Fowler Flaps

3/13/05 -3 hours

The Fowler flaps are the last hurdle in the Palmer plans saga. The B-24 uses Fowler flaps which slide back and down and the model uses them too but slightly modified so they can be built without a machine shop. The Palmer flap tracks and general design is good but needs that real world tweaking. The tracks the way they are drawn doesn't leave room for connecting the two runner halves together. I worked today on making some flap track templates, flap tracks and runners and testing it out. The problem with the plan was the tracks are drawn too low and leave no room for the flap. Also the flap drawings conflict with each other. The side view shows two flap tracks and the top view shows one track and two runners. The correct version is one track and two runners per track.

The tracks attach to the wing and the runners attach to the flap. After making up some tracks and runners I tried it out and it works good. I cut these slots free hand with a dremel cutoff disk. Not bad!


3/17/05 - 3.5 hours

The plywood tracks worked good but I was worried about strength and wear. The bottom slot (top in picture) is very close to the edge, leaving a 1/16 or less strip of material to hold in the pin (and flap). I tried some formica but that too seemed weak at the thin strip.

I made a trip to the local machinery surplus store and scored some perfect track material real cheap. It appears to be phenolic resin but sparks when cut on the band saw. It's a little heavier than the 3/32" plywood but much stronger along the thin edge next to the slot. I bought that plate, 6"x 12" sheet of some great looking fiberglass sheet and some plastic for $1 at Triangle Machinery Surplus here in San Jose. If you like getting your hands dirty looking through lots of surplus machinery, parts and tools, this is the place. Bring hand cleaner!

Since the slots need to be in the same place on three different sized tracks, I have been thinking about how to cut the slots accurately and have experimented a bit and came up with a little table saw to cut the slots. The mini-table saw uses the fiber cut-off disk. I made a jig piece that I tack glued the track piece to and would hold the track piece in alignment for slot cutting so all the slots are the same. The length of the slots is not too critical but the angle and slot spacing is.

Forth iteration on the tracks. Good enough to tack glue in!


flap track

Two flap runner sides getting glued to a 3/32" spacer. I cut off the spacer after gluing. The other piece keeps the runners aligned.

3/18/05 - 3

Today I spent another 3 hours on just the right side flap. I adjusted the tracks a bit for a better flap fit, then cut out the foam for the flap bellcranks, and made a "spar" that fits over the rear mini-ribs and flap tracks (It's more of a rib alignment tool than a spar). After everything is in and works great I will glue on the rear "spar" and then the mini-ribs. I tack glued the tracks in, worked on the flap template, worked on the flap bellcrank system. I am making it hard because I don't want to cut any more holes in the bottom of the wing. Hind-sight is 20-20 and I would like to have put in the flap tracks and bellcranks before sheeting and glassing. I thought I would just cut out the hatches and put in the bellcranks but I'm afraid that would weaken the wing too much. So now I am trying to put in the bellcranks through the opening at the trailing edge and it's tight. I played with the idea of using servos to drive threaded rods as a screw drive on the flaps or using one servo in the middle of the flap but after some tests I decided that Palmers two bellcrank system is the best. I like the idea of a screw drive system but it would be too much work. I cut some notches in the end of a 7/16" brass tube (the brass tube that was supposed to be the wing tubes, but now it gets to do something useful) and used it to drill a hole out to the outer bellcranks for the control linkage to pass through.

3/21/05 - 1.5

I cut slots in the flap and tack glued the flap to the flap tracks to test out the tracks. They work good with no weight on them but drag a bit with pressure on them and feel rough. I will try lubricating the tracks, maybe wax them, and see if that makes them smoother. I'm getting both tired of making tracks and good at it! The flaps need to move freely. The last thing I want is for one flap to stick. I will put in the bellcranks and linkage and see how it all works together before tearing out the tracks though.

3/24/05 - 1

flap linkageSqueezed in a little time and I worked on getting the bellcranks in. I made new bellcranks from the sheet of fiberglass I got at the surplus store because, #1- the plywood was too weak and broke while trying to remove a clevis, and #2 - I had the fiberglass. The new ones are much stronger. I then put ball links on them and made up linkage with an aileron ball link double-head at the inner bellcrank and a regular ball head at the end. Then I tried for about 15 minutes trying to connect the head to the ball and there's not enough room to leverage the head on. I pulled that out, replaced the ball links with EZ-connectors put the bellcranks back in and slid a 1/6" rod in with no problem. Much better!




4/3/05 - 3

Happy that the bellcranks will work fine I re-glued the flap to the flap runners and made up two sets of control arms and glued one set into the flap. I put the flap on and hooked up the linkages. It was binding from the outer bellcrank being very close to the flap. Replaced the clevis with ball links at the flap control arm on the outer bellcranks' long arm and got good movement. So, a mix of ball links, clevis and EZ connectors does the trick. A little wax on the tracks and runners and the flaps work good. Only 14 hours to get one flap working! On the other hand I already have all the parts made for the left wing except for the tracks. Satisfied everything works good I glued in the bellcranks and the tracks for keeps. The servo mount went in without a hitch. I epoxied maple blocks to some light ply and epoxied that into the wing. The servo will be held down by a strip of aluminum and some screws.

top of Flap Flap down Flap up



4/5/05 - 1.5

Flap area wing ribsI put in the rear "spar" and false ribs. The right flap is done except the blending in of the flap top rear to the wing top rear. Onto the left wing!


4/12/05 - 3 - Left wing flap

The left wing went better. Having to make the tracks and the flap were the only things that weren't done so after clearing out the foam, I made the tracks and tack glued them in, then cut and shaped the flap and then realized it weighed way too much to use. So back to the box of wood to find lighter wood. I shaped a new flap, put in the runners and tried it out. The flap sits too low and doesn't sit flush with the bottom sheeting and the runners rub on the top sheeting so a little inspection reveals that the tracks are slightly too small which messes everything up. I drilled the hole through the wing for the linkage.

flap tracks

The flap tracks and runners (left wing)

4/13/05 - 3

flapI had to make a new set of tracks slightly taller. I think the problem was that when I fitted the tracks I didn't clear away all the excess glue and foam from the sheeting which made the tracks fit tight but not right. After making sure all that was cleaned up I fit in a new set, tack glued them in and it all fits good now. The bellcrank assemblies went together and in fast. It's great when you know what you're doing! Next up was to put in the control horns into the flap, make the linkage and hook it up. The tracks are still just a hair too low so the ball links hit the top sheeting. A little dremel action and now the left flap is working good.

4/20/05 -3

With the flap working good I hit the tracks with more CA and a little epoxy and then sealed it up with the rear "spar". On the right wing I slotted the rear spar for the ribs and inset it under the bottom sheeting. This time I made life easier by not cutting slots and not insetting it. I glued it in and sanded it flush with the bottom sheeting. Then I glued in the ribs. The only problem was that now it looked different from the right wing so I did the same to the right wing. So much for making life easier! But at least it was easier to install.

I cut holes in the spar for the linkage to come through and cleaned them up with the dremel. The servo mount was also put in the same as the right side.

4/27/05 - 3

flap topWith the flaps working they now need finishing up. The plans show the flap trailing edge and the wing trailing edge meeting together at a point. But I want it to look scale (i.e., more work) On a full size B-24 the top wing skin doesn't go all the way back to the trailing edge, the flap trailing edge is the wing trailing edge. I had made the flap but I needed to add some balsa to the trailing edge of the flap to maintain the wing contour. After that I added in some filler so it looks normal (no step) when the flap is down. Of course, it would have been much easier to shape the flap tops without the tracks in place but I need those in for alignment.

flap pinI put 1/8 x 1/4" rectangular styrene in the pin slots. After glassing I'll cut a small slot for the pin head to lock into so it can't come out. I then filled all the gouges and dings in the balsa with filler.

That about does it for the flaps other than glassing and pretty much ends the building phase of the project. Now it's onto the finishing phase.

Flap upflap downflap upflap down

The first time the wings were on with the flaps and ailerons in place. Looks good!

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Updated July 14, 2009