Model Aircraft Aerodynamics

(I will stick to equal incidence I think but erring on the side of less on the bottom wing)
Is that a contradiction?

The view is very nice from up here


It would be difficult to build it with exactly identical wing incidence top and bottom so the error will be in setting the bottom wing a nat's less.
Its a bit like setting the front wheel alignment up on my Healey Sprite , which is 0 with the error being towards toe out !!

Sorry, Eric - it's not nit-picking at all. The CofG is a fixed point within the airframe. The only thing that can change it is a change in the a/c's weight - for instance, fuel being consumed. What do you mean by 'centre of mass'? Mass again is a fixed quantity so how can its 'centre' move?
Mike

Andrew - The only difference between a full size aircraft and a model (being pedantic, they are both, after all, the 'real thing') is the size. Fundamentally, the aerodynamics are the same. Increasing dihedral and tail area will help stability in both aircraft, but what we're talking about here is different incidences on the wings of a biplane and it's effect on the stall. This is applicable to both full-size and models.

Here's an aerodynamic view. On our scale model biplanes (and most full-size biplanes), the C of G is in front of the C of P of the wing. The wing lift is upwards, the tail lift is down. When the wing stalls, the wing lift reduces to zero and the only forces we have are the weight acting downwards and the tail lifting down. The weight is the more important, so the nose drops. This causes the speed to increase, which reduces the incidence on the wing and recovers the wing from the stall. If we have differing incidence on either wing, we delay the reduction of lift and therefore delay the nose drop, which, in turn, delays the stall recovery. Ergo, I believe the incidences on each wing should be the same for good natural stall recovery.

John - The Jedelsky wing is a form of all sheet wing construction. It's not really relevant to a discussion on the merits of biplane stall recovery with respect to wing incidences....unless I've missed the point?

Andy Sephton wrote: Andrew - The only difference between a full size aircraft and a model (being pedantic, they are both, after all, the 'real thing') is the size

Err no, one has a pilot, the other doesn't!

I did after all add the caveat that what I was saying applied to free flight models only. The real aircraft can be inherently unstable within the capabilities of the pilot and to a certain extent needs to be unstable to make it manoeuvrable whilst the model is modified to make it more stable and to recover from disturbances the best it can on its own. After all a free flight model is not designed to be manoeuvrable in any way, a full sized or RC aircraft is.

As you know modern aircraft are even designed to be unstable even outside the capabilities of the pilot to make them very manoeuvrable only staying air due to the electronic brain behind the scene constantly monitoring and adjusting things.

I have no further opinion on the aerodynamics, I would have to delve into dusty textbooks to argue. But I just cannot agree that what is good and right for a pilot controlled aircraft is good and right for a free flight model due to the inevitable difference in their control mechanism - otherwise known as the pilot.

Andrew

Andew I can only agree with your sentiments, as there can also be an element of luck in when we fly. A slight disturbance at the wrong time and it all goes to custard. The question of rigging angles to me seems to be bordering on folk lore as there is no documented evidence in the shape of reports at model scale to support them. I am struggling with a peanut Jungmister at the moment that I just cannot get to glide and will be having a hard look at the wing incidences. About the only thing we have to regcognise is scale effect from low Reynolds numbers that has been well researched.
Ricky

By missing out a single word, it seems I may have given you the wrong impression Andrew. Aerodynamically, the only difference between a full-size aircraft and a model is the size. The laws of aerodynamics apply equally to both, whether they be designed to be controlled or not, or to be unstable or not.

I also believe that the original question has been confused. I thought the discussions was on the relative merits of differing incidences on biplane wings. The question of stability enhancements, such as increased dihedral, are a different subject altogether.

When looking at stall recovery with respect to biplane wing incidence, I can't see any reason why, aerodynamically, there should be any advantage to setting the wings at differing incidences. Indeed, theoretically, they should be set the same. However, if there is a practical perception that setting a wing one way or another gives improved performance, then that is what one should do. After all, one has to be content with the aircraft that one has produced. What is clear is whether one setting or another is the best way to go is a moot point!

Andy Sephton wrote: By missing out a single word, it seems I may have given you the wrong impression Andrew. Aerodynamically, the only difference between a full-size aircraft and a model is the size. The laws of aerodynamics apply equally to both, whether they be designed to be controlled or not, or to be unstable or not.

I also believe that the original question has been confused. I thought the discussions was on the relative merits of differing incidences on biplane wings. The question of stability enhancements, such as increased dihedral, are a different subject altogether.

Hi Andy

On the first point I Generally agree.

On the second I would say that that is just the point, the the reason I drew the comparison to dihedral is that I view that the different wing incidence exactly as a "stability enhancer", the mechanism is different but never the less the aim is to try and produce a degree of self correction to disturbances.

Ok I didn't have to refer to my text books, A good think back was required! I agree with much of what you said in your post regarding the Aerodynamics apart from some of the semantics. IIRC the CP moves forward, then rapidly backwards with separation, and the lift drops off rapidly and drag increase hugely due to the turbulent wake. It never really gets to zero, but the full aerodynamic effect is lost. (after all a aerofoil being pushed through the air will create a "lift" simply by the component of deflected air). So we are at the point in your description where the wing isn't doing its job too well, and there is an overall nose down pitching moment due to the CP moving backwards form the C of G and here we both agree. BUT your description fails to describe what is happening with the bottom wing. At the same AOA the bottom wing (disregarding the interference effects between the two just for the sake of simplicity) suffers the same effect and the week rearward lift helps to pitch the nose down too. BUT - and here is the important bit. If the lower wing isn't stalled because it is at a lower AOA, its "strong" lift (and moment arm to the CG) produces a much stronger nose down pitching moment that would have otherwise occurred thus enhancing the stability. I also in contrast to your latest post I didn't say it would improve performance in fact said that the "differential set up" would reduce performance (and duration). This is because neither wing would be operating at it's best AOA, but this is a trade off worthwhile IMHO to improve the pitch stability.

Andrew