lessdragprod(at)aol.com
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 Posted: Sun Mar 08, 2009 11:41 am Post subject: Vibration (was: New Hartzell Available) |
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What HP range is needed to effectively drive a 4 blade MT? That's only half of the question. Altitude is the other half.
15,000' and 260 sea level Hp should be the cross over point. (That is, were the MT Rocket 4 blade propeller becomes more efficient than the MT Rocket 3 blade propeller.)
A F-1 Rocket flew my 4 blade propeller while his 3 blade propeller was being overhauled from a propeller strike.
He only had flight test data at 4,000'. My 4 blade propeller was 2 knots slower than his 3 blade propeller.
However, at his normal cruise altitude of 9,500' and 10.500', he felt the 4 blade was faster than his 3 blade. Unfortunately, he didn't take any flight test data for a comparison. Just some cross country flights that took less time and less fuel.
Apparently, the cross over altitude for the MT Rocket 4 blade propeller is below 9,500'.
BTW, the customers 3 blade MT Rocket propeller lost 3 inches off of the end of each blade, and was rebuilt back to the original length by MT Propellers certified overhaul process. 15% of the blade length can be damaged and the propeller blades can still be overhauled back to their original length. After a ground strike, the propeller hub is inspected before it is returned to service. I normally make this an overhaul, rather than just an inspection, to start the 72 month calendar clock again.
I have done this three times in 2008, so it happens.
John Harmon has flown a 2 blade, 3 blade and 4 blade propeller on his HR3, The 4 blade was the smoothest.
My decision to use the 4 blade on my HR2 was based on two factors.
My normal cruise altitude.
The "quality of flight".
The original O-290-G engine in my RV-3 had a vibration problem I never could eliminate. After 165 hours, the second bearing failed from lack of oil. None of the crankshaft main bearings oil hole line up with the oil hole in the case. The rear two were a quarter hole off, the front bearing was a half hole off, and the second bearing had melted aluminum coming through the bearing oil hole.
With a new crankcase and balanced internal assemblies, the engine ran so smooth the aircraft seemed to be sliding through the air. A real significant change in the "quality of flight".
Jim Ayers
Lycoming has the peak torque at 2200 RPM.
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