Cruise Setting for Normally Aspirated 337

[patrolpilot]

This question has been on my mind, and the same recent topic for the P337 has made me jump in with a similar question for the mouth breather.

What RPM and/or horsepower do you prefer for cruise flight?

I've been using 2600 RPM. That practice was passed down to me from the previous owner, who had it passed on to him. I saw it in practice when I took the seller home, a decent length flight.

I pondered it against a slower RPM, but the bottom line is that these guys ran the engines 1000 hours beyond TBO without issues. I've done enough cross-country flying that I've been able to try different RPMs, with MP that produce 75% or less power. Typically, I fly the airplane at 8,000 to 11,000'. So, in reality, I'm looking at 60% to 70%.

I've tried 2500, 2400, 2300, and 2200. At 2200 and 2300 RPM, while at altitude (reference is probably funny for a "P" flyer), it seems like the airplane is floundering. It has a tough time recovering from chop or turbulence, and the pitch attitude seems higher than it should be.

Here is how my airplane performs against the book at 2600, 2500, and 2400 RPM. At 2600 RPM, I'm always a solid 3 knots faster. 2500 is close to book, and 2400 at least 3 knots slower. Each progressive decrease increases the pitch, of course, but at the 2600 RPM, it seems as flat as a pancake compared to the other two.

I've never experienced "heat" issues, but with the factory gauges, I would say that the lower RPMs are slightly hotter. I think that is a product of pitch attitude and reduced airflow.

As I pondered this over coffee, I thought about comparing just how many times the engine completes a revolution on my typical flight stage length. I didn't consider the climb or descent as the profiles for those segments is the same. It really surprised me how much less the engine turns at the higher RPMs. I made the calculations with the book values, not the gains or losses I've observed. (attached)

So, how do you manage your power choice?

[patrolpilot]

As a note to the above. I just checked my time & fuel figures against ForeFlight's Performance Plus and they draw the same times and fuels. Performance Plus has three profiles: Max Power, 2400 RPM @ 65% power, and 2300 RPM @ 55% power.

[Ernie Martin]

2600 seems too high. Picture below is what I have on my checklist, derived from the POH.

Ernie

[patrolpilot]

Thanks for the reply, Ernie. I'm going to press on based on my considerations above. Attached are the appropriate pages from the '77 G, 2600 RPM is in the normal operating range. I have the O-2 manual; the limitations and the performance considerations are the same. I've also familiar with the T-41, still have the manual, which uses the IO-360-D with the same restrictions, and we flew it at 2600 RPM.

The airplane is coming out of annual, and the mechanic has said to keep doing what your doing (which is what the previous owners were doing). My rear engine has 1418 hours on it, and I'm going to run it until it can't. Honestly, it's compressions, and oil consumption is the same as the front with only 638 hours (11 hours per qt or .09 qts per flight hour). That has been all at a 2600 RPM cruise, 75% power or less.

I've started in on the C414A, and the operator wants them flown at the max cruise RPM as well, that being 2450 RPM. Their experience has been some of what I've covered with pitch attitude; the flatter is better stuff. What the Chief said is that as you enter the FLs, the pitch attitudes at any lower RPM starts to increase significantly. The lifters at the front of the engine are missing out on oil, and cylinder head temperatures are rising on those cylinder heads. With the attitude, the rear lifters are drowning in oil and slinging it out the breather. I guess electronic engine monitoring has been significant in determining what's going on inside the cowling.

I'm looking forward to that as well. Seeing the Insight G4T Skymaster in use and looking at how power settings influence all the recordable parameters. The TAS1000 will deliver a TAS without interpretation as part of the comparison.

[cjordan]

I try to keep things easy to remember. 73 "G" with long range tanks. All give ~6hr range with decent reserves.

2300 23" to 6k 20gph
2400 full throttle to 9k 18-18.5gph
2450-2500 " " to 11k 18gph

No hurry, tooling around down low 2200 20" 14gph

They seem to like the 7-10k range for best TAS and fuel burn for longer trips.

[patrolpilot]

Well, the insight G4T has already solved a big question for me. My Cessna tachometer sucks. The only place it is accurate is at 2800 RPM, and it quickly starts to error short of what is being produced.

I've been running around with 2600 RPM set, but the engines are only turning 2400. That explains the angle of attack issues at lower RPMs because the error is as great. The only hard running would have been down low, like in the picture (2K) as the MP would have been high. That would not be a problem at my normal cruise altitudes.

Yep, I do monitor the idle RPM, and the check indicates higher than the book, but the idle was set by maintenance with a digital tack, and they have that just right sound. I never drew the connection in that the upper is limit is true.

My Cessna Fuel Flow Indicator is spot on. This doesn't confirm my next expense concerning my airplane, a move to the Electronic International Primary Engine Instruments.

I wish we could add inline images on the forum

[Mayhemxpc]

Thanks for another reason to get a G4T. Great picture. I had my tach and prop checked during my most recent annual. Actually it was part of calibrating the fuel system after that was overhauled. One were off by a little less than 50 RPM, which doesn't bother me too much as I am probably not that précise in my control manipulation. 200 RPM is quite a spread, however and I can see where you were having problems.

For what its worth, I have an O-2A and 65% power is about 23"/2300 RPM at 3,000 ft. That is also the power setting the USAF functional checklist says to use for post-maintenance test flight. Other altitudes and power settings are similar to what was posted by Ernie. The O-2 is pretty draggy and, to me anyway, the minor increase in speed at 75% is not worth the noise and fuel burn.

[hharney]

No claim to the content but very interesting formula


Using the Airspeed Indicator as a Fuel Flow Meter

[Mayhemxpc]

As everything with Stick and Rudder, very interesting. I compared it to the USAF Dash 1 for the O-2A. Best glide is 100 kts at 4400 pounds. The performance charts say best long range airspeed is 104 Kts CAS and best glide is 99 kts at the same weight!

Similarly, at 4400 pounds, service ceiling is 17,500 ft. 2/3 of that is 12,000. By the book, at 12k max MP is about 17 to 17.5 which is the power you need to get ~104 kts CAS! (126 KTAS) Seems like 1944 rule of thumb calculations for SEL planes were still valid 25 years later for MEL!

I don't carry oxygen, but the dash 1 shows 110 kt CAS at 8,000 still gets me 124 KTAS at 8.2 gph/engine. That is just a little slower that 12k at 1/2 a gallon more per engine per hour. But I can breathe, which is a good thing.

But...20 minutes longer for a 300 nm trip vs my normal cruise at 65%.

[general]

Aspirated = 25 Cruise all the time, 336, 337, 02