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The Wankel rotary engine seems to have several advantages over traditional reciprocating cylinder-and-piston designs. In general, they have a much higher power-to-weight ratio, less vibration, and drastically reduced rate of catastrophic failure, among many other benefits. I can only find disadvantages related to emissions and fuel efficiency. So my question is, why hasn’t it seen more use in aviation even though it seems very well suited to this application?

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    $\begingroup$ 1) Costs a lot to develop and certify an engine, and there's a very limited market. FWIW, having owned a couple of rotary engine cars, I can say that fuel economy isn't all that bad when you're running at a steady, high RPM, as you would in an airplane or driving long distances on interstates, which I did when I owned mine. They're horrible in stop & go driving, or around town, though. $\endgroup$ – jamesqf Jul 20 '20 at 4:54
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    $\begingroup$ "seems to have several advantages..." that statement is simply incorrect, unfortunately! $\endgroup$ – Fattie Jul 20 '20 at 13:17
  • $\begingroup$ @Fattie please elaborate. $\endgroup$ – Aaron Holmes Jul 20 '20 at 13:19
  • $\begingroup$ @AaronHolmes - my comment is superflous with the excellent answers: unfortunately no matter how "natty" an idea: the connection of the tip of the piston/rotor to the wall of the barrel/cylinder ... is inherently, fundamentally, problematic. Sadly! $\endgroup$ – Fattie Jul 20 '20 at 13:39
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    $\begingroup$ @Freiheit: Well, now I’m even more confused. The Wikipedia section on Wankel aircraft engine makes them seem like a perfect option. $\endgroup$ – Aaron Holmes Jul 20 '20 at 20:08
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They do not have drastically lower rates of failure than comparable piston engines. Their manifest unreliability in automotive applications is one reason they failed so badly in the marketplace. My college materials science TA owned one and had it rebuilt four times. His record for the shortest distance before failure on a zero-timed Mazda rotary engine was 150 miles.

The breakdown mode was failure of the apex seals at the tips of the rotor due to poor lubrication, leading to excessive wear and scoring and the complete destruction of the seals. According to my TA, Mazda went through more than six fundamental redesigns of the apex seal system but never could completely solve the basic problem.

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    $\begingroup$ That’s terrible. By any chance, do you recall what specific problems he was having with it? I’ve always heard that a Wankel could fail, but fail slowly - meaning it would suffer from reduced power output which gradually worsens - but that it was essentially immune to sudden, catastrophic failure in the way a reciprocating engine could seize up. It’s interesting that he had so many issues. Thanks for the answer. $\endgroup$ – Aaron Holmes Jul 20 '20 at 6:06
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    $\begingroup$ @AaronHolmes my rotary RX-8 "failed suddenly" in that it just stopped turning due to the rotor heads - which is the usual failure mode of these engines. They consume oil and if you fail to top them up it destroys the rotor tips. $\endgroup$ – Jamiec Jul 20 '20 at 7:14
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    $\begingroup$ @Jamiec That sounds like (a) a failure to provide proper maintenance (b) an issue specific to automotive engines, and their stop/start regime $\endgroup$ – Mike Brockington Jul 20 '20 at 8:29
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    $\begingroup$ @Jamiec I know that many of these engines, like the Mazda variants feed oil directly into the chamber to provide lubrication and aid in creating a seal between the apexes. I’ve also heard that many people recommend mixing 2-stroke oil into the fuel to further increase the effectiveness. However, several modern manufacturers, such as AIE, claim to have solved the issues of oil consumption and excessive wear. Many of these engines are being used in UAVs. $\endgroup$ – Aaron Holmes Jul 20 '20 at 13:30
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    $\begingroup$ Specifically, the Apex Seals would fail, leading to poor compression in the engine for one or more of the combustion chambers. Not always related to poor lubrication, since the engine was designed to self-lubricate the Apex Seals (which led to a deliberate burning of oil, a trademark of the rotary engine). Once compression was lost, just like in a piston engine, power creation ability declined dramatically. This problem was solved more-or-less in the later years of the RX-8, but nearly every single RX-8 (and RX-7) owner has a story or stories about replacement or rebuild of the engine. $\endgroup$ – SnakeDoc Jul 20 '20 at 17:16
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As another answer points out, the main problem is poor reliability.

In a conventional piston engine, the sliding contact of the piston against the cylinder wall is mostly against relatively cool metal and is constantly lubricated by oil from underneath. The contact is often maintained tightly by piston rings, which have a natural outward spring.

In the Wankel type the sliding contact of the rotor tip seal against the chamber outer wall is frequently against much hotter metal from recent combustion and is difficult to lubricate. A separate spring mechanism is required to hold the seal closed against the firing pressure, so the whole thing is more complicated. The result is that the tip seals are extremely demanding and tend to wear out or crack.

I seem to recall that Norton reckoned to have solved the problems but their bikes and UAVs failed to gain significant business. I don't know if there were other issues affecting that.

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  • $\begingroup$ Thanks for the response. It seems like some of the problems are more endemic to automotive driving, with constant, wide, rapid RPM changes and more frequent engine starts and stops without allowing the engine to properly heat up. $\endgroup$ – Aaron Holmes Jul 20 '20 at 13:36
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I think most answers on this question are fueled by lack of knowledge on rotaries, these engines do have inherent flaws relating to apex seals but 90% of their "unreliability" claims come from pure ignorant owners (at least in the automotive world) and a complete lack of understanding on how to maintain one. Most rx8's can do 100,000 kms before needing a rebuild, (average running time for that many kilometers is 2000 hours) so this is relatively in line with most overhaul windows (thanks google). And least us not forget the rotary engine won Le-mans in 1991 and apex sealing technology has come a long way since then. However to actually answer the question the real reason is due to the effort in getting the engine certified and the costs involved.

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Wankel RCE is still in use today for Ultralight, Parapente, and other uses. Aixro, Wankel Supertec, Wankel GMBH, AIE.uk, are offering Wankel Rotary Engines for sale, research exists in India. John Deere, also Curtiss-Wright, developed a line of Wankel prototypes for Aviation, in several power ranges. The John Deere engine had an SFC around 240 gr/HP/hr, good enough, if you consider that it can work with low grade gasolines, heavy fuel versions of Wankel existed also.

Light weight, compactness, faults being not that total and catastrophic as in reciprocating engines, are Wankel advantages, a 2-Stroke engine, with sudden seizing, plus torsional vibration problems in shaft from engine to propeller killed the BD-5 and some pilots.

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    $\begingroup$ The Wankel engines were tried on BD-5's. They are not used because they are water-cooled engines, generate a lot more heat than 2-strokes and conventional 4-strokes, and it is incredibly difficult to fit the hardware for a required cooling system in such a small airframe, particularly during taxi. The aircraft simply wasn't designed to use such an engine. Torsional issues in the BD-5 drive system were resolved long ago. You just have to follow the plans and use the correct drive system from a knowledgeable manufacturer with a good track record. $\endgroup$ – Juan Jimenez Nov 26 '20 at 21:33
  • $\begingroup$ Good observation. You know the NACA air intakes in BD-5 were tested, and resulted worse than 'Open mouth', the 'Vought F-8 Crusader' style. Small Wankel of today are also Liquid cooled (Aixro,...), but Air cooled Housing with Charge cooled Rotor versions existed. You may like 'Cooling your Wankel', by Paul Lamar, www.rotaryeng.net site, he proposed for Mazda conversions a Cooling similar to Mustang P-51, which added thrust from engine heat. Blessings + $\endgroup$ – Urquiola Dec 17 '20 at 14:51
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    $\begingroup$ What many BD5 builders never understood is that cooling an engine is not about how much air you bring into the engine compartment or onto the face of a radiator, it's about how much heat you take away. The only successful engine cooling designs for the BD-5 when using recip engines work by exiting the air through the bottom of the tail with vents cut out at the rear. The P-51 design was not very adaptable to the BD-5 because the bottom of the bird already sits very low to the ground, particularly with the pilot in the cockpit. $\endgroup$ – Juan Jimenez Dec 18 '20 at 15:13
  • $\begingroup$ A Critique of the BD-5 Concept, Part 1 - PDFHALL.COM in eaa.org/eaa/aircraft-building/kits-and-plans/00---c/bede-bd-5 $\endgroup$ – Urquiola Dec 24 '20 at 15:22
  • $\begingroup$ That is not a critique, that's just the EAA info page on the BD-5. Most of it is accurate but there is some very dated information in it. $\endgroup$ – Juan Jimenez Dec 25 '20 at 16:12

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