I admit I'm in a strange situation: I'm selling a project with a drive system but I am happy to talk naive buyers out of using it. With a good BD-5 jet engine available at $60K and a couple of outfits seemingly willing to help you install it, no one should futz around with the prop-drive system.
https://minijets.org/fr/100-150/pbs-tj100/ https://www.bd-micro.com The engines are less than the cost of a Cadillac Escalade and whereas an Escalade will drop in value like yesterday's newspaper, a well-built jet BD-5 can bring a good price forever.
I notice that the Quinton Campbell-built jet formerly owned by J.J. has been sold to Aerial Productions International for military work
http://www.aero-news.net/index.cfm?do=main.textpost&id=b1e5763c-db7f-4fbc-b721-29207cbcf7c6 and Bob Bishop's jets are contracted out to them, too.
http://www.smart-1.us/?page_id=14 There are at least several airshow jets.
http://flsmicrojet.com/fls-microjet/ There you go. A jet is worth something and worth doing. A prop-drive BD-5 is a future garage project corroding under a tarp. Anyone who has a nice BD-5 should convert it to a jet or pass it along to someone who can, as I tell my callers.
You have to admit that's not very reasonable advice. The vast majority of aircraft builders have some concern for cost, and few of us have even $60K to pony up for the jet engine. Beyond that, a BD-5J is faster and hotter, with half the range and a quarter the flight time, and costs at least twice as much to fly. It's a very different airplane.
Here is a discussion I saved:
https://groups.yahoo.com/neo/groups/bd5/conversations/topics/21229I It amounts to this: Every time that piston engine fires, it flexes the aluminum drive shaft. Aluminum dies a little every time it is stressed and after a certain number of cycles, it fails. Sounds right to me--I'm a Poli Sci major.
Kent, this is technically incorrect. Further, the Yahoo thread is an attempt by the guy who designed the drive system (and did a good job) to point out that the guy who was almost killed in the crash was blaming the wrong piece of the airplane. The fault lay with the propeller, which had been thrice thinned, and with the test pilot, who had not even bothered to look under the cowl, and ultimately with the owner, who had had his wood prop thinned so many times.
I sometimes design electric motor-driven systems with driveshafts and resonances that have to be dampened, and with one long-ago stint on a motorcycle design team. I wouldn't claim anything near expertise, but here's a better explanation, to the best of my ability. (Note that there are better and longer discussions of torsional resonance elsewhere on HBA.)
All mechanical systems have resonant frequencies. In cases where the resonances can cause harm, they have to be moved to a frequency where they won't be harmful. In an airplane, it's a bad thing to have a resonance at, say, cruise RPM.
So the drive system engineer's task is usually to dampen these resonances, i.e., move them to lower frequencies where they won't do harm, say, below the lowest normal RPM of the drive. An alternative is to make the system stiffer, moving the resonance up to a higher frequency that never occurs.
One thing I DON'T notice in the Kauth drive is any sort of dampening devices, so that may need to be added. --> The aluminum driveshaft in the Kauth drive is fine, if and only if the system is properly damped. Note that all metals will undergo hardening and crystallization at some point, not just aluminum.
Also note that a sprag clutch won't solve resonances. A sprag or overrunning clutch's job is to prevent drive backlash from working its way back into the engine. In the case of the BD-5's drive system, which has a driveshaft, that means it's a good idea to have both a sprag or overrunning clutch AND some rubber in the system to dampen torsional resonances.
Failures in the drive-system airplanes have ranged from seized engines to sprag clutch failures,
https://app.ntsb.gov/pdfgenerator/ReportGeneratorFile.ashx?EventID=20101015X85729&AKey=1&RType=HTML&IType=LA to controllable props coming apart. Go to the NTSB site, query "BD-5" and read the six pages of accidents.
Here's a decent list of all the BD-5 accidents:
https://aviation-safety.net/wikibase/dblist.php?AcType=BD5
If you take the time to take a more in-depth look at the list, you'll notice that almost all of the accidents are engine-related. An incredibly-sobering number of pilots died because they failed to do a walkaround. Another thing that struck me is that almost all of the crashed BD-5s use engines like yours that cause the empty weight of the airplane to climb by 55-85% - they flew fine until they had engine problems, but then the pilots had no margin because they were 200-300 pounds overweight in an airplane that's supposed to weigh 350 pounds.
What I take from the list of accidents is not that the BD-5 cannot be built safely without using a turbine. What I take is that redesigning the BD-5 by building it 200-300 pounds overweight is a good way to get yourself killed. Incidentally, my kit came with a Yamaha Nytro engine like Dave Mischke's (the pilot you refer to below). I immediately put it up for sale.
Here is a video of the airplane with the failed sprag clutch. It was a beautiful job but he is luck it didn't kill him.
https://www.youtube.com/watch?v=nzdxxbW5oYM
In the case of Dave Mischke's drive system, I didn't see any attempt to shift resonances, but that might have been there and I just didn't see it.
I think there's a lot of really bad information floating around the community about BD-5s, both among non-owners and owners as well. Here's the advice I would give:
1. Don't redesign your airplane unless you're qualified. Most importantly, don't build it overweight, and you'll have more time to make decisions if something goes wrong.
2. Pay proper design to drive system dampening. If you don't know how to do it, find someone who does.
3. It's easy to test your airplane for resonances before you leave the ground. That's not a cure-all, but it will tell you if you've skipped anything obvious.
4. Don't do sloppy workmanship. Saving a few minutes on your wiring may kill you. Your choice.
5. Develop a checklist for your airplane, and use it, and for *$#& sakes, do a (&*#$(*& walkaround!!!
Last of all, I was struck by the fact that Dave Mischke started flying his first BD-5 at 60 hours of logged time. It's actually a fairly mellow little airplane, and many BD-5 pilots have said so.
SOOOOOOOOO...there's one last thing I'd like to add.
If you allow me to make a model of your Kauth drive in Solidworks, I or somebody else will be able to use Solidworks to do a resonance analysis and improve or fix its shortcomings. Other people could mate various engines up to the drive system and test them in Solidworks for torsional resonance.
That would be a lovely thing to be able to give back to the community. It might make the BD-5 less frustrating to build, and it might save a life or two some day.
