Originally Posted by Trim Stab
How do you measure Vmcg?
My employer just finished doing this for a 19 seat turboprop. The process was fairly straightforward - accelerate down the runway, cut an engine, don't use nosewheel steering or aileron, and see if the aircraft can be kept within the allowed 30 foot deviation.
In our case, we began the testing at a speed well above what we expected Vmcg to be, and then kept reducing the speed. It turned out that the demarcation between "easily keeping the aircraft on the centerline" and "oh-oh, here come the weeds" was pretty sharp - a three knot speed difference was all there was between the two extremes.
The Vmcg for our aircraft turned out to be very low. The only reason we needed to establish this speed (which was never established during initial certification) was to enable development of performance charts for contaminated runways that use a reduced V1.
It's kind of funny to note that although the test aircraft had a huge flight data acquisition suite in the back, multiple GPS receivers, etc., the very best idea for measuring deviation from the runway centerline came from one of the AMEs - he suggested that we just connect a large water bottle to a hose leading out to behind the nose wheel, and open the tap at the start of the takeoff roll. We could then observe the path left on the runway by the stream of water, and measure centerline deviation to the inch... no need for all that fancy GPS stuff.
