I am new to this. Is fully symmetric airfoil better than flate plate in 3D flying? Will fully symmetric airfoil generate more lift and fly slower?

Andrew,

Deep question... many answers. Basically at the wing loadings that we are flying at, there is not too much advantage in a sym. airfoil. Basically the flat wing IS symetrical. A symetrical wing creates NO lift at a zero angle of attack (AOA). it simply holds the plane where it is, like a skier on the water. To create lift, you need to increase the AOA so that the wing simply moves up.

The flat plate "airfoils" do exactly the same thing.

Where the bennifit of a symetrical airfoil comes in is the stall. You can control the stall better with a real airfoil, because you are controling the place that the air seperates from the wings surface. A flat plate airfoil is GREAT for snaps, walls, and transition to High Alpha flight (harrier) the more "airfoil" you get in the wing the more the plane wants to "Fly" through the stall.

Sorry if this seems a little confusing....basically it comes down to a wash. They weigh so little and have so much power, you don't need to spend the time building a real wing.

Andrew,

My partner and I have studied this question quite
thoroughly.

I have debated the virtues of the symetrical, semisymetrical
and flat wing types on thi sforum before. It can get quite
opinoinated.

To me - I would have to say that it depends on what your
looking for. we have built about ten of these things so far
and the best planes that are the most stable have been
the ones with real airfoils.

Please keep in mind that even with airfoil types can get
a little squirly if you don't keep the wingloading down.
Lighter is always better.

Ronster

Okay, throwing in my two bits.

With these small of models, with their low airspeeds and wingloading, you can obviously fly both a flat plate and an airfoil and do well.

What I seem to notice with the flat plates, is that their flying characteristics are really nice as long as either (1) they are slower speed fliers (as in primarily indoor-capable fliers like the Shockflyer series) or (2) their flat plate is THICK, creating drag and greater separation of the air when the AOA is increased, creating lift.

Indoor displays of flat plates (Shockflyer, Funtana mini) are impressive. Some of the outdoor stuff, at higher speeds and adding wind, and the flat plates just don't seem to "feel" like they have the same solid tracking an airfoil plane has. Of course, as pointed out, flat plates do transition into "sub stall" situations differently than airfoil planes, and that can be a bonus in a lot of 3D maneuvers.

I personally prefer the "feel" of a plane with an airfoil, although I fly both. The bonus to the FFF-style planes is simplicity of construction and not a lot of cussin' when a plane gets smashed

There is an article in the February 1982 issue of Model Airplane News by Andy Lennon in which he plots the lift/drag ratios of various airfoils, including a flat plate, curved plate and semi-symmetrical.

It turns out that at low Reynold's numbers (<40,000), curved plates are best, followed by flat plates and finally semi-symmetrical.

At higher Reynold's numbers (>120,000), the lift/drag ratio of the semi-symmetrical has significantly improved, while the curved plate and flat plate remain roughly the same as they were at 40,000.

The formula for Reynold's number at sea level is

Speed(mph) X Chord(in) X 780

A model with an 8" chord, flying at 10 mph would encounter a Reynold's number of 62,400. For this model the curved plate and flat plate would provide more lift for the same drag.

At low speeds curved plate and flat plate are best.

At higher speeds (20 mph) the Reynold's number would be 124,800 and the semi-symmetrical would have a higher lift for the same drag. The lift/drag ratio for the curved and flat plate would be roughly the same as it was at the lower speed.

The key factor in the curved and flat plates is the fact that they are linear and their performance does not change with speed.

A semi-symmetrical airfoil can give a nasty surprise when coming in for landing because the lift that it provides decreases not only because of the decrease in speed, but because of the decrease in lift/drag ratio at low speeds.

For tapered wings, the tips of a semi-symmetrical wing would stall first because of the decrease in Reynold's number at the tips.

For low aspect ratio wings the Reynold's number would be higher at low speeds. Flat plates have an advantage because at slow speed their lift/drag ratio is not only higher, but would be relatively constant regardless of speed.

This helps explain the terrific performance of all the IFO's, Sukalyns and 3dExtras.

One benefit of a thicker section is the increased drag. Drag can be nice at controlling speed in aerobatics during downlines etc. I like the flat section on my Shockie but it is fast during vertical downlines.

Interesting to see the approach of the Germans who seem to have the most experience of small scale aerobatics. Planes like the Shockflyer and the Ultimate bipe used for 3D have flat plate sections, but the molded machines for F3Ai indoor pattern have thick fully symetrical sections like NACA 0012.

--
Dave

If you're a 3D nut, you just really need to have one of each to fly!

IMO the airfoil planes fly more like you'd imagine a plane to fly, with stronger tracking and presence in the sky.

The thin flat plates fly fast and do ridiculous tricks, especially snaps and the like, but they don't have the "feel" of a traditional airfoil plane.

To add to what Martyn McKinney said...

Another affect of the flat plate airfoil at low reynolds number is to linearize the lift curve throughout the angle of attack range, until stall. George Hicks showed me some plots that he did with a conventional aerobatic symmetrical airfoil (I forgot which one it was specifically). He plotted the Lift curve for many Reynolds numbers, and the low Reynolds number tests clearly displayed the non-linearity of lift vs. AoA, whereas higher Reynolds number tests (where the airfoil was designed to work) showed very linear lift curves. The flat plate, on the other hand, is linear in this Reynolds number range, which is why George chose the flat plate wing for the Tribute. From a handling qualities standpoint, any non-linearity will make it feel 'unpredictable'. Again, though, this is dealing with low Reynolds numbers. Anything larger than a foamy is probably out of this non-linearity range unless it flies super-slow.

Baron