ORIGINAL: Siefring
Great Discussion. I understand the definitions of incidence, but don't completely understand the effect of the "tail" incidence. First, can someone define "Angle-of-Attack" stability or "Balance"?
Next,
Based on some more calculations (after all he has gone through 4 years of college and 10 years experience and is supposed to know this stuff) the aero guy says for stability and trim he has determined that the tail needs to be at an Incidence Angle (IA) with respect to the wing of 6 degrees. .... then the tail must be at a -2 degrees.
Why -2 degrees? I assume for a cargo plane the cg is in front of the wing center of lift, so the tail has to push down when in level flight to keep the plane from rotating. Is that all there is to it?
Does the downthrust of the engine effect where the WL or FRL are in level flight?
Carl
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Yes, it's not much more complicated than that. The tail's down lifting opposes the wing's natural nose-down moment, which is a result of the wing's camber.
The actual value for the tail's flying angle relative to the wing depends on a whole herd of things: speed, weight, moment arms, configuration, but the tail is built-on usually to have the least drag at the most economical cruise speed.
In Ben's example of the FRL's position, on the Tristar the floor was set to zero, when the plane was cruising at .92 Mach. The "fuel crunch" of 1972 forced the planes to fly at no more than .85 Mach, which raised the floor angle, and made pushing the food carts a chore for the gals.
The Tristar horizontal is all-flying, so there's no problem "streamlining" it as there is with a fixed horizontal and elevator, with a conventional layout.