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Feb 07, 2010, 04:13 PM
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outsider787's Avatar
Build Log

Variable pitch multi-rotor copter :: Development Stage

I've decided to build a variable pitch quad-copter. (was tri-copter)
It will use 1 motor in the center, driving the 4 rotors.(was 3 rotors)
OLD:For the first prototype I will be using the $13 dollar tail assemblies from HK of a 500 size heli along with the tail boom.
I am going with K450 components instead of 500.
This will be the proof of concept. I can always upgrade to the all metal tail assy if the design looks promising.
Eventually I will also want to duct the prop (for higher efficiency) but for now, start with un-ducted.

Two version are planned: direct drive (where the motor, and gearing happens at the rotor) and shaft drive (aka torque tube)

Belt drive has been attempted with success. See posts on page 9 for details on the setup used.

The tail prop blades that come with a 450 heli sweep a 4.25" diam.
Will use 205mm blades from a 250 heli instead.

Calculated thrust can be up to 3kg per rotor (see page 9 for calculations)

Any advice on what to use?

What I need help with is figuring out how to size some things.
-What RPM should I spin the blades at?
Since I will be using 205mm blades (from a 250 sized heli) they can be spun at 5500rpm-ish. At those speeds, tip speed is getting close to stalling. Need to confirm that the root can support the centrifugal loading at the high RPM.
The 450heli tail spins at over 10,000rpm, so running the system at 1/2 of that speed should be good for mechanical components.
-How do I size the motor?
Since I would like to be spinning 4 blades of a 250 heli, that would take about 200W to produce (see post 122). Make sure to add enough power to cover inefficiency of the system.
-low Kv or high Kv?
Working out the gearing ratios, the tail rotor spins at 1/3 the speed of the motor. So if I want 4000-5500 rpm on the tail, I will need a 1000kv motor (running at 14.8v) That should allow me to do about 5000rpm rotor speed at 4S.

And any advice you can think of giving me, I will gladly take.
UPDATE: I've been going about the design for this quad the wrong way. I've been trying to produce more and more thrust out of a set of blades, when I should have focused more on the efficiency of the system. How efficiently am I using the power I have available? Read my thoughts and findings starting with post #149.
Because I'm looking at the system differently, (or maybe with a different goal in mind) I am resurrecting the belt drive idea, mainly because if it works, it can be a very cheap system to build.

UPDATE: I am abandonning the belt drive idea. I've done all I can (with cheap stock heli components). I don't feel more work on the concept will yield any vastly better results.

Just an update on my mission/vision:
I have decided to fork my build.
I will keep the existing belt driven design on the go, but will also pursue a direct motor drive variable pitch system.

My main goal is to have variable pitch system. How it gets powered is secondary.

UPDATE: I have decided to use all HK450 parts, not 500.
UPDATE: Direction has changed from a tri to a quad. I've been working on the design of the tri, and I can't come up with an elegant design that uses as many stock heli parts as possible, with minimal machining.
Switching the design to a quad makes the design much more elegant, and simple. I like simple.
UPDATE: Added some screen shots of the CAD design in progress. See post #17 here.
UPDATE: I will use the Flymentor 3D heli controller. I don't think there's any controller yet that will control servos for a variable pitch prop (at lease not the ones that I can purchase). The Flymentor gives me what I need, at a reasonalbe cost, without any electronics building. I've talked about it in post #26.
Last edited by outsider787; May 19, 2010 at 02:53 PM. Reason: Changed the title of the thread
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Feb 08, 2010, 03:19 AM
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Hi, I am also in the planning stages of a variable pitch multirotor, but I will use 4 rotors/tails, stabilized with the Flymentor.
I have all the parts in order, and have started to receive some parts.
I decided to use the 3DX500 tail.
In the center I will use 4 6mm shafts, each with a 450 maingear and a pulley for the belt. This way I get 2xcw and 2xccw rotors.
The motor will drive one of the shafts with a belt/pulley, clutch bearing on the pulley.
First i will try with 205mm carbon blades, spinning around 4000-4500rpm.
The "mainshafts" will spin about 2000rpm.
First I will try with this motor:

Pulleys and belts I ordered from
Last edited by krial; Feb 08, 2010 at 09:45 AM.
Feb 08, 2010, 10:03 AM
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capt's Avatar
If you could stack 3 pulleys on the motor shaft you could direct drive the props, eliminate alot of gearing.
Feb 08, 2010, 10:19 AM
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outsider787's Avatar
Originally Posted by capt
If you could stack 3 pulleys on the motor shaft you could direct drive the props, eliminate alot of gearing.
It would certanly make things simple. Haven't tried it but I fear the belt will rub against the boom through which it travels, if the gear is offset in the center of the copter.
Can someone with a 450 or 500 confirm how much room there is inside the tail boom between the stretched out belt and inside of the boom?
Feb 11, 2010, 01:34 PM
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outsider787's Avatar
I just ordered a HK450 kit from HobbyKing.
It was cheaper then getting just the individual components that I needed.
Once it arrives, I'll be able to measure out some components and further the design.

I need to sit down and dream up how to attach everything together in a tricopter configuration.
I am planning on using as many 450 parts as possible, to keep the complexity and number of machined parts of the build as low as possible.
I'll post some CAD drawings when I get something.
Feb 17, 2010, 01:49 PM
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I'd thought the belt tention x three would be too over bearing and vibrational .
Just a thought, no personal expert experiance.

Feb 17, 2010, 02:27 PM
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outsider787's Avatar
I am currently still waiting for the 450 kit from HK, and when that arrives I'm making a rig to do some bench testing on one tail rotor.

Based on my findings I will decide if I want to pursue this project further.
I will make sure to note how much vibration the belt system introduces into the whole system.
Feb 18, 2010, 02:23 AM
Registered User
Originally Posted by outsider787
I am currently still waiting for the 450 kit from HK, and when that arrives I'm making a rig to do some bench testing on one tail rotor.

Based on my findings I will decide if I want to pursue this project further.
I will make sure to note how much vibration the belt system introduces into the whole system.
And the HK500 would be a gear and shaft drive to the tail ??, I'd do that idea personaly.
Feb 18, 2010, 03:23 AM
SebastianJ's Avatar
SCORPION VTOL Prototype v1.0 (0 min 58 sec)
Feb 18, 2010, 03:24 AM
SebastianJ's Avatar
SCORPION SEV2.5 Random Clips 1 (0 min 49 sec)
Feb 18, 2010, 03:25 AM
SebastianJ's Avatar
SCORPION SEV2.0 Prototype (0 min 43 sec)
Feb 18, 2010, 06:15 AM
Registered User
belt or shaft drive ?? that is my question !.
nice video s .
Feb 18, 2010, 06:50 AM
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outsider787's Avatar
Shaft drive would likely be more efficient (I think a timing belt system has more losses then geared) but belt drive is what I have access to.
The 450 Pro is shaft driven tail rotor, but I haven't found any cheap source for parts to make it worth while.
If someone knows of a source for parts on cheap chinese knockoffs of the Align parts for a shaft driven rc heli, please let me know.
It looks like HobbyKing sells the 450Pro shaft drive kit here. There are no parts listed for it yet though.
Last edited by outsider787; Feb 18, 2010 at 07:16 AM.
Feb 18, 2010, 10:04 AM
Registered User
I was checking HK ing out today. Shaft will have some twisting slop but it will only be going forward, ruled that out.
Most airbourne turbine powered platforms back the 1960's failed becuase of a singlular engine failiure issue .Can you get independent motorS attached directly to the collective pitch tailS ?.
Feb 18, 2010, 10:07 AM
Registered User
Last edited by hobbybox; Mar 05, 2010 at 09:55 AM.

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