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.

Cockpit

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: www.rookiercflyer.com 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.

Colin

 

 

 

 

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.

DSC_0001

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 www.rookiercflyer.com 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.

Colin

 

 

 

 

 

 

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March 5

Balancing Propellers For RC Planes

On my website: www.rookiercflyer.com 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 www.rookiercflyer.com 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.

Colin