Best RPM?

[frankh]

I have a 7a and normally run 2400RPM in cruise to avoid the "below 2350 RPM at 22+MP" prop limitation.

I have never done any experiements but wondered if 2400 was the most efficient place to run the prop.

Up high you won't get 22"MP anyway so in theory I can run the prop slower.

Anybody with any performance data to support running the prop slower?

Thanks

Frank 7a

 

[dan]

I typically cruise between 2250 and 2300 (full throttle, ~50 LOP) on my counterweighted (no RPM restrictions) IO-360-A1B6. Just last week on the way home from OSH I was running side by side with another RV and was playing around with RPM versus cruise speed.

At 2400 RPM I was staying with him equally. After pulling the RPM back to 2200-2250, I started pulling away from him. This was up at 12,500' where my full throttle manifold pressure was relatively low.

NOTE: I have a Lightspeed Plasma II ignition. My theory is that as I bring the RPM back, the EI advances further, resulting in more power (and economy). I won't even pretend that I comprehend the prop efficiency effects (Hartzell 2-blade 72" 7666).

I don't believe this RPM effect comes into play as dramatically down low, since I'm typically running higher MP at full throttle. Up high, though, where full throttle MP drops away, the EI is happy to advance. I don't have a timing readout on my Plasma II (only the "Plus" model has that AFAIK), but I'm almost positive that ignition advance is what is accounting for the increased speed & economy.

Not sure if this helps you or not, but it's food for thought.

 

[Ironflight]

I usually level off and then bring the RPM back from where I have it in climb - I'm always amazed at how the airplane accelerates (seat of the pants feeling) when you drop the RPM by a couple hundred - it feels like you've suddenly untied it!

I cruise about 2350 as a baseline.

Paul

 

[frankh]

Interesting

Good point Dan,

Not sure if its the EI advance ( I can vary the max advance with the PMags so that would be interesting to test) or if its the prop being more effiecient 'cus its taking a bigger bite of air as the airpeed increase....In other words the relative angle of attack reduces with increasing airspeed if the pitch remains constant...I think thats right?

Some things to play with for sure!

Thanks guys

Frank