Project Summary - Helpful Suggestions
As the radial project comes to a close, there are a lot of statements such as: "Boy, I wish I had done XYZ instead of ABC!" or "This is slick! I'm glad I incorporated that feature!"
For those of you who are considering a project like this, you may want to consider some of these observations... some of them may be aesthetic, others not applicable, still others flat-out stupid or incorrect, but I would be remiss not to mention them. Quite a few deal with Mr Hodgson's design - these are in no way critical of the design, they just reflect my personal thoughts.
The items discussed here are those which deviated from print, were especially onerous, or were exceptional in some fashion. I will add to the list as I have time.
Select from the following:
The Ignition System was far and away the most troublesome aspect of this project. There was no aspect of machining, casting, research, whatever, which caused as much grief, failure, and anxiety as getting a reliable and potent ignition system up and running. I would conservatively estimate that I have 4 months of spare time and many hundreds of dollars invested in parts, components, books, and research, towards the ignition. Much of the initial reading was from Strictly IC magazine, the universal resource for this hobby; all of them touched upon the horrors of the breaker system and recommended Hall IC activation of the ignition. I redesigned the distributor base to accept my Hall IC chip, magnet, and ferrous vane assembly. The mechanism worked flawlessly on the bench, firing a coil and plug for hours with no troubles. Once installed on the engine, however, strange gremlins crept in and caused the tiny Hall chip to blow.
Once the chip was shot, it was a numbing affair to tear down the distributor, free the Hall IC carrier, strip 3 #30 wires and solder them to a new chip, and reassemble the whole thing, only to either A) Run for 5 minutes, then blow again; or B) blow again with the next flip of the prop. My theory was that the high voltage path was blocked with oil-soaked plugs, and the spark, having nowhere else to go, was backtracking through the most delicate part of the TIM4 circuit, namely the Hall IC. Being Mr. electronics at the moment, I decided to optically isolate the chip. OptoIsolation is the total electronic separation of the hall chip from the rest of the circuit. The signal from the chip is sent via a tiny light pulse inside another chip which activates a phototransistor... this in turn is amplified and fires the coil. I designed 3 or 4 circuits which all worked flawlessly, again on the bench only - once installed on the engine, the hall chip seemed to last a bit longer but still blew! Arrrgggh! That wasn't supposed to happen!
All I can postulate is that the distributor has a lot of arcing and other high-voltage wierdness, which induces spikes in the hall chip and cooks it.
|Summary 1: If you decide to use Hall IC sensing for your engine project, design the chip so that it is mounted somewhere other than the inside of the distributor. You can probably mount it in a separate distributor base, but it MUST be entirely shielded from arcing, preferably by a solid aluminum shell grounded to the engine frame. A better spot would be in an accessory drive area, oil-pump shaft, or for inline engines a cam-shaft carry-through would be ideal.|
In a desperate attempt to get a reliable ignition, I canned the complex circuitry and went back to basics. Gone was the Super OptoIsolated TIM5 with a separate N-sized Nicad pack to drive the hall chip. Instead, I used a straight TIM4, with a very basic pair of breaker points to trigger the circuit. (See the TIM4 page for info on how to do this; it's easy!) This combo drove a pretty little motorcycle coil for quite a few very satisfying runs. In the end, vibrations from the engine shook loose a primary winding wire in the coil which shorted to ground and blew up the TIM4 with 20 amps, and sadder still the coil was shot.
I had spent a few months doing very satisfying research on coil winding. Mr Bob Shores wrote a wonderful little book on ignitions and coils. If this stuff is a mystery to you (it was to me), get his book and the truth shall be revealed. I decided I was going to wind my own coils. To this end, I created a CNC coil winding machine. This project was a LOT of fun - I learned a lot about the subject matter. But the 6V coils I wound weren't "punchy" enough to drive this engine. I know they would work on an inline 4, or a farm engine, but a high-speed radial needs a serious ignition system; and my choice of 1/4" - 32 plugs already hampers my spark. In the end, that meant 12 volts, high current, maximum dwell from the 9-lobed cam. For my engine, at least, three volt model systems need not apply. After the motorcycle coil failed, I gave up on any pretense of scale and mounted a universal automobile coil. These are CHEAP, AVAILABLE, and will take abuse. You can find one in any NAPA store.
|Summary 2: Don't design a minimalist ignition; instead, plan for and design a potent ignition with components that are readily available. Once everything is running well, you can always step backwards and lower voltage, try scale coils, etc. Dropping from 12V to 6V on my system will dramatically reduce performance of the engine. Car coils are hard to beat, and cost less than $20.|
The TIM4 was changed; the initial ampification stage was removed, and the TIP42 was replaced with a 15A PNP device available from NTE electronics. This became the TIM-6 and has proven itself over many, many runs. It uses one transistor, and one power resistor. 300 mA runs through the points, which can be made of brass.
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The Distributor was a fairly straighforward affair with the exception of timing. I didn't put a lot of thought into how I was going to adjust timing. The plans didn't call for variable ignition timing; rather, Mr. Hodgson calls for fixed timing with the spark firing at TDC. Most engines run better at higher throttle settings with the spark advanced... typical settings (from my wonderful 1939 book Aircraft Engine Maintenance) are from 20 to 30 degrees (crankshaft) BTDC. With the engine throttled back, I anticipated a retarded spark would improve idling.
What really happened - the engine runs beautifully with the spark at roughly 30 degrees BTDC. In all cases, minor changes of the spark, especially at higher throttle settings, produced markedly different running characteristics, with the best running to be found in a narrow range of adjustment. Once set there, the engine can be throttled back, and will idle beautifully at the same spark timing. At idle, the spark lever can be moved quite liberally with minimal changes to the engine's performance.
|Summary 3: I believe an adjustable timing is very desireable, bordering on essential. By adjustable, I mean one which can be adjusted while the engine is running and then set and forgotten, as idle characteristics relative to spark timing seem to be pretty decent. A fully variable spark system is good too, because it is simply fun to have one more lever to play with! But trying to guess at the best spark with the engine static is simply too chancy... there is too much good information available to the ears with the engine at full throttle while you set the spark timing. So, incorporate an adjustable spark!|
The first set of breakers I made shredded themselves. I made another cam/breaker set, this time of hardened tool steel, which will run forever with a dab of grease. You can generate a better distributor cam profile with a bit of effort. The plans have a straight nonagon cam. Take that same nine-sided polygon, and at each flat plunge in a 5/16" end mill to create a shape that looks more like a flower. Use some abrasives to smooth out the cam, and be sure to harden and temper appropriately.
If you are using a lever attached to the distributor breaker plate to alter timing, send the arm out the side of the distributor, not out the bottom like I did. The bottom-protruding adjusting arm is almost unreachable, very awkward to hook up to any kind of push rod.
The best ignition wires I have found for any model are 18 guage test probe wires. It is hard to describe how good this wire is... it has a very finely stranded core, a thick flexible insulation, and is essentially perfect scale spark plug cable. It is expensive stuff. A 100' spool set me back $50. Trust me, the cost is negligible compared to the benefit. Use this wire for your ignition!
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The Oil System was built very close to print, especially the pump section, which straddles the rear main bearing. The plans called for an aluminum pump body, with the spur gears running on steel shafts. I know aluminum can be a satisfactory bearing surface, but I really wanted the durability of bronze, so I had the choice of pressing in some bronze bushings, or making the entire pump housing of bearing bronze. I chose the latter for simplicity at the expense of weight and cost.
Spur gear pressure pumps are positive displacement pumps which can generate very high pressures. I was a bit concerned about excess pressure, so I engineered a very simple relief valve in the pump body opposite the pressure outlet. The pressure outlet pumps oil directly into the rear main bearing. From there it is channeled and carried throughout the engine forward of the rear bearing.
|Summary 4: A pressure relief valve is a matter of a couple hours of work and will provide peace of mind. Be sure the oil is vented forward of the rear main bearing, otherwise it will be sucked down into the lower cylinders through the intake valves. Set the relief pressure while motoring the engine on the lathe. Use 10 weight oil, or better yet heat some 50 wt straight mineral oil to 170 degrees or so to simulate a hot engine, attach a pressure guage, and set for perhaps 70 PSI.|
The oil drains from the connecting rods and the cylinder walls of the central crankcase area and collects in the sump, which is a small chamber suspended between the lower two cylinders. Two tubes suspend this sump in place... the rear tube extends into the sump and collects oil via the scavenge pump for return to the external oil tank. The front tube is a simple drain tube. The plans call for these two tubes to be screwed into the crankcase, the brass sump inserted onto the tubes, and then soldered to the tubes. This would, of course, render the sump permanently attached to the crankcase, a situation I wasn't thrilled with. My changes to this area included a milled, mostly rectangular aluminum sump with some cooling fins for appearance, and the tubes leading to the sump are split and secured with a pair of scale compression fittings, which are extensively used in the model steam engine hobby. This allows me to remove the sump with ease, and the appearance is improved. I also added a drain screw on the bottom of the sump, which I can open to drain any oil in the sump.
|Summary 5: Make the oil sump detachable by connecting it to the crankcase via miniature 1/4-40 model pipe steam unions. These can be obtained cheaply from Cole's Power Models in Ventura, CA.|
In use, the oil is metered by a needle valve assembly in the fuel/oil tank, external to the engine. By all means build a sensitive and efficient metering valve, because there is a very fine line between too much oil and not enough. With the valve opened too far, excess oil is pumped and cannot be scaveneged quickly enough for removal from the engine. The oil quantity inside the engine, especially in the front crankcase which houses the cam ring, builds quickly until the oil is ejected out of the front main bearing.
|Summary 6: Early in the project, I would make the path which the "used" oil takes to drain into the sump, much wider and free-flowing than the print. The scavenge pump is larger than the pressure pump and will do a fine job, but only if the oil can actually drain quickly enough into the sump. There are holes drilled in both the front main bearing and the crankcase which allow oil from the front of the engine to drain into the middle section and thence into the sump... these I would open considerably. I would also make the forward drain tube from crankcase to sump significantly larger, so the oil can flow downward into the sump for scavenging.|
There was probably nothing more troubling with the oil system than trying to devise a means to tap the pressure pump and measure its output. I wanted an oil-pressure guage!! In a nutshell, it wasn't worth the effort, as the vibrations of the engine are high enough to shake my little pressure guages and so render the readings meaningless. So early in the testing phase I removed the pressure guage and sealed the fitting on the crankcase (the pressure tap) with a cap.
|Summary 7: While my photos on this site have a cute little pressure guage, the reality is that it was very difficult to modify this engine for a pressure pump tap, and the results were not worth the effort. It is easy to tell when the engine is getting enough oil... you can see it (very light smoke while accelerating from idle, especially off the lower cylinders), smell it, and hear it. So unless you must have a pressure guage, I would forego the modification.|
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The cam ring, tappets, push rods, and rockers form a critical portion of the engine. The entire system, of course, is designed to open and close the valves at the proper moment during the 4-stroke cycle. The cam ring was a tricky bit of machining, but the plans and operations sheet detail this quite well and success is almost assured. The only difficult part is that the flanks of the lobes must be ground a bit by hand, as the machining leaves them pretty acute and would not work fresh off the milling machine. Mr. Hodgson calls for filing of the flanks. A really slick method...
|Summary 8: Rather than file the cam flanks, chuck a fresh dremel sanding drum in a drill press. Run a smear of layout blue around the entire periphery of the cam ring, and now lightly sand/grind by hand the cam lobe flanks. Keep the sanding drum away from the base circle, work the cam lobe flanks only. After a little bit of practice (there is a lot of metal to remove) you will develop a nice technique which will properly profile the cam flanks in no time. I recommend making the flanks less aggressive than the print, meaning the slope up and down needs to be a bit more gradual than the print shows. The blue is used to avoid the base circle and cam peaks. Refresh as necessary.|
The material used was 4140 annealed steel. The plans called for a heat-treatment to 40Rc after the grinding and polishing. This was done in a shop furnace, whose purchase I cannot recommend more highly. My furnace has been a tremendous asset to my shop for precision heat treatment. Inspection of the 40Rc cam ring shows no significant wear after many hours of running.
The tappets were made from A2 tool steel. This is a tremendous tool steel, with a great ability to resist wear, and a very high annealing temperature. These too have shown no wear at all. One problem area has occured - occasionally the pushrods shake loose from the spherical cavities machined in both the tappets and the rockers. Be sure you make these a bit deeper than you think might be necessary.
|Summary 9: I made my engine with exposed push rods, built-up of aluminum rod with steel ends for valve clearance adjustment and appearance. If I had the chance again, I would engineer an enclosed push rod tube, and use the print rocker arms for valve clearance adjustment. This calls for some skilled engineering and machining, as the rocker arms form an odd compound angle relative to the pushrods. But I think the benefits in appearance and function make this a wise choice.|
The rocker boxes have been an unqualified success. I was worried about the lack of lubrication in the RB's, and considered inserting a piece of oiled felt on the RB covers to provide lubrication, but unless you are a valve and ring master, enough oil blows by the piston rings to lubricate the top end, and via the valve stems, to the rocker arms themselves.
Occasionally, after some hard running, one of the 18 valves would stick in an open position, with the rocker fluttering helplessly and the push rod off its seat. This is caused by two things - first, I made the valve stem clearance too tight, on the order of .001 or less. I think .002" is a good value, and while this feels like a sloppy fit, under power and pressure the valves will seat better and not stick so much. Secondly, the use of an ashless, synthetic aircraft oil appears to have helped tremendously with stickiness of the valves and carbon deposition.
|Summary 10: Use an ashless dispersent oil for aircraft engines, available at any small airport FBO. While a bit expensive at around $4/quart, two quarts will last a long time, and I believe the benefits are great.|
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The Engine Mount is another important item often overlooked in the planning process. Often, we are eager after months or years of work, to simply get it secured and fired up. The plans have a mount which is more decorative than functional. Rather than create my mounting to print, I decided to engineer my own. I started with a 3/4" square, welded tube steel structure that was very stout, and as the ignition and oil/gas systems developed, simply found a handy atachment point on the tube steel frame and bolted 'em on. The end result was a rather hideous conglomeration of components, with no real similarity to an actual aircraft firewall.
After mounting #1, I decided to re-engineer the mount to incorporate a finer tube, painted, with a more logical layout of the ignition and fuel components. In the end, this looked so similar to mount #1, that I scrapped the darned thing and started over again! This time I decided to forego shortcuts, and put in the deserved effort to create a classy, functional mounting, one which looked good and performed well for frequent running.
|Summary 11: Early in the project, design a mounting which you know will do your creation justice. At the heart, I would recommend a plate aluminum firewall, with as many components as you can cram nestled on the engine side of the firewall. This engine needs at least a tripod support of 3/4" square tube steel. Route cables and wires as much as practical inside aluminum channels, and use cable clamps and ties to get the rat's nest under control. Spark plug wires can be cable-tied to the intake tubing, as well as the portion of the mount which attaches directly to the engine. The engine will blow some oil, so unless you have an exhaust collector ring, anticipate some oily grime to be cleaned from the firewall and mount, and protect components accordingly|
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