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Post  roddie Mon Nov 03, 2014 6:31 pm

I have been curious about this more unorthodox method of moving the aircraft's prop rearward of the center of gravity. Thinking about it just now.. I came up with the theory... Shocked  that the "thrust" produced by a spinning propeller, is an element in itself.. that has fascinating properties. The "thrust" primarily.. but think about what's actually happening. We take it for granted with tractor/conventional airframes. The prop is grabbing the air that it's pitch and diameter dictate.. and forcing it rearward.. with a force governed by rpm's. This is easy to understand.. and most aero-modelers are focusing on getting the ideal prop to suit the engine on a given model.. for a given design/style of flying.

When I think about a "pusher" design.. the thrust-element is diffused into parts. The pull-push "linear" thrust-element, the gyroscopic effect produced and applied on the roll axis.. and where that force is centered; with regard to the c/g... and expanding on the linear element; the angle of thrust-offset.. for the desired trim.. These are pretty much the factors.. aren't they?

If I want to "push" a wing.. as opposed to pulling it... how will that affect a tail-plane? Could the engine be mounted near the C/G.. and a prop-shaft extension used? Would these elements considered; make for effective trimming on a c/l stunt model?
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Post  RknRusty Mon Nov 03, 2014 9:10 pm

Good questions that I could only give a wild guess at. I might poke around the "Stunt Design" and "Engineering Board" sections over at Stunthanger.com where the scientists and aerospace experts hang out and see if it's addressed.
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Post  dirk gently Thu Nov 06, 2014 7:34 am

Here is an excellent article on the issues with pusher design:
http://www.djaerotech.com/dj_askjd/dj_questions/pushtractor.html
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Post  roddie Thu Nov 06, 2014 9:55 am

dirk gently wrote:Here is an excellent article on the issues with pusher design:
http://www.djaerotech.com/dj_askjd/dj_questions/pushtractor.html

Very interesting read!! It seems that "propeller" efficiency suffers; in most.. if not all designs.. where the airflow forward of it is disturbed by the airframe. Lots of issues for full-scale designs. Flying wings were mentioned as being better candidates. After reading the article, I can certainly understand why there aren't a lot of pusher-designs out there.

Thanks for posting the link Dirk.
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Post  dirk gently Fri Nov 14, 2014 5:03 am

This guy has a lot more interesting articles on his site. He actually explains stuff that you can find in any aerodynamics handbook, but actually does that in a way that let's someone with some modelling experience, but perhaps with no PhD in physics understand it.
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Post  Ken Cook Fri Nov 14, 2014 2:36 pm

The problem with a pusher is that your engine goes rich rather than lean when entering maneuvers. This is a result of the tank being in front of the engine. Not a good thing when you need the power. My friend Dan resolved this issue when he built his scale Farman Bipe. Here's a picture I took of him when we entered a "Fun Scale" at Palisades Park, New Jersey a few years ago. Dan custom built a shaft for the LA .46 which utilized a rag joint similar to a steering column joint. The engine was in the nose with the shaft running directly through a tunnel just like a rear wheel drive car. The tank was constructed with a hull like appearance in which the shaft ran through it. It was remarkably engineered and it worked flawlessly. The plane met it's demise  when the wind came over the trees where we fly at on the weekends. It pretty much splatted the plane into the ground. Ken

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Post  roddie Sun Nov 16, 2014 8:45 pm

That's always sad to hear about Ken. It must have sucked to see happen. I've seen that airplane before.. probably a different model.. but I remember it. I really like bipes... and "tripes" for that matter. I'm trying to visualize the tank in that Farman bipe. Was it like a "saddle" arrangement? Did the extended prop-shaft run over.. or under it? Just curious. I'm sure it was a good size tank for the .46. It looks like the engine is mounted upright with it's cylinder just forward of the wing's leading-edge? It's hard to tell from the photo. Were there any engine cooling issues?

This "rag-joint" I am curious about. Can you elaborate a little bit on the materials Dan used? I've been toying in my mind with extended-shaft drives.. where there would be at least one angle to deal with. One such drive is a "marine" application.. but the other is aircraft. The aircraft is a 1/2A "Stiletto" control-line model.

This plan references a steel universal joint.. pillow-block/bearings type set-up... but there doesn't appear to be any angular changes in the drive-line. Maybe this helps with any slight misalignment when building?

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Of course.. we're talking 1/2A here.. as compared to a big .46.. but I'm just curious about what the coupling consisted of.. and whether Dan ran his shaft in a sleeve.. or whether he used a pillow-block/bearing set-up? I can't imagine trying to flip-start that .46 with those tail-booms in the way.. Shocked
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Post  roddie Sun Nov 16, 2014 8:46 pm

That's always sad to hear about Ken. It must have sucked to see happen. I've seen that airplane before.. probably a different model.. but I remember it. I really like bipes... and "tripes" for that matter. I'm trying to visualize the tank in that Farman bipe. Was it like a "saddle" arrangement? Did the extended prop-shaft run over.. or under it? Just curious. I'm sure it was a good size tank for the .46. It looks like the engine is mounted upright with it's cylinder just forward of the wing's leading-edge? It's hard to tell from the photo. Were there any engine cooling issues?

This "rag-joint" I am curious about. Can you elaborate a little bit on the materials Dan used? I've been toying in my mind with extended-shaft drives.. where there would be at least one angle to deal with. One such drive is a "marine" application.. but the other is aircraft. The aircraft is a 1/2A "Stiletto" control-line model.

This plan references a steel universal joint.. pillow-block/bearings type set-up... but there doesn't appear to be any angular changes in the drive-line. Maybe this helps with any slight misalignment when building?

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Of course.. we're talking 1/2A here.. as compared to a big .46.. but I'm just curious about what the coupling consisted of.. and whether Dan ran his shaft in a sleeve.. or whether he used a pillow-block/bearing set-up? I can't imagine trying to flip-start that .46 with those tail-booms in the way.. Shocked
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Post  Ken Cook Mon Nov 17, 2014 5:27 am

Roddie, the shaft ran under the tank. The shaft indeed was in a pillow block with a bearing and yes cooling and also alignment was an issue to the point that it cost Dan 3 LA .46's until he got it corrected. The engine is mounted inverted and starting the 11 x 4 was certainly an issue. I would start the plane many of the times. You wouldn't really flip the prop like one would conventionally on a open nose stunter. I would grab the prop by the center section and flip it backwards with my thumb and forefinger like I was trying to snap my fingers. It would light off pretty quickly. The joint itself coming off of the engine utilized a couple he made that threaded onto the shaft with a mating flange. The shaft also had this flange and this rubberized cloth he attained was in between the flanges. This assembly was also balanced. Dan and Bob Zambelli worked on this project together. It really was a neat piece of engineering. The tank and shaft still exists and he mentioned that some day he might build another. The struts in the rear were carbon arrow shafts. The projection that you see in front of the leading edge in the picture is Snoopy. Ken
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Post  roddie Mon Nov 17, 2014 12:33 pm

Ken Cook wrote:Roddie, the shaft ran under the tank. The shaft indeed was in a pillow block with a bearing and yes cooling and also alignment was an issue to the point that it cost Dan 3 LA .46's until he got it corrected. The engine is mounted inverted and starting the 11 x 4 was certainly an issue. I would start the plane many of the times. You wouldn't really flip the prop like one would conventionally on a open nose stunter. I would grab the prop by the center section and flip it backwards with my thumb and forefinger like I was trying to snap my fingers. It would light off pretty quickly. The joint itself coming off of the engine utilized a couple he made that threaded onto the shaft with a mating flange. The shaft also had this flange and this rubberized cloth he attained was in between the flanges. This assembly was also balanced. Dan and Bob Zambelli worked on this project together. It really was a neat piece of engineering. The tank and shaft still exists and he mentioned that some day he might build another. The struts in the rear were carbon arrow shafts. The projection that you see in front of the leading edge in the picture is Snoopy. Ken

Thanks Ken. I'm always interested in knowing about how modelers overcome obstacles.. when building an unorthodox model.
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