Re: Nose high during cruise?

From:         drinkard@bcstec.ca.boeing.com (Terrell D. Drinkard)
Organization: Boeing Commercial Airplane Group
Date:         20 Mar 94 22:30:00 PST
References:   1 2 3
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In article <airliners.1994.980@orchard.Chicago.COM>, sdd@larc.nasa.gov (Steve 
Derry) writes:
> 
> 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.
> 
> Why then do I notice this "uphill" effect?  Is the fuselage actually 
> pitched up slightly? 

I love the questions that come off this group.  I always end up learning
quite a bit myself.  Keep them coming!

I cornered one of the senior aero guys and sure enough, the aero fraternity
is responsible for the deck angle you've noticed.

The body, as a surface of revolution, develops a moment when flown through
the air at a positive angle of attack (nose up).  Inviscid effect.  If too
much angle of attack in used, the body begins to develop lift along with
the moment.  Viscous effect.  The body is incredibly inefficient at making
lift, so the angle of attack must be kept quite small.  Also, the cabin
crew has a heck of a time moving those serving carts uphill if the deck
angle is too large.  What does this moment buy us?
Reduction in trim drag through reduction in the amount of moment (lift
times tail arm) generated by the horizontal tail.  Less lift/moment
generated by the tail, less induced drag generated by the tail.

Terry

-- 
Terry
drinkard@bcstec.ca.boeing.com
"Anyone who thinks they can hold the company responsible for what I say has
more lawyers than sense."