Re: Nose high during cruise?

From:         raveling@netcom.com (Paul Raveling)
Organization: NETCOM On-line Communication Services (408 241-9760 guest)
Date:         11 Mar 94 00:56:48 PST
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Steve Derry (sdd@larc.nasa.gov) wrote:
: On many flights on jet airliners, I have noticed that during cruise the 
: aircraft appears to maintain a nose-up pitch angle.  ...
...
: I thought that for efficiency, airliners were designed with the 
: appropriate angle of incidence between wings and fuselage so that at 
: cruise angle of attack, the fuselage would be "level" with the oncoming 
: airstream to minimize drag.

	Actually the nose-high attitude is deliberate a deliberate measure
	to improve fuel economy.  If you look at the fuselage alone it adds
	a bit of drag, but that's far overshadowed by the improvement it makes
	in the wing/fuselage's span loading (spanwise pressure
	distribution).  An additional contribution is that a positive
	deck angle reduces the aircraft's nose-down pitching moment;
	that allows using a slightly smaller horizontal stabilizer and
	slightly lighter structure.

	If you look at an aircraft nose-on, the optimal spanwise lift
	distribution is elliptical.  That means it's a nice smooth curve
	even where it crosses the fuselage.  Typically there's a huge
	area of low pressure over the wings at their roots.  If the
	fuselage doesn't have an area of low pressure over it,  the
	airframe will pump energy into generating a vortex of sorts --
	well, at least some screwy airflow -- at the wing/fuselage
	junction.  To minimize this mess airliner designers deliberately
	gave the planes a significant positive deck angle in order to
	minimize this pressure discontinuity.

	The effect is quite significant on airliner economy.  When
	Eastern Airlines complained about the deck angle on their new
	L1011's in 1974, Lockeed responded with a letter whose second
	paragraph is this quote:

	    "Had Lockheed designed the L-1011 to fly with a level floor
	    in cruise, EAL's operating costs for their present fleet
	    of these airplanes would have increased by three million
	    dollars per year."

	That quote was based on a fleet of 25 L1011's and a fuel cost
	of 22 cents per gallon.

	Lockheed cited these penalties for leveling the fuselage:

	    "In summary, the total drag increase associated with the
	    effect of fuselage rotation is equal to 2.2% of total
	    aircraft drag for each 1 degree that the floor angle
	    is decreased.  ...  on the L-1011 the structural penalties
	    associated with each 1 degree reduction in floor attitude
	    is about 900 pounds..."


-------------------
Paul Raveling
Raveling@netcom.com