Quarter Scale Merlin V-12

[petertha]

Terry, your perseverance through this immaculate engine project & your recent physical hurdles is truly inspirational to me.

 

[Cogsy]

Glad to have you back Terry. No need to rush back into the shop. As I said before, I don't want the build to end too soon as I thoroughly enjoy reading the updates which will obviously stop on completion...

 

[mayhugh1]

Thanks again, all, for your comments. i feel like I'll be back to normal in a few more days.
Dave, I've not yet never tried using tabs on a workpiece. This stick and glue thing just seems easier although it is expensive for multiple parts. I heated the parts in a oven at 190F and while wearing gloves I literally pushed the parts free of the workpiece and rubbed any remaining epoxy off with a popcicle stick. - Terry

 

[mayhugh1]

A panel used to mount the two enclosed ignition modules that were constructed nearly a year ago was machined next. This panel was designed to be bolted to the accessory rails at the rear of the stand immediately below the coolant reservoir. This arrangement protects the susceptible electronics from any potentially oily prop wash, and it allows relatively short and direct connections to the engine. The CDI modules inside the enclosures require 6 volts for operation. Since the current plan is to use a single 12 battery to start and run the engine, a 12V/6V converter will be located elsewhere to provide the required voltage. A copper braid connects the ground planes of the ignition modules to a common point on the rear of the engine. This braid provides the high voltage returns for the spark plugs.

The only machining required on the plate were the cutouts for the Hall sensor connectors and the high voltage and ground wires that run between the ignition modules and the engine. What turned out to be a bit of a hassle, though, was replacing the booted connections on the ends of the high voltage coil wires from the modules. The straight boots I previously made up didn't look quite right in the final assembly, and so I decided to change them to right angle types. I can usually find suitable vacuum fittings in the local auto parts stores to make up just about any ignition or plug boot that I need, but the commonly stocked fittings wouldn't stretch over the large diameter towers that I had machined on the distributor covers. After a couple days of searching, I located some fittings intended for 'ancient' automatic transmission vacuum modulator valves. I was able to cut them down to make a pair of right angle boots that could be reliably and repeatedly installed/uninstalled without coming apart. I've included a couple self-explanatory photos of the boots' construction.

It occurred to me that the Tygon tubing I was using in the coolant loops had a temperature rating of only 145F which was much too low - especially for the runs exiting the heads at the front of the engine. I found other transparent and translucent hoses available with higher temperature ratings, but they were typically sold only in large and expensive rolls. I eventually found some short (12") lengths of reinforced translucent silicone tubing on Amazon, of all places. This tubing has a maximum service temperature rating of 400F and, from a distance, looks a bit like braided metal hose.

I also machined the cap for the coolant reservoir. I drilled a small vent hole in its center which I'll most likely plug later. I haven't yet decided whether I'll allow pressure to build-up in the coolant system since I'm a little concerned about all those o-ring seals between the cylinder blocks and heads holding pressure. I still need to come up with a drain for the coolant system, though, before declaring it finished.

Slowly, but surely, progress is returning. - Terry

 

[nel2lar]

Terry
Very nice and looks like completion is in sight. You have done an amazing job and you documentation is super. I enjoy seeing the different ways you use to complete a part, never thought about epoxy. It is always nice to have the tool in your box for the job at hand.
Thank you and much comfort in your healing.
Nelson

 

[editor123]

Just a word of caution about using dual ignition systems. We tried it on one of the supercharged V-8s and found that electrical crosstalk fired both systems at random times, even using dual distributors.
The other problem we found was that the CDI systems had a built-in timing advance but it was set up for a 2-cylinder engine. With all the pulses coming in for a V-8, full advance was reached before the engine got started. This, of course, created a 'small' problem.

 

[Buchanan]

Terry. That is looking wonderful. I also really like your choice of colors.

It makes it so much more like a full size. The casting look that you have on your machined components like the top of the tank is awesome. I have a daily check to see if there is any more progress. Lovely.

Buchanan

 

[mayhugh1]

The coolant reservoir was mounted at the top of the accessory rails so its coolant level could be set slightly higher than the coolant level in the heads. This will allow the engine to be filled from the reservoir, although any air trapped at the tops of the heads may require some time to work its way out. The coolant pump at the bottom of the engine will take in coolant from the bottom of the reservoir and pump it up into the heads. With reservoir level at the top of the engine, the pump will see approximately the same head pressure at both its input and output ports. With minimum pressure across the pump there will hopefully be a healthy coolant flow through the engine. The return hoses from the radiator will return coolant to the reservoir through fittings located just above the reservoir's coolant level.

The oil tank will be mounted on the front of the oil panel just below the ignition panel. This location will place the pressure pump's input below most of the oil column inside the tank which will aid oil flow (not really necessary) and help prime the pump. A shut-off valve will be added to prevent the tank from draining its contents into the engine during storage. I've learned from my radial builds that this can happen if there is even the slightest leak through the pump. The output from the scavenger pump, on the other hand, will return oil to the tank through a fitting located above the tank's oil level making drain-back through it a non-issue. The return-to-tank line from the low pressure regulator will return excess oil to the tank through a similarly elevated fitting. The bottom of the tank will also have a magnetic drain plug.

The panel to which the oil tank will be mounted contains two oil pressure gages. The high pressure gage will be connected to the pressure pump whose output has been tentatively limited to 15 psi by an adjustment on the high pressure regulator. The low pressure gage will be attached to the low pressure regulator whose output has been tentatively limited to 6 psi.

The space available on the oil panel directly behind the oil tank was used to mount a pair of 12V/6V converters to generate the required voltages for the ignition modules. These took the form of very inexpensive buck converters available in six packs from Amazon:
https://www.amazon.com/gp/product/B01GJ0SC2C/?tag=skimlinks_replacement-20

I chose to power each CDI from its own converter even though the output of a single converter could have supplied more than enough current for both CDIs. A cavity was milled into the panel to completely enclose the converters. It was probably over-kill, but they operate at 400 KHz, and so the shielding provided by the enclosure should improve the electrical system's noise immunity. The enclosure will also protect the electronics from engine soot and oil. After installation, the converters were tested by running them with 7W loads for several hours. There was nothing special about the size of the load - I just happened to have some five ohm power resistors handy.

The eBay oil pressure gages I used were something of a hassle to mount because they weren't designed for the typical U-brackets used on most other panel gages. I bored close-fitting openings through the one inch thick panel and, after testing the gages, I secured them in the openings with a bit of silicone. While I was at it, I also machined a decorative bezel on the panel around each face.

The 'generator' cover for the 12V/6V converters includes a pair of pressed-in light pipes so the blue indicator leds on their circuit boards could be made visible through the cover. The light pipes were turned from clear acrylic rod and polished with automotive paint buffing compound. An extra-fine oil-based Posca paint pen was used to fill in the machined engravings on the finally painted panel and generator cover. Blade terminals were JB Welded into the top and bottom sides of the panel in order to provide tidy electrical connections to the converters later.

The next step is to machine and install the oil tank. Since it appears that it will be competing with the throttle for space, I'll first need to work out the details of the throttle linkage and control. - Terry

 

[Ghosty]

Terry, more great work, a thing with the power supplies, I would fit them with some heat sinks, I use the same voltage regulators and the IC can get very hot. I use little 10x10mm heat sinks and 3M double sided heat sink tape.

Cheers
Andrew

 

[mayhugh1]

Ghosty,
Thanks for the warning. When I ran my four hour 7 watt output test (12V in, 6V out) I checked all three semiconductors and none were more than barely warm to the touch. I'll likely use a third one for the fuel pump which I may not be able to run as conservatively, and so I may need a heat sink for it. - Terry

 

[mayhugh1]

The oil tank was machined separately from the oil panel and bolted to its front. I limited its volume to something under a third of a quart to avoid overfilling the crankcase with oil should the scavenger pump fail to keep pace with the pressure pump. This is a well known issue with the Hodgson radials which also are dry sump engines. A common workaround for those has been to carefully regulate the oil flow into the pressure pump with a custom drip feeder similar to the ones found on medical iv bags.

The tank's form factor had to be massaged in order to avoid infringing upon the area required by the throttle linkage which happens to want to occupy the same space. I haven't yet decided whether I'll machine a quadrant lever or just make a simple push/pull control for the throttle. I cobbled up a test linkage, though, to make sure that I left room for either. The tank's final dimensions worked out to require a four inch deep cavity if machined from a single workpiece. Since this would have required an extra long end mill that I didn't have, I fabricated the tank from two chunks of aluminum to end up with a three inch deep body and a 1-3/8" high cover. The two pieces were joined together with a gasket and a handful of screws.

A sight gage and a provision for a screw-on filler cap were machined into the body and cover, respectively. The sight gage is identical to the one fabricated for the coolant tank and was added to the starboard side of the oil tank which makes it rather difficult to see. This was done so I could access all the controls and gages while standing to the rear starboard side of the engine. I'll likely run the engine while it's secured to a long metal workbench that's in my backyard, and I didn't want to create an unnecessary need to cross over the plane of the engine's spinning prop.

The two previously installed oil panel gages were plumbed to their pressure sources using 1/8" copper tubing. The lines between the oil tank and the two oil pumps in the lower crankcase will be later run using 5/32" copper tubing. The compression fitting for the pressure pump line was drilled/tapped into the lower front side of the tank, and a similar fitting for the output of the scavenger pump was machined into the top cover. The return-to-tank hose from the low pressure regulator will also be plumbed into the top cover through a hose barb. A ball valve was inserted in the line between the tank and the pressure pump. Its main purpose is to shut off the oil supply during storage, but it could also provide some crude flow control if needed. The oil filler cap will be vented so that pressure build-up in the tank created by crankcase gasses being pumped into it by the scavenger pump are relieved. I also added a drain plug at the bottom of the tank.

I'm a little embarrassed to admit that I went so overboard with the design of the tank's cover. In the end, a simple block top would probably have looked better than the over-cooked piece that I came up with. It got away from me and became one of those parts that I made like I did just because I could and eventually soaked up so much time and effort that I just couldn't scrap it.

I'll likely work on the throttle linkage next before finally installing the oil tank. The throttle will be much easier to deal with before installing the oil tank lines. - Terry

 

[SilverSanJuan]

Absolutely stunning workmanship! :thumbup:

 

[Ghosty]

Terry, Looking at your last photo in post #731, you need some covering to cover the pickup wiring, it just looks out of place. I use this https://hobbyking.com/en_us/wire-mesh-guard-black-6mm-5m.html on my electric stuff.
I listed the black, I used the red in my boat as all the drive and engine are red.
The engine and acc look great, can't wait to see it finished.
Hope this helps.

Cheers
Andrew

 

[mayhugh1]

Thanks, Andrew, that's a goid idea. I just ordered some.
Terry

 

[mayhugh1]

Both the Spitfire and the P-51 Mustang used a quadrant throttle located on the wall of the cockpit within easy reach of the pilot's left hand. Although a simple push control would probably be more practical on the rear panel of the Quarter Scale's running stand, I decided to try to use a quadrant control there also. Since I'd already designed one that's been working well on my two radials, I made a third for the Merlin.

The quadrant's base was machined from black Delrin, and the lever was cut from 3/16" aluminum plate. Delrin provides a slick bearing surface for the lever, and so the base was designed so its walls can pinch the flat sides of the lever to provide some rotational friction. A locking nut, embedded in one end of the base into which a SHCS used as the throttle shaft is threaded, allows the amount of friction to be adjusted.

For no apparent reason, when I built them, I set up the throttles on both of my radials to increase the engines' rpm when the levers were pulled back. However, I recently discovered from online photos of both the Spitfire and Mustang cockpits that convention seems to be to decrease the engines' rpm when the throttles are pulled back. I opted to not correct my error at this point, though, so that all my engines will operate identically even if non-conventionally.

After playing around with a suitable mounting location for the throttle, I realized that the best place for the engine's tachometer would be on the control panel just above the throttle. Even though I had a really frustrating experience with the eBay aircraft tach that I purchased during my 18 cylinder radial build, I ordered yet another one for this engine. It's a totally different model, supposedly working, and again it comes from an overseas salvager. Unfortunately, the area on the running stand immediately behind the tach is where I'm planning to mount the fuel tank, and so I need to know the depth required behind the panel for the tach. Some aircraft panel gages can be surprisingly deep, and the seller with whom I'm dealing doesn't seem to waste time answering emails after receiving his payment. So, I decided to wait until I have the tach in my hands before doing any more work on the throttle.

In order to continue making progress, I switched over to working on the fuel pump that will be used to drive a recirculating fuel loop between the fuel tank and the carburetor bowl. Basically, I just repackaged the main components of an RC filler pump that I purchased from a local hobby shop. Inside the unit I used, a composite pump is driven by a 6V-12V electric motor through an Oldham-style coupler. So, I just transferred the motor and pump into my own custom machined aluminum housing. The manufacturer recommends using only alcohol-based fuels with their stock unit, but I've been using their pumps on my gas powered model engines for several years now with no issues.

I also cleaned up a few loose ends that had accumulated during the past few weeks. Since the oil panel was now complete and finally installed, I was able to wire the ignition modules to the 6V generators and plumb the copper lines between oil tank and the oil pumps. I also machined the oil tank's filler cap which I made identical to the one on the coolant tank. I'm still procrastinating, though, over the drain valve that's still needed at the bottom of the coolant system.

The tracking info for the tach shows that it should be delivered in two days, and so my next step will likely be to resume work on the throttle. - Terry

 

[Ken I]

The coolant reservoir was mounted at the top of the accessory rails so its coolant level could be set slightly higher than the coolant level in the heads. This will allow the engine to be filled from the reservoir, although any air trapped at the tops of the heads may require some time to work its way out.

Terry - you might like to try this automotive trick - vacuum the coolant system from anywhere (typically the filler cap) - once it has fully pulled down a vacuum (also proving there are no leaks) you fill via the drain valve (or anywhere else you might prefer) - the vacuum "sucks" all the coolant into all the voids regardless of location and complexity.

This is often how automotive assembly plants fill brake systems and coolant systems without having to bleed them.

You might also need to degas your coolant before "sucking" it in - just to eliminate any dissolved gasses that might froth up.

I know a couple of engines that can be damaged if these "voids" are not bled before use - normally such engines have a bleed valve at the highest point - ignore it at your peril when refilling the engine.

FYI

Regards - Ken

 

[mayhugh1]

Ken,
Good tip... thanks.
Terry

 

[IceFyre13th]

Just outside on a break from work......our place is next to an airport full of older aircraft.

As I am sitting there a MkVIIII Spitfire takes off, I have seen plenty sitting static at museums and such, but never before heard one flying........oh my god, I want that engine in my car......its sounds beautiful!!!!!!

 

[ddmckee54]

Used to be that hearing the sound of these engines was if not commonplace, at least it wasn't a rarity. Now days most of us have to pay for the pleasure of hearing a Merlin, an Allison, or even a P&W radial. IceFyre you are one lucky SOB.

Don

 

[mayhugh1]

I received the tachometer along with a hand-written sketch from its eBay seller describing how it had previously been hooked up. I watched eBay for months waiting for a tach to become available with specs similar to this one. Its description said that it had come off a 6 cylinder aircraft and used a wire-wrap type impulse trigger generated from the coil wire on the engine's magneto. Aircraft tachs come in a wide variety of types, both electrical and mechanical, and the jargon associated with them can make your face hurt. As described, the tach I purchased would have easily dropped into the Merlin with its dual magnetos and twelve cylinders. Generating a trigger by wrapping a few turns of insulated wire around the unshielded coil wire of one of its magnetos would have been trivial. And, it would have provided the same input needed to display the Merlin's crankshaft rpm as it did in the original 6 cylinder single-magneto application.

The first thing I did was to apply 12V to the tach's power pins. This caused the needle to immediately move to zero - a very good sign. The issue that I ran into, though, was that the note now described its input as having come from a cabled magnetic sensor (not included) that had been screwed into the vent port of a Bendix magneto. I suspected the sensor was probably just a Hall device, though, since the sketch showed its cable had three wires: +V, Gnd, and Signal. Fortunately, the tach outputs that I had brought outside my ignition modules were Hall sensor outputs although I had been looking forward to using a much simpler wire-wrap trigger.

I made up a six magnet trigger disk and inserted it into a drill chuck in the spindle of my Tormach. I also breadboarded some test circuitry that duplicated the front-end electronics of one of my ignition modules including its Hall sensor which I connected to the sensor input terminals on the tach. With the disk spinning at 400 rpm, though, the tach read an unexpected 1600 rpm instead of 800 rpm.

I opened up the tach's housing and discovered it contained the CS8190 air core coil driver IC that I had used to make the tach driver for my 18 cylinder radial.

http://www.homemodelenginemachinist.com/showthread.php?t=21601&page=31

Since I was pretty familiar with the circuitry associated with that particular IC, I was comfortable with adjusting the calibration pot to change the output reading. After turning it all the way down to zero, though, the tach still read some 1200 rpm. There was a fixed resistor in series with the pot, and so I soldered a second one across it in order to reduce its net value to something less than half. This modification then allowed me to dial the reading to exactly 800 rpm.

I traced out the trivial signal conditioning circuitry connected to the chip's input pin and concluded it would not likely have handled the ac waveform from a wire-wrapped trigger pulse. Unless my thinking is screwed up, it's hard to see how this tach actualły came from a 6 cylinder engine unless it was a two stroke. There's a lot of additional functionality on the tach's circuit board including a second calibration pot, but I didn't bother trying to figure out what it might be. Curiously, the front of the tach face contains 'Made in USA,' but there's no other markings anywhere on it to identify the manufacturer.

The rear of the tach will ultimately end up in a rather nasty area on the running stand just below the oil tank drain plug and probably near the filler for the fuel tank. Its housing isn't all that well sealed, and so a protective cover for its rear will need to be fabricated. All its electrical connections are made through a DB-9 connector on the rear of its housing, and so I made up a potted assembly covering the wire connections on its mating cable connector. - Terry