Wing Planform Choice

[graupman]

I am continually puzzled by the way foamies are typically built. It appears to me that one of the most popular wing planforms for a foamie is a "half-ellipse." Essentially an ellipse cut spanwise so that the trailing edge is flat. I know that this shape is more efficient in lift per unit of area, but it has horrible stall characteristics, and I would think that this is an important aspect of low speed flight. Anyone have the answer on this one?

[jkettu]

"Horrible tip stall characteristics" is a desirable feature on planes that need to do lots of 'uncontrolled' roll manouvers. :D

[kcaldwel]

The very successful Discus full size sailplane had a planform like that because of its good stall characteristics. Admittedly the low Re of the very small tip on a foamy may do some strange things, but that basic panform shape evolved out of the "Schuemann" planform that is supposed to very good for stall and performance. There is still debate over that though.

http://www.geocities.com/jebbushell/COOKBOOK_FILES/hazel_schumann.htm

[ciurpita]

i thought the shueman planform minimized lateral flow across the wing. what do you think good stall behavior means?

[MCarlton]

I thought the idea behind the "half ellipse" was to gain better lift distribution across the wing?

I think part of the problem has been the modelling industry taking an aerodynamic solution which evolved from full size sailplanes made of rigid and accurate composite materials and then trying to apply that to something hacked out of a piece of foam, where the section at the tip will be anything but accurate and where those tips will probably twist and bend out of shape continuously.

I had an F3b model with "half ellipse" tips which showed no bad habits in the stall, I think because of the greater accuracy and section "thin-ness" that composite construction allows.

We're all still guilty I think of trying to design our models as if they were full size aircraft, aerodynamics work differently at our end of the scale and sometimes full size practice doesn't scale down well.

[ciurpita]

in general, for whatever shaped planform (rectangular to elliptic), the lift distribution is roughly elliptical. in other words, lift gradually reduces to zero near the tips in an elliptical shape as opposed to linearly (triangular shape).

if the lift coefficient is the lift divided by the chord, the lift coefficient will be constant for an elliptical planform. this means each section of the wing produces the same amount of lift per square inch.

for a rectangular plaform, the wing near the tip produces a lot less lift per area than near the root. in this case, the root stalls before the tips, allowing good laterally control since the tips/ailerons are not stalled. rectangular wings are good on trainers for this reason.

presumably, the entire wing of an elliptical planform will stall all at once, and this is considered poor stall preformance. but this might not be bad at all compared to a wing that is poorly contructed or has warped such that one side stalls before the other.

[BMatthews]

i thought the shueman planform minimized lateral flow across the wing. what do you think good stall behavior means?

Oddly enough it works to promote a spanwise flow towards the tips in order to try to avoild or limit the usual spanwise flow that is from the tips inwards THROUGH the tunnels formed by separation bubbles. By robbing the bubbles of this flow the tips help to delay their formation and when they do form to limit the size of the bubbles and promotes their early collapse when the Cl is reduced again.

And as for the foamies I think you're missing out on one other important design choice. One that has nothing to do with aerodynamics and everything to do with crashing. A rounded crescent tip will suffer far less deformation that needs to be flattened back out and taped up than a rectangular one.

It's just not fair to judge flat foamies by more classical aerodynamic criteria. These are models that use their disadvantages to advantage. The very charactaristics of the flat airfoil that prevents us using them in so many other types of model are used to advantage to produce a model that will stall deeply on demand and virtually halt in a blink instead of popping around a loop which more standard airfoils would provide.

[graupman]

ciurpita's thinking is the track I was on... I am not an expert pilot, so I am not sure where the greatest advantages lie, but it seems to me that a wing that loses lift gradually over several degrees of angle of attack would be more predictable and easier to control even in violent maneuvers than a wing whose entire span stalls suddenly at a single angle of attack. Essentially, removing the "hump" on the cl vs. alpha curve. Obviously its less efficient, but who cares about efficiency on a foamie?

Another question from one of the posts above - is there a distinct roll rate advantage to an elliptical wing or a slightly swept wing?

[nmasters]

Another question from one of the posts above - is there a distinct roll rate advantage to an elliptical wing or a slightly swept wing?
Decreasing the the wing's inertia and the distance of the aerodynamic center of the half span from the longitudinal axis is the way to increase roll rate. Taper does that. Elliptical is just fancy taper. Sweep alone doesn't affect the roll rate.

--Norm

[Brandano]

Later marks of spitfires had trimmed tips to improve the roll rate. I think that the distribution of mass is more important than the wing shape for this. Oh, and how far outboard you can get the ailerons to be

[BMatthews]

They are looking for low roll damping rates as well. For that you want a tapered wing. The crescent shape ensured that the ailerons are kept wide all the way out and the shape is quite sexy to look at. And with flat depron being the material of choice it's easy to use the sexy looking option. Add on the ability to reduce damage during a "landing" and it's all win-win-win.

And finally at the heart of the matter these are models made to fly as if they were in a front load tumble dryer. I tend to think we're overthinking the whole thing. The pilots of these are far more worried about a nice non-oscillating airflow that will provide a smooth harrier manuever than they are about lift coefficients.