Rotary or radial?

Hi, I'm quite a novice when it comes to aircraft. My question is what's the difference between a "rotary" engine as mentioned in the engine specs of many of the planes shown on this site and a "Wankel rotary" engine? I know, quite well, from working at a Mazda dealer what the "Wankel" motor is. I always thought that the aircraft engines with radiating cylinders were "radial" engines. Please correct me if I'm wrong.

A Wankel engine is not a true rotary engine although called that. A aeroplane engine such as a LeRhone or Gnome is. A Radial is a stationary engine with the cylinders fanned out around the crankshaft. A rotary the crankshaft is stationary and the cylinders revolve around the crank.
Rotary engines use total loss oiling too.
Cher'o,
Lt. Dwight Rudder, RFC

Thanks for the clarification. I did not realize the crankshaft was stationary on a rotary, although I did know that the whole outside spun. FWIW the "Wankel" is called a rotary because it uses a rotor for compression as opposed to a piston.

I have a copy of the original Wankel patent and have studied it carefully for years. I have also had some experience with a Mazda XR-7. The seals on a Wankel are the weak point of the system, as you well know.
The rotary radial engines of WW-I had the greatest weight to output ratio of any aircraft engine ever designed. The downside was the gyroscopic effect of the rotating cylinders.
The Gnome engine had cylinders that began as a 97 pound billet of nickel steel, and after machining the cylinder weighed 6 1/2 pounds. Lots of blue, curly chips on the floor under the turret lathe.
The effect of engine rotation was compensated for by wing wash in and wash out. A "right dive" was especially hard to correct, requiring real beef on the controls. A left hand climb, however, was an instantaneous thing, due to engine torque.
Having no carburettor, the rotary engine was controlled by a simple "blip switch" which cut high tension current to the sparking plugs, giving a measure of control. Too long a blip resulted in fouled plugs. Pretty dicey by modern standards, but it worked.

[Dan_San_Abbott]

Another disadvantage of the rotary was the engine was not easily controlled with the throttle as an engine with a normal carburretor. The normal engine speed 1200 rpm which was flat out. Through very careful adjustments of the manette (fuel)and the throttle (air) the engine rpm could be reduced to about 900 rpm, however this was dicey resulting in cutting out the engine. What was normally done was to adjust the fuel and air mixture to achieve 1200 rpm and then control the engine with the coupe (ignition cut out)button on top of the stick. Also as long as the engine was rotating whether on or off it is consuming fuel and oil. Ifv off the fuel/oil mixture was ejected out the exhaust valve into the cowl. Thus limiting the engine duration to the volume of the fuel tank as I related in another thread. One reason all aircraft with rotary engine had a firewall was the problem of fuel ignition in the cowling after cutting out the engine with the blip switch. Also it's been said that pilots who flew aircraft with rotaries never suffered with constipation.
Dan-San Abbott

[Dan_San_Abbott]

I forgot to mention in rotary engines they used caster oil as a lubricant, it was not disolved by gasoline or benzin. Some parts of the engine were oiled through oil galleries, other parts were oiled by means of the fuel/oil/air mixture coating the cylinder walls pistons rings and wrist pin, valve guides,etc.
Dan-San Abbott

The rotary was inherently a light engine: as the crank doesn't rotate there is no reciprocating motion and thus no need for any heavy balancing counterweights on the shaft. With the absence of reciprocal movement the usual problems of heavy inertial loads on wrist pins, rod bearings etc are non-existent. Again, the problems associated with ring float due to the piston acceleration found with a rotating crank system are, once again non-existent. So, all in all, there were several remarkable engineering advantages recognizable in the design of the rotary. The cooling of the system was the paramount reason for the rotation of the cylinders but the above mentioned factors (ie "power to weight") were significant. When aircraft speeds began to evolve into the 150 mph range the stationary radials- with proper cowling- cooled satisfactorily and the rotary lost its primary reason for being.

Ira: Should you be reading this thread.Your e-mail is bouncing: I sent congrats. on the big 50 and a few comments re the playoffs. Let me hear from you re the proper e-mail. Regards to all...Lee

Just a couple of additional points on the rotaries.

The Gnome rotaries incorporated more sophisticated "blip" controls, which allowed the pilot to select how many cylinders, within limits, could be cut off when the switch was depressed. I believe a separate dial-type control in the cockpit was used to control the magneto to set numbers of cylinders. This allowed for better speed reduction.

Also, the cooling of the cylinders was not so simple as might be assumed. Because the front (exterior) walls of the cylinder received much more air circulation than the rear, the shape of the cylinder distorted from a circle into an oval once the engine warmed up. This required piston rings that could flex in their shape to accommodate the change.

To come at Ed's original comparison between WWI rotaries and the Wankel rotary from a slightly different perspective than the excellent details already presented, the rotary motion of the earlier aviation engines is of the cylinder heads around a fixed cam, whereas the (revolutionary) rotary motion of the Wankel is of the rotor inside the rotor head.
The rotaries share in the reciprocating (back-and-forth) piston motion of their stationary cousins, whether radials or in-line types.
Rotaries may properly be called radials (although no one does), since the latter term really just specifies the geometric disposition of the the cylinder heads. However, radial has come to denote only the fixed (stationary) type of air-cooled aviation engine that came into its own after WWI as air speeds increased. It should be noted, though, that one of the earliest aviation radials was of the non-rotary sort, the three-cylinder, 25-hp Anzani of 1909 being the powerplant used by Bleriot on the first Channel crossing of 1909. Radial engines are also sometimes called fan-types.
Regards,
Stef

Stef: Imagine a crank shaft of a conventional single-cylinder engine (lawnmower?) mounted in a vise with the rod and piston assembly attached. If you "spin" the piston and rod assembly you would observe it can move solely and only in the peripheral (circular) plane. Ie. it spins, in orbit, without any reciprocation at all.

Back to the "rotaries" under discussion: With the piston assemblies in one orbit, the cylinders of the "rotaries" are in, however, another, second, orbit at a distance- differing in center point- from that of the pistons. This second orbit, of course, centers at a point established by the the center of the crankshaft itself. So one orbit, that of the pistons, follows the periphery based on the radius from the center of the crank journal off-set ("the stroke")while the other orbit, that of the cylinders, is centered on the main shaft itself. These concommitant but offset rotations cause the cylinders to slide up and down over the piston in a function entirely derived from the off-set of these two different orbits. Accordingly the commonly recognized (and traditionally troublesome) inertial forces which occur as a truly reciprocating piston de-accelerates, stops entirely and then acellerates in the reverses direction- do not exist.

BTW: Harold Hartney wrote of a mechanic, formerly on a Indian Motorcycle racing crew, who drilled holes in the piston walls effecting a much better lubrication of the piston skirt/cylinder wall locale. These 160 Gnomes immediately evidenced a much greater TBO and French factory "reps", he remarked, showed up at Toul, puzzled that the Americans were not buying replacement engines at the predicted rate. Regards, Lee