Cessna 172 Rudder/Elevator Control

[deanoaz]

I am building the 172 with plans to only use rudder/elevators for controls. This might be a stupid question but, if there are no plans to use the ailerons, do you build them as shown on the plans or do you just tie them in for a solid wing trailing edge? My devious mind got to thinking that those hinged ailerons flapping in the breeze might effect the flight characteristics.

[Mitch]

My only experience with RC, is I built a 2 channel glider years ago (Hi-Start Launch), then I looked for thermals. My 2 channels were rudder and elevator. The wing was solid. The plane is able to turn, you give a little rudder and up elevator (because the plane will want to go into a down attitude). The proper way to turn is use your ailerons, but 2 channels work. My choice would be solid wing. I expect you will have a speed control, so you would have 3 channels? That is what I would do.

I have no experience with powered RC flight.

Mitch

[Johnny ace]

I used to fly rc, and I agree with Mitch.the ailerons would need to be firmly attached and straight.It does not take much for them to be affecting the flight path of your plane, just like a trim tab would.

[deanoaz]

Being a relative newbie to RC (25 years ago I did some) and being an engineer by education/profession, I just thought you wouldn't want those ailerons hinged and flapping uncontrolled in the breeze to effect the flight characteristics. I think I will build them solid into the wing.
Thanks

[Bill Gaylord]

Unless you plan to add a good bit of non-scale dihedral, the 172 would not be a good model to fly with rud/elev. I've flown a few models with low dihedral and rud/elev, which had to be flown carefully and were not highly controllable. There's a reason they call ailerons yank and bank, as you need to have something to initiate the banking, whether it be dihedral or ailerons. The turns have to be very slow, or they will not coordinate. Being a high winger, it will be a bit better than a low winger, but that's about it. Even with ample dihedral, my second 400 series FW190 build was not easy to fly, until adding ailerons. Fine flyer with ailerons, but had to be flown very carefully with rud/elev. Another issue is that the prop right thrust offset has to be pretty much perfect, without ailerons and ample dihedral, as you need both ailerons and rudder to trim out less than perfect thrust adjustment. I spent a good bit of time adjusting thrust angles on models, especially 3 channel.

The high wingers with dihedral on 3ch fly fine. I'm convinced I could hand a complete beginner the transmitter for the Guillow's C-150, with a bit of pre-flight instruction. It pretty much flies itself, without much elevation loss on reasonably gentle turns, even without elevator fed in.
https://www.youtube.com/watch?v=U7gySPVBtyo

[dirk gently]

The proper way to turn is use your ailerons, but 2 channels work.

Actually, the proper way to turn is to do a "coordinated turn" that for the most part requires both rudder and ailerons.
http://en.wikipedia.org/wiki/Coordinated_flight

This plane, having a tall fuselage and dihedral (well, the dihedral only applies to the model, not the full scale) will however have a decent amount of what's called yaw-roll coupling, and will be able to do a reasonably good turn with rudder only.

You do not want to build movable ailerons and fix them in place, you want to build a solid wing with no ailerons.

One way to do a 2 channel airplane capable of coordinated turns is to do mechanical mixing - using the same servo to move rudder and ailerons, but I would strongly recommend against that for your first build.

[Bill Gaylord]

From my experience with these models, for r/c it's common to only have aileron/elev control. I have a number of them. Once banking is initiated with aileron input, the elevator is essentially a combination rudder/elevator, considering the banking angle that the aircraft is now in. The elevator is essentially a combination rud/elevator, due to the banking angle. With low a low dihedral angle the banking will hold well for some time, where the ailerons are nearly recentered and from that point on, and the elevator is fed in to continue and hold the banked turn and altitude. On higher aspect ratio planes and some biplanes, I've mixed a small amount of rudder in which simplifies flying, since some of those models will not turn well with ailerons alone. The C172 is not one of those models however, and flies well with aileron/elevator. I have a functional rudder on mine, but it is not mixed and is not necessary for smooth flying and good control. The Guillows Rumpler biplane on the other hand absolutely required rudder mixing, as the adverse aileron yaw would actually cause control reversal.

Bottom line here with the C-172 model however is that you really want aileron control, unless built as a non-scale gull wing subject. I started in modeling thinking that rud/elevator would be easier to fly. I quickly realized that is not the case, other than for high wing subjects with reasonable dihedral. Once onto ailerons however, I'd still rather have those subjects with aileron control. Built in more recent times, my C-150 would have aileron control. Single servo with pre-formed .015" music wire adds little weight, for those like myself who are not the biggest fans of micro linear servos. That was used on a recent Guillows Curtiss Robin build.

[Mitch]

Thanks Bill,

Very interesting. I like to answer questions, but should stay away from things I do not have direct experience. My R/C model was a high aspect ratio, 6 foot ws glider called the "Pussycat" It had a polyhedral wing and was easy to fly with Rudder and Elevator.

It crashed when I let someone else try to fly it and when I realized they were getting into trouble I asked for the transmitter back. I had to ask 3 times until they finally gave it back but then it was too late. I made repairs and she flew again, but never as good as when she was new.

Lesson learned for me. I took the time to build it I fly it. You want to fly, build your own plane.

Mitch, Keep Building, Keep Flying and Keep sharing information, but keep your plane for yourself.

[deanoaz]

Thanks for all the valuable input. After many years out of RC flying (25), I have been going out to the nearest field to watch/learn. The club guru there (really good flyer and technical expert) also said USE THE AILERONS! I guess I am rethinking my idea of only using rudder/elevators.

[davidchoate]

I would not add ailerons on anything smaller than a 300 series. If you use rudder/elev only. Add dihedral. All My foam traimers that are 3 channel have alot of dihedral.

[dirk gently]

+1 to what davidchoate said. If you are not an experience flyer dihedral and rudder are the way to go.

[Bill Gaylord]

The problem with adding dihedral to the 172 is that it would most likely have to start from the beginning of the taper point rather than the wing center, making it a gull wing. That would really wreck the appearance of the 172. Of course the wing construction could be modified to add dihedral breaks above the side windows, but again it would not be a scale 172. The thing to consider about ailerons versus rudder, is that it's the ample dihedral angle that really makes the rudder plane easier to fly, and not so much the rud/elev control versus ail/elev control. My Guillows P51 has a fairly heavy dihedral angle, and is almost high wing trainer easy to fly with aileron control.

[dirk gently]

I thought about adding single dihedral point (not polihedral) in the center of the wing. It would not be scale, but hey, if you fly it rubber powered you also need to add dihedral. The good news is, with the tall square fuselage, the 172 will probably require only a minimal amount of dihedral. I even saw similar models fly on rudder/elevator with zero dihedral, the fuselage alone providing enough roll-yaw coupling.

[Bill Gaylord]

I thought about adding single dihedral point (not polihedral) in the center of the wing. It would not be scale, but hey, if you fly it rubber powered you also need to add dihedral. The good news is, with the tall square fuselage, the 172 will probably require only a minimal amount of dihedral. I even saw similar models fly on rudder/elevator with zero dihedral, the fuselage alone providing enough roll-yaw coupling.

It could be done, but it would reduce the strength of the molded canopy top, as the dihedral would force cutting away of center top area of the canopy that saddles the wing center. I cut away a small area for aileron servo clearance, but did not want to reduce the strength of the part. Of course the wing center design could be modified to essentially shave off the dihedral "v" protruding from the bottom, so as to not have to modify the canopy.

When I first started building, I put off what I thought was the "complexity" of aileron flying, until it became completely impractical to build certain subjects with rudder/elev control. Looking back, I would have simply taken the challenge of learning aileron control from the beginning, as it's much more versatile and almost easier, once a person gets onto it. My analogy would be that beginners (speaking for myself when I was one also) want to fly a plane that is controlled like an rc car. A decent high wing trainer with ample dihedral will allow for that. With ailerons, the difference is that you have to have a feel for how the plane itself really wants to fly, and then be a gentle on the controls, voting member in the scenario. Once a person has that feel however, they'll never look back and will want aileron control.

[dirk gently]

It could be done, but it would reduce the strength of the molded canopy top, as the dihedral would force cutting away of center top area of the canopy that saddles the wing center. I cut away a small area for aileron servo clearance, but did not want to reduce the strength of the part. Of course the wing center design could be modified to essentially shave off the dihedral "v" protruding from the bottom, so as to not have to modify the canopy.

I think there is something I'm not getting here. Isn't the 172 meant to be a free flight model (that deanoaz is converting to RC)? As such, doesn't it have dihedral anyway? For rudder-elevator you still need less dihedral than for free flight (I imagine, I never flown FF other than cardboard gliders).