Is this a brain fart or an epiphany?!?!?!

[marbleturtle]

I was leaning toward modifying a Subaru WRX engine until I read about the gains in aerodynamic efficiency vs. increasing HP. (I think it was on Slade's web site.) The same or less HP with much lower cowl drag is better than a big honking engine with a bulging cowl. That swung me over to the Rotary camp. After seeing the web site of the guy who did the ducted fan LongEZ (I forgot who that is, but he seemed to be having teething problems with the system not having unobstructed air access), now I'm thinking... twin small rotaries inside 34" duct fans on a V motor mount above the horizontal line of the strakes, looking to the world like a weird A10 Warthog jet. 6000 RPM, no PSRU, very clean rear cowl, improved cooling for the motors, 2 for redundancy, no prop exposure to the ground, no obstructions to the fans, thrust close to centerline, possible longer arm of leverage against the CG with lighter weight. Hmmm...

 

Okay... Does anyone know where I can get a pair of really small and light 110HP rotaries?

[ekisbey]

If you've been following John's project you'll also be familliar with his fuel system woes. The first thing that occurred to me was that with two engines you're going to have more than twice the complexity of the fuel system, and it won't be something you cna just copy from another rotary powered aircraft. Remember that you're not only going to have to supply fuel into two engines instead of one, but you're going to have new failure modes with switching and controlling fuel flow into both engines, to include fuel pressure and flow issues. Will both engines draw from the same tank? One from each? A sump tank? Will each be independent of the other? A seperate pump for each? Return lines? Getting everything to work together is gonna be a nightmare, because you're designing it from scratch. It might be a bigger bite of engineereing than you think. Best of luck.

[John Slade]

It might be a bigger bite of engineering than you think.

Yep. I'll second that with bells on.

 

Fuel plumbing and management would be the least of the problems. While the idea of a "spare" engine is attractive to us all, adding one to what started life as a Cozy would take some serious engineering. One builder, Charlie Airesman I believe, got part way mounting two small subarus on a Cozy. His approach was wing pods mounted to the end of the spar. There was a picture in CSA not long ago. I think I read somewhere that he abandoned the idea, but I'm not sure.

 

The first problem you'd face using rotaries would be weight. There's no such animal as a small rotary that's easily available. The 2 rotory 13B will weigh in at about 340lb, so two of these is out of the question.

 

There has been one twin cozy built. Two brothers from S. America put two small engines on the same thrust line, then flew it to Sun & Fun. Interesting project, but they will be the first to admit that the new engineering took more time, effort and expense than the plane itself.

 

Does anyone know where I can get a pair of really small and light 110HP rotaries?

Yea - it's called a 13B. Both rotaries come in one package, and it'll get you 200+HP.

 

By the way, I wouldn't call my fuel system issues "woes" - I'd call them learning experiences

[ekisbey]

Something else an aoernautical engineer buddy of mine pointed out:

With two engines you're twice as likely to have an engine failure.

 

That being said, there's a question as to whether one engine on an installation like this can fly the Cozy. Sure, you might have enough horsepower, but will the plane fly with the thrust line out of kilter? From what I understand, there's not a lot of rudder authority to work with in the first place, and less at lower speeds. Combined with the drag from the dead fan and shroud, the off-center thrust line, and the fact that the rudders are independent of each other ond only work in one direction each, the yaw rate such a failure would produce could concievably force you to kill the other engine just to fly in a straight line, though you might be able to get by until final.

 

Point is, if you're twice as likely to have an engine go out, and losing one probably means going without the other, you're probably better off either going for twin counter-rotating props, or a single engine.

 

Or, you could always call Nat. But I think we can all guess his reaction.

[marbleturtle]

yeah yeah... if you have two engines your twice as likely to have an engine failure, if you have three wheels your three times as likely to have a blowout, if you have four passengers your four times as likely to need to make an emergency landing for medical reasons, if you have a backup ignition system your twice as likely to have an ignition failure... et al.

 

Anyway I know there are many considerations. I've thought about the thrust centerline, but maybe your thinking about full sized open props spread far apart like most twin piston engined airplanes. I'm thinking smaller high speed duct fans placed closed together (each roughly 20" inches off of centerline) like a private jet. By my calculations... at 6000 RPM max, open props could only have a 1.5' radius or 3' diameter. The air acceleration effects of a duct fan would drop that down to somewhere around 30" max diameter. (rough calculations) I know that every inch counts when it comes to the thrust centerline but it wouldn't be as drastic as the other twin engine attempt with wing pod mounts and large open props.

 

Fuel management issues could be resolved, not easily, but it can be done. Maybe one engine, one fuel tank... with a simple water level style tube between the two to level out the fuel tanks if one engine uses more fuel than the other. Hmmm...

 

The guys from South America had an engineering nightmare on their hands because of two engines put together and counter rotating twin prop system and counter rotating shaft within a shaft system.

 

John Slade is right though. Where do I get the small rotaries? I remember seeing somewhere a single rotary engine... Do any of the snowmobile or jetski manufacturers use a small rotary? Maybe I saw it on Moller's website... he ain't gonna sell to anyone building an aircraft.

 

Never shy away from a challenge!

[dust]

Teledyne Continental makes one, I think it weighs 75 lbs

[JakeC]

Here is a link to a supposed 100HP single rotor engine built for an aircraft. The people over at YAW power helped build it. I imagine it is a half 13B.

 

http://personal.riverusers.com/~yawpower/onerotor.html

 

I am no expert, but to me one of the selling points of the rotary is that, from what I've read, even if you overheat it and blow the engine, it will still continue to produce some power. Possibly enough to get you to a landing. Of course, this does not account for electrical and fuel failures, but it certainly makes me feel a lot better about having one engine. John, is this true?

 

Jake

[John Slade]

I am no expert, but to me one of the selling points of the rotary is that, from what I've read, even if you overheat it and blow the engine, it will still continue to produce some power. Possibly enough to get you to a landing. Of course, this does not account for electrical and fuel failures, but it certainly makes me feel a lot better about having one engine. John, is this true?

 

 

Generally, yes. My first introduction to the rotary was an EAA meeting in Ft. Lauderdale. Tracy Crook flew in with his 13B powered RV. He let us all look under the cowls, then he brought some spare parts in to the meeting. There was evidence of SEVERE damage to the rotors and housings. The parts were from a 13B which "blew up" during a drag race. The owner then drove it home hundreds of miles

 

The rotary has no valves, pushrods, conrods, piston rings or pistons to break, so it's a lot more robust than you're average piston engine. Sure, you can mistreat it and blow a seal, but typicially it'll still develop enough power to let you limp home. Another nice aspect is that you have redundant plugs (leading and trailing) so you can set up a fully redundant spark supply. You can even run (very roughly) on one rotor. It's provides a relatively high power / weight and generates much less vibration. Negatives are that there's a lot of heat to contend with, but numerous "wanklers" have proved that this can be done. Fuel burn has been proved to be on a par with a Lycoming. First time setup of the engine (mount, PRSU, plumbing, intake, turbo, ignition etc.) will probably cost about the same as a mid-time IO-360 when it's all said and done. A "zero time" rebuild costs about 20% of that for a Lycoming, so ongoing maintenance costs are minimal.

[marbleturtle]

Thanks for the info... I couldn't find information on the Teledyne Continental, but the YAW power single rotor looks interesting. I still like the idea of the duct fans and direct drive without the PSRU. But at 6000RPM the max fan diameter would be about 30". The rotary is small but still roughly 24" square. This would block most of the fan.

 

Does anyone know how Moller installs his engines against the fan? Are his engines smaller?

 

The other thought is engine in the cowling sideways with a drive shaft out and 90 turn into the centerline of the fan.

 

Any ideas?

[Dave Clifford]

I can't remember if it was sun-n-fun or oshkosh that I saw an RV fly with a rotary a couple years ago:confused: I DO remember though that it was the loudest prop plane I have ever heard!! I don,t know if it was for a lack of muffler system or what. Are these things as loud as I remember???

[John Slade]

Are these things as loud as I remember???

Yes. If you run them without a decent muffler - they can be VERY noisy just like any engine. The rotary, however, makes a different kind of noise. Very sharp pulses make it an angry kind of sound. Also, rotarys have a very hot exhaust and they tend destroy mufflers pretty quickly unless they're made of stainless or inconel.

 

The solution is to install a turbo. Running with a turbo and a straight pipe greg Richter's 13B is quieter than a Cessna.