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  #1  
Old 05-13-2007, 10:18 AM
Bob Axsom Bob Axsom is offline
 
Join Date: Mar 2005
Posts: 5,685
Default Best RPM TAS

This has been discussed before but yesterday after failing to land at Rebel's Bluff my wife Jeanine and I decided to use the flight to check steady state effect of RPM on aircraft speed in our RV-6A with an O-360-A1A, 72 inch constant speed Hartzell prop, wide open throttle, leaned to ~120 F degrees rich of peak, autopilot locked in track and altitude (4,100 pressure altitude). I prepared a datasheet with 10 RPM increments five samples and average columns. We selected five different RPMs to test. These are the numbers we came up with:

2450 = 172.6kts
2500 = 172.8kts
2600 = 177.4kts
2650 = 177.0kts
2700 = 176.6kts

How do your RPM vs speed numbers compare?

Bob Axsom (CHTs were a non-issue in the mid 300 F range)

Last edited by Bob Axsom : 05-13-2007 at 10:20 AM.
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  #2  
Old 05-13-2007, 12:10 PM
tin man tin man is offline
 
Join Date: Jan 2006
Location: northern california
Posts: 297
Default

Looks like your'e up against the brick wall there. I get close to a 7 mph gain per 100 rpm until I get up above 210 mph then it starts to all bunch up also.
Tom
RV3 2000+
170 hp
catto 2 blade
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  #3  
Old 05-13-2007, 12:19 PM
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flyeyes flyeyes is offline
 
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Default

Quote:
Originally Posted by Bob Axsom
2450 = 172.6kts
2500 = 172.8kts
2600 = 177.4kts
2650 = 177.0kts
2700 = 176.6kts

How do your RPM vs speed numbers compare?

Bob Axsom (CHTs were a non-issue in the mid 300 F range)
My numbers are here from last year sometime. They are easily repeatable, but I have a significantly different prop than the Hartzell (at least for the moment. I may wind up with a Hartzell, depending on what shakes out with the current mess)

I know that some props are more efficient at certain speeds, but your engine hp should go up fairly linearly with RPM in this range AFAIK.
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  #4  
Old 05-13-2007, 12:22 PM
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hevansrv7a hevansrv7a is offline
 
Join Date: Jul 2005
Location: Detroit, MI
Posts: 1,587
Default Horsepower a factor?

I admit I have a very imperfect grasp of the math, but if this is at a constant altitude (air density) and mixture then doesn't 100 more RPM at 2700 mean about 3.8% more horsepower? If so, then does this suggest that something is countering the tendency of increased HP to produce more TAS? My guess would have to be prop efficiency although I suppose the torque curve of the engine could also be at work. Just a thought.

Quote:
Originally Posted by Bob Axsom
This has been discussed before but yesterday after failing to land at Rebel's Bluff my wife Jeanine and I decided to use the flight to check steady state effect of RPM on aircraft speed in our RV-6A with an O-360-A1A, 72 inch constant speed Hartzell prop, wide open throttle, leaned to ~120 F degrees rich of peak, autopilot locked in track and altitude (4,100 pressure altitude). I prepared a datasheet with 10 RPM increments five samples and average columns. We selected five different RPMs to test. These are the numbers we came up with:

2450 = 172.6kts
2500 = 172.8kts
2600 = 177.4kts
2650 = 177.0kts
2700 = 176.6kts

How do your RPM vs speed numbers compare?

Bob Axsom (CHTs were a non-issue in the mid 300 F range)
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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
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  #5  
Old 05-13-2007, 03:45 PM
Bob Axsom Bob Axsom is offline
 
Join Date: Mar 2005
Posts: 5,685
Default Some thoughts of mine

You would think that the more rpm. the more horsepower, more thrust, more airplane speed in level flight. However, these data indicate there is a peak that is lower than 2700 rpm. I am really thinking that the diameter is too large for maximum cruise speed. As the rpm increasesd the tip speed closes in on the speed of sound/compressability and popular opinion (backed by solid data I'm sure but coming from me it is just hear-say) says the drag increases dramatically and a lot of horsepower is consumed fighting that load. Two ways to reduce prop tip speed are to slow it down or reduce the diameter. If the diameter is reduced the pitch will be increased to absorb the horsepower and the greater pitch depending should increase airplane speed up to the point where the prop blade angle of attack with respect to the prop disc is too great and/or the pitch change cannot absorb the horsepower (think of a feathered ping-pong paddle looking configuration). Has anyone cut down a 72" C/S Hartzell. If so what were the results? How did you handle the rebalancing? Any problems experienced? I have been told about Sullivan Propeller at Hayward repitching and tuning diameter (cutting it off). If they did this for anyone, were there any problems in operation or performance? Just feeling for now, may search out a semi-local prop shop and pursue this if response is positive. I feel like when I increase the RPM in the current configuration the increase in RPM is at least partially achieved by reducing the pitch. Not good.

Bob Axsom
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  #6  
Old 05-13-2007, 04:23 PM
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hevansrv7a hevansrv7a is offline
 
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Location: Detroit, MI
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Default Mach speed?

2700 72 578.3386
The above data is RPM, Diameter, Speed in MPH.
=(C5*PI())*60/12*C4/5280

Bob, is this getting into the area of higher drag you were talking about? My gut tells me it is the prop and not the engine, but I wonder about the Mach stuff being at issue.
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"
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" net of PayPal cost for 2015, 2016, 2017 and 2018 (December).
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  #7  
Old 05-13-2007, 04:25 PM
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Nuisance Nuisance is offline
 
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Bob, I have a spreadsheet for calculating tip speeds that shows you at .79 Mach at sea level for 2700 rpm and a 72" prop. I doubt that you are running into Mach problems.

I think you might run the test again, and watch your manifold pressure to make sure there is no drop-off with increasing rpm (this is a test of your induction system). It is hard to resolve .5 knots even on the same day. I often see larger variations between squares without changing anything.

Have fun, John
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  #8  
Old 05-13-2007, 04:55 PM
HROCKLEN HROCKLEN is offline
 
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Location: woodbridge ct.
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JUST A QUICK QUESTION.DID THE YOU INCLUDE FORWORD SPEED OF THE AIRCRAFT WHEN YOU CALCULATED THE TIP SPEEDS?
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  #9  
Old 05-13-2007, 04:57 PM
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hevansrv7a hevansrv7a is offline
 
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Location: Detroit, MI
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Default No i didn't. Hmm.

Quote:
Originally Posted by HROCKLEN
JUST A QUICK QUESTION.DID THE YOU INCLUDE FORWORD SPEED OF THE AIRCRAFT WHEN YOU CALCULATED THE TIP SPEEDS?
No. Interesting thought. I'd have to know it in advance, too. Complex geometry, too.
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H. Evan's RV-7A N17HH 240+ hours
"
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" net of PayPal cost for 2015, 2016, 2017 and 2018 (December).
This airplane is for sale: see website. my website

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  #10  
Old 05-13-2007, 05:11 PM
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Nuisance Nuisance is offline
 
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Location: Pagosa Springs, CO
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Quote:
Originally Posted by HROCKLEN
JUST A QUICK QUESTION.DID THE YOU INCLUDE FORWORD SPEED OF THE AIRCRAFT WHEN YOU CALCULATED THE TIP SPEEDS?
Yes, I did. It turns out to make less than 5% difference.

I used to have a spreadsheet that also took into account the variance of the speed of sound with altitude, but lost that one in a disk crash, and haven't recreated it. It doesn't change much in the lower levels.
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