Another Edwards 5 Radial

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Apr 22, 2020
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Weirsdale, Florida
I did not plan on building this engine as it is designed for an RC airplane and I never got into that hobby. It not something you can just set on the counter and start it up for a while to watch it run like I do with my other engines. It is recommended to use at least a 20" propeller and produces some serious horse power.
I ran across a set of plans for an Edwards 5 radial engine and was just looking through the pages when the master rod drawing caught my eye. It looked like an interesting part and I thought I will make just the rod for a conversation piece that's all. Well it turned out very nice almost excellent but it didn't demonstrate how the other rods were connected and the path that they followed.
The other four rods were all identical and a rather easy build so I decided to make them but now I had to make the link pins to hold the rods on. Well the link pins are held in position by the link pin retainer plate that captures the pins and hold everything together. Not exactly there is a C clip that holds the retainer plate in place but that clips on to the crank pin that is pressed into the crank shaft arm and there is a needle bearing in the master rod to match the crank pin. I might as well make the crankshaft too so I can attach the crank pin to something and really demonstrate how a radial engine works.

So now you see how all this got started and just keeps going.

I have now decided to try my hand at building this complex model engine and I will give it my best shot.

The crank shaft also has a counter weight that bolts onto the crank throw.

I will be following the plans for the most part but I will be using the materials I have on hand as much as possible.

Thanks for looking
Are you going to stay with glow ignition or convert it to spark ignition? I plan on building one myself as soon as I figure out how to go with spark ignition and gas.
I will start with this 4.5 diameter aluminum round I had in the drawer for the main housing. I turned it down to 4" and started hollowing it out on the lathe.

There was a big pile of chips in the waste basket but after some time this appeared.

Now it goes over to the mill for many more operations.
I mounted the housing on the rotary table with a spoil board underneath so I could drill some holes all the way through and it will be a spacer for future operations..

I spot drilled the ten holes around the outer flange but as you can see there are 14 holes. The four with the black marker are in the wrong place. I kept reading the wrong numbers on the print but no foul as none of them will cause any problems in fact two will be milled away later and the other two will be covered by the rear plate.
There is a valley about an inch wide and a 1/4" deep in the bottom of the housing. At first I thought I would cut it while it was on the lathe but I had second thoughts. I decided to do the valley on the mill using the rotary table and it turned out much easier and more precise than I could have done it on the lathe.

Thanks for looking
It looked like an interesting part and I thought I will make just the rod for a conversation piece that's all.

So now you see how all this got started and just keeps going.
congratulations on getting started !

I did something similar once, wondering how four valves could be fit into one cylinder head so I tried a few layouts in some scrape metal and found some dimensions that seemed to work, and then a few years later ended up with a running engine, so yes it "just keeps going" !!! (one of these days I'll have to get back to the engine with only two valves per cylinder that I was working on at the time I started "wondering" !!!)

looking forward to more of your posts !

Glad to have you along Pete you are so right.

The needle bearing came in for the master rod and it even fit. Now you can see how these pieces go together.

I flipped the rotary table up to vertical and using a milling head cut the flats on the housing.
Because the milling head does not cut to the outside edge of the head I had put a sacrificial piece of wood behind the housing so I could make the cut all the way across the face. There are 5 flats for the cylinders and 5 smaller flats for the mounting brackets.
This went remarkably well with no chatter.

A few views so far.



So far I have mostly stayed with the plans except for the rear crankshaft bearing. (32 X 12 X 10)
I had a bearing that was a little smaller on the OD and 0.312 in width. The bore was 0.500 and since I started with a 1/2" drill rod for the crankshaft I used my bearing. Being narrower I increased the thickness of the bearing lip in the housing in order for things to line up again.
Many more holes to drill and tap but not today.

Thanks for looking
Boring, drilling, and taping day.

I used the boring head in the mill for the cylinder holes.
Then drilled and taped 10 holes of 4-40 size for the rear plate screws.
And 20 holes drilled and taped 6-32 for the cylinder mounting screws.

No drill bits or taps were hurt making this part.

Another view with it all shined up.

I am very happy how this housing turned out. A major step forward.

What do you think?

Thanks for looking
The last couple of days I have been working on the cylinders.


I am not a fan of long bolts that hold both the head and cylinders on to the engine so I modified the cylinder flange so I can bolt them on separately. I will be using only 4 bolts for the cylinder mounting and 4 bolts for the heads. The plans call for 5 long bolts that are not evenly spaced but I think 4 will be fine. You can see where I drilled the holes for the flange and holes for the heads but I have not taped them yet.


The cylinders are ready for the cooling fins but not today.

Thanks for looking
A remark on spark ignition on this engine. It seems that the original engine made by Forest Edwards himself ran on petrol ( or gasoline in the U.S way of speaking). See the picture below .There is a tubular extension on the rear that seems to be the distributor. The drawings of the engine have been revised by Robert Sigler and - seemingly- simplified. The original Edwards had , for instance, offset cam followers whereas the Sigler version has them in line. I suppose the the simplification also involved omitting the distributor.
edwards op een kist 1.jpg

The plane itself , by the way, is a Polikarpov biplane, also made by Forest Edwards. He was clearly a very talented model engineer.

I did find the technical information for a five cylinder ignition module from Rcexl.
It is a little pricey but when I get to that point I will probably use one of those.
Thanks for the information.

I don't have much luck slotting on the lathe with the cutoff blade to make the cooling fins. I can only get so deep and then it grabs and the excitement and bad language begins.
I have a slitting saw that is 0.060 thick and if I clamp the cylinders in the rotary table on the mill it may work better for me.
I will give it a try.
It was not a piece of cake and it took over 4 hours but they turned out quite nice.

I was so relieved it worked I couldn't help but bolt them on just to see how it would look.
And the other side.

I gave everything a good polish as well.
Now the cylinder head fins don't seem so daunting.

Thanks for looking
Josodl, I am not sure when you suggest this is a spark ignition engine. The metalclad leads to a metal clad spark plug seem all wrong to me electrically. And I can't see what could be a carburettor on the inlet pipework...?
But if the distributor is actually a cylindrical fuel injection pump, then what is feeding the engine is not sparks, but high pressure fule hoses with steel braid, and the metal part in the cylinder head is the diesel injector.... Maybe I don't know enough to ask the right questions, but can you explain further, please~?

Ray, Re: Your comment of "Through bolts" versus "flange bolts" for assembly.
I hope this helps anyone who wants to understand why such options are selected by designers:
Usually, in order to accomodate the differential expansion of cylinders (that get HOT!), heads (especially aluminium that expand a lot), and head gaskets (often pressed steel or copper - which do not "spring" to maintain sealing pressure) versus the temperature and expansion of the head bolts, it is necessary to have long head bolts so that the "tension" is maintained close to constant between "very cold in a very cold place", and "Hot in a hot place" (Production engines are usually specified to start at -30deg. C and run up to oil/coolant temperatures of 110deg.C). This "tension" in the head bolts varies too much with very short head bolts. So to get long bolts, and also to minimise distortion of bores from bolts screwing into engine blocks, the manufacturers often find it necessary to run the head bolts right through to the crankcase, to maximise the length and retain head gasket compression even through the wide temperature range experienced or specified.
In a model, where all the dimensions are much smaller, and usually model engines do NOT experience such wide operating temperatures, the longer bolts have "no meaning" so flange mounting of cylinders, and head bolts into the block are adequate. Hope yours works satisfactorily!
When I was in the army, back in the seventies, I was a truck driver and these trucks had petrol engines with a watertight ignition system.
The HT cables looked very much like the ones on the Edwards.
Bu the following will clear things up:

originele  edwards.jpg

Thanks for that picture Jos it clears some thing up for me.

The plans call for a Boston Gear G677 internal ring gear that has 48 teeth, 24 pitch, 14.5 pressure angle. I did find that gear for sale at several different places but with a wait time of 4 to 6 weeks and a price of $150 and up. Now I'm not a cheap skate but I just can't my self paying that much for a brass gear that is 2 1/2" in diameter and a 1/4" thick.
I have gear cutter sets in 16dp, mod 1, and mod 0.7 so I should be able to come up with some combination that will work.
The teeth on mod1 cutters are the closest to 24 pitch so this is what I came up with.
Internal ring gear 52 teeth with an outside diameter 2 1/2".
One spur gear 13 teeth so this gives me a 4 to 1 ratio.
Two spur gears 19 teeth each with one behind the 13T gear on a common shaft and one on the crank shaft.
Now this setup is not exactly perfect but it is within 0.020 total small. I will add a little to each of the 19 tooth gears outside diameter to make up the difference.
I will make a test plate where I can install all the gears to see and correct any issues before installing on the engine.
I had a cad drawing of the ring gear and cut it out on the CNC mill. The final pass was with a 0.03125 end mill at a cut depth of 0.200.
It is a beautiful piece of brass and it should work fine.
I hope to cut the other gears tomorrow using the gear cutters on the manual mill.

Thanks for looking
Making a 19 tooth gear for the crankshaft.

These are the parts that makeup the valve timing for the engine.
The 13 tooth and the thin 19 tooth gears are pressed on to the Oilite bushing. The big 19 tooth gear goes on the crankshaft.

This is the test plate I made up to check the gear spacing. The ring gear rides in a slot and the large gear is in the center where the crankshaft would be. I drilled 4 holes of different distances from the center to see which one would work the best for the idler gears. It turned out that the 0.750 from the center was the best fit. Now I know where to drill the hole for the idler gears on the main housing.

Milled a 1/8" key way in the crankshaft and bored the crankshaft gear to size. Then filed a recess for the key in the gear.

Thanks for looking
I think the slot for the key is too close to the crankweb. Closest to the crankweb is the main bearing, then a spacer, and then the cam gear. See the sketch.

crank gear.jpg

I think you can still use the crank, just extend the slot as far as needed.

Jos you are right I didn't take the bearing into account when I cut the slot.
I am not using a spacer as the crankshaft gear was made wide enough to make up that space.
I think what I will do is cut another key way on the other side of the crank in the correct position or just extend this slot as you suggested.

The plans call for a Boston Gear G677 internal ring gear that has 48 teeth, 24 pitch, 14.5 pressure angle. I did find that gear for sale at several different places but with a wait time of 4 to 6 weeks and a price of $150 and up. Now I'm not a cheap skate but I just can't my self paying that much for a brass gear that is 2 1/2" in diameter and a 1/4" thick.
My 5-cyl Ohrndorf radial is about the same size as Edwards. It's a metric design so uses mod-1 timing gears: 15T-15T-10T-40T ring gear which is quite compact. Strangely you can get a nice quality steel German 40T ring gear from Madler via Florida location for $42 USD. Might not be a suitable match for the Edwards & looks like you are already proceeding along with an alternate plan, but something to keep in your back pocket for next time if you're not opposed to metric.


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