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Author Topic: Flap Configurations  (Read 3310 times)

Offline Dick Fowler

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Flap Configurations
« on: November 26, 2006, 08:12:52 AM »
While reading the latest Model Aviation coverage of the CL World Championships, I noticed what appears to be  a difference in design theory regarding flap configurations.

Most of the US designs use full span flaps and most of these with some taper towards the wingtip with the exception of  the Impact.   A number of the designs from other countries are using partial span flaps and in the case of Beringer's Sukhoi they appear to be approx. 50% of the span.  For those that have been to the World Championships is this a valid observation?

What do you all think? Would shorter spanwise  flaps move the center of lift of the wing panels closer to the CG reducing the rolling moment and if so please explain why the Impact with the full span and a small if any taper flies so well. When you look at the Impact,  the AOA probably increases over the span toward the tips when flaps are used. Sure can't argue that the design doesn't work!

The Yatsenko Shark is another example of short flaps.

« Last Edit: November 26, 2006, 08:52:54 AM by Dick Fowler »
Dick Fowler AMA 144077
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Offline phil c

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Re: Flap Configurations
« Reply #1 on: November 28, 2006, 09:01:09 AM »
I think you answered your own question Dick.  With stunt planes the layout of the wing doesn't really matter that much, given how many different designs have won big contests.  What matters is controllability that suits the pilot, and a good pilot can learn to like almost anything.  Bigger flaps do make the plane less sensitive to weight.  They can carry an extra ounce or two and not even notice it.  A plane with smaller flaps almost always has a lower wing loading and an ounce or two of weight can make a noticeable difference in how sharp the corners can be.
phil Cartier

Offline Ted Fancher

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Re: Flap Configurations
« Reply #2 on: November 28, 2006, 06:13:09 PM »
Dick,

Both your and Phil's observations are accurate.  Yes, the French designs and the Shark (and some well known US designs such as the Shark and the suggested option for shorter span flaps on the Olympic, as well) have smaller or very small flaps.  I suspect you've also noted the observation Warren Tiahrt made about the difference in appearance of the turn on such designs.  All valid.

It's important not to let ourselves overthink the flaps as some sort of magic device.  The really only have one function and that is to alter the camber and, thus, the lift capability of the surface of which they are a part.  Any stunt design could be built with fixed flaps and it would still fly and maneuver.  In fact, if the unflapped version is able to produce the lift necessary to carry the weight of the airplane in high G manevuers it will turn more quickly (be more responsive with the same CG) than will the flapped version of same.

When flaps increase the lift capability of the wing (the coefficient of lift for the deflected version vice the undeflected) they do so only for the portion of the wing span that has flaps.  That portion (all of an Impact and just a small part of the Beringer ships) will dramatically increase it's lift by virtue not only of the increased camber but also by virtue of the angle of attack of that portion being raised to a greater degree than the unflapped portion of the wing.  That is because the A of A is measured from the leading edge to the trailing edge.  The mere act of deflecting the flap simultaneously alters both the wing geometry and the angle of attack.  A very effective device.

The only other thing deflecting the flaps does (other than increase drag commensurate with the increase in lift) is to produce a pitching moment which attempts to pitch the airfoil in the direction of the deflected flap.  I.e., when the flaps go down the airfoil wants to pitch nose down and vice versa.  It is this feature of the flapped wing that makes it less responsive (all else being equal) than the same airplane without flaps.  It is also why flapped designs generally have bigger tails on longer moments as well.

The bottom line is that the major thing flaps do is increase the lift ability of that portion of the wing that is flapped.  This is important if your wing loading is high or if you want to drive a given design into a tighter, higher g pitch situation than could be supported by the wing without flaps.

Stunters don't "need" flaps to fly good patterns.  They just need wing loadings commensurate with the lift which the wing is capable of producing.  I'm sure you've seen some pretty decent full AMA patterns flown by good pilots with airplanes like the Jamison Special.  Really sharp pilots like David Fitz and Bart Klapinski have flown numerous 500 point + patterns with well trimmed, reasonably light and properly powered Ringmasters.

Finally, the reason that we have historically used flapped ships to fly modern stunters is that the flaps with their negative pitching moment tend to provide resistance to pitch changes which provides the pilot with the feel necessary to control pitch changes and fly the "smooth" patterns that -- rightly or wrongly -- tend to wind up with the highest scores.

Ted Fancher

Offline Ted Fancher

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Re: Flap Configurations
« Reply #3 on: November 29, 2006, 04:17:41 PM »
p.s.

At the risk of appearing to pander to the readers, I might suggest you look up the article I wrote on the Imitation in Model Aviation back in the late '70s.  One of the underlying reasons for designing that airplane was to experiment with flap configurations.  The original could be flown with three different length flap set-ups.  The article (actually two of them in consecutive issues) spends some time discussing the results of those tests and why I've personally chosen to always "flap" a fairly substantial portion of the wing span on all of my subsequent designs;  the one exception, of course, being the Doctor whose sole purpose was to perform good patterns with no flaps at all.  The Doctor was published in STUNT NEWS several years ago.  PAMPA products should have copies of both design articles for a nominal cost if you can't find them in your personal archives.

Ted

Offline Al Rabe

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Re: Flap Configurations
« Reply #4 on: April 02, 2007, 10:13:56 AM »
Ted,

I think I disagree with your definition of angle of attack.  I think that angle of attack has to do with the direction of the relative air as it meets the wing as a whole.  It is my opinion that our stunt ships have variable angles of incidence rather than variable angles of attack when flaps are deployed.

Al

Offline Ted Fancher

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Re: Flap Configurations
« Reply #5 on: April 06, 2007, 11:43:49 PM »
Ted,

I think I disagree with your definition of angle of attack.  I think that angle of attack has to do with the direction of the relative air as it meets the wing as a whole.  It is my opinion that our stunt ships have variable angles of incidence rather than variable angles of attack when flaps are deployed.

Al

HI Al,

An interesting observation.  One with which, at least in part, I totally agree.

First of all, under all conditions you are absolutely correct that deflecting flaps alters the angle of incidence, irrespective of the relative airflow.  To make it clear to all the readers, "incidence" is the angle at which a lifting surface diverges from the "datum" line of a vehicle.  The datum is generally the fore and aft centerline about which the airplane was designed and will usually be roughly in line with the thrust line ... not always, but it is a reasonable reference for conventional planforms.

The angle of incidence of, say, the wing (also true of the tail and or canard, if utilized) is the angle at which its chordline differs from the "datum" line.  By definition, the chordline of an airfoil is a straight line from the extreme forward location of leading edge to the the extreme aft location of the trailing edge.  Generally speaking, the chordline of most stunt ships is parallel to the datum and, with no flap deflection the angle of incidence is, thus, zero.  If the airplane has no flaps the angle of incidence will always be zero.

On a flapped ship, any time the flaps are deflected (up or down) the angle of incidence is changed by virtue of the lowering or raising of the trailing edge of the wing (remember, the flap is merely a part of the wing which happens to be "adjustable" ... not a separate surface as many like to suggest).  Thus, as Al properly states, the angle of incidence of a flapped airplane is infinitely variable as the degree of flap deflection is changed.

Our apparent difference of opinion isn't really a difference at all, however.

The angle of attack of any surface, flapped or not, is the angle at which the profile chordline of the surface strikes the ambient air through which it is moving.   It is true, as Al is postulating, that a flapped surface need not act at a positive angle of attack to create lift.  The act of deflecting the flap changes the airfoil from a symetrical configuration to a "cambered" one.  A cambered surface is capable of producing positive lift even when in a slightly negative angle of attack.  In other words, the zero lift angle can actually be "negative".  This is "interesting" to us stunt flyers but not of significant interest because every time we deflect our flaps we also deflect the elevator and, assuming good design and trim the airplane pitches in the desired direction.

It is, nonetheless possible, as I'm sure Al will point out, for a lightly loaded, flapped stunt ship to actually have very little positive angle of attack while maneuvering.  That's one of the reasons we talk about a flapped ship being more visually accurate in turning high G manevuers than an unflapped one, because the body angle (think "datum" from the above discussion) won't diverge a great deal from the direction of travel of the entire ship.  The unflapped ship "must" have the "datum" at a positive angle to the pitching radius in order to achieve the necessary angle of attack to produce the lift necessary to support the G induced weight of the ship in the maneuver.  Thus, the nose will always be at least slightly inside the radius of the track the airplane is following. Very light wing loadings of good unflapped stunt ships minimize this but, no matter how small the angle, it must be there to produce the required lift.

Notwithstanding all the above, however, the act of deflecting a flap on a three or four pound airplane traveling 55 or so MPH in a straight line will produce a positive angle or attack until such time as other aerodynamic forces intercede.  For instance, if the pushrod to the elevator were to fail at the instant of the flap deflection, the cambering of the airfoil due to flap deflection will produce a negative pitching moment and the airplane will actually pitch forward.  Don't ask me how I know this to be true ... on several occassions over the last fifty or so years.

If, for instance, you haven't done a good job of balancing the lift produced to the lift required for the radius of turn, you might well have an airplane that produces more lift than necessary by virtue of not only the increased incidence of the airfoil relative to our datum but also from a greater than necessary angle of attack relative to the ambient air mass.  In the simplest case this could be a ship with a large area, large flaps and excessive flap deflection combined with a tail surface which is too "powerful" (Larger than necessary and with too much deflection).  This situation will result in an airplane which "leaps" through corners rather than tracking around the desired radius as it would do if the lift were not in excess of the lift required.  The "leap" is the result of the excess lift which is directed at right angles to the chordline and thus literally elevates the entire ship "into/inside of" the desired radius.

The opposite of that situation is a wing which can't produce the lift required despite all the flap deflection and angle of attack it can stand.  When that happens the wing stalls.  Stalls occur as the result of excessive angle of attack (exceeding the "critical" AOA as we say in the "biz".)  All airfoils have a critical angle of attack whether flapped or not.  Exceeding the critical angle of attack in your stunt patterns is not a good thing.

So, you can see that it isn't a simple one or the other question.  Angles of incidence and angles of attack are related, but different.  Almost by definition, a successful stunt ship needs to be capable of a reasonable amount of "excess" lift to insure that stalls are avoided.  Thus, almost all will fly at some angle of attack during maneuvers unless they are in that class of ships that have "too much" lift and "accelerate" into corners as a result of that excessive lift.  IMHO, ships that fly that way should have the flap travel backed off or (Sparky will shoot me for this) weight added to the CG so that the acceleration into corners is almost, but not quite, eliminated.  The "not quite" in that sentence is your insurance against stalls.

The bottom line is that I'll agree with Al that stunt ships always maneuver with significant amounts of "incidence".  Incidence is merely a geometric relationship of the datum and the chordline.  Whether or not a little or a lot of angle of attack is also present is a more complex question, the answer to which will vary infinitely with wing loading, air speed, air density and the efficiency of the airfoil (its "coefficient of lift" for a given state of camber).

The length of this response probably tells you a lot about why I generalized in earlier responses which didn't seem to require this degree of specificity.  Pretty much everyone that flys stunt ships is conversant with the concept of Angles of Attack and the distinction for basic understanding didn't seem worthy of the expanded commentary.

Al's comments, on the other hand, lend themselves well to the expansion.  I'm sure he'll have some responses as well.  I look forward to them.

By the way, the experiments with flaps on the Imitation design were very illuminating and my comments in the article relate almost directly to this conversation.  If any of you have found this discussion interesting I think you'll enjoy reading the article, especially the part about varying the span of the flaps and the effect on the flight characteristics.

The article is available through the AMA/Model Aviation link, I believe.

Ted

Offline Dick Fowler

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Re: Flap Configurations
« Reply #6 on: April 07, 2007, 04:55:59 AM »
Good stuff! The only problem is that I have to read it slowly with my lips moving to understand sometimes... wouldn't a "virtual chalkboard" be a great addition to this forum?

By the way Ted, you do have a way with words.... "An interesting observation.  One with which, at least in part, I totally agree." That's a very funny comment!
Dick Fowler AMA 144077
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Offline Ted Fancher

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Re: Flap Configurations
« Reply #7 on: April 07, 2007, 10:20:15 AM »
Good stuff! The only problem is that I have to read it slowly with my lips moving to understand sometimes... wouldn't a "virtual chalkboard" be a great addition to this forum?

By the way Ted, you do have a way with words.... "An interesting observation.  One with which, at least in part, I totally agree." That's a very funny comment!

Hmmm.

Yeah, Dick.  That was sort of "mushy" phrasing wasn't it.  Although it might have looked to have been humorous, my real thought at the time was that I was in total agreement with his comments about "incidence" but less sanguine about his observation about angle of attack associated with flap deflection.  If, however, you got a chuckle out of it I'll take credit for that, too.  What the heck.

And, yes, a virtual chalk board would be a great addition to the internet in general.  I'm not much good at computer graphics but have done a number of "live" clinics with the use of a black (or white, nowadays) board and it makes life much easier for both the talker and the listener.

Glad you enjoyed the post.

Ted

Online Dennis Adamisin

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Re: Flap Configurations
« Reply #8 on: April 08, 2007, 09:43:19 AM »
OK.  Proving my time "away" has not improved my sensibilities I am going to wade into this fray!

While I have been away from CLPA I was flying sailplanes - lotsa different set-ups but the clear winner is the full house birds with flaps and ailerons - typically coupled together.  Learned a couple little ditties that might fit this discussion:

* Flaps on a sailplane have two roles - the first is DRAG as in air brakes.  Watch a modern sailplane in a shallow dive apply full flaps and STOP in midair and you can see how powerful they are!  The secondary role of flaps on a sailplane are L/D adjustment.  That is, the sailplane flyer circling in a thermal will sometimes click in 1/8" or so flaps down to add substantial lift without a lot of drag and enable a tighter circle (sound familiar?) and faster thermal climb.  1/16" of UP flap (reflex) has a profound impact on a sailplanes ability to speed up and traverse big pieces of sky seeking lift.

* If you apply a little flap-only the sailplane PITCHES up and balloons because of the increase in lift.  I think the pitch-up demonstrates that the angle of attack changes; i.e., flap drops, leading edge to trailing edge chord line now sits at a positive angle relative to the fuse and the tail and sailplane PITCHES UP.  Supporting the analogy, when the sailplane flyer mixes in compensating DOWN elevator to re-establish level flight path, the sailplane appears to re-trim with the fuselage in a slightly nose down attitude.  Similar (opposite) reactions occur for the reflexed case, but because the deflections are so small and since the desired outcome is SPPED anyway, not everyone bothers to trim UP elevator to compensate for reflex.

* I do not know from my sailplane experience where/when the Pitching Moment (pitch down for flaps) really manifests itself - the other effects prevent you from continued flight until you adjust!  I will go out on a limb and suggest that the magnitude of the PM change is much smaller than the impact of the AOA ptich change - and it gets lost in the shuffle.  That jibes if you consider that tail moments arm on sailplanes and even with the smaller tails (compared to CLPA) I think the tail volumes are probably higher.

* Also does not hurt that sailplanes need not do square eights, but they WILL turn very sharply - at least once!

As a side bar, I am no longer certain which feature of flaps is more important - added lift or added drag - but you better account for BOTH attributes in the aerodynamic AND power considerations!  I am intrigued about revisiting thin airfoils (10%-12%) with flaps optimismised as BRAKES.

I remain unrepentantly in favor of higher aspect ratio wings - and LONG TAILS RULE!

...and I am having a BLAST in these forums!


Denny Adamisin
Fort Wayne, IN

As I've grown older, I've learned that pleasing everyone is impossible, but pissing everyone off is a piece of cake!

Offline Dick Fowler

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Re: Flap Configurations
« Reply #9 on: April 09, 2007, 05:51:01 AM »
Very informative Dennis. Nothing like real world experience. Do you think the amount of pitching is  a function of the distance the CG is in front of the wing's center of lift? The farther forward the CG the greater pitching moment for a given change in lift.
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Offline phil c

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Re: Flap Configurations
« Reply #10 on: April 09, 2007, 07:23:24 AM »
Ted produced a nice analysis above.  In a nutshell, on many planes too much flap travel can overpower the tail and produce a nose down pitching angle, even while the plane is producing more than enough lift for a tight corner.  The cure is either to reduce flap travel/size to match it to the lift wanted, or increase the elevator travel/tail effectiveness to put the control back where it belongs and force the plane to use the lift available and fly a tighter corner.

Keep in mind too that it is quite possible to move the controls faster than the plane can respond.  Sudden, abrupt, movements that are way faster than the plane can respond will make for a very awkward looking flight.  The flaps especially can stall and actually lose lift if they are moved faster than the plane and air can respond.  Instead of smoothly redirecting the air for increased lift in a turn, they stall and produce a lot of turbulence and drag that just slow the plane down.  The rate at which the controls are moving hits zero, the position of the controls hits max, all at the apex of the turn.  And then it all reverses to smoothly bring the plane out of the turn.
phil Cartier

Offline bob branch

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Re: Flap Configurations
« Reply #11 on: April 11, 2007, 06:55:41 PM »
I'd like to interject a question based on my experience in 100% scale aircraft. In flying planes from high wing trainers to single and multi engine low wing aircraft I have notice that most high wing aircraft pitch nose down with flap deployment. Most low wing aircraft pitch nose up with flap deployment. For the most part all of our stunters are mid or low wing designs. Is it not possible that the position of the verticle center of lift accounts for whether the nose will pitch up or down?

Bob Branch

Offline Ted Fancher

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Re: Flap Configurations
« Reply #12 on: April 12, 2007, 11:47:52 AM »
Interesting comment, Bob.

There is no question the basis of your comment has merit.  Everything that effects the airflow around a vehicle will have a resultant affect relative to the three dimensional location of the CG.

My gut reaction is nothing is quite as simple as your ultimate premise, however.  A lot of things happen when flaps are deflected and all of those things interact with one another in a variety of ways.

A few things that happen are: The flap deflection "will" result in a negative pitching moment based on the cambering of the airfoil.  There may be either a positive or negative additional pitching moment based on the lateral relationship of the CG to the center of lift (if the CG is forward, it'll be a negative moment, if aft a positive). The additional lift will produce a force perpendicular to the chord line which will again result in a pitch axis force in a more or less vertical plane to the vehicle, again acting about the CG.  With lift comes drag which, of course, works to slow the aircraft and "may" have a force about the pitch axis based on how that additional parasitic drag is aligned with the "predeflection" center of drag.  Deflecting the flaps will alter the aerodynamic center of the vehicle, moving it forward the same way it would move forward on an arrow of you moved the tail feathers forward from the tail.  The static margin will thus be affected and if  small enough to begin with may result in some stability problems.

Probably some other stuff going on that doesn't come immedately to mind.

As you can imagine, how the aircraft itself will ultimate react to all the forces place on it by the change in airfoil configuration will vary all over the place.  I've flown a lot of planes over the years (full scale and models) and the reaction to extending flaps has varied all over the yard ... as you might expect from the above discussion.

In our little world of "stunt aerodynamics" I think it behooves us to be aware of the above but not to fixate on them because of the relative impossibility to identify and quantify the variables.  Suffice to say that as long as the design falls within "normal" parameters it isn't going to be a deal breaker.

Ted Fancher

Offline RandySmith

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Re: Flap Configurations
« Reply #13 on: April 12, 2007, 01:21:40 PM »
I'd like to interject a question based on my experience in 100% scale aircraft. In flying planes from high wing trainers to single and multi engine low wing aircraft I have notice that most high wing aircraft pitch nose down with flap deployment. Most low wing aircraft pitch nose up with flap deployment. For the most part all of our stunters are mid or low wing designs. Is it not possible that the position of the verticle center of lift accounts for whether the nose will pitch up or down?

Bob Branch

HI Bob

That is interesing.. In my 152 Cessna, the nose pitches up
In my 172 cessnas  the nose  pitches up
in the 182 cessna  the nose  pitches up
In a Piper cruiser  the nose  pitches up
in a Piper Warrioir  the nose pitches  up

I have yet to find  any plane that pitches down when the flaps are  deployed.
What planes are you flying  that pitch down  when you add  flaps?  I have  yet to find  one

I have taken  dozens of planes in straight level flight  and held every control still, then lowered teh flaps, they all  pitch  nose  up??

The only way I can get a pitch down is to  slow teh plane just above stall, trim it  and  then  add  power

Regards

Randy


Offline Mike Ferguson

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Re: Flap Configurations
« Reply #14 on: April 12, 2007, 01:22:42 PM »
Question regarding flaps - I've noticed that many "older" designs (OK, mostly from the 1970s and 1980s) tended to have large root chords (over 3") and small tips (1" and sometimes under).  When you hit the winning designs of the present (Impact/Trivial Pursuit/Infinity), though, they have almost constant chords from root to tip, usually in the 2.5" range ... sometimes it looks like a thin yardstick hanging off of the trailing edge of the wing!

Just curious if anyone had any insight into this aspect of design evolution (or what the differences between the two layouts might be).

Offline Air Ministry .

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Re: Flap Configurations
« Reply #15 on: April 12, 2007, 06:28:19 PM »
Question regarding flaps - I've noticed that many "older" designs (OK, mostly from the 1970s and 1980s) tended to have large root chords (over 3") and small tips (1" and sometimes under).  When you hit the winning designs of the present (Impact/Trivial Pursuit/Infinity), though, they have almost constant chords from root to tip, usually in the 2.5" range ... sometimes it looks like a thin yardstick hanging off of the trailing edge of the wing!

Just curious if anyone had any insight into this aspect of design evolution (or what the differences between the two layouts might be).
[/quot

 Tip Stall / wake ; Wobble on tight corners.

Peerhaps the modern airfoils are more accomodating,tolerant .So the deflected flap is working as part of the airfoil rather than an appendage .

Offline Igor Burger

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Re: Flap Configurations
« Reply #16 on: April 13, 2007, 03:05:15 AM »
That is all about tail size. Without any question, flaps down on plane with infinite tail causes pitching up, while the same on tailles model will pitch down, so it is only question ot tail effectivity if its lift is able to counterballance the pitching momet of flaps or not.

Randy wrote it always pitches up, I guess it was model with independently controllable flaps and elevator. MODEL with relatively large elevator will ususally pitch up. While (I guess an accidental) case which Ted mentioned is probably diconnected flap/elevator pushrod (yes I did once this test also  VD~) means that elevator has only half the effective area, since evelavor is disconnected, not supported and goes to neutral position (menas no forne, no effect), so huge flaps just win.

And why old models had flaps more on root? I fully agree with Matthew, root should stall earlier than tip and long chord at root gives this advantage, but since we fly actual designs which has better airfoil, but also tails are designed not reach critical AoA, the model never stalls, so that trick with large flap at root is no more necessary, but I do not see too much reason not to do so further.  H^^

Offline Shultzie

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Re: Flap Configurations
« Reply #17 on: April 13, 2007, 09:52:21 AM »
Flaps and their design for stunters has always captured my imagination since my first Kenhi Wildcat and Bob Palmer "open cockpit" Smoothie  waaaaaaaay back in the early days of my kiddydumb!
Check out what Ed Southwick had to say...about flaps. I built an old Skylark just to play with his "larger 1/8 oversized outboard flap. I also feel that so many great high aspect ratio models have been lessened because they didn't take advantage of Ed's flap concepts.
Don Shultz

Offline Randy Powell

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Re: Flap Configurations
« Reply #18 on: April 13, 2007, 03:55:20 PM »
Without a lot of theory, but as Dennis has noted, just some practical experience. I've built numerous high aspect ratio planes (10 or more). They have ranged from a rather modest 6.8 to 1 up to 10 to 1. I must have tried every flap configuation possible (or at least every one that occured to me). Many had several flap configurations tried on the same plane. These wings, for the most part, had very thick, very blunt airfoils. A couple of things I learned: you don't need much flap with these things. The wing generates a tremendous amount of lift. I tried full span, wide flaps, narrow flaps and various configurations in between. Even no flaps, though these didn't generally work as well. I usually settled on half span or 2/3 span, moderate chord flaps. Part of this was to minimize the sensitivity to turbulence that all of them suffered, but also because the things simply didn't need a lot more. All of them had a wicked turn, some more than I could really handle easily. And the last 3 or 4 of them had very long tail moments and very large tailplanes. This was more about controlling the turn than anything, but I suppose it helped to tame the huge amount of lift the wing was generating.

I think that's the main reason that sooner or later, I end up messing with them again. Just can't forget the awsome turn the things will put in. If all contest were flown in perfect conditions, I'd probably never stop flying them.
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