[NSRCA-discussion] Airplane angle of attack

Thu Oct 4 05:33:06 AKDT 2007

It is possible to ruin an otherwise good design with the geometry of the thrustline/wing/stab placement as well as control surface geometry and hinging. Top ranked pilots usually can make most anything look good though.

It is not unusual for full scale airplanes to display the same airspeed for two different angles of attack. The AOA that requires the least power for a given airspeed is generally referred to as being "on the step".  If a pattern airplane is configured in a manner that these two operating points are close together the return to level flight presents an increased workload.  I believe the current trend of high volume fuselages makes it difficult to achieve an "on the step" condition.

Just my opinions of course...

John Ferrell    W8CCW
"Life is easier if you learn to plow 
       around the stumps"
http://DixieNC.US

  ----- Original Message ----- 
  From: White, Chris 
  To: NSRCA Mailing List 
  Sent: Wednesday, October 03, 2007 8:47 AM
  Subject: Re: [NSRCA-discussion] Airplane angle of attack

  Understanding that a lot of misinterpretation can happen in reading or talking about things without actually being directed in person on the subject article, I dismissed the following story. Now after this post it has me curious again and I would be interested to hear comments from people who may have been told the same.  

  The story comes as a result of a couple of local pilots who were working on a well-known $2K arf from 2005-6 era that had no reference lines on the fuse, nor measurements in the plans referring to Thrustline or any clue as to where to start on fuselage angle to engine-wing-stab setups.  Communication with the designer resulted in the customer being told that it should be done by appearance.  (eg: the way you wish to see your fuse angle in level flight cruise.)

  That seems very subject to interpretation and I would think if a person were a few degrees off it would make a significant difference on aerodynamic behavior during maneuvering. (mixing etc)

  There's probably a post on this somewhere, but Bob's comment led me to think of that setup dilemma.   I've never seen or heard of an airplane kit/arf without some reference to thrust line, until I heard my local friends told me about this one.

  Is this scenario familiar to anyone out there? 

  (Gee, do I push send or not...ok I'll send it.)

  Chris White

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  From: nsrca-discussion-bounces at lists.nsrca.org [mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of Bob Richards
  Sent: Wednesday, October 03, 2007 7:13 AM
  To: NSRCA Mailing List
  Subject: Re: [NSRCA-discussion] Airplane angle of attack

  I never meant to draw any conclusion about the knife edge performance of airliners or bombers in my original post. I was merely stating that the reason we PATTERN FLIERS adjust the incidences of the wing and stab (ON OUR PATTERN PLANES) has to do with aerobatic performance. As far as I know, the reason the designers of airliners, bombers, and most full-scale airplanes pick a incidence value has to do mainly with efficiency in cruise.

  Bob R.

  rcmaster199 at aol.com wrote:

    I remember watching a clip of a full size multi engine bomber type in test flight. The test pilot banked hard to knife edge near the ground (maybe 500 ft) for some unknown reason and swiftly proceeded to put it in. Don't remember the plane's or test pilot's names. 

    Full scale fuselages are designed to minimize drag as much as possible (for max range) so they tend to be pencil thin comparatively speaking. Pencil thin fuses do not fly knife flight well nor are they intended to do so. And the higher the weight the worse the problem. At risk of being glib, that test pilot found the outside of the envelope.

    MattK

    -----Original Message-----
    From: chris moon <cjm767driver at hotmail.com>
    To: nsrca-discussion at lists.nsrca.org
    Sent: Tue, Oct 2 4:28 PM
    Subject: Re: [NSRCA-discussion] Airplane angle of attack

    The optimum AOA on airliners is a function of wing design. It's the 
    operator's job to try and stay near the optimum AOA for maximum 
    efficiency. Lighter gross weights require either higher altitudes or 
    lower true airspeed to be most efficient. Likewise, heavier weights will 
    have you faster and or lower. I guess what I'm saying is that the 
    optimum AOA is essentially dictated by wing design and we juggle the 
    other variables in order operate the wing as efficiently as possible. 

    I have rolled the 737, 757, 767 and A320 in the simulator and they make 
    poor pattern planes. I'm sure there is a significant downgrade for a 
    single roll that loses 5000+ feet of altitude. Don't even ask about 
    knife edge performance. 

    Chris 

    Bob Richards wrote: 
    > That makes sense to me. The AOA depends on the load. In an extreme 
    > case, very lightly loaded, I don't think you would want to fly with 
    > the fuselage in a nose down attitude, that would probably be 
    > inefficient. Better to have it slightly nose up in cruise with a full 
    > load. JMHO. 
    > Of course, the reason WE would trim wing incidence would have more to 
    > do with overall flight characteristics during aerobatics, particularly 
    > with pitch coupling in knife edge flight. 
    > Bob R. 
    > 
    > 
    > */chris moon /* wrote: 
    > 
    > 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 
    > 
    > 
    > ------------------------------------------------------------------------ 
    > 
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    > NSRCA-discussion at lists.nsrca.org 
    > http://lists.nsrca.org/mailman/listinfo/nsrca-discussion 

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