Rotary (as opposed to Radial) engines.

[Kurtz]

Was recently explaining to No. 1 son the difference between a Rotary and a Radial engine in aeroplane terms. For example, the type often used in WW1 Scoutplanes like the Camel (Gnome), and why exactly the huge gyroscopic effect gave the Camel such a good rate of turn (in one direction anyway!).
However, I managed to remember that these things had no throttle as such, but were controlled by 'blipping' or effectively isolating the magnetos via a button on the stick.
The question, "Why Dad?" inevitably came up, and I ran out of knowledge. He wouldn't even be distracted by the design features of the oil system, ie that it sprays its entire contents over pilot and fuselage during the flight, and is meant to!

Anyway, does anyone have a technical explanation for why a Rotary is designed to run flat out as opposed to having some sort of manual fuel control system?

Thanks! :ok:

[farqueue]

To get a full answer, you need to find a ref work on the different engines of the time. They came in a wonderfull and weird variety!!

Many rotaries drew the mixture in via the crankshaft. This was VERY rich and was mixed in the cylinder with air that came in via drilling at the base of the cylinder. Thus there was NO controll over these drillings.

LJK Setright's `Power to Fly' is a wonderfull place to get him started if you can find a copy.

[dusk2dawn]

Description of a rotary engine with graphics (http://www.pilotfriend.com/aircraft%20performance/aero_engines/Gnome%20Monosoupape.htm)

[BoeingMEL]

Just think for a moment.... no fuel injection in those days..just a simple butterfly-valve carburettor. Now imagine that same carburettor rotating at 400-odd rev/min with the cylinders! How do you connect linkages and fuel line?

Also, I think you'll find that the gyroscopic effect aided roll-rate - not turn-rate... a big difference! Cheers, bm

[ft]

The reason I've been given was that the carb was mounted on the firewall, with the fuel going through the shaft and crankcase before entering the cylinders.

Were you to wait for the effect of a throttle change, you would have to wait until all the air between the carb and the cylinders had been exchanged. Controlling power with such a delay wouldn't have been good enough. Enter the blip throttle, which gave instant, if somewhat coarse, power control.

I guess this would have applied to later models than the Gnomes linked above - an interesting read for sure! "Hopefully" the mixture in the crank case did not ignite... oh geez!

Regards,
Fred

[Specnut727]

ft

I recall reading that the super rich mixture in the crankcase was above the Upper Explosive Limit. There wasn't enough oxygen for it to ignite until it was diluted with more air in the cylinders, as described by 'farqueue'.

Also I think many rotaries had a conventional throttle. Main problem was the slow acceleration of the rotating mass. If the throttle was slammed open from low revs, the air velocity through the wide open throttle would be too low, to pick up the fuel, and the engine would die. To prevent this, the throttle had to be opened in small steps, allowing the revs to build beween the steps.

No wonder the rotary died out when improvements in metallurgy and design solved the cooling problems of early static radials.

Spec.