There's a fundamental physics problem with helicopters. As airspeed gets higher, the advancing blade (the one turning into the chopper's airspeed) gets closer and closer to Mach 1.0 - adding the rotor's speed to the forward speed of the chopper.
The retreating blade simulatneously gets closer and closer to stalling speed. If the retreating blade actually stalls, most of the lift on that half of the rotor disc is lost. Helicopter then does a snap roll to the left and falls out of the sky.
On most helicopters, the phenomenon occurs at somehwere around 220 knots, less on 2-bladed ships like the Jet Ranger, since the blades are longer. This is the typical failure involved in "unexplained" crashes of Medevac helicopters that are rushing to the scene of the incident.
Getting to 250 knots is a major achievement.
Frank Damp -Anacortes, WA,(DW- Eileen)
ex-pat Brits (1968) and now ex-RVers, as of 08 Dec 14.