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 Post subject: prop ?
PostPosted: Mon Oct 29, 2007 9:50 am 
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A 2 part question for all you A & P experts. In most singles, especially with a Merlin, if you lose all oil pressure, what happens to the prop pitch? Would it go to fine, max rpm, or just remain as you have it? Next, I have friend with a Mooney M-20E with Lyc IO-360, 200hp injected. On the first takeoff each day,he doesn't get full rpm; it wiil be down about 100. All others that day are normal, even if the plane is parked for long enough to cool down.It doesn't seem to be weather/temp related.

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PostPosted: Mon Oct 29, 2007 4:21 pm 
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I think the Hydromatic goes into "Increase RPM" with a loss of oil pressure, but the Ham-Std counterweight prop goes into "Decrease RPM." When you shut down an engine with a counterweight prop you normally go into "Decrease" to move the oil from the cylinder into the crankcase so it can be scavenged. This keeps you from overfilling the oil tank during servicing and protects the piston from corrosion and dirt because it gets covered by the cylinder which is exposed in front of the propshaft.

I belive the Rotol contra-prop also goes into "Increase" with a loss of oil pressure.

Of course if you have a Bearcat or P-51K with the Aeroproducts prop they have a seperate reservoir and pump and the P-38 and P-40 are electric.

Time to break out the old A&P books I guess!


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PostPosted: Mon Oct 29, 2007 4:38 pm 
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The mechanical tachometer and drive shaft on the typical GA aircraft are notoriously inacurate. Try checking the tach for errors before adjusting anything on the engine/prop governor, etc.

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PostPosted: Mon Oct 29, 2007 6:59 pm 
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I believe the prop should go to low pitch (T.O. & Landing condition)

Phil


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 Post subject: Mooney
PostPosted: Mon Oct 29, 2007 7:40 pm 
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Sky, the issue of whether the tach is accurate is one thing. I believe it has been checked. But the real question is why the rpm will not go to the limit only on the first takeoff of the day? I have a little prop check device that is handheld and battery operated so we will try that, but either way the rpm should go to the limit on the takeoff roll, whether that limit is exactly 2700 or not. It does not seem to be just a matter of cold oil or engine temp, it did it even on a mild day. It can also sit all day and cool off and will go to the limit as long as it is not the first takeoff.

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PostPosted: Mon Oct 29, 2007 10:53 pm 
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If the oil temperature is 180F at cruise, are you saying that the oil temperature is at 180F on the first takeoff of the day?

On the early part of the takeoff roll the prop should be on the internal stops. Once it gets rolling fast enough it should get into the governed range. Climbout should definitely be in the governed range. When was the governor last overhauled? Sometimes the engine gets overhauled 3 times and the accessories never do!


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PostPosted: Thu Nov 01, 2007 12:23 am 
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Physics: the Centrifugal force of a spinning propeller makes it want to seek low pitch (high RPM.) The balance between engine oil pressure and prop governor oil pressure in the dome (Hydromatic propeller) is what adjusts the pitch of the propeller. That is why if you blow a prop seal, or if your governor stops pumping you have a runaway propeller (i.e. the RPM exceeds the Max limit.)
Other Propellers use different means of regulating blade angle, but the physics are the same.

Patrick Mahaffey
B-17G "Chuckie" pilot/mechanic
B-25 "Pacific Prowler" Crew


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PostPosted: Sat Nov 03, 2007 9:36 pm 
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Bill: One thing to have your pal check is that when he cycles the prop, that it is going into full decrease and that he is allowing the prop to be at the full decrease prosition for one to two second before going back to full increase. It sounds like the pitch change mechanism is not coming completely back to the low pitch stop. It could be that there is something blocking this movement until the oil is flushing it away after the first takeoff or, there is the possiblilty that the oil pressure in the govenor is a bit too high until the engine is fully warmed and all the cold oil and sludge is scavenged out of the prop and gov.


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PostPosted: Tue Nov 06, 2007 12:57 pm 
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Bill

I would think the prop would sit on the low pitch stops (High RPM) till 2700 is reached (Most wont till rolling) then the prop governor kicks in and takes a bigger bite of air and maintains 2700.

Steve

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 Post subject: prop ?
PostPosted: Thu Nov 08, 2007 3:34 pm 
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Thanks guys, I will likely talk to the Mooney owner in the next few days, and will let you know if we come up with anything. Steve, my thinking is like yours,it should tend toward flat and take oil to force to coarse. That's why this is hard to figure. The Mooney manual says to only cycle it to 100 rpm and only briefly so we didn't give it much of a workout.

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