Seems like the one to ask would be Mr. Rabe.

Sorry.

Please, no "sir"--just...

--Ray

t michael,

Howard Rush, on his Impact, also uses a carbon torque tube as the forward (hinge bearing) surface of the flap. He uses a smaller tube as the training edge, ribs in between and sheets them. Has the advantage of 1) being pretty easy to build straight, 2) very, very torque resistant, and 3) laterally quite stiff. And besides, they are pretty light, too. I think he uses 1/4 tube in front and a 1/16" dowel in the back. That's what I would use (and may on the new plane).

I get such things from either an archery shop (CF arrow shafts make great carbon tubes for pushrods and this sort of thing). Another source is Aerospace Composites. They are on line at http://www.acp-composites.com/

I'm not sure how Howard does it, but if I did, I would plug the end of the tube with some spruce and put a small hardwood block behind and drill a hole for the horn. Probably overkill, but I like to be safe and the weight penalty wouldn't be much. I'd use a rotary grinder and a cutoff wheel to cut the slots for the hinges.

I imagine the all up weight would be comparable to balsa construction. You could use really light balsa for the sheeting. Probably "C" grain, 7lb stuff for the ribs.

I am pretty sure that Al uses a tube near 3/8ths dia, epoxied on the LE.  IIRC, he wraps the flap with fiberglass, too.  Cutting a hinge slot doesn't weaken the tube enough to be a problem.  I remember Al talking about the jig he created to cut the hinge slots.

It is true that given the aerodynamic parameters that Al has to live with in his designs, torsion control is paramount in his flaps.  The flaps play a HUGH role in the ability for his scale planes to create the requisite lift need for his designs.

Thanks for the information.

1. I still do not understand how to split a carbon fiber arrow shaft in half of quarters.

2. Is it a health hazard to saw the carbon fiber arrow shaft?

3. Having a half circle leading edge for a flap would not allow the flap to rotate.  May be the 45 degree champfer is on the wing trailing edge.


t michael jennings

There was a thread on SSW a while back about new planes for the season.  There was a German fellow on there who had a really nice stunt plane and link to the build.  Lots and lots of pics during the build.  He had torque tube flaps and pics on how he did it.  On SSW it was just a pic of his plane but th e link to the build showed how he did it.

However for the life of me I cant find it.  Maybe some of you know what thread I was talking about.

Hi guys,

Lost my password.  It took more tries than I had patience for to try to figure out how to get a new one.

I use 5/16" (.312) carbon tubes that I get from Lone Star Balsa.  It is the only source that I have been able to find stocking a very thin wall tube (and they are cheaper than other sources I've tried).

I begin by drilling through the end of the tube so that the flap horn can penetrate through the center of the tube.  I push the tube onto the airplane's flap horn and tape the other end to the wing tip to hold it while I mark the location of my hinges.

Next, I drill very small holes at each end of each hinge location using a #64 drill (.035").  Each pair of holes is separated by the width of a hinge (5/8") plus another 1/8" on each side of the hinge to define a hinge slot of 7/8".  After marking where I want hinge slots to be and how big they should be, I am ready to drill.  I have a jig consisting of an aluminum block drilled through on one axis 5/16".  Perpendicular to the 5/16" hole are two more drill guides, each 1/4" from the edge of the jig.  One is .035", the other is 1/8" very accurately located, to exactly allow the drill to penetrate the carbon tube within a .001 of center.  The second guide hole is for drilling the flap horn.  The flap horn hole is drilled first.  Next, I align the tube within the jig to drill exactly parallel to the flap horn hole.  I do this by bending a piece of music wire to a 90 degree shape.  One end goes through the flap horn hole.  Attached to the 90 degree side of the wire is a small plastic bubble string level (package of four, I think from Home Depot).  Put the jig block in a bench vice.  Stick the bent wire into the horn hole.  Twist the carbon tube to level the bubble.  Drill the small holes through the aluminum drill jig.   The carbon tube is visually aligned using the bubble level for each hole as the tube is progressively relocated in the jig for each drilling.

After drilling a pair of holes for each hinge,  I cut the slot between each pair of holes on opposite sides of the tube, by eye, using a Dremel cut off disk (.022").  I can usually feel a small bump when I hit the drill holes.  It sounds like a "lash up" and it is.  The alignment and slotting is crude compared to the jig drilling, but results in an excellent alignment of the hinges.

Now we have a carbon tube with a bunch of matching holes for hinges.  The tube has to be sealed.  I take a piece of square 3/8" soft balsa and chuck it up in my drill press, grab the balsa lightly with a piece of sand paper and turn on the drill motor.  It is very easy to make a balsa dowel.  The inside diameter of the tube is about .250".  I sand a bit, check the diameter of the dowel with dial calipers, and sand more until the balsa dowel is about .270" - .280".  I cut the dowel into 1 1/4" lengths.  I compress each dowel a bit with my fingers to start it into the tube, then shove them to the desired location with a threaded rod or piece of wire.  As the dowel is pushed, its progress is visible as it passes each slot.  When it is properly located, I drop a bit of CA through the slots, front and back.  This seals the slots and fills the tube with soft material to support the hinges.  Cut the hinge slot from front to back with an X-Acto knife.  Glue each plug as it is installed.  If a second plug is inserted before the previous plug is glued, it will be moved by trapped air piston effect.  The last plug is installed at the horn location and drilled to accept the horn.  The front side of the carbon tube has to be inletted to allow the parallel portion of the horn to lay slightly inside of the front of the carbon tube, otherwise the horn would hold the tube away from the wing trailing edge.  This obviously weakens the carbon tube, but at this point the flap horn is providing the torsional force to the flap.  There is no apparent loss of stiffness.

At this point, I make a trial fit by inserting all of the hinges into the tube and placing the tube on the horn and shoved up against the wing trailing edge.

Now the 3/8" balsa sheet is prepared.  While it is still "square", I drill it for the flap horn and rout the leading edge to accept the carbon tube.  I use a simple Dremel routing adapter which I have modified by adding two large aluminum angles as guides.  I use 5/16" tubes because that is the largest diameter carving bit that Dremel makes.  The balsa sheet is aligned between the guide angles and several passes are made to rout the flap's circular depression.  A properly located groove will leave only 1/32" flat on each side of the groove.  Adjustments to alignment are necessary using scrap for the alignment process.  I wrap the Dremel screw collar with a couple of wraps of masking tape to tighten the fit in the routing adapter collar.  One side of the collar, is drilled, tapped, and a screw inserted into the hole.  I add a bunch of rubber bands to bull the Dremel tube toward the screw and make screw adjustments to center the routed groove.  There is very little adjustment possible, but enough.

Gosh, I'll bet you are sorry you asked.  Anyway, at this point I cut and shape the flaps.

To assemble, press the carbon tube, with hinges into the routed groove.  The hinges protrude about 1/8" beyond the tube and will press shallow slots into the back of the flap groove.  Mix epoxy.  I use Zap Finishing resin, one part "A", one part "B", one part acetone.  Paint the groove and back of the carbon tube and assemble with many strips of masking tape.  After about an hour remove the hinges or they will be prematurely glued.

The next day, I shape and sand the flaps.  Now we have flaps with carbon tube leading edges but the flaps will be held away from the wing trailing edge by the width of the hinge barrels.  Unless I want a 1/16" gap between the wing trailing edge and flap, I must now add a bit of balsa to the front of the carbon tube and sand to a pointed shape between hinges to fill the gaps.

The flaps are finished by glassing with one or two layers of 1/2 ounce cloth and thinned epoxy resin.  The large flaps on my airplanes weigh about 2 3/4 ounces each.

I figure carbon torque tube flaps add about 4 ounces to the weight of large airplanes but contribute at least 10% to the weight the airplane can carry through competitive corners.  I figure that if a 70 ounce airplane can make a satisfactory corner, an airplane with carbon tube flaps can handle 77 - 80 ounces with a corner of equal radius.  Think of it as insurance to cover the eventuality of my airplane coming out heavy.  Power to pull is never an issue with .76 - .91 engines. 

Obviously, these things are a bitch to build, but after flying the first set, I'll never begrudge the necessary labor or be without them.  I don't make these because I'm obsessive-compulsive.  They make my airplanes fly better.  One last point, if you don't have a specific need for very stiff flaps, then they aren't worth the effort to make.  I would not consider using carbon torque tubes on a typical stunt ship.

I have only two photos.  The first shows my drill jig.  the .035 hole is exactly 1/4" from one edge.

The other photo shows my Dremel routing attachment and alignment fences.

Al 

I would agree that for most designs this is unecessary. But in a twin boom design, where a dual belcrank and horn arrangement is called for. A carbon torque solution would be simpler and quicker to fabricate than all the additional control hardware, there would also be a significant weight savings, so long as the elevator need not be glassed.

http://s122.photobucket.com/albums/o273/frank_wadle/?start=60

At the bottom you will find this guys tube flaps.  You can expand the photos by clicking on them.