November 27

Setting Up Servos For RC Planes

Movement Geometry by Servos for RC Planes

I’m sure that by this time you are aware of the purpose and use of servos for rc planes. When you buy an ARTF model kit the way that the servos are arranged and operate the controlsServo Rotational Extremities should have been thoroughly researched and correctly positioned. There should be no resistance to their operation and to the way they move the control surfaces or throttle.

Having said this, there is no reason for not understanding the way their positioning and settings have been decided. The greater your knowledge of what makes your plane function correctly, the better it will be for your future progress.

In the diagram to the right you will see that the output arm is shown at its neutral central position as well as the extremities of its movement range. The full significance of these positions will become clear as we progress through this post. I will be referring back to this diagram to clarify points we will be considering.

Making the Most of the Servo Motor Output

Most servos made for radio control applications have a rotational range of approximately 90 degrees. This represents the most efficient operating movement and we should try to use as much of this range as possible in order to get the best out of the servo. The majority of modern radio control transmitters have the programming capability to adjust the range of this movement. The servo can be made to move slightly more than the default setting or be taken to virtually zero movement.

Although these adjustments are possible via the transmitter, they Servo Output rangeshould not be used as the normal way to set the range of movement for the various controls. If you look at the diagram above you will notice that the output arm has a series of holes to which the pushrod can be connected. These are provided so that the range of pushrod movement can be varied.

Now let’s look at the diagram over to the right here. You will see that although the servo motor output arm rotates in a circular motion, the actual range of linear movement is the solid line between the ends of the two radii created.

Control Surface Travel

Below this diagram is a photograph of a pair of control surface horns. You will Control Horns & Back Platesnote that the pushrod linkage holes are offset from the mounting plate. This is necessary so that the plate can be positioned close enough to the hinge line of the control surface to bring the holes perpendicular to the hinge axis.

The ideal situation is for these hole to line up with the hinge axis so that the transfer of movement is as accurate as possible and completely linear. The diagram below will clarify this requirement.

It will be evident from this diagram that the actual travel of the pushrod connection at the control horn will be considerably less than the travel ofControl Surface Travel the trailing edges of the control surfaces. What we now have to do is to match the full movement of the servo output arm to the surface control horn so that the control surface movement meets our requirement.

The way we do this is to select the hole in the servo output arm that will give us the same amount of travel as that provided when the pushrod is connected to the outermost hole of the control surface horn. The reason for choosing the outer hole at the control surface horn is to minimise any effect of play or slop due to a lose fit of the clevis pin in the horn hole. This will represent the smallest percentage of overall movement compared to maximum pushrod travel.

Matching Control Surface Movement to Servo Output

With the ideal outer hole selected at the control surface horn we now need to Match Servo To Control Hornmeasure the actual linear travel of the pushrod at its connection to this horn. Once we have this measurement we can find the hole in the servo output arm that gives us the closest linear movement match. This will be the best point to connect the servo end of the pushrod.

Using this method ensures that the maximum output efficiency of the servo is employed. You will need the help of someone to take this measurement. Someone will need to move and hold the control stick on your transmitter at each maximum throw whilst you measure the full travel of the servo arm.

Using A Servo Tester

Alternatively, you could invest in a servo tester as shown in the photograph right. Servo+TesterThis is a very useful little device that makes setting up and adjusting servos much simpler. There are three options with this tester:- a) Automatic Centring of servo. b) Cycling of servo to maximum travel either side of centre. c) Manual Control of servo travel to any position within the full range either side of centre. This feature is most useful for determining the full linear travel of the output arm as the arm will remain where it is set until its position is changed manually by the operator.

The automatic servo centring (a) is most useful for ensuring the servo output arm is fitted in the correct position on the spline for the centralised neutral.

If you would like to purchase this tester, USA readers can click on the photograph. UK readers can purchase by clicking the following link:

 

Servo Tester

 

The power for the tester comes from a 4.8V or 6.0V NiMH battery plugged into the right hand side of the case whilst up to three servos can be attached on the left side. The selected operating mode is indicated by bright blue LEDs above the control knob.

Control Surface/Horn Considerations

As explained previously it is important that, whenever possible, the pushrod Surface Control Hornholes on the surface control horn are lined up with the hinge axis line. This ensures that the travel of the trailing edge of the control surface is equal either side of the neutral centre line.

When it comes to driving Flaps this rule is not so significant. Flaps generally only travel downwards so the alignment of the pushrod holes is not so critical. Admittedly at this early stage of your learning programme you will not normally encounter flaps but it is important to understand the way they operate.

In general the pushrod will hold the flap in alignment with the wing trailing edge and neutral aileron position. This represents the least load on the servo mechanism so ideally the servo output horn should be close to the normal neutral position (see illustration below).

Because we still use the full range of travel from flaps up to flaps down the ideal position of the servo output arm should be close to right angles to the servo body when flaps are up and in line with the servo body when flaps are lowered. Generally the flaps are operated by a two position switch on the transmitter that will cause the servo to travel from one extreme to the other.

The way we set up this positioning is to find the true neutral centre position. This can be done through the transmitter/ receiver link or using our little friend the servo tester discussed above. Once we  have this position remove the servo output arm from the spline and reposition it to a position approximately 40 degrees  further round the rotation away from the flap hinge line.Servo Arms Flaps

When the flaps are lowered the maximum load on the servo occurs when they are at their lowest position. This is due to the pressure of the air being deflected downwards. The greatest load bearing strength within the servo is when the output arm is virtually in line with the pushrod. This just happens to be when it is at its maximum throw when set up as described above.

You will see that the right angle is slightly offset from the servo body. The reason for this is to ensure that the clevis/pushrod does not bind on the servo arm spindle when in the lowered position. As and when you get to the situation of fitting flaps to a plane, I suggest you take advice from a more experienced modeller to ensure flawless operation of the system.

I hope this post has helped you to understand why servos for rc planes are installed and connected to the various controls the way they are. The guidelines I have described cover the majority of installations. There are certain circumstances under which these are changed to accommodate special requirements. For now, so long as you understand the above, you won’t go far wrong.

Don’t forget to visit my website www.rookiercflyer.com for a comprehensive learning programme on getting started in model plane flying.

Till the next post, happy flying.

Colin

 

November 20

Choosing an RC Brushless Motor

Weight and Dimensions

Apart from Power there are two important things to keep in mind whenrc brushless motor installation choosing an rc brushless motor. These are a) its Weight and b) its Dimensions. Having to add extra weight at the front of a model to achieve the correct centre of gravity (CofG) is far from desirable. If we can incorporate at least some of this extra weight by using a heavier and more powerful motor, this is preferable to having a smaller less powerful motor and a lump of lead at the front of a model.

The trainer on the right would fly with a smaller motor but this selection fits well, suits the installation, and provides a surplus of power for emergency situations whilst helping bring the CofG to its desired position.

Occasionally there is no choice but to use additional weight to achieve the correct CofG, but don’t forget about its relationship to the weight of your motor. The dimensions of a motor are obviously important, as it needs to fit within the space provided.

Power Output

So now we come on to the amount of Power (Watts) required for the model to perform as it should. 3D models need thrust to weight ratios greater than 1:1, whilst scale WW1 biplanes need considerably less. The table below provides recommendations of performance in Watts per pound for the styles of model commonly flown. It is important to understand that if you run your motor above its maximum rated efficiency the Watts per pound rule won’t be accurate. The motor will tend to overheat as a result of a higher percentage of the Watts going into the motor producing heat instead of power. This is something that should be avoided at all cost.

  • 70-90 watts/lb.            Trainers 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.

These figures are generally optimum requirements but many model planes will actually fly quite successfully with a lower power to weight ratio. It is not uncommon for lightly loaded vintage style model to fly on 50 watts/lb.

RC Brushless Motor Formats

Brushless Motors are available in two formats. Inrunners and Outrunners. Let us take a look at each type in turn so that we can decide which is best for our application.

The electromagnetic windings of a brushless motor are made stationary (the stator) and the permanent magnets made to revolve (the rotor). The wiring connections are permanent and fixed.

Inrunner brushless motor
Inrunner Brushless Motor

Unlike brushed motors there is no need to make any electrical connection to  the revolving parts and two configurations are possible.

Inrunner Brushless Motor

In this type the stator is in the form of a cylinder around the outside of the motor and the rotor spins inside it. The resulting motor is known as an ‘Inrunner’. They look more like conventional motors, with a fixed cylindrical casing. With a small rotor, they turn at a high rpm but do not produce as much torque as outrunner types.

Outrunner Brushless Motor
Outrunner motor
Outrunner brushless Motor

If the stator with its windings is in the centre, the result is an ‘Outrunner’. Now the outer casing becomes the rotor, with the permanent magnets arranged around the inside of its rim. The casing is fixed to the motor shaft at one end whilst the other end runs in a bearing mounted in the non-moving part from which the coil lead out wires protrude. They turn more slowly and because the magnetic interaction occurs at a greater distance from the motor axis, they develop more torque.

 

 

Brushless Motor Benefits

There are major benefits to be had using brushless motors as an alternative to the brushed types.

  • They are more efficient – Brushless motors are much more efficient. This efficiency has been measured to be between 85% to 95% better than brushed motors.
  • Less electrical energy is wasted as heat – This means that more is used to do useful work.
  • Reduced Noise – Brushless motors have fewer mechanical parts than brushed motors, so they emit less sound.
  • Longer Lifetime – The only parts that are in mechanical contact in brushless motors is the shaft bearings, compared to both bearings and brushes in brushed motors. This results in a considerable reduction in wear and tear.
  • Reduced Electro Magnetic Interference – Brushless motors emit less energy as electromagnetic (EM) waves than brushed motors do. As a result they are more efficient, and create less radio interference.
  • Torque, RPM and Voltage are Linearly Related – This means that the amount of Torque or RPM produced by the motor divided by the Voltage supplied is a constant value. This makes it easy to predict how much power your motor is going to produce.

 Selecting The Best RC Brushless Motor For your Plane

The choice of motor type, either Inrunner or Outrunner, will depend on the requirement of your particular model. Inrunners are more efficient and powerful. Generally they produce higher revs per volt (Kv) compared to outrunners. For models requiring a small prop running at high speed like pylon racers and ducted fans, inrunners are a popular choice.

It is possible to use inrunners with gearboxes to turn larger propellers but this is a more expensive option that adds extra weight and is less efficient due to mechanical losses within the gearing.

For most normal applications the outrunner is a better option. Its ability to turn larger propellers in direct drive mode reduces both mechanical losses and cost and provides greater efficiency.

In the earlier stages of your model flying experience it is unlikely that you will be getting involved with high performance aeroplanes. For this reason I propose to concentrate on outrunner motors suitable for sport and basic aerobatic flying performance.

If we refer to the chart above it is very convenient that a figure of around 100 watts/lb. of model weight is appropriate. I like round figures, they make calculations very simple!

The three most significant figures published by brushless motor manufacturers are:-

  • Kv – This refers to the RPM (Revolutions Per Minute) constant of a motor. It approximates to the number of revolutions per minute that the motor will rotate at when 1V (one volt) is applied to the motor. This figure is called ‘revs per volt’ designated the abbreviation Kv. (Be very careful not to confuse this with kV which is a totally different abbreviation for ‘kilo-volts’ or multiples of 1000 volts). Please understand that you will never obtain the revs calculated from this constant. As soon as you attach a propeller to the motor the load will reduce the RPMs your motor can attain.
  • Continuous Current Rating – This is the maximum value of current that the motor is capable of handling continuously during operation without incurring damage through overheating and potential bur- out of the wire coils. They will also quote a figure of Maximum Current the motor is capable of withstanding for a very limited duration. this figure  should never be used for the purpose of calculating the power figures you will need for your plane.
  • Cell Count – This will tell you the range of voltages appropriate to the motor’s operation. these figure will often include both NiMH cell counts and Lipo cell counts. Not many people use NiMH batteries nowadays so the significant information for us is the Lipo cell count. Most Lipo cells give a typical average voltage under load of 3.3 volts.

Power (Watts) = Volts (Cell Count x 3.3V) x Continuous Current Rating (Amps)

 Using this equation it is possible to calculate the working power figure for any motor. With this figure as a maximum beyond which it is undesirable to operate, the only other variable capable of increasing or decreasing the power consumption of the motor is the size of propeller fitted.

Propeller Considerations

The amount of power required of the motor to turn a given propeller is affected by both pitch and diameter. Below is the formula for calculating power requirements for a given size of propeller:-

power = k x rpm x diameter4 x pitch

 The factor k is a constant depending on the units used to define power, pitch and diameter as well as the airfoil, thickness and shape of the propeller. Because RPM is controlled by the Kv of the motor, the only variables we can control are diameter and pitch.

You can see that changing the pitch of the propeller has only a small effect on power but a change in diameter has a major effect. Because pitch has such a small effect on power, it can be used very effectively to improve model performance at the expense of very little change in current. If we are looking for an increase in top end speed increasing the propeller pitch by a factor of one (e.g. from a 10 x 6 to a 10 x 7) will only increase the power required by 14% hence current will increase by 14%. So if the motor was drawing 25A with propeller one then propeller two will require 29A, a small penalty for the improved performance.

If, however, we increase the diameter of the propeller from a 10 inch to an 11 inch of the same pitch, it would require 1.46 times the power to maintain the same rpm (11/10 to the fourth power i.e. 1.1 x 1.1 x 1.1 x 1.1 = 1.46). If we continue to use the same battery the voltage remains the same so the current would have to increase by a factor of 1.46, i.e. 25A x 1.46 = 37A. An increase of this magnitude could push the current beyond the safe operating parameters of the motor.

This leads us to consider that maybe we can get the same amount of performance from our motor by reducing the propeller diameter but increase the pitch. Lets use a 9″ diameter propeller instead of the 10″. This will reduce the current consumption by a factor of 0.9 x 0.9 x 0.9 x 0.9 = 0.66. So if we take the 25A and multiply it be 0.66 = 16.5A. Now lets increase the pitch from 6″ to 8″ we increase the current by 8/6 = 1.33, i.e. 16.5 x 1.33 = 22A.

Clearly just by reducing the diameter by 1″ and increasing the pitch by 2″ we have been able to reduce the current consumption by 3A. This means that our flight time can be longer for a minimal reduction in overall power consumption. Performance will suffer very little as it is not normal to operate a plane at maximum power continuously. Besides which it is unlikely that the actual power loss will be noticeable  for sport flying applications.

It is always a good thing to experiment with different propellers

Digital Wattmeter
Digital Wattmeter

to find the one that gives the best performance for a particular model. It is normally best to start with the size recommended by the motor manufacturer then make small variations either side of this size.

It goes without saying that you are going to need our faithful servant the Wattmeter to check how the changes are affecting current and power figures. You can buy yours through these links:

RC Wattmeter (USA) or RC Wattmeter (UK)

Summarising  Our Choice Of RC Brushless Motor

Although there are a considerable number of factors to be considered when deciding on which motor to install in your plane, the principle one is the Power to Weight Ratio. By referring to the chart at the beginning of this post you can decide the sort of performance you need and select your motor based on this consideration.

It is always good to err on the high side of the power capability of the motor. You can always throttle back for normal flight leaving a reserve of power for  those occasions when it is needed. A motor that is having to perform at its maximum capacity continuously will not last very long and will not provide the reserve you may need.

The performance you get from your rc brushless motor results from a combination of motor, propeller and lipo capacity and rating, given that your ESC has at least a 30% margin above the anticipated maximum current handling capacity.

Don’t forget to visit our main website at www.rookiercflyer.com for all the help you will need if you are just getting started. If I can help you pleas don’t hesitate to ask your questions via the comments facility below.

Chat soon.

Colin

 

November 13

Maintaining RC Planes For Beginners

This is a follow on from my last post – Preventative Maintenance For RC Planes Beginners Fly – and here we will look at more ways to ensure that any of the RC planes for beginners are completely safe to fly. I propose to deal with such things as ensuring nose and tail wheels are working correctly and, where necessary, modifying main wheels and their fixings.

Trike or Tail Dragger?

Although the majority of trainer type planes featureTrike or Taildragger tricycle or ‘trike’ undercarriages, you will occasionally come across the ‘tail dragger’ configuration. Whichever type you acquire, there are major considerations  regarding whether you use a steering mechanism or not.

Many experienced flyers prefer a trainer to have a ‘fixed’ nose wheel or tail wheel as it simplifies the control of the plane during take-offs and landings. It also means there is no potential damage to servo gears through the abuse caused by difficult take offs and hard landings. This problem is particularly relevant where a grass patch is concerned.

The other school of thought is that eventually you will need to learn to control this function on future planes so you might as well master its control at the outset. Whichever option you decide on, there are ramifications so let’s look at them.

Fixed Steering

A fixed non-steering  arrangement needs to be securely fixed at either end.Fixed Noseleg A non-steerable nose leg will require fixing at the main bulkhead using ‘saddle clamps’ as shown in the picture right.

One of the saddle clamps fits over the upright section of the wire whilst the other fits over the horizontal section. I strongly suggest that you use set screws with nuts and washers behind the bulkhead to fasten the leg. This will give added protection in the event of a hard landing.  Ordinary screws will easily rip out of the bulkhead. It is also important to ensure the coil spring is at the rear of the leg.

If your kit comes with a steerable nose leg then you will need to purchase the above fixed version to replace it.Free Castoring Tailwheel Bracket

A fixed or free castoring tail wheel  similar to the picture right is screwed to the rear underside of the fuselage. The shaft of the wire passes up through the hole and is retained by the collet and grub screw above the bracket. This arrangement allows for a measure of steering control via the rudder.

Steering Control

If you decide to go for the steerable options for either then please refer to the kit manufacturers instructions and be sure to follow them to the letter. The most important items to pay attention to are the security of the servo fixings, the control horns and the free running of the pushrods. The loads imposed on the servo gears is considerable without them having to contend with excessive friction.

Carefree Collets

Wayward collets have been the weak link in many flying mishaps so it is essential to ensure that they are completely secure. They are particularly important when it comes to retaining Wheel Colletswheels and other undercarriage components. We discussed the use of a collet on the free castering tail wheel assembly a couple of paragraphs previously. They are also used on some steerable nose legs and most frequently of all, they retain the wheels on the axles.

When tightening the grub screws in collets it is essential that the  correct size hexagon key is used. To big and it won’t go in, to small and you will remove the corners in the sockets.

I always take the precaution of filing a small flat on the wire to which the collet is to be tightened. This ensures that the grub screw locates in the correct place and prevents it falling off in the event of a slight loosening.

Once the correct position is determined, apply a small spot of Cyano glue or ‘Threadloc'(TM) to the grub screw thread before finally tightening it up.  Vibration, although not so much of a problem with electric planes, can result in the grub screws coming loose and wheels or other components falling off in mid flight. For a learner this is the last thing you want to be worrying about.

Free Running Wheels

It is not unusual for the wheels supplied in an ARTF kit to haveplane wheels different size centre holes to the diameter of the axle wire (usually smaller). If the axle holes in the wheels are too large they can often be brought down to a good fit by bushing with short lengths of brass tube chosen to provide a smooth running fit on the axle.

The best way to do this is to select the tube that provides a smooth running fit over the axle then check whether this tube will fit inside the hole through the centre of the wheel. If it does, cut the required length using a modellers pipe cutter and glue it  inside the hole using Cyano adhesive.

If the tube that gives a smooth fit on the axle is too large to pass through the wheel then refer to the explanation below for enlarging the hole in a wheel.

Tools You Will Need

The holes in wheels supplied are often smaller than the axle so they need to be boredSilverline 262212 Drill Press, 250 mm 350 W out to provide a smooth vibration free fit. This is best achieved using a Pillar Drill or Drill Press (see picture right). If you intend to develop your modelling skills beyond your first ARTF trainer I would highly recommend purchasing this tool. It has so many uses and virtually guarantees accurately drilled holes in anything. It can also be adapted for other uses, for example, as a rotary sanding device.

For security it is recommended that a Drill Press Vice is used with this tool (see picture below right). This vice can be secured to the height adjustable drill table so that vulnerable hands and Silverline 380677 Drill Press Vice, 65 mmfingers can be kept clear of the rotating drill whilst holding the item to be drilled in the correct position.

 

Both of these items can be purchased on-line here. American readers just click on the images. UK readers can obtain them through these links:-

Silverline 262212 Drill Press, 250 mm 350 W

Silverline 380677 Drill Press Vice, 65 mm

 

Enlarging Axle Holes

You will need to place the wheel inside the jaws of the vice and tightenAxle hole resize 1 up the jaws until the tyre (US spelling ‘tire’ I believe!) is depressed somewhat on both sides. This will prevent the wheel from spinning around in the vice whilst drilling out the hole. Make sure the wheel is absolutely horizontally flat by placing a piece of scrap wood under it on the vice base.

Now place the vice on the drill press adjustable table and raise the table until the top of the wheel is about 1″ (25mm) from the tip of the drill bit that you have selected (to create the correct size hole) and fitted into the drill chuck .

Here is a tip for ensuring the centre of the wheel is vertically below the tipAxle Hole Resize 2 of the drill. Initially, fit a ‘countersink’ bit into the drill chuck and lower this into the centre of the wheel. This will automatically centre the wheel beneath the drill chuck. Whilst holding the bit in the un-drilled hole, tighten the clamping nuts and bolts so that the vice cannot move.

Raise the chuck to its retracted position and swap the countersink bit for the selected drill bit. Now you can start to bore out the axle hole confident that it will remain completely central in the wheel hub.

Note:- Most countersink bits are much shorter than drill bits so you willAxle Hole Resize 3 need to position the adjustable table at the correct height for the drill bit, not the countersink bit. There will be plenty of drop on the chuck to wind the countersink bit down to the wheel hub for centring.

When drilling out the hole be sure to select a medium speed setting for the drill press. Turn on the drill press and bring the tip of the drill down to engage the existing hole. Using gentle pressure slowly allow the drill bit to cut through the material of the hub. Most trainers are supplied with plastic hub wheels but occasionally they may be aluminium. Whichever you have, take care not to apply too much pressure. Keep retracting the drill bit and if necessary clean away any waste to prevent binding in the hole. Always turn off the drill press when removing this material.

The end result should be a perfectly centred hole that is a smooth fit on the axle providing  you have chosen the correct size drill bit!

I hope this and the previous post have helped you understand how you can check and improve the standard of finish of any of the ARTF RC planes for beginners you may encounter.

Catch you next time with more useful information.

Colin

 

 

 

 

 

November 6

Preventative Maintenance For RC Planes Beginners Fly

The majority of rc planes beginners select are of the Almost Ready To Fly (ARTF) variety.  These are popular because they get the rookie into the air quickly and because the manufacturer has done all of the design and construction work, They are virtually guaranteed to offer the tutor and student the best chance of success from the very start.

In spite of these advantages, it is not unusual for your pride and joy to startBumpy landing looking a little bruised and battered after a few flights and untidy arrivals. After a while it is going to need some TLC to ensure it remains fit to fly.

This post will look at some of the things that can be done right at the beginning and early in the learning programme to ensure it remains airworthy for as long as possible.

Installations and Connectors

It is not unusual for the fittings ,etc. in budget priced ARTF trainer kits to be of an economy quality in order to keep the price down to an affordable level. As you gain experience you will begin to appreciate that it is often advisable to examine these items at the outset. If you are uncertain as to their unsuitability, ask for advice from a more experienced modeller.

What are known as ‘fast fit’ items are used to speed up the assembly (we all appreciate how impatient you may be to get first flight logged). I have seen some very dubious bits of plastic included to perform quite serious load bearing duties.

clevises & retainers
Clevises & retainers

As you will have come to realise, I always advocate a ‘Safety First’ approach to model flying and do not believe it is worth compromising this philosophy. Because of this I do not automatically assume that the accessories included in a model kit are of a suitable standard in every case.

Such things as horns, hinges, servo linkages, screws, bolts, captive nuts and engine mounts, amongst others, have all proved unsuitable for purpose at one time or another. I have gathered together a range of preferred products from reputable manufacturers that I keep to hand ready for circumstances where replacement is necessary.

This ‘standardisation’ has proved to be a shrewd move as it has meant that my models are all of a standard and I only need to keep a few of the items I need to maintain my planes.

European modellers need to understand that many of the Far Eastern manufacturers cater strongly to the American market and as a consequence, the fittings supplied in many kits use US standard thread sizes for such things as pushrods and connectors.

This is fine if the quality is good but if you are unhappy with the quality of fittings supplied, you will need to replace both pushrods and connectors with European M2 standard versions. This same applies to engine mounting bolts and wing fixing bolts which are often US stock sizes.

Pushrod To Servo Fittings

quick links
Fast Fit Quick Links

Although in some kits the servos are already installed, it is often necessary to connect the pushrods to these servos. To this end a version of what is called a ‘fast fit’ connector is provided. personally I am not a fan of this type of fitting, much preferring the use of  the right angle bend and keeper as shown below.

It is not unusual for this type of ‘fast fit’ linkage to be supplied for attachment to the control surface horn as well. A linkage with this type of fixing at both ends is prone to flexing and also vibration causing the fixing screws to become loose over time.

 

pushrod keepers
Pushrod Keepers

The only advantage to be had from the use of this type of fixing is that quick and easy adjustment can be made to the length of the linkage. I prefer to measure the distance from the servo horn to the control surface horn and tailor the pushrod to suit. Make a 90 degree bend at the servo end and use a ‘pushrod keeper’, as shown here, along with a metal or good quality nylon clevis and retainer to connect to the control horn (see picture above).

There is nothing worse than suddenly seeing your pride and joy plunge to earth despite the correct control being made at the transmitter. This can so easily happen if the grub screw in a fast fit quick link works loose.

It may not even be the grub screw at fault. There is the bottom retaining e-circlip or tiny hex nut that can come unfastened. If you do insist on using this type of connector I suggest you apply a little cyano to the grub screws and bottom retaining fittings.

Having taken these precautions be sure to examine these fixings on a regular basis to ensure nothing has worked loose. Better safe than sorry!

Control Surface Horns

It is not unusual to discover that control surface horns supplied in ARTF kitsControl surface horns are designed to be glued directly into  the Balsa Wood adjacent to the hinge line ( see examples here on the right). This is not really good practice as, over time, the wood can start to break down with the applied loads and eventually the horn and wood could part company. There is really nothing to secure the horn other than the bond between the wood and glue.

A far better type is that shown below with back plates and screws that pass right through the wood and engage in the back plate by creating a thread in the holes provided. Some of these types come with self tapping screws instead of the type of screw shown here. Either are satisfactory and a far better option than the ones above. Some even come with washers and nuts to secure the screws.Control Horns & Back Plates

When you fit these horns be sure to tighten the screws right up to the nylon back plate so that there is absolutely no movement between the horn and wood. Having said this, do not ‘over-tighten’ them so that the wood becomes crushed.

My technique for mounting this type of horn is to hold the horn in the correct position with the pushrod or clevis holes vertical to the hinge line (see picture below) and mark the screw holes on the covering using a fine tipped CD marker pen. I then drill a fine pilot hole small enough for the screw to bite into the wood as it passes through. Correct Control Horn Alignment

Place the horn back in position and start to turn a screw into the wood. When the screw is starting to show through the wood, place the back plate over the end of the screw and continue to turn the screw so that it threads itself into the nylon of the plate. Start the second screw into its hole and, ensuring the hole in the back plate is properly aligned, tighten both screws through the nylon until the horn and back plate are tight up against the control surface without crushing it.

Pushrod to Horn Clevises

My personal preference is for the use of Euro size M2 spring steel types.Metal Clevises M2 Obviously in the USA the preferred types will probably be US 2-56 thread. I am not a great fan of the nylon ones frequently supplied in commercial kits. This does not mean that some may be of a suitable quality but be sure to satisfy yourself that this is the case. If in doubt, get help and advice.

One thing you must ensure is that the fit of the pin through the nylon horn is a smooth bearing fit with little friction but no slop. The best way to ensure this is to select a small drill the exact size of the clevis pin and run  it through the hole in the horn. Excessive friction can cause binding which adds extra load on the servo causing it to draw excessive current. This can lead to your receiver battery to run down quickly or your BEC circuit to overheat or worse still,  to fail completely.

Also ensure that the clevis clicks together over the horn with the pin fitting correctly in the other side of the clevis jaw. If it remains partly open then there is a good chance that it could slip out of the horn under load.

Although metal clevises are not so prone to coming open as nylon ones are, 2 Clevises & RetainersI still take the precaution of sliding a short length of silicone fuel tubing over the pushrod before fitting the clevis. Once the clevis is located in the horn, I slide this tubing over the clevis jaws to help prevent them parting inadvertently during flight. Safety First Eh?!

Secure Control Surface Hinges

We will finish off this post by looking at the integrity of your hinges. Many ARTFs arrive with the hinges already fitted and in some cases they are not well glued. I am not suggesting that this is the case with all kits but it does happen more often than it should.

It is easy to check this by giving each control surface a firm pull. Even if this indicates that everything is as it should be,  take the precaution of wicking some thin Cyano into each hinge line. cocktail sticks

Once this is done I obtain some cocktail sticks that I cut down into lengths that are very slightly shorter than the thickness of the control surfaces.  Using a drill bit equal to the thickness of the sticks, I drill through the underside of the control surface and through the hinge, stopping short of breaking through the top covering. I then insert one of the short lengths of stick and secure it with a drop of Cyano.

Continue this process until all hinges have been secured. I promise you that this will guarantee the hinges will outlive the model.

Flat Mylar Hinges
Mylar Hinges

On  occasion the hinges are set out of alignment during manufacture. In these situations the best thing to do to avoid binding is to cut through the existing hinges leaving them in situ. Cut a new set of hinge slots alongside the severed ones and glue and peg in place a new set of Mylar ones designed for use with Cyano glue (see picture right).

Some models feature torque tubes from a central servo to drive both ailerons.  The torque rods can bind in these tubes if they slightly out of alignment. The way to relieve this is to squirt a small amount of WD40 or similar lubricant into the tubes using the narrow tube supplied with the can.

We’ll leave it there for this post. Next time I will cover some more activities you will need to carry out with any of the rc planes beginners purchase to guarantee their longevity.

If you have enjoyed the content of this post, take a look at my website www.rookiercflyer.com to learn everything you need to know to get started in flying model planes.

Catch you soon.

Colin