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Author Topic: Max Flap % for high lift  (Read 2359 times)

Offline Dennis Toth

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Max Flap % for high lift
« on: October 08, 2007, 10:52:30 AM »
Has anyone used more than 30% of the wing area for flap sizing? I know that Al R was very close to this but I wondered if going to a higher % with a little less throw would give more lift for heavier ships? At one time there was a ship called the Supper Looper that had 50% (or pretty close to that) as movable flap, I wasn't able to find the article to see what the author reported. Since there was no follow-up designs I assume it either didn't make much difference or had maginal performance. The interests is in being able to have a little heavyer plane flying a little faster (for wind penitration) on slightly longer lines (to get reasonable lap times) to get a combination that would handle higher winds without getting blown all over the place. Any thoughts?

Offline phil c

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Re: Max Flap % for high lift
« Reply #1 on: October 08, 2007, 02:56:29 PM »
I believe NACA did some testing in the late 30's on the effect of hinge position and max lift, mainly for control surfaces, but it would apply to wing flaps also.  If I remember right, it didn't make much difference if the flap was hinged anywhere from around 15% of chord to 50%.  The max lift generated was quite close, regardless of the hinge position.  More important, if the flap was moved more than 25 deg. or so it would start to stall and cause drag to go up faster than lift beyond that point.
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Offline Ted Fancher

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Re: Max Flap % for high lift
« Reply #2 on: October 08, 2007, 06:32:59 PM »
Stuntguy,

Probably the biggest practical reason to avoid large chord flaps is that as the chord increases the hingeloads required to deflect them go up rapidly (Wild Bill Netzeband covered this real well back in the '60s).  Since all the force we have available to deflect flaps and elevators comes from line tension anything that increases the amount of force necessary to deflect the surfaces can rapidly become counterproductive.

Beyond that, I can think of little reason to go that route from an aerodynamic standpoint.  I"ve personally actively pursued the reverse approach, minimizing flap chord as a percentage of the wing chord.  I've even had some good success at reducing the chord of flaps on designs which, IMHO, had too much flap area.

Ted Fancher

Offline Randy Powell

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Re: Max Flap % for high lift
« Reply #3 on: October 15, 2007, 07:52:48 PM »
That was the reason for Howard Rush's flap mod. To reduce the force needed to deflect the flaps. I think he would have been better off to just make somewhat smaller flaps, but they look cool.
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Offline John McFayden

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Re: Max Flap % for high lift
« Reply #4 on: October 16, 2007, 07:03:52 PM »
My two cents but your mileage may vary.

I had been flying a profile Cardinal with an LA .46 for a couple of years. In calm weather everything was great. But let the wind get anything much more than perfect and the plane was tough to turn. When the wind was above 10 mph I could barely fly it. I read in Stunt News an article where Ted Fancher had helped someone at the Nats improve the performance of his Cardinal by cutting back the flaps. With nothing to lose I sliced off on each flap 3/8" at the root out to 1/4" at the tip. Didn't seem like much wood........but the difference in flight was dramatic.

The wind no longer takes control of the plane and the turn is still fantastic. The zoom I had experienced is gone and I don't have to fight the load. I have control of the corners. Overhead is noticeably easier to control. I no longer believe that more control surface equals better turn.

FWIW

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Offline Joe Yau

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Re: Max Flap % for high lift
« Reply #5 on: November 12, 2007, 07:54:14 AM »
Quote
by cutting back the flaps. With nothing to lose I sliced off on each flap 3/8" at the root out to 1/4" at the tip.  Didn't seem like much wood........but the difference in flight was dramatic. 

Did you adjust or check the CG before the mod?  When the wing/flap chord is reduced in the rear,  it'll usually moves the CG further back (make it more tail heavy like),  not by the weight diff, just from less wing area to the rear..  which also would make it more responsive, and easier to fly in the wind.   

Offline Jim Thomerson

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Re: Max Flap % for high lift
« Reply #6 on: November 12, 2007, 09:03:16 AM »
Super Flipper and Jolly Flapper,  Larry Scarinzi.

Offline tom hampshire

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Re: Max Flap % for high lift
« Reply #7 on: November 12, 2007, 11:02:19 AM »
To avoid confusion, the super flipper and jolly flapper were designs which Larry S published, each having flaps about one third of the total wing chord.    B U T  the flap throws were very limited, to about ten degrees or so.  I'm sure these would fail terribly if built with a 1:1 flap - elevator throw setup.  Would any of the aero experts care to comment on whether it is better to deflect a small flap 1:1, or deflect a large flap a lesser amount?  I've played with the latter approach, both with flaps and flying tails, proving only that if you use a large surface, its best to build with an adjustment feature so you can change the throws easily.  As to which is theoretically better, I dunno.  Tom H.

Offline Brian Hampton

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Re: Max Flap % for high lift
« Reply #8 on: November 13, 2007, 06:34:08 AM »
I can only go from my experience with the design I use now. I used quite large flaps that basically followed the wing section so were around 1" thick at the root hinge line. With the control system I use I was able to set up the flaps for zero movement while I fiddled with the elevator travel until I was comfortable with it. As a basically flapless model it had a very slight stall particularly on the last corner of a triangle so I started winding in a little flap movement until all trace of a stall was gone. The end result was +-17 degrees for the elevator and +-5 degrees for the flaps. What surprised me was the comparatively huge effort needed at the handle for such a small flap movement compared to no flap.

Offline Bill Little

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Re: Max Flap % for high lift
« Reply #9 on: November 13, 2007, 07:51:57 AM »
For anyone:  Would  anyone like to attempt to explain the differences in "more or less" *elevator* turn?  Maybe someone will understand my question.  ???

I can definitely feel a difference (if the plane is at the right parameters) in turn when there is more elevator to flap movement.  The turns seem quicker and less effort to perform.   Of course, when the wing loading gets very high, it disappears and I have to have a larger amount of flap dialed in.

Anyone understand th enature o fmy question? ;D
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Offline tom hampshire

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Re: Max Flap % for high lift
« Reply #10 on: November 13, 2007, 05:21:27 PM »
Hi Bill - My classic ships with short tail moments always turn better with more elevator than flap travel.  A pollywog Chief set up 1:1 is the most striking example.  It doesn't turn at all when you first feed in some "up".  After about an eighth of a lap, while it climbs higher without seeming to pitch up, it suddenly turns.  Increasing the elevator travel relative to the flaps restores decent control 'feel' and maneuverability.  The modern stuff with longer tail moments does not seem to need the greater elevator travel.
     This would seem to show us that the elevator produces turn, provided the wing does not stall.  If it does, the elevator becomes ineffective and the turn radius widens despite the elevator position.  The flaps are there only to generate the lift in the turn, at high angle of attack.  In addition to added complexity, we pay an aerodynamic  price for flaps, in that the center of pressure of the wing moves aft when the flaps are moved, in opposition to the turn input from the elevator.  This shift is called a 'negative pitching moment'.  So if you can build an airplane with a low enough wing loading, the flaps aren't necessary.  See Al Rabe's article in CLW.  He seems to have understood it all on his own.  But he was forced to go to larger flaps in order to get a scale stunter with fairly high wing loading to turn tight enough.  Make sense?  I still hope that one of the guys who make their living in the aircraft industry jumps in.  I certainly can't be confused with an aerodynamics expert. 
     Maybe this suggests that the flaps should be made just big enough to support the wing loading?  If this is so, then making the flaps too big not only adds drag, but increases the negative pitching moment.  This is about the limit of what I think I understand.  What remains is whether you get a lesser negative pitching moment from smaller flaps and greater deflection, or bigger flaps with only enough deflection to support the weight of the individual airplane.  Tom H.

Offline Kim Mortimore

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Re: Max Flap % for high lift
« Reply #11 on: November 13, 2007, 07:16:11 PM »
Would any of the aero experts care to comment on whether it is better to deflect a small flap 1:1, or deflect a large flap a lesser amount?  I've played with the latter approach, both with flaps and flying tails, proving only that if you use a large surface, its best to build with an adjustment feature so you can change the throws easily.  As to which is theoretically better, I dunno.  Tom H.

Brett Buck discusses this is the Designing/Finishing column of the Sept/Oct '07 Stunt News, part 2 of his "Number Game" series on designing from scratch.  (Actually, I think most all of what he has to say about design is useful whether someone plans to design his own or not).  He says,  "...my experience has been that deviating much from 1:1 has a detrimental effect on the control feel - particularly when you use an overly large flap and then reduce the throw to compensate.  The other direction (too small flap with a lot of deflection) doesn't seem to be nearly as bad.  But I think that you are better off with the correct -size flap and 1:1 ratio."  P.83.

I would like to hear more about the detrimental effect.  As far as I know, Brett doesn't post on Stunt Hanger.  Ted, would you care to comment any further on this?

Thanks,
Kim
Kim Mortimore
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Offline Bill Little

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Re: Max Flap % for high lift
« Reply #12 on: November 14, 2007, 08:20:40 AM »
Thanks, Tom, and Kim!  Maybe it is the "style" of plane that I am use to as Tom is suggesting.  Classic and similar style planes are what I have the most experience with.  Although I have flown several SV series planes, they have been in the minority as far as total number of flights I have flown over the years.  The SV, and similar planes, were very happy at 1-1.  But I found later as I added adj. elevator horns, that dialing in some more elevator really made the turn more to my liking.  Of course, these planes tended to be on the lower end of the wing loading scale for SVs.  Not *ultralights* but fairly light.  So, my own ideas about design, wing loading, and airfoils being the determining factors in flap/elevator ratios.  It seems to me that thinner wings and light loadings benefit from more elevator while the opposite requires more flap.
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Offline Ted Fancher

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Re: Max Flap % for high lift
« Reply #13 on: November 18, 2007, 01:22:07 PM »
snip

The end result was +-17 degrees for the elevator and +-5 degrees for the flaps. What surprised me was the comparatively huge effort needed at the handle for such a small flap movement compared to no flap.

Bingo!

Ted

Offline Ted Fancher

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Re: Max Flap % for high lift
« Reply #14 on: November 18, 2007, 01:56:39 PM »
Hi Guys,

In any discussion of flaps on stunt models it is essential to remember that flaps don't "make an airplane turn".

Flaps serve only a single function.  They allow the wing to be reconfigured so as to produce a higher coefficient of lift when required out of the same wing area.  Yes, they do increase drag a certain amount when doing so, but that shouldn't be a huge consideration since you only need to produce enough lift to fly a corner as tight as you're able to fly it repeatedly and well.  Most stunt ships (especially those whose design goal is to come as close to a zero wing loading as possible) could fly perfectly acceptable patterns with no flap movement whatsoever.  A 720 square inch,  50 oz P-47 being a classic case.

Flaps, when deflected, will actually impede the rate of turn of which a given amount of elevator deflection is capable of producing if they were fixed.  This is the result of the negative pitching moment they produce (this is easy to visualize.  Simply think of the airplane with no tailplane attached so that when the flaps are deflected they are "acting as elevators".  Flaps go up, the airplane goes up ... not down; and vice versa).  Such a tailess plane is often referred to as a "flying wing".  I guess we all know what happens when you deflect the flipper on a flying wing, right?

The tail must overcome this adverse pitching moment first in order to ultimately produce the pitch change in the desired direction.  This is no small part of the reason why flapped ships typically required bigger tails and usually more elevator deflection (see a modern combat ship which maneuvers like crazy with only a few degrees of up and down control).  It is also part of the reason they almost always prefer a more aft CG.

An earlier poster (John McFeydan, I believe) referred to some notes I made about reducing the chord on Paul Ferrel's profile Cardinals.  We literally removed a ful 3/4" from the trailing edge of the flaps and the results were exactly as described,  much improved pitch response and reduced control loads.  By the way, we not only reduced the area but also reduced the amount of movement to the minimum available using the stock control horns.  The reason was simple, the wing was producing way more lift than necessary to support the airplane in the corners.  Even worse, deflecting those controls with the available line tension required "massive" yanks on the control system which resulted in, frankly, terrible maneuvers.

This improvement (please note that removing that much area and movement from the flaps had "zero" effect on any visible tendency to "stall", especially when combined with blunting up the leading edge of the wing a bit) was the result of two things pertinent to this discussion.  First of all, reducing the area of the flaps reduced the pitching moment from deflecting them, allowing teh airplane to respond to elevator inputs in a more timely fashion; second, reducing the chord of the flaps reduced the force required to deflect them ... the control load. 

Think of this experiment.  The lower the aspect ratio of a movable surface, the harder it is to deflect it into the moving air stream.  Say you've got a sheet of plywood two feet by four feet (total area eight square feet) you can stick out the side of your pick-up (like a "flap") while driving 55MPH down the highway.  You can stick it out with the "hinged" portion either on a two foot edge or a four foot edge. 

When extended with the four foot "hinged" section it simulates a high aspect ratio surface and, when deflected only two feet of surface extend beyond the hinge.  The force required to deflect the eight square feet will be large but doable.  Now extend it with the two foot hinged edge and the same eight square feet will be essentially impossible to deflect because the air forces (which are pretty much the same) are centered twice as far from the hinge.

This is what Wild Bill termed hinge load in his classic articles back in the '60s.  Must reading for those who find this thread of interest, by the way.

When you combine the negative pitching moments of large area flaps plus the increased hinge loads of low aspect ratio to produce that area you get the kind of uncomfortable and maneuver debililtating responses described in the earlier post and in the example of Paul's Cardinal in this post.

The bottom line of all of this is that there is "no good reason" to handicap a stunt ship with this sort of penalty unless, (and only unless) the wing loading is so high that that much flap must be deployed to produce the lift required to perform maneuvers of the desired size and corner radius.

If you believe that producing "more lift" is inherently to your benefit, I 'm sorry, but that just isn't  true.  The closer you can "balance the lift produced to the lift required" the better the airplane will follow the desired path while "drawing" the geometry of the stunt pattern; and the less effort will be required of the pilot to place the airplane in those paths.

Ted Fancher

Offline Ted Fancher

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Re: Max Flap % for high lift
« Reply #15 on: November 18, 2007, 02:15:11 PM »
Brett Buck discusses this is the Designing/Finishing column of the Sept/Oct '07 Stunt News, part 2 of his "Number Game" series on designing from scratch.  (Actually, I think most all of what he has to say about design is useful whether someone plans to design his own or not).  He says,  "...my experience has been that deviating much from 1:1 has a detrimental effect on the control feel - particularly when you use an overly large flap and then reduce the throw to compensate.  The other direction (too small flap with a lot of deflection) doesn't seem to be nearly as bad.  But I think that you are better off with the correct -size flap and 1:1 ratio."  P.83.

I would like to hear more about the detrimental effect.  As far as I know, Brett doesn't post on Stunt Hanger.  Ted, would you care to comment any further on this?

Thanks,
Kim

Kim,

I'm of the opinion that chord length should be limited to reduce control forces and the movement adapted to what is necessary to produce the required lift.  Most of my earlier designs for the ST .46 era airplanes used between 15 and 17% of the chord at every station on the wing.  The heavier wing loadings associated with my tuned pipe era stuff generally used two to three percentage points higher, but not over 20%.

It is also true that a couple of the later airplanes had a wing loading that ultimately proved to require more flap than elevator movement (generally thought to be a no no by many but not, in my estimation or experience, a valid concern.  Wild Bill could probably explain it but, it is worth noting that there doesn't seem to be remotely the control loading effect of increasing flap chord when one simply moves a narrower chord flap to greater angles.

Also worth noting is that the use of narrow chord (high aspect ratio) flaps definitely requires the use of a blunter leading edge to the wing.  Merely having a "thick" wing doesn't cut it when comparatively high angles of attack are required.  Flow separation must be delayed adequately and this means no sharp leading edges.  It doesn't need to look like a baseball bat but you definitely don't want the leading edge to cut bugs in half on impact.

Ted

Edited to correct the illogical statement "... between 15 and 15%..." in my first paragraph.
« Last Edit: November 18, 2007, 06:50:59 PM by Ted Fancher »


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