Why symetrical airfoil for acrobatics ?

[Flyfan]

What is supposed to be the advantage of symetrical airfoils ?

I'm slowly learning acrobatics, can do 2 or 3 ailerons rolls of a rolling circle but not knife edge yet.

I built a symetrical wing with a GWS geared 400 for acrobatics. It flew Ok but I have other wings, usually with MH32 airfoil that fly seemingly just as well, especially inverted.

The symetrical airfoil was slower, I think (it went a bit overweight so it's hard to be exact).



Flyfan

[Ollie]

Airfoils with mean line camber like the MH32 can't develop as much lift inverted as they can upright. Because of this, a plane with a mean line cambered airfoil can not do an outside loop as tightly as it can an inside loop at the same airspeed.

Increase the elevator throw so that full elevator is not required to do tight inside and outside loops. Using a stopwatch, time the tightest inside loop that the MH32 will do without stalling and note the size of the loop. Then time the tightest outside loop that the MH32 will do without stalling and note the size of the outside loop. With this controlled experiment, I think you will easily see the difference in the MH32's performance upright versus inverted.

A mean line cambered airfoil has more drag inverted than it does upright for the same angle of attack (using the zero lift angle of attack as a reference). Therefore, the meanline cambered airfoil will be slower inverted than it will be upright for the same thrust. Because the MH32 is not very highly cambered, its differences upright versus inverted are a bit harder to see than if you were comparing a more highly cambered airfoil but the differences are there never the less. The MH32 has much less drag than a thick symmetrical airfoil so the MH32 will be faster for the same thrust. That fact may be making the MH32's difference in performance between upright and inverted harder to compare to a thick symmetrical section. If you compare a symmetrical airfoil of the same thickness as the MH32 instead of a thicker symmetridcal airfoil, the difference in the upright and inverted performance of the MH32 may be easier to see.

Symmetrical airfoils with no mean line camber perform exactly the same inverted as upright.

[RSCherry]

The symmetrical airfoils "Slower" feeling was likely due to the thickness. Many symmetrical airfoils intended for aerobatic use are relatively thick to add drag- this helps control speed on downlines so your loops are consistent, etc. As Ollie said, the symmetrical airfoil should perform as well inverted as upright, just what you want in an aerobatic plane that spends a lot of time upside down. Knife edge is very dependent on the plane you are flying and how it's powered. You have to have enough fuse side area to support the plane in flight.

[Flyfan]

Thanks for the replies, I see the point about inverted/upright being the same. I hadn't thought about the different speed or tightness of loops.

I think the symetrical airfoil that I built was too thick, I might try again but thinner.

Since I posted yesterday I downloaded a few mpegs which by co-incidence had some knife edge flying - the only real life knife edge I've seen were two fast planes and their fuse was horizontal during the knife edge - these mpegs show planes more like mine -

(I'm working on knife edge with my 90% Ultimate Bipe Dual IPS 'B' drive 2S700etec 13x6 GWS prop - 3mm depron and flat plate wings)

- and the fuse is closer to 45 degrees from vertical. I think I'll try that, it looks good on a bipe too.

I'm thinking of getting a brushless for the bipe, maybe the PJS, still researching small brushless, nearly bought a Rotorex 'Phew'

I'm also waiting for a Revolution 3D EPP kit, that seems the most suitable plane to carry on learning.

Flyfan