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Nathan Young wrote: Can you better explain this? My understanding is that a fixed pitch prop is typically a compromise in both takeoff pitch, and cruise pitch. Using the typical car driving analogy - a prop that is stuck in 3rd or 4th gear in a 5 speed transmission. So I would think a CS prop would net gains at both cruise and takeoff/climb. They usually do, for the reasons you stated. If you adjust the controls such that the engine is producing 75% power in level flight and the CS prop settles into a coarser pitch than the fixed pitch prop had, you'll see a faster cruise speed. If the fixed pitch prop was pitched to produce the best cruise speed, the CS prop won't do any better there, but will improve your ROC. George Patterson If a man gets into a fight 3,000 miles away from home, he *had* to have been looking for it. |
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Nathan Young wrote in message . ..
On 7 Oct 2004 13:02:37 -0700, (PaulH) wrote: At any altitude that permits you to develop 75% power with your current prop, a constant speed prop won't gain you an inch unless you want to fly at a higher power setting. Can you better explain this? My understanding is that a fixed pitch prop is typically a compromise in both takeoff pitch, and cruise pitch. Using the typical car driving analogy - a prop that is stuck in 3rd or 4th gear in a 5 speed transmission. So I would think a CS prop would net gains at both cruise and takeoff/climb. The fixed pitch prop IS a compromise, but if the prop is pitched to permit generation of 75% power, it doesn't matter what the angle of the prop is. The only thing the CS prop provides at cruise is a choice of RPM for the same power. I can cruise at 22" and 2400 RPM and get 155 mph in my Arrow or 25" and 2100 RPM. They both generate 65% and my speed is the same. If you want to go faster, you have to generate more power. with CS, I have a choice of increasing either or both MP and RPM. With fixed pitch, you can only push the throttle forward, and the fixed pitch will limit the maximum power you can generate. What the CS prop primarily gives is better climb and increased drag in descent if you need get down in a hurry. I've often thought a CS prop would be very beneficial in long descents. I often cruise @ 8-11k feet, and during descent, it is easy to redline the engine, so I have to remove some power, which decreases the airspeed. Yes, the CS prop will prevent the overspeed so you have one less item to watch in descent. And if you set max RPM and pull the throttle back, you can descend at 2000 fpm while staying out of the yellow airspeed arc. |
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With my Husky, I go fastest with the prop full forward. I am not sure
if this is true with all small GA aircraft with Constant speed prop. The Husky has essentially the same prop/engine config as the Lycoming 0-360 Mooneys. What the CS prop gives me is a smoother ride, better fuel economy and another knob to turn. Nathan Young wrote in message . .. On 7 Oct 2004 13:02:37 -0700, (PaulH) wrote: At any altitude that permits you to develop 75% power with your current prop, a constant speed prop won't gain you an inch unless you want to fly at a higher power setting. Can you better explain this? My understanding is that a fixed pitch prop is typically a compromise in both takeoff pitch, and cruise pitch. Using the typical car driving analogy - a prop that is stuck in 3rd or 4th gear in a 5 speed transmission. So I would think a CS prop would net gains at both cruise and takeoff/climb. What the CS prop primarily gives is better climb and increased drag in descent if you need get down in a hurry. I've often thought a CS prop would be very beneficial in long descents. I often cruise @ 8-11k feet, and during descent, it is easy to redline the engine, so I have to remove some power, which decreases the airspeed. |
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G.R. Patterson III wrote: Nathan Young wrote: Can you better explain this? My understanding is that a fixed pitch prop is typically a compromise in both takeoff pitch, and cruise pitch. Using the typical car driving analogy - a prop that is stuck in 3rd or 4th gear in a 5 speed transmission. So I would think a CS prop would net gains at both cruise and takeoff/climb. They usually do, for the reasons you stated. If you adjust the controls such that the engine is producing 75% power in level flight and the CS prop settles into a coarser pitch than the fixed pitch prop had, you'll see a faster cruise speed. If the fixed pitch prop was pitched to produce the best cruise speed, the CS prop won't do any better there, but will improve your ROC. In the performance charts for my 182 a given percent power always produces the same speed at a given altitude. |
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Nathan Young wrote: Great description - so to summarize - in cruise, the CS prop would only benefit over a fixed pitch in cases where the fixed pitch prop is redlined before running out of throttle. Yes, but also the other way around too. Climb real high with a fixed pitch so even when you're at full throttle you can only get say 2100 rpm in your 172. In my 182 I can always get the full max continuous 2450 rpm, regardless of what the manifold pressure may be. |
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Doug wrote: With my Husky, I go fastest with the prop full forward. I am not sure if this is true with all small GA aircraft with Constant speed prop. Of course it is. Set whatever rpm you want. Add in some more rpm and you will go faster. The Husky has essentially the same prop/engine config as the Lycoming 0-360 Mooneys. What the CS prop gives me is a smoother ride, better fuel economy and another knob to turn. Smoother ride? Never heard that one before. |
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