It is often handy to calculate the amps or watts for a heli. You can use this info to help size components or you can use it to brag to your friends. Whatever the reason, here is the how.
Of course the simplest method would be to get a data logger like one of the EagleTree models or those built into the Castle Creations Ice escs. This is easiest way to get these numbers and in fact it is the best way to get all kinds of numbers. But what happens if you don't have one, or say have a tiny heli that you can not put one on?
Starting with some known numbers
Without a data logger you can't get detailed numbers about a flight but you can get averages that will still be useful. All you need is the length of the flight and the mAh used. Of course lots of error can be involved and it will be your job to gather your numbers as precisely as possible.
Lets start by making up some numbers. Say you are flying a 450 sized heli and the typical flight time is 6min. After a typical flight the charger shows that the 3s 2200mAh lipo took 1500mAh to recharge. The 2 pertinent numbers here are 6min and 1500mAh.
Before I go any further a basic concept needs to be covered and that is Ah. Ah stands for Amp-hour and means just what it sounds like, 1 amp for 1 hour. It is used to express the useful capacity of a battery. For example a 1Ah battery can output 1 amp for 1 hour. A 5Ah battery can output 5 amps for 1 hour.
Finding average amperage draw
Ok so the sample flight used 1500mAh, or 1500 milli-Amp-hour, out of our pack. 1500mAh can also be written as 1.5Ah. This number can be achieved by discharging the battery at 1.5A for 1 hour, but that is not what happened here. Instead it was discharged in 6min. Since 6min is 1/10 of an hour, that means it was discharged 10 times faster or 10 times 1.5A was the average amp draw.
Avg amp draw = 1.5A * 10 = 15A.
Now lets do the same math for a different heli, say a 600 sized electric running a 6s 5000mAh pack. Lets say an average flight is 5min and uses 3800mAh.
In this case 3.8Ah was discharged in 5min, or 12 times faster than an hour.
Avg amp draw = 3.8A * 12 = 45.6A
Likely error in the numbers
Just remember that the numbers calculated using this method are not exact but rather a ballpark estimates. The errors come from both number sources, the time flown and the charger mAh reading. The closer the time is to the actual time the heli was in the air, the better. But that is not easy to do. Most people start their timer and then start the spin-up, check the controls on the ground, hold a short hover all before actually flying. That could add as much as 30sec onto the flight that is different than the actual flight in terms of amp draw. The next error is from the charger. Most people balance charge their packs and that is good thing but the balancing process uses some mAh, raising the total mAh put back into the pack. This error can be as small as 1% or as large as 10% depending on the charger and battery. So keep that in mind when you use these numbers.
Finding average wattage used
Now that you know how to calculate the average amperage draw during a flight, it is time to calculate the avg wattage for a flight. This is actually very simple now that we have the avg amp draw and is nothing more than plugging in the numbers into the wattage equation
Watts = Volts * Amps
For this calculation you will need the average amp draw and the average pack voltage. Now you have no way of knowing the actual average pack voltage during the flight but it is not hard to estimate. A 4.2V lipo under a heavy load will hold somewhere near its nominal voltage of 3.7V. In the case of a 3s lipo, it should hold its nominal voltage of 11.1V under load. In reality it starts off closer to 12V and can drop as low as 10V over the flight, but 11.1V will work as an average.
So a heli drawing 15A from a 3s pack is using how many watts?
Avg watts = 11.1V * 15A = 166.5W
Now how many watts is the 600 using?
Avg watts = 22.2V * 45.6A = 1012.3W
One more example for fun
Take a 700 electric running a 12s 5000mAh pack and using 80% of the pack in a 4min flight.
Avg amp draw = 4Ah * 15 = 60A
Avg watts = 44.4V * 60A = 2664W
Tell that to your plane flying buddies, followed by reiterating that it is the AVERAGE wattage and that the peak is likely 2-3 times as high ... and watch their mouths drop to the ground!
Anyone who says you must have a data logger to get some useful flight data is simply wrong. The average amp draw and watt usage are both very useful numbers that require nothing more than a carefully timed flight and a smart charger. So give it a try for your helis and planes and be that much more informed.