ELECTRIC FLIGHT PRIMER
Written by David Menzies during his time at WRCS.
The following are notes, comments and formula that I have found useful in understanding and designing electric flight systems for model aircraft.
First the disclaimer, this is simple first princlple stuff – a broad brush if you like suited to normal non specialist type models. There are many other more sophisticated programs and information on the web if you require more detailed info.
From Ohms law - Watts are Volts × Amps.
Lipo cell voltage
Battery “C” rating
Every battery has a C rating, this is the theoretical maximum current discharge so a 1000ma 25c battery can deliver 25amps.
Within reason the higher the C rating the better, higher C rated cells have a lower internal resistance and more punch in their power delivery.
Always use a balance charger and for safety charge at a rate of 1c (a 1000ma cell would be charged at 1amp or 1000ma).
The “esc” is the heart of an electric model and if it fails so does the model.
SBEC's are a better choice with digital servos.
A programming card is worth having to set up the esc with brake and timing functions etc.
For a reliable system the 80% rule is a good start point.
take 1760ma from a 2200ma battery.
use 24c from a 30c battery.
draw 24amps from a 30amp esc.
Care for your batteries, check before and after use, store charge for periods over 2 or 3 days.
Use a safety/arming switch on your transmitter.
Invest in a Wattmeter and use it, it is the only way to know is going on with your power system.
Manage the throttle, power can be saved and flight endurance increased.
POWER SYSTEM CALCULATION
Weigh everything - airframe, radio, motor and battery (make an assumption) in lbs, it can be tweaked in a recalc later if required.
The total power can now be worked out as follows – 100watts per 1 lb weight (this is a good all round number that will fly most sport models)
So a 3lb model needs 300watts.
The above can be modified as follows if desired.
70 to 80 watts/lb – Piper Cub style, light vintage model, glider, modest performers.
100 watts/lb – general sports model.
120 to 130 watts/lb – brisk sports perfomance.
150 watts/lb – sports performance with good vertical
175 watts/lb – minimum for 3D
200 watts/lb – minimum for EDF
Battery voltage (lipo cell count)
Modern esc's will generally handle up to 6s so it is better to up the volts rather than the amps in the search for the watts calculated above.
Physical motor size
Sift through the motor specs, they always give either watts or volt and amps (watts is volts*amps) and find some suitable candidates. The laws of physics always apply so no matter who makes the motor you can't get a quart out of a pint pot – the motor has to be a certain size to dissipate the power.
kv of motor
kv is the theoretical speed the motor (unloaded) will achieve so a 1000kv motor with a 3s (11.1v) pack will run at 11,100rpm, whereas on 4s (14.8v) it would be 14,800rpm
Consider the model type, a glider/vintage (slow) model performs best with a large slow revving prop whereas a pylon racer needs a small fast revving prop
A glider on 3s running at say 8,500rpm would need a 766kv motor (8500/11.1) whereas a sport aircraft at say 11,000rpm would need a 990kv motor(11000/11.1). It is unlikely you will get a motor that matches your calculations exactly but it gives a zone to look in based in the type of performance you require.
The battery capacity is “the size of the fuel tank”.
Pick a battery that suits the purpose, a glider only needs 2 or 3 good climbs (each run about 30 secs) for 10 mins flying in average conditions. For example my 2m Spirit draws about 20amps on 3s (220watts) so using a 1000ma 20c battery would give me my 20 amps (1*20c). A 1000ma cell can in theory deliver 1 amp for 60 mins or 20amps for 3 mins(60mins/20amps) 3 climbs equates to about 1.5 mins,well within the possible 3 mins.
For power models I work the numbers on a 5 minute motor run which in practice equates to some 7 or so minutes flying because the prop unloads in the air and the power use drops.