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Jan 12, 2002, 12:31 AM
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hardlock's Avatar
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Single surface undercamber or fat flat bottom better for SF?


Is there a R# value that below it makes the use of a double surface airfoil pointless?

I know that even flat plate wing toy gliders seem to work well at the speed they fly.

I assume that undercambered would be even better than flat, but at what speed or weight does airfoil thickness come into play?

I can save a little weight going single surface, but don't want to sacrifice lifting/handling ability. RTF will be about 10-12oz and don't care about top speed.

Thanks for any help!
Ken
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Jan 12, 2002, 01:00 AM
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davidfee's Avatar
Hi there...
Just so we're clear, how are you defining a double surface airfoil?
A lot of folks seem to be unaware that there is an entire class of undercambered airfoils which are true "double surface" airfoils. Curved plates are used mainly for simplicity... they are rarely the aerodynamic optimum. Airfoil thickness (and camber) are important at all values of Rn, but the smaller things get, the more difficult it becomes to actually build a proper airfoil. Attached is a picture of a typical "undercambered" airfoil section. Sections like this are best at Rn below 50K, but can obviously be used at higher Rn as well. I hope that I've been at least somewhat helpful!

-David
Jan 12, 2002, 01:15 AM
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hardlock's Avatar
Thread OP
Yep, that's double surface. I call single surface a piece of Depron with an airfoil shape formed in.

I'd read somewhere that even though a single surface should have no pressure differential between the top and bottom, that it does achieve it besides just deflecting air down. Didn't know if it's worth adding any bottom (even a partial) considering the extra work involved.

On the other hand, if I ever want any inverted, I'd guess I'd have to have it? As is I can barely keep a Clark Y flying upsidedown.
Jan 12, 2002, 08:00 AM
Old Desert Rat
Arizona Chuck's Avatar
I have played around with just covering the top of a balsa rib wing. What I get is a very high lift, sloooow plane. It flies inverted as well as a Clark Y. Flying is kind of stretching the word, more like a slow fall.
One plus is it is easier to fix a wing if the bottom isn't covered.
Jan 12, 2002, 08:28 AM
Crash Master
Gene Bond's Avatar
Correct me if I'm wrong, but the overall lift is basically the differences in pressure top to bottom, so...

Uncovered bottom gives most drag, thus most lift.
Undercambered bottom has a little less lift
Flat bottom has a little less lift, but much more efficient.
Rounded bottom Semi, or fully simetrical has least lift, but maximum efficiency.

Correct, or is the last one way off base?

Also, is the term Camber the ratio of thickness to chord or is this aspect ratio?

Couldn't find it in the FAQ...
Jan 12, 2002, 09:31 AM
Old Desert Rat
Arizona Chuck's Avatar
The word "efficient" can be misleading. It depends on what you are trying to accomplish. If I put a low power motor with a big prop on my plane I want the lift to be Efficient. If I use a wing that is efficient for speed it would fall to the ground. Can't get enough speed to create the lift I need.
On a small plane that I designed for the GWS A-10 direct drive motor I had to go with a efficiently fast wing so the plane can get up to the speed of the little prop.
All this balancing act is what makes electric planes so interesting and fun.
Besides the thickens of the wing a big thing is how mush of it is above and below the center line of the chord.
I think of an undercamber wing as one with built in flaps.
Jan 12, 2002, 09:34 AM
Registered User
Gene,
Quote:
Correct me if I'm wrong, but the overall lift is basically the differences in pressure top to bottom, so...
Not quite true. This is Bernueli's (sp?) principle, which is actually fluid-dynamics which has been applied to air.

While air has many of the same properties as liquids, this principle is not the "defining" quality of a wing flying.

A great deal of a wing's ability to fly is due to AOA, or angle of attack. The leading edge of wing being canted slightly upward allows the oncoming air to push the wing upward.

The "sucking" caused by air moving more quickly over the top of the wing is actually a small percentage of "lift".
Jan 12, 2002, 09:43 AM
Registered User
Just to define a bit more....

Think about it for a minute; if lift were caused by air moving more quickly over the longer length of the upper surface of a wing, which is what causes the "sucking" of a wing upwards, then a symetrical wing wouldn't fly. The air would move over top and bottom surfaces at equal rates.

How all this effects the reynold's numbers of our GWS type undercambered wings, I don't know. But I would think that, even though these wings are smaller, less efficent, covering the bottom should reduce drag and make them more efficient.
IF you can do it lightly enough!
Jan 12, 2002, 10:23 AM
Registered User
All the experiments I've seen (and done) seem to indicate that covering the bottom of a single surface undercambered wing reduces both drag AND lift. Since you look to be basically halving the overall camber by turning it into a flat-bottomed airfoil that was pretty much what I expected to see.

Unless it is your ambition to force your plane to fly faster just to stay in the air I wouldn't bother .

Steve
Jan 12, 2002, 04:25 PM
Registered User
Steve,

I anticipated having to fly faster. Any idea why the loss of lift?

I guess I'm not clear why an undercambered section would produce more lift than flat bottomed.

Dennis-
Jan 12, 2002, 04:41 PM
Registered User
Bear in mind that I'm an amateur at all this but my understanding is that the maximum lift available is closely related to the camber of the airfoil. Just to be sure we're talking about the same thing the camber is basically a line drawn through the half way point between the top of the airfoil and the bottom. For a single surface wing it's roughly equivalent to the top of the wing.

When you put a bottom surface on a wing that didn't previously have one you have just cut the total camber in half. This naturally reduces the lift.

With luck someone will come along and correct me if I'm talking rubbish.

Steve
Jan 12, 2002, 04:47 PM
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davidfee's Avatar
Quote:
Originally posted by DBCherry
I guess I'm not clear why an undercambered section would produce more lift than flat bottomed.
As Steve correctly mentioned, max lift is directly related to camber (curve at the mean point between upper and lower surface). This is not the most technically correct answer, but you can think of it this way:

With a flat bottom airfoil, the air leaving the upper surface is moving a bit faster than the air on the bottom, so it's deflected downward (adds to the lift). With an undercambered wing the same is true... but now the airflow on the bottom is already being deflected downward... so the two lift components add together and the overall lift is greater.

To give numbers which show that angle of attack is not the only source of lift, consider three scenarios. 1) A symmetrical section needs to fly at a positive AoA to generate lift. At 0 AoA, there is zero lift. 2) A Clark Y type flat-bottomed airfoil needs to be flying at about -2deg. to make zero lift. That is, at 0deg, it's already making lift. 3) An undercambered section has a zero lift angle of around -5deg. But this is already outside the low-drage AoA range. In other words, an UC airfoil is best at low AoA... but it generates a large amount of lift with a small AoA... which means low drag (for the appropriate speed range).

Again, this is not the most technically correct answer (but it might help you).
Last edited by davidfee; Jan 12, 2002 at 05:18 PM.
Jan 12, 2002, 05:32 PM
Registered User
Dave,
Quote:
With a flat bottom airfoil, the air leaving the upper surface is moving a bit faster than the air on the bottom, so it's deflected downward (adds to the lift).
I don't think this is accurate. The creation of lift due to air movement over the top surface is caused by the faster moving air creating lower pressure. It's the lower pressure that creates lift (by "sucking" the wing upward). The air being deflected downward only occurs after it's left the trailing edge and would have no impact on lift.

HOWEVER,
Quote:
An undercambered section has a zero lift angle of around -5deg.
Your "numbers" do make sense to me. Still not sure WHY undercambered requires less AoA to create lift, but I can believe it to be the case.

Thanks,
Dennis-
Jan 12, 2002, 05:34 PM
Registered User
hardlock's Avatar
Thread OP
AZ Chuck - that's right, you have a single surface mini Zagi SF don't you? You must need a LOT of up elevon to offset the neg. pitching moment caused from the hi camber?

Davidfree - So you're saying that an UC airfoil need a lower AofA for best lift than say a flat bottom? I was guessing that due to the neg. pitching moment of that type foil that I'd need a higher AofA incidence set in relative to the tail (declanage - sp?) to prevent pitch divergence tendencies, or a bigger stab or both. (read more drag)
Jan 12, 2002, 05:54 PM
Registered User
davidfee's Avatar
Quote:
Originally posted by DBCherry
The air being deflected downward only occurs after it's left the trailing edge and would have no impact on lift.
Well, sort of. As I understand it, there's an action/reaction situation going on, and a transfer of energy. As the airflow is forced downward, the wing is forced upwards in response. The greater the downward motion, the greater the lift. At least, that's how I understand this portion of the lift generating process. I think they call it "Mass Flow."

The pressure differential and AoA effects are also important. But, for example, it can be shown mathamatically that the pressure differential ("sucking") can't possibly be responsible for generating MOST of the lift, as the required airspeed would be rediculously high. But, that's not the point we're dealing with here.

Hardlock:
Sort of. What I'm saying is that if you have two identical models with the same weight and AOA, etc... the one with the UC section will generate more lift at the same airspeed. In many cases (especially in very slow models like we're dealing with) the flat-bottom setion will need to fly at a sufficiently high AoA to generate enough lift (for a given airspeed) that the drag will be higher than for the UC. Again, the flat-bottom needs to fly faster (or at a higher AoA) to make the same lift as the UC.

Decalage and tail volume are a separate issue... but yes, in general, a model with an UC section will need a bigger tail or increased decalage, etc. to provide the desired degree of pitch stability. However, this doesn't have to be a huge effect.

The wing will fly at whatever AoA it needs to in order to generate the Cl (coef. of lift) being asked of it. In other words, while in flight, the wing determines what angle the fuselage (and tail) is at... not the other way around. The drag associated with a change in incidence of the tail is miniscule (but not totally insignificant) when compared to the drag associated with a change in AoA of the wing. So, for a high Cl (slow flying), the wing which can fly at a lower AoA and still make enough lift will generally have lower drag overall. In most cases, for small/light/slow models, this means that an UC section can be best for efficiency.

But if you want to fly aerobatics, all bets are off!
Last edited by davidfee; Jan 12, 2002 at 06:10 PM.


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