Belted Air Power Chevy V6 ...Indirect Update

Alternative Engine Fans...

Last night I emailed Belted Air Power through their website asking Jess Meyers if he'd come back here and post an update on the latest happenings with Belted Air. Jess replied this morning...answering the questions I had posed but declining to come back over here and "mix it up" again. Like so many he gets tired of the serial skeptics and the unending banter, etc. etc. In follow up he responded in detail to my many questions. He gave me permission to post his replies here. I have edited the paragraph structure just for ease of reading:

From: [email protected] A
Date: Thu, 28 Jun 2007 09:24:02 EDT

Subject: Re: Request for Update

Dan, I didn't know we had anything on the VAF forum's unless your talking about the fellow in Texas's site, it got to be nothing but people talking about harmonic devices that we decided they haven't nor will ever do anything but talk and thought it was a waste of our time.

Thanks for following us though. We are involved with several projects, a four seater from Australia using our FWF, Testing the Vari-Prop which is comming along great, and possibly a new drive for the smaller Chevy V-6 60 degree engines.

Jess

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From: [email protected]
Date: Thu, 28 Jun 2007 14:23:18 EDT
Subject: Re: Request for Update

Dan, I'll try to ally your fears

1. The Vari-Prop has gone through complete engineering and partial re-design to simplify the maintenace of the unit.
2. Yes we trust it.
3. It shortens the distance slightly but will indicate 1500 fpm climb with a density altitude of 5000 msl.
4. Cruise speed is where you want to set it.
5. At 2500 engine rpm and 20" it indicates 130 mph for a fuel burn of 3-3.5 gph. This is one of the things were looking at is endurance.
5. It now can use the Lipps blade profile
6. It made the engine run much cooler at 110 F OAT
7. It will hopefully be on the market by this fall.
8. I think the projected cost was 5-5500.00

We have 75 RV's with the Chevy flying.

I think one fellow in Alabama removed his for a 200 HP Lyc.

Yes we have people one customer who purchased two, but unfortunately he died of an unexpected heart attack 4 months ago.

I'll try to find some in an area close to you. We have one in MN who did not use our whole package and mounted the radiator on the bottom of the plane and is very happy with it.

The planes perform like a 6 or 6A of 180 HP fixed pitch prop, except they use less fuel.

I haven't heard from Dr. Minichan for a while [RV-9A Chevy builder].

We are developing the new drive for the smaller hp requirement planes ie. Zenair, etc. and the Mustang by Titan.

Since the manifold is heated by coolant (190 degrees) or the exhaust crossover...carb ice has not been an issue. Not to say that in a blinding snow storm it wouldn't ice over but the air intake could possibly also, and my question is why are you flying single engine in known icing conditions without de-ice capability and hot props. We have not used carb [heat] and have flown into known icing conditions over a dry lake south of town with a 800 foot ceiling on the breakout. Ice on windshield, slight on wings engine ran fine. Will I do it again? NO. Minnesota is the farthest north where it's known cold and [there is] one in Calgary Canada. No [carb] ice problems. When it's that bad they don't fly anything.

Behind the radiator is an area where we have installed a flapper valve and take 190 degree air into the cabin and on to the windshield works great, no extra connections and clean air.

I still recommend calling ahead, come out and fly in ours, check with a comparable one with a Lycoming and make a valid decision.

Jess

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Great info Dan! I check BAP's site often, hoping for updates...

Thanks for posting!

I was based at Las Vegas VGT in 2004-5 and entertained the thought of using Jessies's setup. I was quite impressed & would see him flying his RV quite often. He demo'd his FWF belted air sytem to our local EAA group. When I visted Jessie's hangar, he was always open to discussions.

I opted to go with a more traditional aviation option.

I appreciate Jess' input on the Chevy conversions and am very interested in his testing on the Vari-prop. I find some other information he has posted as amazing like 3.5gph at 130mph. Doesn't seem to add up as far as an RV drag polar goes with the SFC and available power at 2500 rpm. Comments like the aluminum block being too light also don't make sense. A Subaru is way lighter than even an all aluminum V6 and they always weigh as much or more than a Lycoming, even with a light prop.

The last 7A 180hp Lyco, FP I flew was able to true 174 knots at 8000 MSL on about 9.6GPH. I'm still waiting for an auto engine that can beat that.

Just playing the reality guy here and I'd love to be proved wrong with a side by side flight test against a Lycoming powered RV6A using the "fill the tanks Van's method" of fuel flow measurement. At least Jess is inviting people for rides and to compare.

Exactly

rv6ejguy said:

The last 7A 180hp Lyco, FP I flew was able to true 174 knots at 8000 MSL on about 9.6GPH. I'm still waiting for an auto engine that can beat that.

Just playing the reality guy here and I'd love to be proved wrong with a side by side flight test against a Lycoming powered RV6A using the "fill the tanks Van's method" of fuel flow measurement. At least Jess is inviting people for rides and to compare.

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Ross,
Your 174 knot/ 9.6 GPH just about mirrors our numbers exactly. We occasionally get 177 knots solo and 2700 RPM with the Catto. A nearby friend has the 4 cylinder supercharged Egg in his -7 and goes around 162 MPH on 7 gph of mogas and loves the airplane. I recently gave him a BFR in it and it sure is smoooth.

Is there a geared box anywhere for the 350 Chevy?

Regards,
Pierre

Those are good numbers Pierre!

Lots of gearboxes available for the SBC. What do you have in mind?

pierre smith said:

Ross,
Your 174 knot/ 9.6 GPH just about mirrors our numbers exactly. We occasionally get 177 knots solo and 2700 RPM with the Catto. A nearby friend has the 4 cylinder supercharged Egg in his -7 and goes around 162 MPH on 7 gph of mogas and loves the airplane. I recently gave him a BFR in it and it sure is smoooth.

Is there a geared box anywhere for the 350 Chevy?

Regards,
Pierre

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PIERRE,
There are several GB's for small block chevy. Check Team38.com. There is a Hi-Vo chain drive box as well Gershwinder (sic?) possibly? This looks like a compotent design. The owner has been trying to sell the company through the Contact! magazine though. Ross uses the Marcotte box, and they make one big enough for the Chevy. EPI Inc. did the gearboxes for chevys for the Lancair 4. Beautiful work but expensive. They (EPI) have done a lot of conversions and STC type work. Jack Kane is a real engineering perfectionist and won't release a product until he truly believes it's ready. That can be a PITA, but then you're less likely to make a smoking hole in the ground somewhere.
Bill Jepson

<<..it got to be nothing but people talking about harmonic devices that we decided they haven't nor will ever do anything but talk.. >>

I'm one of those people "talking about harmonic devices", without regret. I've done quite a lot more than talk about it, including designing a drive from scratch, developing a true viscous damper for use in parallel with a soft element, pulling live torsional telemetry, and helping with torsional prediction software. I deadsticked a failed drive (from a vendor), then bought books because a deadstick is a fine incentive for education. I've also flown both good and bad drives to OSH and S&F. My dues are paid.

There are two ways to design a propeller speed reduction unit. One is the brute force method, ie, hopefully make it stout enough to shrug off torsional issues. The other is to apply torsional engineering (torsional vibration is an old science), a path to lower weight and better reliability.

Brute force is fine, if you can stand the weight and have the time (years) for the "fly, break, fix, repeat" development cycles. That's Jess. His drives have torsional issues (all drives do, as the laws of physics vary for no man), but they are stout and Jess judges them good enough. If you're not interested in further development, discussions of torsional engineering are indeed a waste of time.

I have much respect for Jess as a pioneer. I don't think he should dismiss serious engineering discussion as unnecessary. You cannot design the next generation of propeller drive systems without the science.

DanH said:

<<..it got to be nothing but people talking about harmonic devices that we decided they haven't nor will ever do anything but talk.. >>

I'm one of those people "talking about harmonic devices", without regret. I've done quite a lot more than talk about it, including designing a drive from scratch, developing a true viscous damper for use in parallel with a soft element, pulling live torsional telemetry, and helping with torsional prediction software. I deadsticked a failed drive (from a vendor), then bought books because a deadstick is a fine incentive for education. I've also flown both good and bad drives to OSH and S&F. My dues are paid.

There are two ways to design a propeller speed reduction unit. One is the brute force method, ie, hopefully make it stout enough to shrug off torsional issues. The other is to apply torsional engineering (torsional vibration is an old science), a path to lower weight and better reliability.

Brute force is fine, if you can stand the weight and have the time (years) for the "fly, break, fix, repeat" development cycles. That's Jess. His drives have torsional issues (all drives do, as the laws of physics vary for no man), but they are stout and Jess judges them good enough. If you're not interested in further development, discussions of torsional engineering are indeed a waste of time.

I have much respect for Jess as a pioneer. I don't think he should dismiss serious engineering discussion as unnecessary. You cannot design the next generation of propeller drive systems without the science.

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Dan, I believe you are the one that steered me to the Lovejoy site for in-line couplings. Thanks for that. You are correct that the path to a light drive is to handle the harmonics. If you do that you are much less likely to be supprised!

Bill Jepson

Bill, you're welcome. I like the Centaflex stuff.

Some folks like to argue that a particular drive system "has been flown for 1000's of hours", so it must be good. Here's an example of why that argument is false.

The plot you see below is an accurate computer simulation of the drive I purchased from a popular vendor in 1996. It was on a little I-3 Suzuki that made 68hp on the best day of it's life, or around 75 lbs-ft of mean torque.

The left axis is oscillating vibratory torque. The horizontal axis is RPM. At 1800 RPM, wide open throttle oscillating torque is 2300 lbs-ft (!). Mean torque at this RPM might not have been much more than 30 lbs-ft. This is resonant behavior, an unfortunate intersection of exciting frequency and the drive's F1 frequency. Resonance multiplies vibratory amplitude. Amplitude would reach infinity if it were not for the presence of a certain degree of frictional damping present in any real system. In this case the multiple is 76x mean torque.

Now look at a 4000 cruise RPM. Vibratory amplitude is around 50 lbs-ft, possibly less than mean torque, and very easy on the system components.

The failure of this drive would not be predicated on hours of operation. It would depend on the number of times the operator went to full throttle from idle, as in a balked landing or a simple touch and go. You could fly a million hours without failure if (2) you stay in the RPM range on the right side of the graph, and (2) push the throttle up gently every time you pass through 1800 RPM in order to keep the resonant peak as low as possible (resonant amplitude is roughly proportional to manifold pressure). Not a good drive by any means, but....

I failed this drive with 35 hours of T&G's and flight test. 10 hours later I launched for OSH, from Alabama, and had a pleasant trip by operating it carefully. Therein lies a second lesson: just because it flew to OSH doesn't make it good.

Dan makes important points.

In flight testing with the proper equipment is really needed to explore the TV question as Hartzell/ Lycoming does with various combinations. Even these direct drive engines often have avoid rpm ranges at certain throttle settings. The engineers have determined that these are relatively benign in most certified installations and just recommend extended operation in these ranges be avoided.

Rotax 912 engines have bad TV in the lower ranges below 1200rpm depending on prop MIs. You must not run these engines here for any length of time or the reduction gears will be chewed up in short order. Eggenfellner's latest setup apparently has an avoid range below 1200rpm as well.

What would be very helpful is to have an actual plot like Dan's above for all engine and prop combinations that we fly to see where the bad ranges are, either to avoid them or correct them through design changes in the engine/ drive combo. This is unlikely to happen any time soon for most drives due to the sheer amount of time needed to collect data for multiple propellers and engines. It could never be afforded by companies like RWS or Marcotte which cater to several different engines fitted with dozens of different props.

If Eggenfellner only supplied one prop type with all H6 engines and Gen 3 drives, the task is much smaller and probably well worthwhile as the entire package stays the same. One instrumented flight test would reveal the entire TV spectrum.

In the meantime, we all fly along blissfully unaware that a serious failure due to TV could be around the corner at any time.

<<This is unlikely to happen any time soon for most drives due to the sheer amount of time needed to collect data for multiple propellers and engines.>>

Perhaps not quite as true as common knowledge would have you think. Consider the simple two-element model, two inertias conected by a stiffness (think two disks connected by a shaft). If the two inertias are equal they will oscillate in equal opposition. We're a long way from that; propeller inertia is much higher than crank/flywheel inertia. The vibratory motion can be thought of as an oscillation of the small inertia while the large one remains more or less stationary. Put another way, the large inertia is the anchor against which the small inertia oscillates. It makes little difference if the prop inertia is 5x, 7.5x, or 10x the small inertia. An anchor is an anchor.

Here's the frequency equation for a two element model. Plug in a few variations for J1 and see how little it changes F. Use 1, 1.5 and 2 slug-ft^2 for J1, 0.2 for J2. Use 10,000 ft-lbs/radian for K, a realistic shaft number.

F= 9.55 {square root of [ K (J1 + J2 / J1 J2)]}

F is in cycles per minute, so divide by 60 for hertz. Hertz to RPM for a 4-stroke (tells you critical RPM) is [(hertz x 120)/# of cyls].

Aw heck, I'll just tell you. The answers are roughly 39, 38, and 37 hertz. Assume a 4-cylinder 4-stroke. Switch props for one with twice as much inertia and you move the F1 critical to a point in the RPM range a whopping 60 RPM lower. You figure amplitude is gonna change very much?

I just hosted my whole EAA chapter at my shop and I've had two beers, so do check my math <g>

Always enjoy your posts Dan, makes me think. I buy the math and the basic premise.

I have heard some people who have changed prop types from wood to metal or composite and either reduced or increased perceived vibration considerably. I have seen this also recently on a 912 Rotax with three different props on my test stand with all props balanced reasonably well. Do you have some other angle on this observation?

Why is Hartzell doing so much testing on relatively similar props on relatively similar engines if there is theoretically so little difference? A quick calc shows a pretty big difference in MIs for a 60lb metal prop and a 12 lb wooden one as related to say a 25lb. flywheel turning at twice prop speed.

I'm still planning to try measurements with an RF chip and accelerometers- have to get the whole thing done first.

Ross,
<<changed prop types from wood to metal or composite and either reduced or increased perceived vibration considerably.>>

I believe that. The previous example was a simple two-element model, fine for illustrating the point that you have reasonable latitude in prop substitution as compared to a baseline propeller. For example, if a vendor recorded acceptable shaft telemetry numbers with a one-piece wood prop, he could safely assume that any other wood prop of similar construction that would actually fly the airplane will be about the same. That cuts the volume of "required testing" considerably and builders would have safe leeway in prop selection.

BTW, I have some live telemetry plots comparing back-to-back runs of two wood props, one a classic eliptical blade form in hard maple and one a tapered "toothpick" in mahogany. Ain't much difference.

You will start seeing more difference as you swap for propellers of different construction. The key here is construction; the difference in construction is not just a matter of material or inertia. Our real systems have many inertias connected by many stiffnesses, not all of which are shafts. One of them is blade root bending, which is assigned a shaft stiffness equivelent for analysis (same for belts, BTW). Go back and run the two-element equation again, substituting 5000 ft-lbs/radian for 10,000, the same half-as-much or twice-as-much as we used for inertia. You'll see that the frequency changes more than it does with a half-as-much inertia change. Stiffness is a bigger deal.

Let's compare a one piece wood prop to something like a GSC ground adjustable wood prop that happens to have the same inertia. The GSC has small round blade roots to allow pitch changes in the hub clamping, while the one-piece prop has fat, thick roots. Which will bend easier?

Now combine a change of stiffness and a change of inertia. A heavier prop will drive frequency down, but if it has a neatly compensating increase in root stiffness you may not see a change. A heavier prop with really stiff roots might push frequency up, perhaps the substitution of a carbon Warp Drive w/ the aluminum hub for that wood GSC. Substituting a one-piece metal prop with thick roots (a really, really stiff root) might push it up even more...or down. Depends on the balance of stiffness and inertia.

Pushing frequency up leads to the first factor in vibration perception, as well as an important point about actual vibratory amplitude. Remember what I said in the previous post about amplitude being roughly proportional to manifold pressure? The above calculations told us the system's natural frequency. Resonance happens when that natural frequency is matched by an exciting frequency. The primary exciting frequency is gas pressure oscillation, ie, firing events. You've seen lots of torque curves, so you know the force powering the exciting frequency is rising with RPM, sometimes sharply, in the lower end of the engine operating range. Moving the resonance point up the range as little as 200 RPM can increase the resonant amplitude enough to be noticable with weak human perception.

Weak human perception plays out in lower RPM resonance too. We perceive lower frequencies far better than higher frequencies. Might still be the same measured vibratory torque amplitude, but a lower RPM shake is more noticable.

Last, consider the natural frequency of airframe components. If, for example, the natural frequency of the horizontal stabilizer happens to be the same as the resonant vibration shaking the engine, your perception will be that the vibration is much worse. Heck, the whole airplane is shaking like a wet dog!

Human perception is a terrible measure for this work.

<<Why is Hartzell doing so much testing on relatively similar props on relatively similar engines if there is theoretically so little difference?>>

Well, don't confuse propeller life with drive life. Aluminum, unlike steel for example, has no "knee" in it's S-N (load vs cycles) curve. In the words of my engineering mentor, aluminum is frozen mush. If you stay well below the knee with a steel component, theory says it's fatigue life is more or less infinite. Aluminum has no knee; with enough cycles all aluminum components break, as the curve eventually reaches zero. In simple, Hartzell wants to make sure that all points on the blade have loads very low on the S-N curve so the thing will stand a lot of cycles before it turns to mush and ruins your day.

Like that horizontal stabilizer, a prop blade has a natural frequency or frequencies. An aluminum propeller's natural frequency is somewhat higher than the system's F1 frequency we've so far discussed, and by hand methods is a bit difficult to calculate. The primary reason is centrifugal force, which stiffens the blade with increased RPM. The second reason is that blades don't have uniform sections; it is difficult to accurately calculate bending. The higher exciting frequencies that might make it resonate stem from events other than just firing events; note that the conventional aircraft engines known to be easy on props have pendulum absorbers tuned to 5th, 6th, etc orders. I'm sure the propeller wizards have good computer tools for calculation these days, but they still find it sensible to pull live telemetry. A failed drive is bad enough; you find yourself flying a glider. Propeller failures can result in the loss of the whole engine, and the occupants wind up dead.

I've not studied propeller vibration very much, but I think bolting an aluminum propeller to an auto conversion with unknown torsional amplitudes is akin to playing Russian roulette. You put two bullets in the cylinder if you choose a used propeller; some of it's available S-N life has already been used up.

Did you ever buy that copy of DenHartog I recommended? I think it has a good section on propeller vibration.

<<I'm still planning to try measurements with an RF chip and accelerometers- have to get the whole thing done first.>>

Still think you would do better if you went straight to shaft telemetry, but so many guys have talked about using accelerometers that I'd like to see somebody actually try it. You're smart enough to do it. The problem will be sorting through all the other shakes to find the ones that stem from torsional events. You'll need some software.

Ross,
A follow up on your question.

It has been a while since I was into this stuff, so I dug around on my hard drive for an old tool, a simple DOS program for Holzer calculation written by Tom Irvine. Has no inputs for engine output so it won't tell you vibratory amplitude, but it will generate pretty accurate numbers for the natural frequency of as many elements as you wish. They can be used to go further, but I have no plans for that. What I wanted to confirm was the statement about changes in prop inertia making very little difference, but this time with a more realistic 4 element model and some real data from the old Suzuki drive.

All torsional systems have a natural frequency for every connecting stiffness, or put another way, inertias minus one. Thus a four element model generates 3. Generally they increase as you move further along the string away from the propeller. We already looked at F1 with the blunt two element model. The specific issue I wished to review was what happens to the higher frequencies with shifts in prop inertia and blade root stiffness. Could they be the source of your observations?

Answer is "no". Plugging in real inertia and stiffness data from the old bad Suzuki drive generated an F1 of 46 hertz, or a resonant RPM of 1840 with a 4-stroke I-3, exactly as you saw in the graph posted previously (which was generated with far more elaborate software). The F2 is 190 hertz or 7600 RPM, which is why you don't see it as another peak on that graph. F3 would be of concern only if we reached 16,680 RPM <g>

Ok, so triple the prop inertia and cut root stiffness in half. The answer is an F1 of 38 hertz and an F2 of 156 hertz. They would be resonant at 1520 and 6240 RPM with the Suzuki I3.

So, only the F1's are of great interest in this case. The others are out of the operating range, which is highly desirable. An additional 320 RPM (1840 vs 1520) would probably increase resonant vibratory shaft torque because, as previously stated, engine oscillating torque output (the forcing vibration) is higher. "How much" would need amplitude calculations or telemetry, but more is safe bet. Even so, it is entirely possible that an observer would claim the lower frequency combination to be worse, as it may well be reaching down into the range of a lot of airframe natural frequencies. Consider: a steel tube fuselage has a torsional stiffness and mass at each end. What do you have?

Anyway, an anchor is still an anchor. Changing propellers makes a difference but it ain't really a big deal in the strict context of shaft stress in the PSRU. Vibration of the prop blades themselves is another matter.

Without proper measurement equipment at this time, I can only make inferior human based observations. If your example is typical with F1 happening at relatively low rpms and low frequencies, this is what I typically see in my Sube installation and the Rotax on the test stand and apparently on the new Egg H6 packages. Perhaps coincidently, most of these have serious TV below 1200 rpm.

What we don't know in the Sube packages is where F2. F3, F4 etc. are as they are at higher frequencies that we can perceive without instrumentation. The Rotax has millions of hours of flight time so either they have done testing or there is no problem at typical operational rpms. My sources at the Rotax overhaul facility sees plenty of messed up drives and in almost every case, this has been attributed to excessive low rpm idling, contrary to Rotax directives. Engines which are idled over 1400 seem to have few gearbox problems.

I have very noticeable periods at 600-950 and 1100 to 1600 rpm so idle is set to 1000 and taxi above 1600. With the geared prop, this works out fine. In flight stuff is a total unknown at this point.

I'd be interested to hear from any others flying auto conversions with redrives and any noticeable rpm zones.

Ross,
<<Without proper measurement equipment at this time, I can only make inferior human based observations.>>

Of course.

<< If your example is typical with F1 happening at relatively low rpms and low frequencies,..>>

Very likely. Prop size, shaft size, and engine size tend to be relative, yes?

<<What we don't know in the Sube packages is where F2. F3, F4 etc. are..>>

Calculating all the frequencies isn't hard, IF you can take the time to gather accurate stiffness and inertia values.

<< have very noticeable periods at 600-950 and 1100 to 1600 rpm so idle is set to 1000 and taxi above 1600. >>

You mentioned that lower range issue in a previous thread and I didn't think much about at the time. Sorry. Ain't no mystery.

We spend most of our worry on natural frequencies that intersect with the firing frequency. The variation in crankshaft angular velocity due to firing events is by far the most powerful exciting frequency, but it is not the only one. Next on the list is the recip frequency, a variation in crank speed due to decelerating and accelerating pistons. Intersecting F1 with this exciting frequency will give you a resonant RPM just like intersecting the firing frequency, but it usually doesn't generate nearly as much amplitude.

With your 4-cyl Sube, firing events are a 2nd order frequency, meaning they happen twice every crank revolution. Recip is a 4th order frequency. I'll assume a 48 hertz F1 based on your "between 1100 and 1600" observation; that will be close.

The easy way to visualize all this is to plot a Campbell diagram. I've attached one below.


Shot at 2007-07-15

The thread started with reference to a BAP drive, so let's take a quick look at a Campbell diagram for the 6-cylinder.

A 6-cylinder has exciting frequency major orders of 3, 6, 9, and 12. Only 3 and 6 are of interest here. I'll assume an F1 of 48 hertz like the previous example.

As you can see, the system should be resonant at 480 and 960 RPM. You may be tempted to say "Eureka! That's why the BAP drive works!", since the 6th order intersection is pretty much below the operating range and the big, bad 3rd order is merely on the edge of it. There are a few details to keep in mind.

(1) Resonant ranges typically extend across a range from 0.8 to 1.2 times the critical speed, more or less. The peak isn't always in the exact middle.

(2) I've assumed a 48 hertz F1. We don't know the exact F1 of the real system. A little lower would be better, a little higher makes things worse.

(3) You would still have a starting transient, and an operator who sets idle speed too low would still have problems.

As Ross asked, is there anybody here you can report actual operating experience for this system with an eye toward the resonant range? The physics don't lie; it is there. Question is, where?

BTW, you might wonder why I posted this diagram when it seems in favor of the BAP drive. If you have that idea, you may also have a misconception about my goals. I'm not here to knock Jess's work. I'm here to understand, and perhaps spread a little understanding in the process.


Shot at 2007-07-16

Good point, I think we almost assume something bad may be happening in the flight ranges since we don't know without instrumentation. In many cases there may be nothing to worry about but it is not knowing that is scary.

I see that vendors may be reluctant to admit that they have not tested this aspect of their packages but seems to me it would be a selling point and costs would not be too bad and results reasonably applicable to the typical narrow range of generally composite prop types recommended. I was always under the impression that the prop would make a larger difference in range.

With 75 of Jess' packages flying and what must be thousands of hours on them, it would be interesting to get feedback on reliability to date.

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.

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>

Don't Stop!

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?

Yukon said:

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

Click to expand...

You took the words right out of my mouth.

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

Mike

Thanks, Dan

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

DanH said:

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.

Click to expand...

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.

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?

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.

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>

DanH said:

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>

Click to expand...

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!

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>

<<Believe me you don't want to subject yourself or the engine, drive/ prop/ airframe to this one very long.>>

My compliments sir. An honest man is hard to find.

The auto conversion world is covered up in tribal pride; the drive systems always work great, no matter what the reality.

That from a guy who likes auto conversions. I just think they can be a lot better, which isn't gonna happen until folks get educated and more demanding.

DanH said:

<<Believe me you don't want to subject yourself or the engine, drive/ prop/ airframe to this one very long.>>

My compliments sir. An honest man is hard to find.

The auto conversion world is covered up in tribal pride; the drive systems always work great, no matter what the reality.

That from a guy who likes auto conversions. I just think they can be a lot better, which isn't gonna happen until folks get educated and more demanding.

Click to expand...

Very interesting! I have been monitoring the alternative sites for over three years, and this is the first I have heard of a destructive resonance RPM. Can it be assumed that there might be a harmonic to this at higher rpms, maybe less severe, but damaging none the less?

Doesn't gear mesh create frictional drag and heating? Wouldn't torsional harmonics cause the teeth to load and unload, thereby causing even more tooth friction???

DanH said:

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?

Click to expand...

You're right, I am blending the two vibrations together, but I was under the impression torsional vibration can incite blade vibration and failure. No????

Powersport Testing

Dan,

I went back to the Powersport site where they have some actual test data and info on their drive. Over the operational range they have no significant issues even without a damper. Looks like they have just made everything ultra stiff:

http://www.powersportaviation.com/Home/Testing/Testing.htm

http://www.powersportaviation.com/Home/Reduction drive/Reduction drive.htm

Are you aware of a source for commercially available equipment like that used here?

Any other comments on this?

Torsional Data

Fasinating website! Lot's of testing data on torsionals......no fuel flow data!
Guess that's why they are defunct.

Interesting that Egg offers no torsional testing data. Wonder if any has been done? Gen 1, Gen 2, Gen 3.........maybe that's the data!

John,
<<I was under the impression torsional vibration can incite blade vibration and failure. No????>>

Yes. I spoke poorly. Let's try again.

"Torsional vibration" is present in all systems driven by recip or Wankel engines. Crankshaft rotation is not a smooth uniform rotation. The crank (or output shaft in the case of the rotary) speeds up and slows down under the influence of cylinder combustion, acceleration and decelleration of pistons, and a host of other influences. Frequency is a handy way to express the number of "speed up, slow down" cycles in a unit of time. Since time is the measure, frequency varies with RPM, and thus a recip engine outputs a whole range of frequencies.

Here's the important point: torsional vibration is normal and unavoidable. We can minimize it, but we can't fully eliminate it. Thus we gotta learn how to design driven devices so they won't be harmed by it.

Start by understanding that all objects have a "natural frequency". It doesn't necessarily mean the object is vibrating. A tuning fork, for example, only vibrates at one frequency, it's natural frequency, but until it is struck or otherwise excited, it just sits there quietly. When you strike it once, it vibrates until the vibration fades away....entirely at it's natural frequency.

Ok, imagine how hard the tuning fork might vibrate if you could somehow strike it repeatedly and very rapidly, the number of strikes per second being the same as the forks' natural frequency in hertz. The vibratory amplitude would get very large. That is "resonance". Resonance (a large increase in vibratory amplitude) happens anytime an object is excited at the same frequency as it's natural frequency. It is true of both torsional and linear vibration.

Back to our engine. The "strikes" are the variations in crankshaft angular velocity, which we call an exciting frequency. The prop blade is the tuning fork. The blade will vibrate in a resonant manner if the exciting frequency matches a natural frequency of the blade. Vibrate it hard enough for enough cycles and fatigue turns the aluminum back into mush. The nice folks from the prop company don't want that to happen within a reasonable lifespan, so they stick strain gauges all over test prop to check amplitude, frequency and stress. If your certified airplane has a "don't run here" placard, it is because they found an intersection of exciting frequency and blade frequency with an unacceptable amplitude. Material stress is a little too high on the S-N curve. You can run it at that RPM for a short total lifespan, or run it somewhere else a lot longer.

So far we're talking about blade resonance. The engine designer did his homework. He had a pretty good idea what range of possible prop inertias folks might bolt to his engine, and he tailored his engine inertias and stiffnesses so the natural frequencies of the shaft system were not matched by any of the exciting frequencies. Along comes an auto conversion guy without a clue, who merrily designs a shaft system with a critical intersection of natural and exciting frequency. In the resonant range, the shaft system oscillates with considerable vibratory torque. That torque is seen by the prop hub, which tries to whip the blades back and forth, and that puts a very high stress on the blades. The frequency of this huge vibratory torque may not be anywhere near a blade natural frequency, so the blade isn't resonating. It is suffering from plain old overstress.

So, yes, torsional vibration is the root cause of prop fatigue failure. All aluminum propellers will fail eventually, as ordinary, normal torsional vibration is always present and aluminum has a finite fatigue life. However, we can draw a distinction between vibrating the blade gently at low stress, vibrating the blade with a higher stress due to blade resonance, and simply beating the snot out of it with a resonant shaft system. This thread is about eliminating that bad beating, which is shared by everything in the system. That's why I was trying to set prop vibration aside, the "strict context" I mentioned.

Better?

Thanks Dan, facinating subject!

Ross,
<<Looks like they have just made everything ultra stiff:>>

Perfectly reasonable approach. Look at the F1 frequencies for the M14s. Stiffness drives F1, F2, etc upward. Doesn't matter if you push a critical up or down as long as you get it out of the operating range.

Re my previous note to Pierre; these are propeller vibratory modes.

The unsymmetrical blade modes are usually associated with propeller whirl (imagine the common child's toy, a spinning top, with a wobble). We're not going there. Suffice to say they happen sometimes when you have too much radial freeplay in the front propshaft bearing, or pick an engine with significant vibratory moments (block wobble) due to configuration. An inline 3-cylinder with no balance shaft is a good example. Radial engines are prone to both.

Symmetrical modes are the ones common to our experience. They are the result of torsional vibration matching a natural frequency of the blade, as discussed with John.

The shapes describe modes. The little circles are nodes. If I remember my theory correctly, more nodes (the more complicated modes) equals higher frequency.

....and as Forrest Gump said "That's all I have to say about that" <g>


Shot at 2007-07-18

Ross,
Took a look at the PowerSport info.

No I don?t know where to get an encoder or Hall effect sensors, but it can?t be too hard. I have no experience with them or the method, so the following is theory and thought, subject to correction.

Measuring twist by comparing shaft angle at two ends of the system is certainly a valid tool. However, every tool has limitations. A Cresent wrench can be used as a hammer, but that doesn?t make it a good hammer.

Best I can figure, the twist method has a limitation; by itself it is only useful for the first mode of torsional vibration. In the first mode all the inertias divide themselves into two groups, one on each side of the single node. The groups oscillate in opposition; when the initial half of the vibratory cycle rotates the first group clockwise, the second group rotates counterclockwise. The overall angular twist of the system can be measured quite nicely by comparing angles taken at each end.

However, the second mode is a bit more complicated. There are two nodes and three sets of inertias. The first set (perhaps the prop disk, a ?set? in this context can be one) will rotate clockwise, the second set (perhaps the flywheel and the first two crank throws) will rotate counterclockwise, but the third set (perhaps the 3rd and 4th crank throws and the accessory section) will rotate clockwise, same as the first. An overall twist measurement taken at the two ends may show little twist, yet a good bit of twist may actually be present in the system.

The actual forces are negative and positive torques, but "rotate" makes a good picture. Pierre, see why I was polishing the pins about nodes and modes? Knew it was gonna come up sooner or later.

Earlier I said ?by itself?. Clearly the Powersport designer did models. His goal was a stiff system, meaning an F1 way up the RPM scale. His reasonable expectation was if he couldn?t see an F1 resonance (first mode) with the encoder setup, higher modes were moot. The assumption wouldn?t be safe in a system with an F1 below the operating range. If the F2 was creeping down into the upper end of the range, encoders at the ends would not give you a true picture of it. Of course if you had done a lot of calculation you would already know the location of the F2 and the nature of the mode shapes, including angle of twist. Without calcs, I?m not so sure encoders at the ends are a good choice.

In your case (the 4-cyl Subaru RV-6) you already know the F1 resonant RPM.

Had a nice conversation with an Orenda development engineer at OSH some years ago. They did propshaft strain gauge telemetry plus encoders at every main bearing web.

The Powersport comment about ?our initial calculations were off by only 10%? illustrates my previous comment about how a good model may let you skip telemetry?at least in a private not-for-sale design. Don?t be put off by the ?masters thesis? comment. A fellow who is truly qualified to do the mechanical design can learn how to do a torsional model

I think you gotta use a grain of salt about ?the very large pulses of the rotary?. Just perspective. I saw some torque-vs-crank angle plots somewhere that showed rotary in comparison to recip. Recip torque ramped up sharply while rotary had a flatter curve. The real interest is the frequency of the exciting pulse. A rotary fires three times per rotor rev but the planets make that one firing event per rotor per output shaft rev, yes? If so, with a two-rotor engine you have a 2nd order gas pressure oscillation at the output shaft?same as your Subaru 4-cyl. Check me on these points; I?ve never worked with a rotary and you have. And Bill, do you have a thought here?

If you get all the F?s above the operating range, it doesn?t much matter what the power pulses look like, at least in the context of torsional resonance. Is it possible with a flat recip engine? Maybe, maybe not. Like the radial example, the rotary is short and stiff. Exactly how stiff they didn?t say, just that the mainshaft took ?5000 foot pounds of torque to achieve a torsional deflection of greater than 1 degree?. That?s around 280,000 ft-lbs/radian, stiff indeed. Now consider the shape of a typical multi-throw crankshaft. The books say crank stiffness can be as low as 50% compared to a simple shaft of the same length.

Last, note the comment about the propshaft being ?tuned in length and thickness to avoid torsional resonance within the..operating range. Yeah baby, that's how it?s done.

If these guys didn?t make it, I?m sad. Nice work.

We usually treat a 2 rotor Wankel the same as a 4 cylinder 4 stroke as far as EFI software goes.

I was thinking about how easy it would be to install Hall Effect sensors at the prop flange and flywheel and use a micro to split these into very small time intervals. We could then use 2 scope inputs to compare relative deflection. The question is then, relative to what as everything is twisting and relaxing? Do we care about what the crank is doing aft of the flywheel? Seems Orenda was interested in that data although possibly for crank design and life reasons. It seems we should have 3 Hall sensors- one on the aft end of the crank as a reference with hopefully minimal torsional movement ('cept for the pesky balancer), one at the flywheel and one at the prop flange.

Do they make RF strain gauges? This would seem to be the easiest way to get raw data. Alternately we've looked at accelerometers tied to an RF chip. There are some pretty small packages out there.

Your thoughts on the best way to instrument this?

Forgive me if my questions seem lame.

With regards to the Sube crank. Due to the short stroke, it has considerable crank pin overlap unlike a Lycoming crank and the nose to the flywheel flange is many times shorter as well. Should be relatively stiffer even though the shaft is smaller in diameter. I guess it would be best to do a test or calc there. Seems like most of my problems are probably in the pin and bushing "damper" in the drive as the rest of the drive is short and has a massive shaft.
The shaft on a Wankel is really stiff compared to a Lycoming or Subaru I'd say. Massive and short.

Maybe taking the rubber bushings out would be the first experiment to see where F1 moved to?

What have you seen regarding changing TV with higher flywheel MIs?

Ross,
<<I was thinking about how easy it would be to install Hall Effect sensors at the prop flange and flywheel ...>>

You have those bushings between the two, which should show a lot of movement at any critical intersection. Like you said, try it and see what you learn!

<<Do we care about what the crank is doing aft of the flywheel? >>

Probably not, given your description of the crank.

<<Seems Orenda was interested in that data although possibly for crank design and life reasons.>>

Yes. They were on a certification program.

<<Do they make RF strain gauges?>>

Not that I know about.

<<Your thoughts on the best way to instrument this?>>

Got bad news for you...you're surely a better instrument guy than me. I am electronics dumb, and the field is racing ahead.

<<Maybe taking the rubber bushings out would be the first experiment to see where F1 moved to?>>

It would move up the RPM range. How far is the question.

<<What have you seen regarding changing TV with higher flywheel MIs?>>

Flywheel mass does what you would expect...reduces the amplitude of the exciting vibration. Less forcing power, so lower resonant torque. It is what you do if you can't move a critical intersection out of the operating range.

Photometric Analysis?

Regarding torsional vibration measurements, couldn't one put witness marks on each end of the shaft in question, hook up the strobe (sync'ed to engine rpm of course), and run through the rpm range looking for max deflection?

I know that Smokey Yunic did a bunch of this sort of analysis on Chevy small blocks. He put windows everywhere he could, and was able to measure camshaft torsional flex, for example.

Just another idea.


Ted Johns
RV7 plans preview.

Ted Johns said:

Regarding torsional vibration measurements, couldn't one put witness marks on each end of the shaft in question, hook up the strobe (sync'ed to engine rpm of course), and run through the rpm range looking for max deflection?

I know that Smokey Yunic did a bunch of this sort of analysis on Chevy small blocks. He put windows everywhere he could, and was able to measure camshaft torsional flex, for example.

Just another idea.


Ted Johns
RV7 plans preview.

Click to expand...

Could work maybe. Interesting idea to ponder. I'd use my video camera to record though, not standing too close to my prop at full bore!

<<Regarding torsional vibration measurements, couldn't one put witness marks on each end of the shaft in question, hook up the strobe (sync'ed to engine rpm of course), and run through the rpm range looking for max deflection?>>

You would clearly observe only those events that had the same order. The primary exciting frequency for a 4-cyl 4-stroke is 2nd order, a firing event twice per crank rev. To observe resonant behavior due to this order's intersection with a system natural frequency, you would need to trigger the strobe twice per rev, three times for a 6 cyl. Perhaps you could use the ignition as a trigger. However, the practical application gets complicated.

You're interested in a vibration of the system, not just the crank. The system includes a gear reduction. Unlike an encoder, a strobe only shows you what it can freeze in a particular moment in time. With the prop flange turning at an entirely different rate from the crank, you would see an index mark on the flange only when the gearing brings the components to their "repeat position" (I sorta recall 18 turns on my old drive). Resolution would be poor. Maybe you could get around that with 18 reference marks.

You still have the 2nd mode problem if you measure across the whole system. We're really searching for a way to see 2nd mode; we already know the first mode critical RPM on Ross's drive. He can hear and feel it.

If you measure just across the propshaft, the visual resolution would be very small. Twist is minimal.

Last, strobe work is tough outside in sunlight. So, I did lots of it at night. Ever work in close to a live prop in the dark? Nope, can't use a video camera; frame rate.

No, it was not a dumb suggestion. I had to think about it, so I (for one) got something out of it.

Oversampling

DanH said:

<<Regarding torsional vibration measurements, couldn't one put witness marks on each end of the shaft in question, hook up the strobe (sync'ed to engine rpm of course), and run through the rpm range looking for max deflection?>>

You would clearly observe only those events that had the same order. The primary exciting frequency for a 4-cyl 4-stroke is 2nd order, a firing event twice per crank rev. To observe resonant behavior due to this order's intersection with a system natural frequency, you would need to trigger the strobe twice per rev, three times for a 6 cyl. Perhaps you could use the ignition as a trigger. However, the practical application gets complicated.

You're interested in a vibration of the system, not just the crank. The system includes a gear reduction. Unlike an encoder, a strobe only shows you what it can freeze in a particular moment in time. With the prop flange turning at an entirely different rate from the crank, you would see an index mark on the flange only when the gearing brings the components to their "repeat position" (I sorta recall 18 turns on my old drive). Resolution would be poor. Maybe you could get around that with 18 reference marks.

You still have the 2nd mode problem if you measure across the whole system. We're really searching for a way to see 2nd mode; we already know the first mode critical RPM on Ross's drive. He can hear and feel it.

If you measure just across the propshaft, the visual resolution would be very small. Twist is minimal.

Last, strobe work is tough outside in sunlight. So, I did lots of it at night. Ever work in close to a live prop in the dark? Nope, can't use a video camera; frame rate.

No, it was not a dumb suggestion. I had to think about it, so I (for one) got something out of it.

Click to expand...

Well, I said sync'ed, not running at the SAME frequency. Oversampling could certainly be applied. I didn't think about the PSRU application specifically, but in that case I suppose a prop shaft trigger would be useful.

If there was some sort of elastomeric damping involved, looking at that coupling in this manner could be very enlightening.

Without such a coupling, I suppose the prop shaft could be turned down, analogous to a ".505" sample, to provide more resolution. Yes, it would change the resonance picture, but in a fairly predictable way.

Working around a spinning prop in the dark? I suppose certain safety protocols would need to be followed.

Video camera? You would need a high speed video sampler, sync'ed to the strobe. They don't give those away, rats.

Ted Johns
RV7 preview plans

Oversampling, #2

Actually, the more I think about it, a strobe wouldn't be needed. Just a good high speed video capture, a bright light (the sun might do), and a series of (accurate) witness marks. The marks could even be on two separate shafts, but two cameras would be needed. Simply record a few hundred snapshots at the test rpm, and then note the relative motion. In the case of the PSRU, the video sample rate would need to be adjusted to be an integer multiple of the gear ratio, thus freezing ratio induced relative motion. The only way the torsional vibration escapes detection is if the video sampling rate is equal to the torsional vibration rate. Using multiple sample rates would avoid that unlikely issue.

This is almost the same scheme a digital sampling scope uses, btw. Tek calls it asychronous equivalent time sampling.

Ted Johns
RV7 plans preview

rv6ejguy said:

I'd use my video camera to record though, not standing too close to my prop at full bore!

Click to expand...

Chicken!

Allow a practical note, please. Two actually.

Consider the time, expense, work and error inherent in developing a new way of measuring a very dynamic system. It rapidly becomes the experiment, when we really wanted to experiment with the torsional system.

Now consider the time and effort of simply doing a math model. Ross would need a junk Subaru crank (to eliminate taking his engine apart) and a one-time disassembly of his gearbox. He hangs all the parts on a billar pendulum to get inertias. He measures all the shafts to estimate stiffness. He rigs something to measure coupler stiffness. Then he plugs it all into some software, or grabs a pencil and calculator. After a few trial calculations he has a very good idea what shaft or coupler stiffness he needs to move F1 below 1000 RPM, plus he knows the location of F2. Expense would be the cost of new seals for the gearbox. Throw in the $16.95 cover price for a copy of Mechanical Vibrations if wants to do it all by himself.

Still want to measure? Why not just do it the same way the whole torsional industry does it? You don't have to spend a ton of money, although toys are tempting. I have very good data for my old Suzuki drive. Used a Wheatstone bridge strain gauge from Measurements Group, and a borrowed (even then obsolete) analog radio from Wireless Data. The elaborate recording devices were an old 15Mhz BK o-scope that my buddy Bill fished out of the trash, plus a newer true RMS Fluke multimeter for an accurate amplitude number (no capture on the old BK). And a clipboard.

Here's the advantage; there were no unknowns to the measurement theory. The process came with user manuals from MG and WDC. Was it painless? Of course not. Had to glue on the strain gauge just so, and machine a little mounting tube to put the radio in the prop center. Antennas were a pain; had to tape the transmitter antennas to the leading edge of the prop and fiddle with some welding wire on a stand to cook up a high-Q circular dipole for the receiver. Sometimes the transmitter battery would vibrate and drive it's connectors crazy...needed more tape. Agravating, but there was no question about the method and proceedure. Not once did we ever have to wonder if the odd bit of data was a measurement error. Just lovely vibratory waveforms in real time, with real amplitude and real frequency, marching across the o-scope screen.

I learned a lot. If I did it again, I would do it the same way. I would want better equipment for convenience and speed, same as the pros. Digital radio and a recording o-scope would be nice. That would get me up to 1995 technology levels <g>

DanH said:

Allow a practical note, please. Two actually.

Snip...

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No argument, Dan. I used strain gauges back in the college years, on the aforementioned 505 samples. Cool little tools, pretty cheap too.

But stroboscopic analysis is old hat as well. Real easy to convert angular flex in a shaft to torque. The steel properties are pretty well known. If I were to do it it would boil down to resolution and cost between the two systems. To be fair, a high speed video capture system is probably more expensive than a strain guage and transmitter setup. Not sure where the resolution contest would end up without doing more work than I want to right now.

This being said, a simple standard video camera might be useful. If the digital equivalent to shutter speed is fast enough, it is very likely to be able to capture the flex maxima in an elastomeric damper, or between witness marks if enough video is examined frame by frame. What does shutter speed mean to a CCD?

However, unless one had a way to automatically process the video, the completely asynchronous standard video camera would be useless at finding resonance rpms. It would just be suited to looking at known rpm points.


Ted Johns
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