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  #21  
Old 15 November 2007, 07:05 PM
kanazawa kanazawa is offline
 
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Originally Posted by Troberg View Post
I doubt you'll find a single civilian aircraft with counter-rotating props.
Beech Duchess and the Wright Flyer to name two...

http://en.wikipedia.org/wiki/Counter...ing_propellers

I was going to mention something about "the critical engine" before, but I don't know a lot about flying twins.

I've ridden in a Duchess though...
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  #22  
Old 15 November 2007, 07:10 PM
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Hans Off Hans Off is offline
 
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Ha ha! Read my sig line and weep!!!!

They can't include the wright flyer on their list as it has (IIRC) a single engine driving two props by chains!

(I would dismiss it anyway)

I didn't realise the piper seneca etc fitted into that category though.
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  #23  
Old 15 November 2007, 07:27 PM
kanazawa kanazawa is offline
 
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Originally Posted by Hans Off View Post
Ha ha! Read my sig line and weep!!!!

They can't include the wright flyer on their list as it has (IIRC) a single engine driving two props by chains!

(I would dismiss it anyway)

I didn't realise the piper seneca etc fitted into that category though.
Looks like they are counter-rotating to me...

http://wright.nasa.gov/airplane/propeller.html
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  #24  
Old 15 November 2007, 09:34 PM
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Originally Posted by Troberg View Post
...I've always found it strange that they were not used in greater numbers, it was a nimble and powerful aircraft...
I certainly agree that the mid-engine configuration seems compelling from an engineering and aeronautical perspective. There's a lot there to geek out over.

My suspicion is that the reason behind their poor acceptance was servicability. With the big Merlins and Allisons only going a couple hundred hours between overhauls, how easy it is to swap out an engine makes a big difference in aircraft availability. I think that the mid-engine configuration can only have made it harder to do major and minor maintenance and repairs.

The configuration also suffers from the inescapable overhead of transferring power from the engine to the propeller over ten feet away. You need a universal joint and perhaps a torsional damper at the engine to transfer power to the drive shaft that goes past the pilot, then eight or ten feet of drive shaft (a torque tube actually, not a solid shaft), and then a gearbox at the propeller to allow for the hollow prop hub that admits the cannon barrel. There's a lot of parts there that you don't need with a more conventional engine-in-nose configuration. Extra parts add weight that decreases performance, and also increases parts count and introduces more potential points of failure, which decreases reliability.

Sometimes the performance and utility of the buried-engine configuration is worth the complexity and maintenance overhead. Sometimes not.

Fans of the P-39 and P-63 will probably also appreciate the Stemme S-10 motorglider, with a buried engine and a prop that completely retracts behind an extendable nose cone:

http://www.stemme.de/daten/e/index.html

I think it's neat, but not quarter-million-dollars neat. A guy I know has one, and it's so complicated and finely crafted that he considers himself not so much its owner as its curator.

Another source of aeronautical Nerdvana is the Bugatti Model 100, a twin-engine airplane with two contra-rotating props on the nose. It has two large liquid-cooled mid-mounted engines, each driving one prop through a driveshaft on each side of the pilot:

http://pagesperso-orange.fr/morlock68/bugatti.htm

In terms of systems, structures, and aerodynamics it was way ahead of its time. So far ahead that, given the materials and technology available at the time it had only a very limited chance of success. The lone example never flew. It fell into the hands of Bugatti car enthusiasts who stripped it of its engines and essentially abandoned the airframe.

Thanks, Bob "Simplify and add lightness" K.
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  #25  
Old 15 November 2007, 09:43 PM
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Quote:
Originally Posted by Troberg View Post
...I doubt you'll find a single civilian aircraft with counter-rotating props...
Add to the list of counter-examples the Piper Twin Comanche, of which the later PA-39 versions had counter-rotating props.
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  #26  
Old 16 November 2007, 05:23 AM
Troberg Troberg is offline
 
 
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Another source of aeronautical Nerdvana is the Bugatti Model 100, a twin-engine airplane with two contra-rotating props on the nose.
Counterrotating props on a common axle is not that uncommon. It's not very common either, but it's far from unique.
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  #27  
Old 16 November 2007, 01:43 PM
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Quote:
Originally Posted by kanazawa View Post
Looks like they are counter-rotating to me...

http://wright.nasa.gov/airplane/propeller.html
Yes but powered by a single engine turning in a single direction...
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  #28  
Old 16 November 2007, 04:32 PM
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Quote:
Originally Posted by Troberg View Post
Counterrotating props on a common axle is not that uncommon. It's not very common either, but it's far from unique.
No, not particularly unique, just fascinating. Considering that the aircraft was concieved and developed in the 1930s when open cockpit biplanes were still quite common, and then built in a furniture factory, it's a testament to the resourcefulness and ingenuity of Bugatti and his partners and patrons.

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  #29  
Old 16 November 2007, 05:16 PM
Meka Meka is offline
 
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Quote:
Originally Posted by Troberg View Post
It also allowed a 37 mm gun in the nose, giving it a serious punch.

All in all, it was a good construction, even though, as always, there were tradeoffs.
The cannon installation itself actually caused a few problems. The tendency to flat spin was traced to changes in flight characteristics when there was no ammunition or ballast in the nose. Additionally, the placement of the cannon and engine limited the available space for fuel tanks - another reason it never really caught on with the British and American air forces.
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  #30  
Old 17 November 2007, 05:19 AM
Troberg Troberg is offline
 
 
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Considering that the aircraft was concieved and developed in the 1930s when open cockpit biplanes were still quite common, and then built in a furniture factory, it's a testament to the resourcefulness and ingenuity of Bugatti and his partners and patrons.
I find the wing design even more bold. It would be considered advanced today!

When it comes to counterrotating props on a common axle, as far as I'm concerned, one aircraft rules supreme, the Tu-95 ( http://en.wikipedia.org/wiki/Tu-95 ). It's one of the most uncompromising aircraft ever mass produced and has exceptional characteristics.
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  #31  
Old 17 November 2007, 05:24 AM
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Originally Posted by kanazawa View Post
I was going to mention something about "the critical engine" before, but I don't know a lot about flying twins.
My CFI once shut down my critical engine while I was training for my multi-engine rating just so I could see firsthand what would happen. Now THAT was fun.
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  #32  
Old 23 November 2007, 04:40 PM
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The reason the P39, and it's successor the P-63 KingCobra weren't used more by the USA is that the supercharger that was installed in the prototype, at the expense of guns and armor, was removed so that these key components could be re-installed, at the expense of high altitude performance. All of this was due to weight considerations. The action in Western Europe was mostly at high altitude, something the P-39 was wholly unsuited for. That's why the aircraft was never used in any great numbers by any of the Western allies save for the Russians, who used the cannon in the nose of the plane to great effect for ground attack and tank busting, all at ground level. As has been noted, Russian pilots loved the airplane. As for the long propellor shaft and front mounted gearbox, what many thought would be a big source of trouble turned out to be incredibly reliable. They rarely had problems with that part of the aircraft.

One other thing worth noting is the the vertical tail fin and rudder of most single engine propeller driven aircraft are not symmetrical when viewed from obove. They have an airfoil shape, producing a bit of "lift" to pull the nose slightly opposite of the gyroscopic force created by the spinning propeller. Some planes also have a rudder tab to help control this tendancy.

Contra-rotating propellers, while eliminating the torque steer effect, are complex and expensive. The USSR had turbo-prop technology down pat, and "da Bears" are still visitors to the edges of our airspace as the Russian air force test our response time and techniques.
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  #33  
Old 23 November 2007, 04:55 PM
Troberg Troberg is offline
 
 
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Quote:
Contra-rotating propellers, while eliminating the torque steer effect, are complex and expensive.
Actually, they don't have to be so complex, at least not until you start to mess it up with variable pitch props.

Look at this cool design that a fearless Austrian guy made: http://youtube.com/watch?v=xPA2k3qPe-4

I've seen some photos up close of the mechanism, and basically, it's just three 45 degree cogwheels, with the outer two each driving a rotor and the engine coming in on the middle one. Probably built with parts from a car diff.

I'd really, really would like to have something like that. The big problem is that it probably can't do an autorotation, so if the engine fails, you're doing a lawn dart. Still, that could be solved by adding a fourth gear and a second engine, so the basic idea is still sound.
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  #34  
Old 26 November 2007, 07:56 PM
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That is cool. It seems all he's getting is lift. He doesn't appear to have any sort of active control over pitch or yaw or roll, beyond shifting his weight. Essentially, it looks like a high-tech pogo stick.

Also, while I love aviation, I'm not entirely comfortable with the idea of have a 2,000 rpm salad shooter 24 inches away from the top of my head, helmet or no.

And you're right, the bugaboo is the variable pitch propellers. The problem is that for all but the basic single engined Cessna 172/Piper Cherokee type aircraft, modern throttle control demands it.
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