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Pumping the cyclic for rotor RPM's

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Pumping the cyclic for rotor RPM's

Old 1st May 2017, 05:07
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Pumping the cyclic for rotor RPM's

I have heard tale that moving the cyclic back and forth in a fairly rapid manner will help gain RPM's. Is this true? I presume that it wouldn't be affective at low airspeeds? Thanks
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Old 1st May 2017, 05:21
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Doing rapid small movements with the collective will recover RRPM, but rapidly moving the cyclic will just reduce TRT (lift).
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Old 1st May 2017, 09:58
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Originally Posted by fadecdegraded View Post
Doing rapid small movements with the collective will recover RRPM, but rapidly moving the cyclic will just reduce TRT (lift).
Any idea why? I can't rationalise that at all.
Lowering the lever will recover RRPM as long as it is lowered sufficiently for Tq to overcome drag, but how does "pumping" help?
Do movements of the cyclic measurably reduce TRT aerodynamically? How?
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Old 1st May 2017, 10:58
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I'm pretty sure that pumping the cyclic offers no benefit in any circumstance!
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Old 1st May 2017, 11:13
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"I thought," he said, "that if the world was going to end we were meant to lie down or put a paper bag over our head or something."
"If you like, yes," said Ford.
"Will that help?" asked the barman.
"No," said Ford and gave him a friendly smile.
From the Hitchhikers Guide to the Galaxy.
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Old 1st May 2017, 12:18
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Rapid aft cyclic will produce flare effect and increase Nr - rapid forward cyclic will do the opposite. Alternating the two will just make for an unpleasant ride.

Just lower the lever and flare if you want to recover Nr - if you have some speed then rolling and pulling into a turn will help.

Just try the 'pumping' thing in a normal auto and see what happens - not comfortable and not useful.

If you wash some speed off with flare to increase Nr and then gently move the cyclic forward you can get a second bite as long as you still have speed but it's probably no more effective than just holding or tightening the flare in the first place.
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Old 1st May 2017, 12:23
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Pumping the collective won't help you gain RRPM above governed rpm, but it will help prevent the RRPM decaying momentarily. It is a recovery technique used often in low & slow operations to recover from an over pitching situation where the torque required to keep flying is greater than what the engine can deliver(piston that is.) Lowering the collective slightly for a second or 2 unloads the engine slightly & allows it to regain RPM, just to lose it again as you raise the collective slightly for a second or 2 to prevent descending into the trees etc. Repeating this results in a "pumping" action on the collective & if you have left yourself an avenue where you can regain some forward airspeed & fly away &/or turn into wind it could save the day. Note I called it a "recovery" technique, that means you stuffed it up in the first place to be in an overpitching situation, now you need to balance engine power available & aerodynamics for a few critical seconds to keep flying.
Turning downwind while hovering on power line insulator inspections in an R44 is a great Way to get lots of practice with this technique, but if you don't always have a "way out" & screw the throttle to the max stop(don't rely on the governor) you"ll pump the collective all the way to the crash site.
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Old 1st May 2017, 12:38
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There used to be a bit of a fairy tale with the Whirlwind/S55. When you were grappling for lift climbing vertically out of a clearing in Borneo. The theory was that if you slightly stirred the cyclic in an anti-clockwise direction this would encourage the blades, going the same direction, to generate more lift.

I never tried it. The technique I used was to climb almost to the top, then forward cyclic and downwash would would flatten the leaves and branches on the trees so that the rest of the airframe missed it.

A kind of 'tree cushion'.

Last edited by Fareastdriver; 1st May 2017 at 18:57.
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Old 1st May 2017, 13:02
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Heliduck - I assumed the OP was talking about in auto with no engine power - I might be wrong.

FED - yes the same myth was purported to work on the Wessex but for confined areas, using right pedal to offload the TR and coming out in a slight spiral climb seemed to be more effective.
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Old 1st May 2017, 14:05
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Originally Posted by [email protected] View Post
...using right pedal to offload the TR and coming out in a slight spiral climb seemed to be more effective.
Certainly worked coming out of places like Forkhill! Trick was judging when to go for the fence...
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Old 1st May 2017, 14:42
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In the Huey....oft times you were going to turn right on takeoff....no matter what you had in mind to begin with. After a while one planned it so that would take you into Wind as it going to down wind was the exact wrong option.
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Old 1st May 2017, 15:25
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Originally Posted by 212man View Post
I'm pretty sure that pumping the cyclic offers no benefit in any circumstance!
That was my take as well! I couldn't possibly see any feasible reason at all why it would be done.
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Old 1st May 2017, 17:12
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Doing that rapid movement back and forth with the cyclic in a Huey will result in "Pylon Rock". Single or twin Huey. Not fun.

JD
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Old 1st May 2017, 17:30
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Originally Posted by obnoxio f*ckwit View Post
Certainly worked coming out of places like Forkhill! Trick was judging when to go for the fence...
Ah, obviously the pilot of one of those under-powered old Wessex things.

The tailwheel dings in the top of the metal fence at Dungannon always made me smile. Alongside one quite minor one was the pilot's name. The one next to it was quite a bit bigger and it was marked "OC * Flight".

Back to the OPs question:

If you stir the stick, you are over-controlling and creating more drag in the process. It surely can't increase rotor RPM.
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Old 2nd May 2017, 01:01
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My understanding is that "milking the collective" momentarily reduces pitch and there fore drag and let's the RPM recover.
Obviously the RPM has to be low in the first place and only really used it in anger in piston machines, but it works.
Also tried it in a 206 one day and could hold a hover with few percent less tq while milking the collective.

As far as moving the cyclic around goes it destroys lift as the TRT vector is all over the place and not acting in one place or close too it and more power is required
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Old 2nd May 2017, 09:35
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Originally Posted by [email protected] View Post
Heliduck - I assumed the OP was talking about in auto with no engine power - I might be wrong.

FED - yes the same myth was purported to work on the Wessex but for confined areas, using right pedal to offload the TR and coming out in a slight spiral climb seemed to be more effective.
A 900+ft line under a JetBox needed all the offloading of the TR it could muster for the last 100' coming off the ground!

For the OP, undoubtedly bad advice or bad crewroom gossip.
moving the cyclic back and forth in a fairly rapid manner will help gain RPM's
is not only unlikely to help but can be dangerous by leading to mast bumping, as already mentioned
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Old 3rd May 2017, 05:16
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pumping thou self has more merit than pumping the cyclic
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Old 3rd May 2017, 06:18
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Having flown quite a few helicopter types now I would say it's an almost universal constant that pumping any control is not a good idea. In cyclic you might inadvertantly get past what is called the crossover point and find your merry stick waggling gets out of phase with the response (cue huge PIO). In collective you can set off a resonance in the engine control system which can destroy the engine. These are of course the extremes - usually you just achieve nothing but a queasy stomach.

The ways to increase NR are few and well understood: more power to the head at constant blade pitch; less blade pitch with constant (or no) power; flare effect (assuming you have some airspeed); increasing the rate of descent in auto (eg in a turn); and finally turning in the opposite direction to that where you get a torque spike (ie left for French helicopters - this lowers drag on the head).

Another one might be to do a huge input of off-power pedal in the hover meaning that the main rotor briefly gets all the power from the engine and briefly speeds up the head.

Anyone else think of any?
 
Old 3rd May 2017, 13:58
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I've been involved in several flight tests where we "pumped" each control axis at various frequencies, and never has there been any appreciable change in the average NR or torque values during or after the maneuver.

NR and torque do change up and down (symmetrically) in response to the control inputs, but once the input has ceased the parameters are right back where they started.
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Old 3rd May 2017, 20:20
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There is a good chance that a Bell 47 pilot would be familiar with "pumping the collective" in order to regain rotor rpm. I DON'T advise this technique in any helicopter.

https://dms.ntsb.gov/public/47500-47...854/423140.pdf
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