Author Topic: Building Tapered Flaps (Read 6731 times)

Alan Resinger · « **on:** October 01, 2011, 02:58:10 PM »

Several weeks ago I was down in Seattle for the Raider Round-up and got down on Friday afternoon so that I could get a little practice in. When I arrived at Auburn airport, Don McClave, Pete Peterson and Mike Haverly were already there. We flew a bit and I was watching Pete fly his fairly new Cavalier. As most of you know Pete is one of the premier builders and finishers besides being a darn good flyer. I'd seen the Cavalier a few weeks earlier and admired its good looks like all of Pete's planes. The model has an Aerotiger for power but just didn't seem to be a happy flier. Don and I were staying together and were commenting about it that evening either at dinner or after. Neither of us felt the plane was flying well. Next day after Classic I ask Pete if I could take a good look at the model and see if I could see anything that might be contributing to the problems. Pete said that he figured there was some misalignment somewhere but was pretty sure that the construction was as good as he could do. After about 10 minutes I discovered that both flaps had slight twists in them at the tips. One of the elevators was also twisted. Pete thought it might be from the finishing process and vowed to straighten them by the next contest.
I've built flat flaps and stab/elevator assemblies on all my planes for the past few years but my new electric will have tapered flaps so I got to thinking about building straight tapered flaps again. Don and I talked about it and he stated that using wires to sand the taper was the way to go and I disagreed, saying that only if the flap had a constant chord did that method insure straight flaps. For builders with little or no experience building this type of flap (tapered in both chord and thickness) sanding using wires results in a proper shape on the first side but turning the flap over and sanding the opposite side results in a twist being sanded into the structure. To satisfy myself I drew up a flap using AutoCAD with the following dimensions. The root chord was 3 inches while the tip chord was 1.5 inches. The flap was 25.5 inches long. Using 1/4" inch stock and 1/4" wire at the leading edge and 3/16" wire at the trailing edge, the shape of the flap is perfectly shaped when the sanding is complete. Turning the flap over and putting the 1/4" wire at the leading edge and 1/8" wire at the trailing edge and making sure that the shaped side is flat on the building board you will find that now the angles are off. AutoCAD says that there is a 2 degree difference in the flap angle root to tip. Pushing the flap down flat on the building board and sanding means that once you sand the second side, you now have a 4 degree twist sanded into the flap. The only way to insure that you are not sanding a twit into the flaps is to have a board to sand on that has the proper angle for the flap to rest on. Boy building these things right can be a pain. I've included the drawing showing how the angles get off when the flap is turned over.

Howard Rush · « **Reply #1 on:** October 02, 2011, 02:41:45 PM »

Quote from: Alan Resinger on October 01, 2011, 02:58:10 PM

The only way to insure that you are not sanding a twit into the flaps...

I am careful not to have any body parts between the sandpaper and the flaps.

Randy Powell · « **Reply #2 on:** October 02, 2011, 02:46:28 PM »

yep. the flaps I just built I used a core piece and ribs. I used a jig to hold the core flat while sanding in the shape. nearly as I can tell, they taper evenly.

I suppose the other thing you could do would be to build the flap in two pieces. shape one then the other and glue them together.

Tim Wescott · « **Reply #3 on:** October 02, 2011, 04:01:44 PM »

I've considered this, but have not tried it:

Sand the one edge of the flap using 1/4" and 3/16" wires. Then shim the trailing edge of the flap up 1/16" (How? I dunno -- I think I'd try a strip of 1/16" balsa, probably 1/8" wide first) and sand using the 1/4" and 3/16" wires again.

Alternately, make sure to do the top parts of both flaps with the first set of wires, and the bottom parts of both flaps with the 1/4" and 1/8" wires. Then think up some jive about why the slight twist in the flap is actually beneficial to the flying qualities of the plane -- I think I'd concentrate on the fact that you're effectively building in a bit of washout into the wing, reducing the induced drag on the inside corners. Since there are more inside corners than outside ones, and since all the 120 degree corners are inside, of course that slight twist is intentional, beneficial, and (by deduction) you are a genius.

Actually, I think that if you get the top surface right and truly get the bottom surface such that the trailing edge is consistently 1/8" thick, then your flap will automatically be right. This will (maybe) happen because you're actually twisting the flap a bit as you sand, correcting any unfortunate tendencies.

Bob Reeves · « **Reply #4 on:** October 02, 2011, 04:44:15 PM »

Quote from: Tim Wescott on October 02, 2011, 04:01:44 PM

Actually, I think that if you get the top surface right and truly get the bottom surface such that the trailing edge is consistently 1/8" thick, then your flap will automatically be right. This will (maybe) happen because you're actually twisting the flap a bit as you sand, correcting any unfortunate tendencies.

This...

Steve Helmick · « **Reply #5 on:** October 02, 2011, 04:58:05 PM »

I was not there, having surgery the previous Monday. But Pete's Pretty Plane is not a Cavalier, 'tis a Caprice.

Steve

Howard Rush · « **Reply #6 on:** October 02, 2011, 07:02:56 PM »

I've been using built-up flaps, but the same thing could be done for the second side of solid flaps, if necessary. I build them on a base of pink polystyrene foam with the taper hot-wired into the foam. One can use Science to figure out how to cut the foam, but it doesn't hurt to check the result with one of these doodads: http://www.amazon.com/Wixey-WR300-Digital-Angle-Gauge/dp/B001PTGBRQ/ref=sr_1_1?ie=UTF8&qid=1317603722&sr=8-1 .

Allan Perret · « **Reply #7 on:** October 03, 2011, 07:26:02 AM »

I built a Ringmaster Delux last year which had built up flaps. I came up with a relatively simple jigging method to correct for this issue when sanding the taper. Posted an extensive build thread on the Ringmaster Brotherhood website with many pictures. It was all lost when the website crashed several months ago. Will check later to see if I can find and repost pictures of the flap build.

My jigging takes care of the "shimming" of trailing edge that Tim W. mentioned above. But doesnt actually use a shim per se. Pictures will tell the story.

As I am typing this I though of an improvement in my technique that would increase the accuracy of my method. Will implement on next build.

john e. holliday · « **Reply #8 on:** October 03, 2011, 08:32:51 AM »

The tapering of the flaps with wire stops has worked or not worked for how long. If I remember right I think I first seen it in the build of the Gieseke Nobler kit I did when it first came out. Have done it on flaps ever since then. Now I find out it is wrong.

Peter Nevai · « **Reply #9 on:** October 03, 2011, 09:07:41 AM »

??

I use the rod / wire method. A while back I made the mistake of sanding all of the taper into flaps on one side only, realizing that you can not sand just one side to the final thickness (As the first drawing shows) I sand both sides. Say I want a 1/8th inch thickness at the Trailing edge of the Flap / Elevator on a part that is 3/8 thick. I sand one side down to 1/4 inch thickness at the TE, then flip the part over and finish off using a 1/8th wire at the TE. In this fashion you remove a equal amount of material from both sides of the part.

Sanding taper into variable cord parts requires a two sided approach

Allan Perret · « **Reply #10 on:** October 04, 2011, 08:04:11 AM »

As Alan R. explained in the opening post the rod method sands a twist into the first side. The problem arises for sanding the 2nd side when you flip it over try to use that twisted surface as a reference against your flat work surface.

My jigging method uses 2 aluminum flat bars screwed to the edge of building surface. The holes in the bars are oversized to allow height adjustment of the 2 bars individually.
To sand taper into the 1st side, only 1 bar is needed and it serves as the sanding stop (replaces the rod). I add the second bar (stacked right on top of 1st bar) when setting up to taper the second side of flap. The first bar becomes a ledge or shim to keep the flap's centerline parralled with work surface. Second bar's height is set to provide the sanding stop. Important to draw a centerline around the flap stock before starting to aid in setting height of both bars.

In the pictures I am making a set of built up sheeted flaps. Sheeting is 1/16, the 2 sheets come together for 1/8 TE so there is no stick at the TE. I built the framework, tapered the first side, then glued on the first sheet skin. Weighted down to the surface the sheet conforms to the twist. Then flipped it over to taper and skin 2nd side. In the picts you can see the centerlines on some of the ribs are kept parrllel with building board.

This method would also work good with solid flaps. It could be improved upon by leaving an extra inch or two of length on each end of flap stock. By not sanding any taper into the extra end pieces, they would provide automatic parralled reference to work surface when stock is flipped over to set bar heights for tapering 2nd side. Then after finished tapering 2nd side the extra length would be cut off.

Pict 28 indicates the twist that gets sanded into the surface of a tapered chord flap.

Crist Rigotti · « **Reply #11 on:** October 04, 2011, 08:17:50 AM »

Way cool Allan. I'll have to give that a try. Thanks

Alan Resinger · « **Reply #12 on:** October 04, 2011, 04:15:35 PM »

Allan,
That's a slick set-up. I did basically the same thing to get mine sanded. People just don't realize how you can screw up a pair of flaps by simply using wires to make the edges the same thickness. You have to take into consideration the difference in the angle of the sanded side due to the change in chord length between the root and the tip.
Alan Resinger

Mike Haverly · « **Reply #13 on:** October 04, 2011, 08:02:39 PM »

My current project has a tapered trailing edge, that is, from the fuselage to the tip. It is considerable about .190". To make the flaps correctly, they should also taper by the same amount, me thinks. I really should talk to the designer for help on this one, I've been rattling my brain on this and have a couple of ways to do it, just haven't started yet.

Mike

Alan Resinger · « **Reply #14 on:** October 05, 2011, 08:09:46 AM »

Mike,
Are you going down to Salem this weekend? I'll talk to you about tapered flaps.
Alan

Bob Reeves · « **Reply #15 on:** October 05, 2011, 11:26:31 AM »

Quote from: Mike Haverly on October 04, 2011, 08:02:39 PM

My current project has a tapered trailing edge, that is, from the fuselage to the tip. It is considerable about .190". To make the flaps correctly, they should also taper by the same amount, me thinks. I really should talk to the designer for help on this one, I've been rattling my brain on this and have a couple of ways to do it, just haven't started yet.

Mike

The Ringmaster 576 is designed this way, what I did was cut a 1/2 inch x 2 inch balsa sheet to match the wing trailing edge taper on my table saw then sanded in the normal taper free hand.. A little tedious but not all that hard.

Douglas Ames · « **Reply #16 on:** October 07, 2011, 10:45:14 PM »

Quote from: Alan Resinger on October 04, 2011, 04:15:35 PM

Allan,
That's a slick set-up. I did basically the same thing to get mine sanded. People just don't realize how you can screw up a pair of flaps by simply using wires to make the edges the same thickness. You have to take into consideration the difference in the angle of the sanded side due to the change in chord length between the root and the tip.Alan Resinger

Trigonometry 101...http://www.csgnetwork.com/righttricalc.html

IOW- the surface angle washes out as it extends to the tip IF your flap thickness (or elevator) is uniform. I ran into this on my Brodak Corsair elevators. I solved it by tapering the stab spar/ elev. LE from .375" to .280". This results in a constant surface angle along the span. Note the two scrap sticks are the same surface angle.

Allan Perret · « **Reply #17 on:** October 08, 2011, 07:27:25 AM »

Quote from: Douglas Ames on October 07, 2011, 10:45:14 PM

Trigonometry 101...http://www.csgnetwork.com/righttricalc.html

IOW- the surface angle washes out as it extends to the tip IF your flap thickness (or elevator) is uniform. I ran into this on my Brodak Corsair elevators. I solved it by tapering the stab spar/ elev. LE from .375" to .280". This results in a constant surface angle along the span. Note the two scrap sticks are the same surface angle.

Correct me if I am wrong, but it appears in the picture that your flap has a constant chord.

Douglas Ames · « **Reply #18 on:** October 08, 2011, 08:31:08 AM »

Quote from: Allan Perret on October 08, 2011, 07:27:25 AM

Correct me if I am wrong, but it appears in the picture that your flap has a constant chord.

Sorry, bad picture.
It's actually an elevator from my Brodak Corsair, but the same problem arises as tapered flaps.

Allan Perret · « **Reply #19 on:** October 08, 2011, 11:34:03 AM »

Quote from: Douglas Ames on October 08, 2011, 08:31:08 AM

Sorry, bad picture.
It's actually an elevator from my Brodak Corsair, but the same problem arises as tapered flaps.

Interesting hinge treatment. What are you using there ?

Douglas Ames · « **Reply #20 on:** October 08, 2011, 12:33:32 PM »

Quote from: Allan Perret on October 08, 2011, 11:34:03 AM

Interesting hinge treatment. What are you using there ?

Apologies, not intending to hijack the thread.

I'm putting the hinge pivot at the center of the leading edge of the elevator. The gap should stay constant throughout the arc of the elev. I may or may not use gap covers to form a small cove. A strip of Manilla folder paper soaked in fin. resin maybe?

Geoff Goodworth · « **Reply #21 on:** October 09, 2011, 04:41:53 PM »

While I used to be concerned about the very point that Alan has raised, my concern was always tempered by the knowledge that a good many top flyers recommended the two wires process. In practice, the few flaps and elevators that I have sanded that way have come out straight.

That said however, they are only made from 3/16" and 1/4" balsa which may be a factor.

Warps can be induced in a variety of ways so I have to wonder whether the twist induced when sanding the second side is material flex so the part returns to neutral state once the pressure is removed.

Allan Perret · « **Reply #22 on:** October 19, 2011, 06:25:51 PM »

Quote from: Avaiojet on October 19, 2011, 04:44:50 PM

Are you guys using hard wood where the flap horn wire enters the flap?

.125 wire? How far into the flap?

Thanks,

CB

Yes, flap needs a pocket from hardwood or ply to accept horn wire, 1/8 for 60 size plane, 3/32 OK for 40 size.
At least 1" into the flap.

Avaiojet · « **Reply #23 on:** October 19, 2011, 07:18:59 PM »

Quote

Yes, flap needs a pocket from hardwood or ply to accept horn wire, 1/8 for 60 size plane, 3/32 OK for 40 size.
At least 1" into the flap

Allen,

I don't understand? If the thickness of the wood is 1/8" how can you drill a .125" hole into it?

Or, if the wall of the pocket is 1/8" and the wire is also 1/8", add another 1/8" for the other side of the pocket. That's 3/8". What am I not understanding?

If I had a flap that was 1/4" thick and tapered, I could drill a .125" whole to except the wire. leaves me only another 1/8" of wood on each side, which is balsa wood of the flap. The hole has to stop when you reach the 1/8" thickness of the tapered flap.

I'm trying to grasp this?

The model I'm currently working on. When we were kids we just drilled a hole in the balsa and glued or doped cloth over it. Done! "Let's go fly!"

I cannot see myself doing this type of work on my project. 1/4" thick flaps, BTW.

Thanks,

CB

Alan Resinger · « **Reply #24 on:** October 20, 2011, 09:00:30 PM »

He is talking about the size of the wire used for the control horn. Most flyers use heavy duty control systems that includes heavy wire horns and carbon fiber pushrods with threaded inserts and ball links connecting the rods to the horns.

Steve Helmick · « **Reply #25 on:** October 20, 2011, 09:25:57 PM »

Many also use "Lucky Boxes" let into the flap to allow the flap to be easily removed from the horn, and to allow "skewing" to allow for swept hingelines. The flaps, elevators, and rudder can be removed with a one-piece hinge pin (per side). A great plot for R&D purposes, painting, re-painting, repairing, etc. Do a search on "Lucky Box". RSM kits them, and maybe others. They're usually two layers of phenolic or epoxyboard with basswood or spruce spacers.

Steve

Allan Perret · « **Reply #26 on:** October 22, 2011, 07:08:06 AM »

Quote from: Avaiojet on October 21, 2011, 09:56:55 AM

How thick is the LE of that flap you have? Width also?

Nice work.

CB

That is the RMD-3 flap. Root chord is 2.5". Tapers from .5" to .125".
Same flap as shown in other post http://stunthanger.com/smf/index.php?topic=23717.0

Alan Resinger · « **Reply #27 on:** October 29, 2011, 02:45:33 PM »

Earlier this week I got an e-mail from Norm Whittle regarding this thread. Seems he's had numerous requests for plans to build his e-Sultan design and he was busy writing some explanations for some of the building points that were out of the ordinary. He thought of this thread when doing the shaping of the flaps and ask if I'd look his method over and check it out. Upon drawing the flap shapes and profiles out I realized that Norm had come up with a rather elegant solution to the problem that takes into account the fact that using the rod method for sanding taper into flaps it only works correctly for the first side. When the second side is sanded you will sand in a twist due to the fact the plane angle of the surface changed from root to tip of the flap in a section that tapers in chord from root to tip. The rod method only works on flaps that have a constant chord. I've attached Norm's explanation and a drawing that shows how his system works. Any flap can be done his way by making sure that during the initial shaping, the surface that is being sanded keeps the same chord from root to tip.

Mike Haverly · « **Reply #28 on:** October 29, 2011, 04:33:24 PM »

I laminated my flaps last night and should start the process soon. I don't have a good working camera but will post some pictures anyway.

Mike

Mike Haverly · « **Reply #29 on:** October 30, 2011, 02:08:40 PM »

I got both flaps tapered this morning. They are still rough but this process works really well, better than my ten year old camera. One good thing is that no speacial tools or set-ups are required, just a flat sanding surface, a pen, masking tape and a sanding block. Pictures are poor, my better camera is not working.

Bill Little · « **Reply #30 on:** November 01, 2011, 12:23:18 PM »

Hi Alan,

I am guilty of never having used the "rod" method of making tapered surfaces so I can't speak to it.

However, I do use 1/8th" masking tape on the TE and a Solingen Razor Plane and sanding blocks. Takes longer I guess, but I can monitor the progress real easily.

Big Bear

Peter Nevai · « **Reply #31 on:** November 01, 2011, 02:46:33 PM »

While all this works out very nicely on paper, In real life balsa twists, as long as you apply enough pressure to force the wood flat agains the surface for the entire length while sanding, both sides come out just fine, no twist. I did the built up elevators, stab, and solid flaps on my pathfinder LE that way and they are arrow straight and evenly tapered.

Of course it required some elbow grease, but the sanding a taper into 26 inch long solid flaps using nothing more than a 4 inch sanfing block and 150 grit paper kind of requires it. Next time I'm going to tack glue the suckers down to the mirror to keep em from sliding around between the rods. Sanded the silght bend right out of one of em too.