Alchemy
Well-Known Member
I recently stumbled a bit on my multi-engine instrument commercial oral exam on this topic. I was asked by the DPE what critera manufacturers use to determine the value for Vmc in an airplane. I responded:
1. Standard Day Temperature and Pressure at Sea Level
2. Critical Engine at Zero Thrust
3. Critical Engine Propellor Windmilling (max drag)
4. Operating Engine at Full throttle
5. Flaps Up
6. Gear Up
7. Aft Legal Center of Gravity
8. Maximum Gross Weight
9. Up to 5 degrees of bank into the operating engine.
The Examiner then Asked me to categorize each factor as affecting:
A) Assymetrical Thrust
B) Assymetrical Drag
C) Controllability
I categorized them as follows
1) AT
2) C
3) AD
4) AT
5) Neither, Couldn't really categorize this one
6) Neither, "Keel Effect"
7) AT
8) C
9) None of the above
My examiner insisted that I explain why having the flaps retracted increased VMC. The best I could answer was that having flaps extended creates more drag on the operating engine side due to propwash and therefore decreased the overall power of the operating engine and decreased VMC. Anyone have a better explanation?
I got complteley ripped apart on "keel effect". I tried to explain that the lateral profile of the landing gear creates resistance to uncommanded yawing movements and therefore decreases VMC but the DPE wouldn't accept that answer. Can anyone explain to me simply why having the gear down decreases VMC?
Then on the 5 degrees of bank, I explained that holding 5 degrees helps maintain a zero sideslip condition which keeps the stall speed and VMC down. THe DPE didn't buy this either, and told me that 5 degrees of bank can help and hurt. By this time I was thoroughly confused, as the Examiner was contracdicting most of what I had read in the Airplane flying handbook and Multi Oral exam guide.
All in all, my DPE's main argument was that ALL the factors above are designed to increase VMC, whereas I was thinking that 5 degrees of bank and max gross weight decreased Vmc. Anyone have good explanation for how each of the factors listed above increases VMC?
1. Standard Day Temperature and Pressure at Sea Level
2. Critical Engine at Zero Thrust
3. Critical Engine Propellor Windmilling (max drag)
4. Operating Engine at Full throttle
5. Flaps Up
6. Gear Up
7. Aft Legal Center of Gravity
8. Maximum Gross Weight
9. Up to 5 degrees of bank into the operating engine.
The Examiner then Asked me to categorize each factor as affecting:
A) Assymetrical Thrust
B) Assymetrical Drag
C) Controllability
I categorized them as follows
1) AT
2) C
3) AD
4) AT
5) Neither, Couldn't really categorize this one
6) Neither, "Keel Effect"
7) AT
8) C
9) None of the above
My examiner insisted that I explain why having the flaps retracted increased VMC. The best I could answer was that having flaps extended creates more drag on the operating engine side due to propwash and therefore decreased the overall power of the operating engine and decreased VMC. Anyone have a better explanation?
I got complteley ripped apart on "keel effect". I tried to explain that the lateral profile of the landing gear creates resistance to uncommanded yawing movements and therefore decreases VMC but the DPE wouldn't accept that answer. Can anyone explain to me simply why having the gear down decreases VMC?
Then on the 5 degrees of bank, I explained that holding 5 degrees helps maintain a zero sideslip condition which keeps the stall speed and VMC down. THe DPE didn't buy this either, and told me that 5 degrees of bank can help and hurt. By this time I was thoroughly confused, as the Examiner was contracdicting most of what I had read in the Airplane flying handbook and Multi Oral exam guide.
All in all, my DPE's main argument was that ALL the factors above are designed to increase VMC, whereas I was thinking that 5 degrees of bank and max gross weight decreased Vmc. Anyone have good explanation for how each of the factors listed above increases VMC?