I have a GWS Slow Stick which has a totally under cambered (its just a thin skin) wing. If I glued in a flat bottom and some ribs it would be a lot stronger. What are the lift and drag differences between under cambered and flat bottoms?

Hey, don't destroy the good flight characteristics with that undercamber wing, both high lift and slow flight positives. Take some thread and stretch, sew, knot and glue it from leading edge to trailing edge if you want to "reinforce" the wing structure, but don't make it into a "Clark Y" by sheeting the bottom!

Here is a beauty I fly with undercamber, vintage 1911. Can't not stall it! It is probably the most realistic to fly model compared to a full size airplane that I have flown, perhaps because it can fly so slow. Absolute, fantastic contollability at all speeds. Easily have more than 300 flights on it!

Yep, it is an "ARF' because someone else built it and I am the third owner.

Basic characteristics would be more weight, less lift but also less drag. It would need to fly faster and probably at a greater angle of attack to stay in the air. I wouldn't do it.

Steve

japan,

The large amount of camber your wing has allows it to fly slowly. At very slow speeds, a highly cambered wing might well have lower drag than a less cambered wing. At higher speed, the less cambered wing will have less drag. A flat-bottomed wing is generally also a pretty highly cambered wing. If you want less drag at high speed, you want lower camber than a flat bottom. If you want to change the camber of the wing, the cleanest way is to build a new wing with a different airfoil section. Camber is determined by the shape of the 'mean line', which is the curve that runs through the center of the airfoil from the leading edge to the trailing edge. For a symmetric airfoil, the mean is straight, and for a cambered airfoil, it is curved ( cambered ). The shape of the bottom surface is not what determines camber. If the bottom surface is concave, it usually means that the airfoil has some camber and that the airfoil is thin enough to make the camber show up on the bottom. Most wings with moderate camber and moderate thickness remain convex on the bottom. The terms 'undercamber' and 'semi-symmetrical' kind of confuse the issues of camber and thickness. The only advantage of a flat-bottomed wing is that it is easier to build. You can't determine anything about camber from the shape of the bottom surface.

banktoturn

Can't dispute your detailed explanation of airfoil
terminology but it is very common to designate an airfoil that has a concavity to it in the bottom surface as having "undercamber". Perhaps this is "modelese" but even Andy Lennon on page 10 of his treatise on R/C Model Design, refers to the bottom airfoil curvature line as the bottom camber line and further, "the reverse curvature of the lower camber line as being an airfoil with undercamber".
So, I will continue to call my model wings with concave lower camber lines as undercambered airfoils and the GWS Slow Stick sure has that property.

flicka5,

Well, you're right, the term is in broad use, which is unfortunate. The reason that I specifically brought it up in this context is that japan's question was asking about drag as a function of lower surface shape, and that is not the primary factor that affects drag. Camber ( of the mean line ) and thickness have fundamental affects on airfoil performance, and for that reason, it makes sense to discuss airfoils in those terms. Lower surface concavity per se is not as useful in this regard, and the use of the term 'under camber' instead of lower surface concavity leads to some confusing ambiguity. The term 'semi symmetric' has the same problems. It's especially unfortunate that Andy Lennon uses the term inappropriately. This is a case of someone's expertise in one area spilling over to lend him credence in an area in which he does not have any.

banktoturn

Gonna toss in my 7 cents.... (inflation you know)

Japan. Don't bother. The mods will increase the weight of the wing and not make it much better. If you want to go for more efficiency then toss the wing and go for a fully built up balsa wing. Use a proper airfoil selected from the ranks of the free flight design school and buld it light and cover with something even lighter.

Better yet, toss the whole Lite Stik and build a new model from scratch using balsa. You'll end up with an airframe that's half the weight and reacts faster to control inputs. Select one of the slow flyers that; a) gets rid of the performance hampering slot in the center section (VERY bad aerodynamics putting a break in there) and b) has tip dihedral. The tip dihedral puts the roll couple out at the tips where there is more leverage so the response to rudder inputs will be quicker and more positive.

Don't get me wrong. I think GWS did a great job at bringing this model to the masses. With the realities of production and the market it addresses it's a marvel. But it's got a long list of compromises attached to it and the airfoil and wing layout is one of the main ones. Trying to add to it is just going to be an excercise in frustration.

Any comment on the idea of clam-shelling two of the wings to make a symmetrical, super fat airfoil? I am guessing he wants it to be more arobatic...
Dimensions of the resulting airfoil (approximate):
chord 11.5 in
~17% thickness
~30% high point

With CF spar and keeping the weight low, 500 sq inches, reduced dihedral..... ailerons.

Opinions?

Lots of good suggestions have been given.

To improve the basic Stik use wide packing tape and cover the inner gap between the wings.

To improve the strength of the wings and stop the large amount of bending in loops, etc. use a basic guying system.

Go out on each wing about halfway out from the fuselage and 25% aft of the leading edge of the wing. Glue on some 1 inch diameter reinforcement circles on the top of the wing.

Run thich string from one wing down to about 1/2 of the way down on the gear leg on that side.

Tie a knot and put some glue to hold it in place to stop slipping on the gear leg. Run the string across to the other landing gear leg, knot it, and then up to the wing on the other side.

When the airloads are applied to the wing they try to pull the landing gear up which can't because the string keeps them in place.

You can pull any king of up load maneuvers and the wings won't bend.

By the way the airplane control response to roll is much better if you add polyhedral to about the outer 40% (the numbers are only approximates, not critical) of the wing panels. Make it about 1 inch up from the bottom of the wing.

I have had mine now since they first came out. A nice airplane and works in the street in the front of the house. New subdivision and no trees yet.

I hope there is a picture below.

Actually you can determine a lot about the camber of a wing which used to be single surface and then has the bottom filled in which I believe is what the question was about. The camber will very close to half of the original. The thickness will be much greater then the original but exactly how much greater is more difficult to tell.

The difficulty with trying to force people to use the current terminology for airfoil shapes is that the terms are not at all intuitive. If all you quote is maximum camber and maximum thickness you're saying almost nothing about the shape of the airfoil which is all people can see and identify with. Undercambered means something to most people, 6% camber with 8% thickness doesn't. It depends if we're talking to normal modellers or we only want to help qualifed aerodynamicists .

Steve

When you land it try to be use more finesse than I do as evident in the attached image.
If you can stay away from the chairs, basketball backboards, etc., you should not need to reinforce the factory wing!

My original purpose was to install airlerons, and since differentials would probably be better I would want to install them in the wings. Then I would want to hide the wires, so I would add a flat bottom (or reduced cancavity) to hide the wires.
Thus the new om would extend to the airleron area on the root halves and d be reduce concavity.

How would this affect stall?

Would any one like to send me theirross section of the "ideal slow fly?

Correction

My original purpose was to install airlerons, and since differentials would probably be better I would want to install them in the wings. Then I would want to hide the wires, so I would add a flat bottom (or reduced cancavity) to hide the wires.
Thus the new om would extend to the airleron area on the root halves and d be reduce concavity.

How would this affect stall?

Would any one like to send me their cross section of the "ideal slow fly?

Steve,

Actually, in the general case, there's no way to know that filling in the concavity on the lower surface will result in about half the camber. Whether this is true depends on the chordwise thickness distribution. The original question was about the lift and drag differences between flat bottomed and 'undercambered' wings. Because the bottom surface shape per se does not have a strong, well understood, effect on lift and drag, it makes more sense to introduce the conventional, correct, definition of camber in answering the question.

When I go to an auto mechanic, he does not refer to a brake caliper, for example, as a 'doohickey', just because I am not familiar with the correct names of the parts of a car. He explains to me that the problem with my car is related to the brake caliper, explains what a brake caliper is, and proceeds to use the correct term for the rest of the conversation. Can an auto mechanic only help other auto mechanics? I think the situation is the same here. I avoid jumping in every time a post refers to 'undercamber', and correcting the poster just because I am picky, which I guess I am. In this case, to correctly answer the question requires a better understanding of the concepts and terms. As you say, the term 'undercamber' means something to most people. Unfortunately, the amount of 'undercamber' is not the correct characteristic of the wing to consider, so it doesn't make sense to use the term just because it is more intuitive.

banktoturn

Actually since we're presumably quoting the maximum camber (or is that not what a camber percentage means ?) doesn't it depend entirely on the thickness at the max camber point ? In the specific case we were asked about i.e. a single surface wing (thickness approximates zero) it will be half unless I completely misunderstand camber (not impossible ).

I do agree that it's worth trying to educate people to use the correct terminology but not at the expense of turning their fairly straightforward question into a complex generalisation instead of answering it. Certainly it would/has put me off asking anything now I feel that the "answers" I get are more likely to explain why my question is wrong rather than tell me what will happen.

I guess I'm just not suited to this forum.

Steve

Steve,

Perhaps my ignorance of this plane and its wing is showing. You're right, for a thin wing, filling in the lower surface would approximately half the camber. Good point.

I would not want to unduly obfuscate the issue by starting off on a lecture about terminology. What I tried to do was get to the point that I could give my best answer to japan's question, which is that more camber gives lower drag at low speed, less camber give lower drag at high speed, but that it is mean line camber for which this statement is true.

Thickness, as opposed to maximum thickness, is the thickness at each point along the chord. This is why I should have used the term thickness distribution, and been a bit more clear.

I try to be as clear and simple as possible without missing the correct essence of the issue. I reckon that if I sometimes end up with more detail than some want, they can always ignore or ask for clarification ( how was that for some English English? ).

banktoturn

Flew one with aileron mods last week, indoors. Each aileron had its own servo embedded in the foam wing. No special redesign of the factory wing and the servo wires were taped to the bottom of the "concave" wing, span-wise. Flew very well but only have poor digital image of it in the air; otherwise would have provided detail shot of aileron installation. Maybe, I can get you a good shot if the owner brings it to our club meeting in two days; if you are interested? Meanwhile, I will do some 3D aeroelastic computer modeling using normal mode beam simulation to determine what the tensile sigma max. levels are in that foam wing in a 3G stall and spin maneuver. Not to bad for an old Mfg. Eng? Just kidding!

Pics. attached of aileron conversion!

How does it fly?
nm, read the prior post *doh*

I am considering adding a second wing, with the option to go bipe or mono with ease.