Re: Nose high during cruise

From:         David Lednicer <dave@amiwest.com>
Date:         10 Mar 94 13:07:25 PST
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	There has been speculation in the "Nose high during cruise" 
discussion as to the reason for this phenomona.  Its really rather 
simple.  At the beginning of a flight, especially a long range one, the 
airliner is rather heavy, particularly with fuel.  To generate the lift 
necessary to maintain altitude, it most fly at a certain angle of 
attack.  As the fuel burns off, the lift necessary decreases and the 
angle of attack decreases.  The angle that the wings are at (incidence) 
relative to the fuselage is determined by defining a "typical" flight 
condition, but as one might imagine, aircraft only fly at this "typical" 
condition for short periods, hence you can sense a nose up or nose down 
floor angle.

	Incidentally, changing the trim tab setting on an aircraft does 
not affect the stability - it only changes the elevator hinge moment and 
stick force.
        
        The Boeing 727 is notorious for cruising rather nose up.  
This led to the famous "bootleg" manuver that those TWA pilots got caught 
using over Michigan.  In cruise, 727 pilots would throw the breakers on the 
slats and then put the flaps out 5 deg or so to decrease the angle of 
attack necessary to generate the lift required and thereby decrease the 
floor angle.  Legend has it that on the TWA flight, the flight engineer 
didn't know what was going on and flipped the breakers for the slats back 
on, leading to slat deployment in cruise.  The pilots then tried to 
retract the slats, but one stuck from the high airloads, resulting in a 
wild ride before it ripped off.  The airplane landed safely at Detroit 
Metro and the cockpit voice recorder was found to be erased!  Some of the 
debris landed in one of my friend's front yard.

	When we helped Valsan add winglets to the 727, we ran into a 
problem with the loads, so we fell back on the same trick.  We found that 
by rigging the flaps several degrees down, we could move the loads back 
in where they needed to be and at the same time we gained a slight fuel 
burn decrease.  Currently there are two wingletted 727s in Delta's fleet 
and several privately owned ones (I think one is the Limited's - not only 
do they have winglets, but they also have the Valsan reengining on a a 
727-100 - giving them one hot ship!).

ps - I haven't been getting airliners for a while - it seems that some 
weeny deleted my subscription!
				


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David Lednicer             | "Applied Computational Fluid Dynamics"
Analytical Methods, Inc.   |   email:   dave@amiwest.com
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