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Constant speed prop behaviour with power lever at idle.

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Constant speed prop behaviour with power lever at idle.

Old 16th Feb 2021, 18:36
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Constant speed prop behaviour with power lever at idle.

Hi folks,

Wondering if anyone can help answer this query re: prop and governor behaviour and please correct me if I have got anything wrong.

Whenever I have flown constant speed prop aircraft (which hasn't been that often), the engine is usually started and shut down in fully fine pitch i.e. blue fully forward. I assume the prop is on the fine pitch stop at this time. Likewise prop checks are normally done with the prop at the required rpm so that they are in the active range of the governor, therefore the rpm will change as the prop is cycled.

However, what happens if the prop is cycled (not suggesting it should be) whilst on the ground and the throttle is at idle? Will there be an increase in pitch and therefore a further drop in prop rpm or will the rpm remain the same?

Thanks in advance,
Rich
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Old 16th Feb 2021, 20:34
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As always, it depends. Suppose, for a moment, that your prop is adjusted so that full fine is 2700 RPM, while full course is 2100 RPM. And the engine is idling at 1200 RPM, and the governor is set to full fine. The governor is then at the fine stops: It wants to go even finer to reduce the drag and increase the RPM but it can't.

Now adjust your MAP so that the engine makes, say 2400 RPM. At this stage the governor doesn't do anything, so it's still at the fine stops. But now pull the blue lever backwards. About halfway through that blue lever setting it passes the 2400 RPM setting. That will lift the governor off the fine stops. Pull the blue lever even further backwards and the engine will initially be making more RPMs than what the governor is set to. So the governor will change the blade pitch, setting them coarses, increasing the drag, until the RPM matches the governor setting, all the way to 2100 RPM.

However, do the same thing when the engine is idling at any speed below 2100 RPM and not a single thing will happen. The governor will simply not lift off the fine stops.

So it all depends on what your engine RPM is when idle, and what the full coarse stop is adjusted to.

Edited: Have to add that this assumes a normal single engine piston, where the failsafe (no oil pressure) is to go to the fine stops. If you're talking about a twin, with either autofeather or the ability to feather, or an aerobatics engine that defaults to full coarse, then things may be different.

Last edited by BackPacker; 16th Feb 2021 at 20:48.
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Old 17th Feb 2021, 12:16
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What he said.

You mentioned starting in full fine, there are a few reasons for this. It is the preferred setting for take-off as it is the 'low-speed' setting, so you don't need to change the prop setting between startup and take off (other than for the runup). It also (on a generic piston single) means lower drag for the starter motor to work against during the starting cycle.
There you go, more confusing prop facts
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Old 18th Feb 2021, 12:16
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For S/C props on single piston engined GA planes, it's generally "The pilot can't get it wrong". If you need to move the prop control, do it gently. If you were to fly, and never touch it, you'd make more noise, and waste some gas, but not hurt anything. There are different systems, but those differences will be transparent to the pilot. With the exception of coursing out the prop in the case of a prop overspeed (generally a result of a maintenance defect), there's otherwise not a lot that a pilot needs to do to keep things within limits.

As always, there are exceptions, but they are rarely found on GA piston singles. If you're flying a GA piston plane which is different, someone is going to tell you, 'cause it's a feature. The example which comes to mind is that a few MT props on piston GA planes have reverse capability (you'll see the switch). That system is foolproof, though reading the flight manual supplement is a good idea!

I understand that some old military planes can be different, I've only flown one. Again, if you're flying one of these, someone will be explaining it to you.

Piston twins are generally the same, other than they incorporate feathering ability. Again, someone is going to explain that to you before you take the plane, however, be sure that you understand the conditions under which you cannot feather. Some props, and I've read it in the flight manual, cannot be feathered if allowed to slow to too low an RPM. Instead of allowing a feather, the prop thinks it's being normally shut down, and blade pitch latches hold the blades in fine pitch. Nice for a normal ground start next time, but a big problem if you needed to feather it for real in flight. Know your plane.

Turbines are very different. If you're flying a turbine, someone is going to explain it to you, during long, costly training. Overspeeds and surges are more easy on some turbines (PT-6's) with careless operation. The test pilot I was supervising while flying a PT-6 yesterday, had trouble getting the rhythm of the power lever, and this resulted in some undesirable prop speed surges and slight overspeeds. Turbine engines have lots of rotational inertia, and prop pitch which moves differently (sometimes below "fine" pitch). So, training....

The bottom line is follow the training provided to you, and move knobs with considered gentleness. Otherwise, don't sweat it, unless the training has told you that you should sweat it!
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Old 18th Feb 2021, 16:00
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However, what happens if the prop is cycled (not suggesting it should be) whilst on the ground and the throttle is at idle? Will there be an increase in pitch and therefore a further drop in prop rpm or will the rpm remain the same?
The blue prop lever, when put to the fully forward position, applies a spring pressure to its design maximum onto the bob weight leverage which in turn closes off engine oil pressure. The prop is held in the fully fine position by additional springs in the prop hub. Counter to this the oil pressure is used to oppose the hub springs to coarsen the prop which then in turn reduces the engine speed. However with the throttle at idle the oil pressure is at a minimum and by design will not be sufficient to push the oil valve open. If this could happen it would coarsen the prop and possibly stall the engine. With the lever full aft the power lever spring pressure will be at the minimum but will still be sufficient to prevent the low oil pressure overcoming and opening the oil valve and finding a path to the prop.

The manufacturer states the optimum power (usually an RPM indication) above idle which provides the system sufficient effect to exercise the bob weights allowing the oil to pass and operate the prop mechanism. This check is done prior to take off after the engine has been warmed. The manufacturer will also state the minimum RPM change when operating the prop lever during the check.

Last edited by Fl1ingfrog; 19th Feb 2021 at 12:53.
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Old 20th Feb 2021, 18:17
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Originally Posted by Obs cop View Post
However, what happens if the prop is cycled (not suggesting it should be) whilst on the ground and the throttle is at idle? Will there be an increase in pitch and therefore a further drop in prop rpm or will the rpm remain the same?
The short answer is it remains the same because the rpm is below the governed range. Now, I don't know the range of every governor out there so I cannot definitively say what would happen if you tried it but I strongly suspect the answer holds good in general for piston singles. I am talking about idling with the throttle on the stops here circa 800 as opposed to the 1100 rpm that lyco recommend be set with throttle in order to avoid lead fouling. At 1100 you will see from the figures below that you might be at the minimum governed for some units so there is the potential for a little movement.

It stands to reason that the governor has a limited range and hence a minimum selectable rpm. We know redline rpm but the minimum selectable rpm is not information that the pilot needs to know so it is hard to find. Two examples are below, namely, 1100 and 1400. Note the 1400 figure results from reduction gearing so the engine revs in that case will be 1616. There is also an example where the minimum governed rpm is “NA”; that is simply because that unit is for the SR22-T which is a special case because it has only one single governed speed of 2500.




Last edited by oggers; 20th Feb 2021 at 18:28.
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Old 20th Feb 2021, 18:35
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A bit of thread drift but issues often seems to be missing in conversion training from fixed to VP props

1) a bad prop seal will allow oil/grease to escape. It will typically be a thread extending outwards from the hub on the inside of the blade. If you see this have an engineer look at the prop. Certain props have the hub oil dyed red. If you see any red stains on or near the prop hub, the hub is cracked and is unsafe

2) The point of cycling the prop is 2 fold. First to circulate cold engine oil out of the governor and prop and second to check the governor operation. 2 cycles is plenty and you should only let the RPM drop 200 to 300 hundred RPM. Very low RPM at runup RPM is hard on the engine

3j if the prop RPM starts hunting in cruise the first thing you should check is engine oil pressure as low oil pressure will affect governor operations

4) if the prop runs away it has probably gone to the fine pitch stops which means it is a fixed pitch prop with a very fine pitch, if possible enter an aggressive climb to slow down as for most light airplanes the maximum airspeed that won’t over speed the engine will be less than 100 kts.
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