Gentlemen, I have calculated air speed and distance for the 4 pitch and 3 pitch to see what differences I would be looking at for a circumference of a 395.6' circle (63' from bellcrank to my handle). I figured 12,500rpm at launch and 13,000 once flying at speed. I'm not experienced enough to tell RPM from 2 and 4 cycle so I am guessing at flying RPM. I have an RPM meter for before takeoff. So I found this formula on Heads Up Hobby for converting rpm to speed if you know the prop pitch.
Multiply the RPM by the propeller pitch (e.g., RPM 5699 x 6 = 34194)
Divide the sum of the above calculation by 1056* (e.g., 34194 / 1056 = 32.380)
The dividend of 32.38 is the speed in miles per hour.
My Numbers Below
13000 x 4 = 52000
52000 / 1056 = 49.24mph
circumference / speed = time
395.6'/49.2mph= 8.04 seconds lap time
Someone suggested using a 3 pitch prop which changes speed to 36.9 and lap time to 10.7 seconds. Will this bird fly at that speed. All I am looking for is level flight maybe inverted flight so I can overcome my dizziness. Am I missing something? Thanks again guys
I think that equation is quite wrong. It tells you you are getting 8 seconds laps, which is quite obviously too slow.
Part of the problem is you are calculating the lap time from the "speed" incorrectly -you are mixing feet and miles and hours and seconds. 49.24 mph is actually 72 feet/second (5280 feet/mile, 3600 seconds/hour) 49.24*5280/3600 = 72.2 feet/second). This means you would expect a lap time of about 396/72.2 =~5.5 seconds. This, too, is pretty slow, but fast enough that you could gently maneuver.
We already know about what you will be getting with a 25LA/9-4/Flite Streak, its more like 4.5 seconds, which is pretty brisk for a new flier/retread, but not outrageous.
Problem number 2 is that the equation to calculate the speed is wrong, and only comes out pretty close by coincidence. The equation suggests that the prop screws itself through the air at 4"/rev. That's wrong in several regards - It is labelled 4" because if you compute the angle required to go 4" in 1 rev, the back face of the blade more-or-less matches that angle. The actual pitch, that is, the distance it would travel forward in one rev if giving no lift, is probably more like 6.5-7".
But under no circumstances does that, say, 7" pitch, screw it self forward 7" in a rev, because there is substantial drag, so you have generate some thrust, meaning the prop is not putting out zero thrust, and travels much less than 7" in a rev. In this case, the prop is pretty small, so it might only go 50% of 7", or about 3.5", in one rev. A larger prop that is otherwise the same pitch, would travel further in one rev, maybe 60-65% of 7". So, just looking at the pitch, it more-or-less coincidentally works out to about 4, but that depends on many other factors that are not considered.
I think the suggestion to use a larger prop might actually speed it up. That's why I recommend the APC 9-4 instead of a 10-4 - it's too fast with the 10-4, and the otherwise identical 9-4 prop travels forward less per rev, and slows it down. It will not spin the larger prop as fast on the ground, but once it is in the air, it will spin nearly as fast at the 9" prop, and be more efficient, so might actually go faster rather than slower.
Third thing is that your inflight RPM is probably much higher than 13,000 - more like 14, we have tested very similar models many times, the last a few weeks ago, using an audio tachometer from the center of the circle.
So, not to be indelicate, but there are bunch of mistakes in the analysis that coincidentally gives you an answer within about 25% or so of reality.
3" of (marked/measured) pitch will drastically slow it down, in the the 5.5-5.8 seconds a lap (from the current 4.5 or so) which is probably the way to go here. It will not have stellar stunt performance at that speed, but it will help to get over getting dizzy, just be careful trying to do stunts.
Once you *do* get over it, I think you will find the 9-4 and a constant 2-stroke setting to be much better - 4.5-4.6 is pretty fast in level flight, but the speed is so well-controlled in the maneuvers that it seems like you have all day.
You are not treading new ground here, we have done hundreds of tests using airplanes and engines like this, there are no real mysteries about how it's going to go. I think you put on a 10-3 for a while, noodle around until you get comfortable, then speed it up and run it like indicated. It is fully capable of 500+point competition flights and in the right hands would win just about every local expert class contest as long as there were no typical NATS qualifiers around.
You are doing good, please let us all know how you make out. We are here to help!
Brett