University Project - just some general questions for now

[Asidhunter]

So i';m in my final year of university studying product design and my major project is going to be based around a form of multirotor.

I will start by saying i am a complete novice when it comes to RC flight, so i have a few questions based around what i';m hoping to achieve.

First off is there a particular multirotor form that is more stable when hovering? (i.e is octo more stable than hex or quad)

Secondly is there a particular form that is more stable in high winds?

And final question for now what is more important for lift capacity in terms of weight? prop size and pitch of the motor kv?

Thanks for any help 

[chrischapman]

Heya!

I';m new to the forum too but have been building various multi-rotor aircraft for over 2 years and you are in the very same spot I was when I started so thought i';d offer some help to you :)

Welcome to a fascinating and engaging hobby, and the learning curve that comes with it... but to answer your questions;

Generally speaking, a Hex will be more stable than a Quad but the difference in stability is less obvious when you add a further 2 rotor to make an Octo. Really, the stability of the aircraft is down to the quality and set-up of your FC (flight controller). Sure, there are £20 options that will fly your beast but you will need that degree to get them set up right! I don';t think anyone could claim they just pressed a few buttons and away they went.

That is not to say that £20 controller couldn';t equal Naza';s (by DJI)... it';s just not likely. The Nava V2 by the way is very good.

Matching props and motors is where the maths and science kicks in. You also need to factor the battery voltage. There is no right way to do this, most manufacturers suggest a cell count (3s, 4s 6s etc.) for their motors and a recommended prop size too. If you';re really lucky they may give you thrust values but take those with pinches of salt.

Big props need to be spun slower so lower KV';s are required (KV is like RPM, well it is really but RPM per volt e.g.: 300kv would spin at 300RPM with 1V, 600RPM with 2V etc.)

A bigger pitch means the motor has to work harder to drive it through the air so causes heat to be generated if the motor is not up to it. This can lead to damage.

Finding the most efficient combination of power, motor KV and prop dimensions is the key to lift and battery longevity.

Hope this is helpful pal :)

[teslahed]

First off is there a particular multirotor form that is more stable when hovering? (i.e is octo more stable than hex or quad)

If all else is equal more motors provides more stability. A bit like with the stability of tables and the number of legs.

Longer arms provides more stability by increasing angular momentum (the motors are heavy and out on the ends of the arms) such that it takes more energy to start and stop the multirotor from spinning and improving the leverage of the motors about the centre (at the cost of maneuverability, weight and size).

Secondly is there a particular form that is more stable in high winds?

Smaller props have less gust response. But you need a certain amount of lifting power to get off the ground and more weight will improve stability in winds as well. Many people report that a Y6 or doubled up quadcopter offers more lifting power without providing more prop area for the wind to catch by doubling up 2 sets of motors and props one above another (at the cost of some efficiency / flight duration).

And final question for now what is more important for lift capacity in terms of weight? prop size and pitch of the motor kv?

Heavy lifters have larger diameter props spinning at sensible speeds rather than regular diameter props spinning faster. It';s more efficient.

[Asidhunter]

Thanks for the replies, things are starting to make a bit more sense now. My original idea was to use a H-quad as I need to carry an under slung-camera along with various environmental sensors, so the large centre beam would be ideal to mount this equipment too.

I did look at doubling up props and motors but as mentioned this will lower the flight times/efficiency, one of the main requirements for my project is that the multirotor can hover over an area taking reading for a sustained period of time. My guess is that it will be easier for me to choose the rotor setup once I have the figures for the flight weight that is needed.

In terms of parts does anyone know of a reliable UK based store for parts? mainly for the electronics, as the frame, bodyshell and any props will be designed by myself in university.

Last question for today, (you will be sick of this one I';m sure) eCalc is there any point in using it or is it more information than anyone ever needs?

[Hands0n]

I like to use eCalc to get me headed off in the right direction. But there is no real substitute for doing a bit of math, understanding the electronic theory and taking measurements.

The biggest threat to the outrunner motors is heat - that is when calculating and measuring do watch out for power consumption and leave plenty of margin.  Excessive heat will kill the neodymium magnets in no time flat.

[Asidhunter]

The biggest threat to the outrunner motors is heat - that is when calculating and measuring do watch out for power consumption and leave plenty of margin.  Excessive heat will kill the neodymium magnets in no time flat.

Yeah heat build up is one of my concerns right now as to meet safety requirements I will need to enclose the electronics, motors and cowl the props to prevent injury, as you mentioned though if the power consumption is kept in check and I leave a large enough margin this should be easy to combat using vents/ducts to allow air flow around the motor casing etc.

[Hands0n]

The outrunner motors do not get hot if you get it right. Mine are generally neutral to the ambient temperature to touch, even after a fairly aggressive flight. They don';t even begin to get warm.  I don';t really understand why you will need to enclose the electronics in terms of safety.  The danger will be to the electronics and not the humans  ::)

To be used in an enclosed space the ESC will need efficient air circulation within the enclosure, or you';ll need to use ESC that are rated in Amps much higher than any you are likely to draw with your intended motor/prop combination. But then again, adding cowling will increase weight, which will require greater power consumption to lift - it is a vicious circle.

Even prop guards need to be considered carefully in terms of weight - there are many solutions available, limited only by your own ingenuity. 

Back to the outrunner motor magnets - there is some good general reading and theory here --> http://www.kjmagnetics.com/blog.asp?p=temperature-and-neodymium-magnets  Its good bed time reading