Newsgroups: sci.aeronautics.airliners
From: David Lednicer <dave@amiwest.com>
Subject: Re: Nose high during cruise
Date: 10 Mar 94 13:07:25 PST

	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!



-------------------------------------------------------------------
David Lednicer             | "Applied Computational Fluid Dynamics"
Analytical Methods, Inc.   |   email:   dave@amiwest.com
2133 152nd Ave NE          |   tel:     (206) 643-9090
Redmond, WA  98052         |   fax:     (206) 746-1299

Newsgroups: sci.aeronautics.airliners
From: shevell@leland.stanford.edu (Richard Shevell)
Subject: Re: Rear Engined Aircraft
Date: 04 Jan 95 00:40:58

In article <airliners.1995.8@ohare.Chicago.COM>,
drinkard@bcstec.ca.boeing.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.
--
Richard Shevell
Email: shevell@leland.stanford.edu

Newsgroups: sci.aeronautics.airliners
From: shevell@leland.stanford.edu (Richard Shevell)
Subject: Re: Rear Engined Aircraft
Date: 16 Jan 95 21:39:09

In article <airliners.1995.27@ohare.Chicago.COM>, pravelin@us.oracle.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
Email: shevell@leland.stanford.edu