[NSRCA-discussion] Airplane angle of attack

[Jim Alberico]

What Ed said, but even more so....  ;-)   It's not even _that_ simple.
 
For optimum cruise, other factors enter in.   Best range is not necessarily
at max L/D, but usually close.   
 
Well done, Ed, for a structures guy.  ;-)
 
Jim

  _____  

From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of Ed White
Sent: Sunday, September 30, 2007 12:33 PM
To: NSRCA Mailing List
Subject: Re: [NSRCA-discussion] Airplane angle of attack


I work for Boeing, although in structures technology, not aerodynamics.  But
I work with the aero folks enough to know the answer they will give.  Which
will be, "Its not that simple."  I know this because that's the answer I get
to every such question I ask.

There are a lot of factors that will come into play in setting wing
incidence.  Where is the cg?  What pitch moment effect does the fuselage
lift have?  Both these affect how much tail down force is needed to maintain
trim conditions (which affects longitudinal stability but also generates
drag).  Then there is the wing-body interface.  A knowledgeable aero person
once described the flow at the wing-body interface as "problematic" (code
for we don't know for sure until we try it).  Then the fuselage is not a
pure cylinder, the nose is not axi-symmetric (because apparently pilots want
windows to see out of).  The area at the wing-body interface has bump outs
for wing carry through structure and other things, and the tail is usually
not placed on the centerline of the fuselage and the tail cone is also not
axi-symmetric to avoid tail strike on take-off.

All of this and a whole lot of other factors go into fuselage lift and drag.

The simple design objective is to maximize the lift to drag ratio for the
entire aircraft at cruise conditions.  The angle of attack of the fuselage
will be designed to meet that goal as best as possible and may not be 0, and
is likely different for different airplanes.

So now you are all aerodynamics experts.  All you need to know is the easy
to learn phrase, "Its not that simple."  Of course I can find you folks at
Boeing who will claim that when I am asked questions about my real
expertise, structural dynamics, I tend to use the same phrase.  But don't
believe them.

Ed

Jeff Hill <jh102649 at speakeasy.net> wrote: 

This is a question about full size airplanes that has some 
applicability to model design. We're talking about airliners that 
have an essentially cylindrical fuse.

I'm having a debate with a friend at work about whether or not full 
scale airliners fly slightly nose up. I claim they do he claims they 
don't.

I claim they do because the airflow would be more stable about a 
cylindrical body that was at a slight angle of attack, and that if 
you make it nose up you also gain a little lift.

He claims that airliners fly with no AOA in the fuse because the last 
thing a designer wants is lift from the fuse because lift generates 
drag, the fuse is not a good shape for generating lift, and 
consequently it isn't worth paying the drag penalty.

What do you all think?

Jeff Hill
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