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Prop Spacer for 912-S - Has anyone ever seen one like this

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Guy Buchanan

Joined: 16 Jul 2006
Posts: 1204
Location: Ramona, CA

Posted: Tue Dec 26, 2006 9:46 am Post subject: Prop Spacer for 912-S - Has anyone ever seen one like this

At 07:31 AM 12/26/2006, you wrote:

 	  Quote: 	
		  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.


	


Really. The spacer looks so much larger in the picture. The strength 

and stiffness varies as the radius to the fourth power, so any 

increase is quite significant.



 	  Quote: 	
		  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  


	


I guess it's a good thing you've got the BRS.



 	  Quote: 	
		  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 !


	


No, I wouldn't "test" fly it. I'm just too much of a coward. I 

suppose that if I had to use it I would do the engineering I 

suggested, even finding a way to do the vib testing. I have a 

background in structural dynamics and structural analysis, enough to 

know I could never verify the integrity of your spacer without some 

vibration testing. Do not be put off, however, as such testing is 

possible today even at a "hobby" level. (I'll bet Lowell could do it, 

with a little help.) Remember that all you're trying to do is verify 

that your system response is no worse with the spacer. Thus you don't 

need accuracy, merely precision, which is a lot easier.



Our usual method when designing something like this, where we had a 

lot of unknowns, was to make the addition as system independent as 

possible. In the case of this spacer it would mean trying to make it 

dramatically stiffer and somewhat stronger than the output shaft. You 

could use a large diameter steel weldment with some clean-up 

machining, or if you wanted to double the price of your Kolb, use 

titanium. Even the aluminum piece could have been designed a lot 

stiffer and stronger by making it maximum diameter and hollow. It 

wouldn't be as easy to machine, nor as easy to install, though.



I wish I could offer you more. I really hate to question something, 

to degrade your flying enjoyment, to "piss in your Cheerios" as the 

Aussies like to say. Please contact the designer. Hopefully they will 

be able to set your mind to rest and keep you care-free in the air.

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.

JetPilot

Joined: 10 Jan 2006
Posts: 1246

Posted: Tue Dec 26, 2006 11:05 am Post subject: Re: Prop Spacer for 912-S - Has anyone ever seen one like

Hi Guy,



I need the information weather it is what I want to hear or not, im glad you are just telling it like it is.    If you look at the attached picture, you can barely see the output shaft of the Rotax, it is slightly thicker than my spacer at its smallest point...  Then you can see the spacer, which tapers down to a mere 2 inches   



That taper is what worries me,  if they were to have left the spacer diameter thicker, and cut indentations for the bolts, it would be a lot stronger...   I just cant see any way that 2 inches of aluminum will be anywhere near as strong as the 2.25 or so inches of the Steel shaft from the engine.



Mike

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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

Guy Buchanan

Joined: 16 Jul 2006
Posts: 1204
Location: Ramona, CA

Posted: Tue Dec 26, 2006 11:49 am Post subject: Prop Spacer for 912-S - Has anyone ever seen one like this

At 11:05 AM 12/26/2006, you wrote:

 	  Quote: 	
		  That taper is what worries me,  if they were to have left the spacer 

diameter thicker, and cut indentations for the bolts, it would be a 

lot stronger...   I just cant see any way that 2 inches of aluminum 

will be anywhere near as strong as the 2.25 or so inches of the 

Steel shaft from the engine.


	


Doing a little checking I found crank steel strength might run from 

80 - 130 ksi. The strongest aluminum you're spacer would be made from 

is 7075, which in T6 temper is about 83 ksi. 7075 is also susceptible 

to stress corrosion cracking. (I found a fatigue strength of 27 ksi 

for 7075-T6.)

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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