Undercamber Lift and Stall Speed

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3D2

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Yes or no,

IIRC: "all things being equal" on airfoils with an identical upper surface, the one with an undercamber will produce more lift than the one with a flat bottom and as a result in level flight will stall at a lower speed.

Thanks

(Published plans offer both options.)
 

Norman

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Yes or no,

IIRC: "all things being equal" on airfoils with an identical upper surface, the one with an undercamber will produce more lift than the one with a flat bottom and as a result in level flight will stall at a lower speed.
That's really oversimplified. By modifying the lower surface you produce a new, thinner, airfoil with higher camber and that does translate to higher CLmax and shifts the minimum drag to higher CL so, yeah, the one with the concave bottom will stall a bit slower and probably even have a better glide. To explore this I started with an NACA 4417 in XFLR5 and pushed the lower surface up to make 2 airfoils with concave lower surfaces. As you can see in the attached polar depending on how the seed airfoil is modified the high speed drag increase may be pretty small, if at all, and the low speed drag might be significantly lower which will give you a steeper climb, flatter glide, and slower descent.
 

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3D2

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Also stall will be sharper and structural strength lower due to the reduced thickness.
True, but at Part 103's weight limit structural strength is already limited. I won't be doing aerobatics in it.

The published plans I am using show an undercamber of about 0.75 inch at 45% or so of its 48 inch chord as an option. The flat bottom option has a maximum thickness of about 6.5 inches.

I have an EAA reprint of an Air Camper plan with the FC10 airfoil laid out nicely on a two inch grid. It has a more significant undercamber, being 1.5 inches or so at 45% of its 60 inch chord on an 8 inch thick wing. I flew one years ago and it liked to "grab the earth securely" in a three point landing.
 
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3D2

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That's really oversimplified... depending on how the seed airfoil is modified the high speed drag increase may be pretty small, if at all, and the low speed drag might be significantly lower which will give you a steeper climb, flatter glide, and slower descent.
Yes it is...

Thank you for the graphs. (I love graphs.) At Part 103 speed, high speed drag isn't really a concern but bottom end performance is.

Thanks
 

Dana

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Lots of ultralights have thin undercambered airfoils. Most of them also have lots of wire bracing to support the wing.
 

3D2

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Lots of ultralights have thin undercambered airfoils. Most of them also have lots of wire bracing to support the wing.
This won't be a thin wing, just a "less thick" one. The flat bottomed wing has it's maximum thickness of about 6.5 inches at 9 to 16 inches in its 48 inch chord. The undercamber reaches a maximum of 0.75 inch at about 21 or 22 inches. (The top of each airfoil is identical.)
 

3D2

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Dropped leading edge cuffs increase the max angle of attack lift without reduced wing thickness.
Like Robertson cuffs on a Cessna Skywagon.

But with the Part 103 weight restrictions, and the fact that I have two published and flight tested designs to choose from, and the fact that as stated it isn't a thin wing to begin with...
 

TFF

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There is a stepping stone to airfoils. Under camber like USA35B, flat like Clark Y, slightly curved under like 23012, all ending with a symmetrical. Each step has to be flown a little faster to get the same lift. An airfoil next to another, noticeable but not a big jump. USA35B to symmetrical is a big jump.
 
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