Winding Technique

[nulluser]

Hello! I am using the NFFS calculator, and a 10:1 winder but I am running into some issues.
1. What percentage of breaking should a rubber motor, on its 1st use, be wound to?
2. After this, how much should it be unwound?
3. Even with forceps, how can I avoid my knot contacting the motorstick and impeding untwisting?
4. Why does a helicopter untwist too slowly, to the point where shortening the loop from 20 inches to around 6-7 inches is needed?
Thank you.

[coachchuckaahs]

It would help us answer if you provided more details, including the rubber linear density, mass, number of winds, amount of stretch, torque, helicopter kit type, mass, rotor tip pitches, etc.

1. I would do a non- flying break in around 80% max turns, and perhaps a second break in around 90%. My teams look at torque more than Max turns, though max turns is a good indicator of whether you are reaching the potential of the motor

2. Unwind based on torque. For FFM kit properly built, most have found that unwinding to 1.3-1.4 oz-in is best. If you have stability issues at these levels, check you tip pitch angles, and try unwinding to add low as 0.8oz-in or so

3. The know should be placed at a location where it will not turn. If both rotors are on bearings, the knot will be near the middle of the motor stick. You may need to adjust slightly if the rotors spin at different rates. If the lower rotor is fixed to the motor stick, the know should go at that end.

4. What is your launch torque? What is your density and rubber mass? If typical, then a 20" loop is over 3g, which is probably way too large. Most are starting around 2g motor mass (12-13" loop).

Coach Chuck

[nulluser]

I'm stretch-winding to 4x relaxed length and coming in after around 55-60% winds. I'm winding to around 650 turns after backoff. I don't have a torque meter, and I'm using the J&H Twister, at 5.5 grams (note that we are not using this at comp day). The loop is 10 inches, not 20. Rubber is 1/8" Super Sport. Thank you.

[coachchuckaahs]

Without a torque meter it will be very difficult to consistently wind to 90% or more of breaking winds. The max winds calculator gives an indication of how well you are winding, but it does not replace a torque meter.

The launch torque on the helis is critically important. Too much and the blades can go into unstable flexing which can lead to flight instability or loss of duration.

I would suggest looking at the NFFS resources page, at https://www.freeflight.org/science-olym ... resources/, and in particular under the flying tab you will find a video and PDF report on how to make a super simple torque meter. In addition view the video by Coach Brian on winding technique, and be sure you are following that process, including lube, stretching, and torque.

It is also important to know your motor linear density. 1/8" stock rubber has been see to vary from about 0.059 to 0.081 g/in, primarily because the thickness (not the width) varies in the manufacturing process.

I do not have enough information about your rubber to estimate max turns, but it would appear they should be in the range of 1200 for a 1/8" stock rubber, though there is a wide variation depending on density, or on mass of the 10" loop. If this is the case, you are nowhere near the breaking turns.

Coach Chuck

[nulluser]

Thank you for this information. I am planning to go to my hardware store soon for this stuff. Also, Is a 10:1 winder the best ratio, and how can I avoid the twists on the rubber from hitting the motorstcik? Is the blades spinning too slow a aign of low torque/winds?

[coachchuckaahs]

10:1 is generally seen as best for the high torques needed for this year's rubber.

If you mean the knot is hitting the motor stick, then you need to place the knot at the tail hook if a single spinning rotor, or near the center for both rotors free to turn.

If you mean that the knots have "stringers" or "grapevines" to the sides, you need to improve your winding technique. Grapevines are caused by walking in too fast without winding. Most of your winds are put in while stretched, which gets twists without knots. As you walk in, at some point knots begin to form. The combination of walking in and continuing to wind will organize your knots into small knots instead of clusters and grapevines. You cannot add many turns toward the end of walking in, but if you do not add enough then your knots will grapevine.

Why do you think your blades are turning too slow? Yes, low torque would cause that, but what parameter are you measuring to determine your blades are "too slow"?

Coach Chuck

[nulluser]

I'm having grapevining issues, you perfectly described it! Thanks! By too slow, I mean that the rotor (even with turns left) isn't producing enough pure lift to get the darn thing in the air for long. I'm building a Tornado with better consturction techniques and precision, which may fix that.

[coachchuckaahs]

You could need to wind more, you could need wider (higher density) rubber (more torque throughout flight), the heli may be too heavy, or your pitch may be too high on the rotors.

If the heli is landing with a lot of turns remaining (say over 200, depends on the rubber, the heli, etc), then you need to either go to lower pitch or higher density (wider rubber). If it will not climb at the beginning of the flight, you probably need to launch with more torque. Typically you should be launching with 0.8 to 1.4 oz-in torque. Even if your rubber is low density, if you are launching with that kind of torque it should climb, but may come down early.

A heavy heli, or one with high pitch angles, may need excessively large rubber motors.

Coach Chuck

[nulluser]

Thank you! Just to recap:
1. Torque meter is crucial to success. Build right and you can build the NFFS way for cheap.
2. Grapevining = winding in too fast
3. Wind to >80% breaking.

What torque (for a J&H tornado) should I wind to? I don't want to risk damage to my heli (honestly if it breaks after so much hard work I will tear up) once I've finished building it.

[coachchuckaahs]

Grapevining is caused by walking in too fast, or winding too slow for how fast you walk in. You got it backwards. Must keep winding!

Once you are winding with a torque meter you should be able to go to 95% of max turns, or even more on later winds. The rubber will take more winds each time you wind it, but winding this hard it will probably last 4-6 windings. You can probably expect best performance around wind 4 or 5.

The torque you wind to does not depend on the heli, but on the rubber. For typical rubber we are using (in the 1/8" range) you should easily be able to wind it to 2.0 or more. That does NOT mean you put it on the heli at that torque. You then unwind on your torque meter down to your safe launch torque. This may be anywhere form 0.7 to 1.5, depending on your motorstick, and on how your rotors respond to higher torque. Unwinding takes energy out of the tank but is survivable. Putting a full motor on the heli will either bend/break the motor stick, or flutter the rotors which will shorten the flight or even cause a crash.

See NFFS resources on winding to further understand rubber hysteresis, the reason we wind up to near breaking and back down to launch torque, instead of just up to launch torque.

Coach Chuck