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#1
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Prop Pitch Question
Hi All
I fly a Pazmany Pl-2 (see link below for pictures) that had a power upgrade from an 125 h.p. O-290 to a 160 h.p. O-320. I fly behind a McCauley Metal prop. After the engine change, I had the prop repitched from a 7063 to a 7067. Top speed increased and my initial climb rate is about 1400 f.p.m. I recently changed the plugs and magneto leads and saw another 100 r.p.m increase in straight and level flight to 2900 r.p.m. I'd like to increase the pitch of the prop some more. A basic rule of thumb is a one inch increase in prop pitch will reduce the r.p.m. by about 80. I figure a 3 inch pitch change should drop the r.p.m to around the 2700 mark. A fellow in the next hangar flies a Thorpe T-18 with a severely pitched wooden prop. He told me that he can only achieve a top r.p.m of about 2450 r.p.m. behind this prop. My question is this - Is this common to pitch a prop so that full r.p.m. (2700) can't be achieved? His plane has shorter wings and is much sleeker than mine (same engine), and about 30 m.p.h. faster in top speed. What kind of top r.p.m. are all those RV-6/7/8/9 flyers achieving with their big pitched props? Any advice would be helpful. I'm sitting on the fence. Go with a minimum pitch increase to bring the r.p.m. to around 2700 or go with a bigger pitch increase for a real airspeed increase. Blue Skies Eugene http://home.cogeco.ca/~pl2/paz3.html |
#2
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Eugene if it was me,I would just go for the 2700 or so in the summer
air,in level flight,wide open trottle.You will have a climb/cruise prop then.Jim |
#3
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On Sat, 24 Apr 2004 22:21:56 -0400, Eugene Wendland wrote:
Hi All I fly a Pazmany Pl-2 (see link below for pictures) that had a power upgrade from an 125 h.p. O-290 to a 160 h.p. O-320. I fly behind a McCauley Metal prop. After the engine change, I had the prop repitched from a 7063 to a 7067. Top speed increased and my initial climb rate is about 1400 f.p.m. I recently changed the plugs and magneto leads and saw another 100 r.p.m increase in straight and level flight to 2900 r.p.m. I'd like to increase the pitch of the prop some more. A basic rule of thumb is a one inch increase in prop pitch will reduce the r.p.m. by about 80. I figure a 3 inch pitch change should drop the r.p.m to around the 2700 mark. A fellow in the next hangar flies a Thorpe T-18 with a severely pitched wooden prop. He told me that he can only achieve a top r.p.m of about 2450 r.p.m. behind this prop. My question is this - Is this common to pitch a prop so that full r.p.m. (2700) can't be achieved? His plane has shorter wings and is much sleeker than mine (same engine), and about 30 m.p.h. faster in top speed. What kind of top r.p.m. are all those RV-6/7/8/9 flyers achieving with their big pitched props? Any advice would be helpful. I'm sitting on the fence. Go with a minimum pitch increase to bring the r.p.m. to around 2700 or go with a bigger pitch increase for a real airspeed increase. Blue Skies Eugene http://home.cogeco.ca/~pl2/paz3.html If you want the absolute fastest speed, without exceeding Lycoming's redline, then repitch the prop to give 2700 rpm at full throttle at whatever altitude you care about. Increasing the pitch more than that will decrease the rpm, and thus decrease the power produced, so it should result in slower speeds. But, if you aren't prepared to run at 2700 rpm, and you would be pulling the throttle back to reduce to some lower value (lets use 2500 for the sake of argument), then it might be worthwhile increasing the pitch even more so you can run full throttle at 2500. This would give you more speed than if you have it pitched for full throttle at 2700, and then pull the throttle back to run at 2500 with part throttle. But this would likely have a big effect on take-off and climb performance, so you need to pick your poison. As far as I can tell from discussion, most RVers pick a pitch that lets them run around 2700 at full throttle at their typical cruise altitude. But they then pull the throttle back a bit when they cruise. -- Kevin Horton RV-8 (finishing kit) Ottawa, Canada http://go.phpwebhosting.com/~khorton/rv8/ e-mail: khorton02(_at_)rogers(_dot_)com |
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