News:


  • March 28, 2024, 07:45:11 AM

Login with username, password and session length

Author Topic: Transitional Airfoils  (Read 4991 times)

Online Ken Culbertson

  • 24 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 6035
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
AMA 15382
If it is not broke you are not trying hard enough.
USAF 1968-1974 TAC

Offline Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12804
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.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Online Ken Culbertson

  • 24 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 6035
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
AMA 15382
If it is not broke you are not trying hard enough.
USAF 1968-1974 TAC

Offline Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12804
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.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Online Ken Culbertson

  • 24 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 6035
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
AMA 15382
If it is not broke you are not trying hard enough.
USAF 1968-1974 TAC

Offline Howard Rush

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7805
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?
The Jive Combat Team
Making combat and stunt great again

Offline Brent Williams

  • 24 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1260
    • Fancher Handles - Presented by Brent Williams
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 »
Laser-cut, "Ted Fancher Precision-Pro" Hard Point Handle Kits are available again.  PM for info.
https://stunthanger.com/smf/brent-williams'-fancher-handles-and-cl-parts/ted-fancher's-precision-pro-handle-kit-by-brent-williams-information/

Offline Geoff Goodworth

  • Trade Count: (0)
  • Captain
  • *****
  • Posts: 808
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 00xx, 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.
« Last Edit: February 26, 2020, 07:26:55 PM by Geoff Goodworth »

Offline Mark wood

  • 22 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 853
  • I'm here purely for the fun of it.
Re: Transitional Airfoils
« Reply #8 on: March 14, 2020, 08:35:45 AM »
The "flat back", "ice cream cone" airfoils are design for the 4/4 scale airplanes so they can snap roll very easily on command. As a model airfoil they wouldn't be very desirable, especially on the wing tip. The airfoils presented would be done backwards to how I would have utilized them in a design. Since this airfoil has a rapid departure in the Cl/Alpha it is very suitable of creating the differential lift required for the snap roll. To use this type of airfoil to make a more well behaved aircraft using it at the root transitioning to a 00xx at the tip would tame the "tip stall" by causing the stall progression to begin at the root.

The straight transition from the high point to the TE causes a rapid pressure rise in the pressure recovery region which, in turn,  causes rapid separation. Opt instead for a more classic airfoil. If a person claims the airplane is drifting around neutral, the airfoil very well could be the trouble. High pressure air will "leak" up the TE and push the moment around which would result in hunting in level flight. While this airfoil is easy to create, I wouldn't use it in a model opting instead for a size 13 curve in the pressure recovery region.

Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Online Ken Culbertson

  • 24 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 6035
Re: Transitional Airfoils
« Reply #9 on: March 14, 2020, 09:32:27 AM »
The "flat back", "ice cream cone" airfoils are design for the 4/4 scale airplanes so they can snap roll very easily on command. As a model airfoil they wouldn't be very desirable, especially on the wing tip. The airfoils presented would be done backwards to how I would have utilized them in a design. Since this airfoil has a rapid departure in the Cl/Alpha it is very suitable of creating the differential lift required for the snap roll. To use this type of airfoil to make a more well behaved aircraft using it at the root transitioning to a 00xx at the tip would tame the "tip stall" by causing the stall progression to begin at the root.

The straight transition from the high point to the TE causes a rapid pressure rise in the pressure recovery region which, in turn,  causes rapid separation. Opt instead for a more classic airfoil. If a person claims the airplane is drifting around neutral, the airfoil very well could be the trouble. High pressure air will "leak" up the TE and push the moment around which would result in hunting in level flight. While this airfoil is easy to create, I wouldn't use it in a model opting instead for a size 13 curve in the pressure recovery region.
It is apparent to me that you know far more about this than I do.  I did not have the opportunity to fully trim the plane this airfoil came from before it was destroyed by fire.  I intentionally build my planes with at least 45 degrees elevator movement.  When I fly I set my handle to give me 30 degrees with normal maximum handle movement.  I too was confused why it flew so well.  I was able to turn consecutive corners of about 10' (maybe less I never got to plot them) without any stall, even at the tips.  The plane didn't hunt.  There was a slight flap/elevator alignment
that caused it to want to float or pull down a very slight amount but it still grooved.   Tom Niebuhr designed the airfoil.  Unfortunately he is not around any more to explain it.
The more I think about it the superior flight characteristics may have been due to other factors and not the airfoil.

Thanks - I am attaching the airfoil from the plans for the rebuild.  Any comments will be appreciated.

Ken
AMA 15382
If it is not broke you are not trying hard enough.
USAF 1968-1974 TAC

Offline Avaiojet

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7468
  • Just here for the fun of it also.
Re: Transitional Airfoils
« Reply #10 on: March 14, 2020, 11:39:54 AM »
Trump Derangement Syndrome. TDS. 
Avaiojet Derangement Syndrome. ADS.
Amazing how ignorance can get in the way of the learning process.
If you're Trolled, you know you're doing something right.  Alpha Mike Foxtrot. "No one has ever made a difference by being like everyone else."  Marcus Cordeiro, The "Mark of Excellence," you will not be forgotten. "No amount of evidence will ever persuade an idiot."- Mark Twain. I look at the Forum as a place to contribute and make friends, some view it as a Realm where they could be King.   Proverb 11.9  "With his mouth the Godless destroys his neighbor..."  "Perhaps the greatest challenge in modeling is to build a competitive control line stunter that looks like a real airplane." David McCellan, 1980.

Offline Ted Fancher

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2323
Re: Transitional Airfoils
« Reply #11 on: March 14, 2020, 07:58:50 PM »
Ken,

I think it looks as good as 99% of the usual suspects and I'm unsure which of the remaining 1% would be considered #1.

Build it straight and reasonably light, seal the flap (and elevator) hingelines and trim the resulting model and the control system to to allow you to do all the tricks under ideal conditions with finger, wrist (and elbow input only under  difficult air conditions) and go win contests.

Ted

That should get some comments!!!

Offline Mark wood

  • 22 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 853
  • I'm here purely for the fun of it.
Re: Transitional Airfoils
« Reply #12 on: March 17, 2020, 10:22:13 AM »
It is apparent to me that you know far more about this than I do. 

The more I think about it the superior flight characteristics may have been due to other factors and not the airfoil.

Thanks - I am attaching the airfoil from the plans for the rebuild.  Any comments will be appreciated.

Ken

Well, I don’t know about the knowing more part. I am one of those people who have been around a while and airfoils are an intellectual hobby of mine. Several full scale aerobatic designers have endeavored to I,prove the snap characteristics of their airplanes. I don’t think Nieghbower was the originator as it seems to have shown up first in a Sukoy. What they are striving for isn’t necessarily suitable for models.

Your conclusion is probably more correct in that planform makes a huge impact on the model.  The airfoil can make big impacts as well but not on the same scale.

It is fairly easy to eyeball a basic airfoil. The size 13 section generally works and improvement are subtle.  Adding thickness behind 5e high point aids in the transition and helps reduce the sudden stall which is undesirable in a model. My eye says the new airfoil will be better the than the ice cream cone.

My $0.02 FWIW
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Offline Howard Rush

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7805
Re: Transitional Airfoils
« Reply #13 on: March 17, 2020, 04:29:00 PM »
I suspect that airfoil behavior at full-scale Reynolds numbers is a lot different than at ours.  Gary James or Frank Williams or Igor could elaborate as to why.  My airfoil notions are based mostly on experience with models at Reynolds numbers between about 400,000 and 700,000, particularly single-element (no flap) airfoils used on combat planes.  It seems to me for this application that airfoils with the highest max lift have the most abrupt stall.  For several years I used an NACA 0016.5 with the max thickness squished forward to 25% chord.  It worked great, but I had to use bellcrank stops, and the slightest warp made an airplane useless.  Any anomaly around 25% chord significantly killed off lift capability, but I don't remember it causing an abrupt stall.  When I used a foam wing with a spar at 25%, I carefully planed the spar, then filled any remaining little dips with spackle.  Likewise, a pointy leading edge caused reduced max lift, but didn't cause an abrupt stall.  If anything, a pointy LE made the stall gentler. 

I fly an Impact for stunt.  I plotted its airfoil using the NACA four-digit formula, fiddling with the parameters until I matched the root and tip airfoils.  As Brent observed, the trailing edge thickness and slope are the same from root to tip.  The Impact airfoil is very nearly a straight line for the last 20% or so before the flap, causing it to be hard to sand. 

My last two Impacts have a wing structure with slanty ribs that make for pretty bad bumps in the wing cross section aft of the fattest part of the wing.  Although the airplane has significant, audible separation, it doesn't do anything perverted.  It's a pretty good stunt plane.  Vortex generators may have ameliorated the effect of the bumps. 

The aft part of Igor's airfoil has a lot of curvature and fairs nicely into the flap. It wins a lot of stunt contests.  The only comparison I've done between it and the Impact is with XFOIL via Profili, which favored the Impact airfoil.

Aerodynamically I would reckon--based on no theory--that a ice-cream-cone-looking airfoil at our Reynolds numbers could be OK.  I'd use a blunt leading edge and keep the curvature low at the transition to the straight part. Looking at the Endgame II  tip template above causes me to reach for a sanding block--just a wee swipe in front of the straight part.  Structurally, I'd think that a curve in the wing surface would be better than a straight line.

(edited to remove some stupidity)

The Jive Combat Team
Making combat and stunt great again

Online Ken Culbertson

  • 24 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 6035
Re: Transitional Airfoils
« Reply #14 on: March 17, 2020, 07:59:51 PM »
Howard, I think I am going to keep the curve the same to the tip.  I think your description  The more I think about it the superior flight characteristics I had with Endgame I was due to the overall design.  It had a 19.5" tail movement with a 26% sharp LE stab.  710 squares at 65oz. and Keith's CAM rudder (which I love).  In other words it was right.  I am noticing all of that as I redraw the plans for II.  I am agreeing with Ted that the airfoil id good enough to not be a problem.

A word to all of you out there that have spent years collecting plans, airfoil templates, drawings, etc.  STORE IT SOMEWHERE OTHER THAN IN YOUR SHOP!  When mine went up all of my plans went with it.  I almost never build kits and I am having to design totally from pictures and memory.  Camera's distort and my memory at 73 may be a steel trap....a rusty one!
When I get it redrawn I will shoot a picture and give you a good laugh!

Ken
AMA 15382
If it is not broke you are not trying hard enough.
USAF 1968-1974 TAC

Offline Geoff Goodworth

  • Trade Count: (0)
  • Captain
  • *****
  • Posts: 808
Re: Transitional Airfoils
« Reply #15 on: March 19, 2020, 09:47:57 PM »
Ken, have you looked at Hip pocket Aeronautical and Outer Zone for PDF files of the plans you lost?

There are a great many plans on both those lists. You should find something interesting.

Online Ken Culbertson

  • 24 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 6035
Re: Transitional Airfoils
« Reply #16 on: March 19, 2020, 11:29:05 PM »
Ken, have you looked at Hip pocket Aeronautical and Outer Zone for PDF files of the plans you lost?

There are a great many plans on both those lists. You should find something interesting.
I have looked at a lot of plans there.  I really enjoy seeing what others come up with.  But, except for a Twister I built to get back in shape that I totally Fancherized and an ARF Nobler that I bought just to have something to fly a few years back, I have never flown a PA plane I didn't design going all the way back to the early 60's.  None were ever published mainly because I never submitted any.  I love all three phases of our hobby;  design, building and flying pretty much equally so, until I get my shop rebuilt I will content myself with wasting rolls of butcher paper!  The great part of a "II" design is that you get to make all of the things you screwed up and had to do over on the "I" part of the design! y1

Ken
AMA 15382
If it is not broke you are not trying hard enough.
USAF 1968-1974 TAC

Online Ken Culbertson

  • 24 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 6035
Re: Transitional Airfoils
« Reply #17 on: September 17, 2020, 12:41:29 PM »
I was going to start another thread for my latest question but so much of this one is relevant so I decided to revive it.  I was just about to place an order with Bob for a wing with the airfoil we discussed here which is similar to a Thundergazer airfoil with a 1/2" TE to allow for recessed flaps. (Not that they are better, they just look cool).  Then the worst thing that could happen, happened.  I got to watch Mike Scott's new MaxBee fly.  It flew nearly identical to my Endgame, only it seemed to corner faster without losing that "T-Square on a string" tangency to the circle in the corner.  So I decided to see why.  MaxBee nose 1/2" longer, Tail 1/2"shorter, area/span about the same Stab% about the same, stab airfoil close, thrust lines same.  Not enough there to be the difference.  It had to be the airfoil. 

Mike gave me a copy of the airfoil and I overlaid it onto the one I was about to order.  To the high point they are nearly identical.   Length is the same, LE radius is the same, thickness at the high point is the same.  Where they differ is curvature.  MaxBee has a little bit less forward curvature but, the aft curvature of the MaxBee would make Al Rabe blush.  I had always operated under the assumption that such an airfoil was effective when you had high wing loading and large flaps. 

Have any of you out there flown both enough to make a comparison?

Ken

AMA 15382
If it is not broke you are not trying hard enough.
USAF 1968-1974 TAC

Offline Brent Williams

  • 24 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1260
    • Fancher Handles - Presented by Brent Williams
Re: Transitional Airfoils
« Reply #18 on: September 17, 2020, 05:29:56 PM »
Ken, here are Igor Burger's links to his Stunt News articles.  They are a good place to start to understand his philosophies on his airfoil and stunt design concepts.  As I interpret his methodology, it appears that he focuses on the effectiveness and lift of the wing with the flaps deployed in the corner, and he's much less concerned with level flight airfoil efficiency.  The idea is that his design doesn't scrub off as much speed in the corner at higher angles of attack which helps it fly cleanly through the corner. 

Perhaps some more technical observations will develop from the engineers on here.

Quote from: Igor Burger:

http://www.maxbee.net/download/MaxBee_1.pdf

http://www.maxbee.net/download/MaxBee_2.pdf
« Last Edit: September 18, 2020, 05:00:54 PM by Brent Williams »
Laser-cut, "Ted Fancher Precision-Pro" Hard Point Handle Kits are available again.  PM for info.
https://stunthanger.com/smf/brent-williams'-fancher-handles-and-cl-parts/ted-fancher's-precision-pro-handle-kit-by-brent-williams-information/

Offline Air Ministry .

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 4978
Re: Transitional Airfoils
« Reply #19 on: September 17, 2020, 07:40:55 PM »
Quote
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.

People speak of lighter control loads with the aft curvature Vs flatter aft section . And perhaps less ' loading up ' in wind ,  of control loads .

Offline FLOYD CARTER

  • 24 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 4458
    • owner
Re: Transitional Airfoils
« Reply #20 on: September 27, 2020, 05:19:21 PM »
All my designs use the proven NACA 0018 or thicker airfoil.  No point in re-inventing the wheel
89 years, but still going (sort of)
AMA #796  SAM #188  LSF #020

Offline Air Ministry .

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 4978
Re: Transitional Airfoils
« Reply #21 on: September 29, 2020, 11:00:09 PM »
If thinking the flat rear flank , has more unsteady stability. when Stalled . But good recovery . And dosnt 'cept way outside usual airspeed etc .

Good modulation ( correction ) for wind blown round manouvers .

May even rtate ( stall turn ) with wind attacked under but broken off topside ! .


Offline Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 13716
Re: Transitional Airfoils
« Reply #22 on: September 30, 2020, 04:12:42 PM »
All my designs use the proven NACA 0018 or thicker airfoil.  No point in re-inventing the wheel

  How does it compare to, say, an Imitation airfoil? Or one of Phil Granderson's? What did a NACA 0018 do better than Phil's?

    Brett

Offline Howard Rush

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7805
Re: Transitional Airfoils
« Reply #23 on: September 30, 2020, 04:55:13 PM »
All my designs use the proven NACA 0018 or thicker airfoil.  No point in re-inventing the wheel

That’s what the B-17 used.   Not the model, the real one.
The Jive Combat Team
Making combat and stunt great again

Offline Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 13716
Re: Transitional Airfoils
« Reply #24 on: September 30, 2020, 08:15:27 PM »
That’s what the B-17 used.   Not the model, the real one.

   To be fair, Phil wasn't around when they did it.

    Brett

   

Offline Ted Fancher

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2323
Re: Transitional Airfoils
« Reply #25 on: October 01, 2020, 05:41:04 PM »
For some reason I can never restrain myself when these mile long excursion into the "perfect" CLPA airfoil arise...

A number of years ago Big Art Adamisin went down under  to visit stunt folks and judge a contest or two.  One of his more curious /surprising  revelations upon his return was that there were several "competitive" entries with flat plate airfoils...i.e. nothing more exotic than a radius to the leading edge of conjoined sheets of balsa (don't recall if he ever mentioned the thickness) and (most likely although I don't know that I ever heard it from Big Art) a tapered trailing edge...nothing but flat balsa between the two!

One Adamisin or another forwarded pics of the ships following an earlier iteration of the above but, unfortunately, although they're likely somewhere in my office they would have to be somewhere within the multiple stacks of unlabeled or cataloged stunt stuff piles.

Just a hint to remember we're not talking supersonic helicopter rotor airfoils here!

Ted
« Last Edit: October 04, 2020, 04:58:56 PM by Ted Fancher »

Offline Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 13716
Re: Transitional Airfoils
« Reply #26 on: October 05, 2020, 09:57:07 PM »
One Adamisin or another forwarded pics of the ships following an earlier iteration of the above but, unfortunately, although they're likely somewhere in my office they would have to be somewhere within the multiple stacks of unlabeled or cataloged stunt stuff piles.

Just a hint to remember we're not talking supersonic helicopter rotor airfoils here!

   As a counterpoint, we have about 100-post email thread about how adding a .004" thick tripper strip has no effect, and a .006" thick tripper strip has a dramatic effect. .002 is smaller than the ridges around my trim colors.

   I would also note the experiences of The Unknown Pilots #1 and #2 at the inaugural ARF-Off. You and I witnesses,  close-up,  to the effects of an otherwise reasonable size/weight ARF Strega that nonetheless was stalling in the third corner of triangle in 75 degree sea level air, and ended up stalling in *the 4th loop of the 4-leaf* by the time we got over 90 degrees in the afternoon. And how it was fixed by 5 minutes of very crude rasp work to lightly blunt the leading edge.

    Brett

Offline PJ Rowland

  • AUS - 29541 AMA - 809970
  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2058
  • Melbourne - AUSTRALIA
Re: Transitional Airfoils
« Reply #27 on: December 29, 2020, 07:27:29 PM »
I flew in that contest when Big Art came downunder this was 1990.
The top 3 were using very close to flat plate stabs thickness was NOT in vogue so sectional thickness was closer to 1/4"


If you always put limit on everything you do, physical or anything else. It will spread into your work and into your life. There are no limits. There are only plateaus, and you must not stay there, you must go beyond them.” - Bruce Lee.

...
 I Yearn for a world where chickens can cross the road without having their motives questioned.

Offline phil c

  • 21 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2480
Re: Transitional Airfoils
« Reply #28 on: February 04, 2021, 10:59:48 AM »
For some reason I can never restrain myself when these mile long excursion into the "perfect" CLPA airfoil arise...

A number of years ago Big Art Adamisin went down under  to visit stunt folks and judge a contest or two.  One of his more curious /surprising  revelations upon his return was that there were several "competitive" entries with flat plate airfoils...i.e. nothing more exotic than a radius to the leading edge of conjoined sheets of balsa (don't recall if he ever mentioned the thickness) and (most likely although I don't know that I ever heard it from Big Art) a tapered trailing edge...nothing but flat balsa between the two!

One Adamisin or another forwarded pics of the ships following an earlier iteration of the above but, unfortunately, although they're likely somewhere in my office they would have to be somewhere within the multiple stacks of unlabeled or cataloged stunt stuff piles.

Just a hint to remember we're not talking supersonic helicopter rotor airfoils here!

Ted

Flat plate airfoils can work fairly well if they aren't loaded very much.  Dick Sarpolus and I conducted experiments on this when he started his "flat airplane" series of designs for RC.  He also built  a couple of the Too Windys with both a flat airoil(solid sheet.3/8 thick at LE, laminated to about 25% chord) and an actual more or less NACA 0015 airfoil for control line with the same area.  No flaps on either plane.

The flat airfoil flew just fine with smooth control inputs and not trying too hard to do "corners of a sharp radius".  It completely stalled and stopped turning in sharp corners, but just letting up on the elevator movement got it flying again instantly.  It quit flying if I tried a complete, full-control loop and would have fallen out of the air without letting it straighten out.

The "Airfoil" flew almost exactly the same, EXCEPT:  trying to do sharp turns, square loops, eights, etc were no problems.  With enough elevator travel the plane would turn as tight at you wanted until it just stopped turning tighter without stalling. It could turn sharply enough to do a presentable square loop, but remember, this was a simple, profile, flat fuselage plane.  It wasn't big and powerful enough to do a complete pattern.  Just not enough room on the circle.

So, I have no doubt a bigger, more  powerful flat airfoil stunter with enough power could be designed to fly the pattern.  As long as the controls were set up to keep it from stalling it would be fine.  Uneven air or gusts from odd direction would probably be a challenge, but not if the wing was big enough for the flying weight to pull it off.  With enough power it could be flown on long enough lines to get around pushing sharp turns to the limit.

Bottom line, Dick quickly switched to real airfoils for the RC designs.  After some comparison the "real" wing just flew a lot better than a flat plate and was just as strong.
phil Cartier

Offline Serge_Krauss

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1331
Re: Transitional Airfoils
« Reply #29 on: February 06, 2021, 03:58:04 AM »
This is the "Stuntwind". There was another plan set or two of other such designs that have appeared on Stunthanger, including photos. One was an "Imitation" I think. I have the article that goes with these somewhere around here. The thrust offset is apparently important, and the article explained the results of designer experimentation. I think that this forum has also featured accounts of successful slab-winged Australian Nats planes. I'll look further here at home.

Update 1: https://stunthanger.com/smf/stunt-design/sheet-wing-stunter/

Update 2: This is the thread I was thinking about, but the pictures and perhaps a link have disappeared. Descriptions remain.
https://www.stunthanger.com/smf/12-a-building/largest-sheet-wing-for-12a/

« Last Edit: February 06, 2021, 05:11:19 AM by Serge_Krauss »

Offline Chuck_Smith

  • Trade Count: (0)
  • Captain
  • *****
  • Posts: 685
Re: Transitional Airfoils
« Reply #30 on: February 06, 2021, 05:33:12 AM »
Ahh yes, the classic thin vs thick airfoil theory. 

So what's going on here? It's actually a lot simpler than people want to make it.

First, since ALL the airfoils are symmetrical they have the same lift coefficient and same zero lift angle of attack. The difference is in how they stall.

The thick wing has a progressive stall that starts at the trailing edge and moves forward as the angle of attack increases.
The thin wing stalls at the leading edge and stalls all at once.

If you re-read the description in Phil's post above describing flying the flat plate wing you'll have a great description of thin airfoil stall characteristics.

When you get down below 12% the leading edge radius becomes small enough that the stall initiates from the leading edge.

Nothing inherently wrong with thin airfoils. Almost every "tube and rag" light aircraft, Cub, Champ, Pitts, etc., has a very thin airfoil for the tail surfaces. This is possible because the aspect ratio of the tail is significantly less than that of the wing, so even though the airfoil is basically the diameter of the tubing used to build it, the wing will completely stall before the tail and control will be maintained.



AMA 76478


Advertise Here
Tags:
 


Advertise Here