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Propeller position after engine stop

For propellers with no up and down, it may stop in the same place probably, due to gavity and (bellow threshold) symetry/centering of the wholr engine camshaft/prop?

The drawing, is that for a 3 blades or 2 Peter?

I flew a motor-glider TMG it alwyas stopped up/down so I always have to cranck it 90 degrrs before engine off landings

Last Edited by Ibra at 15 Apr 13:59
Paris/Essex, France/UK, United Kingdom

An engine won’t stop with a piston near the top of a compression/expansion stroke. It will continue to turn, driven by the pressure in that cylinder, until counterbalanced by the next cylinder starting its compression stroke. Of course inertia and friction will play a role, but I would expect it to come to a halt roughly with the two pistons in the same position

In a four cylinder engine, the expanding and compressing cylinders are 180 degrees apart, so the engine would “want” to stop roughly around 90 degrees between them. So in a convention where 0 degrees is TDC of the intake stroke on cylinder 1, it would “want” to stop at 450 degrees (5 × 90, halfway down expansion), when the next cylinder is at 270 degrees (3×90, halfway up compression) – at this point the pressure in these two cylinders will be roughly the same. The two other cylinders are in intake / exhaust strokes with valves open, so they are at ambient pressure.

With a four cylinder engine, this situation repeats for all four cylinders, leading to a stopping position roughly 180 degrees apart. As anyone who has pulled through a four cylinder engine can testify: They tend to go through 180 degrees when pulled through quickly. If you pull them through very slowly, the air in the cylinders can escape past the piston rings, and you can put them it into any position.

A six cylinder engine has the same thing occur every 120 degrees.

So – two blade props on four-bangers and three-blade props on six-bangers tend to stop with the same blade pattern.

Last Edited by Cobalt at 15 Apr 14:09
Biggin Hill

The drawing, is that for a 3 blades or 2 Peter?

3 blade – you can see the three blade locations shown.

Anyway, this is what I am thinking (and as I typed it I see Cobalt posted a similar view):

A 6 cylinder 4-stroke engine has three compression points per revolution.

The engine is more likely to stop somewhere between these points, than at other points, and is especially unlikely to stop at one of these points.

So I think the new prop has been screwed on in a different orientation. The typical Hartzell flange looks like this

and this is the crankshaft end on my IO540-C4D5D

and as you can see there are two shorter bushes on the crankshaft so the prop could go on in either of two orientations. Given that these orientations will be 180 degrees apart, a 3B prop will end up exactly as per one of the 2 pics in post #1.

A clever mechanic might well use this option to make the engine stop predominantly at the point convenient for towbar attachment

There is also some old debate out there as to whether the orientation affects the vibration; I cannot see how it could because the prop is symmetrical about its axis and is rotating in uniform air (the same may not be true for a rear mounted prop as on e.g. a Cessna 337 which is getting air pressure pulses from the prop at the front).

I wonder if there is a case for a specific prop orientation on planes which are hand-started? You obviously want the prop to be at the “right” angle relative to when the magnetos fire, so you can get your arm out of the way

Another interesting angle is that the engine compressions should be more pronounced if the throttle is more open when the engine is being shut down; there is more air to compress. You won’t feel this difference when rotating the engine by hand, because the leakage past a “closed” throttle is plenty to fill the cylinders fully. I reckon if you shut the engine down with the throttle closed, it is less likely to stop at particular points. But that is a bad procedure; one is supposed to shut down an IO540 from 1200rpm or even more, not from idle, because that produces less shudder, supposedly…

Administrator
Shoreham EGKA, United Kingdom

Yes you are right on mags/prop for hand start (e.g. super cub and falke), you need to “suck in with mags off” past the ignition point with mags clicking once or twice, then hand starting and ensure that hands are clear when it starts with mags on (no preferred position for handoff, obviously you should not hold on it more than 30 degres)

It does not work otherwise, the mannuals insists on hearing the mag impluse click when rolling back before rolling forward

I dont think it matters where the trottle is, pulling the mixture off will do always the same job but the engine may not like to run bellow 1000rpm for long

Last Edited by Ibra at 15 Apr 15:12
Paris/Essex, France/UK, United Kingdom

PetitCessnaVoyageur wrote:

After propeller governor overhaul, it became like this:

I thank you for your science. Enlightening !

My sentence was wrong. I now think it had no thing to do with the governor overhaul.
It happens that at the same moment, the propeller was removed, and sent to be flushed out (“dépollution” in French). I guess that the change happened when it was set up again

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