This year I have been experimenting with tires. The standard Jungmann can be entertaining on a hard surface runway and I wanted to see how ground handling might be affected by different tires.
This is by no means scientific and certainly there are many with more experience than me on the subject. This only applies to aircraft converted to American Cleveland 6 inch wheels too. I thought I would share some thoughts anyway. Besides, there is nothing like posting something others disagree with to generate some correspondence :)
I may very well be wrong, but I have been privileged to fly about a dozen Jungmann and so far I have not really detected any big difference between standard and extended spreader bars. I have formed the opinion that condition is far more critical. Tight, well lubricated bearings/bushings and springs that are appropriate to the weight of the aircraft seem to be more important. I also like the look of the original gear. Again, there are people with more experience with the extended spreaders than me. It would be good to hear another opinion.
The wheels of a standard Bücker have a great deal of camber (they lean outwards) and I believe this makes the tire shape important. If you choose a modern tire with a more square cross-section like this
the aircraft is going to ride on the edge of the tire and wear very quickly and unevenly. You are in effect riding on a very narrow contact patch. I suspect this is unwise.
The older, more rounded type of tire would seem to be a better bet for a highly cambered tire. This is why motorcycles that have to lean into corners use this shape of tire.
I have found that Specialty Tire "Air Trac" tire seems to be the most round in cross-section. They are also pretty inexpensive by aircraft standards. Another good choice (which I have now switched to) is the Desser "Monster retread". These have a greater tread depth and last quite a bit longer than the AirTrac.
I have tried 4, 6 and 8 ply tires and experimented with high~ish pressure as recommended by AirTrac, but Ed Campbell's advice led to the most success. I am now using 8 ply tires inflated to 18 PSI and this is producing by far the best wear rate. The rolling resistance is a little higher, but I am getting a lot more landings per set of tires now.I'd love to hear of other people's experience in this area.
Don Vance kindly submitted these comments on the landing gear issue:
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From Don Vance
I’ll accept your invitation to comment on Bücker tires and landing gear. Let’s face it, operating a Bucker on hard surfaces is always going to be a problem considering it’s weird camber and toe in. When it comes to tires, I fully agree with what you’re saying. So, I guess it’s up to the owner to replace more often and pay less or pay more and replace less often. But, when you get right down to it tires are not that expensive. You can spend the price of a cheap set of tires taking Mama out to a show and a good dinner. Each has its benefits.
Now, for the Bücker landing gear. It is and always will be a problem operating on hard surfaces. When I first got acquainted with Mira’s N707S in 1973 the landing gear was flat worn out. When fully extended, the wheel end would move back and forth almost an inch. When the strut would compress it was fairly tight. I found the outer cylinder was “belled” just above the gland nut. When you think about it, this could be expected. When you touch down with the strut fully extended there has to be at least a 12 to 1 mechanical advantage of the upper bearing of the lower strut contacting the inner surface of the upper strut. So, I figured we had three choices. Live with it, tool up and build a new landing gear or convert to a Cessna 180 spring gear that wouldn’t wear out and be free from the camber, and toe in problems. Before everybody hollers “sacrilege”, remember, Mira’s airplane was an “air show” airplane and not a restoration. I wanted to go with the spring gear but we couldn’t take it out of service long enough for me to do that, so we just lived with it until Joe Krybus started making new gear many years later.
Now we come to springs and spreader bars. While I had Mira’s landing gear apart, Perry Schreffler had a guy here at Santa Paula make up a set of spreader bars for his airplane about 3 inches (at least) longer and talked Mira into getting a like set. I installed them and found out that the landing gear struts were nearly “bottomed out”. I did not realize at the time that as a coil spring varies from vertical, it’s load capacity decreases. I will attach a table showing angle versus load capacity. It is from an automotive spring manufacturer but it applies to any application.
Mira then got a set of springs rolled somewhere in Los Angeles. That shop used stainless steel wire in making up the springs. I installed them and after just a couple of landings they became “convoluted” spacers and the landing gear was once again bottomed out. I cut about 4 inches off of each and ground the ends flat and installed them as spacers and reinstalled the old springs. We now discovered something new that we had induced. With the widened gear and the effectively weaker springs, during taxiing the aircraft waddled like a duck. In fact, I threatened to paint the landing gear yellow so it would look like a duck. I never did talk Mira in the quacking while S-turning while taxiing. Prior to installing the longer spreader bars, on some Saturday afternoons, Mira would offer to let certain people fly the Bucker, with him in the front seat naturally, if they landed without a bounce he would pay them $10. If they bounced, they would pay one dollar per bounce. My part of the action was to take the proceeds to the local liquor store and buy beer. Dear old 707S kept the refrigerator well stocked. When we installed the extended spreader bars we had to go back to buying our own beer. For that reason and the atrocious taxi characteristics, I reinstalled the stock length bars. Besides that, we felt a Bücker ain’t a Bücker unless it sits on the ground with it’s nose high in the air! While I had the spreader bars off I welded some ½ by ½ “ pointed studs on the lower ends of the spreader bars so you could use a small bottle jack to lift each gear up to work on the tires, brakes etc. I did the same for Perry and he kept his A/C on jacks to keep the tires from flat spotting when parked even overnight.
Coil springs are a rather black art. Coil springs are really no different than a torsion bar in their function. Their strength is determined by the diameter of the wire used, the alloy of the wire and the length of the wire being wound. Many people think more coils equals more strength when exactly the opposite is true. For a given wire size, alloy and heat treat the longer the wire making up the spring, the weaker the spring is. A good example is when people cut coils off automobile front springs to lower the car. What they get is a car that rides hard and bottoms out easily.
707S got new Krybus gear during the overhaul. I believe the spreader bars are extended 1 inch. Mira was unhappy with the gear so he got new springs from Alan Abel. That still didn’t get 707S’ nose in the air as desired so on his next visit to Santa Paula I turned him a set of brass spacers. He got them installed and the results finally satisfied him. I have to add that Mira ALWAYS wheel lands a Bucker. I can never remember him landing even slightly tail low, much less three-point.
Don Vance