Power on / off RRPM limits

[The Nr Fairy]

Taking some examples from a well-known helicopter POH, RRPM limits power on are 97% - 104%, power off the limits are 90% - 110%

The same pattern is repeated for other helicopters, and I've got a query.

I can understand power off high/low limits since we don't want the blades departing the hub at the high end, nor coning too far to be recoverable at the lower end.

Power on at the top end I can understand it's lower than the power off limit since we've now got an engine turning the hub, and the stresses are greater then when the blades are turning the hub in autorotation since it's being tiwsited both by the blades AND the engine.

But I can't understand how the lower power on limit is arrived at - any ideas ?

[donut king]

Sounds like the ole '76!
If I remember, 100-104NR dual engine power on. Single engine power on droop as low as 97NR, limited to VBROC.
I think power off is 90-115NR!
Anyone correct me if I'm wrong.
DK

[Aser]

The Nr Fairy:
But I can't understand how the lower power on limit is arrived at - any ideas ?

The limit is there because if you find yourself with power on and below a normal rrpm, something is wrong...
Ie.: power on AEO 98-101% with a minimum transient 95%
but power on OEI 98-101% with a minimum transient 85%

They allow you to be below 95 because you have still one engine to recover rpm.
But with AEO you must stay above 98%, if not, you or the machine is wrong, so the limit.
It's not a matter of machine limit as I understand, instead is about not be where you shouldn't .
In a power off situation you could be 90% rpm and flying , but it's safer to put the limit at 95%.

I hope to have expresed my self. :}

[The Nr Fairy]

Thanks - useful.

Both the answers though refer to twin engine types - how about singles, specifically R22 (although the answer will be similar for other SE types) ?

[Spandit]

Maybe to do with tail rotor thrust.
If the needles are joined and the engine is providing say 90% of it's power, the RRPM will also be at 90%. The tail rotor would also be going slower, in other words less anti-torque thrust to use.
If you are power off, the engine is not developing torque anyway, so you can afford to let it all get slower.
Perhaps.

[TiPwEiGhT]

I believe that it is to do with tail rotor control as mentioned above and also the efficiency of the main rotor.

At 104% in the R22 the rotor is not working efficient, if you get Nr droop/overpitching, the RRPM will droop towards 97%, where the main rotor is most efficient, making recovery slightly easier (providing you have noticed by this point).

In autorotation, 90% is where the main rotor is most efficient, hence why we use it for max range autorotations in the R22.

TiP:)

[TeeS]

This is drawing from murky distant memory so may be wrong!

For a given TQ, the power transmitted through a shaft is higher for a slower RPM. This is why the RPM is increased as the power train drives the tail rotor drive shaft and then reduced again to drive the tail rotor, which allows the shaft itself to be thinner walled.

i.e. feel free to pull 100% TQ at 100%NR if that is the limit, but don't use 100% TQ to engage rotors from a standstill cos you will twist the shafts into a mangled mess.

Possibly!

TeeS

[Shawn Coyle]

The reasons why there may be different RRPM limits power on and power off are legion, and the manufacturer is not required to tell us why!
Among the possible reasons are:
blade and or head bending moments power on vs. power off, shaft loads, flapping and lead-lag loads, and so on.
Power off, the blade pitch is going to be very different than power on, and that will make a difference to blade and hub loads, as just one example.

[NickLappos]

Nick's stock reasons for the rpm limits:

Power ON upper = mostly engine max qualified N2/Nf

Power ON lower = Minimum droop seen in OEI engine cut at bottom of dead man's curve, often lower than power off

Power Off upper = maximum structural rpm due to centrifugal force (bless you Lu) and or rotor blade tip mach at high speed auto in cold temperatures at low altitude
Power off lower = minimum rpm at bottom collective, low weight, cold temp, low altitude

[Flingwing207]

...and for those of us piston-popper-schooled laddies, minimum power-on also represents where the engine is getting behind the power curve.

[fulldownauto]

As fling said, the reason that there is a lower power on limit, at least in the R22, is that below 104% the engine is producing proportionally less power. And thus the power available to the pilot will be less at 97% Engine (and Rotor) RPM than at 104%. So if I pilot who is used to flying around at 104% and comes hauling in on approach at 95% vs. 104% he will have less power available to him at the end of the approach, and his performance will be reduced (at the same manifold pressure limits); thus the minimum power on RPM requirement. As Robinson describes in his safety video, low RPM is a situation that is self deteriorating due to the associated loss of power at a lower RPM. Pilot droops the RPM, power available decreases, RPM droops further, and so on. Bottom line, you should have the governor on anyways, right? :=