[NSRCA-discussion] Airplane angle of attack

[chris moon]

The optimum cruise angle of attack for jetliners is somewhere between 

2.5 and 5 degrees nose up.  Usually closer to 2.5 or 3 degrees for an 

econ cruise.  As fuel burns off and the gross weight goes down, the 

airplane will need a lower  angle of attack to maintain flight which 

will take us away from our optimum  angle (lower).  So, we will either 

climb to where the air is "thinner" and require a higher aoa (angle of 

attack) to get us back to the 2.5 or 3 degrees or, slow down and 

maintain the lower altitude thus requiring us to increase the aoa back 

to optimum.  The answer to your question is yes, a jetliner flies at a 

nose high aoa in cruise.  Lift from the fuselage would probably be 

negligible other than "impact" lift - the force of the relative wind 

against the raised fuselage bottom.



Chris



Jim Alberico wrote:

> 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 /* 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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>

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