I keep reading and hearing stuff from time to time that makes my hair stand on end, so I thought I would give you an alternative view, and hopefully help lift a little fog.
I have a saying: “If you assume the obvious, you are probably wrong.” Be careful when someone tells you something that makes a lot of sense, appeals to what you already want to believe and costs you money.
I have lots of other sayings that I bore my friends with. For example, “I’ll sell my mother for 200 RPM.” If I had just 10 RPM for every time someone has given me their deepest, darkest secret for picking up 1000 RPM, I’d be King Kong himself. The fact of the matter is, given everything else equal, nothing helps that much except a fundamental change like Nitro, Tuned Pipes, Turbocharging, etc. Not a 2-thousands shim!!!
Then what causes people to believe they have hit the mother lode? Bad data. It’s really easy to get bad data when you only take one datum point. It happens to me all the time. Oops, time for another Jett saying, “Everything works one time in a row!” Sometimes I get some really great results from a test, so I close the shop and go home. Then I get to savor the thought all night long before I go back and confirm the test. Usually I get different and more disappointing results the next time.
How, then, can we get better data? Work harder. Be careful and make sure you are really testing what you think you are testing. Here are some good rules for collecting data:
- Only do your testing on a test stand. Airplanes are lousy test beds.
- Always change only one thing at a time. More than once a test is messed up by sloppy procedure—always ask yourself, what have I changed?
- Repeat the test more than once, and in reverse order. For example, you test a prop and it turns 24000. You unbolt the prop and put on another. It turns 22000. Which is faster. You don’t know. The engine and muffler were cold the first time and hot the second. Try the test over, but with the propellers in reverse order. The same thing holds true for almost any kind of test you run. If you change something and it picks up, then reverse the change and see if it slows down—Sometimes it picks up again!!!!!
- Make sure Atmosphere, engine and muffler temperatures are the same. You can’t spread you tests over several hours. Things change.
- Keeps notes on what you did—you will forget!!!!
- Be very careful what propeller you tach on. Flying props are not good tach props. When you tach too close to the staging point of the pipe just a 200 RPM difference in engines could make the difference between getting on the pipe or not. Thus you see huge differences in engines that are essentially the same. Use a small prop to make most comparisons. Larger propellers can be useful, but make sure you know what you are measuring.
- Repeat your test on a different day. You’ll be surprised that things work one day and not the next.
In summary, everything works one time in a row.
Dub
