Mass Moment of Inertia

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Mass Moment of Inertia

Postby Jack L » Thu Dec 04, 2014 8:17 am

For those interested this informative EAA video explains MMOI and how to test your propeller.

Jack

http://www.eaavideo.org/video.aspx?v=2010764613001
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Re: Mass Moment of Inertia

Postby FlyerChief » Thu Dec 04, 2014 11:16 am

Hi Jack,

Thanks for posting this video! I tested my 74/51 Prince Prop using the method described in the Rotax Service Information sheets and found the prop was just within Rotax guidelines at 5800 kgcmcm. The test considers prop mass (weight) and mass distribution so more mass towards the center will have less effect than mass towards the ends of the blades, but I can't even imagine what a larger prop might register on the test. I suspect a few of the guys may experience premature gearbox wear and failure issues in the future with the oversize props. Having done this test now, next time I will ask the manufacturer to supply MMOI figures before purchasing another propeller. For anyone who already has their prop, it is definitely worth the time to do this test even if it's only for a bit of peace of mind while flying.

Dan
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When everything seems to be going against you, remember that the airplane takes off against the wind, not with it. ~Henry Ford
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Re: Mass Moment of Inertia

Postby R Rinker » Fri Dec 05, 2014 6:28 pm

Not that we 'have' to understand it all, but a little explanation would be great. Maybe someone could explain it. It seems to me this test is strictly dealing with the centrifugal force from the weight distribution of the blades. Would have nothing to do with thrust forces. Since the centrifugal forces of each blade cancel each other out I'm puzzled why this is so critical if the prop is well balanced. My only guess is that they would say no prop is going to be 'perfect' and they would come up with an acceptable average of out of balance force to design into the gearbox? So the 6000 limit would reflect the max out of balance scenario? Or would it have something to do with the gyroscopic effect which is higher with the heavier blade weight distribution? Too bad there was no explanation at all. And yes, I'd love to see some data for these new long two blade props.
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Re: Mass Moment of Inertia

Postby kenryan » Fri Dec 05, 2014 6:51 pm

This doesn't really add much, but at one time I asked Catto what the Mass Moment of Inertia was on one of his props. He shrugged the question off with a statement to the effect "You won't have any problems with that with my props." I should have tried to get him to elaborate, but I just let it go. I would think that since Rotax publishes a spec that prop manufacturers would want to boast that their props comply, if in fact they do comply.
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Re: Mass Moment of Inertia

Postby Jack L » Fri Dec 05, 2014 7:17 pm

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Re: Mass Moment of Inertia

Postby Allan » Fri Dec 05, 2014 8:55 pm

So wants Troys catto come in at? 82x38 I think.
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Re: Mass Moment of Inertia

Postby FlyerChief » Sat Dec 06, 2014 1:08 am

That's the $64,000 question...
Because MMOI is dependent upon the location of the mass distributed throughout the prop, without a Masters degree in Mathematics or P Eng. degree, and knowing the exact profile of the prop, the only real way to know is to measure it using the method described in the Rotax document and shown in the video mentioned above.
Maybe at some point, Troy or someone else with a large prop will take the time to test it. Until then, it is reasonable to assume that it may be over-stressing the gearbox on the Rotax. I suspect the turboed version of the 912uls is making the MMOI effect even worse, but that is another story all together!
Dan
When everything seems to be going against you, remember that the airplane takes off against the wind, not with it. ~Henry Ford
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Re: Mass Moment of Inertia

Postby ScottieB » Sat Dec 06, 2014 10:51 am

The article from sportplanedesign was very informative. Thanks Jack.
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Re: Mass Moment of Inertia

Postby Allan » Sun Dec 07, 2014 7:26 am

As I have the same 912 turbo to fit to my plane I was also keen to know more about the MOI of Steves prince prop in particular. You raise some good points Dan.
Lonnie sent the following reply "Using the first propeller sent to Steve Henry a Carbon Fiber 82/38 @ 7.4 pounds the MOI is 3300. Well within limits, I think you will find our propeller will be the lightest of custom propeller manufactures."

Even with the stronger hp over the 914 I feel that that MOI is quite acceptable as even if it is magnified above the 3300. For my own piece of mind, even if it's a little more effort, I will be running the engine "on condition" early to keep track of what is happening.
Another thing to think about with a turbo is the Rotax say they are "not keen" on a fixed pitch due to the non linear power curve. From what I can understand of the way the prince prop works changing pitch under load depedpndant on forward speed, these props, particularly the carbon fibre, make them more suitable for the turbo. This is illustrated by Steve Henry (and John Levy) indicated he is getting similar performance to the catto on climb but are getting the same cruise performance as his kiev.
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Re: Mass Moment of Inertia

Postby FlyerChief » Sun Dec 07, 2014 9:30 am

Hi Allan,
Very interesting...
Although I have no reason not to trust Lonnie, I find it hard to believe that his 82" prop is only 7.4 lb., since my 74" is 9.7 lb. It really makes me wonder if the hub is a different thickness due to the increased pitch of my prop or if mine is overbuilt. I may have to give Lonnie a call on Monday to see what's up.
Dan
When everything seems to be going against you, remember that the airplane takes off against the wind, not with it. ~Henry Ford
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Re: Mass Moment of Inertia

Postby levyland » Sun Dec 07, 2014 2:27 pm

I emailed Lonnie and Steves first prop 82" was 3300
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Re: Mass Moment of Inertia

Postby FlyerChief » Sun Dec 07, 2014 3:25 pm

Hi Allan,
Again, I have trouble believing it could be 3300, when my 74" prop has a measured MMOI of 5800. MMOI measures the inertia of mass at a distance from the hub. Unless the 82" P-tip has a significantly different profile than the 74", I fail to see how moving more mass away from the hub can give a lower MMOI. We really need Steve to test his prop to be sure... IMHO... :-)
Regards,
Dan
When everything seems to be going against you, remember that the airplane takes off against the wind, not with it. ~Henry Ford
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Re: Mass Moment of Inertia

Postby Allan » Mon Dec 08, 2014 1:08 am

Hi Dan,
Looks like there maybe more clarity developing through all this. Very definitely there is a difference between carbon fibre versions and wood only and no doubt the designs are changing. I think the stol requirements are starting to push the props and designers which is a good thing from my point of view. I think this part of prop design hasn't had that much attention until, as now, aircraft with a relatively wide cruise stall range such as the Superstol from the very low end are beginning to make more impact.
I am watching to see how all this unfolds with everyone!
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Re: Mass Moment of Inertia

Postby Jakkals » Tue Dec 16, 2014 2:15 pm

The current thread on the MMOI specs for propellers is too important to let up on and I hope that it will not fizzle out before some conclusive information is available.
I am at the point where I have to decide on a propeller and I’m unexpectedly running up against a lot of confusing feedback. Because so many of us use Rotax engines with strict gearbox parameters the MMOI specifications of available props have become an important consideration.
My hope is to find a good balance (within reason) between short take-off and cruising and I, frankly, expected more technical data to be available on an item as important to overall performance as a propeller. The desired performance envelope is not the same for everybody and one would think that propeller manufacturers – who all claim to have access to the holy grail of propulsion – would be able to provide propellers generally matched to different expectations. Not everyone is a fanatic about performance at one end of the spectrum and not every SuperStol fan can afford to buy multiple props until a happy medium is found that will not cause avoidable high maintenance or gearbox failures.
Judging by recent posts it looks as if both Prince and Catto may conceivably be pushed to a point where they will have to publish MMOI data on their props. That they may not yet have been contacted regarding Rotax gearbox failures ignores the fact that these are early days and that nobody has had a chance to rack up high hours with big props on SuperStols yet – with the exception of Just Aircraft’s demo planes.
I don’t quite buy the claim that Just, Catto, Prince, and others do not follow the forum discussions and I am hoping that members will keep pushing constructively until full details about props become freely available.
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Re: Mass Moment of Inertia

Postby FlyerChief » Tue Dec 16, 2014 2:45 pm

Hi Guys,

As promised last week, I called Lonnie and spoke to him about my prop and the MMOI figures on other Prince Props. I was pleasantly surprised to find out that he does read this forum and is quite interested in producing props that meet Rotax specifications for MMOI. I asked about the difference in weight and MMOI on mine vs. Steve Henry's longer prop. Apparently, because each wood blank is unique, there can be a variation in the weight and therefore in the MMOI. Mine had a very tight grain,which produces a stronger, but heavier prop, so it is pushing the upper limit of the MMOI for the Rotax 912. Steve's prop on the other hand has a slimmer profile due to the lower pitch and was likely made from a wood blank with a looser grain (less dense) piece of maple, yielding a much lower MMOI. Another change Lonnie has made as this new series of STOL props is developing is that the hub thickness has gone from 3" (like mine) to 2 and 3/4" (like Steve's), which also shaves off some weight and reduces MMOI. With these changes, Lonnie believes all of his props for the Rotax can be made to fall within the required MMOI limits, although it would likely be good to let him know in advance when you order that it is a concern to allow him to pick the best blank for the job.

Dan
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