Props Driving Engines??

OhioStatePilot

OhioStatePilot

Well-Known Member
I think I've got this terminology all muddled up because it's confusing the hell out of me. I continually hear people referring to the props driving the engines instead of the engines driving the props in situations that just don't make sense in my head.

If I'm excluding prop reverse, the way I see it is that you can have a positive, neutral, or negative thrust while in forward flight. Negative in this case, would be that the airframe is moving through the air faster than the current power settings would normally provide. Now, this makes sense as a situation where the props drive the engines, but I can only imagine this occurring during a deceleration or a descent.

The next part of it that's confusing is having heard this specifically in relation to prop settings (the over/under square debate, which I know is BS from the get-go). Logically, my mind tells me that one would most likely have the props drive the engines when at a high RPM setting, low throttle setting, and high airspeed. The confusing part comes in that I've heard this is a concern when running oversquare -- high throttle setting, low rpm setting.

The real question at hand is, how backasswards have I gotten myself?
 
OhioStatePilot said:
The confusing part comes in that I've heard this is a concern when running oversquare -- high throttle setting, low rpm setting.
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The guys saying this will probably also be the ones telling you the engine will implode and vaporize if you run over squared. :) The rest of what you said all appears to make sense.
 
minitour said:
You need a beer.

-mini
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This is probably the closest thing I've heard to the truth all day!

shdw said:
The guys saying this will probably also be the ones telling you the engine will implode and vaporize if you run over squared. The rest of what you said all appears to make sense.
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Yeah, funny you mention that... they are indeed the same people who get uptight about over squared power settings! Thanks!
 
OhioStatePilot said:
The real question at hand is, how backasswards have I gotten myself?
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Don't worry, there are a LOT of myths and misconceptions out there about how to properly operate piston aircraft engines. They are repeated by CFIs, and owner pilots, and uneducated mechanics. The problem is made much worse because today's "experianced profesinoal" pilots have flown turbine equipment for 95% of their careers, and now regard a piston airplane engine as something mysterious and scarey. Never mind that the engine in front of a Bonanza works just the same as a Harley of an old VW bug.


"Never run your engine oversquare" is a complete falsehood. Check your POH, and you will see approved power settings that are "oversquare"

Damage from "Shockcooling" is a freak event usually brought on by severe abuse. If shockcooling were anywhere near the boogeyman that some people fear, then skydiving planes would be blowing engines weekly. They climb at Vy to 12000 getting very hot, followed by a Vne descent rapidly cooling down. Yet, they somehow run just fine despite the abuse.

"Props driving the engine" is just like downshifting in a car. Due to the unique design of radial engines, this can damage the crankshaft. However, inline engines do not have this problem.
 
I figured that was the case. I've seen the power setting charts that are oversquare and the shock cooling just plain doesn't make a whole lot of sense unless extreme circumstances are involved.

So in a radial, what's the care and feeding to prevent crankshaft damage? I'd just imagine that at some phase of flight the prop will wind up driving the engine, at least for short periods of time during speed changes until stabilized.
 
OhioStatePilot said:
So in a radial, what's the care and feeding to prevent crankshaft damage? I'd just imagine that at some phase of flight the prop will wind up driving the engine, at least for short periods of time during speed changes until stabilized.
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Momentary conditions were/are not a problem.

The issue was long power off descents from high altitude in DC-4s and L1049s. The problem was in how the crankshaft was lubricated. The crankshaft in a radial is very different from a in line engine.

The following link explains it much better than I could.

http://www.avweb.com/news/pelican/186778-1.html

As a matter of fact, if you are a CFI (or just curious) read the rest as well.

http://www.avweb.com/news/pelican/list.html
 
OhioStatePilot said:
I can only imagine this occurring during a deceleration or a descent.
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Yes, high speed descent with low throttle setting is the scenario. "Ring flutter" is usually the reason given for why this is bad for the engine. According to "The Sky Ranch Engineering Manual":
You do not want to do a long let-down with the prop driving the engine. This causes ring flutter and broken piston rings. High rpm's increase the up and down inertial load on the rings in the piston ring grooves. Cylinder pressure help to keep the ring seated to the bottom of the ring groove. Reducing rpm to low cruise will be beneficial by reducing ring inertial and increasing cylinder pressures.

(p. 146)
​
high throttle setting, low rpm setting.
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Engines do have a maximum "oversquare" setting they can be safely run. For instance, the IO-360 at 2,000 RPM has a maximum manifold pressure of 26" Hg, according to the Lycoming Engine Operators Manual.
 
USMCmech said:
Momentary conditions were/are not a problem.

The issue was long power off descents from high altitude in DC-4s and L1049s. The problem was in how the crankshaft was lubricated. The crankshaft in a radial is very different from a in line engine.

The following link explains it much better than I could.

http://www.avweb.com/news/pelican/186778-1.html

As a matter of fact, if you are a CFI (or just curious) read the rest as well.

http://www.avweb.com/news/pelican/list.html
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That was a great article! I'm not yet a CFI, but I am an AGI. I'm not actively using that at the moment, but I'm definitely storing that article away in my files for future reference. I think I'm also going to take some time to go down the list of others. Thanks!
 
Huh, also an interesting article. What I found most interesting was comparing the differences between Deakin's article with that one. They both agreed perfectly in some areas, but widely diverged in others, particularly the 1" per 100 rpm rule of thumb.
 
Props driving the engines?

You won't need a DC-4 or Connie, or a big radial engine in order to deal with this.

The geared Continental GTSIO-520 in the Cessna 421 is all you will need to experience this.
 
The engines that have the biggets problem with props driving the engine are direct drive turo props. Like the garret TPE-331. Like on a metro liner. They have whats called an NTS system ( negative torque system). This system tell the pilot when the prop is running the engine so they can make adjustments. Worked on it once or twice but not very strong subject for me.
 
shovel said:
The engines that have the biggets problem with props driving the engine are direct drive turo props. Like the garret TPE-331. Like on a metro liner. They have whats called an NTS system ( negative torque system). This system tell the pilot when the prop is running the engine so they can make adjustments. Worked on it once or twice but not very strong subject for me.
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The NTS system isn't to prevent the props from being driven by the airflow, as it's a drag reduction device. From the POH and some extra additions by me:

Negative torque occurs whenever the propeller tends to drive the engine rather than when the engine drives the propeller. When negative torque is sensed, propeller pitch would (without the NTS system installed) move to a high rpm, fine propeller pitch setting because the propeller governor would sense an underspeed condition and attempt to keep the propeller speed at the commanded setting.

Now with the NTS system installed, as the engine spools down (as in an engine faiure) the gearbox will sense negative torque (it's all mechanical, no electronics involved), and as negative torque is sensed it moves a valve to an open position allowing oil pressure to escape from the propeller hub back into the engine gearbox. And this is a strange thing to watch, as the engine senses negative torque the rpm slows down and the prop windmills, the nts valve opens dumping oil out of the prop allowing it to go into a coarse pitch (towards feather) and the prop rpm now goes up. As it goes up it's no longer under negative torque and the NTS valve closes, allowing oil pressure to go back into the propeller hub driving the prop back to a low RPM, coarse pitch. This "bouncing" happens about 3-4 times a second.

This allows the propeller-engine to maintain a close to zero drag position, to cage the engine and actually feather the propeller you would have to pull the "stop and feather" lever on the pedestol that closes the fuel shutoff valve and opens the feather valve allowing all of the oil in the propeller hub to get dumped into the gearbox and sending the prop to feather.

NTS system=a built in auto feather without the electronics that is needed in planes that utilize a free turbine (aka pratt's). It's ingenious, one less electrical thing to go wrong and it's built into the engine's.

-Brian
 
shovel said:
The engines that have the biggets problem with props driving the engine are direct drive turo props. Like the garret TPE-331. Like on a metro liner. They have whats called an NTS system ( negative torque system). This system tell the pilot when the prop is running the engine so they can make adjustments. Worked on it once or twice but not very strong subject for me.
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Uh...that's almost completely wrong.
 
Roger said:
Uh...that's almost completely wrong.
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Yes it is.
NTS on 331 is there to assist the pilot in feathering the engine after loss of power (giving him some reaction time). When the engine quits and the airflow turns the prop the ring gear inside the gear box moves up actuating the NTS valve allowing oil to route out of the prop (beta tube) and let the counterweights and the spring start moving the prop in the feather position. The prop will not feather by its own so the pilot still has to do it.
 
Usually the engine doesn’t go south on the pilot who runs it overunder..............it ends up to be the other poor bastard.

Anyway...I hope people don’t complain when they get cracks in their exhaust system complete with that heavenly unbreakable bond between every nut and cylinder head stud.
Get that check book ready. :)

To stay on topic: I say the expression prop running the engine is when you notice an increased in RPM above power setting due to aerodynamic forces.

I love old aviation sayings and expressions...........they may not make sense , but there is wisdom in them.
 
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