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How much gap is too much

Old 04-04-2009, 03:55 PM
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spitfire66
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Default How much gap is too much

Hello,

I was looking a little closer at my control surfaces and measured the gap between my elevator and horizontal stab. I have 3mm of space. This didn't look terrible when I first built the thing, but now I am getting ready to maiden the plane and I am having trouble sleeping at night. Will this cause flutter or control problems? It is a 60 size spitfire with a 4 stroke up front and I think it will be a fast airplane. Should I redo the tail?

Shannon
Old 04-04-2009, 04:16 PM
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waterloged
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Default RE: How much gap is too much

If you'r not comfortable use stripes of monocoat and cover the gaps just leave a little extra so the surfaces can move.
Old 04-04-2009, 06:24 PM
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42etus
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Default RE: How much gap is too much

I think that 3 mm is quite a bit of gap. I'd seal it up like waterloged suggests. Take a strip of covering about 1/2 inch wide, put a V shaped crease in it and iron it into the gap. Do it from the underside and it will hardly show.
Paul
Old 04-04-2009, 07:36 PM
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Jester241
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Default RE: How much gap is too much

I honestly doubt it would hurt unless you were doing like full blown competition 3D or something like that,but as long as you glued the hinges well,it should be fine. But the monokote thing those guys mentioned would be a great fix to make you happy and would only take a couple minutes.
Old 04-04-2009, 09:16 PM
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TexasAirBoss
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Default RE: How much gap is too much

I think it should be OK. Just be on the lookout for flutter during your initial flights. If it happens , cut power immediately. And don't go straight for the dive bomb test either. Ramp up your speeds and descents gradually. If you start having flutter, then take it home and fix it. Elevator flutter can disconnect your clevis, so use one with a locking clip. It can also strip the gears in your servo or even jam it, so there is some risk invovled, but I think its unlikely with 3mm.
Old 04-04-2009, 10:02 PM
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j.duncker
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Default RE: How much gap is too much

Whoa 3mm is TOO MUCH ESP IF YOU ARE USING MYLAR FILM HINGES. The elevator will be inconsistant around neutral position IE it will be impossible to trim and flutter is very likely to occur.

Either redo the hinges or seal the gaps using the method described above.
Old 04-04-2009, 11:46 PM
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Default RE: How much gap is too much

Seal the gaps for sure. Believe it or not, it's necessary. I almost lost my H9 P47 because of a fluttering elevator, and I had nearly no gap.
Old 04-05-2009, 10:00 AM
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Gray Beard
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Default RE: How much gap is too much

I can't say anything about the flutter, my crystal ball has a big crack in it and I ran out of CA!! Can't even fix the thing today and I may need it later? I got in the habit of covering in one piece and I fill all the gaps, top and bottom. Before I started doing it that way I flew without sealing and never had any problems that i knew about. Then my advanced instructor mentioned sealing them to me and one way or another I have been doing it ever sence. I keep a roll of clear Ultracote on hand just for gap sealing, I even use it on planes that are glassed and painted. On these planes I can remove the covering without removing the paint just by using my heat gun, much better then using tape. Only real way of knowing if you are going to have flutter is to fly the plane and see for yourself.
Old 04-05-2009, 10:59 AM
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Rodney
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Default RE: How much gap is too much

Contrary to some comments above, a gap does NOT cause flutter. It does lower the response (the smaller the gap the more control you have for the same amount of physical movement) rate. A gap could initiate flutter by stimulating the resonant frequency of the surface. Every thing will flutter if given the proper stimulus. Ideally the only surface that will not flutter is a surface of zero mass and infinite stiffness, a totally unobtainable ideal so the best we can do is make the surfaces as light as possible yet retain a high degree of stiffness. Most sport models will fly perfectly well with very large gaps between stab and elevator but can be somewhat sluggish if the gap is large.
Old 04-05-2009, 11:30 AM
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spitfire66
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Default RE: How much gap is too much

I decided to go ahead and redo the tail. I am going to use robart hinge point hinges and see if I can get the gap near zero. I do like what you all said about using clear ultracote. I will try to get it fixed up today and hopefully have a maiden in a week or so. (crummy weather)
Old 04-05-2009, 05:46 PM
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Default RE: How much gap is too much

[quote][Contrary to some comments above, a gap does NOT cause flutter. It does lower the response (the smaller the gap the more control you have for the same amount of physical movement) rate. A gap could initiate flutter by stimulating the resonant frequency of the surface. Every thing will flutter if given the proper stimulus. Ideally the only surface that will not flutter is a surface of zero mass and infinite stiffness, a totally unobtainable ideal so the best we can do is make the surfaces as light as possible yet retain a high degree of stiffness. Most sport models will fly perfectly well with very large gaps between stab and elevator but can be somewhat sluggish if the gap is large.
/quote]

I have to disagree with the above. I have been flying RC for more than 40 years and have seen a number models with flutter. Anything that gives control surface slop will increase the probablity of flutter significantly.

Most airframe examiners that I know would not allow a model with a 3mm control surface gap to fly.
Old 04-06-2009, 12:51 AM
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Eagleburger
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Default RE: How much gap is too much

The only problem could be hinge failure depending on type used, as pointed out earlier. Some full size have very large gaps.
Old 04-06-2009, 07:12 AM
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sensei
 
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Default RE: How much gap is too much


ORIGINAL: Rodney

Contrary to some comments above, a gap does NOT cause flutter. It does lower the response (the smaller the gap the more control you have for the same amount of physical movement) rate. A gap could initiate flutter by stimulating the resonant frequency of the surface. Every thing will flutter if given the proper stimulus. Ideally the only surface that will not flutter is a surface of zero mass and infinite stiffness, a totally unobtainable ideal so the best we can do is make the surfaces as light as possible yet retain a high degree of stiffness. Most sport models will fly perfectly well with very large gaps between stab and elevator but can be somewhat sluggish if the gap is large.
What he said for sure...

Bob
Old 04-06-2009, 08:03 AM
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Roby
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Default RE: How much gap is too much

Ditto on post #13
Old 04-07-2009, 05:10 PM
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wzak29
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Default RE: How much gap is too much

I have about the same gap I used scotch tape on the bottom works fine.
Old 04-09-2009, 08:58 AM
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Jonathan Ott
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Default RE: How much gap is too much

Gap does not cause flutter, slop in the linkage causes flutter. Gap causes control surface ineffectiveness due to air leak between the fixed and movable surface.

Before you say "I've seen fullscale with huge gaps" consider the type of airplane. I think the Kitfox had detached ailerons, but they are really suplemental movable wings that influence movement about the roll axis.

So, just to be safe all around: Seal your gaps, and make sure you have a TIGHT and SLOP FREE LINKAGE (that is from the control horn clevis through to the geartrain of the servo).

Jon
Old 04-10-2009, 07:03 AM
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sensei
 
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Default RE: How much gap is too much

Sloppy linkage will cause flutter, but gap is not the cause of flutter at all, it will cause your surfaces to be less effective, no doubt about that.

Having a gap only changes the frequency that excites flutter mode; sealing the gaps in some cases can actually induce flutter over an open gap. The only real way to protect your aircraft from flutter is to: 1 mass balance the surface, 2 use servos that will handle the loads, and 3 the likage has to be tight, and slop free, its that simple...

Bob

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