Rotors

[chalker7]

Forgive me if this has been asked, but I didn't see such a clarification listed.


I am in a division where rotors are defined as "surfaces that contribute lift by rotating on a common path around a vertical axis. The helicopter may use up to three fixed pitch rotors, not exceeding a maximum diameter of 35.0 cm."

Does 'Fixed Pitch' mean:

1. Pitch does not change during flight
2. The pitch (Angle of the chord) is constant across the radius of the rotor

# 2. would seem to outlaw many of the interesting designs, like helical or elliptical (optimized) rotors.

This is not the place for official clarifications, but generally within our terminology fixed pitch is the opposite of variable pitch, which is a technique (detailed here: http://indoorduration.com/ftp/VPPropConstruction.pdf ) that allows the pitch of the rotor (or propeller) to change throughout the flight dependent on the torque for the rubber band.

[Kovu]

Would small rotors and a high pitch equal a helicopter that could utilize a thinner rubber. Or do I have the wrong idea, right now were running .155 and getting a minute and a half but I would like to break two minutes.

[chia]

Oh no, it is 2:27. I just tested another one however (still with non-helical rotors) but it had cured rotors and it got a time of 2:49.

Eek, if you'll be at COD, I've really got to step up my game!

I've noticed a problem I keep having with my free-spinning top rotor: it often breaks off either just as I let the copter go or if it runs into anything during flight. It doesn't collapse together like somebody mentioned earlier, it just come off because it can't handle the rubber when wound past ~700 winds. I'm aware the method I use probably isn't the strongest/most efficient (I build my helical rotor completely, then glue the wire hook to the center spar, then another piece of wood on top so that the wire is sandwiched tightly in between), but I'm not sure how to build a better joint (actually, I'm brainstorming as I write this and just though of one way, but I'd still appreciate any suggestions you have to offer).

[illusionist]

What are alternatives to that method? I too build the entire rotor on a jig, then glue the wire to both the top and bottom spars. I think jander14indoor might have mentioned something in last year's thread about it...

[eta150]

What are alternatives to that method? I too build the entire rotor on a jig, then glue the wire to both the top and bottom spars. I think jander14indoor might have mentioned something in last year's thread about it...

One way is to thread the wire through a central spar (sandwich 2 balsa pieces), bend a 90 degree angle at the top and glue it there. You can wet the wire with glue (just a tiny bit, don't overdo it) to make it really strong. It's not the lightest way, but it might help with the breakages.

[illusionist]

Be central spar, do you mean parallel to or perpendicular to the rotor shaft?

[jander14indoor]

This comes up periodically.

Be central spar, do you mean parallel to or perpendicular to the rotor shaft?

Not the original poster, but in the answer I'm getting ready to expand on, perpendicular to rotor shaft.

OK, let me make sure I understand the original problem. You wind your motor, attach it to the "free" prop shaft and when you launch, the prop shaft breaks free from the rotor. Further, let me guess, your prop shaft has a hook for the motor, then goes straight through the bearing, up through the rotor WITH NO BENDS and is attached to the rotor ONLY with glue.

This won't work!!! Remember, you are dealing with torque, the smaller the shaft, the higher the shear FORCE on the glue. Depending only on glue on that SMALL diameter, small area is a recipe for failure. You might have got away with it last year, but the bigger motors trending this year are causing more failues.

<SNIP>One way is to thread the wire through a central spar (sandwich 2 balsa pieces), bend a 90 degree angle at the top and glue it there. You can wet the wire with glue (just a tiny bit, don't overdo it) to make it really strong. It's not the lightest way, but it might help with the breakages.

Yep, that's how to fix this issue. Bold emphasis is mine. Though I actually bend the end of my rotor shaft into a U so it goes through the spar, over to the side, and back down, hooking the spar. Now you have to break the SPAR, not the glue. And the U is wide enough that the leg coming back down to catch the rotor is at least 3-5 times the radius, dropping the force accordingly. Now you actually need very little glue to hold everything in place. A slightly wider U will be even lower force. I've never had a wire break loose set up this way.

Jeff Anderson
Livonia, MI

[illusionist]

I actually use that method. Here's a picture that I uploaded last year of one of my shafts- http://gallery.scioly.org/details.php?image_id=3443

That would be the spar that is furthest from the bearing. The shaft is glued next to the one that is closes to the bearing. The issue is that the shaft becomes free from the spar closes to the bearing and so the rotor collapses. It doesn't break, but rather flattens.

[chia]

Thanks, all of you. I've heard of this method, too, but for some reason had forgotten about it (I don't know why, it seems pretty common sense). I think the reason I wasn't in the habit of using it before was because I usually just cut the wire longer so that it functions as the ceiling probe as well, but I'll definitely start doing it this way instead.

By central spar, do you mean parallel to or perpendicular to the rotor shaft?

Whoops, I meant center shaft, the way jander interpreted it, not spar. That's what I get for posting when I'm too tired to get my terminology straight.

[chalker7]

. Though I actually bend the end of my rotor shaft into a U so it goes through the spar, over to the side, and back down, hooking the spar. Now you have to break the SPAR, not the glue. And the U is wide enough that the leg coming back down to catch the rotor is at least 3-5 times the radius, dropping the force accordingly. Now you actually need very little glue to hold everything in place. A slightly wider U will be even lower force. I've never had a wire break loose set up this way.

Jeff Anderson
Livonia, MI

This is the same technique I use and highly suggest.

[illusionist]

Just rereading that, Mr. Anderson said through the spar. Do you mean poke a hole right in the middle of a 1/16 spar?

[mrsteven]

Just rereading that, Mr. Anderson said through the spar. Do you mean poke a hole right in the middle of a 1/16 spar?

indeed. be careful

Although when you glue it in, if the wood cracks youll be fine. CA will hold that strong

[chalker7]

Just rereading that, Mr. Anderson said through the spar. Do you mean poke a hole right in the middle of a 1/16 spar?

indeed. be careful

Although when you glue it in, if the wood cracks youll be fine. CA will hold that strong

I actually prefer piercing a larger, shorter stick, (1/8" square x 1") and then attaching it to the spar. This way you don't have to worry about breaking the spar and can replace the motor hook or rotors if need be. I just posted a picture to the wiki showing the technique, so once it gets validated you'll be able to see clearly what I'm describing.

[jander14indoor]

I actually use that method. Here's a picture that I uploaded last year of one of my shafts- http://gallery.scioly.org/details.php?image_id=3443

That would be the spar that is furthest from the bearing. The shaft is glued next to the one that is closes to the bearing. The issue is that the shaft becomes free from the spar closes to the bearing and so the rotor collapses. It doesn't break, but rather flattens.

You need a rib or post between your two spars.

Jeff Anderson
Livonia, MI

[illusionist]

I'll try out the method on the current helicopter I'm building.

On a different topic, what's the best way to attach the ribs in between spars? Currently I put the spars on my jig and cut ribs (angled on the ends where they meet the spars) to fit on top rather than in between them. Could I gain something by putting them in between so that they are flush with the surface of the rotor?