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6 Answers

Relationship of Vg to Va

Asked by: 21690 views Aerodynamics

I am trying to find a FAA handbook/manual where the relationship of Vg and Va is explained. I have a student who wants to see it in an "official" document.

I have looked in the Pilots Handbook of Aeronautical knowledge and the Airplane Flying Handbook to no avail.

Where else can I look?

Thank you for your help.

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6 Answers



  1. Brian on Oct 14, 2011

    Are you referring to the VG diagram and how it graphically depicts Va? Or does Vg refer to glide speed? I’m sorry, I am not familiar with the nomenclature you chose. Thank you.

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  2. Jim Foley on Oct 15, 2011

    If you are using ‘Vg’ to mean ‘glide speed’ (actually depicted as Vl/d), then there is no relationship.  Vl/d is the hightest ratio of lift per unit of drag on a particualar aircraft, which is determined by the manufacturer.  Va, or manouvering speed is dependent on aircraft gross weight, in order to avoid imparting too many Gs on the airframe.
    If you are in fact referencing the VG diagram, sorry, I can’t hep you there.

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  3. Bob Watson on Oct 15, 2011

    As I understand it Va is the speed at which the airplane will stall before it exceeds the load limit. The V/G diagram shows the relationship between stall speed and load factor.
    http://www.faatest.com/books/FLT/Chapter17/VgDiagram.htm
    has a good illustration and explanation as does:
    http://aviationglossary.com/aircraft-terms-definition/v-g-diagram/
    as does Flying magazine (which is pretty official)
    http://www.flyingmag.com/pilot-technique/tip-week/ups-and-downs-turbulence
    The request for an “official” document seems rather odd. Like if the FAA doesn’t say, it’s not important or true?
     

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  4. John D. Collins on Oct 16, 2011

    Small aircraft that applied for certification between 1945 and 1965 were required to meet CAR Civil Aeronautics Regulations, Part 3 in order to be certificated. In 1965, FAR Part 23 of the current regulations was adopted to provide certification requirements new designs for small aircraft.  Examples of designs certified under CAR 3 include the Bonanza, the Mooney, the Cessna 172, the piper Cherokee, and other aircraft of their generation.  New designs such the Diamonds or the Cirrus are certified under FAR, Part 23. So based on whether an aircraft is certified under CAR 3 or FAR 23, the requirements for Flight loads including VA and Gust loading must be met.  Both documents have a V-n flight load diagrams and describe the details.

     

    In CAR 3, this can be found in section 3.181 Flight Loads.  See http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgccab.nsf/0/0ad207b847d760f886257822004c9037/$FILE/CAR%203%20(5-15-1956).pdf

    In FAR 23, this can be found in Appendix A23.1 thru A23.13 with the definition of terms in A23.3 the V-N diagrams in A23.13.  See http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/0/3731726874AB80178525668700728785?OpenDocument  for the definition of symbols and  http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/0/3731726874AB80178525668700728785?OpenDocument for the V-N diagram.

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  5. Brian on Oct 16, 2011

     

    Bob says, “As I understand it Va is the speed at which the airplane will stall before it exceeds the load limit.”
     
    Mark on and the formula for finding Va is incredibly simple. It’s expressed as stall speed multiplied by the square root of the positive limit load factor (positive g-limit). Load factor is given the notation ‘n’ and stall speed represented as Vs. 
     

    Check out page 180 of Aerodynamics for Naval Aviators. You can download it free thanks to the wonderful folks here at Ask a CFI. But please, for $5, consider the donation option. 

    To nail it home just try it out. Take the stall speed of the aircraft you’re flying and multiply it by the square root of the positive g-load limit. You’ll find that it to be close to published Va. However, realize published Va does not need to equal calculated Va per certification requirements. See John’s sources for more detail on the certification requirements. Also note that all speed calculations must be done in Calibrated speeds.) 

     

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  6. Nathan Parker on Oct 18, 2011

    “The request for an “official” document seems rather odd. Like if the FAA doesn’t say, it’s not important or true?”
     
    In this case, it’s true, since Va is a speed defined by the FAA.   Interestingly, the definition of the speed does not require that the aircraft stall before exceeding the load factor.  Rather, the only thing it requires is that the flight controls remain attached to the airplane at Va and below.  This speed differs from the concept of “maneuvering speed” found in aerodynamics books and differs from the point labeled “Va” on Vg diagrams.  I suspect at one time, the FAA’s Va conformed to this definition but mutated into something different.
     
    In order to fix this screw-up, the FAA has defined a new speed, Vo, which does require that the aircraft stall before exceeding the load factor limit.  Newer airplanes, such as the Cessna SkyCatcher, defines this speed.

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