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How do manufacturers calculate Vy with the gear down?

Asked by: 2572 views Aerodynamics, Commercial Pilot, Flight Instructor, General Aviation

Hello

I have a question as i am due to take my CFI initial checkride this week with the Orlando FSDO.  My friend went for the checkride on sunday and he failed.

 

He was in the oral and the examiner asked him"so how is Vy calculated", my friend answered with the power available vs power required curve etc.  The examiner then said "ok what about calculating it with the gear down how do they do that" well my friend couldnt answer it and he failed.

The aircraft was the piper arrow i was just wondering if anybody had any insight on this question as i do not want to trip up on the same thing.

 

Michael

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2 Answers



  1. Nathan Parker on Apr 17, 2012

    It is absolutely idiotic to fail someone on a question such as this, IMO.  I’m normally skeptical when I hear such things…..the story that examiners tell often differs from the story the candidate tells.
     
    Anyway, there are a number of ways to address the question.  Real world, the number is probably derived via flight tests.  If you want to pursue the theoretical answer,  it’s the same for gear up or gear down:  Vy is the speed where the quantity of excess power is at a maximum.  The only catch is that the power required curve will change with the gear down.  Parasite drag goes up, induced drag stays the same, the net effect of which is that the power required curve gets higher at faster speeds.  This tends to move the point of maximum excess power to lower speeds.

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  2. Wes Beard on Apr 17, 2012

    I don’t think this is necessarily a bad question.  In fact, I think it demonstrates the need for the examiner to ask questions that are correlative in nature.  If I was the examiner, I wouldn’t be looking for a hard number of what Vy is with the gear down but a well thought out response to how we can interpolate what Vy is with the gear down.
     
    The question asks a question on two separate aspects of aerodynamics and relates them to a specific performance problem.  The applicant needs to know how Vy is calculated in the traditional sense and then realize the parasite drag will raise the the power required curves for  a lower than standard Vy with the gear down.
     
    If the applicant answered all the question correctly up to this point, I would have grace on the applicant but I would imagine the previous candidate didn’t do so well leading up to this question. 

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