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Building Tips and technical articles. => Building techniques => Topic started by: Terry Bolin on November 13, 2006, 01:11:08 PM
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We had our first C/L Fun fly in Neosho Mo. this last Saturday with Good weather (Shock!) and a good turn out but my question is: Jack Docker from Pittsburg Kansas had a Brodak Arf that he flew. Why is it that that plane has no rudder off set and he installed the motor with no off set either? I can understand the rudder because of airfoil like the Nobler but how does no engine off set work out? if the Fuse off set a little?
Thanks. Terry Bolin
By the way, that plane flew sooooo smooth and had a Brodak 40 to boot!
(cardinal, I think)
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No offset in either rudder or engine is fairly common these days. If the leadouts are in the right place and the tip weight is about right, no problem. Offset engine seems to work better than offset rudder.
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Okay Jim, Thanks for that answer so let me understand the next level.
Since I am scratch building most of the CL planes I want now days, what is the most common location for the Bellcrank?......CG.....befor or aft? And is there a common starting place for the lead outs?
Thank you for the time you invest!
Terry Bolin
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Bellcrank location is of little importance, it's entirely the leadout position that makes it work...or not. Still, you want to keep from bending the leadouts, because that's wear & tear, and takes some force to do. The CG is therefore the perfect location, but then that gets adjusted, sometimes a lot. Finally, you need some room, and as much strength as you can find in there, so the spar is the final answer. Just ahead, just behind, whichever fits best.
Engine and rudder offset are pretty much old fashioned notions that have been proven to be a detriment. What you want is constant line tension. To do that, you need to keep the speed constant. A good engine helps, but so does making the plane fly tangent to the circle...less drag. Props make a ton of difference!
n~ Steve
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For older designs like the Twister, Flite Streak, Magician, etc. CG at 15% mean chord works pretty good. More modern designs with larger tails and longer tail moments can have the CG further back. I start out with the leadouts a little bit back of the CG and move them 1/8 in, or less, at a time until I get them right. There is a method of calculating where they should be, but I haven't used it. Basically if the airplane is light on the lines up high, the leadouts are too far back. Also you want your leadouts coming out as close together as you feel comfortable with.
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AmberT,
Note: all the designs Jim T mentioned are w/o operating flaps. Seems a "slightly forward" CG goes well with unflapped models, while flaps and large% tails go nicely with CG further aft.
There is a 'natural' offset compared to the air we fly in when the fuselage has thrust line and vertical fin/rudder area all in a straight line, structurally.
We want as little of that offset as possible - which to me means we want the fuselage centerline perpendicular AT the CG to an imaginary line from the handle ... not the actual line traced by the lines, as they sag aft due to drag, and - much less - down due to their weight.
Since the model flies a curved path, the engine and the fin/rudder are actually pointed 'out' slightly in relation to the air they fly through, when the imaginary line to CG is perpendicular to the fuse C/L. The angle isn't much... about 2° or (usually) less at the prop, and about 2°+ at the fin/rudder. For many models that is plenty.
When the leadout locations 'aim' the pull force to the CG with the model aligned at the right angle mentioned above, leadouts actually will be behind the imaginary line I referred to.
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Lou, the Twister is flapped, and the Magician I built had operating flaps, although they fly fine without same. The point is not flap or not, but small tail volume on those older designs.
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Jim,
Agreed on the "why" (tail volume), and apologize for not catching that you mentioned a few designed-for-flaps types in your list.
To go the other way a bit, the quite aft CGs we see on many recent top level stunters goes with the MUCH larger tail volume that's become dominant. The horiz tail is as effective as stabilizer as as elevator, and has plenty of power in either function.