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  • November 28, 2020, 01:07:38 AM

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Author Topic: Flap Design - Pros and Cons of Extending Flaps All the Way to Tips  (Read 875 times)

Offline jfv

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Just wondering what the pros and cons are of having the flaps extend all the way to the tips rather than having them end a couple of inches before the end of the tips?
Jim Vigani


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Offline Motorman

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Re: Flap Design - Pros and Cons of Extending Flaps All the Way to Tips
« Reply #1 on: November 16, 2019, 05:54:47 PM »
One con is you won't get a trim tab.
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Offline Howard Rush

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Re: Flap Design - Pros and Cons of Extending Flaps All the Way to Tips
« Reply #2 on: November 16, 2019, 07:54:29 PM »
One con is you won't get a trim tab.

I consider that a pro.

I have my flaps go all the way from the fuselage to the end of the wing. Having a movable surface pass a stationary surface invites an irregularity in control response.  Also, the flap hinge moment (the torque it takes to deflect it) is proportional to the square of flap chord. One might be able to minimize flap chord by maximizing span. Not having flaps at the ends of the wing may help prevent the wing from stalling at the tip, but I havenít noticed this being a problem.
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Online Brett Buck

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Re: Flap Design - Pros and Cons of Extending Flaps All the Way to Tips
« Reply #3 on: November 16, 2019, 08:05:51 PM »
One con is you won't get a trim tab.

  As Howard notes, that's a "pro", not a "con". If you have different deflection on the fixed fillers at the ends of flaps, the inboard and outboard flaps line up differently at different deflections, changing the flow around the flap tips from inboard to outboard as you move the controls. This can cause otherwise unresolvable trim conditions, much bigger than having full-span flaps and tweaking them.

    You don't want the "shear" of flow around the tips of the flaps, where the filler is pushing the air one way, and 1/16" away, the flap is pushing it a different way.   Its livable if the relationship of the flap to the filler is the same on both sides, but still not as good, even if everything is straight.

    This is the one and only intentional change I made on my 2006 and now 2020 model. The 1995 and 1999 versions had fillers, the current versions have full-span flaps.

    Brett

Offline jfv

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Re: Flap Design - Pros and Cons of Extending Flaps All the Way to Tips
« Reply #4 on: November 17, 2019, 08:04:53 AM »
Thanks for the info.  I usually  have trim tabs at the tips but my new plane will have removable control surfaces and it would be easier to have full span flaps.
Jim Vigani

Online Brett Buck

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Re: Flap Design - Pros and Cons of Extending Flaps All the Way to Tips
« Reply #5 on: November 17, 2019, 10:54:49 AM »
Thanks for the info.  I usually  have trim tabs at the tips but my new plane will have removable control surfaces and it would be easier to have full span flaps.

  Well, presumably, you would use a single hinge pin for the tabs, too, so you pull out the pin and the flaps and tabs are loose. That also permits you to remove the adjuster and tape them together, so you have flaps go to the tips anyway.    That's how I realized what the problem was - Paul Walker had done this to have an adjuster tab, and found it to cause him problems (as above), and ended up taping the tab to the flap so the whole thing would move. After which, I went back and changed my airplane to have full-span flaps.

    Brett

Offline Tim Wescott

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Re: Flap Design - Pros and Cons of Extending Flaps All the Way to Tips
« Reply #6 on: November 17, 2019, 01:33:18 PM »
One con is you won't get a trim tab.

Unless you get insane with your control horn, you do have trim tabs -- they're called "flaps".  Just tweak them.  Or build a Walker-style flap tweaking mechanism (I'd do a search but I can never find it -- basically one flap drives the elevator, the other flap gets driven by the elevator rod, and you have a 4-40 screw that you can use to tweak the flaps.  It gives you fine and repeatable control of the amount of flap tweak).
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Offline jfv

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Re: Flap Design - Pros and Cons of Extending Flaps All the Way to Tips
« Reply #7 on: November 17, 2019, 03:31:35 PM »
My experience is that flap tweaking has been more effective than a trim tab.  I've read some discussions about the vortex coming off the tips and was just wondering what other flyer's experience might be with the two flap arrangements.   Based on what's been presented here, I'm going to go with a full flap and see how it goes.
Jim Vigani

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Offline Howard Rush

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Re: Flap Design - Pros and Cons of Extending Flaps All the Way to Tips
« Reply #9 on: November 17, 2019, 09:00:21 PM »
My experience is that flap tweaking has been more effective than a trim tab. 

Gary Letsinger, a local wise man, explained to me that ailerons work best about halfway out the wing because they influence the pressure distribution of the most wing there.  He said that the reason full-scale airplanes have ailerons out by the tip is that the flap guys get first dibs at the rest of the wing.
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Offline Curare

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Re: Flap Design - Pros and Cons of Extending Flaps All the Way to Tips
« Reply #10 on: November 17, 2019, 10:26:00 PM »
Gary Letsinger, a local wise man, explained to me that ailerons work best about halfway out the wing because they influence the pressure distribution of the most wing there.  He said that the reason full-scale airplanes have ailerons out by the tip is that the flap guys get first dibs at the rest of the wing.

Does that make the flap guys higher in the pecking order, or lower?!?
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Offline Howard Rush

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Re: Flap Design - Pros and Cons of Extending Flaps All the Way to Tips
« Reply #11 on: November 22, 2019, 02:48:56 PM »
Does that make the flap guys higher in the pecking order, or lower?!?

Decisions like that are often made using a process less mathematically sophisticated than pecking order.
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Offline George Grossardt

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Re: Flap Design - Pros and Cons of Extending Flaps All the Way to Tips
« Reply #12 on: November 23, 2019, 11:29:51 AM »
So what about ďbobbedĒ flaps?  I have plans for Pat Johnstonís Seafire which have flaps 60% of the span.  The explanation is that the elevator is more effective with the bobbed flaps.

George

Offline Howard Rush

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Re: Flap Design - Pros and Cons of Extending Flaps All the Way to Tips
« Reply #13 on: November 23, 2019, 04:04:47 PM »
Do you mean that there is less flap pitching moment to overcome?  Itís usually worth doing whatever needs to be done to overcome it.
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Online Brett Buck

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Re: Flap Design - Pros and Cons of Extending Flaps All the Way to Tips
« Reply #14 on: November 23, 2019, 09:15:20 PM »
Do you mean that there is less flap pitching moment to overcome?  Itís usually worth doing whatever needs to be done to overcome it.

   Pat could explain, but I would presume that it is intended to reduce "downwash" from the flaps depressing the angle of attack of the elevator. Whether or not this actually happens is an interesting question.

     Brett

Offline Chuck_Smith

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Re: Flap Design - Pros and Cons of Extending Flaps All the Way to Tips
« Reply #15 on: November 24, 2019, 04:52:30 AM »
Decisions like that are often made using a process less mathematically sophisticated than pecking order.

Agreed, like controllability and reducing tip stall at high alpha. Plus, an aileron produces a rolling moment. The longer the moment arm the smaller the surface and lower the weight and drag. Ailerons are indeed, more effective at the tip.
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Offline George Grossardt

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Re: Flap Design - Pros and Cons of Extending Flaps All the Way to Tips
« Reply #16 on: November 24, 2019, 07:00:40 AM »
   Pat could explain, but I would presume that it is intended to reduce "downwash" from the flaps depressing the angle of attack of the elevator. Whether or not this actually happens is an interesting question.

     Brett

According to Pat, the bobbed flaps are to allow the elevator to overcome the flaps, presumably to make the aircraft turn better?  I am no engineer so I am wondering if I should reduce the flap width and extend them to the tip. 

George



Online Brett Buck

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Re: Flap Design - Pros and Cons of Extending Flaps All the Way to Tips
« Reply #17 on: November 24, 2019, 01:08:30 PM »
According to Pat, the bobbed flaps are to allow the elevator to overcome the flaps, presumably to make the aircraft turn better?  I am no engineer so I am wondering if I should reduce the flap width and extend them to the tip. 

  Yes, but the real question is the mechanism; how does it accomplish that purpose?

    Brett

Offline Howard Rush

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Re: Flap Design - Pros and Cons of Extending Flaps All the Way to Tips
« Reply #18 on: November 24, 2019, 04:31:08 PM »
According to Pat, the bobbed flaps are to allow the elevator to overcome the flaps, presumably to make the aircraft turn better?  I am no engineer so I am wondering if I should reduce the flap width and extend them to the tip. 

That addresses a different problem.

I'll guess that that particular flap configuration was to preserve the scale wing planform.  One usually starts with flaps optimized for max lift with reasonable hinge moment, then picks a tail size and length big enough to overcome flap pitching moment. 
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Re: Flap Design - Pros and Cons of Extending Flaps All the Way to Tips
« Reply #19 on: November 25, 2019, 12:16:08 AM »




More'n one school of design .





Once you establish one or two prerequisets , things start to align / define themselves .

Underpowered & stalling tips usually result in wing drop. One Wing . Unless they stall together . If your going to get them to stall
and if it matters. Like Control Authority which does . If you want it. In all cases .

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Re: Flap Design - Pros and Cons of Extending Flaps All the Way to Tips
« Reply #20 on: November 25, 2019, 12:30:03 AM »
Reduced area and ' soft ' horns , like these . The flaps are  moe airflow ' trimmers ',  fine tuning the flow in liew of the Std completely altering it .
Where effectively  the A.o.A. of the entire wing goes with the flap deflection .

Looking at the entry point ( line ) and exit , along the full span ( and past ). :P

Offline Shorts,David

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Re: Flap Design - Pros and Cons of Extending Flaps All the Way to Tips
« Reply #21 on: November 26, 2019, 04:10:40 PM »
If I'm not mistaken, those "french flaps" are all on planes with angled trailing edge whereas full length flaps would cause difficult to trim situations. In their situation the flaps are all cut parallel with the elevator hinge. So, my question is, is there a formula for wing area to flap size, or, elevator to flap size?
david


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