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  • February 18, 2020, 11:13:40 AM

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Author Topic: Transitional Airfoils  (Read 510 times)

Offline Ken Culbertson

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Transitional Airfoils
« on: February 08, 2020, 05:25:29 AM »
My first ship to rebuild after the fire that claimed my fleet is going to be the PA ship that I had just finished.  It had a rather unique airfoil, designed by Ton Niehbur  that I would like to understand better before I duplicate it.  It was the typical thick blunt fully curved shape common in most of today's top designs at the root but it gradually tapered to a flat back starting just behind the high point at the tips.  The taper started at about mid span.

Any thoughts - Ken
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Online Tim Wescott

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Re: Transitional Airfoils
« Reply #1 on: February 08, 2020, 08:49:25 AM »
I think you need to document the airfoil for us.  Usually I do this with scrap cardboard, trimmed by whatever method is convenient until it lays nicely along the whole length of the airfoil.  Then I trace it out on paper with a pencil.  Get one at the root, one at the tip, and one in the middle.

And yes -- time consuming PITA.  But sometimes you just need to do the tedious stuff.
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Offline Ken Culbertson

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Re: Transitional Airfoils
« Reply #2 on: February 09, 2020, 06:47:32 AM »
I think you need to document the airfoil for us.  Usually I do this with scrap cardboard, trimmed by whatever method is convenient until it lays nicely along the whole length of the airfoil.  Then I trace it out on paper with a pencil.  Get one at the root, one at the tip, and one in the middle.

And yes -- time consuming PITA.  But sometimes you just need to do the tedious stuff.
Tim:

This is the best I could do without any of my drafting stuff.  It is pretty much freehand.  Actual is a bit more blunt, a bit thicker and the "mid" is a bit thinner at the back.  The point is the transition from rounded to flat.

Ken
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Online Tim Wescott

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Re: Transitional Airfoils
« Reply #3 on: February 09, 2020, 01:00:04 PM »
Thanks Ken.  I know that Brett Buck, who's about my age but spent his whole life flying instead of taking a 30-year break in the middle for college and kids, doesn't like the ice-cream-cone airfoil -- I can't remember exactly what he doesn't like, but I think it's that its jumpy around neutral.

Having it only flat at the tips may make that better overall, but if the tips have a "jumpy" airfoil and the rest of the wing doesn't, it would make the plane jumpy in roll, which can't be a good thing.
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Offline Ken Culbertson

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Re: Transitional Airfoils
« Reply #4 on: February 09, 2020, 01:32:48 PM »
Thanks Ken.  I know that Brett Buck, who's about my age but spent his whole life flying instead of taking a 30-year break in the middle for college and kids, doesn't like the ice-cream-cone airfoil -- I can't remember exactly what he doesn't like, but I think it's that its jumpy around neutral.

Having it only flat at the tips may make that better overall, but if the tips have a "jumpy" airfoil and the rest of the wing doesn't, it would make the plane jumpy in roll, which can't be a good thing.
I am not promoting this airfoil, I am trying to understand it and why it flew so well.  I didn't have enough flights on it when it burned to tell if it had any minor issues.  It did want to float up in level flight a bit which I traced to flaps not neutral.  That was corrected but never tested.  I agree that jumpy at the tips is a bad thing.  Maybe it has to do with stall characteristics.  It is not a true "Ice-cream-cone" since the flat starts well aft of the high point.  It also could have been a bit of block sanding too late in the evening.  I am also not convinced that the exceptional cornering and smoothness in rounds that the plane had was due entirely to the airfoil.

Ken
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Offline Howard Rush

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Re: Transitional Airfoils
« Reply #5 on: February 09, 2020, 05:20:37 PM »
I don't think it would be jumpy around neutral.  All reasonable airfoils have the same lift vs. angle of attack around zero.  Brett's observation (although it's best coming from Brett) is that airfoils like that have high flap hinge moments.  I don't like them flat in the back because they are hard to sand.  I reckon you could use Profili or Javafoil to get a clue as to whether there's a benefit to that airfoil, but I doubt if either would give useful hinge moment information.

On the other hand, Gordan Delaney's wonderful twin Pathfinder has an airfoil like that.

Pat Johnston likes airfoils that are ellipses in the front and straight in the back.  Pat draws a lot of airfoils.  Does anybody remember National Lampoon's Famous Comic Artists' School?
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Online Brent Williams

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Re: Transitional Airfoils
« Reply #6 on: February 10, 2020, 02:09:00 PM »
Just an observation.  Perhaps the difference in perceived curvature aft of the high point can be attributed to trying to fair the dimensionally smaller airfoil into a fixed thickness trailing edge. ie, 1/2", and the usually flat-ish trailing edge sheeting. 
Percentage wise, it requires less curvature to fair the shorter height and shorter length tip ribs into that fixed height trailing edge dimension than the taller and longer root ribs.  There is less distance to make a curve move smoothly into the ~1.5"(ish) of flat 1/16" trailing edge sheeting stock.  So, it can appear to flatten slightly aft of the high point as you move out to the tips ribs.   

I have attached an overlay of the Impact and Trivial Pursuit root and tip airfoils showing this effect.  Interestingly, when comparing the SV airfoils (in this case, taken from a Tempest,) there appears to be more aft curvature as you move from root to tip.  SO...something to ponder upon as you think about your next plane. 
To the extent that it matters, at all, is another conversation.  Pick any of the known, good airfoil shapes, build it straight, and it will probably work just fine.
« Last Edit: February 10, 2020, 03:37:02 PM by Brent Williams »
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Offline Geoff Goodworth

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Re: Transitional Airfoils
« Reply #7 on: February 11, 2020, 09:17:37 AM »
If the section is as suggested in some of the comments, you may find this interesting.

On the comparison drawing, the red line is NACA 00xzx, the blue line is NACA 63A with the max thickness moved forward tp 30% to match NACA 00xx and the white line is an ellipse with straight lines from the TE to be tangent to an ellipse.

The One Design Airfoil Analysis is interesting but only partly relevant to our models because is is the airfoil for full size aerobatic aircraft.

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