Flat plate Vs proper airfoils

[Terry Rigden]

I see a lot of the small park fly and indoor models these days use simple flat plates for wings. I can see the advantage of quick easy to make wings but whats the down side ? I would have thought that compared to a proper airfoil you would loose lots of lift , get a lot more drag and they would have a nasty sudden stall. Or are these effects masked at the low reynolds numbers these models fly at ?

What it comes down to is - If Im designing a small <1m span , light <350g aerobatic model is it worth my while making proper wings or will slabs of foam do just as well ?? I'm looking for a model that will cope with a wide speed range and not have any nasty habits near the stall. And I dont want to have to overpower it so that 50W hauls it round the sky and does normal aerobatics.

Regards

Terry

[raptor22]

I would have thought that compared to a proper airfoil you would loose lots of lift , get a lot more drag and they would have a nasty sudden stall. Or are these effects masked at the low reynolds numbers these models fly at ?

Bingo. They are draggier, and don't produce as much lift, but with those tiny, slow planes, they are very nearly as good as a "real" airfoil. The stall is more sudden, but its nto to hard to cope with, and for 3d a clean stall is better.

--Alex

[Ollie]

"What it comes down to is - If Im designing a small <1m span , light <350g aerobatic model is it worth my while making proper wings or will slabs of foam do just as well ??"

The high thrust over comes drag. So the airfoil drag is not much of a problem.

The light weight and large wing area is not much of a lift problem.

Compare the flat airfoil Cl vs. Cd polars with thin (<9%) airfoils, zero mean camber airfoils. There is no lift problem ( Cl=0 to plus and minus Cl=0.7 near stall). The only problem is higher drag. Then the very high thrust is enough compared to drag.

With the model you describe, it is because it "flies on the prop."

[Salto]

Another consideration is that it's easier to make a wing with some thickness (ie. an airfoil) adequately strong and stiff. Sure, a carbon tube imbedded in a flat sheet of foam can give it sufficient bending stiffness, but it may not have adequate torsional stiffness for crisp, predictable aerobatic flight behaviour. There are elaborate systems using Kevlar threads etc. for imparting torsinal stiffness, but to my mind these solutions are abominations.

The reason that flat plate wings are so popular in kits, and for scratch building is their ease of building and low cost. There's certainly no structural or aerodynamic advantage in using them.

Having said the above, I have to admit to having made dozens of flat plate foamies, but I kick myself every time!

Graham.

[wjbite]

Flat plates perform equally well when they are inverted.
3D pilots spend lots of time upside down.
So all the "minuses" of flat plate are very acceptable if you have enough power to fly straight up. :)

[BMatthews]

If you look at the videos of the flat plate models you'll see that they dart about with shots of high speed punctuated by sudden stops. The flat plate airfoil stalls easier than a "proper" one and it is the drag from these easily produced stalls that halts the model in mid air. To keep going it takes power to fight the drag. But in this usage the drag is a good thing.

If you just want to fly stunts without doing the true sudden stop 3D stuff then a proper airfoil is the way to go.

[Terry Rigden]

Thanks Guys

It looks like I wont be happy with the flight charecteristics of a flat plate wing, so I'm off to start cutting out some Eppler 385 wing ribs.

Regards

Terry

[Salto]

Do you really want to use Eppler 385 for a lightweight aerobatic model? It's got 5.7% camber!

Why not use something symmetrical like Eppler 479? It's moderate Cl will be fine if your model is light, and inverted performance will be the same as right way up.

Graham.

[wjbite]

Jeff:
Found this site that gives some not too complicated calculations to help decide whether to use FLAT PLATE vs. CURVED PLATE vs. CONVENTIONAL airfoils.
Hope this helps.

Walt

[Terry Rigden]

Do you really want to use Eppler 385 for a lightweight aerobatic model? It's got 5.7% camber!

Why not use something symmetrical like Eppler 479? It's moderate Cl will be fine if your model is light, and inverted performance will be the same as right way up.

Graham.

Good point Graham I'm using the E385 on an existing model and it seems to be doing the business.
OK it needs a lot of down to hold inverted. But my reason for choosing it was to get a good climb rate on moderate power (75 brushless W/Lb). But for the next model there is a lot to be said for a fully symetrical section as you suggest.

Regards

Terry

[BMatthews]

Stick with symetrical if you plan on spending pretty much equal time upside down as right side up. Get your climb from low weight instead. In otherwords increase the max available W/lb.

[Chrisbee]

And of course, that's the main advantage of flat plates on small models, low structural weight.

What you gain in efficiency with a full aerofoil, you stand to lose more in adverse wing loading at these sizes.

Chris B

[BMatthews]

And of course, that's the main advantage of flat plates on small models, low structural weight.

What you gain in efficiency with a full aerofoil, you stand to lose more in adverse wing loading at these sizes.

Chris B

For how some folks build I agree... :D But if the airfoiled wing is built using techniques from the free flight world and it's covered in the lightest film or tissue and dope an airfoiled wing can be built to match a depron wing.

But the key point is that the flat and full airfoils just make the plane fly differently. flat airfoils DO have a lot of disadvantages but the way they are being used in the flat foamies the style of flying tends to accentuate the good parts and avoid the bad parts. Like the turning of the normal early and deep stall into a "feature" that is used to advantage as described earlier.

[Mister UHU]

You can get the best of both worlds,
or the worst of both worlds.

As some 3D foamies, you can use flat foam slabs for the tail and fuselage,
and make the wing hollow,
using a spar or ribs and thin sheet foam curved into a nice aerofoil.

Often the wing is pretty thick (and slow)
but for 3D this makes for smoother stunts,
than flat plate "Shockies".

I agree with the comments above,
that a powerful motor can coverup poor aerodynamics.

[wjbite]

Jeff:
Found this site that gives some not too complicated calculations to help decide whether to use FLAT PLATE vs. CURVED PLATE vs. CONVENTIONAL airfoils.
Hope this helps.

Walt

Whoops! Sorry.
Forgot to include the link in the above post
Here it is:
http://jef.raskincenter.org/published/airfoil.html