Mail and Frequently Asked Questions Regarding Model Aircraft Control Systems

These are a variety of questions I have been asked about Aerodynamics and model aircraft engineering and design.

Other Mail and FAQ Pages

• Fiberglassing and Finishing Flying Model Aircraft
• Model Aircraft Aerodynamics
• Model Aircraft Construction Techniques
• Model Aircraft Engines
• Model Aircraft Kit Building
• Model-Building Tools
• Miscellaneous Questions

Also See

• Hinges for Model Aircraft
• Pushrods for Model Aircraft
• Linkages for Model Aircraft
• How to Install Robart Hinge Points

Regarding drilling for hinges in flaps for a Top Flite .60 size Corsair... I've read that the pins should be drilled on an slight angle pointing towards the center of the wing on the trailing edge (TE).  The "hinge part" should be slightly lower than the center of the TE and the end inside the wing should be slightly above the center line.  This is to facilitate the 50° angle the flap achieves at full deflection.  Would you recommend that approach or just drill the hole straight through as a jig would provide?

I would work it out on paper first.  Draw a cross section of the wing (or just use the plan).  Cut a flap cross-section from card stock and then put the hinge over it and see if you can drill straight in and still get enough movement.  If so then that’s the easiest way to go and what I would do.  If not then make your own drilling jig to ensure you get all the hinges drilled on the same line so it doesn’t end up binding.

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I'm building a prototype Sopwith 1-1/2 strutter and i want to make it as scale as possible.  My problem is the pull/pull linkage.

In your article you mention that this system needs to be a perfect parallelogram.  I have somewhat figured out the placement of the servo but I am afraid of binding.

I've attached two pics.  One of the plane so you can get a better picture of what I want to achieve and another of my idea.  Any info/link would be greatly helpful.

As long as it's close you shouldn't have binding problems.  The best way to do it is to have the holes in the servo arm the exact same spacing as the holes in the elevator horn.

Normally what I do is choose the servo horn I want to use and then cut a plywood elevator horn to match the hole spacing in the servo horn.

If that won't work, say the elevator horn spacing is a lot more than any servo arm you have, then you can make a bellcrank from plywood and mount it in the fuselage between a couple hardwood beams going from top to bottom of the fuselage.

Make the holes in the plywood arm the same distance from center as the holes in the elevator horn.

Now hook up a pushrod from the servo to the plywood arm and you're in business.

If you need the pull-pulls to come out of the fuselage in a certain location and can't get it aligned with the arms and horns then you can use pulleys but they tend to be a pain to set up.

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I'm building a rubber band to electric Herr Fokker DVII.

In order to keep it light and scale, I'm installing a pull/pull linkage.  Can I run a single wire from the servo to a 'Y' out to the control horns, instead of two seperate wires?

I would think there would be a benefit to the single as it would be easier to adjust.

Example ----< this setup would be for the elevator only.  Any suggestions?

The setup you propose will work fine as long as the Y part can’t slide around on the single line part.

For example, a bad idea would be to have one string go from the servo all the way to one horn and another string tied around that string so it can go to the other control horn.

I would try having two individual strings going all the way from the servo arm to both tops of the control horns and another pair going from the other side of the servo arm to the bottoms of both control horns. If that didn’t work, then I would move on to the Y idea next.

If you go with the Y idea, just make sure you tie knots or use a small ring or something so the strings can’t slip in relation to each other.  The reason being you don't want the lines to rub and wear against each to the point of failure.

\ /
 O   <- metal ring
 |

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I am interested in your plywood control horns used on Rustik.  What size plywood are you using and how are you mounting them into the control surface?

I make the horns from the greatest thickness plywood that the clevis will close around.  For 2-56 clevises that usually means no thicker than 3/32”.  I simply cut a slot through the control surface by first drilling a hole and then using a small key-hole saw.  I use small files to clean up the hole.

 I use epoxy to glue the horns in after the surface is complete.  The way I do it is to put epoxy in the hole and let it soak in for a couple minutes.  Then I put in some more.  I slide the control horn until it’s almost in position and then coat it with epoxy and slide it into place slowly.  Take your time getting it positioned.  While the epoxy is still wet, I use a brush to go ahead and coat the entire horn with the same epoxy.  That ends up being the finish on the horn.

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I find the strip ailerons on my ARF too small (1-inch) to be desired. I plan to modify it and put 2-inch ailerons . How should I go about this the easiest and most economical way and increase performance?

I see 2 options:

1. Hack away the wing so it could accommodate the bigger aileron.

2. Install the bigger aileron on the original wing recess and build up the wing trailing edge near the fuse so the ailerons won't stick out.

Any thoughts?  The plane in question is a GP Extra 300S 40 size ARF.

Have you tried sealing the hinge line?  If not you will find that it will significantly increase the roll rate.  I would do that before anything else.  Also make sure that the linkages are strong enough to prevent the ailerons from blowing back.  If that’s happening then you aren’t getting as much throw as you think you are.

Assuming you’ve done all of that then I would go the easy route using the second option you are considering.  Make larger ailerons and replace the originals with them.  I consider this to be an inferior choice to your first consideration, but much faster and more economical to implement.

The pure way to do it is to cut back the trailing edge such that the wing maintains its original area with larger ailerons.  If the trailing edge is sheeted then you can draw on the covering with a sharpie marker to mark the cut lines.  Cap the trailing edge with 3/8” stock or 1/4” stock having hinge blocks glued in place.  Sand it flush with the existing wing and cover it.

If there is no trailing edge sheeting or if you will have to cut past the sheeting to get the aileron inset properly, then you will probably have to recover the entire wing.

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I am getting ready to build a Paragon sailplane.  It has 118" WS with a 1080 sq." area.  Advertized flying weight is 50 oz.

The elevator is about 26" x 1-3/4" average and about 52 sq." with 1/2" throw.  The rudder is about 11-1/4" x 3-5/8 average with 1-3/4" x 1-3/4" ahead of the rear tail post, for a approximate 45 sq" overall and about 2" throws.

I was wondering if Hitec HS 85 servos would be enough for this.  I have had some say to use an HS 422 standard but it only has 4 more ozs. of torque and I could save almost one ounce per servo using the 85s.

The only reservation I have about HS-85’s is the gear train.  If the surfaces start to flutter the servos will last all of about a half second.  I think they’re strong enough, but if you plan on doing speed runs with this model then make sure the hinge lines are sealed and you have a very good hinge system and linkage – don’t allow any slop in the setup.  If you can do that then I would trust the 85’s.

If you have any doubts then go to a stronger servo and accept the weight penalty.  The advantage of the standard servo (I’m guessing having never owned one) is not the extra torque but the stronger gear train.  You might look at a mini servo that has a metal gear train as well.

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I'm looking for information on installing a pull-pull system for elevator & rudder.  I have never done this.  It seems simple enough but don't want to reinvent the wheel!

What if any are the pitfalls in going this route?  I want to use it on the next 40 size plane I build.

I have only recently started using pull-pull controls regularly. In the past I only used them in scale models, but I will be using them in more of my sport models from here on. The reason being that they are far lighter and much more positive than a pushrod system.

There are a few gotchas but all are easily avoided if you understand the theory. First, you have to create a perfect parallelogram. The holes in the control horns on the control surface must be spaced the same as on the bellcrank or servo arm to which the other end of the cables are attached.

I make horns from plywood and glue them into the surface. The first time I did that I was concerned about strength, but now I know they are actually stronger than control horns that are attached to a balsa surface.

The second gotcha is that the holes in the control horns should be aligned with the pivot of the hinges. If the horn is ahead of the hinges, then the line on the opposite side will tighten as the other side is pulled.

For example, if you pull up elevator, then the down elevator cable will tighten and cause binding or possibly even breakage of something.

What you can do is put the center of the holes behind the hinge pivot and the other side will slacken slightly. That is called Ackerman.

You can buy everything you need from Wal-Mart except the threaded studs to attach the lines to. I make my own using a micro drill bit on a piece of threaded rod just beyond where the threads end. I don’t know the bit number, but it’s smaller than 1/32”

The threaded studs available through the hobby industry are over-priced, but if you don’t have a drill press and small bits, then that’s the way to go.

If you attach directly to a servo arm, then definitely use a ball-bearing servo. Otherwise, mount a bellcrank on a plywood plate to attach the cables to. Use a pushrod from the servo.

I glue 2” pieces of inner nyrod in the exits and then fish the lines back to the servos. Some people use inner nyrod all the way back to the servo which prevents the lines from becoming twisted in the aft fuselage.

My way is more tedious, but I prefer it because it’s much lighter. But you do have to be careful that the lines don’t get twisted together and that they don’t rub on anything, such as a former.

Lastly, use cable that doesn’t stretch. On small models I use braided fishing line (not metal). It works well and is plenty strong. On larger models, I use braided cable fishing leader. The lines are fed through a ferrule, through the hole, back through the ferrule and looped again through the ferrule. The ferrule is then crimped. It’s very secure.

Take the time to make sure the ferrules are completely deburred. The last thing you want is a cable to get cut.

Take a look at the photos of the tail end of the Herr Piper J-3 Cub I just finished. It should make the hook up clear.

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When making the angle on the aileron, whether it is one or two angles, to be joined to the aileron bay, is it always a 45 degree angle?  Does this angle ever need to be increased or decreased?

If the model is scale, then the system is set up as close as possible to emulate the full scale plane.

Other than that, the bevel on the leading edge of the control surface is cut to give the maximum amount of throw that you think you will ever want.

It is often not possible to get enough throw on a 3D model by beveling only the control surface.  To get around this the mating trailing edge of the flying surface is sometimes beveled too.

I don’t fly 3D planes so most of my designs are set up between 30° and 45° (60° – 90° total throw).  Add a little bit to whatever you think you’ll need to account for binding at the extremes.

For example, if you think you will want 45° of throw and you cut the bevel at 45°, the control surface may start to bind at 40° due to glue in the gap, covering or whatever.  Therefore you might want to cut it at 50° so that you can actually get 45°.

45° is a too much throw for most planes though.  If you actually look at the amount of throw recommended on planes it is almost never even as much as 30°.

The smaller the bevel, the less opening there is between the control surface and the flying surface which means less drag.  Usually it’s not significant enough to matter, but just something to keep in mind.

Another thing you should do is seal all hinge lines.  It will make the control surface more effective.  That means that you can set up less throw to get the same amount of response from the aircraft.  Less throw = less work for the servo, less battery drain, and more efficiency overall.

I cover hinge gap sealing on the Hinges page.

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Would you tell me about setting up for different types of throws for the elevator or rudder, etc.

If you use the outer hole in the control horn and the inner hole of the servo arm do you get maximum or minimum movement?  Do you have an easy way the remember which is which?

What you have to keep in mind is that the servo moves fully from one direction to the other regardless of which holes are used.

If the pushrod is farther from the center of the servo then the pushrod moves farther.

The opposite is true of the horn on the control surface.  The farther the pushrod is from the hinge line, the less the surface moves.

Therefore to get maximum control throw you would use the outermost hole in the servo and the innermost hole in the control horn.

That arrangement also gives the control surface a lot of leverage over the servo which is bad.  The least amount of movement also gives the servo the most leverage.  That would be innermost hole in the servo arm and outmost hole in the control horn.

Normally to get the throw you need you set it up somewhere in between.

I like to set things up to use one of the outer holes in the servo arm and use whatever hole in the control horn that gives the correct amount of throw.

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Having a little problem with my linkage and radio adjusting and trim.  After I test fly a new plane, 4 Star 60, I want to adjust the linkage so that I can zero out the trim buttons on my radio (Futaba 6EXA) and fly again.

That is where I am having a little trouble.  Which way is the best to approach this?  Do I move the clevis at the control surface in or out as needed, move the clevis up or down, or move the arm at the servo connection?

Have looked and read quite a bit but most of what I see is just to use the radio.  That is fine but I would rather get things close and tight on the plane then just fine tune as needed during flight and the ever present changing conditions where I live and fly.

Any ideas or help you have would be appreciated.

It is confusing. I’ll try to explain how it works so that maybe you’ll have a better understanding.

The hole you pick in the horn affects the control response — not the trim. For example, if you want the elevator to be more responsive then either go in a hole in the control horn or out a hole in the servo arm.

If you want to change the trim, then use the same holes you’ve been using, but adjust the clevis in or out. Be sure that you have plenty of thread going into the clevis. If you start running out at one end then adjust the other.

If the plane needs a lot of trim and can’t get it with the setup you have, then you may need to make a new pushrod that is cut closer to correct length.  Normally that isn’t necessary, but it’s better than having only a couple threads in the clevis and losing the whole plane because of it.

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I am currently building a Sig 4 Star 40.  I have never used CA Hinges before and don't want to start now.  The Last Kit I Built was the 4 Star 60 and I used Robart 1/8 Hinge Points.  They turned out great and I've gotten lots of comments as to how good of a job I did being the first time I've used them.

My dilemma is that the control surfaces of the smaller 4*40 are thinner than it's larger sibling and the 1/8" points are the same thickness as the 1/8" Fin/Rudder and nearly as thick as the 3/16" Stab/Elevator and Ailerons.

The next size smaller hinge points are the 1/2A Hinge Points and those don't seem strong enough.  What would you suggest I use?

I would either use flat, pinned hinges such as made by Dubro or the Great Planes small hinge point. The small hinge point by Great Planes is larger than the small Robart hinge, but smaller than the 1/8” Robart hinge. Additionally, the Great Planes small hinge point has a steel pin rather than plastic like the small hinge point by Robart.

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I am building a P-51 Mustang and plan on using Robart Hinge Points.  Your how-to article is great, but it does not help me do what I am trying to do.

The elevator and rudder control surfaces on the P51 are not the same width along the length of the surface.  For example on the rudder, near the top, the hinging surface is only 1/2" wide, while near the bottom it is a full inch or more.

How do I round this surface off and still make the hinges look good?  I want to make them look as scale in appearance as possible.

If you are saying that there is not enough material at the top of the rudder, then you can add filler behind the leading edge of the rudder to capture the hinge.  You can also cut some of the barbs off the hinge, but I don’t do that unless absolutely necessary.

If that’s not what the problem is, then maybe you can explain it in another way and perhaps send a photo or two? I’d like to help, but I need to understand the problem.

I have taken some pictures and have done some research but am still not sure how I am going to do this.

On the plans, they use the usual method for using CA hinges with both sides of the control surface beveled.  If I am using hinge points I will probably want to round the leading edge, but this is difficult due to the odd shape.  Once the rudder is rounded how to I fill the gap between the two surfaces.

In my research I have read about two ways, but have not seen any good pictures or how-to's.  One way was to use triangle stock on the trailing edge of the fin or the stab, but to me this seems difficult to do with the weird shape of the rudder leading edge.

Maybe I am wrong and this is the way to do it, I don't know.  The other way was to use thin plywood to cover the area.  Once again will the plywood conform to odd shape.  If you have any ideas I would appreciate it.

I think I’m clear on what you’re trying to do now. You want to cover the hinge line for a more scale appearance, correct?

First, I would cut that part off the rudder that protrudes from the front.  Removing that piece will make the whole thing a lot easier.  You can glue it back on later.

The pin in the hinge point should be back a little farther so that it is centered in the radius you sand into the leading edge of the rudder.  Depending on how thick the rudder is, that may leave much of the forward part of the hinge unsupported.

You can add pieces of brass tube into the rear of the fin that stick out.  Glue the hinges into the tube.  That will provide a lot of support.

What I would do to cover the hinge line is use a router to relieve an area about 3/8” wide in the sheeting at the rear of the fin. Use pieces of 1/64” plywood or thin plastic to glue in the relieved areas.  Make the pieces wide enough to cover the hinge line.  You’ll have to mess around with it a bit to ensure the rudder can move freely.  Also, don’t forget to take into account the thickness of the finish.

One thing that I consider very important with any model is consistency throughout the project. A lot of guys want to deck out their first scale model, but aren’t really ready to build the model at all. Then when things start not working out they are disappointed with the project overall or put it aside or sell it.

I do not want to discourage you, but advise that maybe you build the model per the plan just to get a feel for it and gain experience.  You can always build another P-51 later.  The reason I mention consistency is that I’ve seen a several scale models that had lots of detail in some areas and no detail in others.  Those models end up looking kind of odd.

My point being that whatever level of detail you decide you want, you should commit to doing to the entire model. Don’t rivet the fuselage if you’re not going to rivet the wing. That’s just one way of looking at it of course. You may think that it’s good practice and who cares if the whole model is riveted.  I can’t really fault that line of thinking.

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Being fairly new to the model scene in the UK I am bewildered by the sheer quantity of servos available on the market.  The prices vary from one extreme to another.

The three areas which seem to have a bearing on price are the Torque, Speed and construction i.e. metal bearings, plastic gears, metal gears and now more recently digital servos.  Where do I start?

There seems to be no real guidance as to what you should need for what job.  If a servo fails in flight it would be 'goodnight Vienna'.

Do you have any advice or rule of thumb to help me select the right type of servo for the job?  How do you calculated the forces being applied to a control surface during flight and then relate it back to the torque characteristics of the servo?  So many questions and a BIG EXPENSIVE MINEFIELD.  Help please.

I asked this exact same question at RC Universe and nobody could give me solid numbers as far as torque to surface area.  A slower flying aircraft can use a smaller servo on a surface than one that is faster due to loads on the surface.

You would have to work out a formula that includes all the following information:

• Airfoil

• Overall Chord

• Control Surface Span

• Control Surface Chord

• Flight Velocity (air speed)

• Control Surface deflection

• Hinge Line location in relation to the airfoil's neutral point

• Probably other stuff that I haven't thought of.

The real answer is that a generic formula just doesn't exist as far as I know.  What you have to do is use your experience to choose servos.  As a beginner, you don't have that experience yet, so best to ask guys in your club about your specific application.

When in doubt, use a stronger servo, or ease your aircraft into maneuvers so you don’t kill your servos and crash your plane.  If the servo seems to be unable to hold the surface, then go to a larger servo.

Today’s servos package much more torque and speed in a smaller package than they used to.  For example, a Hitec HS-85 mini servo has as much torque and is faster than most standard servos that come with a radio control set, yet they are smaller and lighter.

• More information about servos

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I am just beginning to get into designing R/C airplanes.  I had a question about control surfaces.  How do you go about deciding where to put your control surfaces and the dimensions of them?

Also, what is a good way to find out if the servos that you are using are capable of producing enough torque for your control surfaces?  Are there any simple calculations that you know of?  If not, what factors should I consider or am looking at to determine this?

I’ve read this e-mail a few times and unfortunately, my answer for you isn’t an answer.  For new designs, I generally run the numbers of standard design parameters and then adjust until the plane is more what I’m looking for.  For example, if I want a plane that is aerobatic, yet smooth, I’ll start with a standard aerobatic design and lengthen the moments.

Once I have an outline, I start sketching in the control surfaces.  Again, I use "standard" sizes and then adjust until it looks right.  Aerodynamics tend to break down when building smaller aircraft, such as our models.

Now if I’m building another prototype of a plane I’ve already built, then I make changes based on observed behaviors of the model after ruling out everything possible in the prototype.  For example, an inability to spin may be many things, but most of them can be ruled out in the prototype.  Things like the center of gravity or fin and rudder size can easily be changed.

There are just too many different type aircraft and different behaviors to give a solid set of numbers that will work.  A lot depends on how you think a particular design should fly.

There are a few books on the market that discuss the aerodynamics of model aircraft in detail.  Unfortunately, they are way over my head so they aren’t much use to me.  The one I have is Martin Simon’s book, Model Aircraft Aerodynamics.  It covers a lot of theory in simple terms, but then goes into aerodynamics terms I don’t know how to use.  What that means is I can get some rule-of-thumb generalities from the book, such as efficient wing shapes for various aircraft types which is helpful.

The bottom line is that I have to go with my experience, which I have lots of, as well as just standing back and looking at a design and deciding if it looks right or not.  I’ve found that a design that looks right – is.

Last thing – this may seem kind of strange, but I imagine the plane flying.  Sometimes a design that looks like it will fly right, doesn’t look right in my mind when I try to “fly” it in a certain manner.  I adjust the design until it looks like it will fly as I imagine it to and I’m almost always right.  I wonder how many designs came about that way.  For me, it’s almost everything I’ve ever designed – especially those I put on paper first rather than made up as I was building.

• More of my design philosophy

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I had two of four CA hinges fail on my Kadet PT-40's elevator.

I cut the old hinges and thought to clean out the old hinge slots and install pinned hinges.  Of course, the elevator now binds! My idea now is to cut through the new hinges and make Monokote hinges.  I read your technique - good info.

My question is - what do you think about using self-adhesive trim sheet instead of regular iron on film? Do you recommend another solution to my dilemma? Thanks for the help.

I would not use self-adhesive film for hinges.  I'm not sure why, but it just doesn't seem like a good choice.

What I would do in this situation is remove the elevator completely and replace all the hinges with Dubro standard pinned hinges (flat).

I would not necessarily attempt to put the hinges back in the original locations though. It will probably be easier to cut new slots near the old hinges.

If you've never done any hinging before, maybe you can find someone in your club with some experience and have them show you how to do it properly.

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I think I’m kind of clear on what you’re doing.  The filler pieces do not need to span the distance from rib to rib.  About 3/4” wide is good enough if you can hit the middle of it (more or less) when you drill for the Hinge Points.

However, the filler piece should be deep enough to completely enclose the hinge.  If you choose to use hinge points, I do not think 1/4” fill will be enough.  The reason being that when you put glue in the hole and then insert the hinge, a lot of glue will be pushed out the other side of the hole if it’s open.  For that reason, I like to make sure the filler piece more than encloses the hinge and then I take care not to drill all the way through it.

I’m assuming this is an ARF and you’d have to cut away covering to get inside the wing.

You could use flat hinges and then pin them.  I would trust that and then you wouldn’t have to do surgery.  That is probably the route I would take unless the wing is already open and I can get to the trailing edge to add more material.

Also, you didn’t mention how many hinges you are using.  I would space them so that you have at least 4 per aileron, 3 per elevator half, 4 for the rudder and the hinges no more than 5-6” apart.  What that means is that you might need more hinges than the numbers I mentioned if the control surface is very long.

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I wanted to know if you think that it is necessary to purchase a drill press.  I have been doing a lot of reading on line and I am not sure.

I have been using CA hinges and the Great Planes Hinge Machine.  After reading your article on Robart Hinges I am not sure what to use.

Do you think that I should try the Robart Hinges out and do you think since I have not worked with then that I will be successful with them?

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Do you have any sketches, or detailed steps, that you would be willing to share with me about the mechanical flaperon set up?  I only have a 4 channel radio/7 channel receiver.  Thank you so much.

You need at least a 5-channel transmitter to use flaperons unless you don’t use one of the other channels – such as the rudder.  I don’t suggest you do that.  Flaperons require either a computer mix in the transmitter, an after-market device that plugs into the receiver or a mechanical system, such as a sliding tray or the Dubro mixer that attaches to a servo arm.

I have a computer transmitter, so that’s what I use.  Unfortunately, I don’t have any diagrams of a mechanical mixer, but I’ve seen diagrams in various places.  Try searching RC Universe or the internet using the term “mechanical mixer” AND "radio control."

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