Quarter Scale Merlin V-12

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gbritnell

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Terry,
With all the superlatives used many times I'll just say what an outstanding creation. The work, attention to detail and documentation are first class. Congratulations!
gbritnell
 

kvom

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So, what's next? :)
 

dairwin

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<snip> "As a reminder to myself to not blip the throttle and abuse the supercharger, I tightened up the lever in the quadrant throttle to make its action very stiff."

A good idea. I also have a throttle friction control, so I can not bump the throttle. There are two separate throttle return springs in the event of control disconnection.

"The engine wants to rev higher, but its operator isn't quite ready. "

I know the feeling! The max I have run my engine, with low propeller pitch (variable pitch prop) is 2,500rpm and -3psi boost. Torque roll on rapid throttle advance, also has to be avoided.

Nice video Terry. Sounds great. Any more info on the coolant leakage?

DAI
 

mayhugh1

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I don't yet know if the Quarter Scale is going to be prone to overheating, but over time I plan to carefully extend the runs while keeping an eye on the engine temperatures. The cooling system does appear to be removing heat from the heads. Another one of my concerns about this engine has been the capacity of its scaled-down coolant pump whose impeller looks awfully small. After a one minute "prop-less" run at 1500 rpm, my last measurements showed an intake manifold temperature rise of 24F, a header tank rise of 42F, a radiator rise of 23F, and a reservoir housing rise of 9F. After the 1-1/2 minute 1800-2000 rpm prop run in the second video, the intake manifold rose 50F, the header tank rose 60F, the radiators rose 12F, and the reservoir housing rose 19F. That little pump certainly seems to be pushing coolant around the system.

The coolant were a disappointment, but they're fairly minimal and don't seem to be getting any worse. It's not worth going through a complete teardown to try to fix them especially since I'm not sure what I would do differently. After reading Jeff's comment about the danger that ethylene glycol presents to the bearings if it gets into the oil, I drained the oil for inspection. The oil was clear although it had darkened a bit as expected, and I could detect no antifreeze odor. Another piece of good news was that there was only 60 ml of oil in the lower crankcase which indicates the scavenger pump is keeping up nicely with the pressure pump.

Since most of the leaks seem to be occurring just after the engine has been run, it occurred to me that a pressure rise in the system during its hot soak may be overwhelming the o-ring seals located between the heads. I'm currently running the coolant system with an unvented cap, and I may not be allowing enough volume in the reservoir for expansion. On the other hand, there could be a lot of trapped air in the tops of the heads above the seals, and therefore not a lot of coolant to leak out.

So, I machined a neck extension for the filler cap on the coolant reservoir. This neck makes topping off the reservoir a little easier and allows the level of coolant to be increased a bit more which might help displace any air in the system. The neck also allowed me to install a hose fitting so I could add an expansion tank. A temporary expansion tank was cobbled up from a plastic syringe body.

I completely filled the reservoir with coolant and closed up the system with its unvented cap. During a one minute run at 1200 rpm, about 15 cc of coolant was forced into the expansion tank. When the engine cooled, it was all sucked back into the reservoir. I still had a coolant drip, but it was less than before. Even though the overflow tank isn't period, it probably makes sense to keep it and so I"ll machine something that looks better.

Unless I run into something that might be of real interest, this will probably be my last post on this build. I'd like to thank everyone for their encouraging comments and constructive tips during the past 2-1/2 years. I'd especially like to thank John Ramm who patiently answered my email questions about his experiences during his own build. Richard Maheu, the designer of the Quarter Scale, also deserves to be recognized for the labor of love that he spent some ten years creating.

I have a number of things that need tending to around the house and shop. The break will also give me a chance to think about a next project. I hope to be back. - Terry
 

kvom

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I had a discussion on an offroad forum about using just distilled water as coolant, which conducts more heat than antifreeze. With the addition of a product like Hyperkuhl, corrosion is eliminated. Might be worth looking into.
 

dieselpilot

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Excellent work. Cooling systems are tricky stuff. It may be worth monitoring the pressure of the cooling system or doing a pressure test. Though temperature could come into play. I had a car which passed a cold pressure test, but leaked water into a cylinder when parked hot. Some idea of cooling capacity could be determine from the radiator specs (if supplied) and fuel consumption.

As a side note, put together a compilation of your photos and video to post to YT before somebody else does to profit from your work. I saw this the other day and was disappointed to see a video of Keith5700's V-10 had earned 150,000 views in a couple days, half of what Keith's own engine running video has reached in almost a year. The guy made a compilation of his forum photos. Being a compilation it's likely protected and earning ad revenue on a bit of video editing of other's work.
 

Ca-g

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Terry,
thanks for the fun. I've rarely seen such skill and reasoning power...
Chris
 

napoleonb

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Congratulations on your fine build and results!
It has been fun and a great learning experience to tag along reading your posts.
 

editor123

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Terry,
I sent you an email on this forum about featuring your engine as a Centerfold Article in a future issue of Model Engine Builder.
 

dsage

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I knew Jay had lots of cars but I didn't know he also had a Merlin. And a real fancy one at that.

https://www.youtube.com/watch?v=GYcKdK7hmEohttp://

Terry, maybe he'd be interested in seeing yours. I know he liked Lou's Duesenberg. Maybe you can get on one of his Jay Leno's Garage episodes. (On Youtube).
 

mayhugh1

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After accumulating an hour or so of numerous shakedown runs, the engine abruptly stopped one day just after a full throttle test. The prop appeared to be sickeningly trapped between a pair of hard stops separated by what I measured to be some 390 crankshaft degrees. Right after this happened I had to go out of town for a week, and so I had something to think about while I was gone.

When I returned, the first thing that I checked for was a piston striking a valve being held open by a dislodged seat that had become wedged between the valve and the head. This possibility was high on my worry list because during construction one of the seats that I pressed into the head needed suspiciously less force than the others. Repairing this, assuming an irreplaceable casting hadn't been damaged, would require separating a head from its cylinder block, and this was something that I hoped I would never have to do.

After draining the fluids and returning the engine to its rotisserie assembly stand where it would be easier to work on, I removed the valve covers so I could manually exercise each valve. All seemed OK.

My next guess was that a rod bolt had come loose, worked its way out of its cap, and was hitting the oil manifold mounted to the tops of the bearing caps. I rotated the rotisserie 180 degrees so I could remove the lower crankcase (oil pan) and was surprised to see that everything beneath the oil pan looked OK. The symptom had changed, though, and now the crankshaft had much less free rotation. The cylinder bores, as much as I could see of them, had nice shiny mirror surfaces; and only one showed signs of minor piston skirt scuffing.

I turned the rotisserie back over so I could insert a borescope in the spark plug ports of the number two and five cylinders in both the port and starboard banks. The pistons in these cylinders were near the tops of their strokes, and now the crank was completely seized. Everything I could see inside the four combustion chambers, though, looked reasonable.

As an aside, I was really impressed with how clean the Viper spark plugs looked. They looked essentially new. I initially feared I'd have continual fouling problems with these tiny plugs which are mounted on the sides of the combustion chambers. Since they're located in front of the exhaust ports, though, the exhaust heat may be helping to keep them clean. There was a lot of coking, however, on the cooler walls of the huge exhaust ports cast into the heads outside the combustion chambers.

The center bearing cap, which was machined extra wide, establishes the crank's thrust clearance to something just under a thousandth. I then thought perhaps that some debris had found its way into this gap to create a bind. I removed the center bearing cap, which also required removing the oil manifold, but the crank was still stuck fast.

Although it didn't start out feeling like a starting system failure, the symptom now seemed to be pointing to an issue inside the wheel case. I removed the manual and electric starter shafts so I could look inside the wheel case with a borescope. The starter gears still looked good, but my visibility inside the wheel case was too limited to determine much else.

I was mentally preparing myself to begin disassembly of the wheel case when I decided to first remove the front drive cover. With the symptoms I had been seeing, I had no reason to suspect the problem was in there. Inspecting the front drive, though, would at least eliminate the last of the easy stuff. As soon as I pulled the cover, I saw two halves of a mangled screw laying in the bottom of the housing. One of the six button head cap screws that attaches the large driven prop gear to the prop shaft had backed out and fallen in between it and its drive gear. Three of the other screws were also on their way out.

The problem was created by a too small circular pattern for these six fasteners. The screw heads ended up too close to the fillet on the gear hub. When tightened, the heads dug into the fillet around the hub instead of bottoming on the flat surface of the flange. The vibration of the prop load likely caused them to back out over time. Two of the still-loose screws had too much thread damage to be safely removed without breaking them off. I was able to retighten them, but for good measure I first flooded them with a wicking grade of thread locker. Before replacing the other four fasteners, I turned down the diameters of their heads so they no longer touched the hub fillet. I also installed them with blue Loctite. The high spots on the damaged gear teeth cleaned up with a fine file.

I was able to take advantage of some of the unnecessary disassembly to better seal a few annoying areas around the lower crankcase that had been seeping oil. In fact, I discovered my two coolant leaks weren't created by the dreaded o-ring'd transfer tubes between the heads and cylinder blocks after all. They turned out to be associated with the coolant distribution manifolds on either side of the cylinder blocks. These, I was eventually able to fix.

To my relief, after reassembly, the engine fired right back up, and I seem to be back where I started. Actual&#322;y, considering that the drip pan no longer has coolant in it, I'm considerably better off.

So far, I don't believe the engine overheats, but I'm still working on determining that for sure. The cooling system seems to be sufficient for the three minute runs I've made so far, but I could sure use a temperature sensor inside the header tank.

When shutting the engine down, I've learned it's best to first kill the fuel pump and let the engine run itself out of gas before powering down. If not, fuel that was sucked up to the input of the supercharger will rain back down the long vertical intake and drip out through the carb's intake when the engine stops. In this engine that's actually quite a bit of gas. - Terry

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dairwin

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Terry - sorry to hear about the reduction gear bolt problem, and pleased to learn the damage is minor. On the full scale Merlin, these bolts are necked and fitted from the rear side of the gear flange with nuts and split pins. The bolt shaft is machined to fine tolerance and the fit of the gear to the flange very important for the equivalent load across the mesh.

Shutting down the engine; fuel off and let it starve is the best way to evacuate fuel from the inlet. I shut off the fuel and let the revs drop, and eventually the 'low oil press' switch trips the mags to stop the engine itself.

I would like to see a video of a full load run. Something I can never do!

David

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Ghosty

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Terry, very sorry to hear and see the damage, is it repairable, or will you have to cut a couple of new gears?

Cheers
Andrew
 

dsage

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Whew
Terry, you should be a mystery novel writer. You had me on the edge of my seat through that whole read.
Glad to hear it isn't (too) serious. I hope the loctight is enough to hold those screws. But now that there are properly seated you should be good.
Good work.
:thumbup:

Dave
 

tornitore45

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Terry, you should be a mystery novel writer. You had me on the edge of my seat through that whole read.
I was thinking the same. All these write-up should make a good novel, better that the Guy Lutard " Machinist's Bedside Reader".

I hope Terry has a soft copy compilation, saved.
 

ShopShoe

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Terry,

I've said it before, but you continue to amaze...

I followed your inspection and diagnostic post just like it was another part of the build and I continued to be impressed by the way you think and logically work through the process to success. This model engine is just about the most complex mechanism on this forum, but your descriptions keep all of us following along as if we would have to do something similar tomorrow.

Thank you for the additional details about other things you noticed in the tear-down as some of those things will transfer to other things we do.

Thanks again,

--ShopShoe
 

nel2lar

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Terry
I've followed many builds but none like yours. You have a very special way of showing and explaining that anyone can follow. The greatest thing we can feel your excitement and your sorrow when thing are not quite right. You are an amazing person both in talent and words. Maybe one day we will meet. I did not follow when the storms were going on, was your place alright with all the bad weather?
Very impressed
Nelson
 
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