In message
,
JohnO writes
On Jul 17, 8:54*am, "Paul J. Adam"
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

Really?

Yes, if you have two engines each providing half the horsepower compared
to having to develop a new engine. (Changes if there's an off-the-shelf
turbine that puts out the required power, of course)

& easier

Really?

Yes, depending on dimensions and gearing.

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

Really?

Yes, really.

A bit different for the heavies, but light twins such as A109 and
AS355 cost more to purchase, fly and maintain than their single
siblings A119 and AS350, and offer little or no performance benefit
(in fact AS355 has less performance than AS350B3).

Absolutely so. Now, how often do they get shot at, flown through
brownouts, and so forth?

The flipside argument - why do light twins like the A109 and AS335 ever
sell, if they're so much more expensive for no apparent benefit?

"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*.

Yep. That second engine will get you out of enemy territory to a
friendly crash location.

Which is why many of the later UH-1s went twin-engine and so many
military helos like to have more than one powerplant, even if it's less
strictly efficient than a single engine.

Being able to jettison payload and limp home, or at least to pick a
safer spot to come down, can be quite important to the aircrew.

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

Paul J. Adam