RCU Forums - View Single Post - Thrust line, Datum line, Center line?

05-22-2009 | 10:24 AM

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CrateCruncher

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RE: Thrust line, Datum line, Center line?

Buzzard bait writes:<div>Reducing incidence absolutely decreases downthrust. That's not even aerodynamics, it's just geometry. Bruce, the moderator on the Aerodynamics forum got it right. Read his post. You can also read the following passage and try to picture it. It really isn't anything profound or difficult once you see it...</div><div>Gordon Whitehead, in 'Scale Models for Everyday Flying', discusses the Sopwith Triplane as an example,which rigging diagrams show with the wing at +2 degrees, stab at +1 1/2degrees, and engine at 0. "Note that if we set as per the full-sizeTriplane we have effectively generated about 1 1/2 degrees downthrust on theengine, which will reduce the amount of any extra non-scale downthrust we mayneed to use." Note that he goes by the stab, not the wing, but they are very close.

Finally once Bob takes your advice, I suggest he do a simple, honest test. He should go back to the original problem comparing apples to apples. He said, "i can get it to fly perfect at 1/4 throttle then you give it gas and it rockets up at about a 60* angle". So after making the adjustment you recommended, he should fly the Cub at 1/4 throttle and trim it out to fly level that way, just as before. Now when he gives it gas again, what happens? Does it speed up straight and level, or does it climb again just as it did before?For books I use Whitehead, McCombs and Simmons mainly. What do you use?JimBuzzard your obviously unconvinced by my prior post(s). I decided to move this back to the aerodynamics forum because it's confusing people in the scale forum and further discussion seems more appropriate here.MYTH: "Reducing wing incidence reduces prop downthrust."Airplane pitchin level flight is determined by the summation of moments = 0 about the center of mass (of the airplane). It's pretty much irreleventwhat the wing incidence is (recall wing lift has no moment arm)with respect to the datum line because the moment arm for the prop thrust is stillthe line between the prop center and the airplanes center of mass. The thrust angle has not changed with respect to that moment line when the wing incidence is changed.

Ifyou have a copy of Simons, review pages 1-5 "Fundamentals". Then take a look at the car problem at the bottom of en.wikibooks.org/wiki/Statics/Two-Dimensional_Equilibrium

Then take a look at the diagram I attached. Notice the wing incidence has nothing to do with the angle of fuselage equilibrium. The center of mass acts like a fulcrum between the thrust component in the y direction and the - S lift generated at the stabilizer. From this its easy to see why the downthrust prevents the plane from gaining altitude when the throttle is opened.

When the thrust T increases the plane accelerates to a new constant speed as drag D increases to the same magnitude. However, wing lift L does not increase with speed (V^2) because the y component of thrust increases causing an angular acceleration (a slight pitch rotation) which doesn't stop until the stabilizer counters it with increasing lift of it's own. The only way for the stab to increase lift without a pilot input is to increase the angle of attack (the tail rises) which results in the wing coming to equilibrium at a new (reduced) angle of attack. When the plane isproperly trimmed this results in level flight at all throttle settings.

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