Drooping Nr for better efficiency

[jab]

I asked Nick Lappos a technical question, Nick kindly replied and suggested I share the question with other Ppruners.
"With regards to the S-61, Sikorsky recommends drooping the rotor system to 94% after losing an engine after TDP(Take off decision point)/ before LDP (Landing decision point). I do understand that any rotor system is designed to be most efficient at a certain RPM and that this is normally slightly lower than normal operating RPM so that the rotor system will become more efficient as the RPM is drooped in the event of an engine failure. I have seen that it works but dont understand exactly why. Obviously it enables a lot more collective pitch to be used but drag is also increased and lift is lost due to the lower V squared. Coning angle will also increase, reducing disc area and therefore lift. I presume this will be offset by a more efficient lift/drag ratio but is there more to it than that?"
Nick's reply :
"The answer has to do with what we mean by efficiency and at what flight regime. Basically, the higher tip speed is used in high speed cruise, and
actually wastes some power in a hover and at low speed. This compromise is
useful, but when you are OEI, you gain several hundred pounds of performance with lower rpm. It is precisely the same in the S76, and the OEI procedures call for allowing a 5% droop to help the OEI performance."
Anyone else care to contibute?

[Flying Bull]

Hi Jab,

same principle with the EC 155 - you pull the pitch to reduce the Nr down to 330 rpm - from abot 350 in normal flight.
Aural warning should be on and off - then it is abou right....

I think, it is not only to get the most out of the rotor, it is also, to get the most out of the remaining engine.
If the Nr is decreasing, the remaining engine is at its peak power - which you need, to get away from the hazards - or to have the slowest run on speed.

With the FADEC in the EC 155 you have certain modes - so after initial recovery you can select continious OEI power - and drooping Nr to 330 means, that you use all the CT you have available.


For landing you then select high - which is time limited.

Greetings Flying Bull

[jab]

FB
A lot of helicopters require this but I am trying to find out the aerodynamic reasons why Sikorsky, amongst others, say the rotor system is more efficient at a lower RPM. ie What happens to the lift/drag ratio, why it happens and what else happens that I am unaware of? The Puma is also more efficient at about 250 RPM for presumably the same reason, drooped like your EC135, and if any Aerospatiale/EC people can contribute their explanation I would appreciate it.
Jab

[NickLappos]

jab,
Sikorsky doesn't say the rotor is more efficient at low RPM. It is more efficient at low rpm.

The reason why you are confused is because you think there is one measure of efficiency, and one rotor that works best for all regimes. Not so. The rotor that wants to cruise at high speed is a vastly different one that that for a hover and low speed climb, but your one rotor must do both every day, so it must compromise.

For high speed, wide fat blades with high tip speed are best, so that cruise speed is high and maneuver margins are good - the rotor is designed to be far away from stall.

For low speed, a highly loaded rotor, with narrow blades and low tip speed is best, so that the rotor operates close to its best lift-over-drag, and little thrust/maneuver margin is available.

In most twins, in an OEI situation, dropping the rotor speed down gets that rotor closer to the ideal low speed rotor situation, so it creates the thrust you need with less power, and so delivers the most climb for the least power.

[Dave_Jackson]

Jab,

Nick has given the answers that you are probably looking for.

However, if you want some additional information, you will find the US Patent 6,007,298 for 'Optimum Speed Rotor' interesting. The 'Description' section is very easy to read and understand.

http://patft1.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=6,007,298.PN.&OS=PN/6,007,298&RS=PN/6,007,298

[Pofman]

We have to remember that everything in a helicopter is a compromise.between the main rotor,tail rotor, stabiliser, type of head, low speed ,high speed, etc. We will never be at the best efficiency all the time. As Nick and Dave have pointed out it is a balance.Another important point is control response. Generally at high Nr you get a sharper response. Close to the ground that makes me happier. Away from the ground reaction response is not usually that critical, so we could operate at reduced Nr if desired, but lets keep things simple.If you operate with lower Nr you must use a higher blade pitch angle
If you look at a fuel flow diagram for weight /altitude you can see the fuel consumption decreasing up to about 5,000ft. This demonstrates the increased efficiency of the rotor at the higher blade pitch angles required to induce the flow up to the point when the fuel flow starts to increase. This must be the peak point of rotor efficiency as above this higher blade pitch angles mean more rotor drag, hence more fluel flow.The gas turbine engine,meanwhile, will maintain its power output from MSL with improving SFC as a measure of its efficiency as altitude increases until it reaches the design Erpm.
Overall, the designers strike a nice balance.

[NickLappos]

Dave, here is another that discusses the issue of rotor speed for maneuver enhancement. I remember it because it is on my office wall:

http://patft1.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=22&f=G&l=50&co1=AND&d=PTXT&s1=lappos&OS=lappos&RS=lappos

[Dave_Jackson]

One of the inventors is a Mr. Nicholas D. Lappos. Would he be the "NickLappos" on this forum; or is asking this question considered an 'Outing' of a contributor? ;)

[Broadcast Control]

From simulated engine failures (with fixed fuel flow on remaining engine) my experience is that one very important effect of drooping NR is to use part of the kinetic energy in the rotor system during the initial part of the flyaway. Even for a machine that had identical performance at nominal and reduced NR, it would be very beneficial to reduce NR during the flyaway.

As soon as conditions allow keeping torque within limits, I would think that higher NR (more power) is more important than a small increase in main rotor efficiency.

[jeffrey1]

Huey is the same way. When I had loads that were too heavy to lift at max torque all I had to do was beep rotor down and presto, away we would go.

[[email protected]]

Except that for the same engine power, drooping the Nr makes the Torque increase.

[robsrich]

Remember when the RRPM gets to a low setting in an R22 or R44 or a similar piston, you have actually wound down the engine.

The HP lost is roughly the same as the RRPM loss. That is, lose 10% RRPM, engine HP available at full throttle is also down 10%.

No problem at sea level in a derated engine installation, however, at full throttle height or above, the warnig horn can be scary.

Also, low RRPM means the tail rotor is going slower, thus tail rotor thrust is down, a problem in hot, high and humid conditions.

In summary, the little people (machines) are more prone to low RRPM hiccups!

For those younger guys and girls in small things, keep the rotor RPM top of the green,

Later, on the big ones, you can use the preceding tips.

Flying is fun!

[TheFlyingSquirrel]

Can anyone confirm a 10% loss of RPM equates to a 20% loss of lift ?

[puntosaurus]

Well you can do a crude estimate from the lift equation. If lift is proportional to V squared then 0.9V will give 0.9*0.9L ie. 0.81. Close enough ?

[Flingwing207]

Remember when the RRPM gets to a low setting in an R22 or R44 or a similar piston, you have actually wound down the engine.I would say that in any piston helicopter where the pilot is in direct control of the engine RPM (which on paper excludes the Robinson), there is no hazard in running the RPM at just above middle green. For example, the 300CBi's green range is (rotor/engine) 442/2530 to 471/2700. As long as you are running comfortably below maximum power, there's plenty to be gained by dropping the RPM down to something in the range of 460/2640 - reducing pilot workload and fuel burn. I advocate keeping the RPM at the top of the green whenever MP is within 1" of maximum, or altitude is less than 450', and anytime you need performance from the rotor system (climb, OGE hover, speeds below 40kt). Remember that the 300C's redline is 3200/471, but Huges/Schweizer lists 3100/456 as recommended operating RPM.
All that being said, I would not operate a very low inertia system like the R22 (which at redline is operating the engine at 2652 RPM - already on the back of the torque curve for the O-360) at anything less than the top of the green - the margins on that ship are already too close.

[Graviman]

Having read Nick's et al. patent, would it be fair to comment that the collective servo intended for autorotation entry could benefit efficiency too?

My thinking is that the collective servo system would gently "resist" collective change until engine could respond to maintain required RRPM. This would be near instantaneous in a piston helicopter, but long enough in a tubine to keep safe TET. The original intention was as a safety feature, since the system automatically always maintains constant RRPM. A failing engine would automatically result in a lowerd collective in order to maintain RRPM.

Since this is part of the engine govenor system, it would be very easy to vary RRPM as well as torque with collective position. Naturally an "engine response model" would be required in the system, but it would be easy enough to include change of RRPM as part of that response. With some thought, i actually feel it could be done with relatively inexpensive analogue op-amp CR circuitry. This means such a system is not limited just to Comanche levels of sophistication.

Pilot would basically just feel a mild resistance to collective increase while RRPM gained a safe margin. Naturally the response/margin would need to be fast enough to avoid collective PIO in ground effect.

Mart