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Author Topic: A question on flaps efficiency  (Read 2147 times)

Online Peter Germann

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A question on flaps efficiency
« on: December 11, 2018, 04:55:45 AM »
see:

http://homepages.hs-bremen.de/~kortenfr/Aerodynamik/script/node83.html

and attached drawings

"Die einfache Klappe besteht aus einem Flügelsegment, das an einer Stelle gelenkig gelagert ist, und ausgeschlagen werden kann. Üblicherweise beträgt die Klappentiefe dabei etwa 30 Prozent der gesamten Profiltiefe. Die einfache Klappe erhöht den Auftrieb durch Erhöhung der Profilkrümmung. Für ein typisches Profil wird der maximale Auftrieb bei einem Klappenausschlag um 40 bis 45 Grad erreicht."

The simple flap consists of a wing segment that is hinged at one point and can be deflected. Usually, the flap depth is about 30 percent of the total profile depth. The simple flap increases lift by increasing the profile camber. For a typical profile, maximum lift is achieved with a flap deflection of 40 to 45 degrees.


 
"Die Spaltklappe ist eine Einfachklappe mit einem Spalt zwischen dem Hauptflügel und der Klappe. Dieser Spalt erlaubt eine Strömung von dem Gebiet erhöhten Druckes auf der Flügelunterseite auf die Oberseite der Klappe und reduziert damit deutlich die Tendenz zur Strömungsablösung auf der Klappe und erhöht so den erreichbaren Auftrieb bei gleichzeitiger Widerstandsreduktion relativ zur einfachen Klappe."

The slot flap is a single flap with a slot between the wing and the flap. This gap allows a flow from the area of increased pressure on the underside of the wing to the top of the flap and thus significantly reduces the tendency for flow separation on the flap and thus increases the achievable lift with simultaneous drag reduction relative to the simple flap.


True or false?
If true. why don’t we build slot flaps?

Peter Germann
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Offline Tim Wescott

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Re: A question on flaps efficiency
« Reply #1 on: December 11, 2018, 09:35:30 AM »
As far as I know, because we don't deflect our flaps that much.  You look at the modern guidance for flap (and elevator) deflection, and it's in the neighborhood of +/- 20 degrees maximum.  Then you look a bit more and you see that the recommendation is to never bottom out your control system.

The reason for flaps on a full-scale plane is to assist on take-off and landing (which, I assume, is why some German plans label them "Landeklappen").  Our flaps are maneuvering flaps, to make the turns prettier and to let us fly heavier airplanes and get good flight scores.
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Offline Chuck_Smith

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Re: A question on flaps efficiency
« Reply #2 on: December 11, 2018, 10:27:14 AM »
That's a decent explanation, but add that they can be moved backwards as they deploy and create a meaningful increase in wing area which aids in lower landing speeds.

On small airplanes flaps are generally just there to give you a steeper approach - the added drag lets you point the spinner down. The reduction in landing speeds are relatively small on small airplanes usually 10Kts or less.

I used to own a full-size sailplane that had flaps only and not dive brakes. The flaps were purely for drag. They even went up as well as down - to reduce drag so I could boogie between thermals at over 100mph. At 90degrees flaps on final the view from cockpit was, shall we say, impressive. You basically stood on the rudder pedals.

Back on topic, if you'd like to see slotted flaps in action look at a 2018 F1 racecar front wing. Rather than a single element they spit it up into slotted flaps.

Chuck
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Offline Massimo Rimoldi

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Re: A question on flaps efficiency
« Reply #3 on: December 11, 2018, 10:34:23 AM »
Good morning, Peter.
Personally I have no reason to doubt the theory expounded .
However, I think that a good reason that prevents (or at least hinders) the realization of a slot flap is mechanical in nature.
Our flaps are to be moved in both directions, so it is mechanically complicated to realize a hinge system that allows the opening of the slot in both directions.
Maybe someone has some solution in the drawer.

Massimo

Offline Tim Wescott

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Re: A question on flaps efficiency
« Reply #4 on: December 11, 2018, 10:47:09 AM »
Another point to ponder is that most of the top guys seal their hinge lines for consistency.  The few that don't, as far as I know, go to great lengths to maintain a consistent gap between wing and flap.

We are not using flaps to maximize lift!  We're using them so the airplane doesn't stagger through the corners like a drunken rhinoceros.  It's a different application than full-scale airplane flaps, so you have to carefully question whether you should adopt full-scale airplane practice.
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Offline jose modesto

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Re: A question on flaps efficiency
« Reply #5 on: December 11, 2018, 12:47:48 PM »
Frank Williams used slotted flap model that  performed like a typical flapped model
Hope he will see this thread
Jose modesto

Online Peter Germann

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Re: A question on flaps efficiency
« Reply #6 on: December 12, 2018, 02:59:32 AM »
We are not using flaps to maximize lift!  We're using them so the airplane doesn't stagger through the corners like a drunken rhinoceros.  It's a different application than full-scale airplane flaps, so you have to carefully question whether you should adopt full-scale airplane practice.
I respectfully question this, Tim. The centrifugal forces in a tight corner are such that we do in fact need every amount of lift we can get when trying to fly the turn according to the, admittedly FAI,  rule.
Peter G.
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Online Brett Buck

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Re: A question on flaps efficiency
« Reply #7 on: December 12, 2018, 09:50:00 AM »
I respectfully question this, Tim. The centrifugal forces in a tight corner are such that we do in fact need every amount of lift we can get when trying to fly the turn according to the, admittedly FAI,  rule.
Peter G.

    I don't think it has anything to do with the FAI, the AMA has a corner specification of, effectively, 0 feet radius (since I wrote the rule...). The other argument has to do with shifting the emphasis to "corner above everything", including the other 10,000 words (or 10 kilowords for the metric crowd) that talk about shape and precision, not that the corner radius is not important.

    I would be interested in what the intent might be here - do you think the airplanes are currently limited by the maximum lift coefficient in actual practice? Because I think we are nowhere close to that. When we actually do make it to the point of stalling, it's not because of anything happening near the hinge line, but far forward of that.

     Brett

Offline Chuck_Smith

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Re: A question on flaps efficiency
« Reply #8 on: December 12, 2018, 10:43:51 AM »
I've spent way too much time on stunt flaps. Even a lot of "government" CFD time.  Flaps on a CLPA plane are aerodynamic oddities. In theory they cause as many problems as they solve.

Here's the way I see it based on data and heuristics:

1) Yes, they increase the lift coefficient for the wing and add lift - at the price of a bucket load of drag. I mean, like a lot! Did I say," Wow! that's a xxxx-load of drag?!"
 
2) They work counter to the desired pitching moment, meaning you need more elevator with them than without. (again, more drag)

3) The increased downwash they create may counter (2) above if the aircraft design is correct, but that depends on how the stab is positioned WRT the wing.

4) They work - and when the day is done I  often think it may be more about keeping the speed consistent on down lines after corners than actual turning radius.

5) I think over the last 50 years that judges just expect the pattern to look like the way flapped stunt ship flies it. A Flite Streak or Goldberg Shoestring will breeze through the pattern effortlessly with crisp squares but probably won't get you a lot of points.


My 2¢, worth what you paid for it.

Chuck
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Offline Chuck_Smith

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Re: A question on flaps efficiency
« Reply #9 on: December 12, 2018, 10:56:09 AM »
   

    I would be interested in what the intent might be here - do you think the airplanes are currently limited by the maximum lift coefficient in actual practice? Because I think we are nowhere close to that. When we actually do make it to the point of stalling, it's not because of anything happening near the hinge line, but far forward of that.

     Brett

Brett,

Respectfully, I don't agree with that. We fly (very!) thick airfoils with bluff leading edges. These types of airfoils characteristically stall from the trailing edge forward, with a gradual progression forward resulting in a "mushy" stall. (As opposed to a thin airfoil with a sharp LE that will stall from the LE backwards all at once - the "hard break" as it were.)

If anything trips the flow forward of the TE it can bring the onset of the stall at a lower effective AOA.

Also, we are sooooooooo subsonic that the flow field around the aircraft is the sum of all it's parts, i.e., what's happening at the TE of the wing is influencing what's happening at the spinner as well as the tip of the rudder.

Also, the angle at which we stall is more a function of the aspect ratio than anything else, as that controls the induced drag and hence, the aerodynamic AOA.

But - feel free to disagree.

Thanks,

Chuck

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Offline Tim Wescott

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Re: A question on flaps efficiency
« Reply #10 on: December 12, 2018, 02:03:22 PM »
I've spent way too much time on stunt flaps. Even a lot of "government" CFD time.  Flaps on a CLPA plane are aerodynamic oddities. In theory they cause as many problems as they solve.

Here's the way I see it based on data and heuristics:

1) Yes, they increase the lift coefficient for the wing and add lift - at the price of a bucket load of drag. I mean, like a lot! Did I say," Wow! that's a xxxx-load of drag?!"
 
2) They work counter to the desired pitching moment, meaning you need more elevator with them than without. (again, more drag)

3) The increased downwash they create may counter (2) above if the aircraft design is correct, but that depends on how the stab is positioned WRT the wing.

4) They work - and when the day is done I  often think it may be more about keeping the speed consistent on down lines after corners than actual turning radius.

5) I think over the last 50 years that judges just expect the pattern to look like the way flapped stunt ship flies it. A Flite Streak or Goldberg Shoestring will breeze through the pattern effortlessly with crisp squares but probably won't get you a lot of points.


My 2¢, worth what you paid for it.

Chuck

On 1 -- do they increase drag more than an equivalent flapless wing?  My understanding of aerodynamics is simple, but isn't most of the increase just the inevitable effects of induced drag?

On 2 & 3 -- empirically, they add more of a need for pitching moment than the downwash alleviates.  I can tell this because a Flight Streak or a Skyray 35 gets by on an itty bitty elevator that's just 25% of the total horizontal tail area, while an Impact or Nobler or whatever needs 50%.

On 5 -- nah.  They make the pattern prettier, but I don't think it's just the judges.  They keep the fuselage tangent to the line of travel of the airplane, unlike a flapless stunter that needs to point nose-in on corners.  Stunts look much more elegant with flaps, but IMHO the only real advantage is you can have a heavier (and therefore more wind-resistant) plane with flaps than without.
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Online Peter Germann

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Re: A question on flaps efficiency
« Reply #11 on: December 15, 2018, 03:12:43 AM »
I would be interested in what the intent might be here - do you think the airplanes are currently limited by the maximum lift coefficient in actual practice? Because I think we are nowhere close to that. When we actually do make it to the point of stalling, it's not because of anything happening near the hinge line, but far forward of that.
 Brett

Yes, even in a tight corner, we are quite far from stall. To me what seems to happen is that due to insufficient lift the airplane simply widens the radius.
The intention therefore is to discuss design measures increasing lift w/o adding too much drag. From what I've found on the internet (hingeline-) slot flaps could help but I dont know whether, or to which extent, this is so with slot flaps on a wing with symmetric airfoil.
Peter
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Offline Chuck_Smith

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Re: A question on flaps efficiency
« Reply #12 on: December 15, 2018, 06:31:22 AM »
Yes, even in a tight corner, we are quite far from stall. To me what seems to happen is that due to insufficient lift the airplane simply widens the radius.
The intention therefore is to discuss design measures increasing lift w/o adding too much drag. From what I've found on the internet (hingeline-) slot flaps could help but I dont know whether, or to which extent, this is so with slot flaps on a wing with symmetric airfoil.
Peter


Peter, the fun "bucket list" airplane I'd like to build would have split flaps on both the bottom and the top of the wing. According to CFD, it should fly awesome and let's face it - it would look so bizarre in flight that no judge would notice I sloshed the outside-inside turn on the vertical eight, lol! In fifty years I have still not mastered that corner.

Chuck

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Online Peter Germann

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Re: A question on flaps efficiency
« Reply #13 on: December 15, 2018, 07:23:19 AM »
In fifty years I have still not mastered that corner.
Chuck

I do firmly believe, Chuck, that the ongoing search for the seemgly impossible to find is one of the really essential incentives to carry on for many of us. It is this incentive that has brought us airplanes such as the Nobler, Ted' Trivial Pursuit, David's Thunder Gazer, Remi Beringer's Sportster and the brilliant Sharks from the Ukraine. Why then not just going ahead and build the machine, perhaps opening new doors? And possibly pleasing yourself...

Peter
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Offline Chuck_Smith

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Re: A question on flaps efficiency
« Reply #14 on: December 15, 2018, 08:39:51 AM »
I do firmly believe, Chuck, that the ongoing search for the seemgly impossible to find is one of the really essential incentives to carry on for many of us. It is this incentive that has brought us airplanes such as the Nobler, Ted' Trivial Pursuit, David's Thunder Gazer, Remi Beringer's Sportster and the brilliant Sharks from the Ukraine. Why then not just going ahead and build the machine, perhaps opening new doors? And possibly pleasing yourself...

Peter

Because the time and expense of building a new stunt ship makes it difficult to legitimize the effort. I don't know about the rest of you, but by the time I have a new ship done I have about $1000 USD and a lot of hours into it. That makes me want to do incremental changes!

Chuck
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Offline Wolfgang Nieuwkamp

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Re: A question on flaps efficiency
« Reply #15 on: December 15, 2018, 09:08:15 AM »
Another approach would be gurney flaps. Has this been tried?
Regards,

Wolfgang


Offline Istvan Travnik

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Re: A question on flaps efficiency
« Reply #16 on: December 15, 2018, 11:08:30 AM »
I think,
when we leave the trailing edge thick and cut with sharp corners, we made practically "gurney flap" ...
https://plus.google.com/photos/117790355930193335731/album/5715088163776972593/5775826864817467250

Istvan

Offline Wolfgang Nieuwkamp

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Re: A question on flaps efficiency
« Reply #17 on: December 15, 2018, 12:00:29 PM »
Istvan,

basically I agree, but what I really would like is shown at  https://aviation.stackexchange.com/questions/14934/what-is-the-working-principle-of-a-gurney-flap

Regards,

Wolfgang

Offline Igor Burger

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Re: A question on flaps efficiency
« Reply #18 on: December 15, 2018, 12:36:46 PM »
The simple flap consists of a wing segment that is hinged at one point and can be deflected. Usually, the flap depth is about 30 percent of the total profile depth. The simple flap increases lift by increasing the profile camber. For a typical profile, maximum lift is achieved with a flap deflection of 40 to 45 degrees.

Partially true (on our models). First of all 30% is overkill. We use something between 15% to 20%. Defelction is mechanically possible to 45% but inflight deflections are much smaller. My model has mechanically limited deflection to 27deg.

The slot flap is a single flap with a slot between the wing and the flap. This gap allows a flow from the area of increased pressure on the underside of the wing to the top of the flap and thus significantly reduces the tendency for flow separation on the flap and thus increases the achievable lift with simultaneous drag reduction relative to the simple flap.
 

The separation at flap hingeline is caused by 2 reasons - high pressure difference (underpressure on top) and existency of sharp kink on suction side. Properly designed standard flap like we use, will not cause such effect. There are some ways to limit it, I described it in my article in Stunt news.

True or false?
If true. why don’t we build slot flaps?

So 1/ yes, true to some extent and 2/ because it is difficult.

The first reason is we have symmetrical airfoil. If that flow in gap shoud improve lift and drag, it must be VERY well designed. I think everyone who sealed gap between flap and wing knows, that sealing improves lift and drag, so making "some" gap will be usually counter productive.

The point is that if that flow should really improve something it must well controlled. It means the duct for that flow must have well designes shape. It is visible on attached picture. It shows that very small difference of leading edge of flap position has large impact on produced lift. So there are 2 probles to solve:

1/ I do not see easy way how to investigate how to shape it. I do not think that some computer simulators will be enough to make proper design. They are usefull to some extent for simple airfoils or flapped airfoils with sealed flaps, but that duct which will depend on conditons (air properties, surface properties etc) are very difficult to model.

2/ If we can solve point 1/ I do not see easy way how to do it mechanically correct. It is not only problem of precisiion, but also problem of rigidity, if lift on the flap will cause only small fexing of flap and therefore changes of the duct, especially its thickess, it will make LARGE difference in lift (as visible on the picture) and such changes will cause many many trimming problems (that is one reason why we seal flaps)


Offline frank williams

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Re: A question on flaps efficiency
« Reply #19 on: December 15, 2018, 01:46:37 PM »
I’ve used the slotted flap several times.  They will absolutely put some dynamite in the third corner of the hourglass.  I intend to come back and spend some more time with them.  I did do some simple wind tunnel tuft runs with them just to prove to myself that they were going to be real.  One of the pics is sealed and the other has the gap open and active.

First of all … Peter you are definitely right …”the ongoing search for the seemingly impossible to find is one of the really essential incentives to carry on for many of us”.  New doors!

The flaps I have used are simple single slotted flaps. These are just like the Americas Cup sailboats.  I think very similar to the Oracle boat.
http://www.tspeer.com/Wingmasts/airfoils.html

 Wesley Dick has done some really ingenious double slotted flaps, but I haven’t tried them yet.  His mechanical design for the mechanism though is beautiful.
 
From the pictures you can see that it is just an offset hinge point.  Aerodynamically the “secret” is to locate the hinge point at such a point from the actual trailing edge so that the right size gap opens at the right flap deflection.  Too big a gap isn’t going to accelerate the flow correctly to shoot over the top of the flap.  Too small a gap and maybe not enough flow is let through.   Mechanically its not too difficult to accomplish.

Your question is going to be .. “so what about the unsealed hinge line?” ….. “does it produce unrepeatable results”?  From what I’ve seen, I don’t think so.  I do know that the top of the hourglass is “dramatic”.

edited for grammer
« Last Edit: December 15, 2018, 06:05:57 PM by frank williams »

Offline Istvan Travnik

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Re: A question on flaps efficiency
« Reply #20 on: December 15, 2018, 03:21:41 PM »
Adding to Igor's comments on computer designer programs (e.g. Profili-II, which I use) it simply dies most case, when I try to count my airfoils with deflected flap.
And the "Profili-II" is not the worst one amongst them, not at all...  >:(
« Last Edit: December 15, 2018, 03:46:18 PM by Istvan Travnik »

Offline frank williams

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Re: A question on flaps efficiency
« Reply #21 on: December 15, 2018, 06:39:37 PM »
I might add that not only is the gap opening at deflection important, but I also think that the flap leading edge shape is important.

Offline Ted Fancher

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Re: A question on flaps efficiency
« Reply #22 on: December 15, 2018, 06:50:34 PM »
As Tim Wescott said a ways back: "They make the pattern prettier, but I don't think it's just the judges.  They keep the fuselage tangent to the line of travel of the airplane, unlike a flapless stunter that needs to point nose-in on corners." (and in round maneuvers like figure 8s as well!)

I, too, believe that fact is one of the primary advantages of any sort of trailing edge flap on a stunt ship.  The ship's fuselage profile looks better, particularly in intersections of round eights, clovers, etc.  Put another way, the fuselage is more likely to align itself with the path the aircraft following"

I do not believe, however, that massive amounts of additional lift from flaps is "the" secret to the performance of excellent, high scoring patterns with tight corners.  I'm a 15 to 20% of wing chord flap advocate like Frank.

(Definition pertinent to what follows: An adverse pitching moment in this case is defined as a lift force produced by the "shape" of an airfoil such as when a flap is deflected on a "symmetrical" airfoil thus modifying it into a "cambered/lifting" airfoil.  When flaps are deflected "down" the wing will "pitch" nose down unless countervailing force such as more powerful pitch devices i.e. an aft stab/elevator, forward "canard" surface is deployed to overcome the pitching moment OR if the CG of the vehicle is aft of where the lift resulting from the flap deflection is centered. Early combat flying wings and Bill Netzeband's Fierce Arrow stunter and Half Fast combat ship are examples of "flaps" being utilized as "elevators" via the pitching moment they created.)

A "flap" factor I do believe is important is that the adverse pitching moment wingflaps produce when deflected allows us to utilize CGs well aft of where we could fly with accuracy with an unflapped stunter.  The "control load" produced by that adverse pitching moment provides a force "against which the pilot must work during maneuvers which provides "feel" to the input that would not be there were the CG located in the same place (near the center of lift ~25% MAC) without flaps.  IOW, an unflapped airplane (given a large enough tail for stability at a 25%MAC CG) would provide feel only resulting from the deflection of the elevators...a comparatively minor source and such an airplane would feel very skitterish to the pilot and precise manuevers would be very problematic) 

It is also of interest that the unflapped stunt ships we're used to also provides "feel" to the pilot with the CG forward in the 15% MAC or so which is where the CG usually ends up after flight trimming.  That "feel" is also the result of adverse pitching moment by the way...in this case the result of the CG being forward of where the lift to support the vehicle is centered thus increasing the necessary elevator deflection to achieve the desired pitch rate.  Ergo, the pilot gets a feeling of more input effort required to do "tricks" and that feeling provides a tactile sense to properly flown tricks, i.e. you can feel what you're causing the ship to do.  that additional "sense" of performance is missing if the CG and the lift are in the same place on the MAC.

One of the beauties of modern control systems is our ability to adjust ratios/neutrals, etc. to refine the airplane's trim to optimum.  Where response is prompt but controllable and uniform in both directions.  Again, flap chord should be in the "modest range" of 15 to 20% of the chord at any point and deflection relative to the elevators adjusted to provide adequate lift for the pattern the pilot is comfortable with plus a modest amount more to account for adverse conditions when the occasional need to insure abrupt inputs to "save the day" don't result in stalls at inappropriate altitudes...the third corner of triangles, for instance.

An example of that last was my original Trivial Pursuit which flew competitively at many Nats over its years of service.  It was never a light weight tipping the scales at close to 70 oz on 650 or so square inches and was flown for a long time with one to one flap/elevator ratios.  Place second at its first Nats powered by a VF .40 and flew very crisp corners.  At a much later hot and humid nats it displayed a scary stall in the third corner of triangles.  The ultimate fix for this problem was an increase in flap deflection for a given elevator deflection.  I could detect no significant difference in response but the stall issue never resurfaced.

Sorry, too much as usual.  Bottom line is I'm not a big flap, big wing, max possible lift advocate.  I think the flaps are much more important for refining the appearance of the tricks we do...for much the same reasons Tim addressed.
« Last Edit: December 17, 2018, 08:35:58 PM by Ted Fancher »

Online Peter Germann

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Re: A question on flaps efficiency
« Reply #23 on: December 16, 2018, 04:54:24 AM »
I’ve used the slotted flap several times.  They will absolutely put some dynamite in the third corner of the hourglass.  I intend to come back and spend some more time with them.  I did do some simple wind tunnel tuft runs with them just to prove to myself that they were going to be real.  One of the pics is sealed and the other has the gap open and active.

Many thanks, Frank and dear friends for your comprehensive and very instructive contributions. It is very nice to be able to benefit from this kind of experience and knowledge. At this point I would like to explain the reason for my request of 11 December:

•   The FAI rules define the radius of a corner with a maximum of 2.1 m, (7 ft), without allowing any tolerance beyond that.

•   If this radius is exceeded, the rule will not be observed and the judge is obliged to penalize this excess by a deduction of points.

•   I assume that due to insufficient lift in the corner my airplanes are not able to comply with the radius rule and therefore I must accept deductions resulting from this.

•   In order to avoid such penalties, or at least to reduce them, I thought I should add lift by increasing the efficiency of the flaps.

From what I have learned from this discussion, the "slotted flaps" solution by Frank Williams (and suggested by Wolfgang Nieuwkamp, too) seems to me to be suitable to build a test aircraft.

Again, thank you all

Peter Germann.

ps. I tried to determine both the drag and the pitch moments with the help of "JavaFoil", but then very soon I reached the narrow limits of my mathematical capabilities. It should be possible, though:  https://www.mh-aerotools.de/airfoils/javafoil.htm
Peter Germann

Offline frank williams

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Re: A question on flaps efficiency
« Reply #24 on: December 17, 2018, 08:44:02 AM »
In order to benefit from more effective flaps the aerodynamic power of the tail needs to be able to overcome the increased adverse pitching moment.  As Ted points out, more effective flaps and their increased lift, come with unwanted adverse pitching moment.  We limit flap size and deflection to mitigate the effect, kind of like a dog chasing its tail. It’s easy enough to make the flaps bigger to increase lift, but we don’t necessarily always like the side effects.
 
To know that more lift is available (tighter turn) but I can’t use it because the tail isn’t powerful enough to drive the angle of attack to the needed value, is frustrating.  As we try to make flaps more effective we should also look to increase the tail effectiveness.
 
As the plane rotates, the tail is rotated through space and generates an induced angle of attack opposite to what is desired for the turn.  Also there is increased downwash at the tail plane due to increased flap effectiveness, which takes away from tail power also.

I’m told the Hellcat showed signs of limited tail power.   The designers trimmed the tips of the stabilizer (reduced the chord of the stab at the tips).  This makes the tailplane look more like a flying tail. I think Paul did that on some of his Impacts through the years.

The RC Giles ARF, that some of us experimented with for stunt, had a 22.5 inch tail moment (stunt physics - hinge line measurements).  It makes for a very large tail volume coefficient and great damping of maneuvers. But that length is extremely long and the only reason that it worked was that it had about the outer third of the stab turned into elevator.  In other words it had big flippers on the elevators.  About the longest tail moment that can be tolerated on a plane with a conventional tail is about 19.5 to 20 inches.

More efficient flaps and higher lift from the wing is possible.  We shouldn’t stop trying to obtain it.  The answer, I think, is to improve the tail power at the same time and still maintain a configuration that is not too sensitive to fly accurately.

Offline phil c

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Re: A question on flaps efficiency
« Reply #25 on: January 25, 2019, 01:54:47 PM »
A couple friends with experience in aerodynamics and using fluid dynamics programs have said that simply squaring off the trailing edge of the wing(with no flaps) with a thickness of 3-5% of chord improves the lift at high angles and allows a larger angle of attack.  This is a trick used on the original Barnstormer old time stunter.  The article or plans specifically said to leave the trailing edge of the fixed flaps with sharp square corners.

Presumably squaring off the rear of the flaps would do something similar. 
phil Cartier

Offline 944_Jim

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Re: A question on flaps efficiency
« Reply #26 on: January 26, 2019, 04:37:02 PM »
I have about $1000 USD and a lot of hours into it. That makes me want to do incremental changes!
Chuck
Nothing to add, and still reading the thread, but...

Ouch! I've driven cheaper cars. No bust on you, Sir, just slack-jawed at that price-point (keep in mind my biggest plane is a Black Hawk Models Mosquito with two Big Mig .074s). Since I fly alone, I never expected THAT number as a financial descriptor. I'm sticking to my 1/2A toys!  :o

Offline Chuck_Smith

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Re: A question on flaps efficiency
« Reply #27 on: February 03, 2019, 05:40:50 AM »
...

(Definition pertinent to what follows: An adverse pitching moment in this case is defined as a lift force produced by the "shape" of an airfoil such as when a flap is deflected on a "symmetrical" airfoil thus modifying it into a "cambered/lifting" airfoil.  When flaps are deflected "down" the wing will "pitch" nose down unless countervailing force such as more powerful pitch devices i.e. an aft stab/elevator, forward "canard" surface is deployed to overcome the pitching moment OR if the CG of the vehicle is aft of where the lift resulting from the flap deflection is centered. Early combat flying wings and Bill Netzeband's Fierce Arrow stunter and Half Fast combat ship are examples of "flaps" being utilized as "elevators" via the pitching moment they created.)

...

Respectfully Ted, I disagree with that definition. An airfoil has both a lift, and a moment coefficient. Whereas the lift coefficient is a nearly linear function of the aerodynamic  angle of attack (the geometric AoA adjusted for the effects of aspect ratio) the moment coefficient is fairly constant.

Yes, we are changing the camber and hence the moment, but lift and moment aren't quite the same thing.


Chuck
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Offline Chuck_Smith

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Re: A question on flaps efficiency
« Reply #28 on: February 03, 2019, 05:51:57 AM »
Nothing to add, and still reading the thread, but...

Ouch! I've driven cheaper cars. No bust on you, Sir, just slack-jawed at that price-point (keep in mind my biggest plane is a Black Hawk Models Mosquito with two Big Mig .074s). Since I fly alone, I never expected THAT number as a financial descriptor. I'm sticking to my 1/2A toys!  :o

Let's add it up:

Engine and a CF pipe around $550-$600
A couple props and a set of CF wheel pants $200
Balsa with laser cutting - around $150 or so.

Thats before we buy a single can of paint, covering wheels, fabricate a tank,etc.

Stunt ain't for cheapskates.  When you look at what it really takes to build a ship and then compare with the pricey RTF molded ships from Europe - they ain't such a bad deal.

Chuck
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Offline Chris Wilson

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Re: A question on flaps efficiency
« Reply #29 on: June 04, 2019, 07:57:27 PM »
Peter,  would leading edge flaps give what you are looking for?
Surely they would shift the centre of lift forward  rather than rearward and reduce pitching moments.
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Offline Matt Spencer

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Re: A question on flaps efficiency
« Reply #30 on: June 04, 2019, 09:55:16 PM »
' K ' is the Aileron . ( Wing Flaps ) 1906 patent by inventor, used 1903  . Prior was wing warping .



( Direction of FLIGHT is toward upper right , visual confusuion arises from front of ' chassis ' being pictured lowered ) resembles modern hang glider , as built / flown .

Intresting if we put the ' Wing Flaps ' at center chord . As originally enviaged .  S?P VD~ Full Span of course .

« Last Edit: June 05, 2019, 12:11:47 AM by Matt Spencer »

Offline Matt Spencer

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Re: A question on flaps efficiency
« Reply #31 on: June 07, 2019, 09:46:55 PM »
Quote
The Me-163B had an interesting characteristic in the takeoff. The gear configuration did not allow the airplane to rotate to lift off. Applying "up" elevator created a negatively cambered airfoil which glued the airplane to the ground. It was necessary to first give "down" to get it clear of the ground,  Once clear of the ground, with down elevators, the controlshad to be quickly neutralized before the airplane pitched back into the ground.  It could be done smoothly, but required a gentle touch.  When safely airborne, elevator deflections resulted in nose up and down pitch attitude changes which were, more or less, normal. This characteristic of the elevators caused changes in lift as well as changes in pitch applied to flight maneuvers. In an inside square, for instance, hard up elevator creates a negatively cambered airfoil. The airplane would settle first while the pitch attitude was changing to a new direction of flight. This put something of an "S" in every corner. Both the takeoff and maneuver lift problems could have been solved by adding flaps, top and bottom, to the main spars at about the CL. Then there would be a compensating lift while the airplane lifted off or changed attitude in maneuvers. This also explains why there are no decent, tailless, competition stunt ships

Page 243 here : https://vk.com/doc-126621217_483051986?dl=717b8e18c80b78a6f2

o.k. , it'd look a bit odd , but a few sheets of Balsa etc etc and some data would eminate .

Only stirring . At this stage . But ' We wont Know ' if we dont try it . will we . Maybe a few bitts of stiff cardboard & some blown smoke'd be  a start , Then theres the structual bit .


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