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rickofudall
Joined: 19 Sep 2009
Posts: 1392
Location: Udall, KS, USA
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 Posted: Wed Dec 30, 2009 4:58 am Post subject: Propeller inertia |
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Once again the subject of propeller inertia has reared it's head. Has anyone EVER found a propeller that falls outside the range of the "C" and "E" 2 stroke gearboxes and the 912 engine (6000 kg(cm squared)) by performing the test in SI 11ul91 E? My experience is limited by the props I own which includes a 3 blade Warp Drive and a 4 blade Power Fin as well as several 2 blade wood props from TPI and Culver. All of them passed, although despite popular mythology my 2 blade 68" wood Culver tested almost as high as the 66" 3 blade (straight blades, no nickle covers) with the HP-L hub. I don't expect that there are NO propellers that exceed the limit, I've just never seen anyone, except me, (look in the Kolb forum archives from early 2007) post that they actually tested a prop and the result.
Rick Girard
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lucien
Joined: 03 Jun 2007
Posts: 721
Location: santa fe, NM
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| Posted: Wed Dec 30, 2009 7:09 am Post subject: Re: Propeller inertia |
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| rickofudall wrote: | Once again the subject of propeller inertia has reared it's head. Has anyone EVER found a propeller that falls outside the range of the "C" and "E" 2 stroke gearboxes and the 912 engine (6000 kg(cm squared)) by performing the test in SI 11ul91 E? My experience is limited by the props I own which includes a 3 blade Warp Drive and a 4 blade Power Fin as well as several 2 blade wood props from TPI and Culver. All of them passed, although despite popular mythology my 2 blade 68" wood Culver tested almost as high as the 66" 3 blade (straight blades, no nickle covers) with the HP-L hub. I don't expect that there are NO propellers that exceed the limit, I've just never seen anyone, except me, (look in the Kolb forum archives from early 2007) post that they actually tested a prop and the result.
Rick Girard
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The IVO medium in 3-blades is a popular choice on the 912 and I wouldn't be surprised if it exceeded 6000 kg/cm3. When I took mine off my titan it was like pulling a bowling ball off the back of the plane. In fact, the weight reduction from going to my powerfin was so much I had to redo the W&B (9.2 lbs lighter overall) and the plane flies noticeably differently (CG moved forward nearly an inch).
Didn't measure the MOI, tho my 912 was instantly happier at all rpm's and power levels when it was replaced with the powerfin. The reduction in rotating mass was easily felt with a poor man's MOI measurement (rotating props back and forth by hand).
I had a WD 68" 3 blade on the plane for a while too, but it had to go because of a harmonic vibration problem I couldn't get rid of. Its rotating mass was also significantly less than the IVO medium (and yes contrary to myth, the WD is within the 6000 kg/cm3 MOI limit).
Also keep in mind that extensions worsen the situation with any prop, as they add arm to the Coriolis forces exerted on the prop shaft. Rotax limits the permissible length of extension for that reason (a little over 4" IIRC).
So you want as light as possible especially with an extension.
LS
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