Helicopter performance
 

Hi all!!
I'm new here in the forum....a lot of nice topics here!!
I would like to ask and share some considerations about helicopter performance, i'm a little bit confused....
I start with a question:
There are differences to handle an engine loss on a 2 engines helicopter vs on a 3 engines helo??
Any Merlin pilot here?

Simple
 

You lose a third of your power vs a half. The end.

GM

yes , and that means you don't have to carry such a big unused power overhead during normal operation as with the daft modern generation regulation-led twin designs. this is a point that NL makes often, although I can't see him supporting the Wasteland triple.

the maths still doesn't really work untill you get to 4 and 5 engines , but by the time you get there you realise that SIMPLEX is the way to go, as most real pilots already know.

the downside with triples is that the aircraft is so complex that it almost never works, so that is a safety feature, because it is hard to crash when on the ground. (same as the regulator strategy for flight safety:rolleyes:)

Yaaaawwwwwnnnnnn! Perhaps you should get out and do some commercial/HEMS/Police/SAR/VIP/ anything and you might realise the reality of twin helos

It's misguided legislation that dictates the use of twins.
If we were allowed to use common sense and choose for ourselves, I think a lot more suitable singles would become available to choose from! :D

You mean use preference rather than common sense!

The OP wasn't asking about SE ops, regardless of AnFI's inevitable attempt to hijack the thread with his pet rant.

Let's try to keep this thread on topic?

Crab

Are you saying that those who fly singles lack common sense? (And yep, I realize I have opened Pandora's box).

I haven't flown the Merlin, but have flown plenty of twins (with varying degrees of excess power) and I don't think there is any difference in the basic theory. Don't think in terms of how many engines you have, or have failed, simply think in terms of what is the power required (at the moment of failure) and what is the power available. If the power available is equal or greater than than power required, you don't need to do anything much. If the power available is less than the power required, you need to change the power requirement i.e. you need to lower the collective. If the power available matches the power required, but has a time limitation associated with it (30 sec, 2 min etc) then you may need to subsquently reduce the power demand.

At the end of the day - mantain Nr! :ok::ok:

Thanks all,
I did some calculation with meril's performance charts,
For example:

All engines operative
weight 14300kg
pa 0ft
temp 10°
hover height 10ft
torque required 90%

One engine inoperative (twin eng condition)
weight 14300kg
pa 0ft
temp 10°
hover height 10ft
more than 125% (out of chart range!!!)

For the rule of 1/3 with a 3 engines 90% Trq in case of engine loss it has a 30% trq increase....but it seems not correct

On Merlin comunity i heard about this rule trq x 3:2, this means 90% x 3 divided by 2 so 135%.....but this one look to me a rough method.

I think is very important for a pilot in command know precisely about this numbers because it change a lot the resolution of an emergency.
What you think??

Be careful to don't mix the torque require for the rotor (90% with 3 engine will remind 90%, same power and same rpm) and the torque seen by the engine (90% for each engine and the 3:2 rules work)
The two remaining engines(and MGB entries) will see 135% torque.
Keep in mind that AEO torque limitation is to protect planetary and OEI limitation is to protect MGB engine entries.

Hi Laurenson

I would argue with your comment that the OEI limits are to protect the MGB engine entries. I've always been of the understanding that OEI is generally an engine limit and certainly, on my current airframes, if you eat into 30 second power limits then the engine goes back to the manufacturer, not the gearbox.

Cheers

TeeS

You right generaly the engine limts the OEI performance, but the MGB still has OEI limitations, for low altitude and low temerature condition.

esspacially on the new over power helicopter (AW139/EC175)

Mmm, for me torque it's only related to engines, if i talk about rotor i refer as Nr only...
90% (for example) of torque it's the "work" of the engines to keep a costant rotor speed 102% Nr
Am I wrong?

Yes. How do you think that torque gets to the rotor? An MGB is a compromise of weight versus its ability to absorb all the power two engines can deliver, based on the probability of having to. Hence, the engines can easily over torque the gearbox.

In many, if not most, helicopters the engines are still in the green when 100% torque is being delivered. Torque is a limitation on the transmission, not the engine(s).

some definitions
 

Got it!!
Merlin air mission computer has a nice performance page, based on many sensor it can calculate hover performance and capabilities,
A lot of unknow definitions for me like:
Torque margin
Thrust margin
ceiling
reject alt
flyaway
Can you help me out with these??

definitions
 

Torque margin = amount of power left (in torque terms) between max allowable and that required for the flight condition

Thrust margin = same as above but expressed in payload (lbs or kg of capacity left)

Ceiling = maximum allowable operating altitude, normally the point at which a rate of climb of 100 fpm cannot be sustained or the envelope limit if lower

reject alt = maximum distance above the ground at which you can safely land after a single powerplant failure

flyaway = technique used in hover or during low speed flight to avoid hitting the ground after a single powerplant failure.

are you in the Italian Navy?

DM

If so, 'Flyaway' may have another meaning.....:E