In message
,
Charles Talleyrand writes
If the large helicopters can autorotate, and cannot hover with a
reasonable load on just one engine, why do they have two engines?

Because it's often cheaper & easier to use two engines rather than one
for the power needed - you don't end up with an exaggerated hunchback
from one big turbine & gearbox. (Or three engines, as some types - our
Merlin, the US Sea Stallion - go for.)

Even small helicopters often have two engines, such as our Lynx,
especially if they expect to get shot at.

It
would seem that you would have smaller odds of failure with one large
engine rather than two smaller engines. If you have two engines, and
failure of either leaves you kinda screwed, it is not better to just
have one engine?

Or am I missing something?

"It's better to lose *an* engine than *the* engine." Not being able to
hover doesn't mean you can't sustain flight: if you're in cruise, you
get a lot of lift from the rotor disc and can stay airborne on a lot
less power than you need for a hover. (Landing may be interesting, but
autorotating or a rolling landing are both options).

On the other hand, with one engine, losing it means autorotating in
*now*.


--
He thinks too much, such men are dangerous.

Paul J. Adam