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RPM vs MPRPM vs MP

 
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Thom Riddle



Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

PostPosted: Sat Feb 25, 2006 6:06 am    Post subject: RPM vs MPRPM vs MP Reply with quote

Dave,

Here is how I do it in my Titan Tornado which has a 912UL (80hp) engine
and the GSC in-flight adjustable prop.

Take-off and initial climb to minimum turn-back altitude (about 400
AGL).
I set the pitch so that at full throttle and Vy speed, the tach is
reading 5,500 rpm (max. continuous cruise speed). I could set a little
finer pitch to get 5,800 rpm but always stay out of the yellow band.
Don't ask me what that pitch is but because I don't know, but have it
marked on the vernier prop control. This is an advantage of the vernier
over the electric operated props.

Cruise Climb
I dial in two turns more of prop pitch (about 2 degrees more) which
gives me 5,200 rpm at full throttle at a speed of about midway between
Vy and normal cruise speed. I know this because GSC says that one full
turn of the control is about degree change in pitch. The manifold
pressure in this regime is of no concern to me.

En-route Cruise
I dial in enough pitch to lower the rpm to what is comfortable to me
(noise etc) that will allow full throttle operation. For me this is
about 4,500-4,800 rpm depending upon how fast I want to go. Above 5,000
rpm it gets pretty noisy. The manifold pressure is what ever full
throttle allows at the density altitude I'm flying at.

Slow Cruise
This is the only configuration I use less than full throttle. I use the
same prop pitch here as I use for en-route cruise and just throttle
back to whatever speed I want to fly.

Pattern Work
I use the Cruise Climb pitch setting while in the pattern until I'm on
final approach then go to take-off pitch so it will be ready for
go-around if required. Throttle is whatever is required to hold the
speed needed on whichever leg I'm on.

Note the following:
- The primary purpose of an in-flight adjustable prop is to allow the
pilot to control what rpm the engine runs at under all flight regimes.
- The engine is generally most efficient near full throttle operation.
- From best climb to best cruise on my Titan is about 4 degrees
difference in prop pitch (4 turns on the GSC vernier control).
- At full throttle, manifold pressure goes down with increased density
altitude no matter what type of prop you have.
- Power and thrust produced is a function manifold pressure, so as you
climb, power produced decreases.
- To maintain power produced as you climb higher takes about 67 rpm
increase per 1,000' increase in density altitude.
- TAS (not IAS) goes up about 1% per 1,000' increase in density
altitude at a constant power setting (not constant rpm).
- IAS goes down relative to TAS about 2% per 1,000' increase in density
altitude.

I hope this helps. Let me know if you have more questions.

Thom in Buffalo


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Thom Riddle
Buffalo, NY (9G0)



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