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  #51  
Old 01-24-2022, 07:41 AM
PhatRV PhatRV is offline
 
Join Date: Mar 2018
Location: Buena Park, California
Posts: 784
Default Why Oversquared for Speed

Quote:
Originally Posted by OKAV8r View Post
He has to run 25/2500 to stay with me, unless I throttle back....
.
This is a question because I haven't flown an airplane with a CS prop before. If you want high speed, would it make sense to increase the prop RPM? I thought oversquared is to get the best combination of fuel economy and speed.
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Days to Final Installation: 999
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  #52  
Old 01-24-2022, 09:49 AM
theduff theduff is offline
 
Join Date: Jan 2012
Location: L-18
Posts: 372
Default 200 hp RV-4 w/ wood fixed pitch prop

The climb vs speed advantage of a constant speed prop has always intrigued me and the info in this thread is interesting. I would add an interesting counterpoint:
I did a side by side take off and climb with a buddy’s constant speed RV-8. Keep in mind his is a stock 180 hp 360 and mine is pumped up to 200 hp 360. So in acceleration and climb who won ?
It wasn’t even close the RV-4 easily out climbed and out accelerated the 8. At mid altitudes 3,000 - 5,000 ft ( all we flew together at ) the 4 was noticeably faster. Now when we matched up for side by side cruise aprox 180 mph the 8 was burning 1 gph less fuel. I found the side by side comparison interesting and a lot of fun.
I’ve been told over and over again that my 4 needs a CS prop to reach its full potential. I’m not buying it. Admittedly the high compression pistons and port-flowing tipped the equation in my favor but they are a lot cheaper than a CS prop and governor etc. Typically solo my 4 will cruise climb at 170 mph at 1000 ft per minute up to 8000 ft. Not really sure what I’d gain with the CS prop other than the obvious fuel savings.
Just presented as a counter point.
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  #53  
Old 01-24-2022, 07:04 PM
Scott Hersha Scott Hersha is offline
 
Join Date: Jan 2005
Location: Cincinnati, OH
Posts: 2,206
Default

It’s difficult to compare apples to apples with our airplanes because they are all a little different in many ways. Some are a little lighter, some are rigged a little better, sometimes the pilot makes a difference. Thankfully we all have choices and the correct choice for you is what you want. I’ve been down this road before - I’m building my 6th (and last) RV, and my priorities have changed. The RV airplanes I’ve owned, or built over the last 22 years have all been a little different, but they all flew great. I’ve had 3 with constant speed props, and three (including my current “new” RV6) with fixed pitch props. This last one, and also a previous RV4, has ground adjustable fixed pitch props, which for me is a good compromise. I’m pretty sure that this new one will be able to keep up with, or exceed, the top speed of anyone I fly with including those with constant speed props, like all of the others I’ve built, or bought and modified. The CS birds will likely take off a little shorter, and out climb me initially, but in a normal cruise climb - 125 to 135 KIAS - I’ll walk away from them. If we are flying the same airspeed that isn’t wide open, they will most likely burn a little less fuel. But not much. I’m no spring chicken. I don’t think I could live long enough to pay for the considerably more expensive CS prop installation with fuel savings, all things considered. You can’t say that a constant speed prop is better, or that you won’t realized the full potential of your sporty RV with a fixed pitch prop, because we all have different priorities, and different airplanes. When I go some place with my RV buddies, and we’re not in formation (unlikely), we will all get to our destination within about 2 minutes of each other (me first probably), and I’ll probably burn a gallon or two more than the constant speed guys - big deal. My fixed pitch birds have all flown better because they are light, and that’s why I fly. I like to go fast, and I like to have fun along the way. We can all do that, no matter what our configuration is in an RV.
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RV6/2001 built/sold 2005
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Last edited by Scott Hersha : 01-24-2022 at 07:06 PM.
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  #54  
Old 01-24-2022, 11:27 PM
Taltruda Taltruda is offline
 
Join Date: Oct 2017
Location: Las Vegas, NV
Posts: 1,140
Default

Quote:
Originally Posted by PhatRV View Post
This is a question because I haven't flown an airplane with a CS prop before. If you want high speed, would it make sense to increase the prop RPM? I thought oversquared is to get the best combination of fuel economy and speed.
Typically, max RPM produces most power at a given throttle setting, but I’ve found some props have a sweet spot, while others have a pretty broad sweet spot. For example, a MT prop RV-7 I delivered didn’t gain or lose any speed from 2100 rpm to 2700 rpm, however at the lower rpms, the engine could be leaned more and got more MPGs at lower rpms. Other CS planes tend to lose a few knots at the lower rpms.

As for “Over Square”, that is when you have less rpm than manifold pressure, like 24 inches MAP and 2300 rpm. Typically you’ll lose some speed compared to 24”, and 2400 rpm, but you’ll probably be able to lean further at the lower rpm.
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  #55  
Old 01-26-2022, 06:40 PM
Majorpayne317641 Majorpayne317641 is offline
 
Join Date: Sep 2021
Location: Goldsboro, NC
Posts: 59
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I do a lot of cross country flying, generally I am at 157TAS for most altitudes and can sometimes get 163TAS or so at 8k to 12k. I have a catto 3 blade. I run 75% power regardless of RPM, it can be as high as 2650 at the higher DAs.
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