Helicopters C

[jander14indoor]

With both rotors free spinning from the motor stick (as on the Freedom Flight design), probably in the center is the best place for the knot. With both ends of the motor unwinding relatively evenly, that should be a relatively still place for the knot and it shouldn't hit the stick frequently. If it is spinning a lot, you don't have it at the center, or your rotors aren't using torque evenly. Try different spots till you find the stead spot, it does exist. If it does hit some, it shouldn't matter as it will just push the stick around some and the motor on either side is still free to spin the respective props.

If you have a design where one end of the motor is to a fixed hook on the motor stick which spins with a rotor, locate the knot at that fixed end.

The knot itself. The one I like is a double overhand knot (Correction! There is a knot called a "double overhand knot", if you look it up on the internet, it is NOT the not I describe in the following!). Holds securely even with lube and can be located wherever you want it on a loop. Should be tied with the motor lubed. Tie an overhand knot with the open tails, location not too important, but try to make the tails even, the tails don't need to be short (you'll fix that in the next step). Pull the two ends on the loop side of the knot apart to move the knot towards the ends, leaving very short tails, maybe 1/8 inch or less. It WILL move. Now, tie a second overhand knot in the loop close to the first, but loop it the opposite direction. Again, pull the ends on the loop side of the knot to slide the new knot up against the first. It will stop there and NOT slip further. Pull tight to make the knot small. If the tails aren't even and small, trim them now. They can be very short. The tails do no work, so make them SMALL.

This should give you a knot smaller than the cross section of the rubber where you want it with small tails.

Jeff Anderson
Livonia, MI

[BananaPirate]

With both rotors free spinning from the motor stick (as on the Freedom Flight design), probably in the center is the best place for the knot. With both ends of the motor unwinding relatively evenly, that should be a relatively still place for the knot and it shouldn't hit the stick frequently. If it is spinning a lot, you don't have it at the center, or your rotors aren't using torque evenly. Try different spots till you find the stead spot, it does exist. If it does hit some, it shouldn't matter as it will just push the stick around some and the motor on either side is still free to spin the respective props.

If you have a design where one end of the motor is to a fixed hook on the motor stick which spins with a rotor, locate the knot at that fixed end.

The knot itself. The one I like is a double overhand knot (Correction! There is a knot called a "double overhand knot", if you look it up on the internet, it is NOT the not I describe in the following!). Holds securely even with lube and can be located wherever you want it on a loop. Should be tied with the motor lubed. Tie an overhand knot with the open tails, location not too important, but try to make the tails even, the tails don't need to be short (you'll fix that in the next step). Pull the two ends on the loop side of the knot apart to move the knot towards the ends, leaving very short tails, maybe 1/8 inch or less. It WILL move. Now, tie a second overhand knot in the loop close to the first, but loop it the opposite direction. Again, pull the ends on the loop side of the knot to slide the new knot up against the first. It will stop there and NOT slip further. Pull tight to make the knot small. If the tails aren't even and small, trim them now. They can be very short. The tails do no work, so make them SMALL.

This should give you a knot smaller than the cross section of the rubber where you want it with small tails.

Jeff Anderson
Livonia, MI

First before I forget, thanks bernard for the timely response -- we were able to figure out the max number of winds today for the motors we are using.

Now for the motor stick issue...we seemed to have fixed the knots. It is no longer bumping into the motor stick (the clicking noise stopped). We let the heli keep unwinding after it dropped and confirmed this, but the motor stick is still spinning erratically. The other cause you said was that the rotors aren't using torque evenly. What does this mean exactly? Is there anything we can change in winding procedure to alleviate this? Right now we are just following the procedure in the instructions of the FF model - stretching out to 4x the length of the loop and walking in at around 70%. Are we missing some details? Thanks for the help!

[jander14indoor]

Sorry, not clear. Its nothing you are doing with winding that causes the rotors to use torque unevenly. Its in the design of the props themselves and their location. For example, if you have a four bladed prop on top and two below, it will take more energy to turn the upper rotor. Same torque is applied to both (has to be since they are on opposite ends of the same rubber band) but the upper rotor has more drag, turns slower, uses less of the rubber than the lower rotor. Therefore the neutral point on the rubber band won't be in the center.

Motor stick turning. Theoretically it should not as the rotors are free to turn from the stick and the rubber isn't applying torque to the stick. Practically, no bearing is perfect, some small amount of the motor torque is transmitted to the stick causing it to turn erratically. Not a problem, no way to stop it, doesn't use significant energy.

Jeff Anderson
Livonia, MI

[_deltaV]

On the subject of knots, I personally have tied a simple figure eight knot on the end of my rubber bands and this has worked for the past three years. I prefer this knot to any other because it has a very small profile that prevents bumping against most motorsticks. The only breakages I've gotten were from tightening the knot without lubricating the end of the rubber first.

[BananaPirate]

Sorry, not clear. Its nothing you are doing with winding that causes the rotors to use torque unevenly. Its in the design of the props themselves and their location. For example, if you have a four bladed prop on top and two below, it will take more energy to turn the upper rotor. Same torque is applied to both (has to be since they are on opposite ends of the same rubber band) but the upper rotor has more drag, turns slower, uses less of the rubber than the lower rotor. Therefore the neutral point on the rubber band won't be in the center.

Motor stick turning. Theoretically it should not as the rotors are free to turn from the stick and the rubber isn't applying torque to the stick. Practically, no bearing is perfect, some small amount of the motor torque is transmitted to the stick causing it to turn erratically. Not a problem, no way to stop it, doesn't use significant energy.

Jeff Anderson
Livonia, MI

Thanks for the help, a couple more questions...
1. If the splitting of the rubber by the knot on the "neutral point" could be considered with a ratio, for example, length to upper rotor:length to bottom rotor, would the ratio be the same no matter the length of the rubber, if everything else was constant? If yes, then we should be able to take any length of the same rubber, stretch it to X length, and know where the neutral point is to put the knot?
2. The devices we have right now are 3.01 and 3.20 grams; both are FF models. Would it be viable to sand some from the motor stick, or would any other mass reducing procedures work? From experience in bridges/towers, I believe that even a significant amount of sanding wouldn't decrease our masses by much. If this is the case though, our only option to decrease our mass would be to build more helis....

[bjt4888]

Banannapirate,

One possibility for mass reduction is to use a shorter, simpler ceiling bumper. I like t Freedom Flight spinning bumper but it does weih almost .25 grams. The pin "axle" inthe bumper alone weighs almost 0.1 grams. Another possibility is to build a custom fuselage that weighs less.

Brian T

[jander14indoor]

I think you pretty much have it, you're going to be building more helicopters (or major assemblies) to significantly reduce weight. Sanding is a dangerous way to go. Likely to weaken parts in unintended places significantly changing performance or plain breaking early.

Rotating ceiling bumper is neat, and may be a game changer in some locations. What are the ceilings like where you'll compete?

Were you real careful with glue, can you rebuild with less?

What covering did you use? I seem to remember the FF kit comes with indoor mylar, so probably not much room there.

What density did you use for rotor parts? Probably an opportunity there if you are good at wood selection. Same for the motor stick.

Does the motor stick really need to be that long?

Its a matter of careful detail at this point.

Jeff Anderson
Livonia, MI

[BananaPirate]

Banannapirate,

One possibility for mass reduction is to use a shorter, simpler ceiling bumper. I like t Freedom Flight spinning bumper but it does weih almost .25 grams. The pin "axle" inthe bumper alone weighs almost 0.1 grams. Another possibility is to build a custom fuselage that weighs less.

Brian T

Thanks for the tips! We are pretty inexperienced with building outside of the kits so we'll probably be making small/safer changes first. I think we might look at a new bumper though.

I think you pretty much have it, you're going to be building more helicopters (or major assemblies) to significantly reduce weight. Sanding is a dangerous way to go. Likely to weaken parts in unintended places significantly changing performance or plain breaking early.

Rotating ceiling bumper is neat, and may be a game changer in some locations. What are the ceilings like where you'll compete?

Were you real careful with glue, can you rebuild with less?

What covering did you use? I seem to remember the FF kit comes with indoor mylar, so probably not much room there.

What density did you use for rotor parts? Probably an opportunity there if you are good at wood selection. Same for the motor stick.

Does the motor stick really need to be that long?

Its a matter of careful detail at this point.

Jeff Anderson
Livonia, MI

What would be good locations for a rotating ceiling bumper? I don't really get how different testing locations can affect it vs a non rotating one. Our last competition was in a room with a height around 1.5-2x that of a normal classroom. The ceiling was pretty flat, made of the usually seen school ceiling tiles.

For using less dense parts, we're not sure where to look for such wood. The wood in the kits seem to be of pretty high quality already. Just to make sure, all the wood in the FF kits would be around the same density right? So popping out all the pieces and searching for the lightest ones wouldn't work? Just wanna make sure before we actually try that lol.

Also do you have any idea what I was talking about with the ratio on the knot or was I being confusing?

Thanks! Already learned a lot!

[jander14indoor]

<SNIP>
What would be good locations for a rotating ceiling bumper? I don't really get how different testing locations can affect it vs a non rotating one. Our last competition was in a room with a height around 1.5-2x that of a normal classroom. The ceiling was pretty flat, made of the usually seen school ceiling tiles.

Rotating bumper is only useful on ceilings its safe to bump. If you are flying in rafters, you should try to stay below them and a fixed bumper is fine.
The primary advantage of a rotating bumper is to keep from bumping around and moving sideways, perhaps into an obstacle or the wall and shortening your flight.

For using less dense parts, we're not sure where to look for such wood. The wood in the kits seem to be of pretty high quality already. Just to make sure, all the wood in the FF kits would be around the same density right? So popping out all the pieces and searching for the lightest ones wouldn't work? Just wanna make sure before we actually try that lol.

No, you can't assume all wood in a kit, or even a sheet is the same density. One of the things that makes balsa so interesting as an engineering material is the wide range of properties, even from two pieces cut side by side from a sheet. They will be close, but can still vary by 50% or more. Weighing individual parts can be valuable.
As to where I find wood, I always have a scale with me when I go into a hobby shop and sort wood by weight. I'll typically leave with a few sheets. Over time a I have a fairly large stock of good balsa of varying density.

Also do you have any idea what I was talking about with the ratio on the knot or was I being confusing?

Thanks! Already learned a lot!

Jeff Anderson
Livonia, MI

[BananaPirate]

<SNIP>
What would be good locations for a rotating ceiling bumper? I don't really get how different testing locations can affect it vs a non rotating one. Our last competition was in a room with a height around 1.5-2x that of a normal classroom. The ceiling was pretty flat, made of the usually seen school ceiling tiles.

Rotating bumper is only useful on ceilings its safe to bump. If you are flying in rafters, you should try to stay below them and a fixed bumper is fine.
The primary advantage of a rotating bumper is to keep from bumping around and moving sideways, perhaps into an obstacle or the wall and shortening your flight.

For using less dense parts, we're not sure where to look for such wood. The wood in the kits seem to be of pretty high quality already. Just to make sure, all the wood in the FF kits would be around the same density right? So popping out all the pieces and searching for the lightest ones wouldn't work? Just wanna make sure before we actually try that lol.

No, you can't assume all wood in a kit, or even a sheet is the same density. One of the things that makes balsa so interesting as an engineering material is the wide range of properties, even from two pieces cut side by side from a sheet. They will be close, but can still vary by 50% or more. Weighing individual parts can be valuable.
As to where I find wood, I always have a scale with me when I go into a hobby shop and sort wood by weight. I'll typically leave with a few sheets. Over time a I have a fairly large stock of good balsa of varying density.

Also do you have any idea what I was talking about with the ratio on the knot or was I being confusing?

Thanks! Already learned a lot!

Jeff Anderson
Livonia, MI

Ok, thanks again!