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Author Topic: Flap trailing edge shape?  (Read 1087 times)

Offline Mike Scholtes

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Flap trailing edge shape?
« on: March 22, 2020, 09:11:54 PM »
I did a search but can't find a discussion of current thinking on flap TE shape. The fact that Chris Cox is apparently using a square TE makes me think there is more to this than I used to. The options are (1) tapered thickness from flap LE to the TE, the "classic" shape; (2) rounded TE but keeping same thickness from LE to TE; (3) square TE with same thickness from LE to TE, a rectangle in cross section. I am at the point of finishing the flaps on a 660 inch modern stunter so need to make a decision. One advantage of squared TE is that it can later be rounded if performance is not as hoped.

We are basically locked indoors here on the West Coast so I may have more time to work on model stuff. I am one of the fortunate few who actually can work from home, which leaves me more time than I used to have.


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Online Air Ministry .

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Re: Flap trailing edge shape?
« Reply #1 on: March 22, 2020, 10:05:35 PM »
Good stiff quater grain sheet , flat . is stiffer than good stiff quatergrain tapered 'V'eed .

You see all out here . Looks kinda unfinished Square Edge . Specially with a few paint drips .

The Caudron / Hurricane , Flat 1/4 sheet ( think about 5/16th - 8 mm on a 60 ship , onless the 1/4s rock hard )

V front at hinge , round - Half Round actually  LL~ Rear Edge .

The theorys they interact sooner by the angle of a tapered flaps angle , so fly smoother - more effective .
And any extra drags to ones advantage . Someones on about Sq rear edge , on the main forum maybe .

Using 40 / 60 / & 80 weight , sandpaper on flat block , to rough out tapered flaps , these days . Good'n dusty .

  H^^

=====================================================================================

Doing a tapered one ball pen on bench & run flap edge past it , both ways up . Get twoo lines , maybe 1 mm or 1/16th in. apart .
If youre lucky . You can even use black & red pens , get two black with a red on Center spliting them . or Suchlike .

Then I draw a 1/2 ( or 2/3 ) chord line . Block off / sand to outer rear & half chord lines . Both Sides .

Go & have a coffe and rearage the brain cells & eye sight , in case somethings cheating .

Return , eyeball along , hold up to ligh . etc Etc .

THEN .  .  .  .  .      .  Mark at say 1/2 inch back from the front . Not Less ! Nuther line spanwise half or 2/3 fwd on new flank .
MASK the front 1/2 inch . so as not to intrude on its exactude .Could even do it on the rear 1/2 flank too .

Block Sand Down , cross wise mainly , Double Cross Hatch with the 60 on a longish block . Lengthwise with 80 on a long block .

 :P

So you get a thicker stiffer flap at half chord , with a uniform taper . 2 or three swipes on the ' creases ' and theyre gone for ever .

Often sit on a styro fruit box on the bench to raise to eye level . Or on a cold day you stick bent over head at bench edge hight .  LL~

Mark the eqaul thingos for L E cut , Coarse pare on a long block . Carpenters joints free so as excess pressures not applied
to sanding block or razor plane . :(

I leave the flank Fwd around 1.5 deep , till final sanding - radius then .

BEFORE you do ANY whittling , A piece of say 1/8 ply 1/2 wide , to rest a blade on , to thin slot the hinge recesses . then the steak kinfe.
Then the 6 in steel rule end . In 15 mm .  :P
Thisisso theyre all SQUARE and CENTRAL and suchlike . Or its between the knees sitting down , to do em , or clear the dope out ,
with the flaps shaped . Carefull to leave / reistae the centerline & other marks whilst theyre still required .

Right. that deserves a COFFE . But Im at the Library . If anyone thinks theyre getting five dollars outta ME for a COFFE , theyre sorely
deluded .  >:(
« Last Edit: March 22, 2020, 11:33:59 PM by Air Ministry . »

Offline Matt Colan

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Re: Flap trailing edge shape?
« Reply #2 on: March 24, 2020, 07:59:08 AM »
I leave my flaps square at the end. Frank Williams told me once that with a hard, squared off edge, the air will continue off the end of the flap before it drops off and gives the effect of a larger flap. My flaps are also not tapered, 1/4” boards beveled are the leading edge. I hate tapering flaps and once I heard from Paul Walker there is a benefit to straight non-tapered flaps being stiffer than tapered, I never looked back. My last two airplanes have been like this and my new one as well
Matt Colan

Offline Motorman

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Re: Flap trailing edge shape?
« Reply #3 on: March 24, 2020, 09:08:34 AM »
+1 square.
Remove victim to fresh air

Online Ken Culbertson

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Re: Flap trailing edge shape?
« Reply #4 on: March 24, 2020, 12:40:47 PM »
I leave my flaps square at the end. Frank Williams told me once that with a hard, squared off edge, the air will continue off the end of the flap before it drops off and gives the effect of a larger flap. My flaps are also not tapered, 1/4” boards beveled are the leading edge. I hate tapering flaps and once I heard from Paul Walker there is a benefit to straight non-tapered flaps being stiffer than tapered, I never looked back. My last two airplanes have been like this and my new one as well
To some degree I think it depends on the plane BUT recent experience with my Twister (RIP) which I elected to keep slab may be why it cornered like a big ship.  My latest PA ship (RIP) which cornered even better had extremely stiff rather small 3/8" flaps with a 1/8" flat back.  I am 100% on the flat back for aerodynamics but think that the "slab" simply adds stiffness.  It also adds just a bit of additional under camber and slightly less on the top than tapered flaps.  The rocket scientists among us can let us know if it is the shape of the upper or lower surface that matters.

Ken
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Offline Ted Fancher

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Re: Flap trailing edge shape?
« Reply #5 on: March 25, 2020, 09:07:21 PM »
Hmmm?  Tapered versus not tapered flap cross section???

I'm curious what aerodynamic factor one expects to be affected positively or negatively in terms of improving the performance of the stunt pattern of the airplane thus equipped?  ...and why? 

By far the most important question that occurs to me.

Ted

Online Brett Buck

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Re: Flap trailing edge shape?
« Reply #6 on: March 26, 2020, 11:31:40 AM »
Hmmm?  Tapered versus not tapered flap cross section???

I'm curious what aerodynamic factor one expects to be affected positively or negatively in terms of improving the performance of the stunt pattern of the airplane thus equipped?  ...and why? 

   I think it probably makes no difference, or very little difference, to the aerodynamics. The idea is to make it stiffer.

    Brett

Online Tim Just

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Re: Flap trailing edge shape?
« Reply #7 on: March 26, 2020, 03:09:35 PM »
The full size Extra 300’s have a relatively sharp aileron trailing edge.  The stock aileron centering is somewhere between soft to nonexistent in some cases.  The addition of P strip weather stripping top and bottom of the aileron trailing edge dramatically improves centering.  A six inch length on both ailerons is very noticeable.  It takes very little trial and error to get it just right.  The rubber P strip is about 3/16” thick making the trailing edge 3/8”.  I have used it on the rudder as well with similar results.  The negative is a slight but linear stick force increase from zero to full deflection.

Tim

Offline Mike Scholtes

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Re: Flap trailing edge shape?
« Reply #8 on: March 26, 2020, 03:14:53 PM »
Thanks everyone. And to be honored by a reply from a Singer of The National Anthem is especially gratifying! The question Ted poses is the one I am pondering. I can imagine that a thicker TE (whether rounded in profile or squared off) would impact the airflow over the flap with less handle movement and resulting flap deflection, maybe enhancing the light pressure that Ted advocates in controlling maneuvers once initiated. Or this could be complete bunk aerodynamically. I can also appreciate that a stiffer flap is going to give better and more consistent response than a thinner more flexible one, in any conditions, and thus is a good idea from any perspective. As long as I put them on the correct wing panels (see discussion of wider flaps on inner vs outer panel above) I should be OK. The plane is Claus Maikis' beautiful China Clipper, BTW.

Offline Howard Rush

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Re: Flap trailing edge shape?
« Reply #9 on: March 26, 2020, 03:24:48 PM »
I wouldn't compromise aerodynamics for stiffness. If there's room for a torque tube, flaps can be effectively infinitely stiff.  However, as Matt said, whacking them off at the TE is both structurally and aerodynamically beneficial.  Rounding them off at the TE is probably bad, although I can't verify that offhand.  Flap flexibilty may not be bad, but flap twisting would increase as the square of airspeed. I think it could cause trim problems.  Behold the first picture below from Eric Viglione. 

Speaking of flat-back flaps, I am a cautious man, given to moderation in all things.  The second picture shows my Flite Streak with conservative, equilateral-triangle cross-section flaps.  I may try something similar again, depending on whether the upcoming world champs gets canceled before I make the flaps for my new plane. 

Mike, if you want to try something interesting, consider  http://digital.library.unt.edu/ark:/67531/metadc61552/m1/16/ .  The TE shape of the upper two are the ones of interest, I think.  Disregard the hingeline stuff.  That should give you flaps that would be resistant to twisting both aerodynamically and structurally, while reducing flap hinge moment, allowing more accurate stunt.  One caution is to make sure the hinge line is taped, lest there be an instability around neutral. 

Beveling the leading edge is useful when the flap is hinged aft of the leading edge, but I think it would not help flaps hinged at the LE the way American stunt plane flaps are hinged.
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Offline Mike Scholtes

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Re: Flap trailing edge shape?
« Reply #10 on: March 26, 2020, 03:46:08 PM »
Thanks Howard ("Mr. Moderation-in-all-Things"). I did look up the UNT article but I lack the training to understand or interpret the data. I did notice that all the hinge gaps were sealed, though. When you say you would not sacrifice aerodynamics for stiffness, how do you mean? I probably won't go to the extent of a CF tube LE but will stick with the fairly hard balsa slabs, 1/4 inch thickness. It sounds from all the comments that a squared TE with a rectangular cross section is desirable, or at least not detrimental. Maybe enough VGs will make the difference.

Hope the Worlds can go forward on schedule. It is far enough in the future that the contagion may have faded by then.

I had to look twice to be sure my eyes were not playing tricks re the triangle flaps. The clown suit distracted me at first. Eric's model really is showing flap twist. Do you know if that was at the bottom of a loop or some other high load maneuver? Plane appears in level flight in pic.

To Tim Just: I learned akro in a Pitts S2B, then flew an 8KCAB Super Decathlon quite a bit. I am intrigued at what problem the aileron thickeners solved. Is that dead-aound-neutral unique to the Extra, or to monoplane akro designs generally? Did not have that in the Pitts.

Offline Howard Rush

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Re: Flap trailing edge shape?
« Reply #11 on: March 26, 2020, 04:11:41 PM »
The 1/4” square corner slab would be dandy. For fun, though, try the cold-chisel trailing edge.
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Online Tim Just

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Re: Flap trailing edge shape?
« Reply #12 on: March 26, 2020, 04:41:41 PM »
To Tim Just: I learned akro in a Pitts S2B, then flew an 8KCAB Super Decathlon quite a bit. I am intrigued at what problem the aileron thickeners solved. Is that dead-aound-neutral unique to the Extra, or to monoplane akro designs generally? Did not have that in the Pitts.

Mike,
The Extra’s ailerons work perfectly around neutral.  What is sometimes absent is self centering.  The P strip creates breakout force.

Tim

Offline Mike Scholtes

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Re: Flap trailing edge shape?
« Reply #13 on: March 26, 2020, 04:55:52 PM »
So just to create some controversy and draw attention to myself I will give the slab flaps a cold-chisel TE. What the heck. I am not the one heading for the Worlds with a  new plane!

Tim: Thanks for the explanation. My akro instructor sold his Sukhoi, so alas I am unlikely to get to try a monoplane akro ship. And at my age, I shouldn't.

Offline EddyR

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Re: Flap trailing edge shape?
« Reply #14 on: March 26, 2020, 06:54:55 PM »
 Removed my comment as it seemed to go off topic
Ed
« Last Edit: March 27, 2020, 06:54:57 PM by EddyR »
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Offline Matt Colan

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Re: Flap trailing edge shape?
« Reply #15 on: March 28, 2020, 07:36:25 AM »
On the subject of trailing edges of flaps, would these doodads that are on RC Pattern planes help in any way like Tim described on his Extra? As far as I know they only put these on the TE of elevators and rudders
Matt Colan

Offline Mike Scholtes

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Re: Flap trailing edge shape?
« Reply #16 on: March 28, 2020, 11:54:53 AM »
Interesting. Maybe Howard was onto something after all. The RC Pattern guys must have some reasoning and experience that supports the value of this design, wonder what their thinking is?

Offline Ted Fancher

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Re: Flap trailing edge shape?
« Reply #17 on: March 28, 2020, 11:11:17 PM »
On the subject of trailing edges of flaps, would these doodads that are on RC Pattern planes help in any way like Tim described on his Extra? As far as I know they only put these on the TE of elevators and rudders

Matt,

My guess would be that these trailing edge "doodads" are intended to enhance the "effects" of rapid full travel inputs to maximize consistency/predictability of rapid stall of the wing and aggressive yaw input during that stall to improve the rate and predictability of snap roll response.  Not sure of the aerodynamics involved but--maybe--neither do they.  They wouldn't be the first model airplane clique to glom onto something "declared" to be the cat's meow by its inventor.  As long as the airplanes don't crash as a result, who's to say they aren't...the cat's meow, that is.

Preface: I'm no aerodynamics expert but...

I don't think space age "stunt" flaps are an exotic source of instant 600 point patterns.  Remember some "Reb" with a "Ringmaster" kicked butt three ways to Tuesday in Tucson a week or two ago!  Hmmmm. 

I think flaps should "probably" best be an equally tapered "V" cross section; the hinge line sealed (while adding zero resistance to control deflection); near full span and of no greater chord than necessary to insure the airplane to which they are attached won't stall when turning corners as aggressively as the pilot is capable of flying accurately.

I think the triangular cross section coupled with proper wood selection and non-elastic covering (CF) provides adequate stiffness to--especially with a sensible chord in the 15 to 20% of total wing chord range at each station of the span--provide adequate resistance to twisting due to airloads.

or not.... n~ n~

Next! S?P S?P

Ted

Offline Howard Rush

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Re: Flap trailing edge shape?
« Reply #18 on: March 29, 2020, 01:35:20 AM »
I think flaps should "probably" best be an equally tapered "V" cross section...

That’s what I use, because I like how they look.
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Online Ken Culbertson

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Re: Flap trailing edge shape?
« Reply #19 on: March 29, 2020, 10:51:20 AM »
So in summary, flaps should be stiff, about that big, have a TE and be attached to the back of the wing.  Everything else seems to be what works for you. LL~
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Offline Ted Fancher

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Re: Flap trailing edge shape?
« Reply #20 on: March 29, 2020, 01:18:39 PM »

That’s what I use, because I like how they look.


Well...

Oh, what the heck...

I was hoping not to say this in public, butt...is there any other reason for doing anything when building a stunt ship???

C'mon.  Just ask Uncle Jimby!


Offline Ted Fancher

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Re: Flap trailing edge shape?
« Reply #21 on: March 29, 2020, 01:19:40 PM »
So in summary, flaps should be stiff, about that big, have a TE and be attached to the back of the wing.  Everything else seems to be what works for you. LL~

From God's lips, Ken!

Offline Ted Fancher

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Re: Flap trailing edge shape?
« Reply #22 on: March 29, 2020, 03:25:49 PM »
Thanks everyone. And to be honored by a reply from a Singer of The National Anthem is especially gratifying! The question Ted poses is the one I am pondering. I can imagine that a thicker TE (whether rounded in profile or squared off) would impact the airflow over the flap with less handle movement and resulting flap deflection, maybe enhancing the light pressure that Ted advocates in controlling maneuvers once initiated. Or this could be complete bunk aerodynamically. I can also appreciate that a stiffer flap is going to give better and more consistent response than a thinner more flexible one, in any conditions, and thus is a good idea from any perspective. As long as I put them on the correct wing panels (see discussion of wider flaps on inner vs outer panel above) I should be OK. The plane is Claus Maikis' beautiful China Clipper, BTW.

Hi Mike.  Please recognize the following is my opinion based only on having done this thing for a veritable lifetime with some success at both design and competition.  IOW, I could'a just been lucky!

I think...

The primary role of flaps on a stunt ship are to provide the lift "necessary" to allow a competitive pattern to be flown with a small amount in excess for adverse conditions one might encounter.  Such a wing could be smaller in overall area and, thus, less distressing in adverse air.  Remember, it is entirely possible to fly competitive patterns with an unflapped stunt ship under "most" conditions.  Note, however, that for a given amount of needed lift to be obtained the area required for doing so is greater on a wing with "no" high lift devices (flaps) attached.  Generally speaking the necessary lift must be achieved with greater area or higher airspeed--neither of which is generally an asset for our mission...fast patterns are harder to perfect and greater wing area makes for a better kite in windy/bad air!

The greatest advantage of using flaps has been realized/utilized only (ONLY! OMG I'm getting old) in the last three/four decades wherein we've realized that with added tail volume the CG can be moved aft of where it historically had been necessary given a large enough tailplane and still remain stable.  Construction articles of that prior era pretty much all had the same "trim" advice in one or two sentences:  "if it doesn't turn well enough add tail weight"... and vice versa.  "Fill 'er up!"

However, a large tailplane on an unflapped ship, while technically "stable" with an aft CG (as appropriate for a big tailed flapped ship) would provide very little "feed back" (feel due to control loads) to the pilot during maneuvering.  The feedback necessary for the pilot to "feel" and "perfect" his shapes becomes nebulous and flying decent patterns difficult for that reason...although the ship is "technically" stable.  Flapless stunt ships instead regain the necessary "feel" by moving the "center" of gravity forward of where the the wing's lift is "centered".  Thus the need to accelerate the forward CG in the pitch direction desired at the rate desired will require overcoming the natural tendency of the forward CG to pitch the airplane the opposite direct...thus providing "feel" to the pilot.

The big tailed flapped ship, on the on the hand, can, on a ship with an aft CG near where the lift is centered, produce the desired feedback/ feel to the pilot by virtue of the negative pitching moment produced by the flaps' deflection (opposite to the pitch desired for the maneuver) coupled with the air loads that must be overcome to deflect the flaps in addition to the elevators.  This aftward CG location pays great dividends when flying in other than ideal conditions by all but eliminating the need for greater control inputs for a given rate of pitch change and, as a result, a much reduced tendency to speed up...and thus open up...maneuvers..  I.e. the inputs required to fly competitive sizes/corners will be very little greater than under good conditions.

Sorry, way too long, but I hope it makes some sense.

The point with respect to this discussion, re refinement of the flap cross section etc., is that all these "configuration" subtleties under discussion here make almost no difference.  The only flap design factor I consider truly important is that the chord of the flap (as a percentage of the wing chord) be no greater than "necessary" to support the maximum loads the ship is likely to encounter plus a tiny bit in case you get carried away with the clear kote!  A range of from 15 to 20% should be adequate for any ship lighter than a whale.

(One final note about large chord flaps.  The greater the chord percentage the greater the control force required to deflect them and, thus the more important line tension becomes and the greater the negative pitching moment from deflection becomes...both of which are disadvantages to your patterns.) 

In Ted's opinion the answer to the flap question is that there should be one on each side of the airplane, they should be narrow in chord, the hinge lines sealed, and no larger in area (optimally) than necessary to produce the lift needed at all times plus a modest amount for the unexpected.

Whew!  Too damn long as usual.  Sorry about that.

Re the National Anthem thing.  The Director asked for volunteers.  No one volunteered.  I got the short straw!  The director said "go get warmed up".  I went.  The rest is hist....never mind.

Ted

Offline Mike Scholtes

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Re: Flap trailing edge shape?
« Reply #23 on: March 29, 2020, 04:19:58 PM »
Hi Ted: Thanks for the elaboration; not "too long" at all and I bet all the guys reading this who like to understand why stuff works will be grateful for the benefit of your expertise. And as Tina Turner said, "What's luck got to do with it?" or something like that. I have no illusions about there being a magic flap design that will produce 600-point patterns! I just was musing about the effect of TE shape and what effect it might have on the "feel" we want in order to keep the tricks inside their proper shapes and sizes. It is dawning on me that the "magic" element is 40 years of nonstop, serious full-time commitment coupled with an understanding of what works or at least how to make it work better. As Brett says, serious stunt is a lifestyle choice if one wants to really excel.

Back to your comments, it sounds like you just verbally designed the Imitation again! That was 1979 if I recall right. The construction and philosophy article that went with it is a gem that all flyers should have in their field box.

You acquitted yourself well with the National Anthem, especially since it was unrehearsed and was a surprise performance to the performer. Sadly Jimby was not available for accompaniment.

Offline Ted Fancher

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Re: Flap trailing edge shape?
« Reply #24 on: March 29, 2020, 10:12:48 PM »
Hi Ted: Thanks for the elaboration; not "too long" at all and I bet all the guys reading this who like to understand why stuff works will be grateful for the benefit of your expertise. And as Tina Turner said, "What's luck got to do with it?" or something like that. I have no illusions about there being a magic flap design that will produce 600-point patterns! I just was musing about the effect of TE shape and what effect it might have on the "feel" we want in order to keep the tricks inside their proper shapes and sizes. It is dawning on me that the "magic" element is 40 years of nonstop, serious full-time commitment coupled with an understanding of what works or at least how to make it work better. As Brett says, serious stunt is a lifestyle choice if one wants to really excel.

Back to your comments, it sounds like you just verbally designed the Imitation again! That was 1979 if I recall right. The construction and philosophy article that went with it is a gem that all flyers should have in their field box.

You acquitted yourself well with the National Anthem, especially since it was unrehearsed and was a surprise performance to the performer. Sadly Jimby was not available for accompaniment.

Thanks Mike.  As you commented, my opinions were formed years ago.  I just don't think there is any "magic" in flap design configuration that will predictably produce better scores on an otherwise just OK machine.  I love the interest in such things but feel it is important not to make modest factors "primary".  I also, however, appreciate the inventive mind's desire to inquire and, if he/she so choose to say thanks but no thanks.  I opine primarily because I spent a measurable part of my stunt "career" investigating/experimenting with such factors and found  assigning greater importance to such things  unconvincing.  I could be wrong! but, so far don't think so in any significant way.

By the way, one thing I should have added was the necessity to include an adjustable linkage between the flap and elevator horns so as to be able to alter the deflection of one vis a vis the other for fine tuning ultimate flap/elevator trim...altering it as necessary based on significantly different atmospheric conditions when competing as the "visiting" team far away from home.

Re the anthem thing;  I was just fooling around with my smart a$$ responses.  Truth be told, we had a list of soloists, trios etc. culled from the cast lined up to have the honor of doing the anthem nightly for the length of the show's run.  My comments about being "stuck" with the job were pretty much just BS.  I truly enjoyed my several years in community theater.  Like stunt, there are numerous talented and delightful to be around people involved in doing creative things.  It's just a group endeavor of special people rather than a competitive thing between such people.

I did, nonetheless, do my very best to do the anthem justice when it was my turn.  Yeah, it would have been fun to have Jimby on the trumpet although, given his competitive nature, I might have been jockyin' for "air" time!

Ted

Online L0U CRANE

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Re: Flap trailing edge shape?
« Reply #25 on: March 31, 2020, 02:53:02 PM »
Just a few remotely related  thoughts...

A squared TE, rectangular-section flap deflects the air slightly more than one tapered to a fine "point." The taper reduces the surface's net angle to the airflow below the angle turned at its chordline.

Think of the water flowing past the stern of a fairly high speed boat: a square ended transom 'wake' shows a flattish trough for a short distance astern before it flattens back out to level. Boats meet water, a more viscous fluid than the air wings meet. Ships have greater "fluid" surface friction than wings, in air. Reducing hull length in contact with water reduces the hull friction drag. The "trough" just mentioned occurs after the boat has moved past.

Similarly, air 'streams' aft of the physical flap, but only a short distance. When flaps turn to assist lift, the 'wake' doesn't press on the air. Only the physical flap can do that. At low deflections, the 'wake' may have some small influence....

Pressure profiles of wings in steady motion usually show a small zone near the TE where the lower pressure 'above,' and higher pressure 'below' draws air to wrap onto to the lower pressure side. Compared to a rounded TE flap, the 'squared transom' idea makes it less likely that air passing the flap TE rolls 'up' to the other surface. it 'cuts' off contact  more cleanly.

Oh, yes...and the greater amount of wood in a rectangular section helps us make the flap stiffer...

\BEST\LOU

Offline Howard Rush

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Re: Flap trailing edge shape?
« Reply #26 on: April 01, 2020, 02:06:34 PM »
Ted’s pieces above have pearls of wisdom.  As usual, somebody follows up like Mike did with “I bet all the guys reading this who like to understand why stuff works will be grateful for the benefit of your expertise.”  I am one of those guys, and I am indeed grateful for the benefit of Ted’s expertise, but I seldom understand his explanations. 

The primary role of flaps on a stunt ship are to provide the lift "necessary" to allow a competitive pattern to be flown with a small amount in excess for adverse conditions one might encounter.  Such a wing could be smaller in overall area and, thus, less distressing in adverse air.  Remember, it is entirely possible to fly competitive patterns with an unflapped stunt ship under "most" conditions. 

Yep.  If it's not the primary role of flaps, it's at least #3.

Note, however, that for a given amount of needed lift to be obtained the area required for doing so is greater on a wing with "no" high lift devices (flaps) attached.  Generally speaking the necessary lift must be achieved with greater area...

Not quite.  The relationship among wing area, airplane mass, and lift capability is clear when you look at what’s happening in a loop:

Lift = “centrifugal” force

Expanding,

˝ x air density x speed squared x wing area x lift coefficient= airplane mass x speed squared / loop radius

Solving for loop radius, not counting gravity or the cosine thing,

loop radius = airplane mass / (˝ x air density x wing area x lift coefficient)

So for a given airplane mass, air density and wing area, you can turn any size loop you want if you have enough lift coefficient.  You control lift coefficient with your handle.  You need about 0.1 in level flight.  The most you can get without flaps is maybe 1.1, with flaps maybe 1.9.  The extra you can get with flaps allows you to use more paint and have less turbulence response for the same minimum corner size or a tighter corner for the same paint and turbulence response. 

...or higher airspeed...

Ted’s said this before.  Yes, airplanes can get more lift by going faster, but for an airplane turning a loop of given size, the lift it gets by going faster is balanced by the lift it needs by going faster.  To wit, not counting gravity,

Lift = “centrifugal” force

Expanding,

˝ x air density x speed squared x wing area x lift coefficient= airplane mass x speed squared /loop radius

Behold that speed squared appears on both sides of the equation, so it cancels.  There are three caveats:
1. The speed on the left is airspeed, the speed on the right is inertial speed, so they don’t quite match, particularly in an overhead 8 on a windy day. 
2. Gravity has more influence when you’re going slow than when you’re going fast.  If my ciphering is correct, for 70-foot lines and a standard 45° stunt loop, acceleration is 5.1G at the top and 6.8G at the bottom at 50 mph.  Acceleration is 14.0G at the top and 15.7G at the bottom at 80 mph, still 1.7G different, but smaller relatively.
3. In our Reynolds number range, maximum attainable lift coefficient goes up an RCH with speed.

Combat planes can generally turn tighter than stunt planes because they have lower wing loadings, not because they go fast.

However, a large tailplane on an unflapped ship, while technically "stable" with an aft CG (as appropriate for a big tailed flapped ship) would provide very little "feed back" (feel due to control loads) to the pilot during maneuvering.  The feedback necessary for the pilot to "feel" and "perfect" his shapes becomes nebulous and flying decent patterns difficult for that reason...although the ship is "technically" stable.  Flapless stunt ships instead regain the necessary "feel" by moving the "center" of gravity forward of where the the wing's lift is "centered".  Thus the need to accelerate the forward CG in the pitch direction desired at the rate desired will require overcoming the natural tendency of the forward CG to pitch the airplane the opposite direct...thus providing "feel" to the pilot.

Here, Ted writes approvingly of feel due to control loads (hinge moment, we call it).  I take it that some is good, but the amount you get with lots of flap chord is bad.  The dynamic response is way different between flap and no-flap configurations, and I wonder if that is some of what he feels.  The dynamic response (the transfer function between control input and airplane path) feels better with flaps because of direct lift control: you don’t have to rotate the airplane to increase lift.  I reckon this is the second most important benefit of flaps, if not the first. 

The big tailed flapped ship, on the on the hand, can, on a ship with an aft CG near where the lift is centered, produce the desired feedback/ feel to the pilot by virtue of the negative pitching moment produced by the flaps' deflection (opposite to the pitch desired for the maneuver) coupled with the air loads that must be overcome to deflect the flaps in addition to the elevators.  This aftward CG location pays great dividends when flying in other than ideal conditions by all but eliminating the need for greater control inputs for a given rate of pitch change and, as a result, a much reduced tendency to speed up...and thus open up...maneuvers..  I.e. the inputs required to fly competitive sizes/corners will be very little greater than under good conditions.

Having the same control inputs for different conditions is good, but I don’t see how this paragraph explains how you get there.   

The point with respect to this discussion, re refinement of the flap cross section etc., is that all these "configuration" subtleties under discussion here make almost no difference.  The only flap design factor I consider truly important is that the chord of the flap (as a percentage of the wing chord) be no greater than "necessary" to support the maximum loads the ship is likely to encounter plus a tiny bit in case you get carried away with the clear kote!

Ted’s flap chord observation is one of his best wisdom pearls.  I wish I’d learned it sooner.  However, you can lead a horse to data, but you can’t buy him a drink.  Is that how it goes?  My interpretation of figure 19 of the TE bevel report (here’s a better copy: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930092890.pdf) is that a 30° bevel on a 20%-chord flap on an NACA 0009 gives you the same hinge moment reduction as reducing the flap chord by 40%. 

Tailoring hinge moments was a big deal in the days of aluminum airplanes and composite men, and there is a bunch of cool NACA reports available to everybody now, many of which I bet are pertinent to stunt.  I thought the art died when hydraulic boost spoiled everything, but I had an interesting chat with Tim Just the other day about the stuff they do with full-scale aerobatic airplanes.  I don’t know how it applies to toy airplane stunt, but it’s really cool.

I just don't think there is any "magic" in flap design configuration that will predictably produce better scores on an otherwise just OK machine.  I love the interest in such things but feel it is important not to make modest factors "primary".  I also, however, appreciate the inventive mind's desire to inquire and, if he/she so choose to say thanks but no thanks.  I opine primarily because I spent a measurable part of my stunt "career" investigating/experimenting with such factors and found  assigning greater importance to such things  unconvincing.  I could be wrong! but, so far don't think so in any significant way.

Herein is another Fancher pearl.  Guys (me, for example) will sieze on one idea and figure it will make them winners while neglecting other, necessary things.  I’ve seen this effect even more in the real airplane business than with modelers. 

One more thing: what’s with the quotation marks?  Is that a stunt thing?  Bill Werwage used quotation marks like that in his Ares article.  Walt Kelly did something like that in Pogo captions.  Some words had bold lettering, some didn’t.  He explained that he had two pencils, a sharp one and a blunt one.  Sometimes he’d use one, sometimes the other.   
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Offline Howard Rush

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Re: Flap trailing edge shape?
« Reply #27 on: April 01, 2020, 02:27:05 PM »
Well...

Oh, what the heck...

I was hoping not to say this in public, butt...is there any other reason for doing anything when building a stunt ship???

C'mon.  Just ask Uncle Jimby!

I was observing to the aforementioned the other day that I may not be able to get to the paint store because of the lockdown and be limited for my new plane to colors and materials on hand in my shop.  I told him that I'd figure out what to do when the time came to paint it.  "That's backwards!" he yelled.
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Offline Ted Fancher

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Re: Flap trailing edge shape?
« Reply #28 on: April 01, 2020, 05:19:00 PM »
Alas.  Just teed it up for old what's 'is name again.  That's the problem with letting people who know something about what they're talking about participate.

ted

p.s.  The purpose of my quotation marks is to point out the really dumb stuff.  That's why I use so many of them.  It's sort of a self critique mechanism to beat him to the punch.

Offline Chris Fretz

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Re: Flap trailing edge shape?
« Reply #29 on: April 02, 2020, 07:24:13 AM »
I leave my flaps square at the end. Frank Williams told me once that with a hard, squared off edge, the air will continue off the end of the flap before it drops off and gives the effect of a larger flap. My flaps are also not tapered, 1/4” boards beveled are the leading edge. I hate tapering flaps and once I heard from Paul Walker there is a benefit to straight non-tapered flaps being stiffer than tapered, I never looked back. My last two airplanes have been like this and my new one as well
Do you still taper elevators?

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Offline Matt Colan

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Re: Flap trailing edge shape?
« Reply #30 on: April 02, 2020, 08:49:52 AM »
Do you still taper elevators?

Chris

Yes. My new one is 1/2” tapered to 1/4”
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Offline Chris Fretz

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Re: Flap trailing edge shape?
« Reply #31 on: April 02, 2020, 09:52:45 AM »
Yes. My new one is 1/2” tapered to 1/4”

Do you leave the 1/4 end square too?
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Offline Ted Fancher

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Re: Flap trailing edge shape?
« Reply #32 on: April 02, 2020, 05:42:19 PM »
"I wouldn't compromise aerodynamics for stiffness...(Snip)...Flap flexibilty may not be bad, but flap twisting would increase as the square of airspeed. I think it could cause trim problems.  Behold the first picture below from Eric Viglione." 

Snip.

This is probably just dumb Ted making a fool of himself again but I think a close look at the picture of Eric's Thunderbolt will find that what appears at a glance to to be happening isn't happening at all.  The flaps aren't flexing due to being insufficiently rigid.  They are simply deflected down which often happens on a stunt airplane when the elevators are deflected up...which is clearly the case.

Because I don't know a lot about airplanes I searched for three views of the P-47 Thunderbolt to reassure myself that the trailing edge of the T-bolt's wing is elliptical in planform...er, uh...curved in shape.  The search confirmed I was experiencing a rare moment of accurate memory! One of many three views I found is below.

It also appears clear in the picture that the outer third or so of the trailing edge of each wing is fixed and, thus, not likely to flex and clearly appear to be straight because they are...straight, that is. 

With the flaps themselves, however, deflected slightly down (per their frequently natural inclination on CLPA aircraft when the elevators are up) the then visible elliptical shape of the trailing edge of both flaps gives the impression of a straight trailing edge being twisted down (ostensibly due to airloads) when, in fact...or at least in my mind--tends to indicate that maybe there is some possibility that what appears to be a twist may just possibly be an un--twisted elliptically shaped trailing edge viewed slightly from above which, thus, is giving the impression of a flexed straight, tapered surface of inadequate rigidity; which it isn't.

Having previously been taken to task for inappropriately using quotation marks to "highlight" words I wanted to emphasize I have learned not to thus utilize them and, thus, hope that my reader(s), if any, will universally recognize those words important to make clear the writer's intended point of view (if that's not redundant) in the matter of the un-twistted flap image improperly  alleged to  have been twisted in the previous post.

Or not.

Ted Fancher

 


Offline Matt Colan

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Re: Flap trailing edge shape?
« Reply #33 on: April 02, 2020, 05:43:01 PM »
Do you leave the 1/4 end square too?

Normally I round it. The new elevators are square, for no particular reason other than I chose to leave them square this time around
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Offline Howard Rush

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Re: Flap trailing edge shape?
« Reply #34 on: April 02, 2020, 10:28:22 PM »
I think Ted is right about the flaps.
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Offline Dane Martin

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Re: Flap trailing edge shape?
« Reply #35 on: October 23, 2020, 10:16:55 AM »
Pardon my dredging up the past, but this seems to be comparable to what I wanted to ask. Regarding the elevator; my horizontal stab and elevator are 1/4 inch. Would there be any reason to taper the elevator, or simply leave it a flat sheet? It's an old WAM design by Lee Letchworth called the Tigger. It's actually for slow combat, he's around 75mph on his. I'll be using it to fly a brisk pattern and have fun, not using it for combat. But I'll be in the same ballpark on speed.

Online Ken Culbertson

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Re: Flap trailing edge shape?
« Reply #36 on: October 23, 2020, 12:20:49 PM »
Pardon my dredging up the past, but this seems to be comparable to what I wanted to ask. Regarding the elevator; my horizontal stab and elevator are 1/4 inch. Would there be any reason to taper the elevator, or simply leave it a flat sheet? It's an old WAM design by Lee Letchworth called the Tigger. It's actually for slow combat, he's around 75mph on his. I'll be using it to fly a brisk pattern and have fun, not using it for combat. But I'll be in the same ballpark on speed.
Leave it flat.  No need to taper an elevator with a flat stab of the same thickness.

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Offline Dane Martin

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Re: Flap trailing edge shape?
« Reply #37 on: October 23, 2020, 06:49:30 PM »
Leave it flat.  No need to taper an elevator with a flat stab of the same thickness.

Ken

I was thinking it might look neat, and wondering if there were any ill effects. But it hardly seems worth the effort of sanding now. Haha.
Thanks Ken


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