Scioly.org

Hey guys,

What kind of planform shapes are you guys using? (Planform = the shape of the wing from above, right?)
I've been trying out rectangular wings without much success and was wondering if about an elliptical or semi-elliptical design. We're kind of inspired by some F1D models we found online (https://nffsblog.wordpress.com/2012/08/ ... d-model-9/ and http://indoornewsandviews.com/2013/01/0 ... nship-f1d/) but were worried about the lost area and also making the wood curve. Any thoughts/tips?

Also, any veterans have tips on how to design our own planes?

Thanks!

If I've learned correctly, the elliptical wing shape is to minimize drag - it's a trade-off that is optimized so that although lift is lost, drag is minimized. With Science Olympiad models, the wing area is so small I've hardly seen any teams use non-rectangular wings to maximize lift - and for starting, the reduction in drag should hardly make a difference. If you share observations of your plane in flight, I'm sure other fliers on the forum, many experienced, would be happy to suggest adjustments.

If you are having trouble making rectangular wings work then I would advise from trying anything more complicated. For other lighter models the gains from a reduction in induced and tip drag outweigh the increase in wing loading. But I think the loss in wing area from using an elliptical platform would not be worth it for something like wright stuff where the wing loading is already so high. Bending the wood and making repairs would also be a huge pain. I would recommend sticking to a rectangular wing for competitors of all levels.

On the other hand I think the planes this year could benefit from something like a polyhedral, as the instant I made the switch from a normal dihedral my model did not torque roll and it was much more stable.

I'm going to second everything DoctaDave said above. The polyhedral is quite beneficial, and almost no more difficult to construct than standard dihedral. But the rectangular planform with lower wing loading is extremely likely to have benefits over the elliptical wing other than ease of construction. The short answer is that lift/drag ratio matters less and less as wing loading increases at these low speeds. The long answer is this:

Lift equation:

Drag equation:

For level flight, L = mg = constant, so, substituting L = mg and solving for the velocity required to maintain level flight:



Then, setting T (thrust) = D (drag) for constant velocity and solving the drag equation:



Setting the two equal, eliminating the variable v, and solving for thrust required to maintain level flight:



Multiplying both sides by the earlier expression for v, power required for level flight is obtained:



Power required to maintain level flight is a largely accepted measurement of efficiency for indoor models, as the lower the power required to maintain level flight, the less torque required, and thus thinner rubber which can take many more turns can be used optimally.

It's easy to calculate what kind of advantage in drag coefficient would be needed for an elliptical wing to have the same efficiency as a rectangular wing:



Where the subscripts e and r denote elliptical and rectangular, respectively. Eliminating variables and manipulating this equation:



Substituting in values of area for 45x6 rectangular and elliptical wings:



This means that the drag coefficient of an elliptical wing model - not just the wing, but the whole airplane - would have to be well over 30% less than that of a rectangular wing to have any measurable advantage. If the wing were the only source of drag on the model, I could see this being reasonably possible, but given that the stab and rudder are also major sources of drag and the fuselage can actually have quite a bit of profile drag which is unavoidable, I just can't see the (very complicated) construction of an elliptical wing being worth the effort, nor can I see the elliptical wing itself being beneficial. But then again I'm not an aerodynamicist, and my knowledge/intuition of the behavior of drag coefficients is not the most extensive.

I might come back and give some general design tips, but this is enough forum posting for me in one day.

Wow, thanks to everyone for their advice and insightful comments. I'm going to try to decode those equations later but I think I understand why elliptical wings might be excessive now. I think I'll stick to rectangular wings, though now I'm also curious about wings with tapered tips. Is the point of it (pun intended) just to approximate an ellipse? Should I try tapered wings or strictly rectangular?

And I'm definitely going to try the polyhedral! The models I was looking had a elliptical dihedral, which we were going to try to emulate, but I realize now that doing a polyhedral is much much easier.

Right now I'm in the process of drawing up plans for the next plane, so if anyone has design tips they would be much appreciated. How exactly does one decide the exact dimensions/angles for the planes? So far we've been following other people's guides with a few modifications. Is there anything special to keep in mind when adapting someone else's plans? How do we go to drawing our own plans?

Thanks again for all the advice so far!!