Re: Rear Engined Aircraft

From: (Richard Shevell)
Organization: Stanford University, Dept. of Aero/Astro
Date:         16 Jan 95 21:39:09 
References:   1 2 3 4 5 6 7
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In article <airliners.1995.27@ohare.Chicago.COM>,
(Paul Raveling) wrote:

> 	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.

The logic given above is fine.  Both trim drag and induced drag would be
favorably affected by a higher fuselage angle of attack (i.e. less
incidence).  But I think the magnitudes in the Lockheed letter are much too
hish.  Total trim drag is only of the order of 2% so a small fractional
saving would be much less than that.  The lift of the fuselage due to 3
degrees is also small so the induced drag savings are minute especially
when the fact that fuselage lift is very inefficient is considered.  Tail
weights are set by extreme maneuvering or gust conditions so reducing the
cruise trim load may not count for much.  The Lockheed values of 2.2% drag
and 900 lb. of weight for each degree of fuselage angle are grossly high. 
Now, a confession:  It's been a few years, like 25, since I did such an
analysis and the above is a gut feeling, but I believe it is a good one. 
Perhaps more on this later.

Also Torenbeek says, pg.259, "Wing setting during cruising flight ---is
usually chosen such that the cabin floor will be horizontal."
Richard Shevell