Lift versus Thrust

[mbsteed]

I am trying to figure out the proper motors and props for a X8 design. I want to keep my options open so let us say I want to lift a 5D - camera, frame, mount, and electronics weigh in at 3667g

8 motors and each has 820 g of thrust = total of 6560 g of thrust (Rob Roxxi)

However, I have heard one should divide the total thrust in half to determine lift = 3280g of lift

Let us say for argument - 12 inch props.

An X8 configuration is suppose to be a bit less efficient not quite sure where that leaves me. What do I need to lift that kind of weight? Will this kind of configuration handle that or do I need larger motors? It seems to be a bit on overload side to me but what are your thoughts?

 

[matwelli]

ok, if a motor is rated at a maximum of 820g of thrust, probably at 150 watts, and internally the motor is trying to get rid of 15-20% of that as heat (for arguments sake 30 watts, or about the same as a soldering iron) - it can only do that with good airflow.

thats the prob, multicopters are essentially "standing still" most of the time

For that reason, and also so we have enough thrust for manuverability, aim to have 2 to 3 times as much thrust as you actually need, so the motors are working at a nice safe level, that doesnt heat them up too much

In your case, you are a bit over that figure, and you can possibly factor in a 20% loss in the x8 config as well.

I think you will get away with what you need to do, each motor will be using about 91 watts, but they will get warm

I would aim for motors, rated to handle 200-250 watts for your set up

The above is my humble opinion, many will beg to differ

 

[jes1111]

I think mbsteed is a little confused about the numbers. Thrust and lift are (effectively) the same thing. A multirotor will have a maximum amount of thrust (the total of the thrust from all engines at maximum throttle) - say that's 6560g. The "rule of thumb" is that you should be able to hover at about 50% throttle, i.e. all engines running at half throttle, so that you have some spare power available for manoeuvring the craft. There's no mystery to "hovering" - it is simply the point at which the total thrust from the motors equals the weight of the craft.

Mat's estimate of "2-3 times" is right for general craft - in the case of an aerial photography platform you'd be fine with "2 times", or even a bit less - you're not going to be doing aerobatics with it, right? Therefore, if your craft weighs, say, 4000g all-up, you'd want about 8000g or thereabouts of total thrust available. Don't sweat the exact numbers - "thereabouts" will be fine: 7000g total thrust, 9000g total thrust - all good!

The actual thrust produced by a motor will depend on the prop size - larger prop equals more thrust. But larger props need larger motors to spin them, which need more battery to power them, etc.... hence the "black art" of choosing batt/ESC/motor/prop combos.

The X8 "efficiency loss" refers to coaxial motors - the bottom motor will produce a bit less thrust than the top one (at the same throttle setting) because the bottom motor/prop is working with the disturbed airflow from the top one.

To get a good idea of the right size for everything, see: http://www.droidworx.com.au/specifications.html

Hope this helps.

 

[Ryan Thomas]

Just my 2c

I have a normal okto with the roxxys lifting a 550D, weight is around 4kg, I am planing to get bigger motors as the robby's don't have enough power for my liking.

I moved to 5S and this helped but I live at 1700m above sea level and get 7m of flight, I plan to use the avroto

 

[mbsteed]

Thanks for the responses on this question, and your feedback helped clarify and reinforce concepts. The Droidworx spec recommendations is also a useful resource that I hadn't seen before. That helps give me a good sense for the direction I need to go.

 

[CopterCam]

I think mbsteed is a little confused about the numbers. Thrust and lift are (effectively) the same thing. A multirotor will have a maximum amount of thrust (the total of the thrust from all engines at maximum throttle) - say that's 6560g. The "rule of thumb" is that you should be able to hover at about 50% throttle, i.e. all engines running at half throttle, so that you have some spare power available for manoeuvring the craft. There's no mystery to "hovering" - it is simply the point at which the total thrust from the motors equals the weight of the craft.

Mat's estimate of "2-3 times" is right for general craft - in the case of an aerial photography platform you'd be fine with "2 times", or even a bit less - you're not going to be doing aerobatics with it, right? Therefore, if your craft weighs, say, 4000g all-up, you'd want about 8000g or thereabouts of total thrust available. Don't sweat the exact numbers - "thereabouts" will be fine: 7000g total thrust, 9000g total thrust - all good!

The actual thrust produced by a motor will depend on the prop size - larger prop equals more thrust. But larger props need larger motors to spin them, which need more battery to power them, etc.... hence the "black art" of choosing batt/ESC/motor/prop combos.

The X8 "efficiency loss" refers to coaxial motors - the bottom motor will produce a bit less thrust than the top one (at the same throttle setting) because the bottom motor/prop is working with the disturbed airflow from the top one.

To get a good idea of the right size for everything, see: http://www.droidworx.com.au/specifications.html

Hope this helps.

Jeremy,

Thanks for your clarification on the sometimes confusing topic of 'Thrust'............. it has given me a great 'lift'

Sid