? Question ?

popaviator · Feb 4, 2010

I was looking at the poh for the 172 r and I used the va formula with the weights posted but they didn't match. So I'm trying to figure out why. 2450 max gross weight posted in the poh is 99. With the formula I get a different number. Anybody have any ideas?

lhornaday · Feb 4, 2010

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Many aircraft operating handbooks specify Va, the design maneuvering speed, only at the airplane’s maximum weight. A rule of thumb to calculate it at any other operating weight is to reduce the published Va by 2 knots for each 100 pounds below maximum weight.
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Va2 = Va sqroot(W2/W1)

·[FONT=&quot] [/FONT]Va: Maneuvering speed at maximum weight
·[FONT=&quot] [/FONT]W2: Actual airplane weight
·[FONT=&quot] [/FONT]W1: Maximum weight
·[FONT=&quot] [/FONT]Va2: Actual maneuvering speed at the specified weight
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Va = Vs1 X 1.95

MidlifeFlyer · Feb 5, 2010

popaviator said:
I was looking at the poh for the 172 r and I used the va formula with the weights posted but they didn't match. So I'm trying to figure out why. 2450 max gross weight posted in the poh is 99. With the formula I get a different number. Anybody have any ideas?

I don't understand the question. 99 kts at max gross weight is based on aircraft certification. What "formula" are you talking about?

If it's the one that lhornaday posted, you use that with the 2450/99 to calculate Va at lower weights, not use it to come up with 2450/99.

CoffeeIcePapers · Feb 5, 2010

http://forums.jetcareers.com/general-topics/38673-va-calculation-question.html

tgrayson · Feb 5, 2010

popaviator said:
I was looking at the poh for the 172 r and I used the va formula with the weights posted but they didn't match. So I'm trying to figure out why. 2450 max gross weight posted in the poh is 99. With the formula I get a different number. Anybody have any ideas?

51 KCAS * sqrt(3.8) = 99.4 => 99 KCAS.

MidlifeFlyer · Feb 5, 2010

tgrayson said:
51 KCAS * sqrt(3.8) = 99.4 => 99 KCAS.

Definitely, but I still can't tell from the question that he was looking at the formula for the minimum required value of maneuvering speed because a discrepancy between that and actual Va would be okay (so long as actual Va were higher).

tgrayson · Feb 5, 2010

MidlifeFlyer said:
discrepancy between that and actual Va would be okay (so long as actual Va were higher).

Cessnas don't usually have a discrepancy, so the OP's perplexity was justified. The most common mistake is to use IAS rather than CAS in one's calculations.

? Question ?

popaviator

Well-Known Member

lhornaday

Well-Known Member

MidlifeFlyer

Well-Known Member

CoffeeIcePapers

Well-Hung Member

tgrayson

New Member

MidlifeFlyer

Well-Known Member

tgrayson

New Member