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Author Topic: The best stunt airfoil  (Read 28211 times)

Offline Frank Wadle

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The best stunt airfoil
« on: February 06, 2018, 06:43:16 AM »
Hello,

Just recently I came across some pictures a russian F2B pilot posted on facebook. He is currently building prototypes of a new F2B contest airplane, fully take-apart, all build in molds.
On the pictures you can see that he is using a rather unconventional airfoil. It is a Wortmann FX-71-L-150/K25 and the flaps are integrated into the airfoil.
This started a discussion between some friends of mine and myself if this is a good choice.

And this discussion rose the question what exactly is the "best" airfoil for Stunt airplanes?
What exactly makes an airfoil suitable (or unsuitable) for stunt?

I'm not an expert but know about drag coefficient, lift coefficient and moment coefficient curves, but how do these curves have to look like in order to make it a good stunt airfoil?
And how should these curves look like to make it "the ideal" airfoil?


Best regards from Germany
Frank Wadle
« Last Edit: February 06, 2018, 07:02:18 AM by Frank Wadle »

Online Gerald Arana

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Re: The best stunt airfoil
« Reply #1 on: February 06, 2018, 09:54:49 AM »
That looks like a very good airfoil to me.(But I'm NO expert) HB~>

Of course the side of my shoe (Shoeman) looks good too!  LL~ LL~ LL~

Jerry

PS: I like those apple cheek cowls!

PPS: AFAIC, there is NO "Best" airfoil. All airfoils are good in one respect and not so good in another, i. e., it'll go fast and not turn or it'll turn and not go fast. That's my experience in glider racing. (Slope Soaring)

Offline Howard Rush

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Re: The best stunt airfoil
« Reply #2 on: February 06, 2018, 12:01:20 PM »
Dr. Wortmann published a paper in 1973, "Symmetrical Airfoils Optimized for Small Flap Deflection", which included the 71-L-150/25.  I don't know the difference, if any, between that and the 71-L-150/K25.  I presume that K is for Klappe (flap).  He designed these airfoils for surfaces such as vertical and horizontal stabilizers to have minimum drag when the control surface was deflected as for trim.  I don't think this is applicable to stunt, but Phil Cartier and I looked into using this airfoil for combat planes.  I found my hand plot of the 71-L-150/25 this morning.  I think I went so far as to make a flap, but I didn't finish the airplane.  As I recall, to break even for induced drag from the added weight, we would have had to build the flap and mechanism for less than 50 grams.  That seemed impractical.

The "Symmetrical Airfoils Optimized for Small Flap Deflection" (note American spelling) paper heading had the logo of OSTIV, and at the bottom was "Aero-Revue 3/1973".  The 71-L-150/25 and similar airfoils are in Stuttgarter Profilkatalog I, by D. Althaus, Institut für Aerodynamik und Gasdynamik der Universität Stuttgart, 1972.

I think the primary consideration for a stunt airfoil is maximum lift coefficient.  That allows wing area to be minimized, so turbulence response is minimized for a given cornering capability.  Other considerations are thickness, ease of building, and linearity of the lift-vs.-angle-of-attack curve.  The thicker the airfoil, the lighter the wing can be built, and airfoils with high max lift coefficient tend to be thick.  Airfoils that are thick and convex just forward of the flap hinge are easier to build and finish than airfoils such as the 71-L-150/25.  Igor mentioned linearity of the lift-vs.-angle-of-attack curve and wrote about airfoils with a hump in the curve.  I had not noticed such a hump, but looking at the 71-L-150/25 lift-vs.-angle-of-attack curve today, I see that it has that hump at large flap deflections.

I don't think drag is a big consideration for stunt airfoils.  Airfoils with the highest maximum lift coefficient tend to have relatively low drag in maneuvers (which is a virtue) and slightly higher drag in level flight than other airfoils.  Because we fly the same path every flight, I think we could calculate the energy required for a flight as a function of airplane weight, airfoil section drag, and span loading, and find out for sure how much airfoil section drag matters.   I have been too lazy to do that.

Using XFOIL, which I probably don't know how to do, the best stunt airfoil I've found is the Impact's.

Frank's next question will be about the optimal progression of rib spacing from wing root to tip.
The Jive Combat Team
Making combat and stunt great again

Offline Frank Wadle

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Re: The best stunt airfoil
« Reply #3 on: February 06, 2018, 12:48:46 PM »
Optimal rib spacing prigression is out of question a function to the power of 2 ;-)

I agree that the overall drag is not so much an issue.
My theory goes as such (but please dear experts take it easy on me. I'm not an aerodynamics engineer)

I think induced drag has to be minimized (difference of drag in lever to corner flying)
I also think lift should be nicely steadily increasing as you pull on the handle, no "bumps" in the lift curve.
I also think shift of centre of lift should be minimized.
But all this is not really based on textbook knowledge or empirical tests, it's just my intuition.

So I'm calling all the experts out there (hello Igor, Bob, Ted, Dave,....). Please enlighten me. Please tell me that I'm wrong, please tell us what to look for when we look at airfoil diagrams.

Offline Igor Burger

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Re: The best stunt airfoil
« Reply #4 on: February 06, 2018, 03:14:12 PM »

So I'm calling all the experts out there (hello Igor, Bob, Ted, Dave,....). Please enlighten me.

I did, several years before you asked that question :- )))

You can reread my SN article, it is reposted somewhere here on SH or on FB in my profile or on FB max bee page, you certainly saw it :-P ... now it is time to READ it, I mentioned there why that airfoil is not the best way. (and I know several guys proofed that fact :- P )

Offline Frank Wadle

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Re: The best stunt airfoil
« Reply #5 on: February 06, 2018, 03:24:28 PM »
Oh well, I certainly didn't come across it YET!!!
I will dig for it, but the internet is vastly big place.
Can you give us a clue where to exactly find it? A link maybe?

Offline Igor Burger

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Re: The best stunt airfoil
« Reply #6 on: February 06, 2018, 03:29:38 PM »
He designed these airfoils for surfaces such as vertical and horizontal stabilizers to have minimum drag when the control surface was deflected as for trim.  I don't think this is applicable to stunt

Exactly, that airfoil is for tails, not for wings, the main difference is AoA of those airfoils, while wing airfoil operates on positive AoA (regarding produced lift ) tail operates at NEGATIVE AoA ... means its lift is created by deflected flap, not by AoA of fixed part. So quick look to lift curve of that polar shows all problems, while lift curve on left side of 0 AoA axis (negative angles) is nice straight, right side has bumps and other strange excesses.

Result is clear - model is difficult to trim, having very pure tracking, and I have seen model which was not able to fly round loops, it did usually something like hexagons, whatever pilot did :- ))

The other problems of sharp LE were unexpected stalls.
« Last Edit: February 06, 2018, 03:49:13 PM by Igor Burger »

Offline Igor Burger

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Re: The best stunt airfoil
« Reply #7 on: February 06, 2018, 03:32:15 PM »
Oh well, I certainly didn't come across it YET!!!
I will dig for it, but the internet is vastly big place.
Can you give us a clue where to exactly find it? A link maybe?

Simplest way is this I think:

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

http://www.maxbee.net/download/MaxBee_2.pdf


Hope it still works

Offline Air Ministry .

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Re: The best stunt airfoil
« Reply #8 on: February 06, 2018, 04:52:30 PM »
Jees , looks like a P-51 airfoil , He must be trying to get to Berlin. And Back .



Hardly suit a scale stunt Hawker of Supermarine .  >:( cept maybe attacker / sea Fang .  :(

Offline Frank Wadle

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Re: The best stunt airfoil
« Reply #9 on: February 08, 2018, 05:02:17 AM »
Igor,
I read your article now. Thank you very much. That explains it very well.
And it is nice to see that your mathematic proof comes to pretty much the same conclusion as my intuition.

I really like your explanations about what happens in a corner vs. level flight.

I wonder what happens in the transition.
At first there is deflection of flap and rudder, then the plain starts turning and goes into the corner.
Then the deflection is rather abruptly gone and as a consequence the plane stops turning.
Have you had a look at that yet?

Frank

Offline Igor Burger

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Re: The best stunt airfoil
« Reply #10 on: February 08, 2018, 08:29:16 AM »
I wonder what happens in the transition.

Not too much. Hand only hardly makes so quick movement that we can call it "immediate" in comparison to corner time. Corner is not radius, it is more hyperbolic (becaise of that limited deflection speed), and tail (on modern stunters with relatively long tails and large areas) usually follow deflection easily. However still, transcient and especially limted speed of handle movement can make some kind of transcient like you describe. The tail of my max and wing load are designed with respect of 2 things:

1/ Aerodynamic transcient - the efect you described. The point is, that the tail with immediate deflection should not get stalled before the corner - that is easy to analyze, air flow is straight and we have tools for that. The second problem is immediate move of elevator back to neutral while model still flies radius. Since we know AoA of stab in radius, it is also relatively easy. My tail is designed with both on the edge of stall. And since I know I have limited speed of controlls movement, I know it is on safe side. And if pilot does it too strong, it can even do some negative feedback and make turns more repetitive (I mean stall on elevator because of too strong input - when it forms separtion bubble at LE of stab) ... at least I hope.  VD~ I feel something like that when I do stressy turn, looks like model does what it always did when I do "normal" corner.

2/ there is also mechanical transcied. This one is important how to get model fly clean for human eye. This is some kind of cheating, and we spoke about it already in past. Since we know  AoA of model coming from airfoil properties, wing load, flaps deflection etc, then if I know AoA in corner is say 10 degrees, then model will get proper angle for fly off 10 degrees before it does 90 degrees turn. Means absolutely clean fly off will need controll movement taking time 10 degrees of corner radius. Means not immediate movement. Something similar happens on begin of corner. I do not know if it is clear what I say, may be it will need some pictures. I think I posted them somewhere on stuka years ago. I want to say that properly designed model will give chance pilot to make relaxed handle movement while judge can see very quick corner. It has something to do also with that logaritmic unit on flaps (limited flap deflection). This is something we know when model comes too light, model tends to make end of turs somewhat difficult to handle, has tendency to make waves after corner etc. and for surprise some balast in CG can make it fly much better.

And that will probably (here I am not 100% sure with this) define aslso optimal tail arm length - too long and tail will stall in turns and at stop, too short and it will not make enough torque, plus thanks to short distance from wing, the air stream will be paralel with stab and it loose self controll (that stabilizing buble at LE of stab) and probably leads to too "jumpy" model and will need heavier nose (CG at 15 to 18% MAC). ... but here I am only guessing, I did not have time and chance to test.

... well yes I know little bit chaotic answer, but this is certainly not subject for short forum reply  ;D


Offline Tim Wescott

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Re: The best stunt airfoil
« Reply #11 on: February 08, 2018, 09:08:21 AM »
... I think induced drag has to be minimized (difference of drag in lever to corner flying) ...

Other's have thought this, too.  Because induced drag is almost purely a function of aspect ratio and wing tip design, they went out and built really high aspect ratio stunt planes.  The results cornered almost too well (although that may be a training issue) and didn't handle wind nearly as well as the 5:1 to 5.5:1 aspect ratios that you usually see in the winner's circle.
AMA 64232

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

Offline Igor Burger

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Re: The best stunt airfoil
« Reply #12 on: February 08, 2018, 09:14:24 AM »
Amount of induced drag is almost function of "span load" - means weght to span ratio (well +/- something) ... we also spoke about it some year ago

Offline Tim Wescott

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Re: The best stunt airfoil
« Reply #13 on: February 08, 2018, 05:08:55 PM »
Amount of induced drag is almost function of "span load" - means weght to span ratio (well +/- something) ... we also spoke about it some year ago

Sorry, yes -- for a given weight, speed and wing area, it's all down to aspect ratio, but it's really driven by speed and span loading.  It still means that all else being equal, if you want to reduce induced drag you need to increase aspect ratio.  And while I wasn't around for the high aspect ratio stunter era, I've certainly heard stories about the consequences.
AMA 64232

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

Offline Vitalis Pilkionis

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Re: The best stunt airfoil
« Reply #14 on: February 08, 2018, 05:37:00 PM »
Here it is, the Wortmann FX-71-L-150/K25 airfoil. One can find its data here on page 43:
http://www.bilimuygula.com/FileUpload/bs774731/File/deneme_modellflug_profilesammlung.compressed.pdf

K25 was designed for 25% chord flap.


Offline Istvan Travnik

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Re: The best stunt airfoil
« Reply #15 on: February 08, 2018, 06:26:21 PM »
Huh,
It is sorrow to see this beautiful fuselage, designed for wings with FX-71-L150. Why?
To choose this airfoil for C/L stunt, is an absolute, elementary mistake. 
This so-called "laminar" airfoil is designed for fullscale sailplanes' vertical stabilizers, for Re at least 1.000.000, for minimum drag and high efficiency.
(We used to fly sometimes no more than 350.000, pretty below the critical Re value of FX-150)
You can see: the thickest point is very-very aft, at 38% of chord. (Ours is around 25%).
If you modify this airfoil to bring forwards the thickest point, (it is not a big task with "Profili-II" software), the result can be usable. That will be the well-known  "pollywog" airfoil. I use similar, with good results for 30 years, with 30% to 20% moving part (flap) and 16,7% to 15% thickness. It never moves more than 20°by me.
One remark: it is hard task to build correct, warp-free concave flaps by conventional technology. (This question does not touch me since I build of bluefoam, covered by thin glass+epoxy). 
« Last Edit: February 09, 2018, 01:20:56 AM by Istvan Travnik »

Offline Tim Wescott

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Re: The best stunt airfoil
« Reply #16 on: February 08, 2018, 09:45:08 PM »
And this discussion rose the question what exactly is the "best" airfoil for Stunt airplanes?

Al Rabe used to claim that he designed his airfoils by tracing around his shoes.  If I could walk a mile in that man's shoes -- I'd trace them out first.
AMA 64232

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

Offline Igor Burger

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Re: The best stunt airfoil
« Reply #17 on: February 09, 2018, 05:15:42 AM »

>>>Amount of induced drag is almost function of "span load" - means weght to span ratio (well +/- something) ... we also spoke about it some year ago<<<

Sorry, yes -- for a given weight, speed and wing area, it's all down to aspect ratio, but it's really driven by speed and span loading.  It still means that all else being equal, if you want to reduce induced drag you need to increase aspect ratio.  And while I wasn't around for the high aspect ratio stunter era, I've certainly heard stories about the consequences.

The point of that mine statement is that the aspect ratio itself is not whole answer. The induced drag is function of both lift coeffcient and and aspect ratio (linear to both). So if you make shorter chord, you will get proportionally smaller area and thus higher lift coefficient (having all other flight parameters equivalent) and you will get proportionally better aspect ratio. Unfortunatelly one compensate the oyther, so induced drag will not change as expected. Means there is only span which can change induced drag, not aspect ratio itself. Because elongating the span, we will get larger area and thus smaller lift coefficient AND better aspect ratio. So it makes it power of 2.

It means 2 things for induced drag:
1/ you can change aspec ratio withouthout changing span - nothing happens
2/ you can change span without changing aspect ratio a induced drag will change

That means the span defines induced drag (for the model of given weight), not aspect ratio itself. Or better written - weight to span ratio.

Offline Frank Wadle

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Re: The best stunt airfoil
« Reply #18 on: February 09, 2018, 07:57:27 AM »
First I would like to thank everyone for the great input.
This has definitely shed some light on this very interesting topic. I still have a few questions, but I will come to that later.

Meanwhile I would like to clarify what I meant by induced drag.
I believe we have to think in percentages. I think it is important that the drag increases by as little percentages as possible.
This can also be achieved by selection of airfoil.
Many years ago when I designed my Big Kahuna airplane I used the software profili to investigate the influence of different trailing edge shapes.
For structural reasons I wanted to use a “thick square” TE and not a sharp one. And I found out that the thick one has a higher drag overall. And the change of drag in percentages was smaller than with a razor sharp TE.
So there is definitely an influence of the airfoil on this subject.

Let me be the devil’s advocate for a moment…
Why do we want as little increase in drag as possible?
Because we want as constant as possible speed.
This could also be achieved by dragging a parachute behind the plane.
The drag of the parachute is constant, no matter if you fly level, if you fly up, down or if you fly a corner. And that would even out the speed of the plane.
Wouldn’t that mean we have to increase the drag of such parts that don’t create lift as much as possible?
High drag fuselage…
Yak 55…
:-)


Offline Igor Burger

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Re: The best stunt airfoil
« Reply #19 on: February 09, 2018, 08:51:51 AM »
If your model has in-flight drag 10N and drag variation in corner is +5N, then you will balance that in-flight drag by 10N prop thrust. So speed variation in 0,2s corner will change by:

0,2s * (110N - 110N + 5N) / mass of model = 0,2s * 5N / mass of model

If you add a parachute with 100N, the total  in-flight drag will be 110N and drag variation in corner is +5N, then you will balance that in-flight drag by 110N prop thrust. So speed variation in 0,2s corner will change by:

0,2s * ( 10N -  10N + 5N) / mass of model = 0,2s * 5N / mass of model

does it answer what we have to minimize to limit speed variations in corner enough?  H^^

Offline Frank Wadle

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Re: The best stunt airfoil
« Reply #20 on: February 09, 2018, 10:24:45 AM »
I understand.
I thought it goes as such:
The power needed to fly level is
P=F * V
And the power of the motor is always the same for the reason of a simpler math :-)
That means in level and in a corner you have the same power (kW) to fly the plane.
Low drag:
F(level) * V(level) = P = F(corner) * V(corner)
V(corner) = F(level) / F(corner) *V(level)
V(corner) = 10N / 15N * 25m/s = 16,666m/s

High drag:
V(corner) = 100N / 105N * 25m/s = 23,8m/s

I must have looked at the problem from the wrong perspective. I'm sure your explanation is correct and mine is wrong.  HB~>

Offline Vitalis Pilkionis

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Re: The best stunt airfoil
« Reply #21 on: February 09, 2018, 10:29:11 AM »
I'v made a quick analysis with Javafoil of FX71-L-150/K20 airfoil with flaps at 27 degrees. I think a result talk by itself...


Offline Igor Burger

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Re: The best stunt airfoil
« Reply #22 on: February 09, 2018, 10:42:25 AM »
I understand.
I thought it goes as such:
The power needed to fly level is
P=F * V
And the power of the motor is always the same for the reason of a simpler math :-)
That means in level and in a corner you have the same power (kW) to fly the plane.
Low drag:
F(level) * V(level) = P = F(corner) * V(corner)
V(corner) = F(level) / F(corner) *V(level)
V(corner) = 10N / 15N * 25m/s = 16,666m/s

High drag:
V(corner) = 100N / 105N * 25m/s = 23,8m/s

I must have looked at the problem from the wrong perspective. I'm sure your explanation is correct and mine is wrong.  HB~>

I agree ... at least with the last sentence :- ))))))))))

if you expect that the power in both cases the same, you must also expect that the speed is the same, If speed changes, also the power changes.

Offline Frank Wadle

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Re: The best stunt airfoil
« Reply #23 on: February 09, 2018, 10:46:35 AM »
Correct. If speed drops then power output of Propeller drops.
So even More drastic effect.

The difference is that my formular applies to equilibrium only. So I guess it does not so much apply to corner. But maybe more for wingover shortly before we go back to level flight.

Offline Igor Burger

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Re: The best stunt airfoil
« Reply #24 on: February 09, 2018, 10:48:00 AM »
I'v made a quick analysis with Javafoil of FX71-L-150/K20 airfoil with flaps at 27 degrees. I think a result talk by itself...

Yes, that exactly shows what I wrote above.

1/ The lift curve shows that deflected controlls push that airfoil to regime when it abruptly loses lift as it get to higher AoA = just opposite what pilot expect and VERY hard to fly wanted shape path.

2/ The moment curve shows that it has little bit positive feedback in pitchig effect, it means also pitch stability is so low, that it will be hard to conroll at all.

This is just opposite what we need from aitfoil. And it also shows that such airfoil is for AoA less than 0, you can see it is very nice linear airfoir in AoA left of the 0 AoA axis. = great airfoil for tail, but certainly not for wing.
« Last Edit: February 09, 2018, 01:21:46 PM by Igor Burger »

Offline Igor Burger

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Re: The best stunt airfoil
« Reply #25 on: February 09, 2018, 10:49:41 AM »
Correct. If speed drops then power output of Propeller drops.
So even More drastic effect.


Try my props, they pull more, not less when model slows down :- ))))))))))))))

Offline Istvan Travnik

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Re: The best stunt airfoil
« Reply #26 on: February 09, 2018, 12:58:20 PM »
Dear Vitalis,
What do you want with FX-71-L150 at Re=400.000? (instead of 1.000.000)
Dr. Felix Wortmann DID know what he designed, and what is it for...
Fullscale sailplane builders DO know why they use this airfoil...
C/L stunters using this airfoil  simply DO NOT know what they do.
It is so simple.
 

Offline Brett Buck

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Re: The best stunt airfoil
« Reply #27 on: February 09, 2018, 02:23:38 PM »
Try my props, they pull more, not less when model slows down :- ))))))))))))))

   They ALL do that (within usable pitch ranges). What yours does better (for your purposes) is also fall into the drag bucket when slowed down. That's why we liked similar prop airfoils on the 46VF and the PA51 (which run very smoothly with little reaction to drag torque) and had to go WAY off the reservation to get it to work on the PA40/61.  In fact we found the PAs worked better with flat-backed airfoils because the engine itself reacted plenty enough to not have to count on the drag going down.

    Of course you have solved that dilemma by having your feedback system, which senses the slowdown directly, instead of indirectly as it is with an engine, and use the drag bucket issue to partially mitigate the finite bandwidth of the controller. The fact that only a few people even see a glimmer of what you are doing makes it pretty hard to explain, of course.

   Brett

Offline Brett Buck

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Re: The best stunt airfoil
« Reply #28 on: February 09, 2018, 02:28:49 PM »
Correct. If speed drops then power output of Propeller drops.
So even More drastic effect.

     The shaft power may drop but the thrust goes up drastically, and the system absolutely counts on that.  The increase in induced drag from creating more thrust is what makes the RPM/power drop in the first place. Unless you have an undercambered prop with a conveniently-placed drag bucket like an Eather or Igor prop, in which case, you get more thrust and no RPM drop, or at least much less RPM/power reduction.

    Brett

Offline Vitalis Pilkionis

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Re: The best stunt airfoil
« Reply #29 on: February 09, 2018, 02:53:26 PM »
Dear Vitalis,
What do you want with FX-71-L150 at Re=400.000? (instead of 1.000.000)
C/L stunters using this airfoil  simply DO NOT know what they do.
It is so simple.

It's too late, bro, to apologize - molds have been already made and a first ship is almost ready to fly. Let's wait for a flight review.

Offline Igor Burger

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Re: The best stunt airfoil
« Reply #30 on: February 09, 2018, 03:07:17 PM »
   They ALL do that (within usable pitch ranges).

Of course :- )))

I just little bit provocated Frank, he will certainly understand : -)))

BTW, regarding flat back and underchambered props, it is still here also with electric, for example I like flat back props, while some guys especially with Yatsenko models (for example Orestes) like underchambered (I mean both with my regulation). May be it has something to do with model type, but I am not 100% sure.

Offline Frank Wadle

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Re: The best stunt airfoil
« Reply #31 on: February 09, 2018, 04:03:28 PM »
Let's say shaft power is constant. Then the prop output power will decrees with the decreasing velocity of the model.
At stand still (model on ground with prop spinning) the efficiency of the prop is 0%
I've once built a propeller test stand in a wind tunnel that proved this.

Anyway... We are drifting away.
I admire the courage this Russian builder has. He has put a lot of work into making moulds. I keep my fingers crossed that his efforts will be rewarded with a well flying airplane.

In our recent F2B history there was another successful design using a rather unconventional airfoil.... Beringer used something like the CAP21 airfoil for his designs and was very successful with them.
How does this airfoil tie in to Igor's theories?

Offline Igor Burger

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Re: The best stunt airfoil
« Reply #32 on: February 09, 2018, 04:23:46 PM »
Let's say shaft power is constant. Then the prop output power will decrees with the decreasing velocity of the model.
At stand still (model on ground with prop spinning) the efficiency of the prop is 0%
I've once built a propeller test stand in a wind tunnel that proved this.

Well, fact that prop has 0 efficiency at still, is clear from physics, you not need stand to proof it, if velocity is 0 and trust whatever, then power comming from force * speed must be also 0 since the speed is 0

... just the same like when prop does not make any thrust at pitch speed - the efficiency is also 0 because whatever the speed is, multipied by 0 force you have 0 power ... before that point power INCREASES when model slows down.

... so you can see that there is also speed range where that theory does not fit reality :-))



However it is false expectation that power on shaft is constant. Much better expectation is that RPM is constant.


Offline Igor Burger

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Re: The best stunt airfoil
« Reply #33 on: February 09, 2018, 04:30:57 PM »
In our recent F2B history there was another successful design using a rather unconventional airfoil.... Beringer used something like the CAP21 airfoil for his designs and was very successful with them.
How does this airfoil tie in to Igor's theories?

That was actually MUCH better airfoil compared with FX 71. The problem of those models were too small flaps and therefore small lift (may be Keith R. can repost some analyzed triangle which we saw on some meeting) and thus it needed extremly low wing load and thus making troubles in turbullent air.

Offline Vitalis Pilkionis

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Re: The best stunt airfoil
« Reply #34 on: February 09, 2018, 04:38:14 PM »
In our recent F2B history there was another successful design using a rather unconventional airfoil.... Beringer used something like the CAP21 airfoil for his designs and was very successful with them.
How does this airfoil tie in to Igor's theories?

It's been discussed here (one of my favorit topics btw):

https://stunthanger.com/smf/engineering-board/ice-cream-cone-airfoils/

Offline Frank Wadle

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Re: The best stunt airfoil
« Reply #35 on: February 09, 2018, 04:47:01 PM »
Basic problem was, what is better?
A gain in drag of 10% or a gain in drag of 20% due to induced drag?
And I know there are several ways to achieve the lower increase percentage.

Offline Tim Wescott

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Re: The best stunt airfoil
« Reply #36 on: February 09, 2018, 05:24:12 PM »
Basic problem was, what is better?
A gain in drag of 10% or a gain in drag of 20% due to induced drag?
And I know there are several ways to achieve the lower increase percentage.

I think you're dwelling too much on one thing.  And Igor's explanation early in this thread about the importance of not having any sudden changes in lift or drag makes a lot of sense to me.  We have gobs of available power, from our motors or engines -- we don't need super-efficient low-drag machines.  What we need are super-predictable airplanes that go where you point them when you point them.  I do not believe that drag even approaches being the biggest factor in that equation.
AMA 64232

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

Offline Brett Buck

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Re: The best stunt airfoil
« Reply #37 on: February 09, 2018, 10:15:58 PM »
Of course :- )))

I just little bit provocated Frank, he will certainly understand : -)))

BTW, regarding flat back and underchambered props, it is still here also with electric, for example I like flat back props, while some guys especially with Yatsenko models (for example Orestes) like underchambered (I mean both with my regulation). May be it has something to do with model type, but I am not 100% sure.

   Depends on how much "boost" you can tolerate, or want. I have been very surprised at how much boost/brake people have been willing to use, even when it was very clearly hurting their overall performance.  I am not sure why that is, mostly, I think it is that people tend to line up with a particular guru, and just do what the guru says, and aren't even willing to try anything that might incur the guru's disapproval. Or just general unwillingness or inability to make sense of experimental systems.

   People can get to be pretty good by getting their system just good enough and then flogging it with 2000 flights a year. I think it has gotten a lot harder to win that way, but not impossible.

       Brett

Offline Igor Burger

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Re: The best stunt airfoil
« Reply #38 on: February 10, 2018, 01:40:25 AM »
Basic problem was, what is better?
A gain in drag of 10% or a gain in drag of 20% due to induced drag?
And I know there are several ways to achieve the lower increase percentage.

I think the the result from speach is clear, we need minimize CHANGES of drag, not average value of drag, and since we have drag of airfoil which gets higher at higher lift and plus induced drag, we must optimize airfoil for high lift - means with deflected flap - means keep is as small as possible ... while extra drag from inefficient airfoil shape with straight flap is welcome. That is vhy we use "added" flaps while integrated flaps are suboptimal. Just like I show in my article :- ))

But it is slill only about drag, I see TIm wrote already that there are other aspects of airfoil making them good or bad. 

Offline Igor Burger

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Re: The best stunt airfoil
« Reply #39 on: February 10, 2018, 02:48:46 AM »
   Depends on how much "boost" you can tolerate, or want. I have been very surprised at how much boost/brake people have been willing to use, even when it was very clearly hurting their overall performance.  I am not sure why that is, mostly, I think it is that people tend to line up with a particular guru, and just do what the guru says, and aren't even willing to try anything that might incur the guru's disapproval. Or just general unwillingness or inability to make sense of experimental systems.

Probably also a reason, people want it best of the best, strongest boost, largest prop etc ...

So happy as it pulls on top of loop, but prop cannot brake so well as it can pull, so it overspeeds on bottom especially in wind ... so they compensate with underchambered prop which will dump such wild reactions :- ))

Offline Brent Williams

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Re: The best stunt airfoil
« Reply #40 on: February 10, 2018, 03:18:01 PM »
Here's a side question.  Is there a point when these airfoil design arguments become somewhat futile, when all of the "best airfoils" are now, or have been seen sporting the addition of either zig-zag turbulator strips or vortex generators to augment lift?
   
Here's a few notable  examples:
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 Brett Buck

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Re: The best stunt airfoil
« Reply #41 on: February 10, 2018, 03:29:53 PM »
At what point do these airfoil design arguments become futile, when all of the "best airfoils" are often seen sporting the addition of either zig-zag turbulator strips or vortex generators?
   
Here's a few notable  examples:

   Those guys? Meh.

     What it really tells you is that to first approximation, unless you go out of your way to do something silly, it's just not that critical. Maybe, just possibly, a very subtle reshaping of the airfoil might let any of those airplanes fly as well without VGs, but probably not. You have airfoils that are nearly 3" thick (ARF Strega) that have less capability than airfoils maybe 1 3/8' (Diva), so it matters some, but (in decreasing order of silliness):

don't make the LE pointy (known and documented in the early 70s)
don't make the aft portion a straight line/flat to the hinge line
don't make the aft portion concave
don't insist on fairing every LE shape fair into a 1/4 square LE set on edge

    and you will probably be OK.

      Brett

Offline Air Ministry .

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Re: The best stunt airfoil
« Reply #42 on: February 10, 2018, 08:18:17 PM »
But Youve Left Half of Them OFF .   S?P




those airbrakes look rather usefull , too .



not the best illustration , usualy use 1/2 W x 3/8 deep ( 3/4 O.A. ) Double Upholsterers Thread . if you dont countersink it ,
one would presume it must have some ' readhearance ' effect on the airflow . Despite looing centuries old .

The Olde Sailmakers palm , Binding of spars and rigging etc , would convince of the immense strength and coheshion of members .

Offline Vitalis Pilkionis

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Re: The best stunt airfoil
« Reply #43 on: February 11, 2018, 04:59:46 AM »
You have airfoils that are nearly 3" thick (ARF Strega) that have less capability than...

Does anybody know why Brett is so sensitive about Strega, Patternmaster and all the derivatives of Big Jim's legacy? Cmon, Brett, lay down your pain finally   b1

But let's talk about Strega's airfoil particularly. I'v been very curious myself about that since I became an owner of two Strega ARF kits, thinking if they do worth my endeavour as my future project. Now when I'v learned to use Javafoil a little bit, I can think this airfoil is not really that bad. It definitely has a potential, but ofcourse only after drastic LE modification (blunting).

Firstly even just in visual comparison to the classic and proven PW Impact's airfoil, a modified Strega's airfoil doesn't look so much exotic by the means of it's thickness.

And secondly, as Javafoil analysis has shown, up until flap deflection to 22 degrees, it has a very smooth Cl curve, a very reasonable Cm and quite a low Cd up until Cm=2.0.

This and all the flight reports of Stregas with blunted airfoil, convinced me to begin a work on my current project. Still I decided to use only a wing and fuse from a kit, all other parts have been redesigned and built from a scratch.

Vitalis

Offline Frank Wadle

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Re: The best stunt airfoil
« Reply #44 on: February 11, 2018, 07:23:50 AM »
Does someone have an airfoil analysis of the Yatsenko airfoil as used on a Classic or a Yak?


Offline Igor Burger

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Re: The best stunt airfoil
« Reply #45 on: February 11, 2018, 07:32:17 AM »
as Javafoil analysis has shown, up until flap deflection to 22 degrees, it has a very smooth Cl curve, a very reasonable Cm and quite a low Cd up until Cm=2.0.

22 is too little deflection for corners, go over, to 30 or so and you will see forming those bumps on lift curve and abrupt changes of momoment.

Offline Frank Wadle

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Re: The best stunt airfoil
« Reply #46 on: February 11, 2018, 07:38:38 AM »
Okay Igor. I have a question.
Why do you assume we need 30° in a corner?
I have seen in flight pictures of models in a corner and it was visible that the model had far less than 30° flap deflection in the corner.
I have even seen models that weren't capable of 30° flap deflection, there was simply not enough travel in the controls. And they still flew quite nice corners.
Did you take pictures in corners of your plane and analysed them? Or what makes you say that you need 30° ?

Offline Igor Burger

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Re: The best stunt airfoil
« Reply #47 on: February 11, 2018, 07:50:14 AM »
Here's a side question.  Is there a point when these airfoil design arguments become somewhat futile, when all of the "best airfoils" are now, or have been seen sporting the addition of either zig-zag turbulator strips or vortex generators to augment lift?
   
Here's a few notable  examples:

Any VG or turbulator will degrade parameters of airfoil (lift /drag) on airfoil over critical RE and we ARE over. At the same time it will improve controllability if it makes problems mentioned earlier. I never got success with VGs, but I had to use turbulator on wing because of not so perfect surface on my wing happened in one heavy rain on a contest (visible on first picture under the "S"), so I rather installed them on wing and it cleaned some imperfections. Since then I use composite wings and during trimming I always test also turbulators but I always removed them, so I did not have them in Perth and also not on my newest model.

Completely different situation is on tail (as described in article by Dave F. in Stunt news), if you take stab itself, it can be considered on edge of critical RE number. And analyze will show that there can happen very strange effects in boundary layer when transition point can abruptly jump between LE and hingeline what will certainly make some pitching effects like hunting), I also wrote it some years ago, so I not only use those mentioned sharp edged LE as Brett wrote (iy pushes critical number down, because it is source of turbulation), but I also install turbulator close to LE - if it is necessary or not. It will not hurt anything, I did not find any ill effects yet.

Offline Igor Burger

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Re: The best stunt airfoil
« Reply #48 on: February 11, 2018, 08:01:07 AM »
Okay Igor. I have a question.
Why do you assume we need 30° in a corner?
I have seen in flight pictures of models in a corner and it was visible that the model had far less than 30° flap deflection in the corner.
I have even seen models that weren't capable of 30° flap deflection, there was simply not enough travel in the controls. And they still flew quite nice corners.
Did you take pictures in corners of your plane and analysed them? Or what makes you say that you need 30° ?

Take those numbers from polar and make the math, you will see it is not enugh :- ))

... and the next question which you are going to write - yes I did that math, so I do not assume,  :- ))))))))))

You go from totally opposite side of the problem ... you have to go down with area to load the airfoil to its maximal numbers, and it is 30 degrees on some, 22 on another and 15 on even other, but that wich will work well at 30 will give certainly better model, so you can use also that 15 you can even fly without flaps, it is only your choice, however which is better is clear :- ))

Offline Vitalis Pilkionis

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Re: The best stunt airfoil
« Reply #49 on: February 11, 2018, 10:30:09 AM »
22 is too little deflection for corners, go over, to 30 or so and you will see forming those bumps on lift curve and abrupt changes of momoment.

Dear Igor, of course your airfoil is perfect in terms it doesn't have any bumps on ascending part of Cl curve, even with flap at 55 degrees (and I'm not kidding here). And Cl with flaps at 30 is phenomenal.
But hey, all those numbers are just numbers, although you really made me doubt  ~^
As we all know even on your own model flaps are restricted to 27 degrees. At this setup Cl of your airfoil can theoretically reach almost 2.35 at 8 degrees of AoA.
Now.. if another (clearly not so noble) airfoil can smoothly reach Cl=2.1 at the same 8 degrees AoA and stay there up until 20, does that really matters to what angle flaps are deflected 27 or 22 ???
Yes it starts to form a bump with flaps deflected over 22 degrees and probably a model with this airfoil will not be able to perform extremely sharp corners, but I believe it's going to be at least a competitive one.


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