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303. Engine Performance

17/05/2017

I’ve been puzzling over the reasons for the quite remarkable increase in performance in the engine since the refurbishment.

Last year the best we ever got on the ground was 1050rpm. Normally you would expect an increase in the air as the load on the propeller is reduced, but that wasn’t the case; it was exactly the same however fast we were going.

This year the maximum speed on the ground is 1150rpm, increasing to 1200 in the air, which is clearly a big increase in power, and behaviour more like a normal aero engine.

In fact power is a function of the cube of the rpm, so an increase of 100rpm on the ground means we’re getting an astonishing 30% more power from the engine. It means that last year we were flying with the same power output that Grandad had from the 80hp Gnome, before he managed to switch to the le Rhone.

So what’s changed?

We’ve changed from pure castor oil to Castorene R40.

We’ve replaced all of the cylinder liners. A few had been scored by the flight with the oil turned off, but most were astonishingly clean, and obviously damage from the last flight is irrelevant to this discussion. They were all more or less on the outer limits of diameter, however. And three of them had been replaced at some point with ductile iron, instead of cast iron.

We’ve replaced the piston rings. The original ones (fitted by TVAL as part of their restoration) were leaking a little near the slit, and so one would suppose that we were losing a little compression, but in truth the compression always felt excellent whenever we had to depress the exhaust valves for priming. It was very hard to do on those cylinders that were on the compression stroke.

We’ve replaced the steel pistons with aluminium ones. I was concerned that because the thermal expansion of aluminium is double that of steel, we’d need increased clearances.

I knew that the cylinder bores were tapered inwards at the top – presumably so that it would end up as a parallel bore at operating temperature. Knowing the nominal bore and the taper, I could come up with a reasonable estimate of the design temperature of the cylinder head (and therefore, of the piston). I was amazed to find that it was only 80degC. On reflection, however, this would seem  to match our own experience; after a ground run, you can touch the heads for a short time without burning your skin.

When I did the math for the aluminium piston and compared its nominal diameter with the measured diameter of the old steel one, the additional clearance was about four times what was needed. Which was a relief, since as far as I know the only other airworthy le Rhone with aluminium pistons is in New Zealand.

But where is all this extra power coming from?

What about the new liners? Well, these are very accurately honed to the as-new diameter. They are also all of cast iron, which is picked for its ability to provide a ‘slippery’ surface because the intrinsic porosity retains oil. Could the ductile iron have been less good in this respect? When you think of the high friction forces in a rotary engine, with nine cylinders, each of 105mm diameter, 75mm long (that’s a contact area of nearly two and a half square feet), travelling at a maximum of around 20mph, any improvement in the coefficient of friction would have an important effect.

So what about the piston skirts? The original steel pistons were left with a slightly rough-turned finish, the idea being that the very slightly ribbed effect would help to retain oil. The aluminium ones have a more conventional finish, but with an increased clearance, even allowing for the extra expansion of the aluminium.

And while we’re thinking about friction losses, could the change in lubricant be having a beneficial effect as well?

One factor that seems to mitigate in favour of this argument is the fact that the engine is increasing in speed in the air; could it be that the friction losses on the cylinder walls were using up so much power that the poor old thing couldn’t speed up, even if the propeller loads reduced in the air?

I don’t know, and I’d love to investigate the information from the other airworthy le Rhones. Perhaps it’s just a spot of Shuttleworth magic.

But boy, are we chuffed with the result!

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From → Technical

2 Comments
  1. Well, ten 1% improvements really do add up to one 10% improvement, but from what you’ve said in the past rings would be at the top of my list for the most important factor, allied to having the engine put together using the best available components by the best available shop.

    How do you feel about actually getting to fly at Yeovilton – really *the* post war Naval Aviation home? It would have been a really big deal for me, but maybe I have more family connections from the post WW2 era.

  2. Yeovilton was very special, of course, even though it has no direct connection with WWI Aviation. Everyone there was, and is, very enthusiastic about 1264’s presence there, and completely unstinting in their time and expertise to make it happen. The flight itself was very, very high pressure, and there was certainly no time to reflect on any sort of historic connections. I was just glad to get it done safely and on time!

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