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Cruise Power Settings

AviatorJ

Well Known Member
As I'm revising the 'POH' for my RV-10 I got to thinking about cruise power settings. My plane has an experimental fuel injected IO-540x rebuild with 9:1 compression coupled with a 3 blade composite Hartzell propeller. Looking at other builders 'POHs' some use say 2300 RPMs for cruise while others use 2350 and it got me thinking about how I learned in the first place.

For me it all started with different CFIs in different planes transition training. For the 182 I was instructed to 'square everything'... so 24" manifold I should be 2400, 23" 2300, 22" 2200... ect.

When I did training on an A36 it was all about managing CHTs and less about the specific power setting but in extended climbs he instructed me to use 2400 - 2500 RPMs, then once I level out pull it back to 2300 to cruise... he suggested if to be more of an economy cruise leave it at 2300 and just pull back power and as long as it stays in the 'green' it's fine... I think green went to maybe 15". So when you went that low even with full prop you weren't getting 2300.

When I did my RV-10 transition training on a friends plane the CFI didn't care what power/prop combination I was using as long as it was smooth and controlled. I kept it usually squared around 2300 - 2400 RPMs and 23 - 23" MAN... because that's what I learned previously.

I assume each instructor had their preferred method and that's what they passed down. Why do some builders prefer say 2300 over 2350. Is this a limitation on engine/prop settings? I did find a limitation in some Hartzell documentation that I'm not sure if it applies to my engine, prop combo (Emailed Hartzell) that states "Avoid continuous operation below 103,4 kPa pressure between 1950 and 2350 RPM".
 
In practice this is much simpler:

- WOT all the time. Cruise altitude is typically 8K' to 14K' depending on the winds and mountains. WOT will be something less than 24"

- Select RPM for the mission. Typically I run 2400 but if very high I might bump it up to keep the 170+ knot cruise speed. Slower would be slightly better range but shoot fire, who wants that?

- Always LOP. Typically 20-30 degrees or so to know I'm on the backside of the curve to get the reduced CHTs and fuel burn, but not so low to have power drop off faster than fuel flow. Typical cruise is 11.5gph low, 10gph up high. Take the time to balance the injectors and the engine will run smooth all the way to lean cutout.

Carl
 
+1

In practice this is much simpler:

- WOT all the time. Cruise altitude is typically 8K' to 14K' depending on the winds and mountains. WOT will be something less than 24"

- Select RPM for the mission. Typically I run 2400 but if very high I might bump it up to keep the 170+ knot cruise speed. Slower would be slightly better range but shoot fire, who wants that?

- Always LOP. Typically 20-30 degrees or so to know I'm on the backside of the curve to get the reduced CHTs and fuel burn, but not so low to have power drop off faster than fuel flow. Typical cruise is 11.5gph low, 10gph up high. Take the time to balance the injectors and the engine will run smooth all the way to lean cutout.

Carl

As long as the prop will allow the RPM, I think what Carl says is good to go. Some engines work better at certain RPM ranges, otherwise I would say it's hard to mess up at 20-30LOP...
 
Thanks guys. I heard from Hartzell and they've not done testing on engines with 9:1 compression. However if it was a typical 260 HP engine there are no restrictions.

I am still working fuel injector balancing, I seem to have a .5 spread at low levels. Not sure if I'll do LOP or when but I like that option available. I like the 'RPM for the mission' comment.

I have some flight data logs going between 5500 - 11500 at 2350, 2450, ramair open/closed. I'll look through those and see what kind of performance I was getting.
 
Cruise

540 lync in a Comanche, cruise anywhere from
2050-2150, leaned like a diesel, many times
Northway AK to port hill ID direct and vice
Versa 1200 nm.
 
Full throttle and mixture/prop set based on how much of a hurry I'm in. Typical default is peak EGT and 2250 RPM.
 
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