da Rock
Posts: 6707
Joined: 10/11/2005
From: western,
NC, USA
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quote:
ORIGINAL: Strat2003
I've read that most of the drag on a control line plane is from the lines. Any drag savings from a cowl would be a relatively small percentage of the total system drag, so you would'nt expect a big jump in speed. That same drag reduction on an R/C plane would be a larger percentage of the total drag, so a noticable increase might be expected.
Yeah? 
In theory, practice and theory are equal. In practice, they are not. 
Think about the idea that the lines limit the speed. The lines at the handle are creating how much drag? Maybe as much as your arm does? But it's not moving fast at all right. So figure how much of the lines actually affect the airplane's speed, and then start reducing that drag exponentially as you work the theoretical calculations back toward your arm. Yeah, the lines create drag, but it's what it is and the rats were going easily faster than the first R/C racers of that era with the same size engines. If I remember correctly, the junker rat we started with did 160 and everybody passed it but the kid flying the 1/2A stunter. Don't let interesting thoughts that really aren't applicable confuse the issue. Trust me, when we found significant drag reduction we saw measurable improvements.
BTW, keep in mind that the rats didn't really have to deal with the centrifugal forces the R/C racers do. The rat wing never has to contribute drag (lift) like an R/C plane does at least twice a lap and over half it's flight envelope. The rat flight envelope doesn't actually see anything like induced ANYTHING except when it's coming in to land. So let's not try to compare apples with watermellons.
BTW, you might think I'm making fun of your argument. Sorry if you do. I'm not. But it would be good to take it in parts and look closely at it since it seems to refute an earlier suggestion about using C/L observations.
It really wasn't written that most of the drag on a control line plane was from the lines. Actually only the lines out near the plane contribute the drag contributed by the lines. And in actual practice, if you look at the tail rigging on some of the 46size ARFs today that have those wires and such screwed between the vertical fin and the stabs and the stabs and the lower fuselage, think about how much wire and crap that is. And it's out at the airplane, so to speak. Whereas the control lines start at the inside wingtip and head into the center of the circle. And on a rat racer, they were .018 lines and exited the tip one directly in front of the other.
Also, the rat racer wing really had nothing to do compared to an R/C airplane if you considered the L/D deal. The rat flies at basically the zero lift angle. It really doesn't have to carry much weight at all to begin with. And after half a lap it's almost at top speed and has the rest of the time to run almost perfectly level. Even when you have a race winner on the end of your lines, you don't actually change the pitch of the rat more than miniscule bits of degrees to get over the slower plane and then your rat is DIVING..... So don't worry about the induced drag from the rat wing doing any appreciable lifting.
And the rat doesn't even have to carry but half of the weight of the lines. And the lines don't weigh much to begin with. And if the rat pilot was like me, he was "whipping" like a mad man and that unloads all the way to the rat's outboard wingtip.
Actual observation shows that the lines do contribute drag, but nothing really significant or at least more significant than anything else on the airplane. And it also showed that cowling an engine probably produces more benefit from the cooling than from the drag reduction. With C/L it's fairly easy to quantify. With R/C it'd probably be nearly impossible for weekend modelers.
< Message edited by da Rock -- 3/13/2008 7:14:10 PM >
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building a speed cowl - 
