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Q: Calculation of G-Forces ( No Answer,   2 Comments )
Subject: Calculation of G-Forces
Category: Science > Math
Asked by: herkdrvr-ga
List Price: $10.00
Posted: 11 May 2006 03:19 PDT
Expires: 10 Jun 2006 03:19 PDT
Question ID: 727623
I am researching an aviation mishap.  The Vne (never exceed speed) for
the aircraft is 193 KIAS (knots indicated airspeed).  The aircraft, a
Partenavia P68C, was traveling at 220 KIAS.  The pilot initiated an
abrupt pull up, and the wings separated from the aircraft.  The NTSB
calculated that at 220 KIAS and 8 degrees nose high, the resulting
G-force would be 8.3G's.  Can someone please give a detailed, step by
step calculation of this force.


There is no answer at this time.

Subject: Re: Calculation of G-Forces
From: youreh-ga on 09 Jun 2006 09:39 PDT
Not as detailed as you want, but maybe of a little use...

If we assume they included the G-force of gravity, then the G-force
from turning up alone is equal to 7.3Gs. Putting this numbers in
meters (since less conversions are required), the speed is 110 m/s
(speed in mph divided by 2 is roughly the speed in m/s -- if I
remember right, knots are mph, relative to the air). Hence, we get the
following equation:

acceleration = 7.3Gs
             = 7.3*9.8 m/s^2
             = 71.54 m/s^2
             = velocity^2/(radius of turn)
             = 110^2/R
=> R = 169 meters, which is about 558 feet.

So, the radius of the (vertical) turn the plane was trying to make was
about 558 feet. (To be more accurate, it would be important to convert
the speed carefully into m/s -- the "divide by 2" method is just an
approximation and is good for getting an estimate.)

Sorry I cannot explain how this relates to the 8 degree bit. I *think*
I could make a fairly good guess about how they made the computation,
but it would truly be speculation.
Subject: Re: Calculation of G-Forces
From: herkdrvr-ga on 09 Jul 2006 04:00 PDT
220 KIAS = 407.44 KM/H or 113.1 meters per second, so you were pretty close.  

I appreciate the effort you took in demonstrating the calculations,
but it doesn't answer the question.  Here's why.  The NTSB didn't know
he was trying to make a turn of xx radius, so this is an unknown. 
What they did know was that he exceeded the Vne speed by 17 knots, and
initiated an abrupt pullup.  At 8 degrees nose high, the wings snapped
off, with a force of 8.3 G's.  I'm wondering how they derived that.

Any other thoughts?


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