stunthanger.com
Design => Stunt design => Topic started by: Tim Wescott on January 16, 2011, 04:11:38 PM
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Someone asked me months ago if I had plans to make plans for the Waiex semi-scale. Well -- I didn't, but if there's someone out there nutty enough to ask me for plans for a control line stunt airplane, I'll draw them up.
This is not what I have built -- this is what I'm going to build next time, as refinement. The nose is considerably longer, the wing is a hair shorter, the stab tips are curved to match the original plane instead of squared off. The plans do not yet show any nose doublers, but that should be obvious. The only real performance modification is the nose -- I should be able to throw away almost four ounces of nose weight, if I guessed my math right.
Anyone who wants to subject this to a design review -- feel free. My expectation is that this thing should have performance somewhere between 'good' and 'mediocre', with some extra sensitivity to wind thrown in for free because of the Hershey-bar wing and the tips (which are slanted in at 45 degrees, per the prototype).
Edit: Note that work is ongoing. I've added a bunch of annotations, called out the fuselage doublers, and I'm about to put in the trailing edge sheeting that I've been forgetting. The file size has grown by a factor of 2 so I can't post the pdf here -- if you're interested I'll email you a copy, though.
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Great looking design Tim!
I REALLY like the way you disigned the ribs to encase the bellcrank mounts.
WELL DONE! y1
"Tight lines!"
Wes
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Looks very good, Tim. Nice airfoil, and all looks "right".
The "TLAR" method of things really isn't bad if you have seen a bunch of CLPA designs.
Big Bear
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Looks very good, Tim. Nice airfoil, and all looks "right".
The "TLAR" method of things really isn't bad if you have seen a bunch of CLPA designs.
I looked at a lot of plans in Flying Models to settle on the specs for that. I know that's the way to get a plane with average performance instead of outstanding, but how else will I learn to design these things?
It certainly seems to fly well enough, but I am hoping to get it into the hands of an expert or two for comments this year.
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Tim,
Everything is looking right in the ballpark. Good job, thus far. I do have a couple of suggestions before you start cutting wood, however.
One very important thing is missing from the plan...the planned Center of Gravity. Based on my measurements (more or less "modern standard" 25% of the wing area tail, a classic 5/1 aspect ratio on the wing and a tail moment in the noise range), I'd suggest you plan the CG right on the main spar or slightly forward (not more than about 1/2" forward, however. If the airplane seems to respond too quickly for you with the CG there the problem will most likely be a too sensitive control setup...easily cured with external horns). That is right at 25% of the chord. Ideally, the Bellcrank would be mounted at the spar or just slightly forward of it. I'd opt to make the B/C mount extend forward of the spar just far enough to allow a secure mounting of the nuts and washers without cutting away any of the main spar and locate the B/C there.
I suggest forward of the CG because 25% of the chord is probably as far aft as you'll ever want to trim the CG and you could find yourself happier with a CG anywhere between 20 and 25% when all is said and done. Having the B/C in the suggested position will optimize the track of the leadouts through the leadout guide to minimize drag on the guide. I'd also strongly suggest you sketch in the final location of the B/C and the leadouts as plotted and plan the leadout holes in the inboard ribs accordingly. There's nothing worse than finding the perfect location for the leadouts only to discover the rib holes won't allow you to put 'em there!
Plan your leadout guide to allow for the center of their exit to be between the wing spar and 1-1/2" inches aft of it. Plan to locate them for first flights with their center at roughly 1-1/8" aft of your starting CG when balancing the model with a finger at each wingtip. As you play with the CG always remember to track fore and aft movement of the CG with the leadout position. My personal preference is to have the leadouts close together, only about 1/2" apart from each other as they exit the tip.
That airfoil looks like you lifted it from Al Rabe! Am I right?
Ted Fancher
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Tim,
Everything is looking right in the ballpark. Good job, thus far. I do have a couple of suggestions before you start cutting wood, however.
One very important thing is missing from the plan...the planned Center of Gravity. Based on my measurements (more or less "modern standard" 25% of the wing area tail, a classic 5/1 aspect ratio on the wing and a tail moment in the noise range), I'd suggest you plan the CG right on the main spar or slightly forward (not more than about 1/2" forward, however. If the airplane seems to respond too quickly for you with the CG there the problem will most likely be a too sensitive control setup...easily cured with external horns). That is right at 25% of the chord. Ideally, the Bellcrank would be mounted at the spar or just slightly forward of it. I'd opt to make the B/C mount extend forward of the spar just far enough to allow a secure mounting of the nuts and washers without cutting away any of the main spar and locate the B/C there.
I suggest forward of the CG because 25% of the chord is probably as far aft as you'll ever want to trim the CG and you could find yourself happier with a CG anywhere between 20 and 25% when all is said and done. Having the B/C in the suggested position will optimize the track of the leadouts through the leadout guide to minimize drag on the guide. I'd also strongly suggest you sketch in the final location of the B/C and the leadouts as plotted and plan the leadout holes in the inboard ribs accordingly. There's nothing worse than finding the perfect location for the leadouts only to discover the rib holes won't allow you to put 'em there!
Plan your leadout guide to allow for the center of their exit to be between the wing spar and 1-1/2" inches aft of it. Plan to locate them for first flights with their center at roughly 1-1/8" aft of your starting CG when balancing the model with a finger at each wingtip. As you play with the CG always remember to track fore and aft movement of the CG with the leadout position. My personal preference is to have the leadouts close together, only about 1/2" apart from each other as they exit the tip.
That airfoil looks like you lifted it from Al Rabe! Am I right?
Ted:
Thanks for the critique.
The prototype flies with the CG right about where you suggest (except it needs a ton of weight in the nose, due to a miscalculation on my part in the weight & balance department -- I've got a nifty, and totally empty, weight box under the tail, and nearly 4oz of brass in the nose looking back and sneering). I do need to measure exactly where the prototype's CG is and put the CG range on the plans -- I hate it when folks leave that out, so how come I do it???
(Note that the nose weight problem should be solved in this version, or close to it -- I've lengthened the nose about 2-1/2 inches, and the prototype used solid wood for the fuse. So between the longer nose in front and a truss structure fuse in the back, I hope to see the need for just enough tail weight that you know you've got adjustment range).
(Come to think of it, I should call out the weight box structure in the right tip. Hmm.)
I'm going to have to think about the bellcrank location and the lightening holes. It's somewhat dictated by the location of the jig wires, which I put as far forward as I could to make the wing as straight as possible. I don't like the idea of compromising on the ease of achieving a straight structure (nor, for that matter, do I want to make a new rib template), but you've got a heck of a good point about the leadout rigging. So all the planning that you suggested was done, except for really working to minimize the bend at the leadout.
(Dang it, I'm not calling out the shear webs for the D-tube structure either).
The airfoil may be a spiritual descendant of Al Rabe's shoe, but it's not copied from anything of his. I did kind of follow the current trend that seems to call for a good thick airfoil with a nice blunt leading edge, but after that I let my CAD tool's limitations dictate the details. It's an elliptical arc through the leading edge to the back of the spar, and one circular arc each on top & bottom from there back to the trailing edge. This because my CAD program really can't do Bezier curves properly, not because of any deep thought on my part.