Question for the technically minded helicopter people here ?

[brianw999]

During my recent holiday to the Dominican Republic I took this cockpit shot of an R44 Raven. The aircraft was totally shut down so.....can anyone explain why the manifold pressure gauge shows 30" of mercury !!! ?

http://www.jetphotos.net/viewphoto.php?id=5944566

Your answers may prevent me from flying with this lot on my next trip...

[screaming_emu]

Easy, the engine isn't running.

For reciprocating (piston) engines with either a constant speed propeller or a rotor on them, the amount of power the engine is putting out is shown by looking at the amount of pressure in the intake manifold of the engine. A normal tachometer will not work because in a helicopter you can change the pitch of the blades which changes the rate at which they are spinning. With the engine putting out the same amount of power, you can get the rotor spinning at 80% RPM or 100%, just by changing the angle of the blades.

The reason it is showing 30 inches is because the engine is not running and the ambient pressure outside is 30 inches. When running, the pressure in the intake manifold inside the engine is going to be less than the ambient pressure outside the engine because the air is being sucked into the cylinder from the intake manifold.

Hope this helps. I'm also going to move this into the technical forum.

Now for a bonus question to those pilots out there, if the mainfold gauge is reading 30", thus meaning that the pressure outside the aircraft is 30" why might our altimeter setting be different?

[E-Diddy!]

Bonus answer: The manifold pressure gauge reads the ambient pressure at the field. The altimiter setting is the ambient pressure corrected down to sea level pressure.

[screaming_emu]

Bonus answer: The manifold pressure gauge reads the ambient pressure at the field. The altimiter setting is the ambient pressure corrected down to sea level pressure.

10 points for E Diddy!

[brianw999]

Bloody hell !! I opened up a can of worms there, didn't I ?

...and thanks for the answer. All makes sense now. At least I can take a trip with them now without wondering if its a good idea or not.