Re: Rear Engined Aircraft

From: (Paul Raveling)
Organization: Oracle Corporation. Redwood Shores, CA
Date:         12 Jan 95 01:56:04 
References:   1 2 3 4 5 6
Followups:    1
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In article <airliners.1995.12@ohare.Chicago.COM>, (Richard Shevell) writes:
|> In article <airliners.1995.8@ohare.Chicago.COM>,
|> (Terrell D. Drinkard) wrote:
|> > Well, I don't want to argue abstruse technical definitions with you (mostly
|> > because they differ from company to company).  However, studies have shown
|> > that 3 degrees is about all that the cabin crew will tolerate, and that is
|> > what we design to, along with the aero group's demand for additional cheap
|> > lift.  The floor is NOT level in cruise.  Check it out next time you fly,
|> > but you may need to bring something like a carpenter's level with you.
|> Not only definitions but design policies differ from company to company. 
|> At Douglas we usually designed for a level floor.  After the increased
|> emphasis on fuel economy in the 70's, many aircraft flew at a higher lift
|> coefficient than the original design assumed so that the floors often had a
|> positive angle on cruise. 

	Designing for a level floor seems a bit surprising.  The two
	main benefits of a positive deck angle in cruise are...

	--  The fuselage generates a nose-up pitch moment; this
	    reduces the usual download that the horizontal tail
	    must produce.  That in turn decreases trim drag and
	    allows designing a smaller, lighter horizontal stabilizer.

	--  At a positive deck angle the fuselage generates some lift
	    and brings the spanwise pressure distribution over the
	    wing root area closer to the ideal elliptical distribution.
	    Designers can use deck angle to trade off lift over the
	    fuselage for lift generated on outer wing sections; the
	    latter necessarily adds structural weight.

	The best bottom line summary I know of are the excerpts below
	from a 12-page letter that Lockheed wrote to Eastern Airlines
	in 1974 when Eastern wondered why the L-1011 couldn't have
	a level floor, or at least a lower deck angle.

	  "In summary, the total drag increase associated with the
	  effect of fuselage rotation is equal to 2.2% of total
	  airplane drag for each 1 degree that the floor angle
	  is decreased."


	  "In summary, on the L-1011 the structural penalties
	  associated with each 1 degree reduction in floor attitude
	  is about 900 pounds..."

	The letter cites annual fuel burn of 7.5 million gallons
	per year, costing $.22 per gallon (in 1974!)e, notes 25
	L-1011's in Eastern's fleet, and finally concludes:

	  "Therefore a 6.6% increase in fuel burned - due to reducing
	  the floor angle in cruise by 3 degrees - results in an increase
	  in fuel costs of 2.7 million dollars just due to drag effects.

	  "Because of the increased structural weight, the airplane
	  is now heavier (when serving the same route with the same
	  passengers as before and, therefore, drag is increased since
	  more wing LIFT is required).  This adds almost another one
	  percent drag and the total fuel bill increases by about
	  3.0 million dollars per year.

	  "The 3.0 million dollar operating cost penalty is, of course,
	  associated with changing the fuselage floor angle on the
	  L-1011 airplane as it is presently defined.  However,
	  incorporating a 2,700 pound weight penalty and a 6.6%
	  increase in drag would reduce the range of the airplane
	  at full passenger payload by 220 nautical miles.  Assuming
	  that it would have been necessary to maintain the range
	  of the present airplane (not off-load passengers), the airplane's
	  maximum Gross Weight would have to be increased by approximately
	  another 10,000 pounds weight with an additional 1,500 pounds
	  of structural weight to provide the capability for this
	  higher design weight.  Obviously, at these higher weights
	  if it were required to retain the present field length
	  characteristics, the wing area and/or the thrust of the
	  engine would have to be increased.  Obviously, these factors
	  would cause even further increases in the initial price
	  and operating costs."

Paul Raveling