Thinking about a cruise prop

Stingray Don

Stingray Don

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Stingray Don
I have the Air Plains 180 hp conversion in my 172n. Air Plains claims 130 knots TAS. Today, I was at 8,500 and trued out at 125 knots at 2700 rpm (briefly) and still had quite a bit of throttle left. Seems like I am leaving some speed on the table. I was still climbing over 500 / min at 8,000. Static RPM I think was 2400 last I checked. I would love to upgrade to a 182 but my bank account “cessnah”. Thoughts on repitching the prop?

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Stingray Don said:
I have the Air Plains 180 hp conversion in my 172n. Air Plains claims 130 knots TAS. Today, I was at 8,500 and trued out at 125 knots at 2700 rpm (briefly) and still had quite a bit of throttle left. Seems like I am leaving some speed on the table. I was still climbing over 500 / min at 8,000. Static RPM I think was 2400 last I checked. I would love to upgrade to a 182 but my bank account “cessnah”. Thoughts on repitching the prop?

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I think I remember this coming up a few years ago and this guy had some real good info. @Pilawt
 
My 172N (Air Plains 180 hp conversion with Power Flow) must have had more of a cruise prop (Sensenich 76 EM8514-0-60). Static rpm was more like 2300 and change, and full throttle in level flight at 8,500 was about 2650 rpm. TAS was in the higher 120s. The Cessna true airspeed thingy on the airspeed indicator would say 131 or so, but I don't think that was quite accurate.

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Do you have wheel pants installed? Personally I’ll take climb performance over 5 knots any day.
 
Pilawt said:
My 172N (Air Plains 180 hp conversion with Power Flow) must have had more of a cruise prop (Sensenich 76 EM8514-0-60). Static rpm was more like 2300 and change, and full throttle in level flight at 8,500 was about 2650 rpm. TAS was in the higher 120s. The Cessna true airspeed thingy on the airspeed indicator would say 131 or so, but I don't think that was quite accurate.

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What is your climb performance like with the cruise prop?
 
Pilawt said:
Initial climb 1000+ fpm at Vy. Cruise climb 100-105 KIAS, 500 fpm through 6,000' MSL; 85-90 KIAS and 400 fpm through 12,000'. (These are based on 2300 lb MGW; I never installed the flap limiter that would permit increasing MGW to 2550 lb.)
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Still very respectable climb rate. Doesn’t sound the cruise prop hurt your climb performance very much.
 
Just do what Air Planes did to get the cruise value. Load the plane at the aft CG limit with a low gross weigh and you too will have 130 knots.
 
I think if you don’t go out of grass strips / high altitude fields on a regular basis, then why not. I’ve got a cruise prop on the Velocity. My static is poor (2250 RPM) and ground run pathetic (2500 ft) but I operate out of a 5,000 ft runway. At altitude with 2700 RPM will yield 165 KTAS. I don’t run it that hard though and a typical setting of 2500 RPM will give me 158 KTAS.
 
Velocity173 said:
I think if you don’t go out of grass strips / high altitude fields on a regular basis, then why not. I’ve got a cruise prop on the Velocity. My static is poor (2250 RPM) and ground run pathetic (2500 ft) but I operate out of a 5,000 ft runway. At altitude with 2700 RPM will yield 165 KTAS. I don’t run it that hard though and a typical setting of 2500 RPM will give me 158 KTAS.
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This is the answer. 115 kts vs 130 kts is not even 4% difference; why bother. If you want cruise speeds higher, get a plane more optimized for it. I do over 180 kts out of a narrow deck 320 cone.
 
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Dig up the STC paperwork and see what was left undone during the conversion. I bet the original prop was used, the prop from the 160 HP engine. I have seen this before. STCs such as this specify prop repitching or a different prop to take the increased HP. The engine should just reach redline RPM at full throttle in level flight. That's how most airframe manufacturers determine the right prop pitch. If it goes past redline, it's not right.
 
Dan Thomas said:
Dig up the STC paperwork and see what was left undone during the conversion. I bet the original prop was used, the prop from the 160 HP engine. I have seen this before. STCs such as this specify prop repitching or a different prop to take the increased HP. The engine should just reach redline RPM at full throttle in level flight. That's how most airframe manufacturers determine the right prop pitch. If it goes past redline, it's not right.
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Prop was replaced with the conversion. I checked the static RPM today and it was 2350. The manual states limits of 2250-2450, so right where it should be. Can’t figure out why I can’t get close to full throttle in level flight.
 
Stingray Don said:
Prop was replaced with the conversion. I checked the static RPM today and it was 2350. The manual states limits of 2250-2450, so right where it should be. Can’t figure out why I can’t get close to full throttle in level flight.
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Static on mine was close to 2350 as well, but full throttle in level flight in smooth air was still a skosh below 2700 (digital tach). Have you checked the accuracy of the tachometer?
 
Pilawt said:
Static on mine was close to 2350 as well, but full throttle in level flight in smooth air was still a skosh below 2700 (digital tach). Have you checked the accuracy of the tachometer?
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That’s a good point. I’ll look into that during my upcoming annual.
 
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