Variable Prop question

UDaviator

UDaviator

Domo Arigato, Mister Roboto
I am typing up a general checklist for review and want to make sure it is done right. When you do a prop cycle you are looking for a MP Rise and a RPM Drop? Or what else are you looking for? Oil pressure rise? Just trying to review. Thanks guys!
 
Pre flight.....cycle and check for drop in RPM, check for rise in manifold pressure, check for drop in oil pressure. You can check for any oil spray also. What you are actually checking in most single engine aircraft is the prop governor. (non feathering aircraft props, which what I am guessing you mean) Cycling it once is sufficient, some guys do it more, especially if the wx is terribly cold. Follow the POH for the aircraft.
 
UDaviator said:
I am typing up a general checklist for review and want to make sure it is done right. When you do a prop cycle you are looking for a MP Rise and a RPM Drop? Or what else are you looking for? Oil pressure rise? Just trying to review. Thanks guys!
Click to expand...
RPM drop

You just need a 200 or 300 RPM drop. Taking it down further than that is hard on the engine.
 
Yep...did some more diggin n found RPM drop, MP Rise, and Oil Pressure Drop....thanks!!
 
In my experience if it's throwing oil you won't see it the mist is so fine.

Oil pressure will go down and manifold pressure will increase but I never saw a reason to check that.
 
Just a tip...

Look up the difference between a "Variable Pitch" and "Constant Speed" prop.


Something else to think about:

Why does the MP increase when you decrease RPM when making power changes?
 
UDaviator said:
Yep...did some more diggin n found RPM drop, MP Rise, and Oil Pressure Drop....thanks!!
Click to expand...


....... and, as a young man once accidentally demonstrated to me ..... if you mistake the mixture control for the prop control, you will get the same results as you pull the mixture back and put it forward with the change ...... until he did that, I had never thought about it, but it was interesting ........ embarrassing for him, but I thought it was interesting.
 
As the prop slows down, MP rises. Basically the faster the engine turns the stronger the vacuum in the intake manifold. If it slows down, air has a better chance of making it in there, hence greater volume of air and higher MP. If you speed up the engine, it is as if it's gasping for air, it has a hard time getting all that it wants. Hence turbochargers and superchargers... or a bigger intake.

If you change the prop speed and see no change, either some form of charger is doing a really great job, or 99% chance you have a bad MP gauge.
 
fholbert said:
RPM drop

You just need a 200 or 300 RPM drop. Taking it down further than that is hard on the engine.
Click to expand...

Agreed, this is pretty important. The other thing that hasn't been mentioned is by cycling the prop control, you are getting warm oil up into the governor and everything then works happily.
 
On a constant speed, feathering prop (in a twin), usually the POH/checklist will have you decrease to a lower power setting (after the mag check and governor check) and cycle the propeller in and out of the feather detent (The Duchess said no more than a 500 rpm drop IIRC)

BTW, the Dash 8 prop gov is really cranky until you get some warm oil up there...
 
My school checks MP rise, RPM drop, Oil pressure drop, and pull the lever back and hold to make sure it can hold a new RPM.

Hammertime said:
On a constant speed, feathering prop (in a twin), usually the POH/checklist will have you decrease to a lower power setting (after the mag check and governor check) and cycle the propeller in and out of the feather detent (The Duchess said no more than a 500 rpm drop IIRC)

BTW, the Dash 8 prop gov is really cranky until you get some warm oil up there...
Click to expand...

Why is the prop not allowed to have more than a 500 RPM drop in a feather check? (in my case 300 RPM drop (DA42)). The prop feathering on the ground shouldn't be an issue because of the low RPM and the centrifugal locks right? What happens if you go past the allowable drop?
 
the centrifugal lock pins only prevent feathering when the RPM drops below about 900 at shutdown.

too coarse of a pich under power is hard on both the engine, and the prop system
 
Ive never understood checking for those things, if the prop cycles you will get them no matter what. The 182 I fly now and again gets no oil pressure drop, but the prop still cycles ;)

ATPs way actually made more sense to me, throttle to 2000, prop to 1800, increase power to check governing, visual inspection for oil or smoke...

I cycle it until it responds quickly and look for anything scary.


Sent from 1865 by telegraph....
 
Question for a mechanic: if you go below the max RPM drop during a feather check would you have to perform a inspection?
 
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