SUPERCHARGER

Now THAT is cool!
Man I hope it works for you this year.

 

Thats a big carb.


Steve, sorry to hear Sparky is still sick.


I would think that if you have a valve spring problem it will just get worse and worse as RPM climbs. Is that what is happening? Did Sparky get a new set of valves? If so and they happen to be heavier than the old ones that could explain it.

 

I remember that the hydroplane racers talked about something called a "quill shaft" which they found was the weak link in their Merlins. Could that be anything that would worry you?

 

As I know it, the quill shaft (splined coupling) is a problem only when it breaks. If all the bearings are in tolerance and lash is correct, the blower is balanced,and Brant is chewing the right tobacco I just can't see where the problem is with the supercharger. Seriously, what could couple in the supercharger drive system? You've got the same blower and drive mechanism and it all checks out so why all of a sudden does it go bad? I have to think that it could be something down the line. I ain't no expert on Merlins but I have messed around with gas turbine compressors and turbines when I worked on the Boeing 502. That equipment that Sparky showed is rather intimidating to say the least.

 

Good luck Steve.

I know how it feels.


I can loan you a wall to hit your head on.

 

What would happen if you switched to a "cuffed" propeller like certain other Mustangs had?

 

Couple of questions.

1. Does it do it on the ground or is it only in the air?

2. Does it do it at various speeds and smooth out between the resonances or only at one speed?

2. Can you get a freqency analyzer on it, or put a vibe sensor on it and record the data and look at it on the ground? A triaxial vibe pickup on the case with crank position/speed sensor is all you need, that's only four channels of data and there are plenty of portable data acquisiton systems that could get that pretty easily.

Knowing the frequency of the vibration and the engine speed, and you can calc the spercharger, prop and intermediate s/c drive gear speeds, you should be able to figure out what is vibrating pretty quickly.

A good frequency analyisis can determine if it is a cylinder since it will hit at half engine speed and at a particular crankshaft position.

If you can feel the vibration from something as big a a Merlin, it's shaking pretty good. The mass of the engine will eat up a lot of sins, so if you can feel it or hear it, it can do damage pretty quickly.

Swapping out parts will likely find it sooner or later, but that's a time consuming way to do it.

 

I always wondered if anyone had tried to swap out the compressor blades on a P51 supercharger for more modern and efficient blades. Is this possible or are the ones on the original blower as optimal as they need to be for the operating RPM? I would think that someone who has enough money for one of these birds could get Vortech or ATI to produce a new compressor blade. Just my thoughts because I'm a hot rod guy through and through. LOL Hot rodding a P51 would just be awesome.

 

I believe that the Merlin's blower is a one piece aluminium part.

 

If you wanted to just replace the compressor and get better efficiency, there isn't much of a gain to be had. The engine is intercooled so the gain from improvements in compressor efficiency actually shows up as a lower compressor discharge temperature. This means that a slightly lower amount of cooling that has to be done in the intercooler. Net result is a bit less cooling drag, but it's not going to get a lot more speed.

BTW, they already are adding the fuel before the compressor, so during the compression process the fuel is evaporated and that results in a cooler combustion process. Even though this is a pretty old engine, there was a lot of development done in the war on it.

There might be room for improvement in the combustion chamber, since there has been a lot of technology progress in that area in more recent years, but these engines run on the hairy edge of detonation and that's the real limit. It would cost a lot to develop new heads/pistions. More boost and lower compression ratio would make more power, but that would require a new compressor as well as new heads and pistions, and maybe stronger rods and crank... All can be done, just how much do you want to spend.

Since speed is a cube root of the power gain, you would have add a lot of power to make much of a speed improvement. Easier to attack the drag part of the equation than the power side.

 

Sounds like I'm talking out of one ear about the blower. I'm not a big bore expert, just a big bore. I could only imagine that the super charger itself would be with high tensile steel blades. The stuff I worked on had an "aluminium " centrifugal compressor and stellite turbine blades. The Brits didn't have stellite back in the thirty's so a high tensile steel would be my only guess. Solofast, aren't you a gas turbine expert?

 

Sounds like you are working it the right way. Some of the newer analizers are pretty amazing, but a good tech with a scope can tell you a lot too. Good luck.

 

Well, I don't know about expert, but yea, turbines are us...

With high compression comes high temperatures, so for steels, stainless is usually used for this type of duty, the precipitation hardened stainless steels like 17-4 have been around a long time and are good up to 800 degrees, so that could be used. It casts well is used for compressors in engines like the Harpoon missile. After that you go to nickel alloys, but nickel was in short supply during the war and was used primarily in turbocharger turbine blades, so my guess is stainless steel. I had no idea that the pressure ratios of the blowers were that high, that's a pretty high pressure ratio and you are getting up there in terms of compressor discharge temperature so yea, you need a good material to hang on to that kind of tip speed.

 

I dont work on airplanes Steve but we do work on a lot of motors. We run into things like this from time to time and a successful resolution almost always comes about by turning the clock back and looking at when it started and what was done then. This all started when the pistons and cylinders were replaced. Sparky was a little tired but healthy as a horse otherwise. Aside from removing and reinstalling all the other components at that point I am not aware of what other interventions took place. With everything you have tried, inspected, rebuilt, swapped or replaced the problem is still there.

I know you don't really want to hear this but if that motor was one we built I would be telling my guys to take it back apart so I could have a look at the pistons, rods, rod bearings, wrist pins and pin bushings. You know better than I what happens when one of those parts decides to quit warning you.

I don't care how good the motor builder was and how good the parts are, stuff happens.

 

I seem to remember a stainless with high nickel content and maybe chromium. The lack of these elements in Germany in the latter stages of the war is why their gas turbines only had a 2 to three hour TBO. I have seen (through a special window) the turbine blades momentarily turn almost white during a start. Early gas turbines often had "hot starts" when the rpm wouldn't get up fast enough to cool the innards and had way too much fuel burning at high temps that softened things up. Even the Inconel burner cans wilted.

 

I got to thinking about my thinking and I am going back to 1952 and that is almost 60 years. Kinda scary, huh? So, I have to rethink some things at times but I think that I make the necessary corrections most of the time. It's a long trail.

 

Bob,

You're right about hot starts, but they still happen. What happens when you light off a burner is that there is a lot of fuel in the air downstream, and all that lights off and then the flame appears to suck back into the burner and stabilize in the primary zone. If that doesn't happen there is burning going on further back into the engine. That's generally because there isn't enough airflow to make the burner work correctly and burn the fuel where it is supposed to burn. The burning is taking place in or after the first stage of turbine, so the turbine isn't making the work it needs to accelerate the rotor and the engine gets hung there cooking. Adding more fuel just burns things worse, and so does cutting down the fuel flow...

You can cook a hot section in very short order if you don't catch it and shut it down.

Most hot starts are the result of low cranking speed (not a enough battery or insufficient cranking speed for whatever reason). FADEC engine controls have helped a huge amount in that regard, providing better control of the fuel and if the engine doesn't start to accelerate and the temperatures stay too high for too long, it will cut the fuel flow and protect the engine.

Starting engines is tricky if you are starting from scratch and the engine is all new (I can tell you that from painful experience), and it is not uncommon for even the biggest engine manufactureres to fry an engine or two during development from a hot start. GE had a hot start in one of their big development engines not that long ago and it cost them millions and hurt the schedule, so it is somewhat like wheels up landings, nobody criticizes because there but for the grace of god, there go I...

But, once you get a cranking speed and fuel flow sechedule figured out it's pretty routine.