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More HP = more RPM?

bret

Well Known Member
Question, So my IO 360 A1A angle valve and Hartzell CS prop came out of a Mooney, in the logs I see the governor being removed and worked on, so I am assuming they had it set up for a max 2700 RPM. So now it has a full dual EFII setup, when I do a practice launch, ( 3 seconds full power down runway, < 40 Knots ) I see 2850 RPM. I have searched and read where folks dial in the fine pitch stop on the prop 1.5 to 2 turns in, to have the prop slightly more course pitch. Is this what I need to do to bring the RPM down? I am trying to understand the science behind this, Also I am doing this at 4720 MSL, does density altitude-altitude play a part in this max RPM issue? Thanks, :rolleyes:
 
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Question, So my IO 360 A1A angle valve and Hartzell CS prop came out of a Mooney, in the logs I see the governor being removed and worked on, so I am assuming they had it set up for a max 2700 RPM. So now it has a full dual EFII setup, when I do a practice launch, ( 3 seconds full power down runway, < 40 Knots ) I see 2850 RPM. I have searched and read where folks dial in the fine pitch stop on the prop 1.5 to 2 turns in, to have the prop slightly more course pitch. Is this what I need to do to bring the RPM down? I am trying to understand the science behind this, Also I am doing this at 4720 MSL, does density altitude-altitude play a part in this max RPM issue? Thanks, :rolleyes:

Did your governor come out of the same make and model engine as your current IO360A1A? Different engines have different gear ratios driving the governor (often the governor is NOT at 2700 RPM when the prop is). The fine pitch stop can be adjusted on the governor; but I'm not sure if there's enough range for 150 RPM adjustment.
Many people set the PROP pitch stop to limit the RPMs to 2690 when the plane is not moving, no wind, sea level. That sort of gives the max possible protection (which isn't really that much) should the governor fail.
I strongly urge you not to operate at 2850 RPM.
 
Only high speed taxi, not flying yet.....waiting on our not so efficient FAA aeromedical department. The governor is the one that came with the engine-prop combo.
 
Prop RPM

Bret I would highly suggest setting your rpm at something less than maximum fine until you get a couple of takeoffs under your belt. This means you should be operating from a minimum 4000' runway. Your RV should have all sorts of take off performance to easily fly long before you get anywhere near the runway end. I had an MT prop governor destroy itself from a poorly engineered internal bushing that resulted in a prop over speed, and ultimately a crank shaft replacement. Had I had the rpm dialed down sufficiently, I may have saved myself a lot of trouble. I know my IO-540 with just me in it will easily takeoff at lower rpm settings, just takes more runway. Just make sure you talk over your plans with an experienced fellow RV'er who know the performance of your type of aircraft.
Also a higher density altitude does have an affect on engine performance, but I doubt it will make much difference on rpm (not 100% sure on this though).
Rick
#40956
Southampton, Ont
 
Prop pitch

Bret: You need to adjust the low pitch stop to limit the max rpm to 2700 on the take off roll(add some more pitch). This is easily done on most Hartzells with the threaded stem & locknut on the nose of the prop. The Hartzell owners manual explains how to do this. I get home Monday night and can send you a copy of the procedure.

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
looks easy enough, lock nut and Allen screw in to go more course, what rule of thumb does one turn in, relate to as far as decreasing RPM? no worries with runway to play on, 7400 X 100
 
Pitch adjusting

I wouldn't go more than 1 full turn at a time. May take a couple of runs to get it right.

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
There are steps to fix this.

Firstly, you need to adjust the prop fine pitch stop. At sea level, you should get something in the order of 2600rpm on a static full power run. At your sort of pressure altitude, you should aim for something slightly lower - say 2500 - 2550.

Now, when you accelerate, the RPM will increase. This is where the governor comes in and you should limit the RPM to just below 2700 using the high stop screw on the governor.
 
What limits the RPM, is it oil pressure against the prop return spring + dynamic load of the prop blade, and will viscosity of the oil have any affect on these adjustments, ( should I stay with the same wt of oil ) summer to winter time delta of 100 deg here.......
 
Prop Rpm

On takeoff the low pitch stop setting inside the prop limits max rpm until the governor begins to govern. Until that happens your prop behaves like a fixed pitched prop. Exactly when the governor comes into play varies a little, but usually about 1/2 way thru the takeoff roll to right after liftoff is when you see or hear it. When we all flew with mechanical tachs there wasn't much to see because the needle movement was damped and couldn't display rapid changes. In flight the governor controls rpm by continuously changing the prop pitch by sending or bleeding off oil pressure from the prop hub. In our non-aerobatic props the BAS (big a$$ spring) in the prop hub pushes the prop blades toward low pitch and oil pressure toward high pitch. The oil control valve in the governor is positioned by spring pressure you set with the blue knob and opposed/balanced by the governor flyweights. All the forces in balance = a constant rpm. When your engine is shut down and there is no oil pressure the BAS is firmly holding the blade pitch on the low stop. You can run the same wt oil year round, but you might see slight overspeeds on takeoff with really cold oil. One of the Mooneys I used to fly would overspeed 30-40 rpm so I would back out the blue knob 1/2 turn for the first takeoff on really cold mornings.


Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
In a typical single, oil pressure pushes the prop to a coarser pitch, e.g., limits rpm. In most twins, and many aerobatic planes, it's the other way around.
Did you cycle the prop during run Up?
 
I'm no expert, but.......

The engine oil is ported through the governor. The actual internal workings are a bit of a mystery but I believe that spinning bob-weights balance out at the correct RPM and that the fine/coarse ports are both blocked. If the RPM varies then the appropriate port is opened to adjust the prop. Adjusting the RPM moves the datum in the governor which ports the oil to drive the prop coarse or fine until a new balance point is found. Something like that - long time since I did my prop theory! I am sure you can find the material on line.

As for the grade of oil, I believe you use whatever the engine needs. Before flight, you should exercise the prop control 2 or 3 times at your mag check RPM to port warm oil through the governor. Also, make sure everything is warm before doing any adjustments.

Just to summarize the adjustment procedure again. The static RPM should be controlled by the prop fine pitch stop and be around 100rpm below max. Only when the airflow through the prop increases the RPM should the governor kick in and coarsen the prop to control just under max.
 
Governor

In 3 seconds the gov should control the RPM - especially if you made sure it was working before you headed down the runway. The pitch stop on the dome does not set the low RPM - it is there to keep a runaway prop from going to an RPM that causes instant engine damage. If you crank on the low RPM stop, it will still hit 2850 once you get up to 50kts or so.

Methinks your gov is setup for the wrong drive ratio, or it is actually set for 2850. EZ fix either way.

Meantime you can simply pull the lever back a bit...

Congrats on getting this far!!

Carry on!
Mark
 
I'm no expert, but.......

The engine oil is ported through the governor. The actual internal workings are a bit of a mystery but I believe that spinning bob-weights balance out at the correct RPM and that the fine/coarse ports are both blocked. If the RPM varies then the appropriate port is opened to adjust the prop.
.

In most (all?) setups there is only one port - oil under pressure out to the engine. When the governor closes this port, internal leaks back to the crankcase quickly reduce the pressure.
 
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