Absolute Beginner trying to build a UAV

[Ryan Orleans]

Hi there! So, as the thread says, I'm a complete beginner in anything model flying but I have been tasked to design, build, and fly an UAV. This is a group assignment for my final year in university so it's sort of a big deal. We've been given a few pre-determined components which we must base our design around and we're currently near the end of the early stages of our designs. As we are in a group, our lecturer deemed it proper to have the groups assign certain individuals a specialist role regarding a specific aspect of the aircraft. As luck would have it, my group has given me the specialist role concerning Power plants and Systems, which I am admittedly pretty crap in. So far, I've found a calculator online which can help determine the static and dynamic thrust that our prop and engine can provide, and I have come up with a few sketches for the circuit design of our electrical system but I'm pretty much stuck in a rut. The following here is our list of electrical specs:

[Ryan Orleans]

- Motor:

KD 2213 1920KV Brushless Outrunner Motor

Specifications:

* Kv: 1920

* Dimensions: 27.6 x 32mm

* Weight: 59g

* Shaft Size: 3.175mm

- Servo:

Towerpro SG90 9G Micro Servo

Features: Coreless Motor, All Nylon Gears, Connector Wire Length 150mm, Universal plug for just about ALL radio systems!

SG90 Specs:

Length (mm) 23.1

Width (mm) 12.2

Height (mm) 29

Weight (g) 9

Operating Speed(4.8v) 0.12sec/60 degrees

Stall Torque(4.8v) 17.5oz/in (1kg/cm)

Temperature Range -30 to +60

Degree CDead Band Width 7usec

Operating Voltage 3.0-7.2 Volts

- Batteries:

3 cell 11.1v, LiPo 20c 500mAH

3 cell 11.1v, LiPo 20c 800mAH

3 cell 11.1v, LiPo 20c 1300mAH

- Reciever:

DSM2 AR6100E Compatible 6Ch 2.4Ghz Receiver

Technology Specification:

Type: Microlite Receiver

Number of Channels: 6

Modulation: DSM2

Band: 2.4GHz

Dimensions:

Width: 18.37mm

Length: 40mm

Height: 9.80mm

Weight: 4.4g

Voltage Range Input: 3.5-9.6Volt

Key Features

1.6-channel Microlite receiver

2.QuickConnect with Brownout Detection

3.Red LED indicates number of holds

4.Short 30mm antennas for mounting in almost any location.

- Speed controller:

Hobbywing Pentium 18amp Brushless Motor esc speed controller Specification

Output Continuous 18A, Burst 22A

Input Voltage 2-4 cells lithium battery or 5-12 cells NiCd/NiMh battery.

BEC 2A / 5V Linear mode BEC.

Refresh rate of the throttle signal 50Hz to 432Hz.

Max Speed:

210000rpm for 2 Poles BLM,

70000rpm for 6 poles BLM,

35000rpm for 12 poles BLM.

Length 55

Width 25

Height 6

Weight (g) 21

[Ryan Orleans]

What I'm stuck on is that to do my thrust calculations, I need RPM. And to get my RPM, I need my Kv which I have, and I need the voltage that goes into my motor. But I don't know how much voltage goes into my motor if I don't know how much voltage my battery can offer it once the rest of the components have had their fill. Thus, I need a finalised design of my circuit. So what I did is I put my ESC and Rx in parallel together, then connected the motor in series after them, and then had 5 servos that are all also in parallel with each other connected after the motor in series. How much voltage would this leave my motor? Did I do this right? I'm told the ESC is the key to good voltage distribution, but how do I accomplish this? Am I even thinking correctly? I apologise for this very lengthy post, but I've totally exhausted all the rest of my resources now. Any kind of help would be greatly appreciated.

[kc]

It sounds to me as though you are way out of your depth with this! Check the basic elctric flight sections of this forum.
Beware that an electric motor with prop on can instantly start and severly injure you - easily cut off fingers or worse!
A Lipo can easily cause a fire if shorted.
People have been killed by model aircraft going out of control, many injured themselves with props, so join a proper club and get instruction and insurance & learn safety procedures.

Check the proper way to connect ESC, receiver and motor.
Model flying is not a thing to take lightly and insurance is essential!
What sort of university sets people such tasks without giving them basic tuition on important safety matters?

[Andrew767]

Ryan

kc is right....have a look at the basics section as this will help you. Can you tell us about the aircraft this gear is going in as this would help us to point you in the right direction. BTW can any of your group fly RC??....if not, as kc suggests you probably need to find a local club for assistance.

Andrew

[WolstonFlyer]

Ahh University projects, I remember them

Luckily most of the components you have there simply plug together.

Here is my version of a quick explanation, I am sure that others will correct me and add more information!

The three wires from one side of the ESC connect to the three wires on the motor. To change the direction of the motor swap any two of the wires over. Do not put a prop on the motor while doing the setup, you can very easily get hurt, even by a small prop spinning at full throttle!

The side of the ESC with 2 wires connects to your battery, typically using a Deans or XT60 connector plug and socket combination. DO NOT short out a LiPo battery it can very quickly set on fire!

The ESC has a servo plug that plugs into the throttle channel on the AR6100E receiver, this powers the receiver and the servo's. The servos then plug into the receiver. The voltage drop caused by powering the Receiver and Servos is quite negligible, the ESC powers them with 5V with a maximum 2A from the internal BEC (battery elimination circuit because it elimimates the need for a seperate receiver battery). A 2A BEC is actually quite small to power the receiver and 5 servos, you might find it cannot supply enough to power everything, I prefer an ESC with 4 or 5A BEC.

A 3 cell LiPo fully charged will be at a nominal 12.6 volts under no load (4.2v per cell) but this will quickly sag under load from the motor so a nominal voltage of 11.1V is quoted and can be used as an estimate of the 'under load' voltage.

Using that max charge voltage you can work out the theoretical maximum RPM for the given motor kV and the 11.1V to calculate the nominal under load RPM... but as soon as you put a Prop on the motor the theoretical RPM is just that as the motor RPM actually achieved will change for different prop sizes. The motor will try and achieve the maximum RPM for any voltage but due to the various inefficiencies it is never going to get there.

I hope this helps, I am sure other people will chip in with more info.

Edited By WolstonFlyer on 29/10/2013 20:33:57

[SkippyUK]

Ryan, despite some words of doom and gloom, IMHO you've done the right thing in asking experienced people for help - we all had to learn sometime and I'm sure you will get lots of help and advice including any appropriate safety tips along the way.

As Wolston Flyer says, the connections are pretty simple in theory but a bit more of an overview of size, configuration etc. might channel more detailed info your way.

Skippy

[John Privett]

I suspect the lecturer and/or supervisors may well be aeromodellers themselves, and will be on hand to give the safety warnings/advice when the time comes to actually start building the model.

But this is the design phase so whilst Ryan and fellow-students may (or may not) have seen the components they'll be using, I doubt they've had the chance to start chopping their fingers off or setting the campus ablaze quite yet...

So, good advice so far. Your motor (or at least the ESC) will see the full voltage of the battery, the receiver and servos are supplied from the ESC via the lead with the three-pin plug on the end. If you reckon on the motor getting 10 to 11 volts then multply that by the motor Kv to get the rpm - but bear in mind you won't quite reach that figure when you actually put any load on the motor.

You need to consider the current the motor will draw which will go up as you increase the load on the motor - eg. by using bigger props. Higher current means shorter duration, but bigger props will also give you more thrust.  Hmmm... a compromise may be required... Is there any constraint in your design brief on duration of flight required and/or speed/distance covered?

And also you need to consider the maximum current you are able to use. Hint: The motor OR the ESC OR the battery can go "pop" if you draw too high a current. The ESC has a stated max - though 18A doesn't mean it will last for ever at 17.9A and blow-up at 18.1A! The motor will have its own limit though I don't see that stated in the specs. The batteries also have a limit, though it's a little cryptically stated - I'll leave that for you to research...

Good luck!

P.S. Why didn't we have projects like this back in my day?

Edited By John Privett on 29/10/2013 21:30:32

[Mike Etheridge 1]

Some good advice given here and it would seem to me that as long as a basic circuit is adopted for the motor, speed controller,RC receiver and servos then there is plenty of opportunity to log battery EMF readings,operating voltages with motor at various speeds including full speed and associated currents. I am assuming the university can provide the necessary amps/volts/ohms meters and perhaps a rev counter?

There is plenty of information regarding the performance of 'brush less' motors and brushed motors on offer and what size of model they can power. Obviously you have to provide calculations to ensure that your power selection can cope with the model plane in consideration?

Last year I touched on this issue with the conversion of an old Matador plane from IC power to a geared brushed electric motor on this forum. It worked out a success but with some degree of variation to the voltage of the flight battery and modification to the wing and tail incidences.

As others have suggested electric model plane gear can bite back so due care must be taken with your experiments.

MJE

[Ryan Orleans]

Thank you all for the replies. For those raising safety concerns, I appreciate the warnings, however we are merely in the design stage and health + safety risks have been and will be thoroughly explained both before and during the extent of the build phase so no need to worry about that. And I definitely assure everyone that no one is taking this lightly, especially not our university. Any lack of knowledge I am demonstrating here has come from my own inexperience and my poor affinity to the electrical side of my course which shouldn't be faulted to my university. We have a model flyer in our group who has a valid license so the flying bit is of not much concern either. For the other groups who don't have a flyer, they will be provided one by the university.

Thank you for all of the suggestions and advice, I will take all of these into account in our endeavour.

Wolston Flyer, thank you for that very simple explanation. That has actually showed me why my friends from other groups have told me that designing a diagram/schematic for a circuit to be used on the aircraft would be good but unnecessary since everything seems to connect quite well and simply anyway.

Wolston, and John, I suspected that that's how I would get a good estimate for an unloaded RPM, but I just wasnt sure whether or not I should take into account the voltage drops coming from the other components (or am I misunderstanding how circuits work here?). So how much of a drop in RPM would my motor suffer if I had attached a 6x4E prop? (or rather, is there some sort of equation used to find at least a rough estimate?)

John, the primary objective is for the UAV that we design and build to be pass a few tests, namely an unloaded flight test, a few flying qualities tests, and lastly a loaded test where we attach a small camera to it and have it fly around the designated area while taking pictures of some letters and numbers found in open boxes scattered around the floor. The area is 15x15 metres in size, so relatively small-ish. I think the only limits imposed on us are the limits of the equipment themselves and that area size. However, I think my team would prefer to have a top speed of about 6-7 metres per second. Thanks for the heads up on the current!

Mike, the university does indeed have all that measuring equipment and can provide these to us should we require them. Thanks!

[WolstonFlyer]

Hi Ryan, I am happy that my explanation made some sense!

You say the area you will fly in is only 15m x 15m square, this is actually a very small space to fly a fixed wing aircraft inside, especially for a UAV that I assume will be positioned by GPS.

In my opinion you want something that can fly very slowly and that can be positioned accurately over the locations to photograph... so... have you considered carrying your camera under a small(ish) Quad or Hexacopter? They are much more suited for the kind of test flight it sounds like you are trying to do. They can hover on the spot over an object, fly to another fixed location and hover then move off etc. There are several off the shelf GPS systems available that can be used and they include PC software for plotting waypoints for the aircraft to follow and fly your pre-programmed flight path.

Here is one example of such a system.
**Link**

Edited By WolstonFlyer on 30/10/2013 12:42:08

Edited By WolstonFlyer on 30/10/2013 12:45:36

[Delta Foxtrot]

Ryan,

I would recommend getting hold of the Gibbs Guides on motors and LiPo's if you are new to electric power systems. These contain a lot of useful information on safety and how to spec powertrains for electric flight. These are available as ebooks and cost around £7 each.

You would learn a lot

Cheers

dave

[John F]

interesting stuff. Do you have a model in mind or is it designed by yourselves?

[Martin Harris - Moderator]

I'm a bit surprised that no-one has mentioned Motocalc - it's a comprehensive bit of software that gives pretty accurate performance forecasts and allows you to compare many different configurations - and you can try it with no commitment for 30 days!

Download the demo version and have fun!

[Seajay]

Alternatively try drivecalc free download and your spec'd motor is in the database. This is pretty good for a freebie and you can play with different props batteries etc and see the effect, shows battery volts under load etc, a lot of the stuff you need. Recommended.

Good Luck!

[kc]

I think you need to be aware of the dangers of testing motors with props on. Dont even think of holding a motor in your hand with a prop on if it s connected to a battery. Even your small motor will be about 1/5th HP and chop terribly into whatever part of your body it meets! Juggling chainsaws would be a near equivalent. MIke's photo seems to show a motor under test, but what you may not realise is that he seems to have 2 heavy (?) paint cans restraining the tail. And you should have everyone standing behind the prop and ensure it turns the right way too. Also note that going into the memory of a Tx to even look at settings can allow a motor to start suddenly ( on Futaba at least- it happened to me and also to a colleague ) this can be disastrous if the prop is on. Fortunately when my colleague looked into his Futaba memory his model instantly started up and went in a 20 foot arc around the club members missed them all and their cars but only took a big chunk out of another members model. If it had been someones legs it would have been an ambulance job.
If your lecturer has not already warned you of these dangers then he should have done!

Edited By kc on 30/10/2013 19:49:58

[John F]

Oh yes, paint cans are recommended all the time to hold down model aircraft, very safe!

I'm pretty sure, as Ryan says, they've got the safety aspect covered. The uni would be liable should anything were to happen so their H&S folk are probably quite heavily involved.

[Mike Etheridge 1]

Yes the almost full paint tins were positioned to restrict any movements of the plane, and at full throttle there was no movement of the plane. The paint tins were also a constant reminder of the fact that I should have been decorating and not modelling!

The photo shows the plane after the geared brushed motor was replaced with a brush-less motor,the last part of the experiment.

As I have mentioned before I have had my Twinstar 2 start on full throttle with the transmitter stick set to 'off'' when the battery was connected. This has happened twice with failure of the speed controller(s) and on the last time it happened the small plastic propellers sliced through the nose supports on my new glasses which were on the same table as the plane!

MJE

[Simon Chaddock]

A 15m square and a flight speed of 6or7 m/sec? That is a definite no! no!

At that speed I doubt you could even keep such a plane in the required area even flying in a continuous tight circle.

If you really are limited to a 15m square and it has to be a aero plane then you will have to fly very slowly indeed and that brings its own problems.

Before you design a plane you have to establish what it is required to do. More than anything else this will determines its design.

I presume there is a 'specification' for what your UAV is required to do?

I don't wish to 'teach granny to suck eggs' but the classic approach (unless your team are already leaders in the technology!) is to do some research to find out what exists and how close it gets to meeting the required specification. This should give some valuable pointers on likely methodologies.