Flightglobal:

Full circle for Rotary...

Date: 16 July 2008

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I am deeply disappointed in Flight Global, to put it mildly.  For an aviation magazine with a long and distinguished history of technical excellence to get thier basic facts about aircraft engines so wrong is inexcusable.

The engine described in the article being developed by Mistral Engines is a Wankel rotary, in which a triangular wedge rotates within a fixed crankcase to provide power to a rotating shaft.

The rotary engines from World War I and the early history of flight are piston engines with a fixed crankshaft and a crankcase and cylinders that rotates around it.  The propeller is fixed to the crankcase and cylinders and thus rotates with them.

Even though both engines share the name 'rotary,' there is little in common between them.

 And by the way, the airplane that Louis Bleriot flew across the English Channel was powered by a radial engine, not a rotary engine!

 And by the way... WHERE'S UNCLE ROGER WHEN YOU NEED HIM!!!!

Zounds and gadzooks...now I have heard/read everything! A triangular wedge rotating inside a crankcase???? Surely this is entering into the realms of fantasy. I mean surely it would vibrate hugely and smash everything into smithereens? And just how, pray tell me, do the poppet valves function? This device is not also designed to run on just water perhaps?

Oh; by the way, the Possessive Thier is actually slelt Their....I before E, except after C. (whoops; being English, that must be an exception....).

Uncle Roger is on hoilday for the week at the Hampshire Air Fete, he was in good spirits when I saw him yesterday

Aren't we comparing apples and oranutangs here?

1. A Gnome or Bentley rotory engine was in essense a P&W Wasp Jr wirling around a fixed crank shaft. Highly inefficent (read spuing unburned fuel and Caster oil out the exhaust) though light weight with limited life and Housepower. {not to mention shedding pistons, cylinders, and valves.} Air cooled.

2. A Wankle design with a fixed crankcase and moving crankshaft with with triangular "pistons". the design is efficient (Though the early ones did have blowby problems with the seals - gone now) and light and about 2/3 the size of the equivalent piston engine. Liquid cooled, thus far.

Let us not use World War One technology to explain a 21st century motor which can be built to atleast 600 hp (I think Curtiss-Wright made one in the 1980s, but there was no market.)

Tom Anderson

http://auto.howstuffworks.com/rotary-engine.htm That link should clear up any issues with the understanding of how a Wankel rotary engine works. The wankel rotary engine has many unique advantages and disadvantages.

 General Pros vs piston engines on equal fuel:

Fewer moving parts for less maintenance.  Low weight.  Lower vibration. Smaller size.  Generally better fuel consumption at upper spectrum of the RPM range (due to lack of pumping losses @ higher rpms vs pistons).  Wide applicable RPM range (some RX7 rotaries are operated daily within 2K-14K RPM).  High Horsepower per engine displacement size due to longer combustion cycle (not a stroke as things move circularly).

Cons vs piston engines on equal fuel:

Higher cost of initial construction (economy of scale advantages far less than piston engines).  Poor fuel consumption at lower RPM ranges.  Poor engine response and power at low rpm ranges.  Higher emissions in general.  Possibility of "flooding" the engine during stops and restarts (a challenge to overcome in aviation). 

I know a couple of people with experimental planes flying 20B mazda rotaries.  Their greatest fear is the difficulty of inflight restart due to the flooding issue.  This threat is greatest during cooler weather and engine temperatures.  However, ensuring the proper warmup of the engine prior to departing is a part of the checklist. 

Their secondary fear is lack of quickly available power during final approaches, however both of the pilots have special gearing in their props which hold the engine over 4000 rpm or so at landing speeds to keep the engine as close as possible to its 6000-8000 rpm main powerband.

As one who drives a Mazda Rotary RX8 and have done for 41/2 years i can vouch for their reliability, smoothness, lightness and power to size ratio.

 I'd happily fly an aircraft powered by one.

" So your telling me your piston go's up, stops, go's down, stops, go's up, stops, go's down and stops..for just one revolution ???? How quaint !  

Please, please, will someone finally explain to me why it is that most quality motorcar producers stubbornly continue to install powerplants into their vehicles that use this "quaint" reciprocating mode of producing torque? My German car uses pistons and runs silky smooth, uses almost no oil, is very fuel-efficient, has a remarkably long life in terms of hours run and is amazingly reliable. The Germans have long made fine engineering products. Why do they not seem to learn from Mazda? Are they perhaps dumpkofs?

Some of my best friends have owned Mazda cars and they have been unable to explain why most car and light aircraft producers refuse to fit Rotary engines. Surely there must be a logical explanation? 

A fine point you make. In reality its only in the last 10 years that materials technology has allowed rotary engines to reach their potential. Previous incarnations have suffered from poor fuel consumption and premature apex seal failure and the use of Turbo's to deliver the power.

With the use of advanced materials these powerplants offer normally aspirated high power in a low weight package. The reliability has been sorted and proved by Mazda using a number of high endurance tests and indeed le mans 24hr races wins.

So...whats the point.

In a car it allows the engine to be placed low down and towards the centre of the car giving the RX8 pure 50/50 weight distribution. 220 BHP from an engine the size of a clouple of loaves of bread ! This in in comparison to a BMW straight 6 which is 5 times the size for the same output.

The application for this type of engine is not however universal. It's high revving but still relitivley thirsty so the weight advantage needs to be able to offset this. A plus is that, as its more like a turbine it can run on hydrogen and avgas without (m)any modifications to the combustion chambers.

In reality the standard rociprocation engine hasnt changed much since it was invented so this is just a compact and smooth alternative. In the big scheme of things no engine/powerplant is perfect, each type has a weakness but the rotary design still has huge potential thanks to Mazda's investment. 

In summary, great for light sports cars, great for light aircraft.