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Guy Buchanan
Joined: 16 Jul 2006
Posts: 1204
Location: Ramona, CA
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 Posted: Sun Dec 24, 2006 9:20 am Post subject: Prop Spacer for 912-S - Has anyone ever seen one like this |
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At 08:54 PM 12/23/2006, you wrote:
| Quote: | Has anyone ever seen a prop adapter / spacer like this ? Is this
very likely to fail and let the entire prop fly into the back of my
Kolb ? Would you guys trust this ? Any thought you may have would be
appreciated.
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It looks nicely made, with adequate radii. Certainly the engine-side
flange looks big enough! I would guess it's turned from billet but
Clem's right, the material is important. Jan's right too, that the
static thrust and torque loads are generally insignificant in the
design of drive components. Much more important are the loads
resulting from harmonic amplification of the engine's natural torque
pulses. Given the right combination of engine / spacer / prop it is
possible to generate enough resonance to be a problem. In the
certificated world there are some engine / prop combinations which
are forbidden for just this reason. Others have cautionary operating
regions where resonances occur that are harmful, but not catastrophically so.
The answer to your question is that the spacer can only be considered
"safe" if your exact engine / spacer / prop combination has been
tested, either on the bench, or in flight, for a given number of
hours. (The airframe might also contribute to the response, but these
engines are typically so softly mounted that you might be able to
discount the airframe. Certainly any test will want to use a similar
mounting system so the engine vibrates similarly to your aircraft.)
The spacer could then be considered "safe" up to, and only up to,
some fraction of that number of hours. If you can find no such
testing then, guess what, YOU'RE THE TEST PILOT! Neat, huh?
Looking at your spacer, and based on my experience in the world of
recreational engineering, (that's engineering FOR the recreational
industry, not engineering AS recreation,) a really good designer
would start with the 912 output shaft dimensions. If they were
really, really good they might do some hardness testing of that shaft
to estimate its hardness. They then could design an aluminum "shaft"
by similarity to give equivalent fatigue life. Unfortunately this
design would completely ignore any system resonance modification
induced by the spacer. And more unfortunately, the aluminum spacer
will act as a relatively low frequency spring with negligible
damping, capable of dramatically increasing the system resonance. I
suppose a really, really, really good engineer would mount the stock
engine / prop combo and run it festooned with accelerometers in an
attempt to determine the system harmonics. They could then install
the spacer and confirm the system response was NO WORSE across the
entire operating range of the engine. You could then feel confident
the spacer had not decreased the reliability of the system FOR THAT
ENGINE / SPACER / PROP COMBINATION. You would, of course, have to run
this test for every engine / prop combo.
Finally note that I have quite obviously neglected the increase
bending loads in the 912 output shaft caused by the increased prop
offset. This works to reduce the fatigue life of the output shaft and
bearings, particularly if the prop is not perfectly balanced. You
would definitely want to make sure your system was dynamically
balanced fairly often to mitigate this detriment.
I've given you all this information mostly because I like to hear
myself talk,  but also to give you some questions to ask the
spacer's designer. It's even odds you're operating as a test pilot.
What do you do then? First you get parachutes for you and your
passengers. (A BRS obviously counts.) Then you always fly high enough
to use them. (You'll be at risk during take-off and landing.) Then
you decide how long you want to do the test. To do that you note that
you're going to replace a big chunk of the Rotax output system plus
the spacer at some hours interval. (Let's say the output shaft,
bearings, and spacer.) The cost of that replacement will help you
determine how many hours you want to run between replacements. (Note
that you don't get to fly up to the number of hours you tested, but
only to a fraction of those hours to account for variations in
fatigue life. What's the fraction? I don't know off the top of my
head, but we could probably come up with a conservative value with
some investigation.)
Now I'd really better shut-up. Good luck!
Guy Buchanan
K-IV/1200 w/ 582 C-box & Warp 3 blade
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_________________
Guy Buchanan
Deceased K-IV 1200
A glider pilot too. |
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JetPilot
Joined: 10 Jan 2006
Posts: 1246
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| Posted: Tue Dec 26, 2006 7:31 am Post subject: Re: Prop Spacer for 912-S - Has anyone ever seen one like |
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Thanks for the response Guy,
The spacer is very close to the same size as the 912-S Gearbox output shaft, but the sapcer is aluminum, and the shaft is hardened steel, so im sure that the spacer is the weak point in the system.
What mkes it even worse, unlike a Kitfox, on my pusher Kolb the prop will fly into my wing if the spacer breaks, most likely rendering it uncontrollable as the aileron actuator tubes are on the trailing edge of the wing. If I'm really having a bad day, the rotating prop might even cut my tail boom in half which is a couple inches below 
The good news is that the Kiev prop is much lighter than the Warp Drive props are. Kiev Props are well within Rotax Specs for rotational mass on the output shaft. You seem to know a lot about this, I think the material is milled billet, but I am not sure. It was expensive.... Bottom line, given what you know about materials, would you fly this, or test fly it ? If it lets go, its much more than an engine out, It would surely create an opportunity to test my BRS !
JettPilot
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_________________
"NO FEAR" - If you have no fear you did not go as fast as you could have !!!
Kolb MK-III Xtra, 912-S |
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