Safety Speed.
A very poorly taught and little known factor in assymetrical engine flying is "Safety Speed " If, with your wheels down and full flap, you open up full power on the good side be it one or two engines and assuming a complete loss of power on the other then there is a speed at which you can no longer keep the aircraft straight. That, very simply put, is your Safety Speed and if you let the aircraft get below that speed you will start turning towards the lost engines, you will start losing height in the turn due to the added lift required and you will stall and lose control of the aircraft. It was, probably , the cause of most of the accidents at OTU's and HCU's during assymetrical flying because frankly, the Instructors themselves were not sufficiently trained to know this and therefore very little, if any, instruction was given until much later when Bomber Command Instructors School came into being and , very quickly, taught recognition of Safety Speed as being of vital importance to the people who were going to be Instructors. When I had my crash in a Mosquito , returning from a raid on a German Airfield in Holland on one engine I am now sure that it was because I let the speed drop too low and below the Safety speed and was unable to stop it from turning and crashing into the trees that saved our lives by lowering us more gently to the ground. I had never had the words "Safety Speed" given to me in any of the instruction that I had been given. Hope that this helps. It goes without saying that you had a different safety speed for each configuration of the aircraft i:e Wheels down 30 Flap etc. Two good engines on one side, one good engine outboard , differing from inboard and even the direction of rotation of the propellors played a part because of the Gyroscopic effect acting 90 degrees in the direction of rotation. (The cause of many a swing on take-off in tail wheel aircraft when pushing the stick forward to raise the tail thus exerting a forward force at the top of the circle of the prop rotation which is transmitted to a sideways force of 90 degrees in the direction of rotation.) Reg.
I found this in a copy of pilots notes for a Halifax 3 (Hercules engines)
Engine failute on take-off. The aircraft can be kept straight on any three engines at take-off at full load provided a safety speed of 155 kts, has been attained.
In the event of an outer engine failure below safety speed, control will be lost unless the opposite outer engine is immediately throttled back, at least partially. Feather the propellor of the failed engine, retrim and re-open the throttle of the live outer engine.
After control has been regained as described above, it will be possilble to climb with flaps in the takeoff position and U/C up on three engines at take-off power at 140 kts. IAS at light loads.
At heavy loads or if the engine failure has occurred at a low height immediately after take-off it will be necessary to land straight ahead using the two inner engines to control the rate of descent.... And the best of British luck ! I hope that this clears things up. For Kts. read MPH....Sorry, but that's how accidents happen ! Reg.
Last edited by regle; 1st Sep 2009 at 15:11.
Reason: Safety speed found in Pilot's notes Halifax 111