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Can you have too much camber in an airfoil?

Old 07-31-2012, 06:10 AM
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flyyy
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Default Can you have too much camber in an airfoil?

The videos of Ed at experimental airlines showing how to build a strong and light true airfoil out of Dollar Tree foam board looks like what I have been looking for in a quick build. I am going to add camber built into the wing. In a earlier post I was told that the camber of the wing has more lift than the wings having a thick cross section. I first tried the door to my bar-b-cue as a mold to fiberglass the curvature into the foam; However, that looked too tight a circumference. I have now shaped a mold that has a radius of 15 inches with a 12 inch constant chord and in my opinion it looks bout right. I want slow with control. One thing that I have noticed in videos of the Slow Bipe is that the camber appears to not be constant and it looks as if the leading edge of the wings have a tighter radius. I can't really tell because I haven't seen one in person. This kind of wing might have too much drag, but I am more interested in lift than speed. Anyone out there ever tried something like this?
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Old 07-31-2012, 08:47 AM
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JetPlaneFlyer
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For our slow flying planes about 4% - 6% camber is about right.

% camber is the height of the camber curve divided by the wing chord, so 4% would mean that (on a 12" chord) the camber highest point would be about 1/2", 6% camber would be about 3/4".

4% camber would need a 36" radius former, 6% would be about 26" radius. Using a 15" radius would produce about 10% camber which is really too much to work well.
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Old 07-31-2012, 09:10 AM
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slipstick
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I don't know Ed and I've never seen his videos but it sounds like you're talking about just bending a flat sheet to produce the cambered shape. If that's so then 10% is a bit much and if you're planning on making it a constant radius bend (so the high point is at 50% of chord) then it's likely to have really vicious stall characteristics.

A 12" chord flat bottomed airfoil may often be 10-15% high at the high point but remember that will give it an effective camber of only half that amount.

Steve
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Old 07-31-2012, 11:27 AM
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JetPlaneFlyer
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Steve touches on a good point.. it's important in these discussions to have a common understanding of the terminology, otherwise things get very confusing.

By 'camber' we would normally be refering to the 'mean camber line' which is a line drawn along the centre of the airfoil thickness:




  • The Chord Line (1) is a straight line connecting the leading and trailing edges of the airfoil.
  • The Chord (2) is the length of the chordline from leading edge to trailing edge and is the characteristic longitudinal dimension of an airfoil.
  • The Mean Camber Line (3) is a line drawn halfway between the upper and lower surfaces. The chord line connects the ends of the mean camber line. The shape of the mean camber is important in determining the aerodynamic characteristics of an airfoil section.
  • The Maximum Camber (4) (displacement of the mean camber line from the chord line) and where it is located (expressed as fractions or percentages of the basic chord) help to define the shape of the mean camber line.
  • The Maximum Thickness (5) of an airfoil andwhere it is located (expressed as a percentage of the chord) help define the airfoil shape,and hence its performance.
  • The Leading Edge Radius (6) of the airfoil is the radius of curvature given the leading edge shape.
taken from: http://www.dynamicflight.com/aerodynamics/airfoils/

Steve

Last edited by JetPlaneFlyer; 07-31-2012 at 04:28 PM.
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Old 07-31-2012, 04:08 PM
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flyyy
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That diagram helps me to know what your explaining to me. The best example of what I had in mind would be the upper surface and line number 3. In other words the cross section of the wing would be curved [both top and bottom] the entire distance of the wing. I take it that maybe the example outline in the diagram might be a better choice of the airfoil where only the trailing bottom of the wing has under camber. In an earlier post someone suggested if you want under camber just build a flat bottom airfoil and simply add flaps. I don't know if that would have the same effect. As for the airfoil I am talking about in the video can be seen at:

http://youtu.be/qJZoqGHAIDE
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Old 07-31-2012, 07:53 PM
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JetPlaneFlyer
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Flyyy,

If you want a very slow flying high lift airfoil then something with about 5% camber and just enough thickness added around the camber line to add structural integrity. Thick airfoils like the one in the diagram are no use for slow flying models.

A slow-stick type airfoil would take some beating if slow is all you want.

Steve
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Old 11-18-2012, 11:06 AM
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Chuck Plains
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Thumbs up My first post, hope it's useful.

Guys, have a look at a windsurf sail.
It's only as thick as heavy fabric for most of the chord apart from the fromt where the mast is accommodated in a tapering sleeve to help reduce turbulence. And virtually all the camber is oriented towards the front when the most lift and less speed is required (ie: in light wind or 'tacking' upwind).
Race sailors will actually trim this effect out on the water, as they alter their course. So that when they want best speed, they will flatten out the camber by tightening the 'outhaul', that stretches the sail out more and shifts the 'belly' of the foil rearwards.

Hmm, adjustable wings? Oh yes, morphing wing, seen that somewhere. How about genetics? Just grow wings in a bucket.

Windsurf sail adjustment video
don't watch the whole thing, you'll get bored, I did. But check out the shape near the mast, all the camber appears to be right up front. Though it will shift somewhat as the guy does the 'outhaul' bit. And you can see that he has further to pull it as the wind speed increases. That flattens the foil.
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