NWOC Merlin Topic

[Bill Greenwood]

One of the discussed topics at NWOC was Merlin operation, especially in P-51. There seem to be lots of theories around, some may be well founded in experience or engineering, others more like myths. I wonder what the facts are?. A lot of this is re: rpm and power settings. In discussing the LSFM Hurri they mentioned it didn't have much range. When I asked about the fuel tankage and consumption, turns out they are cruising at 2300 rpm, so the consumption is quite a bit higher than the 1800 I use enroute economy cruise. Some say running a Packard Merlin at low rpm makes it rough. The Spit pilot manual gives cruise settings for the Rolls at 1800 or 2000, no fear of roughness except to clear plugs periodically if on high lead gas. We use 100ll now, I don't see plug fouling as a problem except maybe at idle. Jack Roush says 100 ll runs much different than 115. He advocates using 2350 rpm, about 36 inches, to avoid detonation in cruise. I did not get to discuss this with him, so I don't know what he bases this on. Are there any gauges that show such detonation, perhaps an aftermarket EGT or computer? Does it just feel rough, and can one be sure it is not prop inbalance or nose bearing play? I just know my engine, a Rolls 76,is smooth at 1800 and 2000, and 2000 is a common Spit power setting in UK. For sure, if you use 2350 in a Merlin over the life of the engine you are going to have a lot more piston travel and thus wear, as well as burn more fuel. I don't recall so much concern for 100ll when I came into warbirds 24 years ago.
Another theory was on low power descent and landing patterns in a 51, with some guys wanting to keep power driving the prop sort of like a radial. Again, ideas from experienced pilots, but is it fact or supposition? I know in a Spitfire if you keep power much over idle, its pretty hard to slow to approach speed, the manual just says avoid any setting that are rough., nothing about min power.

[T33driver]

Bill,

We've had a brutal winter in the midwest and a lot of warbirds have been stuck in hangars waiting for good weather to fly. A Mustang owner buddy of mine hasn't flown his airplane since late last fall and sought advice from his engine guy as weeks went by and he hadn't flown. He was advised to pre-oil a few gallons every few weeks but not too much so it that it would interfere with the movement of the crankshaft. Do you have any other maintenance practices related to a long non-flying spell?

[Bill Greenwood]

Paul, if you talk to the oil reps, they would probably advocate draining the oil when it went to storage and putting in preservative oil which has extra anti rust. I don't actually know anyone who has done that in a Merlin. Ray did my oil change before the winter so at least got rid of any acids built up in the oil. Since spring is coming, I would certainly pump a few gallons of warm fresh oil through before first start and have it in a heated hangar or well preheated unless it is a pretty warm day, perhaps above 50f. Of course we as pilots need some time to come out of storage also. Missed you at NWOC, are you coming to Lakeland?

[T33driver]

Paul, if you talk to the oil reps, they would probably advocate draining the oil when it went to storage and putting in preservative oil which has extra anti rust. I don't actually know anyone who has done that in a Merlin. Ray did my oil change before the winter so at least got rid of any acids built up in the oil. Since spring is coming, I would certainly pump a few gallons of warm fresh oil through before first start and have it in a heated hangar or well preheated unless it is a pretty warm day, perhaps above 50f. Of course we as pilots need some time to come out of storage also. Missed you at NWOC, are you coming to Lakeland?

Bill,

Sounds like lots of valuable forums at NWOC, sorry I missed it and seeing friends. I'm probably not going to make it to Lakeland but I might try to squeeze in a few days. My airplane will be down for annual so I'd just grab an airliner jumpseat and maybe come Thurs, Fri, Sat.

My Mustang buddy also mentioned putting a heater on the engine along with pre-oiling. I'll ask him if he drained the oil. Maybe so as the weather continued to prevent him from flying for several weeks. I've only managed to fly the T-33 three times since November due to the crummy weather but at least do a ground engine run every two or three weeks.

[Matt Gunsch]

Bill,
Do you think the difference between a merlin driving a metal ham standard and a wood Dowty Rotal prop would make a difference with cruise RPM and fuel consumption ?
The props are the biggest difference between a 51 and Spit.

[dhfan]

I think it was in Stanley Hooker's "Not Much of an Engineer" that low revs and high boost were recommended for economical cruise.

[Bill Greenwood]

Paul, many, if not most of the Mustang, Spit owners have put on engine preheaters( a simple glue on electric pad, about $150, that sticks on the oil tank) as well as the Hovey canister type oil filters, and a preoiler pump. Total cost might be around $3000, and worth every penny. They have been found to be pretty effective. Even if you have a heated hangar the filters and preoiler are beneficial. There are a few holdouts, but most realize the value of the Merlin Fingers(cam followers) also. They are about $6000, less than the cost of two cams, that is if you can find the replacement cams. For $10,000 you can protect an engine that cost more than 10 times that much. Other than an little extra plumbing and a few pounds, there is no downside to these add ons.

[Bill Greenwood]

Matt, I hadn't thought much about the metal vs wood prop. The heavy metal one might act more like a radial when using hi rev, low power as in landing. I know one of the good things about the wooden Spit Dowty props, other than looking vintage cool, is that they accelerate and decelerate much faster than the 51 prop. It makes the Spit feel livelier than a 51, even if the engine is much the same, and may have been handy in combat. It has been some time, TOO LONG, since I have had any 51 time, but I remember that it feels like a bigger plane than a Spit, more of a cruiser, and I'll bet the prop effect is part of this. The 51 is heavier and has more wing loading, but it's not that much difference, and I bet the prop feel is a factor.
As for economy, I doubt if there is much difference in cruise between a wood and metal prop. It is possible the Ham Std is a little more streamlined, or the Dowty lighter and easier to turn, but probably little difference in result.
The big point Jack Roush is making seems to be that because of the differenc in how 100 ll burns compared to 115, that we need to run more rpm and richer mixture in a 51. I am looking at this with interest, but some doubts. I'm sure Jack's Nascars have instruments/computers that can tell the mechs what the driver had for breakfast, but is there anything in a 51 that really confirms the engine not running well at 2000 or so on low power? It's hard for me to believe a 51 Merlin may detonate at 2300 rpm in criuise at moderate, say 36 in, power. It seems the Rolls in my Spit is happy to run at 1800 or 2000 in cruise at 30 to 38 inches in auto lean, but I can't see inside the engine to be sure.
Some of this is style, the 51 guys have lots of fuel and like to run harder and go faster. The Spit or Hurri usually have an hour less fuel and need the low cruiise rpm, 1800 or 2000 to get cross country range. Spitfire mixture control is all automatic, it richens with more power; ie less than +7lbs, 44 inches it is lean, so that is one factor we don't worry about.

[Rossco]

IIRC the wooden props would absorb the vibrations of the higher powered merlins better and were much better in nose over situations.The prop blades would shatter and not cause too much damage to the fragile redugear as a aluminium blade would.

[sdennison]

Paul, many, if not most of the Mustang, Spit owners have put on engine preheaters( a simple glue on electric pad, about $150, that sticks on the oil tank) as well as the Hovey canister type oil filters, and a preoiler pump. Total cost might be around $3000, and worth every penny. They have been found to be pretty effective. Even if you have a heated hangar the filters and preoiler are beneficial. There are a few holdouts, but most realize the value of the Merlin Fingers(cam followers) also. They are about $6000, less than the cost of two cams, that is if you can find the replacement cams. For $10,000 you can protect an engine that cost more than 10 times that much. Other than an little extra plumbing and a few pounds, there is no downside to these add ons.

Bill, you bring up an interesting point. Back in the "olden days" when I was a race mechanic at Indy, the first thing we did was plug in the oil heater. A nasty long heater rod that stuck into the oil tank for hours. We then pulled the plugs and cycled the engine to get the warm oil through the system. Before practice, qualifying, or race we then installed "warm up plugs" and ran the engine up to all proper temps. Later we discovered (or invented, don't remember) the glue on heating blanket/pads. Also added water heaters. All to make sure that the engine and systems were at temp prior to "standing on it".

Obviously, not all of this can happen with Warbirds such as the Mustang and your Spit but doesn't it make sense that the oil heaters and perhaps water heaters be employed? Do I understand correctly that the pre-oiler is a pressurized reserve tank that forces oil through the system? Is it "refilled" by normal engine oil pressure once the engine is started?

What is the oil tank volume for you Spit? We had about 5 gallons on the Indy cars.

Thanks for the info. If an owner actually goes through all this, what is the expected life extension of the engine or is this just a lot of wasted energy?