Hi Everyone,
Just wondered if anyone with a better mechanical mind than me can help me understand an aspect of the subject.
Recently while doing a MEP rating (BE76 Duchess) , I was taught during the feather check for the propeller you set 1500rpm, then feather the propeller for a max 500rpm drop before putting it back to high rpm. While doing this I should be watching for RPM to increase, manifold pressure to decrease and oil pressure to increase.
Can anyone help me understand exactly why these 3 things happen? RPM increasing I can understand. Manifold pressure decreasing I would be grateful of an explanation for to build on the basics that i already have. The main one I need further explanation for is the oil pressure increasing.
The POH explains that centrifugal forces and dome pressure move the propeller to coarser positions, while boosted oil pressure fine the propeller. So to my knowledge Oil will be leaving the propeller when its being feathered and will then be reentered when its returned to high rpm.
Two explanations I have been given is that the higher rpm drives the engine driven pumps faster and thus creates higher oil pressure, very good, that makes sense to me. I have also been told that its more to do with the accumulator pushing the oil out and that is given the increased pressure. Is it either, neither or a combination of both?
Thanks greatly for any help.
With the power remaining unchanged, varying the RPM (propeller pitch) will result in an inversely proportional change in torque, which is approximated by MP. Increasing one will decrease the other, as long as you don't adjust the throttle.
As long as you are indicating appropriate oil pressure, I would not be too worried about watching for oil pressure changes. If the flight manual tells to see them, then you must, but I have never seen a FM which required this (Never flown a Duchess though...). If you see oil pressure change, within the green arc, that's fine, but I would not be worried if you do not, as long as the props are doing what you expect they should.
Some props and feathering systems work a little differently, but at the pilot end, things are the same.
On the subject of feathering, assure that are are familiar with any feathering limitations for the engines. You might find that the engine will not allow feathering the props at less than 800 RPM or so. This is important to know, in that if you let the engine stop, or it seizes, you might never get it feathered. Some flight manuals state this, others don't. I had it added to the IO-360 DA-42 FM, when I reviewed it. Also know if you can unfeather a feathered prop in flight - some yes, some no.
