Quote Originally Posted by aeroschmitz View Post
A very neat bird! I did a search for the 20-20 episode but could not find it. Thanks for the link, it clearly shows how the engine is mounted upright in the open. this is what I wanted to do with my challenger to get away from the inverted engine issues. what climate did your fly in? I had researched the F33 hirth (spendy) and they do make a pretty slick kit to mount on a challenger. I think this is probably a better way to go since that engine is in use in a pusher.

I fly in Northern California. Never gets really cold here but it can get really hot in the summers. The engine originally had an aluminum air scoop on the head to improve the cooling. It cracked from the vibration. I made another one. It cracked from the vibration. I made another one out of thicker aluminum. It cracked from the vibration. I gave up and just ran the engine as rich as it would stand without a scoop and found I could keep the temps in range. Carbon build up was a little more than before but decarboning a free air 277 takes all of about an hour. A scoop fabricated out of composites might hold up to the vibration. Aluminum won't.

After the second piston seizure, I had the cylinder bored out about 0.002 larger than spec to provide more clearance for the piston. I managed to avoid any further seizures but it was always a worry. The CHT would get down below 250 on the descent to landing. Whenever I was doing touch and go, I was at risk of a cold seizure, which is when the aluminum piston heats up and expands faster than the cast iron cylinder and seizes in the bore.

I think the F33 would be a better option than the 277, but it is spendy. It has a Nicasil cylinder so it's not nearly as prone to cold seizure as a steel-sleeved Rotax. In my experience and from experience of others I know, Hirth engines often require a lot of fiddling to get them set up right. Rotax did their homework and provided charts with the proper carb jetting for different altitudes. Hirth just seems to send engines out with a random selection of carburetor jets installed. They often will barely run when received straight from the factory, so you should plan on buying a selection main and needle jets, and jet needles and spending some time get it running right before you attempt to fly it. Once he got it set up properly, my friend has had very good service from his F33 engine and he wouldn't trade it for a Rotax. He's got it mounted on a ultralight trike and frequently shuts down the engine for soaring. He has the electric start which is real boon for air starting when the thermals give out. An F33 weighs a lot less and runs smoother than a 277, but probably doesn't put out quite as much thrust. I think it would probably get a Challenger in the air, but don't expect spectacular climb rates. Mounting the engine upright should give you the room to swing a big prop, which is key to getting high thrust out of low HP. I was getting over 200 pounds of thrust from my 277 with a 60" IVO prop. I felt pretty good about that until I read that the Wright Flyer produced about 130 pounds of static thrust from 12 hp. It was turning two 102" propellers at only 300 rpm. I doubt if there's been any airplane since that time that's had a better thrust to HP ratio. Static thrust is a pretty good indicator of performance for slow flying ultralight aircraft. Maybe not so much for faster airplanes.
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