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Best power to weight ratio setup


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dav1d
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Joined: 03 Dec 2015
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Location: Texas

PostPosted: Wed Nov 23, 2016 8:13 am
PostPost subject: Best power to weight ratio setup
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I hope that this is a reasonable question to ask, as I am a total beginner at the selection of suitable components for electric rc helicopters.

I am writing to ask for recommendations on a combination of motor, ESC and battery pack for these candidates for an (idealized) rotor and power/weight is a critical factor. The concept employs two identical rotors.

What I'm working on is an experimental VTOL proof-of-concept prototype. I have written a brute-force Python script which has produced "candidates" for the design of rotors that develop a given amount of thrust per watt absorbed. I've picked 6 of the most promising looking rotors, all having the same diametrical dimensions and rotor RPM but which differ only in their leading and trailing edge angles (all fixed pitch) and the amount of power they (theoretically) absorb. At this point the design has halted until I know at what RPM a suitable motor will turn before I can proceed with gearing design.

Candidate (a) theoretically absorbs 91 [watt]. That particular rotor develops 2118 [gram], (74.7 [oz], 4.67 [lb]) thrust. That wattage absorbed seems a little low compared to the rest, but if it flies, then I'll take it.

The other 5 candidates theoretically develop the same thrust at the same RPM but differ in the amount of power they absorb:

(b) 176 [watt]
(c) 257 [watt]
(d) 333 [watt]
(e) 405 [watt]
(f) 474 [watt]

Any help you can offer would be very much appreciated.
Thank you in advance. ~ Dav1d
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Burgess
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Joined: 27 Mar 2012
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PostPosted: Wed Nov 23, 2016 7:17 pm
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Hello Dave and welcome to the forum.
Now that's a very interesting but intriguing question.
Brute-force Python script and helicopters, Are you somefing t' do wiv Monty-python flying circus? Wink

Sounds like your looking to design a twin rotor drone, and thinking along the lines of using cambered or under cambered foil section blades, so no 3D flight capabilities.

You have theoretical figures for thrust, rotor disk size, head speed, and mechanical pitch angle. With this size really does matter, so without these figures it's difficult to arrive at any meaningful power train (battery, ESC, and motor) recommendations.
Can you provide:
#1 rotor diameter
#2 rotor head speed
#3 all up weight of proposed craft
#4 configuration, tandem, side-by-side, or coaxial.

Burgess
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dav1d
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Joined: 03 Dec 2015
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Location: Texas

PostPosted: Wed Nov 23, 2016 9:12 pm
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Hi Burgess,
#1: 11.4 [in]
#2: 1000 [RPM]
#3: won't know until motor speed is known
#4: parallel
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Burgess
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Location: Wales

PostPosted: Thu Nov 24, 2016 8:43 am
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Hi David


#1: 11.4 [in] 290mm
#2: 1000 [RPM]
#3: AUW, won't know until motor speed is known
#4: parallel (I'm assuming you mean transverse).

You can use your projected thrust figures to approximate a maximum all up weight. The more the AUW is below that the faster it will move.


Brushless out-runner motors: shaft rpm designated by the motor kv, that is (1,000 revs per volt), for a given series of motors the weight will vary a few grams depending on the windings, -the number of turns and number of strands per turn of transformer wire.
Within a given series, the number of windings determine the output shaft rotation speed and the wire gauge determines the current rating, and the strength of the magnets the efficiency of the motor. The type of winding star or delta effects the spool-up and torque.

A slow revving brushless motor can produce unwanted vibrations and so for your projected head speed using a high kv and gear it down will get round that problem and well balanced motors come at a price.

Here is a link I think you'll find useful, you can plug-in you figures and change the variables.

http://dhrc.rchomepage.com/calc.htm

With that size of rotor diameter and fixed pitch, propellers might more advantageous than rotor blades. With rotor blades there is always some initial out of balance wobble/vibrations, with two rotors this might prove very unstable, especially with such a low head speed, whereas props will be more stable on initial spool up.

Your theoretical rotor head speed seems low for the amount of thrust produced.

Here's a couple of links to an Align 150 and 250, these will give you info relevant to rotor heads above and below your intended rotor disk size. Also the head speed and power train specs.

http://www.align.com.tw/helicopter-en/trex150/

http://www.align.com.tw/helicopter-en/trex250/

Alternatively the new Walkera Pandora Warrior might be a good case to study for design and functionality.


Here's another couple of links i think you'll find useful and inspiering.

http://www.fpvguy.com/2013/09/pandora_warrior_v1.html

https://www.youtube.com/watch?v=AJQ8c7KOYsg




Burgess
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dav1d
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Joined: 03 Dec 2015
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Location: Texas

PostPosted: Thu Nov 24, 2016 9:22 pm
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Thanks Burgess, I'll check out those links. Cool
David
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dav1d
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PostPosted: Thu Nov 24, 2016 10:08 pm
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Hi Burgess,
The rotor dimension, I gave you earlier(11.4 [in.]), was radius.
The rotor diameter therefore is 22.8 [in.].
Sorry for the confusion.
David
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