I helped a friend take apart his motor to figure out why it wouldn’t start. The symptom: The flash-start pull-start wasn’t “flashing,” and the motor wasn’t starting. He had already taken apart, reassembled and replaced the flash starter. We also replaced the carburetor, which is often the cause of troublesome motor issues (but not in this case). Spark plug replaced, gaskets replaced, piston/head cleaned, etc. — nothing standard was fixing the problem.
There was no resistance when he pulled the pull-starter handle. The flash-start mechanism isolates the user from the compression of the engine, but you still feel *something*. We were getting nothing. I hooked up a compression tester: 0 psi compression when we rotated the piston– I’ve never seen that before. We took apart the cylinder a second time and looked at the rings. They were not springing out to contact the cylinder wall, and they were stuck flush(!) with the piston sides, allowing air/fuel mix to easily blow by the piston, completely voiding the combustion chamber. The flash-start needs this compression to wind its “flash” spring.
I don’t know if the motor would have started even if we could get the first bang to happen. Even if it did run, the power would be extremely lacking. I’m not sure exactly what would happen with so much blow-by (combustion gases migrating from the front of the piston to the crankcase). My friend has a new piston on the way.
The most common cause of stuck piston rings is excess heat when the engine is running, usually due to a lean condition. Not using a 2-stroke lubricating oil might also cause something like this to happen. In my friend’s case, I don’t think he ran it too hot; there was no other indication of heat damage (and perhaps someone with more experience reading pistons can tell me what the picture of the piston tells them). There was an amount of carbon built up on the piston and head consistent with 30 hours of use. He might be changing oil brands though, just in case.
Tim got his first flight at the end of June. After 6 towed launches, he was more than ready for motorized flight. His first flight was on the full Moon, so I had him fly a few more laps than usual for a first flight so he could take in the scenery. He’s had a couple more since then, but we ran into issues with his used motor: it wouldn’t shut off when he landed– the kill switch wasn’t working. Now he gets to learn about motor maintenance.
High humidity and temperature
Temperatures have been in the 80s and 90s over the 4th of July weekend. We flew a few days, and those launches were brutal! There was no wind to help, What little wind we did receive was switching directions, and we ran into issues with the high density altitude. Once off the ground however, flying was calm and warm. I even flew a tandem (one successful launch, one non-successful), which was surprising that we could launch in this stuff.
I’ll briefly explain Density Altitude. Think of it as “the altitude that the weather conditions make it feel like.” High Density Altitude means the conditions are making it seem like you’re launching at a high altitude. Low Density Altitude makes it seem like you’re launching at a low altitude. Note, it’s not (High Density) Altitude, it’s High (Density Altitude)– See what I did there?
Hypothetically just varying altitude, planes have a harder time launching at Denver (5000 feet above sea level, more or less) than in Kennebunk (10 feet above sea level). This is because, assuming equal temperatures and humidity, the air at 5000 feet is less dense than the air at sea level. Less air density at high altitude results in less lift force, and fewer oxygen molecules for the motor to burn. You end up needing to run faster to get the same lift, and at the same time the motor produces less power than it normally does. Mostly it’s the running that sucks.
Besides high altitude, two other things can make the air less dense: High Humidity and High Temperature. The result of higher humidity or temperature is the same as the result of high altitude– faster run, less power from the motor.
In a nutshell:
High Temperature AND High Humidity -> REALLY High density altitude. Launches will be hard.
either High Humidity OR High Temperature -> SOMEWHAT High density altitude. Launches will be somewhat difficult.
65ºF temperature, 50% humidity -> density altitude feels approximately like the actual altitude. launches will be easy.
Low Temperature AND/OR Low Humidity -> (NICE!) Low density altitude. Launches will be really easy.
I finished the USPPA Tandem Trainee program– Thanks to all my test passengers, especially Chris Parish, who was in the seat for 15 of my test flights! I am now a Tandem Foot-Launch PPG Instructor. I’m excited to add this new dimension to training. Now your first solo flight doesn’t have to be your first-ever flight.
If there’s one thing I’ve learned about doing Tandems in Maine in the summer, it’s that our summer’s high Density Altitude is not helping. If you’re going to fly with me this summer, you’re going to need to run hard for at least 30 feet. To be on the safe side, I’m not taking anybody over 200lbs until I’ve had a chance to test a heavier passenger who has flying experience. I’ve had my first tandem flight attempt during a windless, hot and humid day wherein the motor was not producing enough power to lift us off the ground. Earlier in the evening, I had a marginal launch that took ages to gain altitude.
I’m using an Air Conception Nitro 200 for tandems. It’s adequate, but a more powerful motor would help, so maybe next year I’ll be wearing an Air Conception Tornado 280 for tandems. However, I really like that I have a motor that I can fly solo, and then also use the same motor for tandems. I also like that the Nitro 200 is so light on my back. I like it so much that I have a brand new electric-start, clutched Nitro 200 coming to me in the next week. I may use it for tandems (that was my intent when I ordered it), but I would consider selling it brand new to a former student (warranty included).
The Tandem wing (MacPara Pasha 6 42m with T-Ride risers) has been amazing! It’s like a giant EN-A-rated school wing that rises easily and points straight. I think I’ve flubbed one launch in the 27 I’ve done so far, which is all the more amazing since most of them were done in no wind. This wing has our backs.
Interesting observation: Flying the 42m tandem wing with a 130-lb passenger and my 185-lb self results in the same fuel consumption as when I fly a solo 20m wing; we just fly at a little more than half the speed.