Ibra wrote:
I had the impression VMC also applies on the ground?
VMC as it was defined in CAR 3 and even part 23 is with the undercarriage retracted. 
With 50s era stuff even Vr is not defined in the VMC regulations. If the engine stops before you reach Vyse you may crash, you still may crash after you after you reach Vyse , this is not hugely different from a single engine CAR 3/part 23 certified aircraft. 
It is up to the pilot to manage this sensibly which in many cases is to land/crash straight ahead.
If you find yourself in the air below VMC after lift off you could expect a heading change of more than 20 degrees and the inability to hold a heading with less than 5 degrees of bank*, it of course all goes down hill from that point, UNLESS you can accelerate to a speed above VMC. (* by inference based on the what you know from the certification testing)
I guess there is VMCA & VMCG you have to be above both to keep it straight with the rudder
It up to pilot to manage this sensibly which in many cases is to land/crash straight ahead.
Most of the time it’s the case unless one has gone past Vyse & 200ft agl they are better off landing ahead than die hard trying, not many pilots have walked away from a real low & slow engine failure (many have crashed simulating them)
The most scary sight I saw on aviation was a ferry pilot who flew 10km shallow climb over Southend in DA42 with one dead engine (the right engine was U/S after an emergency landing and he decided he could ferry it after some extensive performance calculations), he lined up and took off with one engine all the way on a hot afternoon, I watched the whole takeoff with a friend (who decided not try), the pilot was 2 inches from death and that was on 2km runway !
I am surprised he managed to taxi and keep the aircraft on the runway with one engine inop!
To take off in a DA42 with OEI is just a bad idea.
However where do you get your 200ft from. Surely most have got gear up well before that.
Even a heavy DA42 will climb OEI at 82kts Vyse or Vxse with gear and flaps up.
So rotate at 79kts gain 3kts gear up, engine fails you can continue to climb at around 200ft a minute. If the gear is still down you might not climb but you won’t necessarily descend either.
Figures do of course depend on weight and density altitude like with any other aircraft.
.
However where do you get your 200ft from
I think above it’s the height where you no longer feel ground effect
Also the typical height where you are no longer scared of the ground to do something stupid in the heat of the moment, most people who got killed in twin after they got scared from obstacles (even while training) were bellow that height…
Figures do of course depend on weight and density altitude like with any other aircraft
When I did my MEP at Southend with 2pob & 1/2 fuel you get +100fpm at 82kts once you have cleaned aircraft which I found very shallow at 100ft agl: you will be travelling 1nm to make another 100ft
gallois wrote:
No planes don’t normally flip over at Vmca
I fear the Vmc roll LOC is quite prevalent and there are several examples on YouTube showing an actual EFATO Vmc roll with fatal consequences. The Vmc demo is an instructor demo, and there to show the student the effect of correct technique, ie using horizonal component of lift with a bank up to 5 degrees towards the live engine (Vmc in certification only allows 5 degrees, and optimal bank is actually only around 3 degrees with half ball slip), provides some more rudder authority. Wings level you will run out of rudder at speeds higher than Vmc, around Vsse, you then bank and make a slight side slip, and some rudder authority is restored.
The reason for the roll into LOC in an actual EFATO, is that the poor currency or startled ME pilot may attempt to maintain track with aileron and altitude with pitch. This sets up a potential tail fin stall due to excess beta/side slip, usually when the startled pilot remembers he should use rudder – speed is too slow, beta too high, and the increase camber on the tail fin with rudder applies goes beyond Beta critical angle.
Cue this excellent YouTube explaining the principles of Vmc in a clear and professional manner.
RobertL18C wrote:
Vmc in certification only allows 5 degrees, and optimal bank is actually only around 3 degrees with half ball slip)
Depends what you mean by optimal, in the context of VMC i.e. controllability, then 5 degrees outperforms anything less than 5 degrees. Interesting the certification does not specify what direction the bank needs to be, because it is possible to design an aircraft were the limiting factor is actually aileron authority in which case you would want to bank the other way. Perhaps something like a DHC-4 with massive flaps and big propeller in front of them though I don’t actually know of any, it is certainly a consideration.
At some none zero bank angle there will be the zero side slip condition often 3 degrees at this speed. This will give you the best climb performance but you could still fly slower and remain in control if you increase the bank angle to 5 degrees. Why stop of 5? my guess its a sensible limit as with much bigger side slip you are introducing other problems.
AFAIK most light twins have used 5 degrees to calculate VMC. Though the certification allows them to use zero side slip if they choose.
Optimal climb performance is with zero slip (which is not wings level) but only happens at Vyse, which is hopefully more than VMC…
When I did my MEP at Southend with 2pob & 1/2 fuel you get +100fpm at 82kts once you have cleaned aircraft
That’s sounds pretty low. I easily have 300+ FPM at MTOW with OEI at 86 KIAS (DA42 TDI with CD-155 engines, MTOW 1785 kg, rotation speed 72 KIAS). With 1/2 fuel and 2 POB I can get 500+ FPM OEI.
RobertL18C wrote:
Cue this excellent YouTube explaining the principles of Vmc in a clear and professional manner.
The very first animation is the one I referred to in post #39.
I have done many GA OEI with different da42, most of these were done with 3 pob + 30usg jet, but all with cd135 engine and there are clear differences between planes. The heavier (mtow increased, deice, sick engine…) couldn’t achieve more than 200 fpm climb at Vyse. Although another one that is 60kg lighter could sustain more than 400fpm. I suspect engines were in better shape although it’s a very old one (for a da42, to be fair…)
