Prop as an air brake

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Mtns2Skies

Final Approach
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Mtns2Skies
I read on another thread (I don't recall which) that using full-forward prop(with throttle at idle) as an air brake is rough on the engine due to it pulling a vacuum on the intake stroke leading to the piston rings rattling.

I don't fully understand this (if it's even true), because by definition on the intake stroke there is a vacuum just to get air into the cylinder which would place load on the other side of the piston rings regardless so why would using the prop as an air brake be any worse than normal operation?

Neglect shock cooling.
 
Color me skeptical.

What happens in a car or motorcycle every time you downshift and take your foot off the gas or roll off the throttle? To no ill effect.

But I'd be happy to be corrected if there's something different in aircraft.
 
People are scared of aircraft engines. This is probably an old wives tale that has some radial origin.
 
Sure doesn't seem legitimate to me. I'm not scared of airplane engines though :)
 
The term of art is "flat plate drag," and if it is harmful to engines there must be a graveyard full of airplanes I have flown while using it as a tool to lose altitude under specific conditions. Refer to Figure 12-3 in the Airplane Flying Handbook. It is used for only a matter of seconds on final, not at cruise altitudes. The key is to keep about 11"-12" manifold pressure with the prop full forward; you don't want the prop driving the engine. It acts like a flat plate bolted to the crankshaft. If you are on-speed but high on final it works like a dream to lose 50-100 feet...but you must add MAP after the altitude is where you want it because if you don't, the disturbed air over the horizontal stabilizer makes flaring an adventure.

It is definitely not the tool to use when you are at cruise altitude and are given lower.

Bob
 
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bobmrg said:
The term of art is "flat plate drag," and if it is harmful to engines there must be a graveyard full of airplanes I have flown while using it as a tool to lose altitude under specific conditions. Refer to Figure 12-3 in the Airplane Flying Handbook. It is used for only a matter of seconds on final, not at cruise altitudes. The key is to keep about 11"-12" manifold pressure with the prop full forward; you don't want the prop driving the engine. It acts like a flat plate bolted to the crankshaft. If you are on-speed but high on final it works like a dream to lose 50-100 feet...but you must add MAP after the altitude is where you want it because if you don't, the disturbed air over the horizontal stabilizer makes flaring an adventure.

It is definitely not the tool to use when you are at cruise altitude and are given lower.

Bob
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I'll bite. Why don't you want the prop driving the engine?
 
Fearless Tower said:
Covington (gold standard in P&W radial overhauls)
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A company that still uses ball bearings in their impellers can hardly be called the gold standard. Old standard, maybe. o_O

If going full forward caused ring flutter or damage, cropdusters would be screwed.
 
tyndall said:
A company that still uses ball bearings in their impellers can hardly be called the gold standard. Old standard, maybe. o_O

If going full forward caused ring flutter or damage, cropdusters would be screwed.
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Whatever. There is no better company for an R-985 or -1340.
 
Mtns2Skies said:
I'll bite. Why don't you want the prop driving the engine?
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There are a whole lot of bearing surfaces that come into play when the engine is driving the prop (or idling), including those in the prop hub. Those relationships would be altered when the pistons are being moved up and down by the crankshaft and the whole drive chain (including timing) would be adversely affected. This is Tom-D territory, though...I am not a mechanic. Let's see what he has to say.

Bob
 
ElPaso Pilot said:
Lycoming clearly states the following in multiple operators manuals. Here's from the IO-360:

http://www.lycoming.com/content/operator's-manual-IO-360-n1a

CAUTION: DO NOT DO CLOSED THROTTLE DESCENTS. CLOSED THROTTLE OPERATION DURING DESCENTS WILL CAUSE RING FLUTTER WHICH CAN CAUSE DAMAGE TO THE CYLINDERS AND PISTON RINGS.​
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Okay so back to my original question. Why would that cause ring flutter when a standard intake stroke would not?
 
Mtns2Skies said:
Okay so back to my original question. Why would that cause ring flutter when a standard intake stroke would not?
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If you subscribe to the theory, during a normal intake stroke, the throttle plate is open and the fuel/air mixture flows relatively freely through the intake valve. During closed throttle operation, especially at high RPMs when the throttle plate is closed, the engine is pulling a much more significant vacuum on the intake stroke, followed by a pressure reversal on the compression stroke.

Here's Deakin:

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

Ring Flutter

During most of the time the engine runs, the cylinder rings are forced "down" against their lands, which also pushes them against the cylinder walls. If the RPM is very high, and the MP is very low, there is a large, negative pressure created in the combustion chamber during the intake stroke, due to the closed throttle plate and the piston trying hard to suck air in. This may lift the ring off its land during that stroke. The next stroke is the compression stroke, and while the pressure will be greatly reduced because not much air got in, it's still enough to push the ring back down again. This repetition may well cause the rings to "flutter," beating up and down within the land, and this may well cause damage.​
 
ElPaso Pilot said:
If you subscribe to the theory, during a normal intake stroke, the throttle plate is open and the fuel/air mixture flows relatively freely through the intake valve. During closed throttle operation, especially at high RPMs when the throttle plate is closed, the engine is pulling a much more significant vacuum on the intake stroke, followed by a pressure reversal on the compression stroke.

Here's Deakin:

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

Ring Flutter

During most of the time the engine runs, the cylinder rings are forced "down" against their lands, which also pushes them against the cylinder walls. If the RPM is very high, and the MP is very low, there is a large, negative pressure created in the combustion chamber during the intake stroke, due to the closed throttle plate and the piston trying hard to suck air in. This may lift the ring off its land during that stroke. The next stroke is the compression stroke, and while the pressure will be greatly reduced because not much air got in, it's still enough to push the ring back down again. This repetition may well cause the rings to "flutter," beating up and down within the land, and this may well cause damage.​
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There's still a significant vacuum pulled during the normal intake stroke especially on these big bore engines of ours, followed by the compression stroke. I'm not so sure I'm buying this ring flutter thing. Also even if the throttle plate created a perfect seal the intake manifold is big enough that the change in volume created by the piston going down wouldn't be that large, thus it wouldnt cause that much of a vacuum. Atleast not much more than a standard intake stroke.
 
Lets put my engineering degree to some use. Let's say we have an O540. Let's approximate the intake manifold to also equal 540 cubic inches(I'm happy to change this if someone knows better).
Boyle's law states Pressure*Volume1 = Pressure*Volume2. Pressure 1 = 1atm, Volume 1 = 540in^3(intake manifold) Volume 2 = 540+(1/6)*540 for one cylinder being open. This gives us a pressure of 0.833atm and a relative vacuum of 0.167atm.

On a regular intake stroke the piston is lowering in a sinusoidal velocity profile peaking at about 44mph. Assuming an intake size of ~2in^2. At the middle of the cylinder's stroke it's pulling in 15959in^3/s of air. Doing some fancy math I don't feel like typing into PoA we get a vacuum of about 0.12atm at the peak of the velocity curve.

I dont really find that little difference in vacuum enough to justify ring flutter. So in essence the piston ring flutters on EVERY stroke. Or it never flutters even with a closed throttle and high RPM.
 
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Mtns2Skies said:
There's still a significant vacuum pulled during the normal intake stroke especially on these big bore engines of ours, followed by the compression stroke. I'm not so sure I'm buying this ring flutter thing. Also even if the throttle plate created a perfect seal the intake manifold is big enough that the change in volume created by the piston going down wouldn't be that large, thus it wouldnt cause that much of a vacuum. Atleast not much more than a standard intake stroke.
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The engine manufacturers need some unverifiable reason to blame owners for engine failures prior to TBO, don't they? :eek2:

Regardless, here's another interesting forum debate on the topic.

https://www.euroga.org/forums/maint...ter-applicable-to-our-aircraft-engines?page=1
 
Never heard of ring flutter. Have heard of de-tuning crank counterweights. Never been an issue for me but while I reduce power and push the prop flat I doubt the prop is "driving" the engine. I feel a deceleration from reduced thrust and increased drag but I doubt the prop is pushing the engine even a little bit.
 
Again, other motorized vehicles routinely coast with the wheels driving the engine, to no ill effect.

One might say the difference is the looser tolerances of air-cooled engines, but I don't recall older VW's or Corvairs or Porsches needing special handling to avoid "ring flutter".

I generally descend with cruise or near cruise power settings, but in training there are numerous scenarios where descents are made power-off - engine out practice, power-off accuracy landings, etc. There are still pilots whose standard patterns are power-off from abeam the numbers as a matter of course.

I'm trying to be open-minded but in the absence of hard data, I'm still skeptical.
 
There's a lot of counterweight detuning info on the web. Some includes references to Lycoming and Continental service bulletins and ooerating instructions. Like this one.
http://mechanicsupport.blogspot.com/2012/04/lycoming-or-continental-counterweight.html

One of the new developments in high-output 4 cyl Lycoming clones is counterbalanced cranks to help with stroker and high compression engine stresses. I suspect the detuning warning will apply to 4 cyl engines with counterweights, too. My new Superior EX engine uses a conterweighted crank.
 
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Stewartb said:
...while I reduce power and push the prop flat I doubt the prop is "driving" the engine. I feel a deceleration from reduced thrust and increased drag but I doubt the prop is pushing the engine even a little bit.
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If you pull the throttle to idle at speed (or cut the mixture), the prop still merrily spins along.

What do you think is "driving" the engine? Would it do the same on the ground?
 
Stewartb said:
I feel a deceleration from reduced thrust and increased drag but I doubt the prop is pushing the engine even a little bit.
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Exercise:

In level flight, come back to idle. Temporarily hold altitude and note the rpm.

Now begin a descent. What happens to the rpm?

The obvious fact that it increases show pretty clearly the prop is driving the engine to some degree.

Not saying it's a bad thing, just kind of obvious.
 
Never heard it being bad for engines in general. Some geared engines like my old gopher-35 you were advised not to let the prop drive the engine as it causes wear in the gear box.
 
"• A detuning of counterweights on balance weight-equipped crankshafts is another source of overstress for the crankshaft. Many engines are fitted with balance weights mounted on pins running in precision ground bushings as an integral part of the crankshaft. The counterweights are designed to position themselves by the inertia forces generated during crankshaft rotation and effectively absorb and dampen crankshaft vibration. If the counterweights become ‘detuned’, the normally expected torsional vibrations are not properly dampened and crankshaft failure can occur. Counterweight detuning can occur from rapid opening and closing of the throttle, excessive speed, excessive power, operating at high engine speed and low manifold pressure and improper feathering procedures. Proper operation of engines with counter balanced crankshafts is vital for safety. Usually this type of crankshaft is fitted to higher power piston engines."

That's relevant for counterweighted cranks. Specific enough?

In Cubs (4 cyl, fixed pitch for this example) one of the primary advantages of long flat props is for air braking. That's a big part of short landings. Still, nobody I know goes from full throttle (implies gaining altitude) to closed throttle (implies losing altitude) in a rapid or instant manner. At least not anyone with respect for their engine.
 
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i recall i had a a65 small bore Continental with a 74 inch prop on a champ the engine was set to a very low idle like somewhere near 400rpm it acted as a speed brake as it would kind of fall out of the air in the flare as at such a low airspeed 35mph it started to act as a brake fixed/set to spec the idle and it was a different airplane in the landing
 
Stewartb said:
...dampened and crankshaft...
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I know you didn't write it and just were quoting. Wow did my Dynamics of Machines professor have a stick up his butt about dampen. "Dampen/dampening/dampened" means to make wetter. "Damp/damping/damped" is to control vibration. ;)
 
unsafervguy said:
My vote goes to Rex at Tulsa aircraft engines
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Tulsa overhauled one of mine. It had to go back due to making metal within the first hundred hours. They reworked it under warranty and then it started making metal again. After that it went to Covington.....

Tulsa used to be good, but I no longer have confidence in them.
 
flyingron said:
Never heard it being bad for engines in general. Some geared engines like my old gopher-35 you were advised not to let the prop drive the engine as it causes wear in the gear box.
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Definitely a concern with 421's.

Bob
 
Mtns2Skies said:
I know you didn't write it and just were quoting. Wow did my Dynamics of Machines professor have a stick up his butt about dampen. "Dampen/dampening/dampened" means to make wetter. "Damp/damping/damped" is to control vibration. ;)
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How about "damper", as in yaw damper or aileron/elevator damper?
 
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