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Custom Autopilot development - Theory through Implementation

Started by Wil Selby. Last reply by Stone1295 6 hours ago. 2 Replies

I started this project wanting to learn more about the ArduCopter software that was powering my APM board and 3DR Quad-C frame. I found …Continue

Tags: equations of motion, MATLAB, autopilot, Control System, ArduCopter

GPS err in altitude vs. flying a drone at low altitute 3 mts

Started by Darius Jack yesterday. 0 Replies

Hi,I would like to study GPS err in altitude,since Cala suggested problems flying a drone at 3 mts altitude in agriculture.Laser Developer offers Lidar technology solution.Let me know your  opinion…Continue

Motor spin slow than others

Started by Cristian Rougier. Last reply by Khoa Vu yesterday. 19 Replies

Hello, all , im working with APM 2.5, in X configuration, when i arm the APM, one motor (motor 4) spin slow than others 3.When i calibrate the ESC's in automatic mode, all spin with same speed and…Continue

Noob APM/PixHawk RC transmitter coming soon to HobbyKing

Started by Tony Kenward. Last reply by Khoa Vu yesterday. 3 Replies

I am not interested in purchasing one of these, but I thought y'all may be interested. [Chinglish description text spell-corrected and posted below]--------------------------------------The Quanum i8…Continue

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Comment by Rui Manuel Cravo Marques on February 24, 2012 at 8:50am

Chris, I appreciate the facts ... and yes, I like beeing corrected, since lot of the math, units and facts miss me and I like to learn. I only tryied to put it in not mathematical terms since I've been using some different motors and the lifting and static thrust they can achieve are somehow far from what expected, so I started using ecalc and that was the best thing I encountered to help me. :)

Comment by Jani Hirvinen on February 24, 2012 at 8:04am

In generally those calculators and others are always wrong as they miss many important parameters like temperature, humidity, general air pressure, altitude of testing. We at jDrones are testing every week many many motors with different propellers and batteries to find out how to tune motors and how to get bet results out from those. 

Theres one picture of our motor test rigs and Gap (our engineer) is running test runs for some of our test motors. 

In last month itself we ran different tests more than 200 hours, tests are performed always with same propellers and fully charged batteries to get as accurate results as possible.

Tests what we run are as close as possible to hover conditions and they give us a lot of good data to tune our motors.

So far we have been running tests for over 150 different motors (our own custom and reference motors) and 8 most commonly used propellers (for multicopter uses). Based on these results there will be new even better motors coming soon.

Motor KVs have been from 300kv to 1500kv and propellers from 6" ABS to 15" CarbonFiber, Batteries 3S, 4S and even 5S.

Besides of technical and scientific data it's really nice to run those motors, as most of you know we are located in hot Asian country so some wind movement is always welcome :)

Ps. No animals were harmed during these tests but few flowers have gotten hit and several batteries....

Comment by Chris Gough on February 24, 2012 at 7:27am

@Rui Manuel Cravo Marques, I agree that motors marked "suitable for a 1 kg plane" are not suitable for 1/4 of a 4kg quad. A 1kg plane requires a lot less than 1kg of thrust to maintain altitude or climb at a reasonable angle, so the motor probably produces less than 1/2 a kg of "thrust".

Yes I have pushed a car, and yes, I was doing all the work. What's more, I have manually maneuvered an 11 ton boat too. That didn't require 11 million g of "thrust"!

Thrust is pushing force that can be used to accelerate a mass and/or counteract drag, and is properly measured in Newtons. Measuring it in weight is a OK on the surface earth where counteracting gravity’s force against a 1kg mass requires 9.8 N in the opposite direction. That doesn't make sense somewhere with a different gravitational force though, like in space, weight is relative to gravity but thrust is thrust is thrust.

When pushing a 2 ton car on the level, the ground is pushing up against gravity, you don't need to supply that force yourself. It would take an extra ~20,000 N to hold it over your head (Aaarg!), but accelerating it along the level ground only requires sufficient force to overcome inertia (accelerate the mass) + rolling resistance (counteract the drag). This is similar to a propeller pushing a plane, where the wings provide lifting force that carries the weight of the plane (but inducing some drag that has to be overcome in addition to the parasitic drag).

I put it to you that a set of four motors/props that actually produce 1200g static thrust each IS almost enough to hover (but not climb) a quadcopter with an AUW of ALMOST 4800g. Not lift (climb), and only "almost" enough because some spare capacity is required for stabilisation (not all motors can produce maximum output all the time unless the airframe is inherently stable).

I'm not suggesting anyone should take a quad with a theoretical maximum thrust of 4.8kg, load it up to 4.5 kg and expect it to fly. Given the the need to stabilise and navigate, prop unloading as airspeed increases, operation in a rotating column of air etc. etc. maybe 2kg is a more practical AUW limit for that quad (depending on how you want to use it, prop-speed of the motor/prop combo, etc).

I think you and I would probably both agree about what constitutes a sensible amount of thrust for any given plane or multicopter. I only got all pedantic and gave you the physics refresher because you said "correct me if I'm wrong" :)

Comment by Ante Vukorepa on February 24, 2012 at 7:17am

The thrust stated in the specs is usually "static" thrust. Meaning, under perfect conditions, with 100% efficiency, you will be able to *prevent* the equivalen load *from falling*, not do anything useful with it. Which is why a hefty margin on top of that is usually a good idea.

Comment by Ramesh Tahlan on February 24, 2012 at 7:14am

About the issue of thrust, yes, if ur motor has 1000 g thrust, it is supposed to be able to lift a 1000g wt, however, because u never get 100% efficiency, it better to take 80% of that value, so it will lift 800g physically, that is how u calulate the motor thrust u need for a planes. so for a trainer for which u could use a 600g thrust motor for a all up flying wt of 1000g, or 700 to 800 g motor thrust for sport flying of same fly wt  of 1000g or 1200g to 1500g motor thrust for 3D. That is the direct equation and u will never go wrong. There is something called thrust to wt ration, if u have a plane of 1:1 thrust to wt ration, (good enough for sport but not 3D) then if fly wt of plane is 1000g , ur motor thrust is 1000g, but do remember that motor thrust to be 1000g do take efficiency into account so u would actually take a motor thrust of 1200g for 1:1 ration.

Comment by Rui Manuel Cravo Marques on February 24, 2012 at 6:20am

Chris Gough, yes me too. Correct me if I'm wrong, but have you tryied to push a car ? It has about 2000kgs, are you making that force ? I seriouslly doubt that 1000gr of thrust in spec sheets would be lifting capability but more "suited for a 1000gr airplane model", so it accelerates untill it has a speed where air can sustain the weight. I see spec of motors with 1200gr thrust with a 10" prop... put 4 of this on a quad and see if you can lift 4800gr :)

Comment by Marco Robustini on February 24, 2012 at 5:01am

Ecalc work fine, when I designed my X8 got it right:

Comment by Chris Gough on February 24, 2012 at 4:45am

Pay attention, sometimes motors have in their specs the thrust, but it's not lifting thrust but horizontal thrust (for planes)

??? I thought thrust was thrust.

motocalc works well for me.

Comment by Rui Manuel Cravo Marques on February 24, 2012 at 4:38am

Gjango, I don't know if you're familiar with this site : 

The thrust depends on several factors : motor power, motor kv, propeller used. A 100w motor with 800kv and a 13" propeller will give you less thrust than a 300w motor with 800kv and a 13" prop, since the last one will be able to maitain higher rpms with such a "eavy" prop, but the motor will also be heavier. Pay attention, sometimes motors have in their specs the thrust, but it's not lifting thrust but horizontal thrust (for planes)

Comment by Gjango on February 24, 2012 at 4:28am

hi to all

How can I calculate the thrust produced per motor in kg?


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