Remote controlled aircraft: Biplane efficiency

[check]

I was wondering about the benefits of building a biplane for an aero-design contest. The rules state a particular wingspan limit, but there is no limit on number of wings. There is also a limit on the type of engine. The plane that’s able to lift the heaviest payload and still be controllable wins.

Any thoughts or suggestions? Thanks.

 

[Rothiemurchus]

Go for a single wing with a large surface area.Then the ground effect will
help get the plane in the air ( the ground effect occurs close to the ground where
there is less downward reaction force on the wing - a reaction to upward deflected air - than normal).For a large full scale plane you would need a material that is stronger than a biplane needs while being light too.
But for a model there should be no problem.

 

[Owen]

I agree with large wings but if wing span is limited then an ever decreasing aspect ratio with increasing area would be a bad thing (harder to control, in roll especially, as well as a drop in efficiency). I'd say that if your looking for high lift to carry plenty of weight then a bi plane is the way to go. make sure the wings are staggered to minimise interferance between the upper surface of the lower wing and lower surface of the upper wing. and to aid controllability ailerons on both wings would be an advantage.

I haven't built/flown any RC models in quite a while now but i always favoured biplanes for their smaller spans and higher roll rates.

Hope it goes well

PS. if the engine is driving a propeller are you able to change it? I found in the past that swapping props (with no real scientific thought, just trial and error) could lead to quite an increase in performance, especially at low speeds.

 

[check]

Thanks for the suggestions,

Owen, I'm not sure about swapping props. The team is meeting on thursday and I'll bring that up. Thanks!

 

[Owen]

I seem to remember it working quite well, basically if you run out of lift, increase the angle of attack to an absurd amount and nail the throttle. hangs quite nicely on the prop, with plenty of authority on rubber and elevator, another advantage of higher aspect ratio, roll control with rudder is easier.

if you need help with prop choice (if you are allowed to that is) then let me know and i'll find my old notes.

let me know how it goes

Owen

 

[check]

Ok, well the first meeting was just an orientation thing. The next meeting will likely be the same thing. We probably won't start working on designs for a couple more weeks :S but I'll keep you posted.

 

[ChrisHarvey]

A few thoughts...

If heaviest payload and controllability are the only criteria, I think the key will be to try and minimise on the weight of the rest of the aircraft and make it as fast as possible. A compromise must be made here with engine weight. This way, for a given wing area, you get more lift. Therefore speed will cancel out your wingspan limit disadvantage, and remember, lift increases linearly with velocity squared !

Do you have to carry different loads from next to nothing, to as much as possible? Or is the aim just to carry 1 massive unchanging load?

If it is the latter - it should be easy. Store it close to the centre of pressure for the aircraft (will stop unwanted pitching), and use canard forplane - this will improve stability, and this way both wings will contribute to lift! A tailplane arrangement would cancel some of the main-wing's lift.

If it is the former (different loads up to a maximum), I think you may have problems with stability here. Unless you keep the payload in the same place as the engine, you will find the centre of gravity moves quite a lot as it increases. This may well be what limits the controllability. It will be easier if you can move the payload up and down the fuselage to try and put the CoG at the CoP and still use the foreplane idea. You may find a canard foreplane easier to design than a biplane.

Make the wingspan as long as is allowed, but don't give it too long a chord - you want as little drag as possible, so you can get maximum efficiency and acceleration from the engine. You want to reach those higher speeds!

 

[Owen]

If the type of engine is limited then surely so is the power output? for a constant power output i would think that when considering the relativley small range of speeds available (with a heavy payload and so low acceleration) a greater angle of attack with as large a wing area (within reasonable aspect ratio limits) would be of more benifit than a greater speed. not only is lift increased with angle of attack but the vertical component of thrust also increases, with purpose built models (as well as some full scale aircraft) it is possible to fly well beyond the stall with sufficient lift provided by the propeller. whilst ths apparoach isn't recommended due to the difficulties with control (unless helicopter type controls are employed) it may be used to some extent with large, but unstalled, angles of attack to increase lift at no cost in drag.

I agree that canards are more efficient but from my (rather limited) experience with model aircraft i'd say it is rather more complicated fitting a canard system. weight is all important and because a canard nearly always has less of a moment arm about the CofG considerably more structural reinforcement is required to cope with higher loads, all this does is increase weight, and move the CofG further forward. also, fitting a canard requires moving the main wing (or wings) further back, to avoid any unwanted interferance. while this does move the CofG aft again it also means the fuselage between canard and wing must be reinforced, to cope with the large loads from the wing. depending on the method of construction a canard could be made to work, but if conventional methods (balsa wood, ply wood, etc..) are used then it would be impractical.

keeping the bulk of the reinforced structure towards the front. the firewall to mount the engine on, obviously needs to be strong, and a strong structure from firewall to wing root, from the wing to the tail need not be as strong because of lower loads at the tail. if a biplane design is used then mounting of critical parts such as control system (servos) and fuel tank can usually be between the wings taking advantage of the structure already in place.

by all means use a canard but it needs considerable work to refine the design for both good aerodynamic and structural properties

as usual I may be completely wrong so feel free to challenge my ideas.

Adios,

Owen

 

[check]

Thanks for all the great advice Owen. Though some of what you have suggested won't be possible because of the rules of the competition (which can be found here: http://www.sae.org/students/aerorules.pdf) I will bring up some of the things you've mentioned at the next meetings. Thanks again!

 

[Owen]

I'll have a look over the rules later and offer any help I can, I'm off to university tomorrow though so may not reply for a few days. did you say that changing props is not allowed? that's a shame, but provided the prop you are given has enough pull (anything above about 8" pitch) the use of high angles of attack will still give quite a boost in vertical thrust component. No need for thanks as i love this sort of competition (i've was involved in one myself some 3 and a bit years ago, was only 15 at the time, slightly more relaxed rules, came 4th)

 

[ChrisHarvey]

I agree that power output is limited by the engine type (should have read the first post more carefully!), so maybe speed isn't the best idea.

Given that higher speed is now ruled out, you are left with 2 options: wing area and changing the lift co-efficient. Wing area is limited span-wise, and increasing the chord too much increases drag and reduces controlability, so the chord musn't be too big. Angle of attack will undoubtably need to be a little higher, but given the low speed, the model will be in danger of stalling and I'm not sure how well the prop. will supplement any lift from a stalled wing, given that the plane is likely to be quite heavy! Too much camber will lead to the same, so really more than 1 wing providing lift seems to be a good idea.

I don't know much about biplanes, so I'm not sure if I'm right in saying that effective angle of attack is decreased in using them (due to downwash from the other wing, top or bottom)? There are of course 2 wings, so is the loss of lift here always more than countered, or under certain conditions, is there cause to be careful?

I hadn't realized the structural implications of a canard on a model aircraft. I just thought you could get the benefit of both extra lift and stability, whilst removing the tailplane. Unless the CoG is forward (which it usually is) of the CoP (basically put) when using a tailplane, you can have run away pitching - so you would need to find space in the front of model for the payload too. The canard would have allowed some flexibility here, because the CoG could move back a little.

One more thought, although the canard idea has probably been discarded, if you completely discard the tail (thus removing weight), you could sweep back the wings (and move them back at the same time) and give your model vertical surfaces on the wing tips (like on the Beech starship) - these would act as fins, and would reduce vortex drag. I do however realized the difficulties in implementing control surfaces on the fins!

I should add that my experience in model aircraft is less than limited, and I know owen knows quite a lot about it.

 

[ChrisHarvey]

Actually, now I think about it, that last idea was pretty terrible because it reduces what valuable airspeed you already have. Sorry !