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Author Topic: Flap Design  (Read 945 times)

Online Ken Culbertson

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Flap Design
« on: January 19, 2021, 02:20:18 PM »
This post is to explain my flap design and how I build them for one of our subscribers since you can't attach pictures on a PM.  Hope I don't bore you to death. ::)

So here goes.  I flat got tired of slab flaps warping after the plane was finished.  My favorite hinge is the recessed Roberts for no other reason than I love the look.   So I found a way to make a light flap that is easy to make for the Roberts and stiff in both directions.  I am one of those builders that tends to skip over the complicated stuff the closer the plane is to being finished so I make my flaps, stab and fuselage first.  Electric has helped that a lot since I can test fly it as soon as it is airworthy.

Back to the flaps.  I build all of my structures except the wing, without a jig.  I use glass and CA to keep things straight.  That way I do not build in internal tension during assembly.  To build the flaps I cut the balsa LE to length and lay it on the glass.  If there is any warp at all I cut another piece.  Once I have that I cut the end rib from 1/4 strip to length and the same for the end rib.  CA them to the LE using a square.  Lay out some 1/16" shims then CA the 1/8 x 1/4 TE to the end ribs.  Next the center rib.  I use small weights to keep it in place if necessary but never to force them into place. Take the bare frame off and flip it over and lay it on the glass.  If there is any warp, fix it now.

Next I add the "geodetic" (I know that they really aren't) in one direction.  Cut/sand everything to fit but not be forced.  I use a "slow" cure CA like JB Weld for this.  When they are all in flip it over again.  I have done the other direction by both sanding slots and cutting to fit.  Both are a pain but I think the cut to fit ended up stronger.  Avoid the temptation to leave this direction out.  The gap on the long end makes the 1/32 sheeting weak. You guessed it, flip it over.  Still flat?  If you are going to be tweaking then you might want to fill in an inch or so on the fat end.

Sand the taper into the ribs with a wide block.  I use a 4 x 8". and I do it with the flap on the glass.  I sand the two end and center ribs on one side last so I can do both sides on the glass.  It should sit perfectly flat on both sides using a 1/16" shim on the FRONT of the TE which should not be sanded and still be 1/8".

If you are using Roberts hinges, add some doublers behind the LE where the hinges will go.  The pockets can go in either the wing or the flap.  I put them in the wing because it is thicker but that is another story.  I hate pockets and I am going to experiment on a way to make the flaps removable without pockets on the next set but that again is another story.

You should have a very stiff frame in your hand.  Time to add the CF tube.  I use cheap target arrow shafts.  Light and very strong.  More expensive options are out there that are stiffer BUT it is really the torsional value of the tube and the roundness that we are after.  Take a piece of 3/16" dowel and CA some sandpaper to it of sufficient grit to hollow out the LE.   The sandpaper adds just the right thickness. Keep it centered.  I make a tool by gluing two pieced of 1/16 to the top and bottom of a 1/4" piece forming a "U".    The tube should indent as close to 1/2 way as possible.  You want to use a very thin coat of slow cure Epoxy to attach the tube.

Find some really straight grained 1/32 and mold two layers of it around another CF tube.  Don't use the flap - Wet warps.  Trim the smaller one to cover 1/2 of the width of the LE and glue it in place using a thin coat of slow cure Epoxy.  Save the larger one for the wing.

Plank it, square up the TE and glue the 1/8 CF cap on the TE and you are done.  I will leave the horn piece and the hinges to your preference. (I cut slots for the Roberts before planking and I use a 1/4" Bass end rib drilled for the horn)

Not much need for lots of surface stiffening.  One layer of thin glass is all I needed.

Hope this helps.

Ken

I would be curious if there are any others out there who have built Jig-less.




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Offline Craig Beswick

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Re: Flap Design
« Reply #1 on: January 19, 2021, 03:43:26 PM »
That is great Ken.
thank you so much for taking the time. Beautiful drawing too!!

Craig
AUS 87123
"The Ninja"


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