Belted Air Power Chevy V6 ...Indirect Update

[DanH]

Ross,
You brought up the question "If your example is typical with F1 happening at relatively low rpms and low frequencies.."

I believe it is, more or less, for the typical auto conversion. However, I don't want you to think it is typical for all aircraft systems. it's not. The difference is mostly in the length and thickness of the shafts, ie torsional stiffness. A comparison might be an eye-opener.

My most recent torsional project is a fixed pitch, non-geared version of the Russian-designed M14 radial. The idea is to push the weight of the installation down into the same ballpark as an IO-540. So, no planetary gearset and no heavy (about 90 lbs in this case) constant speed prop. There are two prototypes in the US, one of which is in my shop. It goes on a biplane when I get this RV done.

Radical changes in shaft configuration and propeller inertia required a look at torsional issues. Compared to an auto conversion, this system has a very short single-throw crank, a short, thick propshaft, and nine low-compression cylinders.

The result (using a simple hand-calculated three element model) is plotted below. The 4.5 order is the firing frequency, and the 9th is the recip order. As compared to an auto conversion, the short stiff system pushes F1 way up the frequency scale. F1 is never intersected by the firing frequency. This is a happy result.

The concern of course is the intersection with the 9th order. How bad will it be? I did not do amplitude calculations as the real system is very, very complex; the blower and accessory sections for example are a whirlwind of shafts and gears. Unlike a Sube, there is no supply of available parts in the junkyard down the street, ie, no practical way to do bifillar inertia measurements or measure the shafts. Without a reasonable degree of input accuracy, I didn't think amplitude calculations would be accurate enough to justify the effort.

Instead I made some (I hope) reasonable assumptions. First, there is old data from the 30's and 40's (the heyday of radial research), including a calculation of harmonic torque contribution as a percentage of mean torque. The contribution of the 9th order is only 8%. Second, I did three models to compare with the common, proven M14P/PF version as well as the M14D, the subject of considerable dyno time in Romania. They work just fine. Third, the front end of this engine is very robust, the old "make it stout" principle. Propshaft material stress, for example, is much lower than what you have with an AEIO Lycoming. Last, this is more or less a private project. It's my risk. You can buy one, but nobody is gonna assure you that it is "proven"...at least until later. Yeah, before it is all over I may put some telemetry on the shaft.

Anyway, compare to the previous plots.

BTW, we have over 600 views of this thread, but very little contribution. Is anybody interested in this stuff or should I just shut up? <g>

[Yukon]

Dan,
Don't stop talking about this stuff! I think many people are interested, just few of us are qualified to comment. Should be of significant interest to anyone considering an untested engine/prop combination. I remember a few years back when people were shortening larger, certified props and runniung them on their RV-4's and finding that the blades would snap off due to destructive resonance. Van has pretty much educated everybody now about the evils of random prop "mix n match".

Is it possible that the higher GB temps Egg is experiencing on the G3 are related to harmonics? Does it ever manifest itself in heat as well as vibration?

[Mike S]

Originally Posted by Yukon

Dan, Don't stop talking about this stuff! I think many people are interested, just few of us are qualified to comment.

You took the words right out of my mouth.

Remember, part of the legal description justifying the FAA to allow homebuilding is "education".

Mike

[pierre smith]

I agree with you guys as well. I do appreciate the education since I used to have a Cassutt Formula 1 racer. The cut down Sensenich prop that was retwisted for racing at around 4000 RPM had significant resonant "nodes" as I recall, around 2900 RPM, and weren't supposed to be used for ferrying. One owner (a Jim Wilson) was ferrying back to Texas when a blade let go. The saving grace was the cable that was required on all F-1 racers that went from the upper motor mount down between the cylinders to the other side. His engine hung there and stayed on during the forced landing to a road.

Appreciate the good stuff,
Pierre

[rv6ejguy]

Originally Posted by DanH

Ross,
Again back to a previous conversation;

You now understand why I had reservations about switching to harder urethane bushings in your drive. A lower torsional stiffness is necessary if you wish to reduce shaft load by eliminating the need to pass through a resonant peak every time you come up off idle. That may not be possible with urethane bushings, but they are a miserable choice of soft element anyway.

Lowering F1 would probably lower F2 also. The goal is to drop F1 as far as reasonable without allowing F2 down into the top of the operating range. I don't know where it lies right now.

An accurate model might let you pass on telemetry. You gather stiffness and inertia data, then compute frequencies. If you're sure F1 is below idle and F2 is above redline, you might reasonably decide there is nothing critical to measure. If there are no critical intersections, even amplitude calculations are moot.

Apparently there are several types of urethanes, some suitable for vibration isolation and some certainly not. I'd be more inclined to stay with rubber, varying durometer and testing the results.

I have to warn people taxiing my RV to push the throttle up quickly to pass through the evil range without delay. Fortunately with the 2.2 gear ratio, 1600 engine rpm gives about 700 at the prop so brakes are not needed much on level ground, takes about 2000 engine rpm to get moving from rest. Your choices for power on the ground are either idle at 1000 or above 1600.

BTW, facinating subject and I really appreciate your examples and thoughts. The radial looks like a neat project to be working on.

[DanH]

John,
<<Should be of significant interest to anyone considering an untested engine/prop combination.>>

Keep in mind that we're pretty much just talking about torsional vibration, oscillation of the rotating masses in relation to each other. Within that strict context, the propeller is a just an inertia combined with an equivelent shaft stiffness. Vibration of the propeller blades and blade failure is a different subject. Torsional vibration can be related to blade failure, in that high amplitude torsional resonance obviously stresses the prop very hard.

<<Is it possible that the higher GB temps Egg is experiencing on the G3 are related to harmonics? Does it ever manifest itself in heat as well as vibration?>>

I have no knowledge of the G3 box, so general stuff only; generating heat pretty much requires friction. The friction can be rolling, sliding, internal to a flexible or fluid substance, etc. Sliding and rolling friction are obvious. Less obvious might be a rubber doughnut soft element run constantly in a bad resonant range. (BTW, that means you probably picked the wrong one; the idea was to pick a stiffness value that moved the resonant point somplace where you don't run). The doughnut has a small true damping value. In the torsional context the strict definition of "damping" would be "removes energy from the system", usually as heat. When worked very hard the doughnut can overheat and even melt; it can't shed heat fast enough.

Does the Egg box contain only gears, or is there also perhaps a decoupling device, maybe a ramp and dog set like the one in a Rotax B or 912 box?

[rv6ejguy]

To my knowledge the G3 Egg drives have no coupling per se but use a relatively heavy damped dual mass flywheel, similar to what BMW uses on their cars. Maybe David can give us the straight scoop on this since he is now flying his.

Interestingly my Marcotte drive has never exceeded 90C on the oil even in the climb on a hot day. It has only a single gear mesh. I have a 3/8 gap between the spinner and cowling which lets in high pressure air flow over the drive casing. There is no dedicated cooling duct.

[DanH]

Ross,
<<I have to warn people taxiing my RV to push the throttle up quickly to pass through the evil range without delay.>>

Tell them to push the throttle up gently, using the minimum throttle necessary to get through the range.

<<I'd be more inclined to stay with rubber, varying durometer and testing the results.>>

Yes, but I don't think you can get enough frequency reduction with a pin in any bushing. As you've seen, things are not what you might expect. That setup probably has a rather high stiffness value. I actually did a shop setup to measure the torsional stiffness (ft-lbs/radian) of some Subaru clutch plates as well as a stupid urethane element of my own design. Trust me, it is a whole lot better to simply buy a good soft element from Lord, Lovejoy, or Goetz. They come with a stiffness value right next to the part number <g>

[rv6ejguy]

Originally Posted by DanH

Ross,
<<I have to warn people taxiing my RV to push the throttle up quickly to pass through the evil range without delay.>>

Tell them to push the throttle up gently, using the minimum throttle necessary to get through the range.

<<I'd be more inclined to stay with rubber, varying durometer and testing the results.>>

Yes, but I don't think you can get enough frequency reduction with a pin in any bushing. As you've seen, things are not what you might expect. That setup probably has a rather high stiffness value. I actually did a shop setup to measure the torsional stiffness (ft-lbs/radian) of some Subaru clutch plates as well as a stupid urethane element of my own design. Trust me, it is a whole lot better to simply buy a good soft element from Lord, Lovejoy, or Goetz. They come with a stiffness value right next to the part number <g>

Believe me you don't want to subject yourself or the engine, drive/ prop/ airframe to this one very long. Throttle up! With the prop load, you have to add enough power to take it up from 1000 to 1600 within a half second or so.

Lots of machining to adapt a linear coupling but I'll do it if I have to. I have a big polymer place right across from my shop which will pour any type of neoprene or urethane in any durometer if I machine the mold and provide the bushings. I'd try this first and test to see what the results are. I might waste my time but I'll learn something. I do a lot of that!

[DanH]

Pierre,
<<The cut down Sensenich prop that was retwisted for racing at around 4000 RPM had significant resonant "nodes"...>>

Probably meant "modes". "Nodes" and "modes" are both vibration terms, easily confused for obvious reasons.

A node is a particular point on a vibrating shaft or beam. A torsional example would be a system with two equal inertias connected by a shaft. Each inertia will rotate in oscillation opposite the other, twisting and relaxing the shaft. If you took a magic marker and drew a line along the shaft, you would see the line twist and untwist in a sprial around the shaft. At the exact midpoint of the shaft, a small part of the line would not have any apparent movement. That's the node.

BTW, the F1 node on our auto conversions is usually just rearward of the prop flange. The node moves closer to the big inertia in proportion to the difference in inertias.

A mode refers to the motion of the system. The above torsional system has a single mode, an opposing oscillation of the two inertias.

I'm pretty weak on linear vibration of beams, but if I remember my theory right a beam (like a prop blade) has an infinite number of nodes because it can have an infinite number of modes. For example, a prop blade can vibrate by bending only at the root (a mode with one node) or at the root and somewhere along the blade (another mode, two nodes), or it can even form an S-curve (three nodes, root, midspan, and tip).

Who named this stuff anyway? <g>