[NSRCA-discussion] Airplane angle of attack

[chris moon]

Tried to post this before but it did not go through. 



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





Matthew Frederick wrote:

> 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 /* 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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