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Torsional Dampening

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wsimpso1

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Cars (and trucks) have another card up their sleeve - soft engine mounts. This means that the engine block can effectively "rotate" about the crankshaft to some extent, which helps in the "soft spring" approach. And some modern mounts are pretty trick, fluid damped units, not the rubber blocks of old.
ps I have seen failed engine mounts in rally cars which I would swear have died due to overheating, not overload.
Engine mounts in both cars and airplanes are sized to give 1st order resonance of the block moving in torsion below idle speed, so it is isolating during operation. Usually, you want to put the resonance between firing order at cranking and firing order at idle, so the engine quickly passes through resonance during start accel and after fuel cut-off, but does not run there even while cranking. You see this when you shut down many piston airplane engines - the shudder as it slows below idle.

Engine mounts don't really help much with torsional vibration of rotating internals. During firing pulses, the engine block is accelerating one way while the crank accelerates the other way. The acceleration of the internals is much bigger than of the block/heads/accessories. The pressures happen way too fast and the block accels are way too low for it to influence the crank accels much.

If somehow they get into extended operation near resonance, all mounts will heat up and cook if the temps get in range for the mount materials.

Billski
 
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rv7charlie

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OK; I'm back after our annual Pumpkin Drop this weekend here at Slobovia Outernational. Tracy Crook in his RV-4, and a Renesis powered RV-10 attended. Tracy OK'd publishing his name as the heretic using a directly coupled input shaft to a planetary. He said he has around 50 hours on it now (yeah, I know it's not 50000 hrs of testing that the car makers do, but what alt engine product for a/c has that?). He said that he actually sold a few drives with that setup before retiring, but hasn't had any feedback from the buyers. He said that he welcomes any discussion. (If you aren't on the Flyrotary list, he might be hard to get, however.) Tracy does fly a low power profile on cross countries, but not to protect the drive (max efficiency is his goal). He did say that the drive is operating about as well as it's ever operated, using the AL flywheel/no damper setup.

Billski,

You mentioned that Hatch had broken multiple planetaries. Did he tell you what part(s) he was breaking? (There's a reason for asking, as you might expect.)

Charlie
Bobby and Tracy.jpg
 

wsimpso1

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Billski,

You mentioned that Hatch had broken multiple planetaries. Did he tell you what part(s) he was breaking? (There's a reason for asking, as you might expect.)

Charlie
View attachment 55943
Ok, I have been sharing big, and you are playing games and not sharing.

I asked. We talked about it. Want to know more? Be forthcoming yourself.

Billski
 

rv7charlie

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Sorry; didn't intend to play games. But we're both 2nd or 3rd hand sources of info in this case, & I don't want to pretend to be an authority, or have 1st hand knowledge. I asked Tracy if he'd heard about Everett breaking planetaries. Tracy said that he'd talked with Everett back then, and when pressed for details on how they'd broken planetaries, Everett told him that they were drilling the sintered metal carriers for mounting/attachment, and the carriers were what they were breaking. It's not unreasonable to wonder if the mods they were making to the gearset itself were contributing to the failures, instead of the torque peaks & resonances of the rotary. The sintered metal parts seem to survive just fine (unmodified) in much higher torque/power applications in heavy duty trucks, but were breaking after mods when attached to rotaries. I wonder if they would have survived attached to the V-8s in the trucks after the same mods.

So, was Everett breaking the gearsets in ways other than than the modified sintered iron parts?

I want to be clear that I'm not asking this stuff to be argumentative. I'm typing this while sitting next to the RV-7 fuselage that has a Renesis with a RWS redrive in the final stages of FWF work, and my and my passengers' lives will be depending on stuff working correctly and reliably. So far, I've seen little that concerns me (I know personally quite a few who have hundreds of hours on the gearsets), but I'm always willing to learn.

Charlie
 

rv6ejguy

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Any time there are variations in flywheel, propeller inertia, backlash and damper construction, what failed (or lasted) in one application, may have little or no bearing on the next.

I believe PS was doing some testing with heavy metal props at one point, this can seriously change the TV picture according to our math models where we keep everything the same but double or triple the prop MMOI.

As I've said numerous times, theory is great and can save you a lot of time and tears but in the end, only several hundred hours of trouble free flight time behind a certain engine/PSRU/prop combination really validates any design whether eyeball designed or professionally engineered. Tracy has many dozens of units out there with cumulatively thousands of flight hours. You don't hear of many problems with them. He did consider TV when designing his units and had some unique solutions.
 

rv7charlie

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Thanks, Ross. That (heavy metal prop) would seem to be a very significant data point. The one new restriction that Tracy did put on the no-damper version of his drive was *no metal props*.

It seems very common in aviation to hear 'X (or Y) won't work; my friend Bubba tried it and it failed,' when there are dozens or even hundreds of X's and Y's being operated successfully. I always try to ask, 'What's different? What did they change?'

Charlie
 

wsimpso1

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Sorry; didn't intend to play games. But we're both 2nd or 3rd hand sources of info in this case, & I don't want to pretend to be an authority, or have 1st hand knowledge. I asked Tracy if he'd heard about Everett breaking planetaries. Tracy said that he'd talked with Everett back then, and when pressed for details on how they'd broken planetaries, Everett told him that they were drilling the sintered metal carriers for mounting/attachment, and the carriers were what they were breaking. It's not unreasonable to wonder if the mods they were making to the gearset itself were contributing to the failures, instead of the torque peaks & resonances of the rotary. The sintered metal parts seem to survive just fine (unmodified) in much higher torque/power applications in heavy duty trucks, but were breaking after mods when attached to rotaries. I wonder if they would have survived attached to the V-8s in the trucks after the same mods.

So, was Everett breaking the gearsets in ways other than than the modified sintered iron parts?

I want to be clear that I'm not asking this stuff to be argumentative. I'm typing this while sitting next to the RV-7 fuselage that has a Renesis with a RWS redrive in the final stages of FWF work, and my and my passengers' lives will be depending on stuff working correctly and reliably. So far, I've seen little that concerns me (I know personally quite a few who have hundreds of hours on the gearsets), but I'm always willing to learn.

Charlie
Sorry, I was feeling taken advantage of. I'll play nice.

Ev Hatch had already moved on from planetaries when I was talking with him, but we did talk about them a little. He had used and got failures on both factory and aftermarket gearsets. The racing gearset carriers were billet parts not the powder metal ones from the factory. When you break any little bits free in an automatic trans planetary, the little bits go through the gear mesh and pretty much make a salad out of the thing, which is what Ev had described to me. Badly scarred up gear teeth, broken ring gear, and sometimes, break or bend the carrier. We were familiar with that story at Ford, with bonus planet gear needles and broken off bits of gear teeth going through the mesh. I did ask Ev if any of bearing parts were blued or coked up, looking for symptoms of poor lube/cooling and bearing overheating, and he did say "no" and described shiney bearing and shaft surfaces. I suppose his machining to adapt the gearset might have weakened the carrier, but he was pretty savvy about this sort of thing, I would't have expected that he would have made that sort of mistake more than once.

Water under the bridge. If they can fly this system and get a bunch of hours while doing aerobatics, that will be the proof.

Bill
 

BoKu

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...a single row 7 cylinder radial would have a 1-3-5-7 firing order on the first revolution followed by 2-4-6 on the next so there's going to be a cyclic imbalance in power production...
I think that the "cyclic imbalance" is an artifact of the cylinder numbering. The thing to observe about radials is that, generally speaking, every other cylinder fires in sequence. The result is fairly smooth distribution of power pulses.

However, there is a caveat. The connecting rod articulation used in most radials, with a master rod that anchors all the other rods, results in small but perhaps significant variations in some combination of the location of TDC and the swept displacement between cylinders. And that must have at least some effect on the smoothness of power delivery.

--Bob K.
 

wsimpso1

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Sorry, I was feeling taken advantage of. I'll play nice.

Ev Hatch had already moved on from planetaries when I was talking with him, but we did talk about them a little. He had used and got failures on both factory and aftermarket gearsets. The racing gearset carriers were billet parts not the powder metal ones from the factory. When you break any little bits free in an automatic trans planetary, the little bits go through the gear mesh and pretty much make a salad out of the thing, which is what Ev had described to me. Badly scarred up gear teeth, broken ring gear, and sometimes, break or bend the carrier. We were familiar with that story at Ford, with bonus planet gear needles and broken off bits of gear teeth going through the mesh. I did ask Ev if any of bearing parts were blued or coked up, looking for symptoms of poor lube/cooling and bearing overheating, and he did say "no" and described shiney bearing and shaft surfaces. I suppose his machining to adapt the gearset might have weakened the carrier, but he was pretty savvy about this sort of thing, I would't have expected that he would have made that sort of mistake more than once.

Water under the bridge. If they can fly this system and get a bunch of hours while doing aerobatics, that will be the proof.

Bill
I let my memory work on those conversations with Ev Hatch. In the Ford planataries, he had fatigue cracks visible in gears, gear tooth face pitting on gear teeth and pitting on bearing needles. These normally indicate things like dimensional errors in the gears or carriers (if true we would have had BIG problems with those parts in diesel trucks - we did not - so dimensions were probably in range) or impact/overload. Given he was running less than 200 ft-lb in gearsets known to be good above 600 ft-lbs, that left impact. We talked about big vibration, resonance, and going through lash, and I told him he was probably in resonance for at least some of his duty cycle. I was supportive of a soft system, but he did not like that idea. His perspective was that a low lash high stiffness system would work, and it was already coming together. That path seems to work, so how would I ever say it was invalid?

Billski
 

wsimpso1

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Off the top, I want to say I really appreciate these technical threads - they make me think. They also give me a headache, but there you go.
I was wondering if there was a particular engine layout that reduces TV.
For instance, a single row 7 cylinder radial would have a 1-3-5-7 firing order on the first revolution followed by 2-4-6 on the next so there's going to be a cyclic imbalance in power production. Also there is only one node on the crankshaft.
If a two row radial is configured so that it runs 4/3 one revolution and 3/4 on the next is that ideal since the crankshaft is always equally loaded or is it a problem that it is going to be loaded mostly at the front on one revolution and then mostly at the back on the next?
And so on for in-line engines. Is a V-12 a better solution because there are six impulses per revolution or is the longer crankshaft a problem? Is a flat four a better solution because the crankshaft length is reduced over an inline four?
You are talking like the crank and gearset in a radial engine KNOWS how many pulses come in during a turn. The vibrating system only knows what frequencies it can freely vibrate at and how often input vibrations come in. In a seven cylinder radial, it runs at 3.5 firing pulses per turn, but that is a convenience we have for stating how many oscillations it will make per second when we know how fast it is turning.

The way to express vibe input in any piston engine is to recognize that:

Every time a piston changes direction there is a pulse;

Crank and rod rotation result in the accels at the top and at the bottom of piston travel are different, giving a pulse per turn on each piston;

Everytime a piston makes a power stroke there is a pulse;

The arrangement of cylinders, crankshaft, and firing order determine the orders we see.

In piston engines, the pistons and rods are fairly light, the non-firing pulses are fairly small and firing dominates.Add in how fast the thing is turning, and you get frequencies for each of the orders. Lets look at the details for the 7 cylinder radial. In your example 7 cylinder radial, for every two revolutions, you get a bunch of vibe orders each evenly spaced:

1 accel per piston per every other rev for power, so that is 7 pulses, and 3.5 order. This one is big;

1 accel per piston per rev for crank and rod configuration, so that is 14 pulses, and 7th order of rotation. This one is usually about 25% of the power pulse;

2 accels per piston per rev, so that is 28 pulses, and 14th order of rotation. This one is measurable, more like 15% of the power pulses.

in total, the big radials really were a pretty elegant way to do a piston engine on an airplane. They took little length, could be integrally cooled nicely and with reasonably low drag, when stacked into multi-row engines were compact and low drag, and were well set up for efficient turbocharging and turbo-compounding. And you are talking about solving problems that did not exist.

Now, get into inline engines and things change. Firing order in inline and V's (which are still inlines, you just have two of them paired up) is scattered around to do a bunch of stuff, but mostly to allow even firing and keep weight down. If you used current crank stiffness (sized to give resonance safely above 2x firing order at max rpm) and ran an inline engine with its firing order running from one end to the other, the crank would wind up significantly and then release every two turns and your crank pin error at the cylinder furthest from the cam drive would be out of time with the cams enough to notice, giving really big 1st order and 1/2 order vibrations. You could beef up the crank even more to reduce these nasties, but to what end? Scatter the firing pulses and other vibe about around crank, run lighter stuff, and be happy.

Billski
 

wsimpso1

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Yeah, sounds too! In the car business, we did not break stuff very often, and we usually had everything well contained with ballastic pads and the like, but the dyno guys have described:

BANG! (most common)

A long sequence of bangs (as it spins down with parts flailing around);

Drum solo sequence of varying noises and sequences of bangs and booms (flailing parts and stuff flying off);

A brief clatter followed by a irregularly timed crashes, bangs and booms, and then pause before the last crash as the last part finally falls from the sky;

All of this is usually superimposed upon the engine screaming up in rpm until the rev limiter cuts spark or the dyno's controllers chop fuel and spark.

So what about gory details?

Billski
 

AdrianS

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I can't go into much detail (I probably shouldn't be posting pics at all ;)), but -
BANG! was about it. 5,000 rpm to 0 in an instant.

Followed by me swearing because the shock caused a power plug to disconnect, crashing the datalogger before it had saved the last, critical few seconds.
Much destruction, but the safety guards caught all the flying metal.
 

wsimpso1

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I can't go into much detail (I probably shouldn't be posting pics at all ;)), but -
BANG! was about it. 5,000 rpm to 0 in an instant.

Followed by me swearing because the shock caused a power plug to disconnect, crashing the datalogger before it had saved the last, critical few seconds.
Much destruction, but the safety guards caught all the flying metal.
Bummer about loosing the data... Knowing the rpm where one or more speed sensors goes crazy would be really good info to have. Hang in there.

Bill
 

AdrianS

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Back of the envelope calcs say about 130 kJ of kinetic energy in spinny bits when it went bang. But that's me doing it in my head, so I may be completely wrong (I=0.94 kg.m^2, w = 5000 rpm = 525 rad/sec).
Online calculator says that is equivalent to about an ounce of TNT (who says we antipodeans can't use imperial units :)).

The problem with dynos is the the variety of engines a customer may couple to them - anything from a 3 cyl diesel (bad) to a 6 litre v8.

Anyway, back to redrives and flying stuff - I just thought it was educational to show what happens when things go wrong.
 

wsimpso1

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Back of the envelope calcs say about 130 kJ of kinetic energy in spinny bits when it went bang. But that's me doing it in my head, so I may be completely wrong (I=0.94 kg.m^2, w = 5000 rpm = 525 rad/sec).
Online calculator says that is equivalent to about an ounce of TNT (who says we antipodeans can't use imperial units :)).

The problem with dynos is the the variety of engines a customer may couple to them - anything from a 3 cyl diesel (bad) to a 6 litre v8.

Anyway, back to redrives and flying stuff - I just thought it was educational to show what happens when things go wrong.
Thanks for sharing!

At Chrysler our dyno labs had a couple dampers based upon engine torque. I argued for more based upon number of cylinders.

Bill
 

rv7charlie

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I let my memory work on those conversations with Ev Hatch. In the Ford planataries, he had fatigue cracks visible in gears, gear tooth face pitting on gear teeth and pitting on bearing needles. These normally indicate things like dimensional errors in the gears or carriers (if true we would have had BIG problems with those parts in diesel trucks - we did not - so dimensions were probably in range) or impact/overload. Given he was running less than 200 ft-lb in gearsets known to be good above 600 ft-lbs, that left impact. We talked about big vibration, resonance, and going through lash, and I told him he was probably in resonance for at least some of his duty cycle. I was supportive of a soft system, but he did not like that idea. His perspective was that a low lash high stiffness system would work, and it was already coming together. That path seems to work, so how would I ever say it was invalid?

Billski
Thanks for sticking with this discussion. I, for one, really appreciate you letting us 'take advantage' of your expertise. :)

Did he tell you what (if anything) he was using for dampers when he was breaking the gearsets? What the low freq tuning was? Or whether, as Ross mentioned, he might have been running metal props at the time? I'm no genius (but not completely stupid either) and have no training/expertise in this field. But when approaching the data logically, we've got:
1. an inherently low torque engine
2. peak torque that has to be less than 1/2 that of an equivalent HP 4 cyl 4 stroke piston engine, since rpm is over twice as high and the net torque never goes negative like it does with a 4 cyl engine
3. 2 per rev power impulses, like a 4 cyl 4 stroke (but with no torque reversals; see #2)
3. a gearset designed for 600 lbs/ft average torque from a diesel V-8; I assume that it isn't a 2 stroke, so 4 per rev power impulses that will be significantly higher than 600 lbs/ft

So, please help me wrap my mind around the processes that could result in resonance when driving a prop. Everett obviously had problems, but where? Metal prop? Grossly misdesigned dampers? misalignment in assembly? Allowing the system to operate 'unloaded' for extended periods? Too low an idle, causing gear slap?

Charlie
 

plncraze

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Also if anyone has any more to tell about George Graham's experiences with the Mazda and the tranmission locked in second gear for a reduction drive please share. I thought had to be the cheapest setup one could have for a homebuilt.
 
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