and Dave Stillinger shared this:
I have been challenged by some landing stability problems and often wondered what specifications/adjustments are in order to gain the best results when on the wheels. I’m not sure I can answer that, but I have some observations on toe in/toe out to pass along.
Never had much issue with my Jungmann of 10 years as the previous owners apparently had everything up to snuff and it seemed to track fine. I purchased my Jungmeister 5 years ago and that thing was possessed on ANY landing. Very unstable and took tons of footwork to keep it rolling anywhere near straight. So began my investigations into toe in/toe out issues.
Luckily for me I have access to several Jungmanns and John Hickman’s beautiful Jungmeister here at Gillespie field to refer to and measure.
Your article eloquently shows how complex the geometry behaves and thus firstly the need for a common way to infer where a particular wheel geometry is set.
Since I was measuring several aircraft I took the easy route. My definition of wheel toe angle is taken when the aircraft is sitting three point, on its wheels, full fuel, but no other loads.
I built a small jig that picks up the wheel rim, so tire variation would not affect measurements and the fenders could remain in place.
Degrees are calculated relative to the aircraft centerline.
I measured my two airplanes first:
- Jungmann- 0.9 degree tow out. Standard spreader bars (29.5 inches between mounting bolt C/L’s)
- Jungmeister- 1.3 degrees tow in. Standard spreader bars
Went to the other hangars and measured.
- John’s Jungmeister- 0 degrees tow. Standard spreader bars
- Jungmann #3- 1.3 degrees tow out, extended spreader bars (2 inch addition each side)
- Junmann #4- 1.5 degrees tow in, extended spreader bars ( 1 inch addition each side)
- Jungmann #5- 0.3 degrees tow in, standard spreader bars
I felt my Jungmeister had excessive tow in and that could be the potential problem. The discrepancy was in the landing gear tripod. Again measuring all the available units on site I discovered the location of the lower pivot joint on the tripod, where the spreader bars connect, was 1 inch further aft than any of the others, which were within a 1/8 inch of each other. That rotates the spreader bars and thus the wheels into an excessive tow in position. I had a new tripod built to spec and installed it.
This resulted in 1.1 degrees of tow out when measured using the jig.
The character of the airplane during the landing phase completely changed. Lands like the Jungmann and is quite well behaved.
So by my personal experience I believe toe in of the wheels, measured when the aircraft is sitting three point, is NOT GOOD. I think neutral is ideal, with a small degree of tow in/out acceptable. I don’t have extended spreader bars, but Jungmann # 4 seems to land OK with toe in using extended spreader bars. That I don’t understand other than the wider footprint of the main landing gear with less wheel caster may be helping. Damping, spring condition, spreader bar and tripod geometry, all obviously play a role in the dynamics. All I can say is a stock landing gear should be near neutral toe when in three point configuration and that some tow out is better than toe in!
It is interesting to look at the log books of my Jungmeister. It was with Albert Ruesch in his aerobatic club, then rebuilt and taken to Austria for their aerobatic club and then back to Switzerland. Probably most, if not all, of its flying was from grass runways. When it was sold to the USA it went to Idaho and was reassembled and flown off an asphalt runway. It was badly ground looped on its first landing and had to have its tail feathers reworked! It was put in a museum for many years and not flown again by those folks! It was purchased by a crop duster friend years later. He can fly anything off of any surface and he flew it without incident but not many times. I don’t know how often he was on grass or asphalt. I bought it and reassembled it at Gillespie field and flew it off of asphalt. That first landing was not fun. No crosswind and I still kissed the wingtip. I struggled with it for a few years and managed to keep it in one piece, but it took changing the toe in condition to a slight toe out to make it a more enjoyable airplane on asphalt.
Dave