Weird pattern in motors

[Stechnoblade]

So I was flying today and I noticed that for two of my motors, when I hit 2:07 (in 8ft), the next time I wind harder by about 20-30 turns and similar launch torque, it decreases a maximum of 9 seconds.
Motor 1:
33.3 cm, 0.125 in, 2.12g (0.767 g/in)
Launch turns: 1240
Launch torque: 0.95 in. -oz
Flight time: 2:07.52

Next flight
Launch turns: 1260
Launch torque: 0.96 in. -oz
Flight time 2:03.87

I don't recall any interesting factors that affected the flight time to decrease so much.

Motor 2:
33.7 cm, 0.125 in, 2.11 g (0.764 g/in)
Launch turns: 1260
Launch torque: 0.91
Flight time: 2:07.04

Next flight
Launch turns: 1295
Launch torque: 0.94
Flight time: 1:58.94

These two patterns of winding harder but having lower flight times are really interesting and weird. However, when I wound motor 2 another time, I got an even higher flight time! (I didn't wind motor 1 another time because I saw small cuts)
Launch turns: 1310
Launch torque: 0.98
Flight time: 2:12.15

I don't know why this is or if it's just a weird coincidence, but it certainly is weird. If anyone has any possible explanations I'd appreciate it! I'll also give an update if I see another pattern or not when I fly tomorrow.

[bjt4888]

So I was flying today and I noticed that for two of my motors, when I hit 2:07 (in 8ft), the next time I wind harder by about 20-30 turns and similar launch torque, it decreases a maximum of 9 seconds.
Motor 1:
33.3 cm, 0.125 in, 2.12g (0.767 g/in)
Launch turns: 1240
Launch torque: 0.95 in. -oz
Flight time: 2:07.52

Next flight
Launch turns: 1260
Launch torque: 0.96 in. -oz
Flight time 2:03.87

I don't recall any interesting factors that affected the flight time to decrease so much.

Motor 2:
33.7 cm, 0.125 in, 2.11 g (0.764 g/in)
Launch turns: 1260
Launch torque: 0.91
Flight time: 2:07.04

Next flight
Launch turns: 1295
Launch torque: 0.94
Flight time: 1:58.94

These two patterns of winding harder but having lower flight times are really interesting and weird. However, when I wound motor 2 another time, I got an even higher flight time! (I didn't wind motor 1 another time because I saw small cuts)
Launch turns: 1310
Launch torque: 0.98
Flight time: 2:12.15

I don't know why this is or if it's just a weird coincidence, but it certainly is weird. If anyone has any possible explanations I'd appreciate it! I'll also give an update if I see another pattern or not when I fly tomorrow.

Stechno,

Good questions and good data. Do you also have data on max turns and max torque and backoff turns in your log? These are critical data points in analyzing this situation.

If max torque, max turns and backoff are the same or similar for flights with same motor, then the lower flight time with 2nd use could be due to the motor average linear density being too low or the overall mass of the motor being slightly too high. Maybe try .078 g/in for the same 2.1 -2.13 motor overall weight.

Brian T

[Stechnoblade]

Hey Mr. Turnbull,
I'll copy and paste the flight details again with the max turns. I didn't include then at first since I thought they weren't as important as launch details.
Motor 1:
33.3 cm, 0.125 in, 2.12g (0.767 g/in)
Max turns: 1270
Launch turns: 1240
Max torque: 1.26 in. -oz
Launch torque: 0.95 in. -oz
Flight time: 2:07.52

Next flight
Max turns: 1290
Launch turns: 1260
Max torque: 1.4 in.-oz
Launch torque: 0.96 in. -oz
Flight time 2:03.87

Motor 2:
33.7 cm, 0.125 in, 2.11 g (0.764 g/in)
Max turns: 1280
Launch turns: 1260
Max torque: 1.23 in. -oz
Launch torque: 0.91 in. -oz
Flight time: 2:07.04

Next flight
Max turns: 1310
Launch turns: 1295
Max torque: 1.2 in. -oz
Launch torque: 0.94 in. -oz
Flight time: 1:58.94

Next flight
Max turns: 1330
Launch turns: 1310
Max torque: 1.19 in. -oz
Launch torque: 0.98 in. -oz
Flight time: 2:12.15

My motor-making abilities are not very consistent since I have no way to control the density of my rubber so I cut about 1 inch more than needed and evaluate it from there. I have just received a bag of 0.122" and 0.135" rubber today and am looking forward to experimenting with the different widths and densities. Some general tips would be helpful.
Thanks,
Stechno

[coachchuckaahs]

Stechno:

Did you break in the motor before flight, or is flight #1 the first winding of the motor?

I suspect this is the case (that it is first wind).
The motor will take more winds on later winding, each wind getting more than the prior, but especially going from first wind to second, as the motor breaks in. The first wind on the motor will be rather limited in winding, but will exhibit higher torque than any subsequent winding.

Since you are getting your best flights on the later winds, it is highly likely, as coach Brian notes, that your rubber is too low density (too narrow) for your helicopter. It also helps to check remaining winds at the end of the flight, that can give some insights. I suspect these went up on later flights.

I suggest trying denser (wider) rubber next, and recording remaining turns. Only the stopwatch matters, but remaining turns can be an indicator as to which way to go.

The wider rubber will take fewer turns but run at higher torque throughout the flight. Once broken in it may give you closer to optimum for your craft.

If you have limited or no ability to cut rubber widths, you can try shorter loops (lower mass). This will take fewer winds, but will result in lower total mass in the air and may increase your times. You will find an optimum mass for each rubber motor density (fully broken in). Ideally your best flights are around flights 3-6 on a rubber motor, where the motor is fully broken in but not worn out.

If you do have ability to strip rubber to density, you optimize the combination of density and mass. I normally hold density constant and change loop mass to get an optimum, then change density, re-optimize mass, and see which way the stopwatch says to try next (did it improve or get worse).

Coach Chuck

[Stechnoblade]

Hey Coach Chuck,
The motors had already been broken in on the first flight, but the remaining turns have increased for the flights that had lower times. I just tested the motor that got 2:12 today with similar winding to a previous flight that got 2:07 but this time it only got 1:56. It's probably worn out already and not useful since it's the rubber's 7th flight by now. Unfortunately I don't have a rubber stripper so I'll just use the method you outlined with the 0.135" I have.
Thanks,
Stechnoblade