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One of the primary components of the thrust system is, of course, the engine. I was going to use a Polaris 440 racing engine but thought better of it since it was a free-air style of engine and was guaranteed to over-heat on me and be a consistent source of aggrevation. So, off I went to my father's snowmobile boneyard and picked out an older Hirth 634cc 2-cycle fan cooled engine. Heavier but more reliable according to pops. So, I yanked the beast and hauled it home. One of my concerns about the 12R design is that it doesn't really consider nor offer solutions regarding engine vibration isolation so off I went in search of the perfect vibration isolator. I wasn't happy with what I found out there due to high price ($15/mount!). The engine mount consists of a heavy engine mount plate, 4 isolators, and a lighter base plate. This photo is of the engine plate mounted to the engine.
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This is an exploded view of one of the vibration isolators. The big rubber piece on the right is placed directly on the base plate with the lip pointing up. The engine mount plate is then fitted on top of the large piece with its top being run into a large hole on the engine mount plate (see above picture). The smaller rubber piece is placed with its lip down into the top of the engine mount plate. The metal sleeve in the center of the photo is pushed down through both rubber pieces to hold them together and prevent shearing. The sleeve also provides a place for the compression of the rubber to stop when the bolt is tightened. The bolt is placed up through the base plate through the entire assembly and through the large washer at the top and a nyloc nut is tightened on. Not shown here are rings that I placed around the lower rubber pieces to hold them together in the event that they start to split or break apart.
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You can see in this photo how the engine mount is assembled. In this photo you will also notice the engine pulley (sheave) that drives the prop via belt. Got it from and it fits a standard (snowmobile) 30mm tapered shaft. It has 5 grooves but I was only able to use 4 of them since the back cylinder of the engine gets in the way. No biggie, it was the same price as a 4 groove pulley.
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I then started work on the propeller. It was purchased from UH and was as rough as the lift fan (see lift system) and needed the same attention. Sanding, balance, and glass. The balance was completed the same way as the lift fan using a spindle and rails. The prop was balanced, the light side weighted, and balanced again until the prop stopped spinning on its own. For weight, I drilled out a 3/8" hole, and melted some lead with a torch and poured it into the hole. As you can see here, I had to do this 5 times. The holes were covered with glass and sealed with epoxy when I glassed the prop. I don't know if this was the best idea but it seemed the least likely to affect the aerodynamics of the prop at the time.
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The hub was fitted to the prop and the upper pulley was mounted on the shaft with the bearings. I placed extra locking collars to the front of the bearings in case the locking bearing collars start to give way. The lock screws seemed pretty small to me.
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Finally, I mounted the thrust engine and drilled out the 6 3/8" engine mounting holes. I ran into a bit of a problem when I found that I'd measured the belt length wrong and the 56" belt required me to raise the engine 3/4". I treated 3/4" x 2" x 13" wood strips and placed them on top of the craft's wooden mounts. Why didn't I just order a longer belt you ask? Because it took a week to order the first one in! :-)
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Judgement day! I nervously filled my gas tank with 40:1 fuel, pumped the fuel bulb, choked the engine, and pulled the recoil starter cable. The darn thing wouldn't start so I fiddled with the carb settings until it finally fired up... reluctantly. After some more adjustment, I got the engine to spool up. Slowly at first to ensure that the prop didn't contact the duct and then faster and faster until I blew all the leaves from my yard into the neighbor's prize winning roses. Just kidding, but she did blow up a hurricane... and made alot of noise. I discovered that the previous owner of the engine used a blow torch to cut a hole through the baffles in the muffler so that it was a straight shot. I've since replaced the muffler.
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That test was good enough for me! I hovered her around to the back yard, cranked her up and flew for the first time! Around the trees, between the bushes, through the flowers (oops!). It was very maneuverable. I stopped by cutting lift, cranked the rudders hard to one side, kicked up lift, and hit the thrust, and she would come about. I could move around the yard in short bursts like this and eventually only had to cut lift to stop on a couple of occasions. Otherwise I would just use the thrust and rudders to turn. Woo Hoo!
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| About a week was spent installing guards on the lift and thrust ducts to keep objects out of the prop and fan, and keep my fingers on my hands. :-)
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