Helicopters C

[wx4caster]

We are using the 2017 FFM design, but are having trouble cutting the copter weight. Our copters are consistently coming in at around 3.5 grams. (2 g. for rotors, 4 blade X 2 blade / 1.5 grams from stick).

Does the FFM rotor stick need to be that long??

We are worried that a shorter stick will not allow us to put enough winds in our rubber. BTW, we are using about 2 g. of .094 rubber.

We know we are too heavy, but want to find ways to cut the weight. We thought about shortening the stick by half. I would rather not shave or sand the stick much, as this led to a catastrophic break (tangled mess) to one of our copters.

With the 3.5 g. copter we are consistently getting 40-60 secs. But worried this isn't long enough.

[jander14indoor]

As a general rule, the only way to effectively lose weight, especially that much, is to build new components. You kind of found that out by trying to shave your motor stick already. You are going to have to look at each sub-assembly and figure out where you can use lighter components. Make a bill of materials with every parts weight before building and adjust wood density/sizes so the parts add up to the target. Building to weight takes PLANNING

Motor stick length will not affect how many turns you can get in the motor. The main risk as hook to hook length gets too short (and that is much shorter than you think) is motor hanging up on stick. You should be able to shorten some without any significant constraint on winds. You should also look for lower density wood. NOT just a lighter stick, but much lower density and a larger stick, but in a ratio that lets you maintain stiffness and lose weight at the same time.

Rotors. Are you sure 4 blade rotors are worth it? If it would allow you to drop the weight by half I'd seriously consider 2 blade rotors.
Or, find lighter wood and go with 3-blade rotors. Really light wood and accept some breakage to get back to 4 blade rotors.
What density wood are you using for the rotor spars? How much does that plastic hub in the FF kit weigh vs some balsa sticks doing the same job?
Can you lose some weight on the spinning top bumper?

Jeff Anderson
Livonia, MI

[calgoddard]

The 2017 FFM helicopter kit uses carbon fiber (CF) composite rods for the spars of the rotor blades. I would not reduce the diameter of these CF composite rods from the provided .020" OD rods. You won't save any significant weight using balsa wood spars if you use balsa wood spars that are strong enough to survive the inevitable whacking against beams.

[jander14indoor]

I have to admit it will be hard to beat that with balsa, but you need to lose weight SOMEWHERE. Extra weight has to have a BIG payback and I doubt the extra blades are worth it. I'd really look at building a two or three bladed set of rotors and comparing. And there is no reason to say you have to have the same number of blades at each end. Try four top, two bottom. Three top, two bottom. Whatever it takes to hit the min weight.

Jeff Anderson
Livonia, MI

[calgoddard]

.020 inch OD carbon fiber (CF) composite rod weighs .0075 grams per inch.

1/16 inch x 1/16 inch balsa wood stick, 8# density, weighs .0083 grams per inch.

Stability in a rubber powered helicopters is of paramount importance.

Achieving that stability very much depends upon the configuration of the upper and lower rotors and their relationship to each other.

The 2017 FFM helicopter building instructions show a 2-bladed rotor on the top and a 4-bladed rotor on the bottom, which is not recommended. Those instructions recommend more blades on the top than on the bottom.

[wx4caster]

Reply to jander and others,

We are gonna try reducing the weight by going with a 2 and 2 configuration. Going with the 2 blade on top reduces the weight by almost 0.48 grams. So we are now closer to 3.0 grams. We are still worried about stability of the 2 x 2 configuration. An initial test flight with a not fully wound motor led to some very erratic action. (think of a very mad may fly on a ceiling).

We are also gonna try a shorter motor stick (remnants of a previous catastrophe) and see if we can lose the other 0.5 g. this way, and still get enough winds out of it. We may even try a 2 x 3 config, and see how that goes.

Test and restesting is certainly the key. Our regional is still a few weeks away.

[Eureka314]

Hey, what's a good length for the rubber motor length? The thicknesses I have are .094'' and .085''. Thanks!

[P2P]

Freedom Flight recommends around 2g, I use less than that. Try different values to find what works best for you, it shouldn't be too hard (maybe an hour or so)

[yew]

Besides shorter motor sticks saving weight and longer ones helping stability, do y'all think motor stick length (or more of hook to hook distance of motor) has any other factor when flying?
Like, assuming the motor weight/length/width is the same, after winding would a shorter or longer hook to hook distance affect how fast the winds unwind or torque or anything at all?

[jander14indoor]

Very little effect until you get so short that you have trouble with the unwinding motor hitting the motor stick, or so long that you can't pack in as many winds.

Jeff Anderson
Livonia, MI