IO-360 prop cruise speeds

[NotarPilot]

What do you guys find the best RPM for speed and fuel efficiency at altitudes of 6500 to 8500’?

Whats the lowest RPM you’ll use in cruise at full throttle at those altitudes? I’m usually around 2230 to 2250 RPM and wondering if I should or can go lower to save a little bit of gas.

[PT20J]

Higher rpm = more power

Lower rpm = better efficiency, to a point. At higher rpm there is more tip loss and at lower rpm the angle of attack is not optimum. So, there is an optimum point. But, the whole point if a constant speed prop is to make that optimum broad, so we’re not talking about a big difference. 

Bob Kromer once said that Mooney optimized the M20J prop for 2500 rpm.

David Rogers analyzed Bonanza props and came up with a rule of thumb that the optimum rpm is in the range of 14 to 15 times KTAS. 
https://www.nar-associates.com/technical-flying/propeller/cruise_propeller_efficiency_screen.pdf

Skip

[A64Pilot]

Try it yourself, but I don’t think it’s going to make much difference, IF you maintain speed as the constant.

Personally I don’t look at RPM to some extent and run what is smoothest, 2500 and above is loud to me and just busy feeling. I seem to gravitate towards 2300 RPM, but airplanes will differ somewhat. Some of it depends on what RPM the prop was balanced at, tiny bit. I’ll buy the logic if you have to pick one RPM 2500 is it, a little off the tops, but high enough for power.

Tip loss isn’t significant in our props in my opinion as our props are so short, if we have a 74” prop tip speed is only .77 Mach, and that’s low.  For reference the Ag plane I built had a 106” prop at 2200 and it’s mach number was .90

https://www.warpdriveprops.com/propspd2.html is a prop tip speed calculator and I think is accurate but I have not validated it.

At 7500’ altitude you should only roughly be able to get 22.5” MP or so, so 2300 is “square” and most accept square as fine.

Of course assuming WOT  the lower the RPM, the lower the fuel flow and speed, so I think it’s more of what do you consider more important speed or fuel burn?

There is some increased wear with RPM, but your flying for less time so they may wash out, another who knows or it’s just not significant?

 

 

[Hank]

2 hours ago, A64Pilot said:

Of course assuming WOT  the lower the RPM, the lower the fuel flow and speed, so I think it’s more of what do you consider more important speed or fuel burn?

Personally. I care about time and fuel used for a trip. There's a balance point where the additional fuel savings per hour use more total fuel fuel due to longer flight time. Try to not do that!

[201er]

5 minutes ago, Hank said:

Personally. I care about time and fuel used for a trip. There's a balance point where the additional fuel savings per hour use more total fuel fuel due to longer flight time. Try to not do that!

What, 90 knots? Wouldn't even know how to get it down that slow with gear and flaps up.

But yeah, there's little to be gained in terms of fuel savings going below Carson's speed.

[A64Pilot]

I’ve found 120 to 130 kts fuel over distance to be close and even 140 isn’t bad, above 140 the curve steepens.

135 kts roughly returns about 17 NMPG, where 150 is roughly about 15 NMPG, and above that likely requires ROP, 150 may I usually cruise about 140, but fangs out hair on fire she will make 168 kts at takeoff power and fuel burn.

Those are my numbers roughly.

I can’t imagine a cruise speed where you burnt more fuel than if you went faster. burn as NMPG

 

[NotarPilot]

Interesting information here. Like I said, I actually get my RPM down to 2250ish and see about 9.9 GPH and 143 knots true.

I might start playing with the prop and see what speeds I get at 2100 RPM.

[ArtVandelay]

I don’t cruise below 8000 unless it’s a short flight, since I want to run LOP around 65%, I’ll select RPMs to get there, usually 2400 or 2500.

[201er]

28 minutes ago, NotarPilot said:

Interesting information here. Like I said, I actually get my RPM down to 2250ish and see about 9.9 GPH and 143 knots true.

I might start playing with the prop and see what speeds I get at 2100 RPM.

That’s not good. I get higher TAS and lower fuel flow at a higher RPM and LOP. I’d probably be down to 7-8GPH to run that kind of speed.

[jaylw314]

You can get some idea looking at the POH?  Looks like leaned out you can get a reduction in fuel flow of about 4% at the same power settings from 2200 RPM down to 2000 RPM.

The spacing between the 200 RPM lines looks pretty linear to me, if anything it looks like 2200 might be slightly better than expected

 

Edited October 27, 2022 by jaylw314

[graham28105]

https://resources.savvyaviation.com/wp-content/uploads/articles_aopa/AOPA_2022-08_what-price-speed.pdf

This is a great article for what you're asking about.

[A64Pilot]

I think your going to find that at any normal cruising altitude even for us flatlanders that it’s likely your at WOT, and yes slowing the prop will reduce fuel flow, but also will slow you down.

If you want to see the correlation of RPM vs fuel flow you need to be down low where you can increase MP to recover the HP lost by reducing RPM and keep the speed, if mixture and speed are kept the same, then any decrease in fuel will be from lowering the RPM.

‘I think your going to find that this is another one of those things that are real, but the difference is slight

[DCarlton]

On 10/27/2022 at 7:48 AM, PT20J said:

Higher rpm = more power

Lower rpm = better efficiency, to a point. At higher rpm there is more tip loss and at lower rpm the angle of attack is not optimum. So, there is an optimum point. But, the whole point if a constant speed prop is to make that optimum broad, so we’re not talking about a big difference. 

Bob Kromer once said that Mooney optimized the M20J prop for 2500 rpm.

David Rogers analyzed Bonanza props and came up with a rule of thumb that the optimum rpm is in the range of 14 to 15 times KTAS. 
https://www.nar-associates.com/technical-flying/propeller/cruise_propeller_efficiency_screen.pdf

Skip

I have Bob Kromer's article printed and filed on this very subject if anyone is interested.  I actually saw it yesterday.  And yes, he recommends 2500.  I have a three blade McCauley though; not sure what difference that might make.  I usually go for what feels the smoothest; somewhere between 2400 and 2500.  I'm no expert though...   

[MikeOH]

Meh, 8000-10000, WOT, 2500 rpm, 65% LOP. Get what I get.

You guys are really overthinking this

[Ragsf15e]

1 hour ago, MikeOH said:

Meh, 8000-10000, WOT, 2500 rpm, 65% LOP. Get what I get.

You guys are really overthinking this

I agree.  I think up that high giving up more power by reducing rpm isn’t worth it.