After more experiments of adding and/or removing the filter, I can confirm I have experienced all the points you mentioned above. But, another thing that I’ve experienced is when I have the same RPM number whether it’s with/without the filter on, I got the same power, break, tension, lap times, except the fuel consumption/flying times.
I thought the one with the filter would produce lower power even when we close the needle to get the same RPM number.
Is this something to be expected?
Yes, it's absolutely critical thing to understand. But, I sure hope you did as I suggested, because while you were adjusting the needle a priori, you would also note that the engine run got "flatter' as you leaned it out. A restrictive venturi, run leaner, will have much less variation in flight and under load. With lots of restriction - in this case, extreme restriction - will require you to run the engine much harder and closer to peaked, meaning you will get very little power variation with load. Running it richer with less restriction "frees up" the engine to respond to load. The entire process of developing the engine setup is playing off the various factors to get the best combination of overall power and power variation, without also killing the airplane performance in other ways (trim and turn capability).
But of course, the more restrictive the venturi, the less peak power you have, and you would either get less RPM at the maximum with a given prop, or less prop at a given RPM. Your range of testing probably never hit the far end, and in general, you never care very much about the maximum power, since almost all engines now provide far, far more than enough, and at RPM you can't use. You could fly your airplane with reasonable effectiveness with anything from a 25FP to a PA75, it only needs about .35 shaft HP, and they can all greatly exceed that. What's why over the range you tested, you didn't see any effect of the venturi on the power, because even the wimpiest setup is far more than you could possibly use.
What you may have missed about it is the responsiveness changing, that is not terribly surprising because that takes some feel to achieve. My suggested test was to allow you to see the range of variation, and maybe get some of that feel. And - since you were about 2 needle clicks away from a very good usable system many posts ago, you would hit that on the way.
Brett
p.s while you were trying different degrees of restrictions, you should have noticed that you needed to run the engine closer to peak/leaner to get the desired power/lap time. That's the problem with shotgunning multiple changes, because you could have easily confused yourself trying to shoot out one change after another. For instance, if you are going to try different venturis/restrictions, you would want to hold the fuel/prop constant, and adjusted the engine to whatever it needed to get a particular fixed lap time. Note that a particular lap time always requires the same power to overcome drag, and holding the prop constant and the lap time constant means you are controlling the in-flight shaft power. Then, as you change the venturi/restriction, you will see how running it richer or leaner affects the variability of the run. That's how you pick venturis - not to alter the maximum power (which you aren't going to sue anyway), but to move where you are in terms or rich/lean.
I would add - I am having trouble following what you did, and what your results were for the individual things you tried, and under what conditions. If you really did all the suggestions above, and were pretty careful about it, I would guess it would take maybe 200 flights, that is a lot of flights since July 11.
p.p.s. As an example (bearing in mind I have been doing this pretty successfully for 40+ years...), this weekend, I changed my prop. It was the same design/diameter as the prop I had been running since 2017, but, instead of 3.75" of pitch, it was 3.9 - a mere .15" of pitch different. The reason I wanted to try something different was that the engine was, to my ear and several learned observers, "too flat", that is, it had too little variation in flight and wasn't helping me when I need the power to change. More pitch allows me to run it slightly richer, moving it further down on the torque curve to free it up (allow it to vary more due to load in flight). So, I to a single test flight on the old engine, launch revs about 10,000-10100 in the cool morning air, I got the expected 5.25 second laps. As soon as I landed, I switched the prop and went right back up again, no waiting so I could get the same air conditions. I guessed at about 9700 as a target, I fired it up, it was 9700 ish, sounded pretty good, so I just left it. Flew, marginally too fast (5.2), next round about 1/2 hour later, warmed up, so I arbitrarity backed the needle off about 10 degrees (no clicks, unfortunately) and it came up about 9500-9600 toggling back and forth. This was too much, because it was now too slow (5.3) Another back-to-back, I put it back about 5 degrees, checked it on the ground, solid 9600. Good, back to 5.25.
OK, great, after 4 flights over an hour, I can begin to evaluate the change. Seemed pretty good, so I left it, chased the needle in as it warmed up during the session, from maybe 65ish to 80 degreee. Generally successful, it is running richer in level flight and responding more (richening up when unloaded, leaning out when loaded). So, generally good change and that is as far as I can go until I drive 2400 miles and fly it in Muncie.
But that is me, with an engine that I have probably flown 1000+ stunt patterns in every condition over the last 18 years, in a series of nearly-identical airplanes, all over the country, and I can manage to evaluate one single tiny change in about 7-8 flights. I note that this is on the heels of another change that David and I made *6 months ago* that had the effect of "flattening" the run, to good in his case, and overdone in my case.
Topic: Need Help: to Identify Engine Run (Read 12296 times)