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Building Tips and technical articles. => Building techniques => Topic started by: Dennis Toth on September 08, 2014, 02:12:42 PM
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Guys,
I am working on a new ship that has an elevator between booms that will be driven from one end. I am looking for a torsionally rigid way to connect the tail horn to the elevator so that one end doesn't move more than the other when control is applied. I've looked at CF rods but they seem to be very strong in tension and compression but not twist, unless you go to pretty large diameter then you have a weight problem. Anyone have any experience with this and could suggest a method I would appreciate it.
Thanks, DennisT
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CF made out of woven sleeving material. You'll have to make it yourself, or pay $$$. Howard Rush may get on here and explain his method -- he has done (or still does) it for flaps.
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Spread tow carbon fiber cloth with the layers on 45 degree bias, applied top and bottom. Same material and technique that many of the European manufacturers of r/c sailplanes use both in the larger F3B. F3F, and F3J models and as seen in smaller DLG models as well.
Take a look at this: http://www.rcgroups.com/forums/showthread.php?t=2161133&highlight=carboweave
r/
Dave
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Dennis,
Interesting, I'm also working on a twim boom. We should compare notes. ;D
I'm not attaching the control horn on the elevator's end.
I don't know the design of your booms? Hollow?
I'm thinking an aluminum tube or CF leading edge and have the horn attatched to the tube. The horn would be inside the boom.
I'm taking that route. Here's my effort so far. Just moments and construction. Nothing yet is carved in stone.
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There seems to be two approaches, one to use 45 deg bias CF on the elevator to stiffen it to resist twist the other to get/make a CF 45 deg bias torque tube. I am wondering if one could use a combination of the CF arrow shaft or hard wood dowel with the 45 deg bias CF cloth over the surface.
C,
Interesting design, we can talk at the field Saturday.
Best, DennisT
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Dennis, I think that the 45 degree bias cloth over an arrow shaft would work fine.
I've never bought from these people -- I just ran across them in a web search. But this stuff looks like just what you need: http://www.cstsales.com/carbon_braid.html (http://www.cstsales.com/carbon_braid.html).
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Any old carbon tube should be stiff enough for an elevator. You can also get 3/16" aluminum tubing arrow shafts, which are light and made out of good stuff: 7075 T-something.
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I would be tempted to drive it from both sides, using something like strip aileron bellcranks in the stab to get the actuation on the "far" side.
Brett
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I am looking for a torsionally rigid way to connect the tail horn to the elevator................
This part I would tackle by at least triangulating the pivot point to the very top of the horn, to the trailing end of the elevator - in fact a full carbon disk rather than a triangular horn would be the ultimate if you could stand the look of it if not hidden.
Drill a central hole in the control arm disk to suit your drive rod and epoxy in place to make it as 'one piece' as possible.
Can't think of a more torsionally ridged way of connecting your drive point on the 'horn' to the drive shaft.
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How bloody much twist do you get?
I would be tempted to drive it from both sides, using something like strip aileron bellcranks in the stab to get the actuation on the "far" side.
with 1/8" steel pushrods. You just can't take a chance on elevator twist.
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I flew my original Ephesian for five seasons and scored some good flights with it. I attached the pushrod at the outboard end only. The LE of the elevator was .014 carbon sheet sandwiched between two 6-8 lb balsa strips that were 1/4 X 1/2 with the 1/2 being the depth. I used warren truss construction on the elevator and then planked it with 1/32. Finish was silkspan and dope. I took the pushrod end outside of the boom at the very back because the boom was so narrow. The booms were molded balsa with no formers except for carbon tow strips evenly spaced inside and were 3/4 wide at the widest spot.
That ship was a big one with 720 squares or so and gave me no quirky characteristics at all. I later built another, which is somewhere in the south doing(I hope) patterns for another flyer. It flew fine, but I wanted to downsize back to the TP sized ship.
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How bloody much twist do you get?
with 1/8" steel pushrods. You just can't take a chance on elevator twist.
?? Are you yelling at me? I can see a lot of problems with the idea of multiple bellcranks, but one of the beauties of using the aileron bellcranks is that it's potentially much lighter than trying to brute force it. This may well be a silly application, but I have seriously considered using something like 3 separate aileron bellcranks per wing to drive the flaps from the both ends and the middle, and no conventional control horn. Mostly to improve the overall rigidity and get rid of the 2 ounces or so of spring steel and whatever the double-covering of the flap, exotic magic torque tubes, etc, cost me.
Brett
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Dennis is sitting back digesting. LL~
How long can his elevator be? 16" max? My guess is 14" for a 35-40 size model. IMHO. n~
I'm usung an aluminum tube as a evevator LE for my twin boom.
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I have seriously considered using something like 3 separate aileron bellcranks per wing to drive the flaps from the both ends and the middle, and no conventional control horn. Mostly to improve the overall rigidity and get rid of the 2 ounces or so of spring steel and whatever the double-covering of the flap, exotic magic torque tubes, etc, cost me.
That's interesting. You could have different flap deflections for different segments if you wanted to. It's exotic magic torque tubes for me, though.
My outburst was just a reaction to the conclusions people reach in the absence of measurement or calculation.
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My outburst was just a reaction to the conclusions people reach in the absence of measurement or calculation.
Measurement and/or calculation gets in the way of reaching fun conclusions. What are you, an engineer or something?
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Lou Wolgast built a Charles Mackey "Junkers Gobbleswantz" (sp?) ('58 NATS Winner) and flew it in several VSC's. As a HS Freshman, I was enthralled by the magazine article, and talked to Lou about his control system. As I recall, he said he tried double BC's/double pushrods, but found that they bound up some and he abandoned the double elevator pushrods. I expect this was due to angular errors on the horns and/or minute pushrod length differences.
I'm not sure exactly how he had the thing rigged, but I may eventually find out, because it's hanging on the shop wall downstairs! Burt Brokow flew it for awhile, after Lou sold it. Then, Burt sold it at Eugene NW CL Regionals about 10 years ago or more, and I pounced on it. Silly boy! If anybody has the article ('58-'59 AM?) and could scan and email it to me, I'd be grateful! I've got the plans. H^^ Steve
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Well...I dug out the plans for the "Junkers Gobbleswantz". The solitary BC is at the inboard boom, and there is only one pushrod. I still think outboard would make more sense (less tipweight offsetting longer leadouts), plus if there is more flap travel (due to torsional flex), it would be better to be outboard than inboard (similar to having a larger outboard flap). H^^ Steve
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Ok... please dont get out the pitchforks and torches... but in theory this sounds like a great application for a pair of Kims RC servos and stator BC instead of Bretts pair of flap BC's. S?P
Not sure its worth the added weight penalties... I'd be tempted to sacrifice the esthetics and run a middle pushrod externally or do like Barons Humbug or have triple boom. Hard to make suggestions withought knowing the overall design and intended use. If this is a proven design or classic then you pretty much go by the plans... if its a scratch build you have a lot of options.
Eric:V