Choosing a power system

I like this from Himax:

Choosing a power system:

Power system can be chosen based on the type of flying expected of the model and all up weight of the aircraft. Sedate flying from a hand launch requires 35 watts per pound (W/Lb). Taking off the ground needs approximately 50 W/Lb. Aerobatics and good climb performance, 75W/Lb. Anything more than 75W/Lb will result in excellent performance. Based on the weight of the model and the flying desired, the power required can be calculated. Select the voltage of the battery being used. It is best to use a loaded voltage of about 90% nominal. Now, calculate the current required. From the chart, pick a motor at the voltage you intend to use and find the prop that pull the required current.

Except, none of the motors I have, comes with its chart or the information about A and propeller size, and I don't have a wattmeter to test out the current draw on the propellers I use.

28/1/2014: Well, got to start somewhere, so I placed order for a Hobbyking's X1 Wattmeter

This from Hobbyking's website, much higher W/Lb is recommended:

A basic guide to Electric Flight

An under-powered model is a disaster waiting to happen, here is a rough guide to choosing the electric power train needed for various model types, bear in mind that over-powering is fine but the penalty is additional weight, and a good model is one that is balanced in terms of power, flying weight and build quality. This guide is as the title says, a ROUGH guide and offers a basis from which to choose a power train for your model, it is not intended to be a definative guide but will help to get you into the air with performance that will make your introduction to electric flight enjoyable and reliable.

 

MOTOR POWER CHOICE(base on reccomended AUW, or Flying Weight of model choice):

Vintage types and many non-aerobatic indoor flyers -

50w~70w per 1lb

Trainers, gliders and high wing scale -

70w~100w per 1lb

Sport flyer with general aerobatic performance -

100w per 1lb

Warbirds -

120w~150w per 1lb

Multi engined models -

100w per 1lb (thrust from Multiple props gives in effect, more than 100w per 1lb performance)

EDF Jets -

150w~200w per 1lb

3D, F3A and high performance Models -

150w~200w per 1lb

 

LIPOLY VOLTAGE CHOICE

Based on the above, we now need to work out what voltage we are going to need to use, generally, to keep Lipo's in good order, try and keep max amps to around 50~60% of the capacity/C rating of the Lipoly Pack, for example, if you purchase a 2200mAh 20c pack, then it is rated for 44A constant discharge, so keep the max amps at around 20A~25A IF possible, it isn't always! Choose the capacity of pack based on reccomendation for the model by model manufacturer and in conjunction with the size/weight data published with all our advertised Lipoly packs, for low powered models, choose 20c packs, for general flying choose 20c~25c packs, for high performance models 30c + packs;

 

Up to 50w:

1s~2s

up to 100w:

2s~3s

100w Up to 500w

: 3s (This is the practical upper limit for 3s Lipo's, so basically, models of 5lb AUW)

500w up to 800w:

4s (This is the 0.40~0.46 glow equivalent range favoured by many club flyers)

800w up to 1000w: 5s

• 900w up to 1500w: 6s (this is the 0.60~0.90 ic equivalent range)

8s~10s packs are for very large and generally specialised models.

 

 

MOTOR CHOICE - KV or RPM per volt

Which actually means, what prop size! If you are used to IC, the simple analogy is to treat low kv motors as 4 stroke engine equivalents and mid-high kv motors as 2 stroke engine equivalents, if you are not used to IC then we can give you some examples of the approach to take, this is an important choice as you can literally choose how your model flies, however, their are practical considerations, the most obvious is ground clearance. Please refer to motors such as the NTM range, which give you prop data as well as power, dimension and weight data.

Example 1:

Trainer/Sport Model, 1lb AUW, we want 100w motor (3s 20c Lipoly) mid kv for general flying, probably around 1200kv~1400kv, so around 8" prop

Example2:

3D/F3A Model, 1lb AUW, we want 150w motor (3s 20c~30c Lipoly) low kv, 1000kv or under, spinning 10~11" prop, highly efficient at low throttle openings giving lot's of prop wash over control surfaces at all times, high thrust for low rpm and low amps draw at higher throttle openings.

Example 3:

Warbird/scale Model, 1lb AUW 120w motor, kv choice, either of the above, it is personal choice

Example 4:

High Speed Delta type model, 1lb AUW, 200w motor (3s 25c~30c Lipoly) 2200kv~3200kv motor, 5"~6" Prop, high speed/low torque, low thrust at low throttle openings, high speed from high rpm at full throttle.

 

 

FINALLY, ESC CHOICE

You have decided on your motor, so look at the MAX AMPS figure given by the motor manufacturer in the data section and generally add 25% headroom, so, if a motor is rated to 15A, then choose at least an 18A ESC, better still a 20A and so on. Next make sure that the ESC voltage is compatible, in other words, if you are using a 4s Lipo, that the ESC is rated for 4s voltage. Next, check if it has functions you desire, if you are flying a glider for instance, you will want a brake facillity so that the prop stops when soaring un-powered, allowing the prop to fold by not windmilling, we strongly advise purchasing a programme card to make programming the ESC easier. Also look at BEC rating, the BEC supplies radio reciever power for servo's without the need for a seperate reciever battery, however, the can be limited in the number of servo's they are capable of powering, if the servo count is over 4, as it is on many models these days, then consider purchasing an ESC with a high AMP rated SBEC, or a seperate UBEC, OPTO type ESC's (they have no BEC, keeping the ESC seperate from RX suply) are reccomended for large models that require a seperate reciever power supply, they are also safer in high powered, large models as they will not arm until the RX is switched on.

 

This document is a work-in-progress. Check back regularly as we expand this document.

TURNIGY® Batteries explained

Zippy: Great value for money. Average Cycle Life* (100+) and minimal voltage sag under load.

TURNIGY Standard: Excellent value, Longer Cycle life* (160+) and very low voltage sag under load.

TUNIGY nano-tech: Unbeatable performance, Longest Cycle Life* (250+) and almost 0 voltage sag under load.

*Cycle Life results from discharging at full C rate to 3v. End of life when battery has 80% capacity.

        

And also from HobbyKing's website:

Lithium Polymer (LiPo) Basics
It can sometimes be difficult to know which battery is best for your application.
For R/C aircraft there is a huge variety of batteries available and while many may suit your application your ultimate goal is to purchase a battery pack that will;
-be within your budget
-have a long cycle life
-have the correct size and weight
-give you the longest flight times
-be able to deliver the correct voltage/amp (Power)
We hope this simple guide helps you understand the different types of LiPoly (Lithium Polymer) batteries and which is right for your model.

You may have noticed by now that batteries have different ratings, sizes, plugs, wire, charge rates and chemical makeup. Lets decipher;

Capacity (mAh).
This is usually the biggest number shown on the pack and is measured in mAh (Milliamp/hour) or Ah (Amp/hour). The capacity is the first indicator of the batteries size. To keep things simple, think of capacity (mAh) as the amount of fuel in your cars gas tank. A higher capacity tank will run your car for longer. A 4,000mAh battery will run for twice as long as a 2,000mAh battery.
A 2,000mah battery will (in theory) run for 1hr if drained at a constant 2,000 Milliamps.

Discharge (C)
Discharge is the amount of power the battery can 'push' out and the number shown '20C' is an multiplication of the capacity. For example; A 20C battery can discharge at 20 x 2,000mAh which is 40,000mAh or 40Amps.
This is an important number if you know your motor requires a certain power level.
In addition to this, batteries have a 'Burst' rate, which is the amount of power the battery can discharge for a short period, usually 10-20 seconds. A typical battery label may show 20-30C, this would mean a 1,000mAh battery can discharge 20,000mAh constantly or give a sudden and short 10-20 second 30,000mAh (30A) burst of power.
Tip: A higher 'C' rated battery will last longer if run at a lower 'C' rate. Example: a 30C battery run at 20C maximum will have a longer cycle life than a 20C run at 20C each flight.

Voltage (S)
All lithium Polymer cells in any industry have a nominal voltage of 3.7v per cell. When fully charged a LiPoly cell should be 4.2v and when discharged it should never be below 3v.
You will notice that LiPoly RC packs are made up of layers of multiple cells. If the battery's rating is 3S this means it is 3 x 3.7v which is 11.1v. It has 3 layers of 3.7v each. In other words, its a '3 cell pack'.

Weight/Size
For a battery to be right for your model it must fit within the models battery compartment and also balance the plane correctly.
It's temping to choose the biggest and most powerful battery your model can handle, but this will sacrafice flight performance and if your packs voltage is too high; destroy the ESC or Motor.
Check with your ESC and Motor specification to ensure you have the right voltage pack then check the models CG (Center of Gravity) to decide on the right battery weight.


LiPoly Charging
Always use a lithium Polymer battery charger and never charge the battery above 4.2v per cell. (example: 2S, never above 8.4v)
Never leave a charging battery unattended.
Never allow the battery's voltage to fall below 3.2v per cell. (example: 3S, never below 9.6v)

This document is a work-in-progress. Check back regularly as we expand this document to include battery chemistry, dig deeper into battery technology, battery sales tricks and production methods.

TURNIGY® Batteries explained
Zippy: Great value for money. Average Cycle Life* (100+) and minimal voltage sag under load.
TURNIGY Standard: Excellent value, Longer Cycle life* (160+) and very low voltage sag under load.
TUNIGY nano-tech: Unbeatable performance, Longest Cycle Life* (250+) and almost 0 voltage sag under load.
TUNIGY nano-tech A-SPEC: Competition level cells, strongest voltage hold in the industry.
*Cycle Life results from discharging at full C rate to 3.3v. End of life when battery has 80% capacity.