R975 Radial engines

[Bob Phillips]

Thank you gentlemen for your input and comments!
The idea of using a -46 in a tank is tempting but I think it has fundamental dimension issues because the crankshaft is significantly longer. Then the cooling shroud would have to be redesigned to the longer length, unless... you machined the front crankshaft shorter. I think it could be done ( will post c/s pics later). The engine in post #12 is probably intended for an automatic transmission vehicle (M18 ) and has no clutch. The cooling fan around the front shaft is for the oil coolers. I had one of these engines and (sadly) I let it slip away.
Here are some more photos the C1éC4 carb ( stromberg NA9RG) versus the much larger Stromberg QD 9A1 of the -46. Also magnetoes , two types used on the -46, the SF9RN which looks very similar to the C1 and C4 mag (VAG9DFA), and another type SF9RN-8 which was an automatic retarding mag.
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[Bob Phillips]

On the bottom mag picture the C1 C4 mag is on the left and has the small rotating aluminium cap. The -46 was used in some crop duster aircraft applications and I understand they used a smaller carb and just covered up the two barrel intake with a plate with a single hole.
Inside link rods are interchangable but the knuckle pins and master rods are not. The diameter of the end of the knuckle pins is different.
The -46 is an impressive engine and the culmination of decades of refinement on the 975 engine. The 1950s was the era of overkill in radial engines with mega engines with as many as 28 cylinders arranged in four rows were built. What a mechanical marvel but also a mechanics nighmare!

[Bob Phillips]

P & W 3460, 28 cyls 3500HP
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[Bob Phillips]

Piasecki helicopters;
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you tube video, hopefully will work

www.youtube.com/watch?v=5gTyZ7grjeA

[Perry Kitson]

Here are a couple photo's of a new C4 supercharger impeller. Outside diameter is 8.875", 16 vanes, .750" 6 tooth spline.

In my opinion, the bane of the radial is the seperation of the two angular contact bearings on the supercharger impeller shaft. Angular contact bearings are meant to be used in pairs, as they are on the impeller shaft, BUT they are designed to be installed back to back to give the bearings the proper preload. Seperating the bearings on the impeller shaft necessitates very precise shimming of the housings so the bearings run neither too loose or too tight, as they run up to very high speeds.

[Bob Phillips]

Perry thanks very much for the impeller info! Looks a whole lot like the -46 impeller, does anyone know the significance of the holes through to the back?
As I mentioned earlier in this post tghere were weak points in the 975 engines, the impeller shaft/ bearings were certainly one. A second was the master rod/ crank bearing which often failed due to abuse or possible oil starvation. Any other known issues?
I also need to thank Perry for the opportunity to photograph his C4 cylinders next to a -46 cylinder. Please note -46 cylinder does not have an exhaust elbow attached.
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[David Herbert]

I would suggest that the holes in the impeller are to more balance the pressure on each side of it as without them it would draw itself towards the side with the vanes on it (as it sucks air from that side) causing high axial load on its bearings. A certain amount of recirculation will happen but it may be that that was judged to be the lesser evil.

In my experience the primary cause of failure of these engines (in preservation) is that people let them idle at too low a speed and the master rod to crank bearing fails due to oil starvation as most of the lubrication pressure of this bearing is from centrifugal force within the crank itself and that is negligable below 800rpm. They will idle nicely at 500rpm so people think that that is better - wrong !

David