correct. Most Covering materials will attach to a balsa wood surface when the iron temperature is at around 150 to 170 degrees Celsius.

Attaching & Shrinking

With the covering pinned in place the first step is to run the shoe of the iron all the way around the edge so that it adheres to the wood. Once firmly held in place, I slightly increase the temperature of the shoe so that the material will start to shrink when it is moved steadily over the surface.

I try to keep the shoe just above the surface to avoid scratching the high gloss finish. Moving slowly the covering will shrink with the applied heat. Using the soft cloth pad I follow the shoe, gently rubbing the covering down on the surface to eliminate air bubbles and cause the adhesive to adhere to the wood. Any small wrinkles can be removed by re-heating the surface and gently rubbing the surface down with the soft cloth.

Once the complete area has been shrunk into place the excess material can be trimmed off and the edges smoothed down with the hot shoe. Corners need to be carefully trimmed so that the covering can be sealed across them. Curved edges, such as wing tips, need to have a surplus of covering left. This allows it to be pulled over the curvature whilst shrinking down with the shoe. Initially this technique takes aIroning Covering Joint Line 2 bit of practice to perfect.

Where two colours overlap, having first sealed the underlying covering, the first part of the top covering to be sealed is the actual join line. Here I touch the sole of the iron gently on to the overlap to tack the edge down. I take care not to apply to much heat so that the edge does not shrink way from the intended join line.

Having completed this tacking I seal all of the other edges before shrinking the covering down to the surface. I take care not to run the iron sole over the join line as the softened adhesive could cause the edge to shrink back from its intended position.

Nearly There

With just the ailerons and wing underside centre section to cover, she’s beginning to look much how I envisioned when I first chose the colour scheme.

Next week I hope to have finished these small tasks and installed the receiver, ESC, Lipo retaining straps and linked up all of the servos. I have to say that as an example of building balsa wood rc planes, this project has given me as much pleasure as I anticipated it would.

I do hope you are enjoying following this post and may be encouraged to have a go yourself at some stage. There is nothing more rewarding than the feeling of achievement and pride when the finished article takes to the air.

My website is available for those needing help getting started in our wonderful hobby. Please feel free to share this post and the previous five posts with anyone you think would appreciate them.

Talk next week,








April 17

Stage Five – How I Scratch Build RC Planes

Last time I said that I hoped to be able to show youFully Glazed Cockpit the Hawklett partly covered. Well, later in this post I will have  a photo showing just that.

Build progress doesn’t always go exactly to plan due to unavoidable delays and every day life interferences. This last week has been one of those weeks so unfortunately progress has not been as good as I’d wished.

Never-the-less, this demonstration of how I scratch build rc planes has made some progress so let me talk you through it.

Finishing The Cockpit

As you can see from the first photo, having received theCockpit Catch new bottle of ‘canopy glue’ I have been able to finish off the glazing.

It’s quite amazing what you can do with a nice clean soft drink bottle, a plug made from scrap balsa wood, a hot air gun, bits of scrap balsa for the interior furnishing and Humbrol matt paint! Eventually I will be outlining the canopy frame using lining tape.

Initially I had intended to rely on the strength of the magnets to hold down the rear of the canopy/battery compartment cover. On reflection I decided that it would be prudent to latch down the rear of this component to avoid loosing it in high ‘G’ manoeuvers. Consequently, I have fitted a spring loaded catch at the top of the fuselage immediately behind the cockpit. This latch engages with a tube let into the rear former of the cockpit assembly.

Covering The Wings

The picture on the right shows the top of the wings fully coveredWing Topside Covered in 1.5mm (1/16″) balsa sheet.  Two sheets of 100mm wide balsa were joined edge to edge. My method of doing this is to apply a run of masking tape along one edge of the first sheet. I then place the second sheet hard up against the first one pressing down on the masking tape.

I then fold the masking tape in half with the two sheets back to back and run a bead of white glue along both mating edges. When they are laid flat on the bench the excess glue is squeezed out of the join line and wiped away using a damp cloth. I then pull the edges together tightly using pieces of masking tape across the joint. This is left to dry weighted down with various heavy flat objects.

Once dry the sheet was placed over one half of the the wing surface and lightly pinned in place whilst the outline was marked for cutting to size. Once cut it was attached to the curved surface of the wing using white PVA glue applied to the complete frame and rib edges. Whilst drying it was held in place using copious pins and masking tape.

The underside outer sections were done in the same way. Full length sheetsRetract & Servo Plate were not practical due to the need to cut out the wheel wells, leg recesses and undercarriage mounting plates, to say nothing of the aileron servo mounting/access plates.

These areas were covered using cut pieces of 1.5mm (1/16″) balsa sheet joining edge to edge and glued with thin CA adhesive. You can see from this photo that this area is fairly busy and needs care to get the covering correct.

Having completed the covering, small cracks and gaps were filled with lightweight filler which was sanded smooth when dry.

Checking Incidence of Wings

As this model is an aerobatic type with fully symmetrical wing cross section, the wing is set a ‘0’ degrees of incidence. This means that the centre line through the leading edge to the centre of the trailing edge is parallel to the model’s datum line.Datum Line

I know, you’re going to ask me ‘what is the datum line?’ Well here’s an official dictionary definition: “The horizontal or base line from which the heights of items are reckoned or measured as in the plan of an aircraft, etc.”  

Often this reference line will be drawn through the centre of the thrust line unless the engine or motor has down thrust built in. In this case a Spirit Level on Stabilizerdatum line is usually drawn below or above the main drawing and everything is measured and positioned above or below this line.

OK, enough of this technical stuff, let’s get back to the incidence check. The stabilizer of this model is set parallel to the centre line so in order to check the wing incidence I had to set the fuselage inverted on blocks so that a spirit level laid across the stabilizer showed ‘0’ degrees. I placed the wing on its seating and checked that it was also at ‘0’ degrees.

I did this using an incidence meter I made myself. Here is a photo of it in position. The level indicator is a smart phoneHome Made Incidence Meter on which I have an app. that makes the phone into a spirit level. It has a simulated bubble and digital numeric indication of the actual angle (most useful).

The two Grey/Blue slides have a ‘V’ cut out, one for the leading edge and the other for the trailing edge. The deepest part of the ‘V’ each side coincides with the centre line of the wing chord. The small table on which the smart phone sits is parallel to the slide bar. This ensures that the phone reads an accurate angle of incidence for the wing.

Fixing Wings To Fuselage

Prior to finishing the wings I had marked and drilled the Nylon Wing Bolt Locationsplywood former either side of the retracted nose wheel to take the two short wing locating pegs.

The matching location for the pegs were marked on the wing face plate through the holes in the former and drilled out.  The pegs were then glued into place and the final fit checked. So far so good!

Once the underside fuselage fairing was glued in place and sanded to a smooth profile a 1.5mm (1/16″) ply plate was glued to the rear of this fairing to take the wing bolts. The hole positions were marked and 3mm pilot holes drilled through.

The wing was placed in position on the inverted fuselage and, using the pilot holes, the appropriate positions for the blind nuts were marked on the 6mm (1/4″) plywood plate epoxied into the fuselage. These pilot holes were then Blind Nuts Clamped Up Using Steel Boltsenlarged to take 6mm nylon wing bolts as shown here.

The locating holes were drilled to take the centre boss of the blind nuts. These were placed into position with a thin smear of slow setting epoxy resin applied to the face that is pulled up into the plywood. Steel bolts and washers were used to pull the nuts and their barbs into the topside of the plywood plate. these were left to allow the epoxy sufficient time to set hard.

When the epoxy had cured the steel bolts were removed and I was able to fit the wings on place and  secure them using the nylon bolts.

She’s really starting to look like a flying machine now – quite satisfying I think! I promised you some covering at the beginning of this post. Well I’m a man of my word so as a token gesture I have covered the motor access hatch, the anti-glare panel on the cockpit/battery hatch assembly and also used black lining tape to finish off the cockpit! Here’s a photo of the full assembly so far.

Wings Attached

Moving On

We’re progressing quite nicely and next time should see some serious covering done. I do hope you’re enjoying this build blog, I certainly am.

I think there is a great deal of satisfaction to be gained and I strongly recommend it to you.  To scratch build rc planes does not have to be expensive or onerous and you will learn techniques that will make you a better modeller in the long term.

If you have dropped across this post accidentally and enjoyed it, please take a look at my main website Also you can find the earlier posts in this series to see the full build log. Number one is here, take a look and enjoy.

Catch you next week.



April 8

Stage Four – Building Balsa RC Airplanes

Steady progress is being made on my Hawklett andJoining Wings I now have the second wing panel built and joined to the first one. The retracts are fitted into the wing panels and connected to the centrally mounted servo.

A certain amount of patience is required when building balsa rc airplanes as glues have to be given adequate time to dry. Some people are happy to use fast setting CA glues but I prefer to use White or Yellow aliphatic glues for the majority of wood to wood joints and slow setting Epoxy glue for high stress joints. These do require that the joints be left for the glue to penetrate the wood fibres and dry completely.

You will see in the photo right how I clamped the wing panels to my work table and used some old six cell Lipo packs to weight the joint down while the slow set epoxy glue dried. The trailing edges  are blocked up  so that the centre line of the ribs are horizontal all the way along each panel.

The circular item near the joint area is one of the wheel wells for the retracts. there is another on the other panel hidden from view in this shot.

Strength Considerations

Parallel chord wings are relatively simple to brace across theWing Dihedral Brace centre join line using what is known as a ‘dihedral brace’. This usually takes the form of a piece of plywood that is glued to the front or rear of the main spars, full depth across the centre joint line and extending outward several rib spaces either side

When joining swept back (or forward) wings  there is no straight line available between the two wing halves to which the brace can be glued flat. Instead the brace has to be cut to a length that fits across the centre glued ribs and is slotted to accommodate  one or two ribs before terminating at the junction of a pair of outer ribs and the main spars. Hopefully the illustration here will help you understand this arrangement.

Further strength is ensured by gluing a web of 1.5mm (1/16″) balsa between the top and bottom main spars with the grain running vertically the full length of the wing apart from the outermost three bays on either side.

Installing The Retracts In The Wing

This process is a little involved and care had to beWing Centre section With Retract Servo taken when cutting the pushrods and outer support tubes to the correct lengths. The linkage to the central servo is via ‘z’ bend 2mm rods set into Sullivan ‘Gold-N-Rod’ look-a-likes whilst the linkages to the retract activator rods are via metal clevises. These are mounted on threaded rods so that fine adjustments are possible.

When retracted the wheels fit into round wells that I made from rolled cardboard cut from the sides of a cereal packet. These are rolled round a suitable diameter spray can and glued. once dry they were mounted on false bases glued between the appropriate pair of wing ribs. The retracted Retracts & Servo Installationwheels would actually interfere with the inner ribs so cut outs had to be made in these ribs to accommodate the false bases and wells.

Before connecting the pushrods to the servo I connected up my trusty servo tester to both the wing servo and the fuselage servo that drives the nose leg. This enabled me to determine the correct way to connect these push rods to the servo and match the operation of the nose leg.

In the past, I have used a single wing mounted servo to operate both wing legs and nose leg on a trike retract arrangement. The only problem with this is that there has to be an easily connectable linkage for the nose leg when fitting the wing to the fuselage. This can be quite difficult to arrange so I decided to go for two servos connected by a ‘Y’ lead.Nose Wheel Retracted A much simpler arrangement although carrying a small extra weight penalty of the additional servo.

You may have noticed in the last two photos that there is a cut out in the centre leading edge where the wing sits against the fuselage. This is to accommodate the nose wheel where it extends beyond the limit of the fuselage underside when retracted. The sketch on the right shows this more clearly.

Adding Trailing Edges & AileronsWing Tip Block

Referring to the pictures above, you will see that I have attached the central trailing edge section and the wing tip blocks. The next step is to carve and sand these to shape prior to cladding the wings with the 1.5mm (1/16″) balsa sheeting.

The photo on the right shows one of the wing tip blocks glued in place. The full depth of the end rib at its widest part is 35mm (approx. 1 3/8″). The depth of block I required was achieved by gluing together three laminations of 12mm (1/2″) soft Balsa.Aileron Servo Plate Mount

My favoured approach to installing the aileron servos is to fit two plywood rails across one of the rib bays. The servos are attached to 1.5mm (1/16″) ply plates on their sides so that the output arm protrudes through slots in the plates. The plates screw down to these rails.

All that needs to be done before sheeting the wings is to face the frontal area that fits up to the fuselage bulkhead with 1.5mm (1/16″) plywood prior to drilling and fitting the wing locating pegs.Aileron Servo Plates

The rear wing bolt holes will be drilled through a ply reinforcing plate. This will be fitted once the wings have been sheeted and the underside fuselage fairing has been glued in place.

The ailerons are cut from 12mm (1/2″) medium balsa and sanded to the correct profile, tapering to 3mm at the trailing edge.


We’re Getting There

Next time I hope to be able to show you the model in some of here colours. The covering has arrived and just as soon as I have finished the woodwork I will be getting down to some serious heat shrink covering. This is not a scale model so I have adapted an early RAF/ Swiss Air Force colour scheme for my plane.

I have to say that I really enjoy scratch building balsa rc airplanes and so far this build has not disappointed. There have been a number of modifications necessary and these challenges add to the enjoyment of creating the finished article.

This is a totally traditional build process using wood and other materials common to models going back to the earliest days of rc model planes. Maybe one day I’ll get round to using more modern materials but for now I just love the feel of these traditional materials.

Don’t forget to check out my main website,, especially if you are a newbie. Everything you need to know about getting started is there. This is the fourth post in this build series so if you want to go back to the beginning and follow it through from the start, go to “How To Scratch Build RC Planes“.

Come back next week for the next episode of this build blog.



April 1

Step Three – Scratch Building Model Airplanes

Battery hatch Cover

Last week I managed to reach the stage of finishing the basic build of my fuselage and create the cockpit/removable battery compartment cover.

As I said at the end of the post, I hoped to have the cockpit glazing finished. Unfortunately, I am a simple human with foibles and occasionally forget to check the status of certain material requirements. I have to admit to running out of canopy glue!

Who said that scratch building model airplanes was easy and always ran smoothly? I have ordered a bottle of said glue but I don’t expected it to arrive until next week.

Finishing The Tail Feathers

I explained in my last post how I created the rear end Alignment of Finfairings between the stabilizer and fin. The next step in this part of the build was to glue the stabilizer to the rear of the fuselage which, again, was explained last week.

Once the epoxy glue had dried I fitted the fin into its slot and checked for vertical alignment and that it was true to the centre line of the fuselage. This simplified diagram shows how I check this centred alignment.

Once everything was true I mixed some slow setHawklett Fin Installation epoxy glue and set the fin in its correct position. Using pins, masking tape and rubber bands I secured it and left it to dry.

Once dry, I was able to fit the previously made fairing blocks either side of the fin. a little lightweight filler rubbed into the join lines and sanded when dry gave a smooth streamlined shape ready to take the final covering.


Building The Wings

There is no actual dihedral built into the wings but a naturally created angle is obtained by building the wings inverted. This occurs because when the top spar is fastened down flat on the building board, the wing taper causes the underside of the wing to take on a positive dihedral angle from root to tip.

The wings are swept back at quite an angle so each panel has to be built separately. They are then glued together inverted with the spars flat on the board and the trailing edge blocked up to the correct height at the centre and at each tip. We will look at how this is done in due course.Undercart Plate

You may recall that I had decided to use a set of mechanical retracts I found in the spares box. This has meant re-engineering the undercarriage mounting arrangement. Instead of fixed leg hardwood blocks being glued into the underside of the wing, I have had to cut 6mm (1/4″) plywood plates with the appropriate cut-outs  for the legs when in the retracted position.

In the first of these posts I said that I had cut out the wing ribs following re-drawing the wing plan as a built up construction rather than balsa clad foam cores.Trailing Edge Spacing

Now I was ready to start laying down wood to create the first wing panel. The first thing to do was to pin the top main spar down over the plan, not forgetting to cover the plan with glue resistant clear polythene sheeting first.

Next I cut two small blocks of scrap balsa to act as spacers for the trailing edge. These were cut so that when spaced off the bench, the centre of the trailing edge was at the same height as the centre line of the ribs.

Once all of the ribs had been glued to the top spar and to the trailing edge, I fitted the bottom spar (remember the wing is upside down so this spar is on the top). When all joints are firmly set the leading edge was glued to the front of the ribs.

It is worth mentioning at this point that the root ribsBasic Wing Structure (the ones that will be glued together when the wing halves are joined) are from 3mm (1/8″) plywood as are ribs 3 & 4 to which the retract plate will be glued.

So far, so good! The wing is now a strong rigid structure ready to have the retract unit and pushrod fitted. Structurally it will be finished with vertical 1.5mm (1/16″) webbing between the main spars. A central retract servo housing and supports for the aileron servo mounting plate will be fitted. Once these are all fitted and the two panels have been glued together and braced, the whole wing panel will be covered with 1.5mm (1/16″) balsa.

In the photo (right) you can see the cut outs for the retract plate, pushrod clearance and for the wheel-well. Its interesting to see that although the wing tapers quite dramatically, it looks to be almost parallel because of the angle at which it was taken.

Till The Next Time

Next week I hope to have built the second wing panel, joined the two together and fitted the various accessories within the wing structure ready for the covering.

My fresh supply of canopy glue should be available so that I can fit the rest of the canopy glazing to the battery compartment cover.

I hope you are enjoying following my explanation of scratch building model airplanes. If you have just discovered this post, please take a look at the previous two posts covering the start of the project, starting withhow to scratch build rc planes

Please feel free to share this post and the others with anyone you think could benefit from its content. If you are new to rc model planes and want to know how to get started successfully, please visit my website You will find everything you need to know.

Catch you next week.






March 25

Build RC Airplane Scratch Progress

Last week I had created the basic fuselage for my new project andHawklett motor installation installed the servos for all of the controls associated with this part of the plane. So let us move on to the next stage and look at how the build rc airplane scratch build progress goes.

Installation of the motor was next on my agenda. Now this model has a very narrow tapered nose profile so I had to be very careful with internal shape of the motor compartment. It needed to offer freedom for the rotating part of the motor without fouling the casing and power wires.

The smooth profile of the nose will involve a spinner of the same diameter as the end of the fuselage. This is exactly 50mm (2″) and, as the front end will be totally hidden behind this spinner, I drilled the centre shaft hole well oversize to allow as much air flow as possible into the motor front end.

Motor & Battery Location

Hawklett Motor Installation

The motor is fastened to a 6mm (1/4″) ply mount to the front of which is a further 6mm (1/4″) Balsa and Ply sandwich to bring the final face plate to within 1.5mm (1/16″) of the spinner back plate.

My choice of motor is an EMP N4250 – 950 KV. This is equivalent to a 50 size glow/nitro engine so there should be plenty of power to perform the aerobatics within my capabilities.

Previously the model was intended for use with a glow/nitro engine and what is now the battery compartment was then the fuel tank location.

The main modification here was to provide a false floor and rear stop plate for the lipo battery above the wing centre section. This meant opening up the cut out in the former F3 ( F1 being the motor mount and F2 just behind the motor) so that the lipo can be moved fore and aft to help with the balance position.

Fairing In The Tail Feathers

The stabilizer is positioned about 40mm (1.625″) belowHawklett Fin Fairings the top of the turtle deck at the leading edge. This means that, once installed,, fairing will be required either side of the fin to continue the shape of the upper fuselage through to the rear.

To create this fairing after the stabilizer and fin have been installed is quite difficult. To avoid this problem I made up a dummy fin/stabilizer ‘T’ section from 7.5mm (5/16″) soft balsa, slightly higher and wider than the tapering turtle deck.

This was ‘tack glued’ to the stabilizer mounting plate to simulate the finished items. I then built up the fairings either side of the dummy fin, ‘tack glued’ them to the dummy and sanded the whole assembly to match the taper of the turtle deck.

Once the profile was correct, I slid a sharp scalpel blade through the ‘tack glue’ and separated everything. I now have two perfectly shaped fairings to fit once the stabilizer and fin are finally installed.

Furnishing A Cockpit

As I explained earlier, the whole of the cockpit assembly needed to beBattery Hatch Cover 2 removable to make for easy access to the Lipo battery compartment.

The base of this assembly is a sheet of hard 3mm (1/8″) balsa to the underside of which are glued 6mm (1/4″) square balsa runners to locate the base flush with the fuselage sides.

The front edge has a hard balsa 6mm (1/4″) cross member that butts up flush with F2. A 3mm dowel set into this cross member centrally locates in a matching hole drilled through the top of F2.

At the rear a balsa plate has two Neodymium disc magnets set into the surface that mate with two similar magnets set into a plate that stretches across the fuselage.

To my mind there is nothing worse than a large cockpit (as this is) without appropriate furniture and at least one pilot. A quick search through my junk box produced a suitable head and shoulders pilot bust. The two in-line seating areas and instrument panels I fabricated from 1.5mm (1/16″) balsa suitably painted in pseudo military colours.


Now, I am nothing if not frugal when it comes to spending my pension so rather then searching on-line for suitable cockpit glazing, I decided to make a plug from scrap balsa and mould my own from a disused soft drinks bottle.

Because of the length of the cockpit this required two bottles. A quick visit to our local grocery store furnished two suitable bottles for the princely sum of 0.56€. Whether or not I drank the contents was irrelevant, the bottles were all important!Hawklett Rear Cockpit Glazing

The rear portion was a simple cut section of curved bottle side to be glued to the rear cockpit former and a balsa laminated support ring over the rear cockpit instrument panel (see picture right). The front section was formed over the balsa plug using scraps of balsa to tension it within the body of the bottle and my trusty heat gun to bring about the necessary re-shaping.

The glazing is attached to the frame using canopy glue. I like this product as it dries totally clear and does not effect the clarity of the cockpit.

Tail End Assembly

Hawklett Tail Assemblies

Now comes a tricky bit! Setting up and fixing the stabilizer and fin into the fuselage rear can be quite challenging as care must taken to obtain correct alignment. The stabilizer must be glued horizontal with relation to the fuselage. Then the fin must be set so that it is vertical with respect to the stabilizer.

The first job was to set the fuselage up with the sides vertical on the flat work bench. I dry fitted the stabilizer to its mounting plate and checked that the tips were equidistant from the bench top. Fortunately my building had given the necessary level but if this had not been the case, I would have gently sanded the mounting plate until correct.Stabilizer Setup

I mixed up a quantity of slow setting Epoxy Glue and applied it to both the mount and the underside of the stabilizer. I then positioned the stabilizer in the correct position and clamped it firmly to allow the glue sufficient setting time.

Whilst this was happening I clamped two previously cut balsa blocks, the same depth as the fuselage, under the stabilizer halves and in line with marked positions on the fuselage sides to keep it horizontal. The slow setting Epoxy allowed me plenty of time to ensure all distances and angles were correct.

Surplus glue that was squeezed out of the joint was wiped away using household rubbing alcohol. This works very well and dries away quickly through evaporation leaving no undesirable residue.

To Be Continued

In my next post I hope to have completed the glazing of the cockpit and have a set of wings built ready  to be mated to the fuselage.

In the meantime, I hope you are enjoying my “build rc airplane from scratch” progress. If you have any questions about this build or there is anything that is not clear to you, please don’t hesitate to ask through the ‘comment’ facility at the end of the post.

please visit my main website: if you need any advice or information on getting started in rc model plane flying. Everything you need to know is there.

Thanks for following me, catch you next week.






March 18

How To Scratch Build RC Planes

Hawklett Plans

Over the next few weeks I’m going to diversify from my tutorial style of posts to take you through the build of a new model that I’m currently working on.

Although the majority of my visitors are newcomers to flying rc model aircraft, there will hopefully be some of you who will be sufficiently interested to want to know how to scratch build rc planes.

For me a major part of the pleasure in our hobby is actually deciding on a subject and then obtaining or drawing up a set of plans for it followed by the building process to completion.

The end result is a completely unique model that no-one else will have when you turn up at your field. I find it a little disappointing when I turn up at our club field to find several identical ARTF models lined up. I can honestly say that none of my scratch built planes are duplicated at our club.

Now I don’t expect you all to start drawing plans but there are a good selection of model subjects available from plan publishers. The range of subjects covers simple designs right through to very advanced scale models that require particular skills and expertise to complete.

I have been designing and building my own projects for some years now however, my current build is one designed by a friend of mine back in 1975. Although the original was for 40 size glow engines, I have adapted the design to take electric power. I have a particular affection for models with a retro feel to them and this one is just such a subject. As you can see, the plan has suffered the effects of time and wear.


Another modification to the original, besides the electric conversion is the inclusion of mechanical retracts. The only reason for this is the fact that I happen to have a set of suitable trike retracts lying idle. There is no good reason why they should not be replaced by a set of the latest all electric retracts so readily available now or left as a fixed undercart as the original drawings.

Once the model is completed and flown I will be re-drawing the plans to show the changes required to accommodate the electric components and retract installation.

Getting Started

You have seen the condition of the original plans and because I didn’t want to cause any more damage to them I decided to re-draw the Wings, Stabilizer, Fin & Rudder. The original design called for foam cored wings covered in Balsa or Obeche veneer .

Where I live in Spain there are no foam cutting facilities and I don’t have a ‘hot wire’ foam cutter so I decided to draw up a set of wing ribs. Now this requires a certain amount of geometry knowledge for which I have to thank the perseverance of my school maths teacher all those years ago.

Having re-drawn the wings, projected the rib set and cut them out, I decided to build the fuselage first. Don’t ask me why! When you are scratch building you are free to do whatever you prefer unless there is a good reason to build in a particular sequence. This will usually be indicated by the designer either on the plan or in any additional instructions included.

Main Components

There are four basic components that make up the main structure:

  1. Wing
  2. Fuselage
  3. Fin & Rudder Assembly
  4. Stabilizer & Elevators

The simplest constructions are the Fin, Rudder & Stabilizer so these are the parts I decided to build first.

In each case the structure is a frame from 6mm (1/4″) Balsa covered on either side by a skin of 0.8mm (1/32″) balsa. Prior to covering the Rudder it was sanded to a tapered section at its trailing edge. the Stabilizer and Fin are flat section with rounded leading edges.

The Elevators are solid medium density 7.5mm (5/16″) balsa tapered to 1.5mm (1/16″) at their trailing edges. The method of moving the Elevators is quite unusual in that, because they are swept backward, there is no connection between the two surfaces.

A Fuselage mounted 90 degree crank right at the rear drives a split connector to ball links mounted on the inner edge of the elevators as shown in the diagram below. This means that they can be driven from a single fuselage mounted servo via a pushrod.Elevator Drive

Here is a photo of the end of the crank protruding from the rear fuselage.Elevator Drive Crank

The crank is cut from a piece of 2mm PCB resin board. The connection to the servo forward in the fuselage is by way of a length of 4.5mm (3/16″) Ramin with wire pushrods at each end terminating in clevises to both servo output and crank.

The ball links on the inner ends of the Elevators are mounted on 1.5mm (1/16″) marine ply extensions. I will show photos of the completed installation in due course.

Building The Fuselage

The sides are cut from 1200mm (48″) x 100mm (4″) Balsa sheet, 3mm (1/8″) thick. It can be difficult to source this length of sheet so I took a standard 1M (39″) sheet and spliced an extra short length on to one end.

To do this I cut the main sheet at an angle of 45 degrees and from another sheet cut another piece again at 45 degrees and glued the angled joint using aliphatic resin glue.

My technique for doing this is to lay the main sheet on the bench and apply a length of masking tape along the join line. I then place the mating piece hard up against the angle line of the main sheet, pressing it down firmly on to the masking tape.

I then fold the mating joint edges back and run a bead of glue along this joint line. I then bring the additional piece up into line with the main sheet and add strips of tape across the joint line to hold it together. I then place weights on the sheet to keep it flat on the bench until dry.

The beauty of this method is that once dry the masking tape can be peeled off cleanly as the glue will not adhere to the masking tape adhesive.

1.5mm (1/16″) ply doublers run from the nose to just behind the rear edge of the wing mounting area on each side. These are stuck to the Balsa sheet sides using contact adhesive.

Whilst these side panels and doublers were drying out fully under weights, the fuselage formers were cut out from the appropriate thicknesses of plywood where required and the rear formers made up from strips of 4.5mm (3/16″) Balsa as directed on the plan.

Once the sides & doublers were fully dried the front formers were glued to one side laid flat on the plan (covered in clear polythene sheet to protect it from glue) ensuring each was truly vertical. These were allowed to dry before the other side was glued to the other sides of the formers, again checking for true square setting, and clamped in place using weights.

basic fuselage

The turtle deck formers were then added and stringers glued in place ready to take the 1.5mm (1/16″) covering. The photo here shows this basic structure with the stabilizer, fin & rudder placed in position.

At this stage, to enable me to locate the exit points for the rudder closed loop control wires I decided to install the elevator and rudder servos in the space above the wing seating.

Whilst doing this I also decided to install the front nose leg and its retract servo. Having done this I cut and installed the pushrod to the steering arm from the rudder servo.Servo instllation

Then I made up the elevator pushrod and installed it inside the rear fuselage connected to the crank and to the servo output arm.

This picture will give you some idea of the internal servo layout as viewed from the underside. The front of the fuselage is to the right and the rear to the left.

The servo to the right is for the nose leg retract operation whilst the top left servo is for rudder and steering. The nearest one drives the elevator.

Next Time

Over the next 7 days I will be making progress on the rest of the fuselage and assembling the rear flying surfaces to it. I will go into detail on how I complete the removable cockpit assembly and fabricate the moulded glazing.

The cockpit has to be totally removable for the purpose of changing the power Lipo after each flight.

In the meantime please feel free to visit my website where you will find all the basic information you will need to get started in RC model plane flying.

I hope you have enjoyed this initial insight into how to scratch build rc planes. So join me again next time and we’ll progress with the build together.











March 5

Balancing Propellers For RC Planes

On my website: I discussed how to selectBroken Propeller propellers for rc planes. Once you have your propeller it needs to be balanced before you fit it to your model and attempt to fly with it.

An out of balance propeller produces excessive vibration that can travel through the entire airframe and, if bad enough, can affect the handling of the model. It can endanger the structural integrity of your model, loosen nuts and bolts and at its worst cause the engine or motor to part company with your plane.

Propellers that are badly out of balance can self destruct by shedding a blade (or blades) which in turn can cause the power plant to be ripped from the model. I know this because it has happened to me!

Why Is Balancing Necessary?

All synthetic materials that are used in the manufacture of propellers for modelBalancing a Propellor aircraft can vary in density throughout the mix. Wooden propellers, because they are made from a natural material will vary in density throughout their length.

To prove this just take a length of medium density balsa wood and try cutting through it across the grain with a sharp blade. You will feel the change in material density as variation in resistance at different points across the cut line. This is due to changes in the grain of the wood.

Wooden propellers are machined from solid wood and, again, because of variations in wood density throughout their blades, each blade if identical in profile will be different in weight. Even the hub can weigh more on one side than the other.

Because synthetic propellers are mass produced in moulds, minute differences in material density or mould inaccuracies can cause one blade to be heavier than one or more of the others. Such a propeller spinning at several thousand revolutions per minute will develop considerable vibration because of the imbalance of the centrifugal forces produced by each blade.

Not only this but a good deal of power is lost, either in the form of current in electric models or fuel in Glow/Nitro models. The effect of this is to shorten your flight times.

What is a Centrifugal Force?

The best way to explain this phenomenon is to imagine eachCentrifugal Forces blade of your propeller is a ball with the weight of each blade at the end of a piece of string. If we spin these weights round at the speed of the average model power unit the balls will rotate around the central axis point that represents the motor shaft.

Since the balls are traveling in a circular path, an outside force must be acting on them to keep them moving in a circle instead of flying outward. That force is the string which is pulling the balls back toward the axis, acting as what we call centripetal forces.

Centrifugal force is actually not a real force. If the centripetal forces that pull the balls toward the centre stop working (i.e. the string breaks), then the balls’ inertia takes over and sends them travelling in a straight path.  If centrifugal forces were real forces and the string broke, the balls would move straight away from the centre at the point where the string let go its hold. This does not happen, however, the balls follow their paths of inertia and move in a straight line that is a tangent to the circular path (as shown in the diagram).

Balancing Methods

Many people think that if a propeller is balanced so that the blades remain horizontal when positioned that way then the job is done. This is definitely not the case! Sure, it will be much better than one that continuously drops a heavy blade but it is still not totally correctly balanced.

The first objective is to check that the hub is correctly balanced. This is done by setting the propeller in the vertical position on an accurate balancer. I say “accurate” here because the accuracy of the balancer is the true measure of the final balance of your propeller.

It is not unusual for the hub to be heavier on one side than the other, irrespective of how the blades are balanced. Take at least two checks and correct the imbalance by filing away a little of the hub material on the heavy side or, alternatively, add a very small quantity of epoxy resin to the indent in the back of the hub on the lightest side. Don’t worry if you add too much, you can always use a small drill to remove some of the set epoxy. The propeller should not move from the vertical position you place it in if the hub is correctly balanced.

The next stage is to balance the propeller horizontally. To do this place the propeller on the balancer with the blades extending outward in the horizontal position. If correctly balanced the blades should remain in the selected position. Double check by rotating the propeller through 180 degrees. A heavy blade should be corrected by gradually removing small amounts of material from the front face of the blade without changing the aerofoil shape of the blade.

Progress slowly whilst continuously rechecking. Once you reach a situation where the blades remain in the position you place them irrespective of the angle, do a final recheck of the hub balance, making any fine alterations to ensure a perfect balance.

Choice Of Balancer

There are numerous models on the market varying in price from a few pounds or dollars to really sophisticated types the will represent a considerable financial investment.

Here is a small selection of prop balancers demonstrating the variety and range of available. The unfortunate thing about the first three is that they will only help you balance the blades (horizontally). They are not tall enough to enable you to carry out the hub balance check we discussed above.

The deluxe balancer is completely flexible and will enable you to test both horizontally and vertically. Not only this but you can also check the balance of spinners. The adjustable height is useful for checking propellers with longer blades. But should you need to check propellers with blades longer than the height extensions provided, you can arrange the balancer so that the prop sits outside the supports. You can stand it on the edge of a table with the blades able to rotate beyond the edge of the table.

Here is a video provided by the distributors of this model that explains the full benefit range of the product:

If, having seen the above video, you would like to purchase this product please click on either of the following links:

DuBro True-Spin Balancer (UK)          or         DuBro True-Spin Balancer (USA)

Final Thoughts

Trouble free flying is the ultimate target for any radio control airplane flyer so anything we can do to eliminate problems and anything that reduces the efficiency of our models is worth committing to. As I mentioned earlier in this post, vibration is a killer and can result in catastrophic end results.

Both glow/nitro engines and especially gas engines produce more than ideal levels of vibration on their own. The last thing you want to be doing is to add to this by fitting an out of balance propeller.

Because the electric motors fitted to rc planes are virtually vibration free, the airframes tend to be more lightly constructed to save both weight and cost of materials. Because of this it is totally undesirable to introduce unnecessary vibration by fitting an unbalanced propeller.

So I will close this post by stating once again that it is essential to balance all propellers for rc planes.

Don’t forget that this is just one of a series of informational posts  under the umbrella of my website aimed at helping newcomers to our hobby gain knowledge and improve their progression.

Please feel free to share the link to this post with anyone you think would find it useful.

See you soon.