Warbird Information Exchange

I started the Merlin oil change by asking Bill Greenwood a question so I thought I would ask a couple more about engine operations. My frame of reference in general aviation where 540 or 550 cubic inches and 350 horsepower is considered a lot and where there is a lot of opinion and speculation passed off as fact.

I have seen two specific engine management techniques get a lot of attention in past few years. One is operating "over square", this means running the throttle or manifold pressure in inches "higher" than the propeller control or RPM (less 100). For years we were taught that while you could run at 24" MAP and 24(00) RPM you should never operate at say 24" MAP and 22(00) RPM as it would damage the engine. We were even told that you always led with the prop when advancing power and with the throttle when retarding power. But as I understand it operating "over square" was at the heart of Charles Lindbergh's fuel saving techniques for Corsairs and P-38s and indeed was how he flew the Spirit of St. Louis, and not just CA but most of his generation of long range pilots used the same technique.

So what technique does a modern warbird pilot use?

Another technique getting a lot of play is operating lean of peak. Again in GA we were taught to lean the engine to peak temperature and then richen by 50 to 100 degrees to avoid damaging the engine. In the last few years there is a technique of operating lean of peak, basically leaning to peak temperature and then continuing to lean to about 50 degrees cooler than peak. This obviously uses even less fuel than the other technique. The major caveat with this technique is that the operator needs to know the temperature in each cylinder as each can have a different range of temperatures and is possible to lean past peak on one cylinder while having another stay at peak which really is dangerous. So without Cylinder Head and Exhaust temperature readings for each cylinder operating lean of peak is pretty much a crap shoot. About the only aircraft types of the warbird era with such instrumentation were post war airliners, maybe some of the bigger bombers had it but I doubt any fighters had room for analog CHT and EGT gauges for twelve or eighteen cylinders.

So what leaning techniques does a modern warbird pilot use and are there any modern fully instrumented EGT and CHT installations on any V-12s or twin row radials out there?

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"I can't understand it, I cut it twice and it's still too short!" Robert F. Dupre' 1923-2010 Go With God.

2nd question is simple for me, I don't lean a Merlin at all, in Spitfires it is all automatic, there is no mixture control and nothing to adjust. As the throttle is opened and boost goes up the carb, on the Rolls engine automatically enrichens the mixture. The Pilot Notes say it is in lean below 7lbs boost, ( 44 inches) and rich above that, don't know if this is progressive or a one step thing, but it seems to work fine for me and during the war.

Mustangs have a mixture control, and I hear their pilots often debate about where to set it. Maybe North American or Packard got paid more to include more gadgets. Or maybe Mustang pilots were apt to get bored on those long Pacific escort missions and needed something to monkey with.

As a VERY general premise, you want to use high rpm when you are using high power, thus takoff in a Merlin at 3000 rpm, full rpm.. This allows you to develop full power and protects the engine against detonation. My Bonanza takes off at 36 in and 2700 rpm. The downside is that this burns a lot of fuel,also wears the engine more. When you reduce power for long range cruise, you can use low rpms and not hurt the engine. Thus my Spit 1800 rpm at 0 boost or 30 inches, my Beech at 28 in and 2300 rpm. Mustangs went as low as 1600 rpm for ultra long range cruise in the war. Fuel burn is, Spit at full throttle 180 gal per hr, econo cruise 47 gph U S. For my turbo Bonanaza and TSIO 520, 36 gph full and 18 gph cruise,

Some big radials are very sensitive to this over boost or undersquare thing, and you have to be more careful of them.

I have not run my Beech lean of peak, but it seems to be all the rage among many new owners, like Cirrus.

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Bill Greenwood
Spitfire N308WK

Of note-
All P-51B, C and Ds equipped with a Stromberg carb with a Throttle, Prop Control and Mixture Control. Carb changes along the way changed how the mixture operated and selections avail to the pilot.
Spit Mk XVIII with a Griffon 65 with a Stromberg carb (different model than P-51) has a Throttle, Prop but on/off lever for fuel. It is a spring loaded arm actuated by a cable similar to a brake cable on a bicycle. Originally the Throttle and Prop were controlled by one lever. It didn't work the best so the Throttle and Prop were separated. Of course this is an airplane that still had a morse key device in the cockpit.
Any change to the mixture was done on the ground with the Spit.

Rich

_________________
Rich Palmer

Remember an Injured Youth
benstear.org
#64- Stay Strong and Keep the Faith

BOOM BOOM, ROUND ROUND, PROPELLER GO

Don't Be A Dilbert!

The simple answer is follow what the book says, but with today's fuels add 50-100rpm to every boost/rpm setting that the book suggests. Jack Roush suggested that to me and it makes sense. Also there's no need to test the blower on the ground (you can hear the solenoid click when the engine is off and that's all you need to check) and there is no need to go to max boost (i.e. 61" for a merlin), again due to 100LL.

To manually lean a KC-97s R-4360 engine the following procedure is used.
1. Set engine at desired cruise power. RPM, Manifold pressure (MAP), and torque (TPSI).
2. Set base TPSI to 109%.
3. Check mixture by moving mixture control one knob lean and note TPSI.
a. If TPSI remains constant, Auto Rich is best power.
b. If TPSI rises, Auto Rich is richer than best power, adjust mixture to highest TPSI, retard throttle to 109% TPSI.
c. If TPSI drops, Auto Rich is leaner than best power, hold primer switch to apply prine, adjust mixture to highest TPSI, retard throttle to 109% TPSI, return mixture to Auto Rich.
4. Increase MAP by 1 inch.
5. Lean mixture to 109% TPSI.

This gives you the max cruise mixture setting. the engines are actually getting more air than peak EGT and it helps cooling.

That's the key

You either need a Torque Meter, BMEP Gauge, or EGT on each cylinder to properly manually lean an engine without hurting it. It's much cheaper to just buy fuel in most cases!

Leaning - schmeaning.........what value is there in leaning a Merlin these days? Most flights are very local, so the need to lean for 'Lindberg' type range maximization is not existant. Very rarely do we go on epic cross-Atlantic long range flights. It seems better to stay in rich and conserve the metals/mechanics of the motor.
Just as Glenn said, to get the best (anal retentive) leaning, you'd need lots of extra instrumentation. However, as most engine guys say run it rich or a little higher RPM to protect the motor. For the cost of 5 GPH (or $25 per hour) more fuel burn, the engine is given some extra protective margins.
And just as Jim said, run the motor an extra 50 to 100 more RPM.
The ususual power settings are:
36" & 2300 rpm.+auto lean...cruise
40" & 2500 rpm.+auto rich...max continuous (cruise/climb or high speed cruise)
46" & 2700 rpm.+auto rich...climb (also akro power)
55" & 3000 rpm.+auto rich...takeoff (some use 61" for takeoff)
One can add 50 -100 rpm for protection and/or stay in auto rich, for the same benefit.
Theoretically, the Merlin carb should run in auto rich at the higher power settings without switching to 'auto rich', but that's trusting the automatic fuel schedule will work correctly.
(Good thread here + please correct me if I'm wrong.)
Thx,
VL

Vlado is correct for operations today. The KC-97 procedures were for long range missions back when taxpayers were paying for engines. The procedures for the B-29 FIFI is to operate in Auto-Rich while doing pattern work or airshow stuff and using Auto-Lean when in cruise power. The price of fuel is cheap compared to an engine rebuild.

In regards to low RPM and high MAP for mileage think of it this way; Each time a cylinder fires it is an event using fuel, the fewer events, less fuel is being used, thus low RPM uses less fuel. By increasing the MAP you regain the power (torque) you lose by lowering the RPM.

b29flteng,

You wrote:

"In regards to low RPM and high MAP for mileage think of it this way; Each time a cylinder fires it is an event using fuel, the fewer events, less fuel is being used, thus low RPM uses less fuel. By increasing the MAP you regain the power (torque) you lose by lowering the RPM."

If I understand what you're saying , wouldn't this also inturn create greater cylinder pressure during the combustion cycle and increase the probablility of detonation...hence Jack Roush's suggestion to increase RPM?

Thanks,
John

John asked:
b29flteng,

You wrote:

"In regards to low RPM and high MAP for mileage think of it this way; Each time a cylinder fires it is an event using fuel, the fewer events, less fuel is being used, thus low RPM uses less fuel. By increasing the MAP you regain the power (torque) you lose by lowering the RPM."

If I understand what you're saying , wouldn't this also inturn create greater cylinder pressure during the combustion cycle and increase the probablility of detonation...hence Jack Roush's suggestion to increase RPM?

Thanks,
John

In today's enviroment ( only 100LL fuel available ), that would be my immediate concern. Jack's increased RPM suggestion is based on the uncertainty in today's 100LL fuel quality and the uncertainty of the 100% mechanical operation of all the Merlin components; it was a risk tolerance suggestion. The 'Lindberg' type fuel management was done in combat conditions with the best fuels of the day available. --Historically, weren't the fuels in the WW2 combat theatres better quality and grade than available WW2 stateside?--curious.
In today's enviroment, I would not want to fly that low RPM profile due to the risk of engine damage. The concept is understandable and has merit in a different circumstance.
VL

Is there any hard evidence that 100 ll is worse than the fuel Merlins ran on for 5 years of war, plus many years of peacetime since?
I know fuel at the start of the war was 100 octane, Maj Doolittle was part of the development of it.,if so 100 ll should work fine now.
It was only the last year or so of the war that the very high octane , 115/145, with high lead was available. It allowed high boost pressures, 18 lbs, even 25 lbs, for high power climbs, fast speeds, and high loads.

Nowadays, we operate Merlins mostly at moderate boosts, 61 in max, and more likely 55 in. More important we cruise at perhaps 0 to 3 lbs, or not more than 36 in manifold pressure. So why do we need more than 100 ll? I hear lot's of opinions on that, but no facts that I know of.

And the idea of using lower rpms, perhaps 2000 or 1800, at lean cruise, is ONLY FOR LOW POWER CRUISE, JUST LIKE THE CHARTS IN MUSTANG AND SPITFIRE PILOT MANUALS FROM WW II, and not for anything different or at high boost calling for high octane, or rich mixture.

Some very bright Merlin engine guys hold the view that 100 ll is inferior and harmful to engines. BUT, I don't know of any data to support that opinion. I don't know of a single blind type Merlin engine test, nor on any other kind of engine. I don't know of any information from the oil companies that make the av gas that 100 ll has any such difference, nor any such info from an informed neutral and expert source like the society of engineers.

Just because an expert or even just a pilot or a&p has a hunch does not mean it is wrong, maybe they are picking up something before it is accepted as fact.

BUT, if we are going to be objective, we must, if we can, try to prove or disprove hunches. I have a hunch that if I wear orange on football game days, then the Longhorns will win. It seems to work pretty well. Would it work if I wore the blue of Rice or the gold of Colorado? Tiger likes to were black or red in golf tournaments, seems to work pretty well. Would it work if Charles Barkley wore the same colors?

I give expert opinions some weight, but I don't assume that is the same as fact beyond any doubt.

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Bill Greenwood
Spitfire N308WK

There are some excellent books put out by RR Heritage Trust written by engineers that were involved in the development of the Merlin and Griffon.
Without a doubt they write that the development of the higher octane fuel was the most important part of getting the horsepower out the engines.
The higher octane fuel reduced detonation and that allowed higher power to be generated.
The higher power output then showed any structural weakness of the powerplant and then further development would take place.
IIRC Merlins were about to be put in production at the end of the war that were putting out power that equalled or exceeded what some of the Griffons were making with 600+ more cubic inches.


Today the Carb is an important device to make sure is working properly. It seems we end up replacing or just sending ours out for bench check every couple years.
With rubber diaphragms inside the flow characteristics can change over time.
Engine builders have the flow rates changed for todays fuel rather than just running WWII flow rates.

Rich

_________________
Rich Palmer

Remember an Injured Youth
benstear.org
#64- Stay Strong and Keep the Faith

BOOM BOOM, ROUND ROUND, PROPELLER GO

Don't Be A Dilbert!

If you look at the program that Jack Roush has developed over the years, it does show a methodical process being followed.
It has involved testing and teardowns within an environment of test runs on a truck and also many hours being flown in his own airplanes followed by teardowns and inspections.
Other engine builders have followed similar paths of development.
Obviously the final decision is up to you as the owner/pilot to make using the process that you feel comfortable with to find an engine builder.
But what exactly do you want to see to get a fact beyond any doubt?
Rich

_________________
Rich Palmer

Remember an Injured Youth
benstear.org
#64- Stay Strong and Keep the Faith

BOOM BOOM, ROUND ROUND, PROPELLER GO

Don't Be A Dilbert!

Maybe I am lucky, the carb on my Rolls 76 is the same one Jack Hovey put on years ago when he overhauled the engine. We have never removed it for bench check or overhaul, and now have about 1000 hours SMOH. Maybe the Packard ones get worn out by the 51 pilots fiddling with the mixture control so much!

I have and have enjoyed THE MERLIN IN PERSPECTIVE, by the Heritage Trust. I don't see anything that says you need very high octane fuel for normal slow cruise at low power or that high rpm is needed for low power cruise. But maybe these guys didn't know about it then.

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Bill Greenwood
Spitfire N308WK

Why would 51 pilots be "fiddling with the mixture"? It's either in RUN or AUTO LEAN or AUTO RICH depending on the carb installed.
I admit a 51 driver transitioning into a Spit might spend the first five hours in the cockpit " fiddling around" LOOKING for the mixture but that's about it as I remember anyway )))))))))
Hope you've been well Bill.

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Dudley Henriques
International Fighter Pilots Fellowship