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Author Topic: Blunt leading edge  (Read 8197 times)

Offline Shorts,David

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Blunt leading edge
« on: November 15, 2021, 04:35:51 PM »
Hi, so how blunt is too blunt and what will happen if the LE is too blunt? My dad thinks my LE is too blunt.

Online Ken Culbertson

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Re: Blunt leading edge
« Reply #1 on: November 15, 2021, 04:51:38 PM »
Hi, so how blunt is too blunt and what will happen if the LE is too blunt? My dad thinks my LE is too blunt.
Maybe your Dad is just being Blunt! LL~
A lot depends on power.  I would think that any radius over half of the thickness of the wing is approaching too blunt.  Just for reference the Thundergazer and MaxBee airfoils have a LE radius about 1/3 the thickness of the wing.

Ken
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Online Brett Buck

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Re: Blunt leading edge
« Reply #2 on: November 15, 2021, 07:57:33 PM »
Hi, so how blunt is too blunt and what will happen if the LE is too blunt? My dad thinks my LE is too blunt.

   It's pretty difficult to make it "too blunt", as long as it is not also "too thick". What airplane are we talking about, with what engine?

      Brett

Offline Dennis Toth

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Re: Blunt leading edge
« Reply #3 on: November 15, 2021, 08:25:06 PM »
Dave,
Here is a thread about stab leading edge shape. It points out from test data on top stunt models that sharp leading edges on the stab gives positive results in shapes and control.

https://stunthanger.com/smf/open-forum/stab-aerodynamics-question/msg608996/#msg608996

Best,  DennisT

Offline Shorts,David

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Re: Blunt leading edge
« Reply #4 on: November 15, 2021, 11:07:15 PM »
Hi Ken, I would guess it is approaching half the thickness of the wing. When I made it I thought it'd be great, but now looking at it I'm second guessing.
Hi Dennis, this is the wing LE, my stab is pretty sharp.
Hi Brett, this is a beech staggerwing, I'm planning on using an fp .40. it's about 525-550 squares.
I may have to try it and then modify the LE if it what...maybe can't penetrate on a windy day?

Offline Avaiojet

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Re: Blunt leading edge
« Reply #5 on: November 16, 2021, 09:42:13 AM »
Hi Ken, I would guess it is approaching half the thickness of the wing. When I made it I thought it'd be great, but now looking at it I'm second guessing.
Hi Dennis, this is the wing LE, my stab is pretty sharp.
Hi Brett, this is a beech staggerwing, I'm planning on using an fp .40. it's about 525-550 squares.
I may have to try it and then modify the LE if it what...maybe can't penetrate on a windy day?

What kit is your Staggerwing?
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Online Brett Buck

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Re: Blunt leading edge
« Reply #6 on: November 16, 2021, 09:53:29 AM »
Hi Ken, I would guess it is approaching half the thickness of the wing. When I made it I thought it'd be great, but now looking at it I'm second guessing.
Hi Dennis, this is the wing LE, my stab is pretty sharp.
Hi Brett, this is a beech staggerwing, I'm planning on using an fp .40. it's about 525-550 squares.
I may have to try it and then modify the LE if it what...maybe can't penetrate on a windy day?


   If it is just a pure radius, that by itself might be OK (albeit not that pretty). It depends on how it fairs into the rest of the airfoil behind it. If it is a nice smooth curve, OK, if the radius has any sudden changes, then, maybe a big problem.

     Brett

 p.s. how thick is the wing (in percent)? The thicker it is, the more difficult it will be to ensure fair surfaces.
« Last Edit: November 16, 2021, 10:51:18 AM by Brett Buck »

Online Perry Rose

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Re: Blunt leading edge
« Reply #7 on: November 16, 2021, 12:42:12 PM »
You can use the wing knock out for the radius or a nickle.
I may be wrong but I doubt it.
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Offline Shorts,David

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Re: Blunt leading edge
« Reply #8 on: November 16, 2021, 01:20:35 PM »
What kit is your Staggerwing?
Jack Sheeks, 1964 Staggerwing.

Offline Dan McEntee

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Re: Blunt leading edge
« Reply #9 on: November 16, 2021, 01:31:30 PM »
Jack Sheeks, 1964 Staggerwing.

      Doc Holiday and a few others have built his Staggerwing Stunt model for VSC. It was a Flying Models Beamer of the Month Sheeks article. It should have a good side view of the airfoil which should be a typical I-Beam wing.
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John A Miller

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Re: Blunt leading edge
« Reply #10 on: November 16, 2021, 01:51:23 PM »
Basically a Pathfinder/Ice cream cone type of airfoil. Mine flew real good.

Online Ken Culbertson

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Re: Blunt leading edge
« Reply #11 on: November 16, 2021, 02:57:09 PM »

   If it is just a pure radius, that by itself might be OK (albeit not that pretty). It depends on how it fairs into the rest of the airfoil behind it. If it is a nice smooth curve, OK, if the radius has any sudden changes, then, maybe a big problem.

     Brett

 p.s. how thick is the wing (in percent)? The thicker it is, the more difficult it will be to ensure fair surfaces.
I just crudly checked the radius and thickness on the three airfoils I have templates for that are closest to what I have in use on my designs - 7/16".  The thickness ranges from 2 1/4" on the Thundergazer to 2 3/8" on the MaxBee Airfoil.  I would consider an airfoil that had a definable "radius" to be blunt if the tangent to the forward curve was greater than about 25 degrees.  Most airfoils are parabolic and have no definable LE radius, only an approximation.  I had a parabolic airfoil on my last Sandpiper design with a 2" thickness and a 40% highpoint.  It did not fly exceptionally well under IC power and I was mad at myself for using my classic airfoil on my classic.  Then I converted it to Electric and it really performed.  I have heard others commenting that electric power does not need the drag of the blunt LE and the more traditional parabolic airfoils are better suited.  They may be right.  I want to learn a much about them as I can since it is the one thing that cannot be easily changed after the plane is built.

Ken


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Offline Dennis Toth

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Re: Blunt leading edge
« Reply #12 on: November 16, 2021, 03:31:58 PM »
Ken,
Seems with electric we get much more motor braking then we thought. If the rpm is set it will just keep backing off current draw and only use what it needs to maintain the set rpm and no more. The smaller radius wing leading edge being a little cleaner can get good penetration with less power. As a bonus we can use more efficient props with a better pitch to diameter ration (more pitch than current trend in IC) and have the same lap time at lower rpm and less current draw another plus. How far we can go with the  prop we'll have to see.

Best,    DennisT

Online Ken Culbertson

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Re: Blunt leading edge
« Reply #13 on: November 16, 2021, 03:41:01 PM »
Ken,
Seems with electric we get much more motor braking then we thought. If the rpm is set it will just keep backing off current draw and only use what it needs to maintain the set rpm and no more. The smaller radius wing leading edge being a little cleaner can get good penetration with less power. As a bonus we can use more efficient props with a better pitch to diameter ration (more pitch than current trend in IC) and have the same lap time at lower rpm and less current draw another plus. How far we can go with the  prop we'll have to see.

Best,    DennisT
I have always suspected that it was things other than the airfoil that added to the performance of electric but I think it is what you are pointing to.  It is not so much an ability of electric as it is a deficiency in IC that makes the thinner airfoil possible and perhaps even desirable.  I hope someone who has seriously looked into this and has some answers chimes in.  I have my 2022 ship on he drawing board.

Ken
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Offline Howard Rush

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Re: Blunt leading edge
« Reply #14 on: November 16, 2021, 03:42:57 PM »
What do stunt people mean by “penetration”? 
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Offline Howard Rush

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Re: Blunt leading edge
« Reply #15 on: November 16, 2021, 03:46:30 PM »
Show me why you think electric planes can get away with thinner airfoils.
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Offline John McFayden

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Re: Blunt leading edge
« Reply #16 on: November 16, 2021, 06:36:28 PM »
Penetration in stunt refers to the distance travelled by the prop nut in the time between impact with the earth and the end of forward motion.

Offline Howard Rush

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Re: Blunt leading edge
« Reply #17 on: November 16, 2021, 06:44:36 PM »
Penetration in stunt refers to the distance travelled by the prop nut in the time between impact with the earth and the end of forward motion.

Chuckle. Thanks for clearing that up.
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Offline Dennis Toth

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Re: Blunt leading edge
« Reply #18 on: November 17, 2021, 06:21:44 AM »
The thinner electric airfoil thing has been written about by Bob Hunt in a few columns, others may also have noted it. Electric wants to go to a set point rpm, it doesn't unload as the ship launches and doesn't want to unload as the wind pushes the plane, it holds rpm. It does what the pipe does for IC but better since is looking for a set rpm/voltage from the ESC/Timer. On IC the pipe is holding the IC back by detuning the power point that the engine wants to go to for a given amount of fuel/air entering the engine. To further control the natural unload of the IC low pitch props allow it to spin-up a bit without to much change in airspeed. This control of unload is the same objective of the high rpm/low pitch and 4-2-4 run styles. To aid in controlling airspeed variations designs have evolved that use the thick airfoils to slow the rate that a ship will change speed due to momentary unloading. For electric it naturally reacts to the increase in load very quickly but some improvement in load control with the newer timers give a bit of a power bump as the load comes on but still aims to hold the airspeed pretty constant.

Seems that if you build/finish reasonably light the old Nobler airfoil is a pretty good one for stunt. Not needing to hold the plane back with drag from a 20% thickness gives electrics a benefit of reduced current draw and allows more pitch less rpm better prop efficiency, again a benefit in lower current needs. With the thinner airfoil holding airspeed in higher wind seems to most a bit more consistent. I think what Ken stated is somewhat to the point as not so much that electric by itself creates the benefit as much as allows these features/benefits and IC seems to need some of those features. Not that IC can't compete with electric it just needs different design features that let it perform at its best.

Just my understanding on the how and why for thinner airfoils on electric as told by others.

Best,   DennisT

Offline Dwayne Donnelly

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Re: Blunt leading edge
« Reply #19 on: November 17, 2021, 06:30:50 AM »
Penetration in stunt refers to the distance travelled by the prop nut in the time between impact with the earth and the end of forward motion.
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Offline frank williams

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Re: Blunt leading edge
« Reply #20 on: November 17, 2021, 07:21:21 AM »
Hi, so how blunt is too blunt and what will happen if the LE is too blunt? My dad thinks my LE is too blunt.

This set of data from the Illinois University wind tunnel work clearly demonstrates the effect of too small a leading edge radius.  The ultimate lift of the smaller leading edge radius airfoil is less than the larger radius.

Offline Dennis Toth

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Re: Blunt leading edge
« Reply #21 on: November 17, 2021, 09:34:51 AM »
Frank,
Looking at the two examples they are not  the same airfoil. The Ultra-Sport 1000 is a non-flap, the Trainer 60 has a trailing edge straight flap sections. The Trainer 60 will get the benefit of the fixed flap as it's Angel of Attack (AOA) is increased. On the Barnstormer OTS ship Andrews told Dave Cook that the trailing edge flap should have a straight flat shape to get the most benefit from the flap, it gives the effect of a 10-20% flap deflection.

What is needed is data from a airfoil like the Ultra-Sport 1000 as is then with several radii to see if how just the leading edge radius impacts the lift (Cl) and the stall point. I do agree that a very sharp wing leading edge will contribute to less lift because it will likely stall at a lower AOA. I think the other extreme is the ice-cream cone airfoil with I think would also be a lower performing PACL airfoil. One of the best IMHO would be the airfoil on the Detroit Stunter series, nice smooth and was a proven contest winner.

Best,    DennisT

Online Ken Culbertson

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Re: Blunt leading edge
« Reply #22 on: November 17, 2021, 09:49:49 AM »
Frank, I am not advocating a sharper LE at all.  A thinner airfoil section with more of an elliptical section but not a small radius LE.  Did you have a chance to study any of Bob G's ships over the years.  He attributed the improvement in lift to the thicker airfoil but look at the LE on the Gieseke vs the Blue Box.  Bob rounded it off nearly double.  I never told him but I used his wing in my mostly sky blue Surveyor design that I flew in the late 70's.  You may remember it. That plane would turn corners 50% better than I could fly them and didn't wind up in wind.  I am hoping to find some data that supports the theory that an 10" airfoil (w/o Flaps) with similar LE "radius" and chord curvature would be better at 2 1/8" or even 2" center thickness tapering to 1 1/4" tip thickness vs 2 1/2" tapering to 1 3/8".  My gut says that either will produce more lift at our AOA's than we need but I have absolutely no data to back that up or the resources to test it. The airfoil I envision looks like a slightly thinner version of Al's or Igor's.  With an active timer I should get all of the braking when I need it and better penetration when I need it.

Ken
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Offline frank williams

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Re: Blunt leading edge
« Reply #23 on: November 17, 2021, 07:01:17 PM »
Dennis
I really don't think that the sheet flap on the Trainer-60 airfoil  versus the Ultra-Sport 1000 airfoil, that proceeds on down to a sharp point, makes a whit of difference in the results.    The max coefficient of lift (cl) for the Ultra-Sport is about 0.90 versus the max cl for
the Trainer-60 is almost 1.2.  I'd bet the farm that changing the rear of the Ultra-Sport to have a finite sheet flap trailing edge would make a big change at all.  The stall of the airfoil is pretty well determined by loss of ability of the flow to make it around the leading edge successfully.   This effect of the minor alteration of the airfoil trailing edge on the overall flowfield I think is minimal.

Ken
Bob got blunter through the years .... and thicker too.  Remember also that his planes were weightless, they were more flexible than II like, but then they were weightless.

Offline Mark wood

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Re: Blunt leading edge
« Reply #24 on: November 17, 2021, 08:22:21 PM »
What do stunt people mean by “penetration”?

I always considered Penetration a good thing.
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Online Ken Culbertson

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Re: Blunt leading edge
« Reply #25 on: November 17, 2021, 08:43:33 PM »
Ken
Bob got blunter through the years .... and thicker too.  Remember also that his planes were weightless, they were more flexible than II like, but then they were weightless.
He once told me that if you could feel the plane in your hand as you carried it to the car it was too heavy!

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Offline Howard Rush

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Re: Blunt leading edge
« Reply #26 on: November 18, 2021, 12:03:31 AM »
Here is an XFOIL comparison between a standard NACA 0018 airfoil and one with a sharper LE.  Re = 400,000. 
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Online Brett Buck

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Re: Blunt leading edge
« Reply #27 on: November 18, 2021, 12:19:11 AM »
What do stunt people mean by “penetration”?

   Lack of airspeed regulation. At the Golden State meet, second round, I would guess it took my airplane 50% longer to do the top legs of the square 8, compared to the bottom. That's because it holds the airspeed very steadily, so the ground speed varies all over the place, depending on the vector sum of the wind and the airspeed. Relatively poor pentration. A Nobler, by comparison, holds it's airspeed poorly, so it takes much longer to bleed off momentum, so it will carry itself further before slowing down to the equilibrium speed, which you can take advantage of in some places in the pattern in the wind.

    First time I flew the first Thundergazer, it was a bit windy, but it seemed pretty normal compared to my airplane, or the Trivial Pursuit/Star Gazers, until I got to the overhead 8. Up it went, pretty normal until about halfway through the first loop, then, suddenly, it was so far ahead of where I was expecting it, I almost lost sight of it. Turning back into the wind, I was used to the airplane slowing down, and anticipated it, but it barely changed speed at all, so it penetrated much better than our usual "blimp airfoil" airplanes - even with exactly the same engine that David was flying the last Star Gazer with (PA75, same pipe settings, same prop).

    Of course that doesn't mean good or bad, necessarily, either way, it just means that you have different problems in different places.

     Brett

Offline Dennis Toth

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Re: Blunt leading edge
« Reply #28 on: November 18, 2021, 08:53:45 AM »
Interesting Brett, would you say that the Thundergazer had more line tension overhead then the Trivial Pursuit/Star Gazers in the wind? Seems since groundspeed give line tension the better the wind penetration the more line tension.

Howard, I don't remember enough of my aero courses to have the inputs for XFOIL (and I don't have the app) but it would be interesting if it is possible to take a simple airfoil, like used on the Andrew's Barnstormer, (also used as base for Still Stuka), NACA 00xx of 1.5" high point, thickness with 9" cord (just airfoil section) then add a flat flap, 5/32" thick by 1.5" (makes it like a polliwog) and do the comparison between a sharp and blunt leading edge to see the impact of the flat flap at various AOA's. Could also compare just a straight NACA 00XX 1.5" thick high point with total cord of 10.5" no flap, sharp and blunt. This would answer if the fixed flap has any impact.

Seems that the flat fixed flap should have minimal impact of lift at low AOA (as in rounds) and more impact in high AOA (as in corners) which almost give a variable flap ratio, would need to then determine how much weight the ship should be to get best penetration with best maneuver performance (CG weight box?).

Best,     DennisT

Offline Mark wood

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Re: Blunt leading edge
« Reply #29 on: November 18, 2021, 10:03:34 AM »
Interesting Brett, would you say that the Thundergazer had more line tension overhead then the Trivial Pursuit/Star Gazers in the wind? Seems since groundspeed give line tension the better the wind penetration the more line tension.

Howard, I don't remember enough of my aero courses to have the inputs for XFOIL (and I don't have the app) but it would be interesting if it is possible to take a simple airfoil, like used on the Andrew's Barnstormer, (also used as base for Still Stuka), NACA 00xx of 1.5" high point, thickness with 9" cord (just airfoil section) then add a flat flap, 5/32" thick by 1.5" (makes it like a polliwog) and do the comparison between a sharp and blunt leading edge to see the impact of the flat flap at various AOA's. Could also compare just a straight NACA 00XX 1.5" thick high point with total cord of 10.5" no flap, sharp and blunt. This would answer if the fixed flap has any impact.

Seems that the flat fixed flap should have minimal impact of lift at low AOA (as in rounds) and more impact in high AOA (as in corners) which almost give a variable flap ratio, would need to then determine how much weight the ship should be to get best penetration with best maneuver performance (CG weight box?).

Best,     DennisT

The impact of flaps on the LE section of the airfoil is negligible. XFOIL and other airfoil models struggle with determining the effect of flat plate sections. The funny looking TEs that Frank showed are the result of the digitization of the sections. We take the validity of these models much more seriously than they truly deserve. They are a tool that is good at gaining an understanding of the dependencies and the answer changes with the model assumptions input. Take the same dat Howard used in XFOIL and put in Martin Heberles Java Foil or Hanley's Visual Foil and you get a different answer for the magnitude of the Cl albeit similar profiles. Changing the radius impacts the curve in all of them.

How the LE behaves is dependent on the smoothness of the contour. Contour drives the pressure variation and steep pressure gradients are what cause separation. Small radii cause steep pressure gradients.  Here are three pressure plots. What is desired in the pressure curve is smoothness and low slopes. In the smallest radius notice how the pressure curve goes up steeply then back down and then back up again. That rapid change in slope is what causes the separation. Imagine a marble rolling on the curve if the curve were turned upside down which is basically what they are. The Cp is actually presented as -Cp vs T/c. The steeper the curve coming out of the well the more difficult the marble has getting back up.
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Online Ken Culbertson

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Re: Blunt leading edge
« Reply #30 on: November 18, 2021, 11:28:47 AM »
How the LE behaves is dependent on the smoothness of the contour.
If I am following this properly you are saying that a fairly "blunt" parabolic curve up to the highpoint with a gradual curve to the TE (less flaps) is where we are headed. This leaves thickness not only at the highpoint but for the aft part of the wing.  Right now I am torn between the Rabe Mustang/MaxBee style and the Thunderbolt style.  Not a good place to be when you are getting ready to order the wood.  It is the same issue as always, corner vs rounds.

Ken   
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Online Brett Buck

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Re: Blunt leading edge
« Reply #31 on: November 18, 2021, 11:44:20 AM »
With too blunt l.e. it’ll be more difficult to glide the 1 full lap from 1,5m height to touchdown, as required in FAI-rules. L

  I don't think David is particularly concerned with that problem.

     Brett

Online Ken Culbertson

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Re: Blunt leading edge
« Reply #32 on: November 18, 2021, 11:52:39 AM »
With too blunt l.e. it’ll be more difficult to glide the 1 full lap from 1,5m height to touchdown, as required in FAI-rules. L
Not a problem.  Most of us here could easily whip a dead horse. LL~

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Offline Mark wood

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Re: Blunt leading edge
« Reply #33 on: November 18, 2021, 12:06:53 PM »
If I am following this properly you are saying that a fairly "blunt" parabolic curve up to the highpoint with a gradual curve to the TE (less flaps) is where we are headed. This leaves thickness not only at the highpoint but for the aft part of the wing.  Right now I am torn between the Rabe Mustang/MaxBee style and the Thunderbolt style.  Not a good place to be when you are getting ready to order the wood.  It is the same issue as always, corner vs rounds.

Ken

Not necessarily. Look at the E472 section. Round and blunt and not desirable due to the sharp stall break. To properly design an airfoil you need to be somewhat versed in the pressure distribution behavior. From that you can "message" the pressure distribution. This a kind of inverse design methodology.

Here's an example of the airfoil I've been using. It has a fairly "blunt" LE but not overly. And it features a smooth transition and pressure recovery ramp. My flight testing does not confirm the Cl max and as we know I don't use predicted Cl max as more than just something interesting. More important Cl/Cd vs Cl polar. From that you can see at higher Cl's the NACE is better than the 0018. That is discernable in flight. Here's the reality of Cl max of a symmetrical section on an airplane using flaps. We never operate at that angle of attack therefore Cl max is not a design parameter but just a curiosity. We operate at some intermediate AOA and create the required Cl by changing the contour of the airfoil by using flaps.

The NACE airfoil originate 15 years ago when I was doing a Stolp Acroduster II wing redesign. When that airplane was designed, they wanted an airplane that would "snap good" so they took a 63-0012 section and sharpened the LE. The result was much improved snap characteristics. Much too improved. So, I spent a bunch of time to make a better airfoil. The variation for the model is different than the one for the biplane and different than the one for the Frankenlaser. Each has some tuning for its mission. The models airfoil is the only one fully symmetrical and the biplane has two different sections designed for top and bottom wings.
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Online Brett Buck

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Re: Blunt leading edge
« Reply #34 on: November 18, 2021, 12:20:07 PM »
Not necessarily. Look at the E472 section. Round and blunt and not desirable due to the sharp stall break. To properly design an airfoil you need to be somewhat versed in the pressure distribution behavior. From that you can "message" the pressure distribution. This a kind of inverse design methodology.

    Why do you care about how sharply it stalls?

     Brett

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Re: Blunt leading edge
« Reply #35 on: November 18, 2021, 01:10:56 PM »
I have a couple planes equipped with the Selig-Donovan SD8020 airfoil scaled to 22% minus flaps.  Works really well.  Norm Whittle used this airfoil on his Sultan-e scaled at like 17 or 18% minus flaps.  Just one of many good choices out there.  Looks a lot like most other good modern stunt airfoils.

Dr. Selig designed the 8020 for good performance at low Reynolds numbers.  I would be interested to hear details about how this section compares against other known, good sections.
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Re: Blunt leading edge
« Reply #36 on: November 18, 2021, 01:16:09 PM »
    Why do you care about how sharply it stalls?

     Brett

That's a good question Brett. When there is a sharp stall with AOA as in the E472 there is a significant difference in Cl. An airplane in a skid will have one wing with a higher AOA than the other typically. When a section that has a sharp stall is stalled like this, one wing stalls much greater than the other causing a snap roll or spin. I have seen some wierdness hinging "events" which after a while thinking about it decided what is occurring in essence an attempt for the airplane to do a snap roll ona string. It's highly likely that some of the difficult hinging cases can be traced to this kind of stall characteristic. Wing sweep can exasperate this behavior as well. The basics are in my spin course presentation.

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Offline Mike Alimov

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Re: Blunt leading edge
« Reply #37 on: November 18, 2021, 01:35:09 PM »
In every airfoil debate that I've read here, we keep referencing polars for fully symmetrical airfoils and looking at high AoA region, whereas polars for undercambered foils are more appropriate, as 99.999% of stunt planes enter the corner with a flap deflected.
We also fail to mention wing loading and air conditions. A 15 oz/ft^2 plane flying in hot, humid, still air (summer morning in Muncie?) is much more likely to stall than a 12oz/ft^2 plane on a cool windy afternoon.
Reynolds number includes viscous properties of the media, if I understand correctly, but we somehow keep assuming that air never changes.

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Re: Blunt leading edge
« Reply #38 on: November 18, 2021, 02:15:50 PM »
In every airfoil debate that I've read here, we keep referencing polars for fully symmetrical airfoils and looking at high AoA region, whereas polars for undercambered foils are more appropriate, as 99.999% of stunt planes enter the corner with a flap deflected.
We also fail to mention wing loading and air conditions. A 15 oz/ft^2 plane flying in hot, humid, still air (summer morning in Muncie?) is much more likely to stall than a 12oz/ft^2 plane on a cool windy afternoon.
Reynolds number includes viscous properties of the media, if I understand correctly, but we somehow keep assuming that air never changes.

Exactly, sorta. The common Cl v AOA plot isn't actually a polar since the Cl is  dependent on AOA. The polar is the CL v Cd both of which depend on AOA and not each other directly.  Actually the flapped section instead of an underchambered airfoil although the analogy is correct. This is what I've said many times that there is an over attention to the Cl max of the symmetrical airfoil, at least when it comes to a PA airplane with flaps. Wing loading is important but doesn't change the L/D only the speed at which the airplane will fly at a given L/D. Reynolds number does change ever so slightly for a PA model which works in the roughly 500,000 zone. Taper ratio can have a much larger effect on Rn than does the in flight velocity variation. If you look at the chart on page 8 of my presentation, therein is a diagram showing lift curve as a variation of flap angle.
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Re: Blunt leading edge
« Reply #39 on: November 18, 2021, 02:17:36 PM »
That's a good question Brett. When there is a sharp stall with AOA as in the E472 there is a significant difference in Cl. An airplane in a skid will have one wing with a higher AOA than the other typically. When a section that has a sharp stall is stalled like this, one wing stalls much greater than the other causing a snap roll or spin. I have seen some wierdness hinging "events" which after a while thinking about it decided what is occurring in essence an attempt for the airplane to do a snap roll ona string. It's highly likely that some of the difficult hinging cases can be traced to this kind of stall characteristic. Wing sweep can exasperate this behavior as well. The basics are in my spin course presentation.

     All that is correct, however, you can never let the wing stall in the first place, if you hope to be successful. So what matters is not what it does afterward, it's (at most) how much Cl you get before it stalls, or alternately, how much AoA you can get at the point of a stall.

     Same reasoning applies to the various theories about increasing the section percent thickness as you to toward the tip, to avoid the dreaded "tip stall". Stall the root first, you lose anyway. Make sense, maybe, for full-scale, but not for stunt. There may be *other* reasons to alter the section thickness from root to tip, but not stall characteristics.

    BTW, we have long suspected/known that some of the very blunt/forward high point airfoils used in some of our designs have violent stall characteristics. Some of the most successful stunt airfoils are one short of putting a beer can on the paper and tracing around it to get the leading edge. The idea, correct or not, being to get enough Cl to avoid stalling in the first place. The only downside I have see with it is that they tend to be ugly.

         Brett

Offline Steve Helmick

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Re: Blunt leading edge
« Reply #40 on: November 18, 2021, 03:03:25 PM »
What do stunt people mean by “penetration”?

Probably different for everyone, because of equipment differences and personal preferences?   LL~ Steve
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Re: Blunt leading edge
« Reply #41 on: November 18, 2021, 05:12:49 PM »
     All that is correct, however, you can never let the wing stall in the first place, if you hope to be successful. So what matters is not what it does afterward, it's (at most) how much Cl you get before it stalls, or alternately, how much AoA you can get at the point of a stall.

     Same reasoning applies to the various theories about increasing the section percent thickness as you to toward the tip, to avoid the dreaded "tip stall". Stall the root first, you lose anyway. Make sense, maybe, for full-scale, but not for stunt. There may be *other* reasons to alter the section thickness from root to tip, but not stall characteristics.

    BTW, we have long suspected/known that some of the very blunt/forward high point airfoils used in some of our designs have violent stall characteristics. Some of the most successful stunt airfoils are one short of putting a beer can on the paper and tracing around it to get the leading edge. The idea, correct or not, being to get enough Cl to avoid stalling in the first place. The only downside I have see with it is that they tend to be ugly.

         Brett

Brett speaks wisely, as usual.  I used to fly a little combat.  The airfoil I used (more on that later, maybe) had a high Clmax, but a violent stall.   Airplanes using it had to be built straight.  I put stops on my bellcrank to limit stabilator (hinged at the LE without flutter) travel to keep the wing just below stall.  I would test airplanes on a windy day so the right wing would stall first until I got the airplane trimmed.  I could have used a sharper LE to give a gentler stall.  It would have also saved me the bother of fixing string slits in my wing leading edge. 
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Offline Howard Rush

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Re: Blunt leading edge
« Reply #42 on: November 18, 2021, 05:17:05 PM »
Wing loading is important but doesn't change the L/D only the speed at which the airplane will fly at a given L/D.

Pertinent to Lauri's issue, but not to the part of the stunt pattern when the engine is running.
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Offline Mike Alimov

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Re: Blunt leading edge
« Reply #43 on: November 18, 2021, 06:30:48 PM »

Bob got blunter through the years .... and thicker too. 

Doesn't this describe most of us...

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Re: Blunt leading edge
« Reply #44 on: November 18, 2021, 06:53:52 PM »
Seems I'm at the end of what I have to contribute.
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Offline Howard Rush

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Re: Blunt leading edge
« Reply #45 on: November 18, 2021, 07:03:58 PM »
We also fail to mention wing loading and air conditions. A 15 oz/ft^2 plane flying in hot, humid, still air (summer morning in Muncie?) is much more likely to stall than a 12oz/ft^2 plane on a cool windy afternoon.
Reynolds number includes viscous properties of the media, if I understand correctly, but we somehow keep assuming that air never changes.

Reynolds number is proportional to air density / viscosity.  As temperature goes up, density goes down and viscosity goes up, hence Reynolds number goes down.  Lift capability varies more with density than with Reynolds number.  I found this chart showing how temperature affects various things at Muncie's elevation compared to their values at a sea level standard day (59F, pressure 29.92 in Hg, density .002377 slugs / cubic ft.).

Definitions:
SLSD is sea level standard day.
Sigma is the ratio of air density / SLSD density.
Re / Re0 is Reynolds number / SLSD Reynolds number for a given true airspeed and wing chord.
0018 is NACA 0018 airfoil
Cl is lift coefficient, useful for comparing airfoils and airplanes. It's nondimensionalized lift, lift / (dynamic pressure * wing area)
Dynamic pressure is air density * true airspeed2

Details:
Impact data are for the Impact mean aerodynamic chord with flaps 20 or 30 degrees (I forget which) from Javafoil.
NACA 0018 data are from NACA TR 586, which was such an influence on me as a kid. 
Humidity doesn't have much of an effect. I forgot what I assumed for humidity. You can see its effect and other cool stuff at https://wahiduddin.net/calc/calc_da_rh.htm .

Moral:
Reynolds number changes a lot with temperature.
Maximum Cl doesn't change much with temperature, hence with Reynolds number over the speed range stunt planes fly.
Lift capability changes with air density.  Minimum loop size in inversely proportional to density. 

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Offline Howard Rush

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Re: Blunt leading edge
« Reply #46 on: November 18, 2021, 07:11:07 PM »
But like many things, a good landing glide has more variables than just that, like how engine quits (fits, tank..).

I've noticed that European engines have a significant burst of speed when they run out of fuel.  I'm counting on electrickery. 

And Ken, the more you can manage without whipping, the better. L

For one thing, you might wander out of the center circle and hit the fence.  Fortunately it was my old airplane.
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Offline Howard Rush

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Re: Blunt leading edge
« Reply #47 on: November 18, 2021, 07:21:27 PM »
Not necessarily. Look at the E472 section. Round and blunt and not desirable due to the sharp stall break. To properly design an airfoil you need to be somewhat versed in the pressure distribution behavior. From that you can "message" the pressure distribution. This a kind of inverse design methodology.

Here's an example of the airfoil I've been using. It has a fairly "blunt" LE but not overly. And it features a smooth transition and pressure recovery ramp. My flight testing does not confirm the Cl max and as we know I don't use predicted Cl max as more than just something interesting. More important Cl/Cd vs Cl polar. From that you can see at higher Cl's the NACE is better than the 0018. That is discernable in flight. Here's the reality of Cl max of a symmetrical section on an airplane using flaps. We never operate at that angle of attack therefore Cl max is not a design parameter but just a curiosity. We operate at some intermediate AOA and create the required Cl by changing the contour of the airfoil by using flaps.

The NACE airfoil originate 15 years ago when I was doing a Stolp Acroduster II wing redesign. When that airplane was designed, they wanted an airplane that would "snap good" so they took a 63-0012 section and sharpened the LE. The result was much improved snap characteristics. Much too improved. So, I spent a bunch of time to make a better airfoil. The variation for the model is different than the one for the biplane and different than the one for the Frankenlaser. Each has some tuning for its mission. The models airfoil is the only one fully symmetrical and the biplane has two different sections designed for top and bottom wings.


NACE looks dandy.  I don't agree that Cl/Cd is important, but you might fix the scale on your polar so we can see what it is.
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Offline Howard Rush

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Re: Blunt leading edge
« Reply #48 on: November 18, 2021, 07:41:10 PM »
Howard, I don't remember enough of my aero courses to have the inputs for XFOIL (and I don't have the app) but it would be interesting if it is possible to take a simple airfoil, like used on the Andrew's Barnstormer, (also used as base for Still Stuka), NACA 00xx of 1.5" high point, thickness with 9" cord (just airfoil section) then add a flat flap, 5/32" thick by 1.5" (makes it like a polliwog) and do the comparison between a sharp and blunt leading edge to see the impact of the flat flap at various AOA's. Could also compare just a straight NACA 00XX 1.5" thick high point with total cord of 10.5" no flap, sharp and blunt. This would answer if the fixed flap has any impact.

Seems that the flat fixed flap should have minimal impact of lift at low AOA (as in rounds) and more impact in high AOA (as in corners) which almost give a variable flap ratio, would need to then determine how much weight the ship should be to get best penetration with best maneuver performance (CG weight box?).

I can't even say "flat fixed flap". 

XFOIL comes with Profili, https://www.profili2.com/ .  There are some YouTube tutorials on it, which I oughta watch. Mark mentions some other programs that might be more recent.  I had trouble getting XFOIL to accept flaps the way we use them on stunt planes, so I'm a little suspicious of the results.  I did compare an Impact airfoils with flaps at zero (a flat fixed flap) with some good combat airfoils.  XFOIL liked the Impact airfoil better, which was embarrassing.  You may have a point.  Flite Streaks fly well.   
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Online Brett Buck

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Re: Blunt leading edge
« Reply #49 on: November 18, 2021, 07:47:51 PM »
Reynolds number is proportional to air density / viscosity.  As temperature goes up, density goes down and viscosity goes up, hence Reynolds number goes down.  Lift capability varies more with density than with Reynolds number.  I found this chart showing how temperature affects various things at Muncie's elevation compared to their values at a sea level standard day (59F, pressure 29.92 in Hg, density .002377 slugs / cubic ft.).

Definitions:
SLSD is sea level standard day.
Sigma is the ratio of air density / SLSD density.
Re / Re0 is Reynolds number / SLSD Reynolds number for a given true airspeed and wing chord.
0018 is NACA 0018 airfoil
Cl is lift coefficient, useful for comparing airfoils and airplanes. It's nondimensionalized lift, lift / (dynamic pressure * wing area)
Dynamic pressure is air density * true airspeed2

Details:
Impact data are for the Impact mean aerodynamic chord with flaps 20 or 30 degrees (I forget which) from Javafoil.
NACA 0018 data are from NACA TR 586, which was such an influence on me as a kid. 
Humidity doesn't have much of an effect. I forgot what I assumed for humidity. You can see its effect and other cool stuff at https://wahiduddin.net/calc/calc_da_rh.htm .

Moral:
Reynolds number changes a lot with temperature.
Maximum Cl doesn't change much with temperature, hence with Reynolds number over the speed range stunt planes fly.
Lift capability changes with air density.  Minimum loop size in inversely proportional to density.

       So, here's a question (and this is a real question, not an "assertion in the form of a question") - why is it that we have so much more problem with marginal airplanes stalling and losing engine power in Muncie, rather than Davis or Tucson, even though the nominal air density is the same?  It gets pretty darn hot in Davis, and even at 85 degrees the air is pretty thin in 2200 Ft Tucson.

     We talked about this before, and you suggested the viscosity difference, but this suggests it is a nearly negligible effect. I had previously speculated about humidity, that makes the air less dense (and I think it causes the "vacuum air" spots you sometimes get in calm afternoons in the Midwest), but it seems like a pretty weak effect.   

    Brett

   


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