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Bob Axsom

Well Known Member
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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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
 
Bob Axsom said:
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)
Click to expand...
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.
 
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.

Bob Axsom said:
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)
Click to expand...
 
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
 
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.
 
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
 
JUST A QUICK QUESTION.DID THE YOU INCLUDE FORWORD SPEED OF THE AIRCRAFT WHEN YOU CALCULATED THE TIP SPEEDS?
 
No i didn't. Hmm.

HROCKLEN said:
JUST A QUICK QUESTION.DID THE YOU INCLUDE FORWORD SPEED OF THE AIRCRAFT WHEN YOU CALCULATED THE TIP SPEEDS?
Click to expand...
No. Interesting thought. I'd have to know it in advance, too. Complex geometry, too.
 
HROCKLEN said:
JUST A QUICK QUESTION.DID THE YOU INCLUDE FORWORD SPEED OF THE AIRCRAFT WHEN YOU CALCULATED THE TIP SPEEDS?
Click to expand...
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.
 
The question remains..

hevansrv7a said:
No. Interesting thought. I'd have to know it in advance, too. Complex geometry, too.
Click to expand...
I went back and assumed 200 mph. Then I used forward speed as one leg and rotational speed as the other and computed the hypotenuse. That gave me 581 mph. I guess it's not a big factor if my design is right.
 
For sure - more testing will be done

More testing will be done especially in the 2550 to 2700 rpm range. Digital electronic tach resolves to 10 rpm but even dialed in prop is a little hard to keep in on the same RPM for a long period without a single change. Still this test was run is one continuous steady state flight with the only change being the prop control and the important observation here is relative not absolute speed.

Bob Axsom
 
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