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Allowable flap twist

TASEsq

Well Known Member
Patron
Well, my first (left) flap turned out great - as I riveted the bottom skin along the spar I checked the angles of the flap brackets as I went. They started out true and stayed that way.

However, the right flap is another story. It started out true but after the first round of rivets (every 10th one) it was about 0.2 degrees out of true. I checked after each round of rivets and it ended up 0.5 degrees out of true.

It seems an odd way of checking for no twist - I deburred the aft edges of the flap hinge brackets as a pair - did I induce a half a degree difference?

If there was a half a degree out of true on one flap - is this a lot? I’m hoping not.

Once the trailing edges are done etc I can use a laser to check them but can’t think of a way at this stage that it would be helpful.
 

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Another way to check is to mount them and ensure that the leading edge of the flap is fully against the rear spar. Then you look at the distance first row of rivets on the flap as compared to top wing skin.

Using this method - one of my flaps was off maybe 1/16" from the first rivet to the last and I have had no issues with a heavy wing.



****

Just read read the post - Save the above until the flaps are fully completed
 
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I thought the reason for checking frequently was if there was twist, one would apply a little pressure to straighten out the twist , then apply a few more rivets and check. Repeat as necessary, not continue if there is twist. If continuing was the protocol then why bother checking, just finish and wait to see the results.
I'm no expert but that is what I've been doing.
 
I did try and remove the twist - removed all the clecos and twisted the flap. But I could not get it to come back no matter what I tried.
 
When I did mine I put a four foot level tight against the bottom skin at each end of the flap and stood back and sited the edges of the two levels. I found that to work better than the plans method of checking for twist. Late now, but I always check the spar before starting. I’ve found the spars almost always have a bit of a twist. Tuning those up before assembly is always a good idea.
 
I tried the winding stick method and by eye they look fairly twist free to me?

Bit hard to translate into a photo.
 

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RV-7 single data point

The RV-7A project I bought had quick build wings. The flaps had about 3 degrees of twist and Van's was fine with that.

I wasn't, so we built a new set. Spent a lot of time leveling the table, being careful during assembly and the new ones ended up at about 0.3 degrees. I thought that was nearly perfect.
 
The RV-7A project I bought had quick build wings. The flaps had about 3 degrees of twist and Van's was fine with that.

I wasn't, so we built a new set. Spent a lot of time leveling the table, being careful during assembly and the new ones ended up at about 0.3 degrees. I thought that was nearly perfect.

Thank you - that is good info. My TC also advised that i should ditch my digital level and build on - so will do so! Trailing edges next up.
 
Is this ok?

Just mounted and aligned the ailerons. All is well. Then, mounted the flaps, and while the outer portion on each side aligns perfectly with the ailerons, at the very innermost point, beneath the fuselage, one is about 1/4" lower than the other one. The curve of the fuselage makes it a bit obvious if you really look for it, which we were doing. The fairing will probably hide it, so I'm asking now.

Also, not at all sure what could be done about it now, so that'll be the next question if I need to address this.
 
I did a prebuy on an old RV-4 that had 1" of twist in the right flap and it flew straight. Splitting hairs here.
 
I hope I'm splitting hairs! It's just so hard to know. Visually, the inboard end of the flaps are up against the curved edge of the fuselage, so a small difference is visually not small. Thanks.
 
Just mounted and aligned the ailerons. All is well. Then, mounted the flaps, and while the outer portion on each side aligns perfectly with the ailerons, at the very innermost point, beneath the fuselage, one is about 1/4" lower than the other one. The curve of the fuselage makes it a bit obvious if you really look for it, which we were doing. The fairing will probably hide it, so I'm asking now.

Also, not at all sure what could be done about it now, so that'll be the next question if I need to address this.
Flap twist is one of the causes for heavy wing and in a 14, and unlike other models you cannot squeeze the ailerons to take care of the heavy wing.
I would check it again by the correct method to make sure there is no significant twist in it unless you’re ok with a possible heavy wing
 
I can't find any reference to a "correct method" for evaluating twist in the flaps in the manual. Neither could I find any reference to what "significant twist" might be.

I measured the angle of the top of both flaps now that they are mounted and are aligned with the ailerons at the outer end where they are the closest. Interestingly, both flaps have a 0.7 degree difference between the inner and outer top surface. So each of them has a "high" end and a "low" end. I guess that surface they were built on wasn't perfectly flat after all.

Also interesting and maybe complicating, the Left flap has the higher end outboard and the lower end inboard. That was what was noticed first. The Right flap has the higher end inboard and the lower end outboard.
 
Flap twist is one of the causes for heavy wing and in a 14, and unlike other models you cannot squeeze the ailerons to take care of the heavy wing.
I would check it again by the correct method to make sure there is no significant twist in it unless you’re ok with a possible heavy wing

I just helped someone with a heavy wing issue. In his case, there was at least 1/2" of twist in one aileron. It was a 6, but was fully resolved without touching the TE. In his case, the builder didn't see the twist and used only one side to align and therefore didn't properly align the flap. There are several methods to eliminate rigging issues and I have never used TE manipulation to do it. I am a firm believer that the TE should net perfectly straight skins and rigging dealt with in other methods. The planes handle noticeably better when the TE's leave proper skin surfaces.

Just une mans opinion.
 
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I had some twist in one of my flaps on my 9A. It caused a heavy wing. In the end I bought a quick build flap from Vans and it fixed my heavy wing. At that point I wanted to fly more than continue to build.

By the way before buying the quick build flap, I borrowed a flap from another 9 builder in the area to verify that it would fix my problem. It did.

That was 8 years ago. I don't know if you can buy a quick build flap today.
 
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I can't find any reference to a "correct method" for evaluating twist in the flaps in the manual. Neither could I find any reference to what "significant twist" might be.

I measured the angle of the top of both flaps now that they are mounted and are aligned with the ailerons at the outer end where they are the closest. Interestingly, both flaps have a 0.7 degree difference between the inner and outer top surface. So each of them has a "high" end and a "low" end. I guess that surface they were built on wasn't perfectly flat after all.

Also interesting and maybe complicating, the Left flap has the higher end outboard and the lower end inboard. That was what was noticed first. The Right flap has the higher end inboard and the lower end outboard.

First, I am not implying you have a twist in your flap or aileron. Only suggesting it that if you do have a significant, more than a degree or two, this will likely result in a wing heavy which is undesirable.

Question, were these QB control surface or you build them? The plan has a fairly good description of how to check for twist during building and even if it is a QB you can check the plans and use the method for checking for twist.
Unfortunately, I have read reports that VANs allows up to 3 degree of twist in their QB.
Some folks reference the top row of rivites of the leading edge of the flap when it is fully retracted against the trailing edge of the wing skin. The skin is roughly aligned with the dimple of the rivets dimple and see if they are all the same.

I just helped someone with a heavy wing issue. In his case, there was at least 1/2" of twist in one aileron. It was a 6, but was fully resolved without touching the TE.

I believe manipulating the trailing edge should be the last resort and only after making sure all rigging is correct.* And my point was that, in models*such as 6,7,8 and 9, that option exists for the last resort but on the 14 it does not.
 
I'm fine. Also, I searched for "flap twist" and landed here, without noticing that this is the -14 forum, so I apologize if my -8 questions get in the way here. It's funny. When aligning the leading edges of the wings, for drilling the rear spar, Van's says that within 1/2" is fine, with a "get real!" comment to boot. So, in some places 1/2" is fine, but here in flap-land, I'm down into fractions of degrees. I'll probably press on into final completion and see what I get on the far end.
 
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