Quarter Scale Merlin V-12

[josodl1953]

This Merlin was on display on the Model Engineering Exhibition in London, way back in 1982. It seemed to me to be scale 1:5 or even smaller. It was an amazing piece of engineering.

Jos

 

[mayhugh1]

I delayed the starter wiring as long as I could in order to avoid accumulating a lot of unnecessary play time on what might be a limited life starter. However, there's something really neat about an electric starter on a model engine, and I ended up running down two 12 Amp-Hr batteries while 'testing' it. I could be using a battery with more appropriate capacity (and terminals), but the ones I have might be a little less stressful on the engine's starting system.

Afterwards, I removed the manual starter shaft and bearing housing so I could inspect the starter bevel gears which are all visible from behind the cover. I didn't yet see anything suspicious about the teeth - just the expected wear patterns on their phosphate coating. The patterns looked normal, and so I just re-coated the gears with gray moly grease and replaced the cover. I also filled the oil tank so any further cranking could begin pumping oil throughout the engine. I allowed the engine to sit for several hours with the oil tank's ball valve open so gravity could help prime the pressure pump. The copper oil distribution lines were previously filled with oil when they were installed, and so all that was left to do was to pre-fill the oil filter.

The good news was that the high pressure oil gage was now registering a couple psi while cranking the starter. The bad news was that I had a pretty significant oil leak on the underside of the engine. Even though it's been in plain sight ever since the lower crankcase was assembled, this leak has been difficult to pinpoint because of its totally unexpected location. Oil has been leaking from the bottom machined surface of the scavenger pump because of an imperfect fit of one of the bronze bearings pressed into the aluminum pump housing. The bearings for the pump shafts were pressed into the housing's workpiece before any machining was done. The floor of the housing ended up rather thin after machining, and this evidently uncovered latent damage created by a poor pressing operation. I had been assuming all along that the oil on the underside of the pump had been coming from far more likely places above it.

My fix was to JB Weld an aluminum plate to the machined bottom surface of the pump housing. The outer contour of the plate was machined to match the pump housing so it would completely cover the bottom surface of the pump and look as though it belonged there. After installing the plate, the engine's underside finally remained dry.

I then completed the machining of the rear and front plates that make up the electrical panel. Cutouts for a panel voltmeter and four toggle switches were machined into the rear plate. The voltmeter monitors the battery voltage; and, in addition to a main power cutoff, the switches apply power to the magnetos, fuel pump, and radiator fans. The outputs of the switches on the rear plate connect to their loads through blade terminals on the front plate. A pair of connectorized cables between the front and rear plates compete the circuits between the two while allowing the panels to be easily separated for maintenance.

The fuel pump was mounted to the front side of the front panel. A buck converter located on the inside of the rear panel supplies voltage to the pump motor so the fuel flow can be regulated. A screwdriver adjustment, accessible on the rear panel, controls this voltage and the fuel flow.

Including the tach, two CDI's, and three buck converters, there's a lot of electronics associated with the electrical panel that could be damaged by an inadvertently reverse-connected high capacity battery. Therefore, I added a protective diode in series with the main power switch. I was able to come up with a Schottky power diode that had only a .3V forward drop at 3 amps of current.

During testing, I ran into a problem with the starter wiring. When I wired in the starter relay I had forgotten that one side of the motor's brushes was grounded to the motor housing. I managed to wire the relay into the wrong side of the motor, and all was well until the negative side of the battery's accessory loads were connected to the chassis. This inadvertently bypassed the starter relay and caused the starter to be energized as soon as the battery was connected to the terminal block. Pretty much all the starter wiring had to be re-done to correct the problem.

The build's final steps are soon coming up and will include the fuel tank and a system to fill and drain it. -Terry

 

[dairwin]

Terry - interesting post. I do not have knowledge of this engine or castings, but the real engine also leaks from various points including the scavenge pump assembly. The oil finds its way to sit on top of the water pump impeller housing.

Regarding your wiring harness; do you have a wiring diagram? Will you use an oil pressure cutout switch as a safety?

David.

 

[tornitore45]

Love the vintage voltmeter, all analog gauges and classic look switches. Digital gauges would be so out of place.

Terry, slow down we are out of state till October and would hate to miss the premier.

 

[ddmckee54]

OOOOOOHHH, tied wiring bundles!:thumbup: You ARE going old school on this one, very appropriate.

Don

 

[Buchanan]

Terry , I really like your period engraving on your panel, not to mention everything else that you have done. It is beautifull.

 

[mayhugh1]

Terry - interesting post. I do not have knowledge of this engine or castings, but the real engine also leaks from various points including the scavenge pump assembly. The oil finds its way to sit on top of the water pump impeller housing.

Regarding your wiring harness; do you have a wiring diagram? Will you use an oil pressure cutout switch as a safety?

David.

David,
I didn't originally include a wiring diagram because I didn't think it would be of interest. Here is a sketch of what I did, though. No, I'm not planning for an oil pressure switch. - Terry
p.s. Did you work on these engines in a previous life?

 

[dairwin]

Terry, I work on them in this life, but only for ground running and just as a hobby. I have a small collection of RR Merlins which I research and rebuild, with their correct propellers. I am interested in anything Merlin, hence my appreciation of those on this forum who endeavour to build working examples.

https://www.youtube.com/user/dairwin2000

David.

 

[dsage]

OMG. What a video. And two engines at once if you watch one of the other videos. That must be something to hear / see in person. Fantastic enough just in video !!

Thanks for the link.

Great work Terry and David.

 

[dairwin]

We try to run two engines together; the resonance of two Merlins is certainly a memorable. However, we can align with other engines to produce the same harmonic. This year I have also displayed my engine with a Bristol Hercules radial.

I am part of a small group of fellows around the UK who own and restore WW2 aero engines for ground running. We attend large public shows on a non profit basis, so that enthusiasts of all types can get close access to the engines after they are run and ask questions, etc.

It's great fun and I get to meet many veterans and others with an interesting story to tell.

David

 

[dairwin]

Terry - thanks for the circuit diagram. I was interested to see how you powered the various loads, and note you use a power relay. Are you intending to use fuse protection?

There is some similarity with my harness, below. I also have a charge circuit side, as the engine drives an original generator which charges through a carbon pile voltage regulator. A bit unnecessary, but completes the electrical loop back to the batteries.

David

View attachment DAI Sheet 1.pdf

View attachment DAI Sheet 2.pdf

 

[mayhugh1]

It looks as though the mounting hardware for the fuel tank will be the last machined parts required for this build. I didn't have any suitable stock on hand to use for turning a cylindrical metal tank, and so I used the same 10 oz high density polyethylene tank that I used on both of my radials. These particular (Sullivan) tanks are usually available from my local hobby store. I like using them on model engines that consume lots of fuel because their translucence provides continual feedback on the amount of remaining fuel. The downside is that they look somewhat RC-ish. On my 18 cylinder radial I hid the tank inside an aluminum shell, but the backlighted slot that I added to retain some visibility didn't work as well in bright sunlight as I had hoped. For the Quarter Scale, I designed a set of banded support brackets to give the tank an industrial look.

These tanks come with a few accessories that were designed for use with glow fuels. For my application they had to be replaced with gasoline compatible parts. One of these was a very flexible silicone pick-up hose terminated with a 'klunk'. A klunk is just a heavy metal nozzle that keeps the end of the pick-up hose submerged in fuel during flight. The klunk wasn't heavy enough to ensure the stiffer gasoline-compatible Tygon pick-up tube would always remain in contact with the bottom of the tank. So, before inserting it into the tank I soldered the three metal tubes together in a subassembly with the pick-up angled downward. This kept the pick-up, return, and vent tubes properly oriented even while the tight-fitting external hoses were being twisted onto them. The stock silicone stopper used to seal the tubes to the tank was also replaced with a gasoline compatible version.

Another RC fuel-related component that I like to use is a filler valve designed to remotely fill the tank without disturbing the metal tubes sealed into the stopper at its end. This valve has a FILL port that's accessible at the end of a manually extendable o-ring'd nozzle. In its RUN position the valve is simply a pass-thru for the tank's output line.

It's important to provide a way to drain the fuel system for storage since pump gas has a short shelf life. This is complicated by the fact that the fuel tank is mounted near the bottom of the Quarter Scale's eighty pound assembly. Fortunately, the electric fuel pump can assist with this. The heights of the various components in the fuel loop were chosen so the fuel in the carb bowl will drain back into the tank when the fuel pump is shut off. In order to drain the tank, however, the return hose from the bowl must be pulled from the fuel tank and directed into an external container. The pump is then run until the system is dry. In order to avoid constantly messing with the tank's metal return tube, an alternative and more robust return port was mounted on a bracket next to the tank.

Finally, an inline fuel filter was inserted in the input line going to the carburetor bowl. This line also contains a .018" diameter restrictor to reduce the turbulence inside the carb bowl. The size of this restriction was empirically determined several months ago during the design and initial testing of the fuel loop. Once all the fuel components were installed, the tank was filled, the pump control voltage optimized, and the system carefully checked for leaks.

With the fuel loop finished and running, it occurred to me that all I had to do was bump the starter switch to see if the engine would 'pop.' The stand was sticking off the edge of my messy and overcrowded workbench, the propeller wasn't installed, coolant hadn't yet been added, and the carb's settings hadn't been touched since taking it out of its package. Against my better judgement, though, I couldn't resist pressing the starter switch. I was startled when the engine fired right up and ran. All I recall now is thinking how great it sounded, but in all honesty I was too dumbfounded to appreciate what was going on. After three or four seconds of hot exhaust in my face that ended up blistering a lip, I spotted the oil tank valve which I had forgotten to open. I hit the power switch to shut everything down, but I'm sure I kept on smiling. The next thing I did was engrave a warning placard about the oil valve which I bolted to the engine stand next to the starter switch.

I finished off the valence on the rear control panel, but I have a few more cosmetic ends to tie up before focussing on running the engine. If I don't tend to them now, I'll be too busy playing with it, and they'll never get done. Then, they'll end up being the only things I see later on when I look back over the finished project. I also need to round up a camera for a video. - Terry

 

[kvom]

Magic moment is fast approaching.

 

[Buchanan]

So you started it,th_confused0052 And you only give us a 3 line description of it running! Congratulations!

Buchanan

 

[michael-au]

Will be nice to see the video of it running when you make one

Fantastic workmanship

 

[Charles Lamont]

I'm not moving out of this chair.

 

[dairwin]

Terry - the return line from the carb to the tank; is that a pressure relief return from the pump or a ullage drain?

David

 

[prophub]

Terry,
Awesome work! Glad to hear it fired right up. Add me to the list of people waiting for the video! I've been following this build and your radial builds since I'd like to build the radial as my skills progress. You should be able to drain the tank from the fill valve. To fill the tank you could use something like this http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXJGS5&P=7#mults by rotating the handle forwards and to take fuel out, just rotate the handle backwards. It would save some wear and tear on the engines fuel pump.
Shawn

 

[ShopShoe]

Slight OT

For those following this, Jay Leno recently posted a video of demonstration Merlin (Packard) that he has acquired. He does a pretty good walk-around of its features and then gets it running. Whether you love or hate Jay Leno, the engine is pretty well described. It shows how Terry’s model is so incredible to incorporate so many of the things that make a Merlin a Merlin.

[ame]https://www.youtube.com/watch?v=GYcKdK7hmEo[/ame]

BTW, Terry, I like your “dashboard” better than what Jay has.

—ShopShoe

 

[ICEpeter]

Hello Terry,
Congratulation on your success. You are very close now. At some point in your thread there was a discussion regarding the cooling fluid to be used in your Merlin engine and various possibilities were suggested.

One possibility that I did not notice in the discussion is the possible use of a low temperature heat transfer oil and I wonder whether it might be possible and could be suitable for your Merlin if the Merlin heat load matches the heat transfer capabilities of a heat transfer oil. Don't know much about heat transfer capacity etc. of low temperature heat transfer oils but they are extensively used in food processing applications for example.

Peter J.