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R22 MAP drop in forward flight, no collective movement

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R22 MAP drop in forward flight, no collective movement

Old 24th Oct 2012, 20:06
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Question R22 MAP drop in forward flight, no collective movement

Hi guys,

I think I know the answer to this, but I'd like some clarification from the many collective hours of experience on this forum:

On the R22, as you start your climb out, lets say you are pulling 22" MAP. Now lets say you have climbed at 55 knots and are now at your cruise altitude. So you push forward on the cyclic to attain 80 knots straight and level, but you have NOT lowered the collective, but the MAP drops. Why is that?

I am assuming that better blade efficiency in the increased horizontal airflow has lowered the amount of 'work' you are asking the rotor system to do and to maintain 104% rotor & engine RPM, the governor has lowered the throttle slightly, thus causing a reduced MAP?

I have also noticed something similar when turning crosswind to downwind - my MAP has dropped with no lowing of the collective although I'm not so sure why that might be. In an auto I know a turn loads up the disc and ups the RPMs but in powered flight?

I have always been taught the collective correlator affects MAP and roughly approximates correct RPM, while the governor fine tunes the RPM, with no mention of the governor affecting MAP - but it makes sense that it would.

Thank you for your thoughts,

JB77UK is offline  
Old 24th Oct 2012, 20:21
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Ask yourself What happens when you increase fwd airspeed from a climb ?
Ac will want to yaw to the left with increasing airspeed so you apply right pedal which reduces power the tail rotor needs and so on
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Old 24th Oct 2012, 20:55
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with no mention of the governor affecting MAP - but it makes sense that it would.
yep tha's how it does it by changing the throttle setting.

Fly in turbulence with the Gov' on, very lightly hold the throttle and feel it moving all the time - observe the MAP fluctuating up to one and one half inches.

Spooky - if you are not used to a gov' and are a very keen observer of a fluctuating MAP which often predicts a valve failure.
pucker tucker.
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Old 25th Oct 2012, 08:37
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RPM Changes

Its probably too far back in your memory but revisit Effects of Controls (UK PPL syllabus Ex 4) which covers effects of disc loading and airspeed on RPM. With the governor turned off, forward cyclic will initially reduce disc load and RPM will transiently drop, then the increase in airspeed will lead to an increase in RPM. With the governor on, it will sense this RPM increase and reduce throttle to maintain 104% RRPM. This is evident by the reduction in RPM.
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Old 25th Oct 2012, 14:24
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The main reason for the drop in MAP is that if you change airspeed at the same collective setting, you're changing the induced velocity through the disk, and hence changing the drag on the rotor blades.
The engine is there to overcome the drag on the blades (lift being a very useful by-product).
If you fix the collective, start at 60 knots, and increase airspeed to 90 (in a dive naturally), you'll see the MAP/Torque decrease. With the same collective position slow to 20 knots or so, and you'll see a torque/MAP increase.
Same thing happens when going through translational lift- at the same time that you see a large increase in 'performance', you'd also see a decrease in torque/MAP when you didn't move either the collective or pedals.
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