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Constant Speed Prop.

LewisU_Pilot

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Do I have this right? As the prop control is pushed in the speeder spring tightens and the flyweights move in. which pushes the piston valve down which allows the oil from the engine in and moves the props to a low pitch high rpm.

As the prop control is pulled out the speeder spring loosens which moves the flyweights out pushing the piston valve up allowing the oil from the hub back into the sump.

An over speed condition will cause the fly weights to fly out which will increases the blade angle.

An under speed condition will cause the fly weights to fly in which will decrease the blade angle.

Practicing teaching a commercial student tomorrow and haven't looked over the prop since my ride. Just want to make sure I fully got this stuff so I am not confusing another student.
Thanks
 

U-I pilot

Relaxation....
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Seems good. I call the "piston valve" as you call it the pilot valve.
This is also assuming you are talking about an arrow or alike. Constant speed props on multi's like a seminole are reversed in a sense....assuming its a single since you indicate instructing a commercial student that sounds good.....
Try to find a good diagram which will help make it more clear for both you and your student.....

If you are confused.....a confused student will do wonders for your teaching....
Hope that helps...
Talk to RTO or any of the other fine U-I grads now instructing there ;)
 

avbug

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The mechanical action inside the propeller governor is relatively unimportant to the understanding of the prop system in use, compared to understanding the function of oil pressure, and what forces are actually moving the propeller.

Explaining ATF (aerodynamic twisting force), counterweights, oil pressure, the function of internal springs, cams, and gas pressure is very important. After all, these are what move the prop, what determine where it goes and what it does, and can be critical issues in an emergency.

Understanding when the engine is driving the prop and when the slipstream is driving the prop is important, and how and why the prop reacts to each, is also important. Equally important, if not more so, is the aerodynamic response and properties of the propeller in each condition.

Teaching the use and properties of the high and low pitch stops, their functions during the runup and low RPM checks, etc, are also very important...and are seldom taught.

I've known folks who died because they didn't understand these things, and sadly, most CFI's don't understand them.
 

MauleSkinner

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avbug said:
I've known folks who died because they didn't understand these things, and sadly, most CFI's don't understand them.

Personally, I agree with Avbug on this, but unfortunately most of the examiners I've dealt with only care about the internal workings part.

If you can find somebody with a steam engine, you can really show how a governor works...they're exposed, and easy to see and manipulate. Failing that, I use a corkscrew (if you know how a governor works, you'll know the type I mean).

As far as the oil moving back and forth, generally single-engine airplanes require oil pressure to go to low rpm/high pitch on the propeller.

Multiengine airplanes are generally the opposite...oil pressure pushes the prop towards high rpm/low pitch so that you can feather them after the engine quits and oil pressure is lost.

I have flown a Baron (about 1985 vintage), however, that required oil pressure to feather the prop...never did figure that one out.

Fly safe!

David
 

avbug

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A lot of prop installations have used oil to feather the prop, the most notable being the ham standard hydromatic. Lose your oil, you can't feather, and you're stuck on the low pitch stops. In many installations, you can't feather if you let the RPM drop, either, once the prop is resting on the low pitch stops.
 

minitour

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avbug said:
In many installations, you can't feather if you let the RPM drop, either, once the prop is resting on the low pitch stops.
Is there a reason for that? like in the Duchess...you take away oil to feather...but when you shutdown (and oil pressure is removed) the prop doesn't feather because of the pins. Why the pins? I know they're there and what they do, but I don't know why vs the King Airs I've seen shut down all but the 100 feathers (due to the type of turbine engine I would suppose)...but why some feather and some don't on shutdown? Is there some benefit? Anyone

-mini
 

Tinstaafl

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LewisU..., it depends on the type of installation. There have been numerous variations across many different aircraft types. Some hydraulic, some electric etc.

Speaking generically for typical Cessna/Piper/Beech singles: All their CSU single engine types I know about default to fine pitch/high RPM and use oil pressure + Aerodynamic Twisting Moment to drive against the strong Centrifugal Twisting Moment that causes the blades to have a fine pitch/high RPM tendency ie oil pressure + ATM drives the blades to coarse/low RPM. This means that a failure of the governing system still leaves full power available.

Typical Cessna/Piper/Beech light multi-engine a/c with feathering props use some combination of counterweights/feathering spring/pressurised gas or oil in combination with the ATM to overpower the CTM. This causes the prop. to default to coarse pitch/low RPM and eventually into feather.

A failure of the governing system/engine at least allows the failed unit to be feathered to reduce drag for the remaining engine.

There is a slight problem introduced by this feathering system in that the prop will try to go into feather when oil pressure is lost after the engine is shutdown normally. Not good for then next start!. This problem is prevented by latches that engage when RPM drops below a certain critical value (typically 800 or 1000 RPM or so). Given a real engine failure, it's important to identify if the RPM is reducing in any immediate actions so feathering can be done before the ability gets locked out.

Unlike light aircraft that tend to use single acting designs described above, many larger aircraft use a double acting design where oil pressure can be applied to either side of the actuating piston to move the blades to coarse or fine pitch.
 

VNugget

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It would take quite a bit of torque to turn a feathered prop. Free power turbine turboprops don't care since the core runs independently from the prop, but a piston engine would have too much trouble.
 

Vector4fun

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Actually, I'm curious if anybody actually knows how much stress is placed on a piston engine by starting it with the prop feathered. I've seen it done when the stop pins didn't engage for some reason. I understand it's not the ideal way to start an engine, but if the throttle is carefully managed, is it really that stressful?
 

Tinstaafl

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Oh yes. The poor engine shakes like buggery and the time to reach idle is longer. Think of all that mass getting shaken back & forth and the effort the starter motor has to do. More throttle is needed to get the thing to run so higher MP as well.
 

avbug

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Starting a piston engine from feather doesn't take much more engine effort than out, but getting it out of feather can be an issue of it's own accord. The prop doesn't always want to come out of feather, and without an unfeathering accumulator on some light piston engine installations, coming back out can be a drawn out process. It's not the greater torque loading that's hard on the engine, as there isn't a significantly greater loading, or rather, it doesn't provide a great hinderance. The problem is the excessive starter useage that may sometimes be required before the prop reluctantly comes out of feather...and most of us have seen times when the prop wouldn't unfeather, too. It's not "overtorquing" the engine that's the issue, but starter duty cycle limits.

In a hydromatic installation as identified earlier, the feather button can be held in to drive it out of feather. Going into feather is tougher, because you need to be able to shut it off if it doesn't shut off when feathered, lest the pump drive the prop through feather and right back to turning again. When coming out of feather, one merely lets off once the airstream takes over driving the prop. Introduce spark and fuel, and it's a done deal.

Conversely, I remember trying to relight an engine on a King Air 90 once, and being unable to get it out of feather, or to light off. On another occasion in the same airplane, the prop wouldn't feather, and that produced an enormous amount of drag.

Even on four engine airplanes, I've had difficulty maintaining altitude, even at low altitudes, if an engine failed to feather.

Why do the antifeather pins insert at low RPMs? To prevent the engine from feathering on shutdown under normal circumstances. There's no reason for it to feather on a normal shutdown on the ground. If one is going to feather, it needs to be done at higher RPM's, such as in flight. The engine shouldn't need to be feathered on the ground, though you can...the whole point is that the pins shouldn't insert so long as adequate centrifugal (centripetal) load is on them in flight, and that's a function of RPM...with adequate airspeed, the prop should never get that slow, so it shouldn't be an issue.

The problem is that these are mechanical objects, and they do malfunction. Props don't automatically feather every time with loss of oil pressure. With a governor problem, they can occasionally feather on their own. Aircraft equipped with autofeather have the unfortunate side effect of disabling themselves if the pilot proceeds by habit with the usual identify, verify, feather drill...pulling back a power lever can disable the autofeather system, negating it's value and preventing it from doing it's job.

You want your piston engine to revert to it's low pitch, high RPM setting in the case of a governor failure. If the governor can't control the propeller, then it acts like a fixed pitch prop, and you control it with the throttle. Decrease RPM, merely decrease throttle, just like any fixed pitch prop installation. This allows you to continue using power. A full feathering prop on a single might or might not provide any great advantages, but it does provide more things to go wrong, more errors for a pilot to make in flight, and adds expense that isn't really necessary, not to mention complexity.

In a turbine installation, you have a different animal, with different blade area and energy, and different results to the aircraft in the event of an engine failure or shutdown. The drag from a windmilling prop that fails to feather on most turbine installations can be considerably higher, with considerably more dire results. The same can be true of a runaway prop on a turboprop, vs. a piston installation. The wider blades that operate at lower RPM's and different angles tend to create a lot more drag and absorb a lot more energy. Additionally, these systems don't stop at the low pitch or high pitch; these feather, and go right through low pitch into beta and reverse...with the potential for severe overspeed related consequences, as well as directional and pitch control issues for single engine airplanes using turboprop installations.
 
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