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The Truth About Rotaries

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I've never heard of Historynet, but after googling them, I guess they're owned by the same people who published Armchair General magazine, which I used to see in the pharmacy and convenience stores in my area.

Strange how they don't date anything...

I don't know enough about the history of rotary engines, or how they operate or perform, to say what I think about the article.  But its definitely fun reading. :)


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From what I know about them (which is just a bit of mechanic research... and who knows if what I read in other areas was true) it seems pretty spot on and plausible. I never really believed the whole schmear about the high degree affects of gyroscopic force. Due to how close the cylinders are to the center of rotation it shouldn't be really that much. Though watching some ground runs on a Camel while blipping the engine did make the whole plane move... but that's sudden acceleration / deceleration. Still, the large 4 blade prop on some SE5s should make probably the same gyro force as the smaller rotaries I would think.

And what about a turbine like a PT6 spinning the same relative mass with just slightly smaller diameter at more than 10,000 rpm. We never feel any gyro force from them. P factor and a bit of torque from the prop... yes, but nothing that I would describe as really noticeable gyroscopic precession.

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The gyro effect is so extreme due to weight ratio between engine and the rest of the airplane.

The engine and prop in Camel and DR.1 is the most heavy object in the plane. The rest is only light wood and canvas. So when rotating at such high speed it behaves like a gyro and when the rotating object has more mass, the gyro forces are more significant.

How close the valves are to the shaft has little influence. The rotation and the mass of the object are key here. We don't have centrifugal forces but gyroscopic forces here.

The blade of SE5 is light in comparison to the engine and the rest of the plane. You can feel the gyro effect like in all prop planes but much less significant because the rest of the plane has more mass to compensate.



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The effects were noticeable but not overwhelming. I remember reading as a kid that pilots could only turn out of the pattern on take-off against the force or THEY WOULD DIE and thinking,,, huh?  How critical it was that they had to respect this powerful force. That overwhelming desire to make things more dramatic than they are seems to be in everything you see or read. Everytime an airplane was involved in a story line of a TV show in the 60s or 70s the guy flying the thing would be "IN TROUBLE" and on short final would be giving the rudder hell back and forth so the audience could conceive how dangerous it was.  If you make a significant power application in a high performance piston you will "torque it around" some and in some cases you can pull the power off rapidly and get the reverse effect ever so slightly. 

Unfortunately the devs use these anomalies to excite their game players and in worse cases "play balance" the game to make it more fair<sic>.  My only experience with rotaries is you better like castor oil because you are going to smell like it for longer than you would think. When you see those crude moving pictures of pilots from that age suiting up before a flight... it's not because of the cold... it's trying to keep clear of the oil. 😎

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3 hours ago, Butzzell said:

Rotaries did not go away. With better metals for cooling, rotaries became radials. It is the exact same mechanical design only the body of the engine is bolted to the plane and the prop is on the crank shaft. Over the years new valve arrangements were developed.

I didn't realize that there was so little difference.

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12 minutes ago, Heinrich said:

and the Sherman, didn't it had a radial engine in some versions? Radial and Rotary nephew/niece? 

Radial... yes. Rotary... no.


In the years leading up to World War II, as the need for armored vehicles was realized, designers were faced with the problem of how to power the vehicles, and turned to using aircraft engines, among them radial types. The radial aircraft engines provided greater power-to-weight ratios and were more reliable than conventional inline vehicle engines available at the time. This reliance had a downside though: if the engines were mounted vertically, as in the M3 Lee and M4 Sherman, their comparatively large diameter gave the tank a higher silhouette than designs using inline engines.[citation needed]

The Continental R-670, a 7-cylinder radial aero engine which first flew in 1931, became a widely used tank powerplant, being installed in the M1 Combat Car, M2 Light Tank, M3 Stuart, M3 Lee, and LVT-2 Water Buffalo.[citation needed]

The Guiberson T-1020, a 9-cylinder radial diesel aero engine, was used in the M1A1E1, while the Continental R975 saw service in the M4 Sherman, M7 Priest, M18 Hellcat tank destroyer, and the M44 self propelled howitzer.[citation needed]

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Some things I learned from the article:

The Gnome 100hp was the only rotary in common usage that didn't have ways to control fuel and air to the engine. I thought most early-war rotaries had this deficiency.

Some people think that the rotaries were 2 cycle not 4 cycle.

I never considered the logistical problem of keeping enough castor oil on hand to feed the rotaries.

There was a geared rotary in which the engine crankshaft rotated in the opposite direction from the prop, the Siemens-Halske Sh.III. I looked it up elsewhere, and in theory, a very promising engine. The counter-rotating crankshaft would slightly balance out some gyro precession; the 2:1 reduction gearing would allow greater prop efficiency; and the IIIa high-compression version (6:1) would give better high altitude performance. The IIIa was rated at 160hp (nominal), very strong for a rotary, and reportedly capable of 240hp max. Here is Mikael Carlson's Pfalz D.VIII with a Siemens-Halske:


As they prime the cylinders and pull the prop through prior to start, you can clearly see the engine rotating faster than the prop.

As for the discussion of handling problems due to gyro precession, I always suspected it was exaggerated in the story telling, as Vonrd and Ludwig suggest, and due mostly to lack of experience of new pilots. The Dr.1 and Camel were exceptional, especially the Camel. They are both unstable, with the Camel obviously dangerous when you get too close to the low-speed, high AoA edges of the envelope. When you watch Chill31000 videos that show his stick and rudder movements in the Dr.1, you see how busy and extreme the inputs are, but the stall characteristics are relatively benign and recovery is quick.


My first time on the controls of a sailplane, even though I was pre-advised by the instructor, I was very surprised at the amount of rudder needed to roll in and roll out of even gentle turns.

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