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7 Answers

Pressure v. Gravity

Asked by: 14638 views Flight Instructor

CFI-Airplane Oral Question on Practical Test:  (Need Help) Explain of the relationship between center of pressure and center of gravity? Please provide source reference.

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7 Answers



  1. Nathan Parker on Apr 03, 2011

    The answer they’re looking for is that the CG must lie in front of the CP in order for the aircraft to exhbit longitudinal static stability.  (In actuality, the CG must like in front of the Aerodynamic Center; the CP isn’t useful in this regard because it changes with AoA.  But the Aerodynamic Center doesn’t exist in the FAA world.)
     
    You will find a discussion of it in the Pilot’s Handbook of Aeronautical Knowledge.

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  2. Brian on Apr 04, 2011

    Nathan, Aerodynamic Center is a derivation of the average location of the center of pressure through various AOA. It is useful for simplifying complex calculations, asside from that it is relatively useless. I bring this up only to better define the term you brought up for the students reading this topic. 
     
    As Nathan mentioned, the importance of this relationship has to do with static longitudinal stability. However, understanding the relationship, and the reason for it, goes beyond just stability. This relationship is the premise for designers needing to place a horizontal stabilizer on the airplane and explains why this surface exhibits negative lift (tail down force).
     
    Here are some sources for you:
     
    http://www.faatest.com/books/FLT/Chapter17/LongitudinalStability.htm – A copy of the FAA text on this subject.
     
    http://www.grc.nasa.gov/WWW/K-12/airplane/short.html – NASA index on aerodynamics. Relavent sections include: Trimmed Aircraft & Center of Pressure (two seperate sections)
     
    If you own the book Aerodynamics for Naval Aviators you can start on page 250. The information is discussed pretty thoroughly in this text. Also, if you own The Illustrated Guide to Aerodynamics, chapter 6 covers this information in detail as well. If you feel the urge to purchase either of these books, in my opinion, the second book is a better choice. It is less technical and just as detailed as the first, except in areas of super sonic flight. 

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  3. Wes Beard on Apr 04, 2011

    Since this is a FAA practical exam; the examiner will require an answer form the FAA reference material.  In the Pilots Handbook of Aeronautical Knowledge starting at page 4-14 has a good discusison on longitudinal stability.  I will let you read, the pages but there is no mention of aerodynamic center.  This center is the average of the center of pressure as the angle of attack changes.  The handbook has a good discussion on why they use center of pressure instead of the aerodynamic center.
     
    The location of the center of gravity with respect to the center of pressure directly relates ot the longitudnal stability of the airplane.  The farther forward the CG is the more stable the lateral axis becomes.  The closer the CG is to the CP, the less stable the lateral axis becomes.
     
    If you have a forward CG, there are certain characteristics you can count on.  A higher stall speed, slower TAS, easier to recover from a spin.  On the contrary, a rearward CG will have a lower stall speed, faster TAS and a more difficult time recovering from a spin.
     
    As Nathan said, the key is understanding this question is “stability”.

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  4. Nathan Parker on Apr 05, 2011

    “The handbook has a good discussion on why they use center of pressure instead of the aerodynamic center.”
    The words “Aerodynamic Center” do not appear in the handbook, according to my Summit Aviation CD.  What are you referring to?
    Regardless, you’re not limited to FAA material on a practical exam, but I do think the student needs to be able to defend a particular point of view.  Explaining how the Aerodynamic Center differs from the Center of Pressure is difficult, so it’s best not to go there, unless the student has a strong command of the subject.

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  5. Nathan Parker on Apr 05, 2011

    <<It is useful for simplifying complex calculations, asside from that it is relatively useless.>>
    Useless?  It’s probably the most important concept there is in the field of stability and control.  The Aerodynamic Center is not merely the average position of the Center of Pressure; that would indeed be a useless concept.
     
    The AC is the location where the pitching moment coefficient does not vary with the AoA.  This is approximately the 1/4 chord point on an airfoil.  We can assume that lift acts through this point, as long as we add a constant, AoA independent pitching moment to the airfoil.
     
    Even that is a simplification, though, because it’s not even the AC of the wing that’s most important, but the AC of the entire aircraft, which is called the “Neutral Point”.  This point may lie behind the AC of the wing.

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  6. Brian on Apr 05, 2011

    “It’s probably the most important concept there is in the field of stability and control.”
     
    Yes, because, “Using the aerodynamic center as the location where the aerodynamic force is applied eliminates the problem of the movement of the center of pressure with angle of attack in aerodynamic analysis.”
     
    Source: http://www.grc.nasa.gov/WWW/K-12/airplane/ac.html
     
    For an engineer curriculum, I would agree with you. For pilot training, I’d argue that one can accurately present stability regardless of the term chosen to compare with CG. I’d be intrigued to learn of any error in this belief, however.
     
    Thank you in advance.
     
     
     
     

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  7. Nathan Parker on Apr 05, 2011

    << For pilot training, I’d argue that one can accurately present stability regardless of the term chosen to compare with CG. I’d be intrigued to learn of any error in this belief, however.>>
     
    You can certainly convey the gist of the idea by using the CP, but it will be an obstacle for the rare student who wishes to purue the topic in depth.  It’s only for CFI students that I hammer away on the distinction; for the pilot certificates, I do use the term Aerodynamic Center, but don’t make a huge issue of the difference between that and the CP.   They at least won’t be able to claim that I taught them incorrectly.

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