Constant Speed Prop: Over/Under Speed

Volitation

Volitation

Pre-takeoff checklist
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Volitation
Im working on the HP & Complex endorsements for a new to me R182. Right now studying the workings of the prop governor, and likely causes of over and under speed conditions, and I’m having a hard time getting it. Met with my instructor today on the ground, and I know the results when the control is moved, but I want a better grasp of the mechanics of the system and why the blades do what they do.

I’d like to try to explain my basic understanding, grossly oversimplified, and would appreciate comments and corrections, or any other input for that matter. Lol.

At high rpm setting, the oil valve from the governor to the hub is closed, oil pressure is low, allowing the hub spring to put the blades to the stops in a low pitch, high rpm condition. Or, Low OP = Low Pitch = High RPM. Mentally I picture this as the ‘default’ condition.

At lower rpm settings, the governor oil valve opens, allows oil under pressure to flow to the hub, overcoming the spring, and moving the blades to a higher pitch, lower rpm condition. Or, High OP = High Pitch = Low RPM. (Centrifugal force acting on the flyweights is the mechanical force opening the oil valve against the force of the speeder spring holding the valve closed. )

So, trying to apply this, a likely cause of over speed might be low oil pressure, and a likely cause of under speed would likely be mechanical, ie a broken spring.

Ok, fire away!
 
High RPM setting isn’t against the stops. Otherwise it wouldn’t be able to overspeed, right? :) Loss of oil pressure is usually the reason for an overspeed. (Pumping it overboard from a leak, failure of the high pressure pump, etc.)

Underspeed is more likely mechanical in terms of the speeder spring breaking and now the flyweights can open the valve wide open all the time and allow the high pressure oil into the hub constantly.

For a single engine airplane anyway.

All of that is reversed for a multi usually. The prop goes to feather when oil pressure is lost and it takes pressure to keep it in fine pitch.
 
denverpilot said:
High RPM setting isn’t against the stops. Otherwise it wouldn’t be able to overspeed, right? :) Loss of oil pressure is usually the reason for an overspeed. (Pumping it overboard from a leak, failure of the high pressure pump, etc.)

Underspeed is more likely mechanical in terms of the speeder spring breaking and now the flyweights can open the valve wide open all the time and allow the high pressure oil into the hub constantly.

For a single engine airplane anyway.

All of that is reversed for a multi usually. The prop goes to feather when oil pressure is lost and it takes pressure to keep it in fine pitch.
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Right, duh, smh. Lol.

So, vastly oversimplying again, the mechanics (springs) will hold the blades at highest rpm, lowest pitch the oil in the hub will allow. The high rpm pitch setting still requires oil pressure, if that pressure isn’t there, pitch moves even flatter, thus over speed?
 
Volitation said:
Right, duh, smh. Lol.

So, vastly oversimplying again, the mechanics (springs) will hold the blades at highest rpm, lowest pitch the oil in the hub will allow. The high rpm pitch setting still requires oil pressure, if that pressure isn’t there, pitch moves even flatter, thus over speed?
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Okay, it overspeeds so you pull back the throttle yet the overspeed remains. Now hat do you do?
 
Clark1961 said:
Okay, it overspeeds so you pull back the throttle yet the overspeed remains. Now hat do you do?
Click to expand...

Okay, let me work thorough this.....prop is over speeding, throttle doesn’t do it, since I can’t increase load on the prop with prop pitch, I think I have to raise the nose to lower the airspeed and get some resistance against the prop. Which is not helping with a problem that’s likely oil related, telling me I need to be thinking about getting this thing on the ground fairly quickly. But, first things first.

Anywhere close to the right answer?
 
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Volitation said:
Okay, let me work thorough this.....prop is over speeding, throttle doesn’t do it, since I can’t increase load on the prop with prop pitch, I think I have to raise the nose to lower the airspeed and get some resistance against the prop. Which is not helping with a problem that’s likely oil related, telling me I need to be thinking about getting this thing on the ground fairly quickly. But, first things first.

Anywhere close to the right answer?
Click to expand...
Right on target. It’s one of those ‘interesting’ questions that folks might not have heard. Do you pull the mixture while descending? Would you switch off the mags? Any other options? Is overspeed really a problem?

Anyway, I like the question better than the canned: it’s overspeeding what is the most likely cause?
 
Clark1961 said:
Right on target. It’s one of those ‘interesting’ questions that folks might not have heard. Do you pull the mixture while descending? Would you switch off the mags? Any other options? Is overspeed really a problem?

Anyway, I like the question better than the canned: it’s overspeeding what is the most likely cause?
Click to expand...

Interesting questions to consider. A few hundred rpm probably isn’t an immediate catastrophe for the engine itself for a short time, hadn’t even thought about components like mags, etc.

As I consider it, I’m trying to,picture what’s going in side the cylinders with fuel flow reduced with the throttle, but being pulled to high rpms by the prop. Mags would fire on time, but no or little fuel to ignite... cutting the mix or mags would create resistance against the prop...

Ok, more to work though and think about, thanks!
 
Volitation said:
Thanks Dan, that’s a clearer explanation of what I’ve been muddling through!
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It operates on the same principal as this:

There are three main parts to a governor an adjustable spring, that you set tension on, a slide valve, to direct oil pressure, and a fly weight assembly that feels speed and operates the valve against the spring tension, you set.
 
Tom-D said:
It operates on the same principal as this:

There are three main parts to a governor an adjustable spring, that you set tension on, a slide valve, to direct oil pressure, and a fly weight assembly that feels speed and operates the valve against the spring tension, you set.
Click to expand...

Thank you Tom. If it’s good enough for a steam engine, it’s good enough for me. Lol.
 
This technology has been around a long time.


all of our machines have a beginning some where.
 
There are high (prop control forward) and low (prop control pulled out) speed stops on the blades so if you get an overspeed pull the throttle back and pull the nose up until below redline. Lower the nose and now it works like a fixed pitch prop so just keep it under redline and fly to the nearest port. Now to muddy the waters an aerobatic prop works just the opposite. Loss of oil pressure causes the prop to go to low (coarse) pitch.
 
yakdriver said:
There are high (prop control forward) and low (prop control pulled out) speed stops on the blades so if you get an overspeed pull the throttle back and pull the nose up until below redline. Lower the nose and now it works like a fixed pitch prop so just keep it under redline and fly to the nearest port. Now to muddy the waters an aerobatic prop works just the opposite. Loss of oil pressure causes the prop to go to low (coarse) pitch.
Click to expand...

Why are do the Aeros revert to low pitch?
 
If the prop cable breaks, the prop behaves as if the prop knob is all the way in.
 
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