Re: hydraulic problems with DC-10s??

From:         greg@saltydog.dpsi.com (Gregory R. Travis)
Organization: Data Parallel Systems, Inc
Date:         10 Dec 92 16:07:13 PST
References:   1 2 3 4 5 6 7
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In <airliners.1992.121@ohare.Chicago.COM> barr@ash.mmm.ucar.EDU (Keith Barr) writes:

>greg@octopus.dpsi.com (Gregory R. Travis) writes:
>> BTW, just to be pedantic:  The wings do not each contribute exactly
>> 50% of the total lift.  Remember that that fuselage itself contributes
>> a SUBSTANTIAL amount of lift at cruise as do the horizontal stabilizer
>> surfaces (in certain flight regimes!).

>What flight regimes might those be?  Unstable flight, as in the F-16?

>Actually, in most aircraft the horizontal stabilizer is downlifting, so
>the wings have to create more lift, and the body contributes to the
>pitching moment, but contributes very little lift (unless we are talking
>about the B-2).

As I understand it (and I'm not an aero geek, just an historian geek :-))
there are flight regimes in which the horizontal stabilizer on conventional
(i.e. dynamically stable) aircraft contributes POSITIVE lift, even when
said aircraft is loaded within acceptable C.G. ranges.

It is possible (and likely) during slow flight for
the center of lift to move forward of the center of gravity on the wing.
At such time, the horizontal stabilizer becomes a LIFTING surface, restoring
normal stability to the aircraft and the pilot's never the wiser.

If you look at many horizontal stabilizers, especially on small GA aircraft,
you'll note that the airfoil is completely symmetric top and bottom.  If
the horizontal stabilizer contributed ONLY to negative lift, one would think
it more efficient to design it as an upside-down wing and shorten its overall
span.

I've talked to a couple of Cessna engineers about this (when I was completely
incredulous that it actually happened) and they agreed that the horizontal
stabilizer does contribute positive lift and that Cessna routinely measured
this lift with strain gauges when evaluating stabilizer structure.  The
lift is not present only during extreme maneuvering (such as a violent
pitch-down) but also during straight and level flight.

My good friend Moshe Braner (braner@emily.emba.uvm.edu) can give you a much
better technical explanation of the phenomena and, no doubt, correct all
my mis-explanations and errors.

greg
--
Gregory Reed Travis				D  P  S  I
Data Parallel Systems Incorporated   greg@dpsi.com (For MX mailers only!)
Bloomington, IN			     greg@indiana.edu (For the others)