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Do I Countersink or Dimple the FL-703 Flap spar?

eh_v8_tor

Active Member
I understand that Vans want's me to Countersink the bottom of the FL-703 flap spar where it goes together with the bottom skin and the piano hinge. In fact you use the piano hinge to back up the countersink tool so that it doesn't wander.

The wording in the manual is a bit vague about what to do on the top of the spar. Should it be dimpled? (It looks too thick to me..) Or should it be countersunk?

Thanks in advance,

Steve Wolfe:confused:
 
With nothing to back up the countersinks, I chose to dimple the top of the flap spar. IIRC it's 0.040, the same thickness as the wing rear spar, and most of those are dimpled.
 
I understand that Vans want's me to Countersink the bottom of the FL-703 flap spar where it goes together with the bottom skin and the piano hinge. In fact you use the piano hinge to back up the countersink tool so that it doesn't wander.

The wording in the manual is a bit vague about what to do on the top of the spar. Should it be dimpled? (It looks too thick to me..) Or should it be countersunk?

The intent of the manual is to follow the recommendations given in Section 5 unless told to do differently (such as was done on the bottom of the flap spar).
Section 5 recommends dimple countersinking .040 or thinner material unless specified otherwise, or special circumstances make dimpling difficult.
 
Dimple 'em both at the top, csk 703 at the bottom

The top joint only consists of the 701 skin and the top flange of the 703 spar, there is no need to countersink either, since it would just remove metal for no reason, with the result of not leaving a very strong underside to the 703 spar flange for the shop head of the rivet to bear on.

The bottom was another story, since the hinge shouldn't be dimpled because of the distortion it would cause, and the 703 spar flange is sandwiched between the 702 skin and the hinge, so the removal of metal from the countersunk area has no effect. Thus you should dimple the 702 skin, and machine countersink the 703 spar (bottom only) web, which may or may not get very slightly into the hinge material, otherwise nothing but drill and debur the hinge material. You still have a very solid surface for the rivet shop heads to bear on, i.e., the upper side of the hinge.
 
Thanks for the replies!

Thanks for your replies. I went back for a re-read of section 5 of the builders manual. I decided to dimple the top of the flap spar. More material left there is probably a good idea, and it was the easiest option.
I was worried about causing distortion, but this seems to be a non-issue. The flaps are clecoed together and they seem to fit perfectly.:) riveting has begun!
 
I'm at this point now. It seems that to countersink the bottom spar flange to accept the dimpled -702 skin would give a knife edge hole from the countersink on the bottom -703 flange/web. Is it possible to countersink the spar flange deep enough to accept the dimpled skin and not cause a knife edge on the countersink hole?
 
the knife edge is thought of as less important...

If my aging brain remembers correctly....

the sandwich assembly with the knife edge on the inside of the sandwich is not a concern.

If you were to dimple the hinge, you could risk a wavy hinge line that you'd have to fix. I wouldn't know how to begin to help you find a way to to that with repeatable results.
 
Thanks Bill. I agree on the build sequence but logically creating a sharp hole doesn't make sense. But, as you pointed out, the spar web will be secured between the skin and hinge.
 
When in doubt, follow the plan

Check drawing 14A, plan view of the flap at the top of the drawing, rivet callout about in the center of the flap drawing. It says to dimple the bottom skin, countersink the bottom flange of the rear spar. Flap hinge is just drilled and deburred, no dimple or countersink. Pretty much in line with the above comments.
 
Just for reference, Dimple countersinking the piano hinge material gently using a hand squeeze does not distort the material and ruin the hinge. I did this to all my piano hinges with the pin installed in the 2 hinge halves on the bench. You get a gentle curve over the length of hinge piece, but nothing that causes a problem. You can simply straighten it out and cleco it back into position for riveting.

If you tried to the same with a pneumatic squeezer set to close too tightly, I suspect it may cause an issue.

On my flaps, the skin, spar, and hinge are all dimpled and riveted together with no problem whatsoever.
 
Just for reference, Dimple countersinking the piano hinge material gently using a hand squeeze does not distort the material and ruin the hinge. I did this to all my piano hinges with the pin installed in the 2 hinge halves on the bench. You get a gentle curve over the length of hinge piece, but nothing that causes a problem. You can simply straighten it out and cleco it back into position for riveting.

If you tried to the same with a pneumatic squeezer set to close too tightly, I suspect it may cause an issue.

On my flaps, the skin, spar, and hinge are all dimpled and riveted together with no problem whatsoever.

Good to know!
 
Success

Yesterday I machine countersunk (? grammar?) the first spar. Took my time to properly set up the depth on the countersink, and in the end the spar turned out nicely. The depth on the spar does leave a sharp edged hole but as noted, this will be the middle layer on the skin/spar/hinge assembly. Checked fit on the skin to spar and it sits real nice. Thanks for the words of wisdom.
 
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