Helicopter Rotor Direct Drive

[Quadrotor]

Hi,

I have started working on a multi rotor project.

It uses a 4 blade rotor form a size 450 helicopter, with 4 325mm carbon blades.

My problem is driving the rotor. I planned using direct drive, thus eliminating the use of a gearbox. Only after purchasing motor (turnigy 3536 910KV) I read up on how fast they spin.

I need my rotor head to turn at about 3400 rpm. However the motor I have will be turning at about 10,000rpm @ 11.1v. This is about 3 times to fast.

My question is, is this how it works out in reality or will the weight/drag of the rotor and blades reduce the speed of the motor. If it doesn't then s it feasible to only use the first 30% of the motors speed capability with the use of ESCs.

[Planehazza]

Hi mate, it depends on the ESCs. Most don't like to be run so low.  You would really need a lower Kv motor or gear it.

[Quadrotor]

Hi planehazza, looks like your not far from me unless your in Newcastle under lyme.

I have had a look for lower KV motors, they run much higher voltages and seem to defeat to point.

I found a 350kv motor but it required 6 - 10s. That's 22v - 37v.
So even at 6s it will run at 7700Rpm, about twice the speed I need. I imagine the torque on that would be massive too.

Looks like im going to have to design a gearbox.

[Varelco]

You are going to be extremely hard pressed to find a motor that will run this heli direct drive. You simply won't have the torque unless you run a big motor which isn't going to be feasible because of weight.

[Quadrotor]

Am i correct in thinking this is the reason that smaller high speed motors are used. The speed is then geared down therby increasing the torque.

Im looking into planetary gearing at the moment. Should be quite interesting.

[RcRotaryMadness]

To run the rotor directly from the motor is a bad idea in general...

To get the right head speed you would need a really low KV motor running on the right voltage.. Generally this means the current draw is going to be sky high and the motors bogging is going to be more noticeable.

Electric motors (for this purpose at least) like to be run near full speed/power which usually means anything from 30K to 50K rpm on the motor (depending). You need the gearbox to convert that into 3,500 rpm Headspeed or whatever but with more torque behind it.

It just doesn't work hence why all helis have some sort of gearing involved.

[Quadrotor]

Ok thats fair enough. Makes perfect sense.

I am looking at how i can gear it without having the motor offset from the centre of the craft. Plastic gears are obviously used as there lighter but how long do they last at these speeds??

[trillian]

How many 450 size rotors are you using? I don't think you'll need 3400 rpm unless you anticipate some serious heavy lifting. One rotor at less than 3000 rpm carries an 800 gram 450 heli very easily (hovering you could probably get down in the low 2000s).

I assume you're planning to use the cyclic pitch / swashplates? (otherwise a full main rotor is overly complicated and you could use something like a tailrotor mechanism instead). If you use four rotor units, of whatever type and number of blades, you'll need two run in the opposite direction to counteract the torque.

My thought would be if you want to use variable pitch (not cyclic) the easiest way to go is using tailrotors like the Next-D Stingray and those you can spin around 10,000 rpm if you want but obviously they are much smaller, like 110/115mm blades.

[RcRotaryMadness]

How many 450 size rotors are you using?

Thats the same thought I had but I think the OP means 1 rotor with 4 blades on it... not 4 rotor heads like his post implies.. I think lol

[trillian]

Thats the same thought I had but I think the OP means 1 rotor with 4 blades on it... not 4 rotor heads like his post implies.. I think lol

Given the forum nickname of 'quadrotor' I interpreted it as some kind of quad / multi rotor device using several 450 size rotorheads.

[Quadrotor]

Sorry if i wasnt clear. I am going to use 1 size 450 rotor this will have 4 x 325mm blades attached.
Counter rotation will be provided by 3 smaller props, these props will also give me the ability to yaw, pitch and roll.

The main 450 rotor will have variable pitch and speed. Thats all i require. The 450 rotor is to provide lift only.

[RcRotaryMadness]

Given the forum nickname of 'quadrotor' I interpreted it as some kind of quad / multi rotor device using several 450 size rotorheads.

Yeah I did too but his post is a bit unclear..

"I have started working on a multi rotor project. " - Suggests a quadcopter type machine

"It uses a 4 blade rotor from a size 450 helicopter, with 4 325mm carbon blades." and the reference to "rotor" rather than "rotors" suggests one rotor.

Perhaps the OP could clarify lol

Edit: Oops he just did while replying.

OP: How come your not just building a 450? Why use 3 props instead of conventional methods? Is it a science project or some other project?

[Quadrotor]

Its not a science project no, Just a project for me to play with really.

I like to try different approaches to things, I like the idea behind quad copters but I think that there just too inefficient.

@rcrotarymadness, i didn't want to just go and build a standard helicopter, i like a challenge. This project of mine will incorporate mechanical design and electronics, both of which i am skilled at.
I am a toolmaker by trade so can pretty much make anything i can imagine, and i play with electronics as a hobby.
I have written software for accelerometers, magnetometers, gyros, and barometric pressure sensors. On top of that i can also get GPS units and ultrasonic distance sensors that i can write software for.

[RcRotaryMadness]

...All sounds very interesting - will you be posting a build thread? =)

[Quadrotor]

I will if you want me to