rotary question

[Dale_R]

Probably beating a dead horse, but ...

 

In the automotive industry, engine exchanges are commonly talked about in terms of "long block" and "short block". A long block is a complete engine, minus the oil pan, manifolds (induction & exhaust) and ignition; a short block is just the basic engine block with all its internal components, but no cylinder heads or oil pump.

 

Comparing like to like, a rotary engine "short block" does, indeed, have only three moving parts (I'm not counting either piston rings or rotor seals as "moving parts"), compared to a four cylinder reciprocating count of 23.

Add in the cylinder heads (which on an aviation reciprocating engine are

integral with the cylinders) and the count jumps up to 55.

 

I'd call the oil pump a wash, except the rotary has two - sort of.

 

Coming from the John M. Browning school of engineering, what is far more

important to me is what do the parts do? The eccentric shaft on the rotary is massive, compared to a crankshaft from a reciprocating engine of comparable horsepower - not impossible to break, but about as close to it as possible and still get reasonable torque. The motion of the rotor is a trochoid, which while not truely linear, proceeds with a relatively steady change of vector rather than the sudden start/stop of a reciprocating piston. If balanced to equivalent standards, the rotary assembly is less prone to vibration, and wear.

 

Of course, there is a downside to low parts count. A failure of any of the 3 moving parts will be a flight-stopper. But then again, isn't that what will happen with the failure of any of the 23 core parts of the 4 cyl. reciprocating engine?

 

What about failure of a pistion ring? 25% loss of power? Failure of a rotor seal is a little more taxing - 33% (loss of power on two rotor faces). Rotor side seal failure isn't quite so bad - 16% loss, but the risk increased that

the rotor will wobble and eventually seize.

 

Then there is failure in the valve train - power loss is still only 25%, IF the valve doesn't come in contact with the piston. Umm, there is no comparable failure mode for the rotary engine.

 

Then there is the issue of "how often do any of these failures occur?"

Although aviation reciprocating engines are typically behind the technology curve of automotive engines, their technology is pretty sold - provided adequate QA is being excercised. That's a big "provided", considering the recent spate of Lycoming recalls.

 

Some folks would look at the "teething pains" experienced by some of the better-known rotary flyers, and conclude that the Wankel engine isn't any more reliable than reciprocating technology. But how many of those failures involved fundamental failures in the "long block"? In most cases (all?), catastrophic failures involved upstream failures that would have doomed a reciprocating engine as well.

 

Of course, the only analysis that is really useful is a "systems" analysis.

The standard, cert. aviation engine installation is mature and pretty much offers a predictable service life.

 

There isn't, as yet, any real standard for rotary installations, but there is evidence [via Perry Mick, et al.] that the factory, normally-aspirated, automotive installation will work as an aviation application - and give acceptable service - essentially unmodified.

 

What is left is cost of ownership, and that isn't just a matter of resale value. It's the total cost of all the maintenance items that will occur leading up to (and including) TBO. It's all well and good to tout getting $20K more for an airplane because it had a cert. engine. It's another matter, if it turns out that it took $20-30K more in expenses to get it to that point.

 

YMMV,

Dale R.

COZY MkIV #1254

Mesa, AZ

[ondix80]

there more than 3 moving parts. how about psru. thats at least another 5 to 7 parts depending on a design, just my guess. did anybody ever experience any psru problems? whats tbo of these units?

[CBarber]

No, the engine itself has three moving parts. The PSRU is separate and is needed on other auto conversions as well. Agreed, they are not needed on most aircraft engines. Also agreed that for me, I do kinda count the PSRU as part of the "core" engine and everything else such as alternators, starters etc hang off the two. My PSRU from RWS does not have many parts...your quess, I beleive, is in the ball part. Still less than a "banger" engine.

 

All the best,

 

Chris

[argoldman]

Probably beating a dead horse, but ...

 

 

Of course, the only analysis that is really useful is a "systems" analysis.

The standard, cert. aviation engine installation is mature and pretty much offers a predictable service life.

 

There isn't, as yet, any real standard for rotary installations, but there is evidence [via Perry Mick, et al.] that the factory, normally-aspirated, automotive installation will work as an aviation application - and give acceptable service - essentially unmodified.

 

YMMV,

Dale R.

COZY MkIV #1254

Mesa, AZ

 

For a good semi-scientific discussion of the attributes of the rotary, go to the Cozy Girrls site and look at their rationale for selecting the rotary.

 

The aircraft engine, as you have stated, is reliable as long as it is properly maintained... I wonder if Dick Rutan's 150 engine was as well maintained as possible. One of the things that sticks in my mind is that most of the "short block failures of a rotary just decrease HP (of course they may be trashing the engine, but give you a good amount of time to find a landing place, even loss of cooling fluid will not stop the engine since the trochoid (cylinder) expands more than the rotor. Loss of rotor seal oil also does not cause catastrophic failure.

 

The standard aircraft engine, on the other hand has various modes of catastrophic failure, put a rod through the case, etc, etc. They are manufactured to enable the remanufacture of the engine i.e. easy (costly) replacement of cylinder assemblies, as a normal course of things. (at least 3 companies making these replacement parts).

 

Is that dead horse rotating or reciprocating:p ?