[NSRCA-discussion] Airplane angle of attack

[Ed White]

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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