Fly by Wire questio0n

[Chris Scott]

Thanks for your point on the inherent limitation of canard main-plane lift (CLMAX), Brian Abraham. I guess it doesn't matter at cruise speeds, e.g., VMIN-DRAG or above, where main-plane alpha would be only 2 - 5 degrees?

fantom has identified at least one point that I don't follow in Mad (Flt) Scientist's admonition of my pitch-stability argument. M(F)S seems to be overlooking the fundamental fact that, in both canards and conventional aeroplanes (forgetting hybrids, like the Gripen), the C of G is normally FORWARD of the centre of lift of the mainplane, assuming natural pitch stability is required.

So on a conventional aeroplane at 1G (one with what I have called a tailplane), a CofG near the aft limit means the download on the tailplane is lower, the mainplane lift for 1G flight is (consequently) also lower, the fuel consumption is lower, BUT the A/C is less stable in pitch – as fantom points out. [It's noticeable, I can assure you.]

Continuing with the 1G case; as the CofG goes forward, the tailplane download increases until, ultimately, it will stall and the aeroplane will depart in bunt. This would happen sooner with a small, efficient tailplane.

On a canard, the aeroplane presumably also becomes more stable as the CofG goes to the forward end of the envelope; the upload on the foreplane being higher. So in both types of configuration, I propose: maximum pitch stability is achieved when the stabiliser load is high; minimum stability when it's low; instability when its load (lift direction) is reversed.

STICKING WITH CONVENTIONAL AIRLINERS

Quote from M(F)S:
take a REAL aircraft. Trim in 1 'g' flight. There's probably a small download on the tail, especially if we have a fairly aft cg... [Unquote]

An aft CofG REDUCES the download that the tailplane needs to generate for 1G flight.

Quote from M(F)S:
...now start to bunt. Push forward on the stick. The load on the tail reduces and eventually will be negative. The aircraft doesn't suddenly become unstable when the sign of the tail load changes. [Unquote]

It's normally negative, so I presume you meant positive? It may reverse and become positive, but only because of the down elevator. As soon as the elevator is neutralised, the tailplane will produce negative lift (download) again, restoring pitch stability. If you maintain down elevator and existing stab trim, the aircraft will obviously continue to fly at less than 1G (i.e., bunt).

Broadly speaking: for a given IAS, slat/flap/gear configuration, thrust setting (i.e., thrust pitching-moment), and CofG position, there is only one tailplane (stab trim) setting for 1G flight, unless you are happy with a constant elevator input. [The trick is to find it.]