Robinson low rotor

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Bitch Slapper
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Robinson low rotor

Postby Bitch Slapper » Fri May 29 2015, 05:35

Just thought I would bring this up as I remember reading a few threads on it.
R44/R22 low rotor. What should you do first second and maybe third?

From my experience I have rarely seen low rotor light or horn or rotor taco needle below 98% unless the collective was under your armpit, or pulling more than 26 inches, or and I should say or you had a very slow governor but you still need to be pulling lots of power,

The basic of it as I understand it is, the correlation between the collective and the throttle works as we know raise the collective and the correlator increases throttle, and lower the collective and the correlator decreases throttle, then in the middle of this is the governor, it overrides the correlator and opens or closes the throttle to try and maintain 102% RRPM, no matter what you are doing with the collective! Every engine will produce different amounts of total horsepower, which in turn will then top out at some collective setting, normally well above 26 inches MAP. If your engine is a little tired then this topping out of horsepower will happen at a lower collective position or MAP, when you pull say 28inches you may start to lose rotor RPM, and as you lose RRPM I have seen that the MAP decreases right down to say 21inches, it looks like something is massively wrong, (I’m only pulling 21 inches!) why is the horn on? When you go into the recovery mode of lowering the collective, when you get it low enough for the RRPM to recover or start to recover you will then see the MAP increasing, this might go up to 27 inches,
So as far as I can tell the correct procedure is to lower the collective and then wind on throttle which may not move because it is or was at full open position anyway.
This is a different story when you have a slow governor, or a pilot who holds the throttle so firmly that they stop the governor from governing.

Your thoughts people?
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froginasock
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Re: Robinson low rotor

Postby froginasock » Fri May 29 2015, 05:56

Wrong for R22/44. Give yourself an experiment with the governor off to prove it. The correlator is basically too 'strong' .. So when you raise the collective the rpm goes up as too much fuel is passed to the engine. The reverse it true when lowering collective. The correlator takes away too much fuel. Simply turn the governor off on the ground when light on the skids and lower the collective to prove it ... The low rrpm horn goes on you MUST roll on full throttle ( overrides correlator) and then lower collective. .. The reason the governor is there is because it's opposite to what happens in other helicopters (I.e bell47) ... Also note if you pull 28 inches and over pitch the rpm will decay but the MAP will remain high as the throttle butterfly is wide open and the MAP guage is directly reading the pressure of the inlet manifold. You must be clear on all of this. The wrong actions won't save the day ..
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Re: Robinson low rotor

Postby Bitch Slapper » Fri May 29 2015, 06:00

Ok then Frog, why is the RRPM low in the first place I'm wondering?
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Re: Robinson low rotor

Postby froginasock » Fri May 29 2015, 06:16

Overpitching would be one guess . Or doughy governor and pulling collective too rapidly .. Could be hot hot hot when heavy operating low and slow ... Could be pushing the nose too far forward when heavy and on take off .. You might have dropped a valve .. Recovering from overpitch or low rrpm with engine running will still be best recovered by rolling ON throttle and lowering collective .. If you've got foreard airspeed aft cyclic at the same time will also help (it all depends on the situation) ..
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Re: Robinson low rotor

Postby froginasock » Fri May 29 2015, 06:26

.. I'll add .. The risk is you've possibly put yourself in a bad spot RRPM on the way down .. In the r22 or 44 lowering the collective without rolling on throttle will initially see LESS fuel to the engine .. And the ENGINE will have a short period of time with less power than it did before the collective went down!

This will initially decay the RRPM and could put you in a worse spot (unrecoverable) .. It's a correlator issue.
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Re: Robinson low rotor

Postby Bitch Slapper » Fri May 29 2015, 06:43

Agreed sort of, but I have only ever seen it from over pitching. hence what I posted.

Saying that I have seen pilots freeze on the throttle, so increasing throttle would fix pilot error, the problem is they have frozen !!

doughy governor
well you should know it has a doughy governor and tread carefully wouldn't you, or that's a special or aircraft malfunction I would say, and if that was the problem then only winding on throttle is the fix.

dropped valve
the governor will still try to fix that up until its at full throttle then you get Low RRPM and the only fix is still lowering the collective first.

pushing the nose too far forward when heavy and on take off
becomes over pitching.

Now my though is that we are talking about a normally functioning flimsy copter, which will if all being put together properly, be at full throttle when RRPM is low because that is what or partly why the governor is there? I'm don't mean to say do not wind on throttle, I'm just thinking that it is normally at full throttle when this happens so when that is the case then the only fix is to lower the collective first, and keep the throttle wide open till the RRPM recovers?
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Re: Robinson low rotor

Postby Helicoil » Fri May 29 2015, 08:13

froginasock wrote:Wrong for R22/44. Give yourself an experiment with the governor off to prove it. The correlator is basically too 'strong' .. So when you raise the collective the rpm goes up as too much fuel is passed to the engine. The reverse it true when lowering collective. The correlator takes away too much fuel. Simply turn the governor off on the ground when light on the skids and lower the collective to prove it ... The low rrpm horn goes on you MUST roll on full throttle ( overrides correlator) and then lower collective. .. The reason the governor is there is because it's opposite to what happens in other helicopters (I.e bell47) ... Also note if you pull 28 inches and over pitch the rpm will decay but the MAP will remain high as the throttle butterfly is wide open and the MAP guage is directly reading the pressure of the inlet manifold. You must be clear on all of this. The wrong actions won't save the day ..


I'd just like to clarify Frog's comments on the R22 correlator - yes with the governor off, collective floored, E/RRPM top of the green and aircraft on the ground, the raising of collective will supply too much fuel resulting in a significant rise in E/RRPM above the green range. Likewise governor off, collective around 20" MAP, RPM top of the green and aircraft on the ground, lowering the collective will decay E/RRPM significantly (to around 75%), or in other words the correlator will over-compensate. However in flight its a different story altogether. The reason behind this is that the correlator is designed to co-relate the position of the collective with the throttle butterfly when the weight of the aircraft is on the disc, ie the aircraft is in flight. This over-compensation occurs when the helicopter is on the ground because part of the weight of the helicopter is supported by the ground. This is evident during lift to the hover with governor off - the throttle no longer needs to be wound off when raising the collective after reaching around 20" MAP, or in other words, once most of the weight of the aircraft is now on the disc. Try this for an experiment - in flight, say 60-70 knots, governor off, RRPM top of the green, MAP = 23" lower the collective (not rapidly) to 13" MAP without adjusting throttle, then back up to 23" MAP (again, not rapidly). You'll find that the RRPM remains more or less at the top of the green range, which is testimony to design of the R22's correlator - Robinson got it pretty much spot-on.

So in summary - in flight the correlator works very well, with virtually no need to roll throttle prior to moving collective, however when the R22 is partly supported by the ground, the action of the correlator is to over-compensate.
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Re: Robinson low rotor

Postby Heli Bloke » Fri May 29 2015, 10:03

From memory correlator works well between 19-22 ", generally lags over 22" and over compensates under 19"
Bitch Slapper
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Re: Robinson low rotor

Postby Bitch Slapper » Fri May 29 2015, 10:09

Ribbit. Ribbit
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Re: Robinson low rotor

Postby AgRattler » Fri May 29 2015, 22:18

I found the the most successful method of dealing with Robinson low RRPM is to avoid flying them.

Thoughts?
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froginasock
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Re: Robinson low rotor

Postby froginasock » Sat May 30 2015, 01:11

Regarding all of this .. My comments are in relation to the horn going off and rrpm decaying .. You are in a poor spot and the governor isn't keeping the rrpm where it needs to be .. Cause could be many - varied - or obvious - but the rrpm is low ... I'm rolling on throttle and lowering collective (and aft cyclic if I have airspeed) because (whatever the cause) the systems are not functioning or my actions have put me in a position that needs to be corrected.

For Slapper: I don't stay on the site all day/night .. So forgive my slow response.
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Re: Robinson low rotor

Postby headcase » Sun May 31 2015, 21:47

AgRattler wrote:I found the the most successful method of dealing with Robinson low RRPM is to avoid flying them.

Thoughts?


What a silly comment. That could be said for any aircraft type. The anti robbie sentiment on here is tiresome.
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Re: Robinson low rotor

Postby County » Mon Jun 1 2015, 01:00

I didn't know Low Rotor RPM was type specific. General rule of thumb is if you don't know what your talking about....its best to say nothing.
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Re: Robinson low rotor

Postby AgRattler » Mon Jun 1 2015, 02:39

It was tongue in cheek.
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