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Aerodynamic of undercambered vs. flat-bottom airfoil?

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Old 05-02-2005, 02:45 AM
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abufletcher
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Default Aerodynamic of undercambered vs. flat-bottom airfoil?

I'm working on a scratch-built scale Nieuport 11 from Joe Nieto drawings (supplemented with info from technical drawings of the Nieuport 17. I was thinking of simplify things by using a flat-bottomed airfoil produced by drawing a line across the bottom on the scale undercambered airfoil. But then I assume I'd also have to change the incidence angle which might wreck the scale appearance.

Could someone explain the basic aerodynamics of a flat-bottom airfoil at zero degrees and an undercambered airfoil at a positive incidence?
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Old 05-02-2005, 10:21 AM
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Default RE: Aerodynamic of undercambered vs. flat-bottom airfoil?

Flatbottom wing isn't at 0 incidence angle. Incidence angle is measured at the most forward part of the curve of the leading edge, which is usually a couple of degrees incidence. If you use the same incidence as shown on the plans, I doubt if there would be much difference in the way the two airfoils fly. In fact it would probably fly better with the flat bottom as undercambered airfoils are very speed sensitive.
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Old 05-02-2005, 12:11 PM
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Default RE: Aerodynamic of undercambered vs. flat-bottom airfoil?

Think of an airfoil this way - it is a streamlined profile wrapped around a curved line, called the mean camber line.

If the line is straight, then of course the airfoil is what we modellers call a symmetric airfoil, ie. it is just the streamlined profile. This is a non-cambered airfoil.

If the line is curved then effectively the streamlined profile is bent like a banana and we have what modellers call a semi-symmetric airfoil. This is a cambered airfoil.

If the line is bent enough, the bottom of the airfoil will be bent upwards and this is what modellers call an undercambered airfoil. This is another cambered airfoil.

It is clear that an undercambered airfoil is merely a cambered airfoil with a lot of camber and not much thickness. A flat bottomed airfoil is one where the thickness and camber are matched so that the bottom of the airfoil is flat.

To see the effect of thickness and camber, get a copy of FoilSim 11 (do a web search). You should even be a able to use this program to do a smart airfoil selection for your biplane.
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Old 05-03-2005, 07:13 AM
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Default RE: Aerodynamic of undercambered vs. flat-bottom airfoil?

Here's a photo of the two airfoil templates I've made up. The top template is the 100% scale airfoil (on the Nieuport 17) shown at approximately the incidence used on the original (and the incidence I'd like to use on my scratch-built model). The bottom template is the shape I got when I drew a line across the bottom of the undercamber.

Using the flat-bottomed airfoil would simplify construction BUT I'd want to be able to mount it at the scale incidence as a more or less level mounting would be very noticeably non-scale. However, I would think that mounting the flat-bottomed airfoil at what amounts to almost 10 degrees of incidence would be a big no-no.

But then as I look back at FlyPaper's explanation (angle measured at leading edge) then the angle would be much less (maybe less than 5 degrees) and maybe the would be ok.
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Old 05-03-2005, 10:57 AM
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Default RE: Aerodynamic of undercambered vs. flat-bottom airfoil?

You should be OK with the flat-bottom. on my Nieuport 27, which uses the same wing, I used a flat-bottom, with both wings mounted at about zero incidence.
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Old 05-05-2005, 04:10 PM
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Default RE: Aerodynamic of undercambered vs. flat-bottom airfoil?

Abu,
I am trying to understand WHY you want to "simplify" the wing section on this airplane. Having seen your other thread on this highly detailed, scale-accurate model, why in the world do you want to blow it by cheaping out on the wing, especially for a very dubious benefit of making the wing so slightly easier to build?
The cambered wing is a big part of the scale look and it would only be consistant with everything else you have done on this model to preserve it. The cambered ribs will lie nicely on the building board, just as well as a flat bottom rib set so there really is no building advantage WHATSOEVER.
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... just my opinion.
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Old 05-05-2005, 05:11 PM
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Default RE: Aerodynamic of undercambered vs. flat-bottom airfoil?

Until you cover it -
but a scale junkie should not mind that task-
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Old 05-07-2005, 12:56 PM
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Default RE: Aerodynamic of undercambered vs. flat-bottom airfoil?

Well, a number of other voices have also convinced me to stick with the scale airfoil. If I'm going to be involved with WWI models I'd better get used to building scale wings and that means undercambered! Thanks for the advice, guys.

Any tricks for how to manage the covering of an undercambered wing? I'll be using Solartex.

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Old 05-07-2005, 04:05 PM
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Default RE: Aerodynamic of undercambered vs. flat-bottom airfoil?

I don't know the spar arrangement on your scale subject, but undercambered wings are easier to cover when there are several spars to stick the covering to, rather than just to the ribs.
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Old 05-08-2005, 06:41 AM
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Default RE: Aerodynamic of undercambered vs. flat-bottom airfoil?

I will be using rib caps so that should help a bit. But that also means that none of the spars will actually be touching the covering. I've read about people actually doing real rib stitching on undercambered wings.

Anyway, I'll just try it and see what happens. That's the nice thing about building from scratch: there's no kit to "wreck."

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