Manifold Propeller Check

bc2209

bc2209

Well-Known Member
I've basically been taught to exercise the propeller control three times during the runup. While doing this we are looking for a rise in manifold pressure, a drop in RPM, and a drop in oil pressure. That's fine and dandy because they all move and do what I'm looking for.

However it was brought to my attention that, "where does it ever say to do that". Uhhhh ?

So I was thrown into the discussion of why manifold increases when you exercise the propeller control.

Does anyone have a good explanation to this? I've been trying to work it out in my head but I'm getting stumped.

Since we are changing he blade angle, slowing the RPM, is the result sucking less fuel through the intake manifold?

That could be wildly off.

Thanks guys
 
The lower the RPM, the higher manifold pressure will be. When the RPM is zero (engine off) the the manifold pressure will match outside air pressure.

When you start the engine (RPM sucks) the engine is trying to pull air, however the (idle position) throttle gate is restricting air. This results in a low pressure in the air intake. You can increase that pressure by opening the throttle which allows more air into the intake, or reducing RPM which results in the engine sucking less air out of the intake.
 
Should you one day fly a turboprop, you'll note torque (a measure of power which is basically the turboprop equivalent of manifold pressure) will do the same thing - rise with a reduction on RPM.

In the 600-hp Cessna Caravan for example, the torque limit for takeoff is 1658 ft/lbs at a prop RPM of 1900. That combo produces 600 shaft horsepower.
Once in climb/cruise, the prop RPM can be reduced as low as 1600 RPM, with the torque limit rising linearly as prop RPM decreases, until it hits a minimum RPM of 1600, where the torque limit is 1970 ft/lbs...which also gives you 600 shaft horsepower.

The common car formula works for turboprops as well (you just have to know the actual torque rating, not a percentage):
Horsepower = (torque x speed) / 5252

There's also a little extra power from the jet thrust of a turboprop, but that's another discussion. ;)
 
Yeah, once is enough. If the prop changes RPM with the prop lever once, you're good to go. Doing it several more times isn't going to show you a malfunction. The only reason I can think of for doing multiple prop cycles is to dump cold oil out of the hub on the first flight of the day.
 
Spoke with a McCauley propellor guy one day who said exercising it is very hard on certain parts of the hub and they recommend once. One cycle will get the warm oil in there and allow you to make sure it works. If your propellor needs a feather check, you can do this in one fell swoop. All this assumes you're in a piston airplane.
 
Roger Roger said:
Yeah, once is enough. If the prop changes RPM with the prop lever once, you're good to go. Doing it several more times isn't going to show you a malfunction. The only reason I can think of for doing multiple prop cycles is to dump cold oil out of the hub on the first flight of the day.
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And the only place I can see getting warm oil in there beneficial is up in Alaska. So twice should be more than enough.
 
Roger Roger said:
Yeah, once is enough. If the prop changes RPM with the prop lever once, you're good to go. Doing it several more times isn't going to show you a malfunction. The only reason I can think of for doing multiple prop cycles is to dump cold oil out of the hub on the first flight of the day.
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mshunter said:
And the only place I can see getting warm oil in there beneficial is up in Alaska. So twice should be more than enough.
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Indeed.

Most folks here are not messing with radial engine airplanes, or big turboprops where really flushing the domes is required; consequently, once (and a feathering check, for multiengine airplanes) is adequate. "Does it work? Y/N?"'

If anyone cares, we actually feathered the Brasilia's props three times on taxi-out on the first flight of the day (twice manually, once electrically); on the Martin, two full-range checks of pitch control were performed along with feathering both propellers once.
 
Once is enough, and check the feather detent if the checklist calls for it.

Multiple prop cycles is up there with other OWTs along with avoiding oversquare and running lean of peak burns valves.

Richman
 
Reason why three times is because you are looking to note three separate things the rise in MP, drop in Rpm, change in oil pressure etc. If you could note all those in one prop cylce then by all means, but for most students/pilots tend to follow the three cycle and note each individually. Once the habit is developed then it stays with them, and so the next guy.
 
Is there ever a time that one will happen without the other? If not then why not look for the RPM drop and move on.
 
belizepilot said:
Reason why three times is because you are looking to note three separate things the rise in MP, drop in Rpm, change in oil pressure etc. If you could note all those in one prop cylce then by all means, but for most students/pilots tend to follow the three cycle and note each individually. Once the habit is developed then it stays with them, and so the next guy.
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I would hope that a halfway competent pilot could determine the functionality or lack thereof of the engine instruments by their normal behavior through the runup, without cycling the prop multiple times.
 
Roger Roger said:
I would hope that a halfway competent pilot could determine the functionality or lack thereof of the engine instruments by their normal behavior through the runup, without cycling the prop multiple times.
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I agree, however many student pilots I've known are not halfway competent, yet. Once they've improved to a point that they don't need to cycle it three times to catch all the instrument indications they've already developed that habit.

Of course it doesn't help if, as an instructor, your hands are tied. The SOPs of my last job required us to cycle it 3 times every time we started the engine, and 1 additional time for a feather check. Any less and your student would probably fail their stage check.
 
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