Geared Engines and OWTs.

[Roger, Roger]

So I went flying in a 421 the other day, with the infamous hand-grenade GTSIO-520s. On final, the guy sitting next to me was talking about how you can feel the engines switch from engine driving props to props driving engines as you reduce power. He also mentioned something about how bad it is for the gearbox to have the prop driving the engine. My BS flag immediately went up. This is a simple spur-gear arrangement. Both sides of the teeth are cut the same. How is it soooo bad for the torque to be going the other way?

So what say the all-knowing interweb crowds? Old Wives' Tale? Gospel Truth? Shock Cooling will make your engine asplode?

[ppragman]

Originally Posted by Roger, Roger

So I went flying in a 421 the other day, with the infamous hand-grenade GTSIO-520s. On final, the guy sitting next to me was talking about how you can feel the engines switch from engine driving props to props driving engines as you reduce power. He also mentioned something about how bad it is for the gearbox to have the prop driving the engine. My BS flag immediately went up. This is a simple spur-gear arrangement. Both sides of the teeth are cut the same. How is it soooo bad for the torque to be going the other way?

So what say the all-knowing interweb crowds? Old Wives' Tale? Gospel Truth? Shock Cooling will make your engine asplode?

Ehh, I dunno. But you don't see a lot of geared motors up here. I think that stress hardening can occur in motors that run at cruise a lot, and that might be the problem. But I doubt it, its probably an owt.

[sharkey]

You know they say the same thing about the props driving the engine on the radials I fly but no one, Mx included, can't give me a better explanation of why than "it's just really hard on the crank shaft."

This may or may not be related to your original question since our engines are not geared as far as I know.

[ppragman]

Originally Posted by sharkey

You know they say the same thing about the props driving the engine on the radials I fly but no one, Mx included, can't give me a better explanation of why than "it's just really hard on the crank shaft."

This may or may not be related to your original question since our engines are not geared as far as I know.

That's what I've heard, stress hardening on the crank which can cause the gear box or the whole motor to seize. That's what I've heard anyway.

[trafficinsight]

IIRC, In terms of the radials, it's the bearings that don't like it because when the prop is driving the engine the load is reversed on them... I would bet that the situation is the same for the gears in the propeller reduction on the 421.

[RomeoTango]

I think that if the engine drives the prop through a reduction gear [ small gear on the engine turning larger gear on the prop ] an analogy can be made with a car coasting downhill in first gear.Heard from experience 421 drivers that they limit the props to 1950 on apch and power above 19'' till the flare to avoid negative load the gear box and yes shock cooling is a big deal

[tprops4me]

I do not know about the gear box thing, but there is a charter 421 on my field that has had three or four engine failures in the last few years. That is a high percentage to me.

[Roger, Roger]

Originally Posted by RomeoTango

an analogy can be made with a car coasting downhill in first gear.

And how does that hurt the car?

The gears work the same either way.

Now, it may be that Continental did a crappy job designing the anti-thrust bearings in the engine, and that bearing surface just can't handle load very well. Or it could be that the power pulses from the engine cause the gears to switch between one loading direction and another (this actually makes the most sense).

The GTSIO 520 is known as a rather temperamental engine and I think a lot of the things that people do to "be nice to the engines" are really just rituals that have no scientific basis. But hey, what do I know...

[USMCmech]

The prop driving the engine issue only applies to radials.

In line engines (geared or not) don't have any problem with this. Every time you downshift in a car, you are doing the exact same thing. Auto and airplane engines obey the same laws of physics.

For a much more in depth explaination follow the link.

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


There are tons of myths about how delicate many engines are. IMHO, the main reason is that they sit unflown for much too long, and when they do fly, they are run too hot.

[mshunter]

Had a friend giving instruction in a 680 with the geared engines. He told him to reduce power slowly on after crossing the fence. The guy chopped the throttles, and the gearbox blew up, and the prop was hanging off the front of the engine. This was like 20 years ago. So take it as second hand knowledge. I do hear that you have to be gentle with power adjustments, but I cant see how the load would be worse when coasting than at high power settings. I think some of it is urban legend.

My take, the harder you spin the engine, the faster it wears, so I would stay away from anything geared. No sense it buying something that you know is going to be short lived, and problematic.

[minitour]

I liked my time in the 421, but we always kept some power in to avoid that whole prop driving the engines thing. I heard it form the mx in charge of the plane so that's what I did.

If someone has any other advice to dispute it, I'd love to hear it. Shock cooling is about as bad as swimming 27 minutes after you ate a hotdog and not waiting the whole 30.

-mini

[wjmiller3]

We should all shock cool our engines.... It will stop global warming, and save the icecaps..

[minitour]

Originally Posted by wjmiller3

We should all shock cool our engines.... It will stop global warming, and save the icecaps..

But you'll die when your engine explodes because you got within a 10' radius of it.

-mini

[wjmiller3]

Originally Posted by minitour

But you'll die when your engine explodes because you got within a 10' radius of it.

-mini

Crap, forgot about that.... But its the price we will have to pay to save the world.

[minitour]

Originally Posted by wjmiller3

Crap, forgot about that.... But its the price we will have to pay to save the world.

True. Those poor polar bears.

-mini

[Roger, Roger]

Also, what about Turboprops? their gearboxes are many, many times more stressed than the box on a GTSIO-520 and you don't hear the 'Van drivers talking about how you can't have prop driving engine on their motors.

[minitour]

Originally Posted by Roger, Roger

Also, what about Turboprops? their gearboxes are many, many times more stressed than the box on a GTSIO-520 and you don't hear the 'Van drivers talking about how you can't have prop driving engine on their motors.

....because the prop doesn't drive the engine. The engine doesn't drive the prop....

Escaping hot expanding gasses do.

-mini

[Berkut]

Originally Posted by Roger, Roger

Or it could be that the power pulses from the engine cause the gears to switch between one loading direction and another

I have no reliable information on the subject, but this would be my guess.

Also, what about Turboprops? their gearboxes are many, many times more stressed than the box on a GTSIO-520 and you don't hear the 'Van drivers talking about how you can't have prop driving engine on their motors.

I'm not sure that they're more stressed, since there is much more to gearbox stress than just power and RPM. A big-bore recip engine is a vibration nightmare when compared to a free-turbine turboprop. Not just the vibration that you feel, but all the dynamics of that heavy crank twisting and relaxing, power pulses, variation between cylinders... and all that while trying to be a light as possible and lubed with non-optimal oil. I'm surprised they last as long as they do.

[Roger, Roger]

Originally Posted by Berkut

I'm not sure that they're more stressed, since there is much more to gearbox stress than just power and RPM. A big-bore recip engine is a vibration nightmare when compared to a free-turbine turboprop. Not just the vibration that you feel, but all the dynamics of that heavy crank twisting and relaxing, power pulses, variation between cylinders... and all that while trying to be a light as possible and lubed with non-optimal oil. I'm surprised they last as long as they do.

Yeah, that makes sense. I think that what does it is the power pulses. I'm still not sure that prop-driving-engine will make your gearbox asplode though.

As to the Commander, it seems more likely to me that one of the shafts in the gearbox was already on it's way out. A simple pulling of power isn't going to solely precipitate a catastrophic failure like that. Now, it's likely that the change in loading was the straw that broke the camel's back, but I wouldn't worry about my engines doing that every time I reduce power.

[Roger, Roger]

Originally Posted by minitour

....because the prop doesn't drive the engine. The engine doesn't drive the prop....

Escaping hot expanding gasses do.

-mini

Right, but you still have opposite loading on the gearbox and driveshaft, since all of that is on the power turbine half of the engine, right?

[minitour]

Originally Posted by Roger, Roger

Right, but you still have opposite loading on the gearbox and driveshaft, since all of that is on the power turbine half of the engine, right?

Which has what to do with driving the engine?

-mini

[shdw]

Originally Posted by minitour

But you'll die when your engine explodes because you got within a 10' radius of it.

-mini

Not me, I bring my parachute for shock cooling missions.

OP: My worry would be the gearbox, as USMCmech pointed out there is nothing to worry about the driving the engine, that is crap. Though after reading about how radial engines run and work in the A&P book I can certainly see why they may have an issue with it. The question with current engines is what kind of loads can be put on the gear box. Rapid power changes would leave opposite directions of torque applied to either side of the gear box. Prop still spinning fast torquing that direction while the motor was spinning very slowly causing opposite torque. In a car this is fine, but in a car the gearbox I would bet is built much stronger than in an aircraft for weight purposes.

I don't know that the prop driving the gearbox/engine would matter much though as long as it got there smoothly. But it would also depend on the gears used, strength, and set up. Is it exactly like a car gear box or are there extra parts in there?

PS I am not a mechanic, merely applying a physics type thought to this. Does it make sense to you mechanical nuts out there?

[Roger, Roger]

Originally Posted by minitour

Which has what to do with driving the engine?

-mini

Right, you're not driving the entire engine, but you still have negative loading on the gearbox, which is what makes all the old timers go "oh teh noez".

[Mike H]

Originally Posted by RomeoTango

and yes shock cooling is a big deal

Now you've done it

[Nigel Croker]

Originally Posted by Roger, Roger

Also, what about Turboprops? their gearboxes are many, many times more stressed than the box on a GTSIO-520 and you don't hear the 'Van drivers talking about how you can't have prop driving engine on their motors.

Uhmmm, because the prop and the gas generator are not physically connected......everybody sing "the backbone connected to the tailbone, the power turbines connected to the gearbox, and not the compress-or sec-tion"