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HeliTorque :: View topic - Mainfold Pressure Question
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HeliTorque Forum Index » Flight Dynamics

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Mainfold Pressure Question
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PilotWolf
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PostPosted: Sun Apr 17, 2005 1:54 pm    Post subject: Mainfold Pressure Question Reply with quote

Try and answer this one without searching elsewhere...

Quote:
This has puzzled me for a while now. On the 44 I fly the Man pressure drops by a couple of inches in steep turns. This without adjusting power at all. Rolling out of the turn the pressure comes right back up to the pre-turn level.

Am I missing something basic here??
'

Now the poster got one sharp response to it and some sensible ones... but I have to admit that until someone posted an explaination I couldn t for the life of me figure it out Sad

PW
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weekend warrior
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PostPosted: Sun Apr 17, 2005 8:28 pm    Post subject: Reply with quote

.

Last edited by weekend warrior on Mon Jun 05, 2006 4:30 pm; edited 1 time in total
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R22-Adam
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PostPosted: Sun Apr 17, 2005 10:45 pm    Post subject: Reply with quote

That makes sense....the governor would reduce the engine RPM to avoid overspeed, as the rotor speeds up slightly, hence the manifold pressure reduces with the engine RPM reduction.

Right?
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Thomas Coupling
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PostPosted: Tue Apr 19, 2005 3:57 pm    Post subject: Reply with quote

Confused

In the turn the blades cone, it also flaps (or bends) upwards. The centre of gravity of each blade is closer so the rotors speed up slightly because the angular momentum can't change, therefore the RPM increases to match the stored energy in the system. As the RPM speeds up the governor reduces.

Got it?
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aviatorjames
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PostPosted: Wed Jan 25, 2006 10:09 pm    Post subject: Reply with quote

I'm not sure blade coning could account for a couple of inches difference. (I too, have not flown a piston helicopter in a while) ..but that seems like a large change.

Air is light, but is affected by gravity and load factors. It sloshes around in tubes and gauges and stuff. It may just be an indication error from the load factor.

Interesting experiment while driving home from a party...

First, steal two helium balloons from the party.
Then, with one balloon floating happily on the ceiling...

Exclamation !!!keep your eyes on the road!! ..and no, this won't work with the top down!!! Exclamation

...step on the brakes and watch the balloon zoom to the back of the car. Make a turn, and it floats to the inside of the turn.

Now, after you've explained all this to the police that has pulled you over for erratic driving, you will have a renewed appreciation for the fluid nature of air!!

The second balloon is for inhaling just before you start talking to the police.

james


Last edited by aviatorjames on Fri Jan 27, 2006 5:53 am; edited 1 time in total
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Flying Foxy
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PostPosted: Wed Jan 25, 2006 10:30 pm    Post subject: Reply with quote

This brings in my quandary about the draught in the car when turning corners even with the windows shut...

FF

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aviatorjames
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PostPosted: Tue Jan 31, 2006 4:07 am    Post subject: Reply with quote

FF...

Yeah, air is pretty fluid stuff!! The balloon thing is fun.
Really gives one an appreciation for the air "sloshing" around like beer during a toast!!!
Cheers!

Cheers!!!
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LR
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PostPosted: Sun Mar 19, 2006 10:00 am    Post subject: Reply with quote

The same thing happens when pushing forward on the cyclic during normal flight. The manifold pressure will increase and when pulling back on the cyclic the manifold pressure will decrease.

Reason: When loading the disc by pulling back the coning angle increases and the blades tend too speed up due to the conservation of angular momentum (Coriolus Force), but it also decreases because the induced flow is decreased when tilting the disc backwards, which in turn reduces the rotor drag and therefore the governor must roll off some throttle to maintain a constant Rrpm, which you experience as a decrease in Manifold pressure. The reverse happens when you push forward on the cyclic. Now turning is exactly the same thing.

I normally demonstrate this to students during their first few sorties after having discussed it in the lexture room.
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Hawkdriver
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PostPosted: Tue Mar 21, 2006 12:14 am    Post subject: Reply with quote

Im going to quote from the maneuvering flight handbook because it can explain better then I could in summed up sentence.

It occurs in aircraft with FADEC/ECU/DEC equipped aircraft also.

Quote:
Transient Torque
Transient torque is a phenomenon that occurs in single rotor helicopters when lateral cyclic is
applied. For conventional American helicopters where the main rotor turns counterclockwise
when viewed from above, a left cyclic input will cause a temporary rise in torque and a right
cyclic input will cause a temporary drop in torque.

Transient torque is caused by aerodynamic forces acting on the rotor. At the rear half of the
rotor disk, downwash is greater than that seen at the forward half of the rotor disk. This effect is
more pronounced for heavier aircraft, which will exhibit greater coning due to their weight,
which caused even greater downwash at the rear of the rotor disk. If a left cyclic input is made
by the pilot, the following events occur which will lead to a temporary increase in torque:

1. Pilot makes a left cyclic input
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