Bob - S50 Posted September 18, 2017 Report Share Posted September 18, 2017 In another thread somebody brought up the props driving engines topic. Rather than continue down a side thread, I thought I'd start a new one (and food fight). I started thinking about the topic after I read it on the other thread. It has to do with descents at low MP with high RPM. The argument is that doing that creates a big negative pressure above the piston which may pull the rings up off the landing during the intake stroke only to have them slammed back down during the compression stroke, thus creating flutter and potential ring/landing failure. So how low can the MP be or how much spread can be allowed before this happens? It hit me. You MUST have SOME reduced pressure above the piston or no new fuel/air mixture will flow into the cylinder during the intake stroke. How much is too much? I don't know. However... If I'm cruising at 12,500' in my NA '78J, the maximum MP I can get is something around 17" of MP, maybe lower depending on the air filter. If I'm cruising at that altitude using 2600 RPM to get as much speed as possible, what's the difference between that and using 2600 RPM with the power pulled back to 17" during a descent at lower altitudes? Nothing. Pressure is pressure. It doesn't matter whether it gets that low because of my altitude or because I've choked the airflow with the throttle. If 17" at cruise doesn't cause flutter, neither should 17" in the descent. The mighty J has a service ceiling of around 18,000' where the MP would be about 15" or less. If I can theoretically cruise at that altitude and MP, I feel pretty comfortable using 15" during a high RPM descent. Might just be me though. Let the games begin... Quote Link to comment Share on other sites More sharing options...
ArtVandelay Posted September 18, 2017 Report Share Posted September 18, 2017 The pressure inside the cylinder is what, 1000s psi during combustion? And you're worried about the MP 15 vs 25? Quote Link to comment Share on other sites More sharing options...
Bob - S50 Posted September 19, 2017 Author Report Share Posted September 19, 2017 8 minutes ago, teejayevans said: The pressure inside the cylinder is what, 1000s psi during combustion? And you're worried about the MP 15 vs 25? I'm not, but others are. And it isn't the absolute pressure they worry about, it is the change from negative to positive. Personally, with as little space as there is between the piston and cylinder wall for the pressure to act on, and the friction between the ring and the cylinder, I think it would take a lot of pressure to move that ring. Besides, the friction between the ring and the cylinder is already opposing the downward movement of the piston and will be more likely to move the ring in the groove than a small relatively negative pressure will. Quote Link to comment Share on other sites More sharing options...
RLCarter Posted September 19, 2017 Report Share Posted September 19, 2017 The only thing I've ever heard is on geared engines, but I thought it had to do with shock cooling but not sure Quote Link to comment Share on other sites More sharing options...
jetdriven Posted September 19, 2017 Report Share Posted September 19, 2017 The GTSIO-520s used in 421s and the like would chatter if the prop drove the engine. After a short time this cracks the cases. We had a 404 at the freight place I worked at where one had less than 100 SMOH and both cases were cracked. They didn't teach to avoid that because I did my 135 ride in the 421 and, while maneuvering to intercept the ILS I accidentally pulled the power back a little too far and the airframe shudder was unmistakable. I said "oh" and bumped the power up a little and it went away, he looked at me and said "what was that about". I tried explaining but it was clear the disconnect. 1 Quote Link to comment Share on other sites More sharing options...
Mooneymite Posted September 19, 2017 Report Share Posted September 19, 2017 So....what happens when, at Vapp, with prop full forward, you pull the power to idle? Are we not driving the engine everytime we land? I'm not aware of any prohibitions. Quote Link to comment Share on other sites More sharing options...
mike_elliott Posted September 19, 2017 Report Share Posted September 19, 2017 Apples and oranges, but what happens on an Indy car or F1 car when they reach their braking point after a long straight? The wheels drive the engine big time. Yea, they explode every now and then, (but not as often as our "reliable" aircraft engines it seems. :() Quote Link to comment Share on other sites More sharing options...
Guitarmaster Posted September 19, 2017 Report Share Posted September 19, 2017 The GTSIO-520s used in 421s and the like would chatter if the prop drove the engine. After a short time this cracks the cases. We had a 404 at the freight place I worked at where one had less than 100 SMOH and both cases were cracked. They didn't teach to avoid that because I did my 135 ride in the 421 and, while maneuvering to intercept the ILS I accidentally pulled the power back a little too far and the airframe shudder was unmistakable. I said "oh" and bumped the power up a little and it went away, he looked at me and said "what was that about". I tried explaining but it was clear the disconnect. A 404 is pretty rare. What company were you working at that operated those?Sent from my XT1585 using Tapatalk Quote Link to comment Share on other sites More sharing options...
N201MKTurbo Posted September 19, 2017 Report Share Posted September 19, 2017 Two things One, the piston rings move from the top of the groove to the bottom on every stroke no matter what you are doing. The biggest problem with letting the prop drive the engine is the crankshaft oil film strength. The oil hole in the crank is placed so it is at the point in rotation where the max pressure is, giving the highest film strength. When back driving the engine the oil hole Is 180 degrees from where the pressure is being applied and the film strength is the lowest. This can lead to scuffing of the rod bearings and crank. 2 1 Quote Link to comment Share on other sites More sharing options...
N201MKTurbo Posted September 19, 2017 Report Share Posted September 19, 2017 I suspect the problem with the GTSIO-520s is they have counterbalanced crankshafts and gearing. The counterbalance weights are expecting a power pulse at a specific angle. When the power pulse is 180 degrees off they will start swinging totally out of sync with what they are supposed to do. This can get the gears in the gearbox banging back and forth through their backlash. I can see how this could be very destructive. 3 Quote Link to comment Share on other sites More sharing options...
Vance Harral Posted September 19, 2017 Report Share Posted September 19, 2017 2 hours ago, N201MKTurbo said: The biggest problem with letting the prop drive the engine is the crankshaft oil film strength. The oil hole in the crank is placed so it is at the point in rotation where the max pressure is, giving the highest film strength. When back driving the engine the oil hole Is 180 degrees from where the pressure is being applied and the film strength is the lowest. This can lead to scuffing of the rod bearings and crank. My understanding is this was a legitimate, long-term reliability concern on radial engines, where multiple pistons apply pressure to a common crankshaft journal and bearing (e.g. for a 9-cylinder radial, 4.5 pulses per revolution. The practice of not letting the prop drive the engine stems from operating manuals for airlines, running engines like the R-2800, in very long descents at 300 KTAS speeds. John Deakin argues this is a non-issue with the typical flat engine used in the modern GA fleet (0.5 pulses per revolution). I don't know that he's right. What I am confident of is that it's a long-term issue either way, not an instantaneous one. There certainly is no harm in handling occasional events like a slam-dunk approach or emergency descent (real or practiced) by pulling the power to idle. 2 Quote Link to comment Share on other sites More sharing options...
N201MKTurbo Posted September 19, 2017 Report Share Posted September 19, 2017 55 minutes ago, Vance Harral said: My understanding is this was a legitimate, long-term reliability concern on radial engines, where multiple pistons apply pressure to a common crankshaft journal and bearing (e.g. for a 9-cylinder radial, 4.5 pulses per revolution. The practice of not letting the prop drive the engine stems from operating manuals for airlines, running engines like the R-2800, in very long descents at 300 KTAS speeds. John Deakin argues this is a non-issue with the typical flat engine used in the modern GA fleet (0.5 pulses per revolution). I don't know that he's right. What I am confident of is that it's a long-term issue either way, not an instantaneous one. There certainly is no harm in handling occasional events like a slam-dunk approach or emergency descent (real or practiced) by pulling the power to idle. I agree completely. It is like running your engine in the yellow RPM range. It won't instantly explode, but over the long term it may shorten the life of your engine if you do it habitually. 1 Quote Link to comment Share on other sites More sharing options...
carusoam Posted September 19, 2017 Report Share Posted September 19, 2017 Backlash... 1) The amount of free space between gear teeth, when one gear drives the other... 2) When the drive gear becomes the driven gear, the trailing face of a gear tooth closes the gap and bangs into the leading face of the other gear's tooth. 3) In our engines, with no gear box... We get that same backlash effect by pulling on the prop vs. pushing on the prop.... 4) under heavy forward acceleration, or even at low power all the engine parts are pushed or pulled in a constant direction.... 5) there is one situation where engine braking can become very noticeable... Fast speed in the traffic pattern, gear down, flaps at T/O position... Throttle reduced to achieve 90kias (approximately) Press the prop control all the way in, quickly... Sense the deceleration... This is very similar to... putting the firebird's six speed into second gear (while going 30mph) while letting off the throttle Then letting out the clutch pretty rapidly Sense the deceleration, that comes with the raise in rpm and exhaust noise..... to be extra harsh...use full throttle, then zero throttle, repeat...a couple of times... rpm range from 1500 to 5000+ The differences with a Mooney... no clutch Continuous gears (prop angle change) rpm increases only a few hundred rpm at most... 2400 to 2700... Since The prop is mostly pulling all the time, and at some times it may be pushing back...there is one condition that the prop may be inclined to flip flop between these two states.... low power, descent, in rough air, where the engine is neither pulling or pushing decisively....but is free to flip flop between states, and can be induced to flip flop because of oscillations in the rough air. Looking at the thrust bearing design for the engine will give a hint of how well the engine can take being pushed into the hangar and pulled out of the hangar by hand. Ever sense the gap when you pull the prop forwards and the push it back? My O360 felt like it had a giant gap, while it gave a firm clunk when being pushed and pulled into the tie-down... My IO550 TopProp doesn't get pushed or pulled. The thrust bearing is the parts that keep the crank shaft from exiting at the front or back of the case... Anyone have a picture of those parts for an IO550 or an IO360? PP thoughts specifically for this thought experiment only. Observations over the years... Best regards, -a- 2 Quote Link to comment Share on other sites More sharing options...
Yetti Posted September 20, 2017 Report Share Posted September 20, 2017 I think of all the BMW motorcycle Boxer Engines and all the downshifting letting the tire drive the engine..... Quote Link to comment Share on other sites More sharing options...
jetdriven Posted September 20, 2017 Report Share Posted September 20, 2017 You're forgetting the vast difference in tolerances between these air cooled engines and the BMW motorcycle engines or race cars. Airplane engines are thrown together compared to those. Quote Link to comment Share on other sites More sharing options...
AndyFromCB Posted September 20, 2017 Report Share Posted September 20, 2017 2 hours ago, Yetti said: I think of all the BMW motorcycle Boxer Engines and all the downshifting letting the tire drive the engine..... Maybe it's time to learn how to ride a motorcycle and rev mach before dropping the clutch...As my father always said, brakes are a lot cheaper to replace than clutches... Quote Link to comment Share on other sites More sharing options...
bradp Posted September 21, 2017 Report Share Posted September 21, 2017 On 9/19/2017 at 8:53 AM, N201MKTurbo said: When back driving the engine the oil hole Is 180 degrees from where the pressure is being applied and the film strength is the lowest. Not sure I'm following the relationship between oil oriface and degrees of pressure-rotation... Quote Link to comment Share on other sites More sharing options...
N201MKTurbo Posted September 22, 2017 Report Share Posted September 22, 2017 6 hours ago, bradp said: Not sure I'm following the relationship between oil oriface and degrees of pressure-rotation... I'll see if I can find a diagram. Quote Link to comment Share on other sites More sharing options...
Yetti Posted September 22, 2017 Report Share Posted September 22, 2017 On 9/19/2017 at 11:40 PM, AndyFromCB said: Maybe it's time to learn how to ride a motorcycle and rev mach before dropping the clutch...As my father always said, brakes are a lot cheaper to replace than clutches... Oh I can totally go up and down the range without using the clutch. can do it in a non syncromeshed tractor transmission. Double clutching is also a skill. 1 Quote Link to comment Share on other sites More sharing options...
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