[NSRCA-discussion] Airplane angle of attack

[Matthew Frederick]

I only read the first sentence that Ed wrote and felt like chiming in. I have to agree with Ed here in the assessment that "It's not that simple." The airfoil has to produce lift... PERIOD. If the airfoil is symmetrical the only way it can accomplish that task is with a positive angle of attack. The flatter the airfoil is on the bottom, the less positive angle of attack required. This, however, has NOTHING to do with the attitude of the aircraft's nose as long as the fuselage is not a required component of lift (which I hope and pray it's not). Think of it like this: most of us have adjustable incidences on our wings and stabs. You can go ahead and move your wing, stab, and thrust line to a completely new line, and as long as the relative movement of all surfaces was the same compared to the center of mass of the aircraft the plane will fly the exact same in straight and level flight. The one big difference you might notice, however, is that if you put more positive incidence in your wing, your airplane will appear to fly nose-down at the same airspeed that it used to look level, and vice versa. For those of you having trouble with that "vice versa" thing... the plane will appear nose-up if you put everything negative. Granted, the question asked was regarding airliners, and I can't really speak for that better than Ed did other than to agree that it is a delicate balance of form and function. I'd much rather just talk about our small planes... much easier to diagnose. I don't know about you, but I prefer to have my aircraft to appear to be flying straight (not nose-high) when I make a level pass. Considering most of our aircraft have pretty much symmetrical airfoils, that might give everyone some insight into what goes into designing pattern aircraft. Remember the first people setting up with 0-0-0 on their airplanes? I don't know about you, but to me they never looked level... until the fuselage design was changed of course. Anyway, guess I'm done for the night...

Matt
  ----- Original Message ----- 
  From: Ed White 
  To: NSRCA Mailing List 
  Sent: Sunday, September 30, 2007 12:33 PM
  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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