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  #1  
Old 07-21-2022, 03:07 AM
KayS KayS is offline
 
Join Date: Jul 2009
Location: lake constance
Posts: 469
Default Propeller Efficiency at different RPM

within the process of flight testing my RV7 for range/endurance, i was interested in the efficiency of the prop alone (independent from the engine).

i thought that the lower you set the RPM, the more increases the propeller efficiency. i asked that question to Hartzell directly to get an confirmation but learned that this is not entirely true. many of us use these CS props so maybe it's also interesting for you.

here's the response from Hartzell tech support, i hope they don't mind posting here:

It is generally true, but not specifically true. The airspeed, altitude, and power will influence what RPM setting is the most efficient. For example, assuming 8,000 ft ISA, and a power setting of 55% on a stock 180 HP Lycoming IO-360-M1B, if your airframe drag is such that you’ll go about 140 KTAS, then the prop is most efficient at ~2200 RPM. However, if your airframe drag is such that you can go ~160 KTAS on that power, the most efficient RPM for the prop is ~1950 RPM. More power tends to shift the peak efficiency RPM higher, and less power lower. More speed for the same power (less aircraft drag) also tends to shift the efficiency peak lower.

At fairly low RPMS, 1800-2200, the efficiency curve is often fairly flat, so even though the peak may technically be at some particular RPM, there may not be much advantage over a different RPM. Again, for example, at the same 160 KTAS and 55% power, by reducing RPM from 2700 to 2200 increases the operating efficiency by ~5%; the peak is at ~1950 RPM but is only ~0.3% more efficient than 2200 RPM. And the engine may run smoother at a particular RPM, or the cabin vibration and noise may be more pleasant at yet another RPM, and the governor may not have the authority, or the propeller the pitch range, to govern at very low RPMs.

To truly determine the most efficient setting analytically would require a good model of the airframe drag, the propeller performance, and the engine performance. Even then, models have assumptions in them. The best way to determine it for your airplane is through careful experimentation and testing.

While there aren’t specific limits for low RPM operation, any propeller operating restrictions specific to your propeller and engine configuration must be adhered to.


i think Hartzell's answer is very interesting. some food for thought.

what i understand here is that if you run the prop in the lower regime (below 2200), the differences in prop efficiency (within that range) is almost NIL. if you lower the RPM to reduce internal engine friction, pumping losses etc., this will basically not counter balanced by poorer prop efficiency as the engine itself should have an much greater effect.

Last edited by KayS : 07-21-2022 at 03:17 AM.
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  #2  
Old 07-21-2022, 01:01 PM
scsmith scsmith is offline
 
Join Date: Jan 2008
Location: Ashland, OR
Posts: 3,199
Default

AND...

Of that 5% improvement that they mention between 2700 and 2200, almost all of that occurs between 2700 and 2400. The difference between 2400 and 2200 might be 1% or less (speculating here based on experience), and then just an additional 0.3% from 2200 to 1950.

Even for those that do not have RPM restrictions, higher RPM is probably better for the prop from a torsional vibration standpoint.
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  #3  
Old 07-21-2022, 04:12 PM
hevansrv7a's Avatar
hevansrv7a hevansrv7a is offline
 
Join Date: Jul 2005
Location: Detroit, MI
Posts: 1,657
Default

Anyone who is interested in this can download Jack's books for free, now.
www.propellersexplained.com
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  #4  
Old 07-23-2022, 02:30 AM
Cumulo Cumulo is offline
 
Join Date: Apr 2015
Location: KHMT
Posts: 107
Default Javaprop

There is a solidly scientific prop design program that is free - Javaprop. I have used it for quite a while.(years) Its a good learning tool.

Its all metric though, so 70 inches is 1.778 meters, etc. It has a very good graphic interface. Primarily written for model aircraft, it works just as as well for full size props, but again, input needs metric numbers. Doesn't do mach.

http://www.mh-aerotools.de/airfoils/...pinstaller.msi

Interestingly, simulating a typical prop with an efficiency of 85%, then reducing blade drag to an impossible zero will only increase the efficiency to about 92%. So aerodynamic refinement may not have the potential improvements many would hoped for.

And finally, don't mess around with this stuff if you are in the middle of a build. Just follow the tracks of someone who has had good results already with a certain prop.
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  #5  
Old 07-23-2022, 04:48 AM
hevansrv7a's Avatar
hevansrv7a hevansrv7a is offline
 
Join Date: Jul 2005
Location: Detroit, MI
Posts: 1,657
Default Jack Norris's Computer Program.

Quote:
Originally Posted by hevansrv7a View Post
Anyone who is interested in this can download Jack's books for free, now.
www.propellersexplained.com
I should have mentioned that Jack's and Andy's prop design program which is mentioned in the book is still available. It was originally written in GW BASIC for a (now) primitive computer and, worse, a dot matrix line printer.

But don't despair because I converted it to be able to run using a page printer which means any printer you are likely to have.

It needs to run on a DOS emulator because it's an 8 bit program. I still have a copy of that around, somewhere.

Jack was keeping it to himself in order to control its use. But since his family has put the book online for free I am volunteering to provide it, with some help, to anyone who is interested. The help would include my short rant on why the program can only be used academically, not practically.

Just a note. Jack told me that the original Whirlwind props were designed using a re-write of the same program. Whirlwind (Mr. Rust, I think) did a full conversion to modern computers but that's just name dropping, right?

PM if you are interested.
__________________
H. Evan's RV-7A N17HH sold at 268+ hours
Bought N512RV (RV-12)
"
We can lift ourselves out of ignorance, we can find ourselves as creatures of excellence and intelligence and skill. We can be free! We can learn to fly!" -J.L. Seagull
Paid $25.00 "dues" 2021.

my website

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