Without using carbon fiber which is the best design for flaps that are roughly 3" at the big end 1.5" at the tip and 28 inches long? The leading edge is 1/2 inch thick. Lightness is needed too. They are for a .60 size profile stunter. Maybe 65 ounces when finished.I would use a built up sheeted arrangement. I build the frame using stiff light 3/8 square for the LE and 3/8 x 1/4 for the TE with the 3/8 being the tall dimension. Draw a center line on both on the outside. Full depth light 1/16 ribs every 1" followed by zig zag ribs inbetween. End rib is 1/8 bass. Epoxy brass tube for flap horn to inside recessed 1/4" and add a 2nd 1/8 bass rib. ( sandwich the tubing) While still on the board, block sand the top to the line on the TE then fill the space over the horn tube with more epoxy and sheet the top with soft straight grained 1/16. It will be stiff enough to flip it over and do the same to the other side and still maintain the double taper. The 1/4 TE will all but disappear leaving you a 1/8" thick TE and 1/2" LE. Gobs lighter and stiffer and warp resistant than solid.
Conventional built up construction but fully sheeted with 1/32” on a 45 degree bias. Top and bottom 45s opposite directions.That cross grain is a good idea but I wouldn't use 1/32. Warps too easy with the glue and CA will make sanding difficult. 1/16 and sand like hell.
That’s my two cents worth and you get what you pay for. ;D
Matt
Maybe 65 ounces when finished.
Well, if your weight budget is 65 ounces for the flaps, just machine them out of 2024 -- you'll come in much lower than your target, and they should be plenty stiff.
Conventional built up construction but fully sheeted with 1/32” on a 45 degree bias. Top and bottom 45s opposite directions.
That’s my two cents worth and you get what you pay for. ;D
Matt
That would not be a good idea as the stifness would not be same in both directions in torsion.I am not sure why having the grain "X" pattern is any different from having the glass in an "X". With a 1/2" tapered flap you are not going to get much torsion. I use the cross grain on things like rudders with good results. With sheeting you get a triangle shape that resists torsion twisting.
The structure must be symmetrical.
Fully sheeted with +/-45 degree glassfiber (I would use at least 50g/m2, not the lightest 25g stuff) helps a little, but carbon would be better.
With balsa only, best way to get stiffness is to make flap thicker. L
The difference is wood grain goes one way on top the other way on the bottom. Fiberglass grain goes both ways top and bottom.That was a brain fart on my part. Sorry, but I still do not get how putting the sheeting on the same way is going to produce uneven torsion.
If you're going to put the sheeting on at 45 degrees, you would want to put the grain the same direction both top and bottom since the flap is being driven from one end. Otherwise the side with the right grain would take the load in compression or tension and the other side would just get folded. You want the grain going from front inboard to rear outboard on both sides, right?
Motorman 8)
I think what we are finding here is that there is no one best way to skin this cat. I have just finished a set using the method I added above so I checked them for torsional twist and there is virtually none. Maybe a 1/16th of an inch. They are 3/8, 1/2 would be even stiffer. I can get that much twist in a solid sheet of 3/8. Neither have covering yet but one has 2 coats of sanding sealer and the other one is still naked. The sealer cuts the twist in half. Maybe the direction of the grain on the sheeting doesn't make that much difference (mine is parallel to the flap span) but I am willing to bet that the covering does.
I was going to post exactly this, this morning. I was thinking about it last night laying in bed and came to the same conclusion. Since this thread popped up, I keep thinking about building several flaps with different construction and compare them torsionally and weight. Also only considering construction, no glassing or covering. I think the sheeted blue foam would be stiffest especially if the sheeting is done on a 45 but it will likely also be the heaviest.
Matt
I’ve had a lot of aggravation in recent years trying to keep built up- sheet covered flaps straight for very long due to shrinkage of the covering and dope. I’ve replaced the flaps more than once on several airplanes. I’m now back to solid carved 3/8 medium balsa with a 3/32” bass trailing edge and NO covering material- just non- tautening dope. So far so good. I even had the issues with carbon covering though not as bad or soon.I flew a lot of hand launch glider back when my arm was not so ancient. We never covered the wings but we did put a glass like finish on them using lacquer sanding sealer followed by a thin coat of lacquer when you could no longer see the grain then rubbed out. Incredibly durable, didn't warp like dope and very light. Vibration would tear that apart on an IC but with electric these types of finishes are possible again. Curious, why brass vs carbon on the TE? Good idea though and I am going to shamelessly steal it on the set I have on the bench. Do you round out the TE to "embed" the tubing or just glue it on and fill in the trough?
Dave
The basswood is easy to work with, cheap and the LHS has it. A carbon strip would be OK too. Never thought about that application. Sorry if I made it unclear.I have a 3/32 carbon tube in the shop. I will try it and let you know how it works. I find myself using more basswood. Our LHS's carry it too, more than balsa and it is not that bad. 1/16 bass weighs about the same as 1/8 medium balsa. I have used it in fuselages and I don't see much wrong with it.
Dave
Ok, back to the first post, how to get best results without help of composites.
Two kind of unwanted deformation has been mentioned; deformation under stress, meaning torsional stifness and deformation during test of time, meaning warping.
The dilemma with balsa(or any)wood is that regardless of density, the individual wood cells are allways more or less the same. This means that in denser, heavier, wood there is more wood cells per volume unit.
Because of that, denser wood is more prone to warping, so that's the price you pay for using heavier wood to increase stifness.
And the other way, lighter wood is more stable but with lower stifness.
So the best way would be to use light, quarter grain cut wood and to keep thickness as thick as possible.
Reinforcing the trailing edge with something harder, carbon or hardwood, is nice especially against handling damage but as it is located at the central axis of flap, it won't do much to increase stifness.
If you really want to do it right, the best way is to use skin with modulus of elasticity higher than balsa's, kept solidly apart from each others with a lightweight core. The core can be dense ribs or some lightweight foam, like Rohacell, or if you want to go a step further, honeycomb.
The skin could be of thin balsa/glassfiber laminate, thin plywood or thin carbon sheet. There are really nice thin carbon products easily available, starting from under 50g/m2 for a nice biaxial product. Please don't confuse them with carbon veil, the veil has nothing to do with correct use of carbon fiber. L
Good question, Jerry.
I mean, in so many other categories of aeromodelling it's more like natural evolution, and has caused many revolutions. Basically new technology comes and walks over old, no matter how well the old has been made.
Of course, especially in free flight we go quite deep into the newest technology, cnc'd metal molds etc, in things like stunt flaps it does not have to be so complex for excellent results. L
Without using carbon fiber which is the best design for flaps that are roughly 3" at the big end 1.5" at the tip and 28 inches long? The leading edge is 1/2 inch thick. Lightness is needed too. They are for a .60 size profile stunter. Maybe 65 ounces when finished.
If you want the stiffest, lightest flap don't taper it. Make it constant thickness, and glass it top and bottom with bias fiberglass.
Rectangular sections are stiffer than triangular sections.
What effect does more flap area create?
How is "veil" different than "mat"?
Edit: I have been wondering about wrapping a flap in a hunk of that "CF braided sleeve" stuff. I guess the angle of the weave would depend on the size of the sleeve and taper of the flap, so maybe not so wonderful.
It would be nice if 4" wide tape was made on the bias.
Another rather obvious way to prevent twist is to drive the flaps from both ends - haven't seen this done but it is possible.Then we would have to start another thread to deal with "Center Sag" LL~
OR you could build your flaps like Howard.
Home made +/- 45 degree carbon tube LE. Very rigid in torsion. Very wimpy in bending, which is exactly what you want.
Maybe he will elaborate on his construction process.....
Googled for both Textreme and Carboline. From Sweden and Russia, respectively, and I didn't see a way to order a couple of square meters of anything on either's website. I noticed that Textreme makes a version of their product in a 45+/- fabric, which would be splendid, but obtaining this stuff might be expensive or nearly impossible.
I'll have to check US sources for composite materials and see if I can find them here, or something similar. I've only dealt with CST in the past, and don't know the name of the one in the LA area. If anybody can post the links for those two, please do. My old tower has those links, but apparently needs a new power supply, so until I can get that fixed, I'm having to use DuckDuckGo or Google search engines. Wondering why they are not called "search motors", since they're friggin' electric? ;) Steve
Here's what you might try. 2 layers of unidirectional CF at opposite 45 degrees bias. Expensive but light and strong.
https://shop1.r-g.de/en/art/192029060-CBA310
Here's what you might try. 2 layers of unidirectional CF at opposite 45 degrees bias. Expensive but light and strong.
https://shop1.r-g.de/en/art/192029060-CBA310
I understand what you mean, but that sounds like you want the wing to flex:)
I am absolutely fascinated by all of the responses that indicate that the first four words in the original post have no meaning.Maybe Perry knew that if you get 10 of us in a room you will have 12 different opinions and just wanted a good laugh.
I understand what you mean, but that sounds like you want the wing to flex:)
Not what I meant. Fact is the wing will flex. If you build an infinitely rigid flap it will keep the wing trailing edge from deflecting. The hinges that connect the two will attempt to transmit the forces necessary to make the wing comply to the flap. Hinge loads increase as does the friction in those hinges and it will increase the moment required at the bellcrank to move the control surfaces. None of this is what you want.If I understand your argument, you are saying that flaps should be chord wise stiff and span wise flexible with as little torsion twisting as possible. This all but eliminates carbon and makes any form of angled rib (like I currently build) built up problematic. As far as I can see, only a moderately thick flat solid flap with a traditional covering fits that definition. Sometimes the easiest to build is actually the best.
Instead, make the flaps wimpy in bending and rigid as heck in torsion, and the hinge loads are minimized, and the flaps don't twist the wing.
One way to do that is with a Howard torque tube, or....WITHOUT carbon, make the flaps out of 1/4" flat sheet (no chord wise taper) made with "perfect" "C" grain balsa. The last Predator had those and they are more rigid than a 3/8" tapered sheet or built up tapered flaps. With the 1/4" wood, they are not very rigid in bending also. Good wood is hard to beat.
" Actual fabric would cost a lot, and putting it on a bias isn't very cost effective."
This all but eliminates carbon and makes any form of angled rib (like I currently build) built up problematic.
Ken
.... I was trying to answer the question he asked .....I didn't think we did that anymore! LL~