xray328

My Feedback: (1)

I've seen some homemade birds that are using that pink insulation foam as wings. Another time I saw a pizza box flyer, again totally flat. How can they fly like that? Doesn't a wing have to have some degree of curvature to provide lift?

gboulton

My Feedback: (15)

In the case of those aircraft, they are light enough, and the wing area large enough, that the impact of the air on the bottom of the wing provides sufficient lift.

In this instance, the analogy of "stick your hand out the car window and tilt it" really does work.

kuga220

Symmetrical airfoils produce lift when the angle of atack is positive.

xray328

My Feedback: (1)

Oh, ok. Now i get it.

hillbillyflyer

why wouldnt a larger more comon weighted plane fly with a flat airfoil with a positive angle of attack?

BMatthews

It would but flat airfoils have some nasty charactaristics that make them not the best choice for many designs. If you watch videos of the flat foamie 3D flyers you'll see that when the input a lot of elevator the model's lift to a high Angle of Attack and pretty much stop like they hit a brick wall thanks to the drag of the deeply stalled airfoil. A thicker and more proper shape does not stall as easily and the model will tend to loop or fly through a corner rather than pivot and halt.

It's all about what you want the model to do.

rmh

they will and very well indeed in fact -not opinion- many guys have put the wing on backwards on rubber band attached wings -and found they flew just fine (no ailerons of course.)
much of the test book information on airfoils relates directly to man carrying machines which are typically purpose built and have low power because that isall is that is available andmost are heavy aircraft relative to sport model airplanes because strength is needed .
The reason the Wright brothers plane looked like it did- was that weigh to power was a HUGE problemand they had no practical data to fall back on - so they developed an "airfoil which worked on the large box kite they knew would stay together and fly.
Their layout for a good aircraft was abandoned in less than 15 years. through experience - others quickly found much improved power plants and designs --and airfoils.
Now here is the bottom line -- at small sizes -typical models -- the order of importance for performance goes like this:
1 It has to be strong enough to fly in the manner it may be used
2 it should be as light as possible -it is practically impossible to make one too light if rule one is followed.
3 power is optional according to preferrence
3 the airfoil means very little .
That may sound crazy but actual experience -not just mine - bears this out.
The fine tuning of airfoil is meaningless except for certain speed setups
Your instincts are correct -
typically a 400 square inch model which weighs 2-3 lbs will fly just fine on a thin flat board for a wing as longas th board does not twist or break.
It has been proven many times by countless modelers
Not withstanding all of the formulas and wind tunnel data and NASA airfoil numbers - --the realized difference on the model described is small.
All of this data was developed for specific conditions -that is why there are so many airfoils.
The perfect airfoil for the F22 Raptor looks nothing like the airfoil for a Piper Cub . Each fit a specific application.
That the flat board is un scientific and horribly inefficient means not a darn thing in this case.
the laws developed over the years regarding how lift is developed have been re anayzed countless times.
Some versions are just plain goofy

any time you can get a shape to develop a difference in pressure -relative to the Earth--that is higher underneath -than on top --you -get -lift.
How you do it is up to you and the job at hand - it can be thru moving it ( good speed ) or making it super light ( very little speed) - not a balloon -that is a different thing.
If all this is interesting
read through some basic books on aerodynamics but be careful- some are -like all things - not worth the paper they are printed on.
Some recent stuf I have seen is quite good!

Tall Paul

Backwards wing.. I fly this one every once in awhile to shake up the troops...
A bit sensitive in pitch, but otherwise flies just fine..

B.L.E.

My G. P. Cub 20, with it's flat bottom airfoil, flies quite well inverted. Can an airfoil be more "wrong" than that?
Butterflies, wasps, bees, and other flying insects have flat wings, the reason they have not evolved airfoil shaped wings is that they have no significant advantage over flat airfoils in that small size. Scale a bumble bee up to the size of a buzzard and it probably would not be able to fly.
In the pioneering days of aviation, everyone assumed that airfoils needed to be as thin as possible. Thin wings have no strength and need extensive bracing to support the weight of an aircraft. For this reason, most of the early designs were biplanes. It wasn't that the designers were unaware of the advantages of a monoplane, they believed that by the time you made a monoplane wing strong enough to support the plane, the weight gain canceled out the increase of the wing's higher efficiency, and, if you braced the monoplane with a whole bunch of flying wires, the extra drag was so great that you might as well have built a biplane. This all changed when NACA wind tunnel testing showed that an airfoil can be surprisingly thick and still have low drag if the shape of the airfoil is optimized. Thick airfoils can have spars strong enough that the wing needs no external bracing and the slight increase of the thick airfoil's drag is more than offset by the drag reduction realized by getting rid of the flying wires that the thin airfoils needed. This resulted in monoplanes replacing biplanes almost completely.
When airplanes are scaled down to toy size, the short wingspan and the light weight means that the cantilever loads on the wing structure are very low and a flat airfoil is more than strong enough for the flying loads. Scale one of the flat wing foamies up to Cessna size and the design would likely be a disaster.

britbrat

They don't have flat wings -- the leading edge support structure is many times thicker than the trailing edge & the ancilliary support structure gradually thins toward the trailing edge. On the down-beat the bee/fly/grasshopper, etc. wing assumes a partial undercambered shape. On the up-beat, the leading edge structure is rotated upward to allow the wing to follow, in trail, with minimum lift & drag.

rmh

abso friken lutely-
I get asked many times "what is the best aifoil for my xxx design.?"
One which is strong enough -is my first reply
Having tried everthing from 20% stick n Monokote fun fly to 3mm foam wings -I find weight is the biggest concern -after strength
last--- the airfoil
When the first monoplanes were done -there had to be a lot of head shaking going on -oddly enough - one flew the English Channel a few years after the Wright Bros. craft flew ---an abberation .
As a thought - supos'n you could make a thin flat wing to refit your old Cessna 150
just suppose --that new wing could reduce the weight of the entire craft by ---50%!
Would the 150 fly like a turkey (status quo)-or would is be better -faster - lower landing speed etc..
-------------just suppose.

jamie_duff

It would fly like a brick same as all modern fast jets with extremely thin, sharp aerofoils yet retaining generous wing area. For example the F-15. Landing speed anyone???

rmh

Oh come now !
weight means a LOT . The F15 is not even comparative .
how about 150 kts for the F15.
sharp thin airfoils are just fine as long as the weight is low .

Edge 540

The F15 also has very little wing area. Relatively...
Thats part of the problem. You cant even reasonable compare them. They are designed to operate in opposite ends of the speed and maneuverability envelope.

Edge 540

Oh yeah..I forgot to mention.
I think it would fly reasonably well considering the substantial weight loss. [8D]

adam_one

Hi Dick!
Could you tell us the weight above which the thin airfoil is no longer fine?

rmh

gosh -I don't know - in looking over the Blackbird at Hill Airforce museum-I noted it had a thin airfoil.
but then -you knew that didn't you
you changed horses in midstream
I was originally speaking of low speed use .
You guys -have got to get out more often!

adam_one

Yes I knew that Dick, I just couldn't help teasing a little bit...
But I understood your point.

jamie_duff

Can I add that an F-15 wont be too much fun at 150kts without flaps??? Which are there to increase the camber at low speeds

rmh

Glad you did not say "adds lift"---

jamie_duff

No, they don't add lift, but they do increase the Coefficient of Lift of the wing whilst they are lowered. That kinda means that it's not a flat plate any more

rmh

of course.

Skidmarx

I thought flaps added both drag and lift - each in proportion to the degree of flap application? A few degrees of flap for extra lift at takeoff with minimal drag and high degrees of flap for landing with much drag and extra lift. The extra lift in landing mode is controlled by use of elevator to keep the glide path steep - no?

jamie_duff

Lets not bring drag into this too. The reason I brought flaps into the conversation was to illustrate that a near flat plate aerofoil such as an F-15 wing will not carry the F-15 at it's landing speed (called 150mph for arguement's sake). For these speeds lift augmenting devices are absolutely mandatory because the coefficient of lift is extremely low. In fact the F-15 wont be much fun flaps up below about 250kts, maybe more.

The near flat plate aerofoil is neccessary for management of shock waves at very high speeds. For low speeds, the designers would much prefer to use a much more cambered aerofoil. Since it's not easy to sway wings over in flight, the lift augmentors are used to compensate for the thin aerofoils extremely poor lifting abilities at low speeds.

Whilst you're right, drag does increase with further deployment of flaps etc, that's not why I brought the subject up, if you see what I mean.

Also, whilst on the subject, you never use elevator to control your decent rate on approach....... trim to fly at your approach speed, then adjust RoD with power. Featherweight r/c models will let you off with murder, warbirds and real aircraft wont be so forgiving![X(]

adam_one

The correct answer is:
lowering flaps increases the Coefficient of Lift of the wing.
This because the Lift force itself may remain constant as the airspeed decreases during the landing approach, just when the flaps are usually used.
The Lift force is dependent on several factors:

Lift force = 0.5 * air density * airspeed^2 * wing lift coefficient * wing area

So, you may increase the Lift Coefficient by lowering the flaps, but may keep the same Lift Force if the airspeed decreases accordingly.