Question: Is there a quick and dirty way to estimate the approximate flight time in the air for my electric powered RC aircraft? I'm pretty good with a calculator but I don't work for NASA... can you give me some simple tips?

Answer: Sure. Here is a simple way to calculate the approximate flight time in the air for an RC model equipped with a fully charged Lithium Polymer (LiPo) battery in good condition, an Electronic Speed Control (ESC) and a direct drive (no gearbox) Brushless Motor. (We can't attest to this working for other technologies or setups because we have not tried it)

Here is a simple estimating equation:

Flight Time [min] = (.06 * LiPo Capacity [mAh]) / Current Draw [amps]

where:

• Flight Time is in [minutes]. Remember this is an estimate. See caveats below.
• LiPo Capacity is in milliAmphours [mAh].  
• Current Draw is in amperes [Amps]

To determine LiPo Capacity, look at the label on your LiPo battery. Capacity is measured in [mAh] and is usually a number from say 500 to 4000 or more.  

To determine Current Draw you will need to measure the current flowing from the LiPo battery after about 1 minute of full throttle operation. By this time the peak voltage and current associated with a fully charged LiPo have been burned off the battery and you will be able to measure a more stable and typical Current Draw. Use a medium current DC amp meter between the LiPo and the ESC. Be careful to avoid the propeller at all times. The easiest way to do this is to use an AstroFlight Whatt Meter or a Medusa Power Analyzer. These are both great tools for the electric flight enthusiast!

Example:

• Battery with LiPo Capacity = 1800 mAh
• Current Draw after 1 minute of full throttle = 11 Amps
• Flight Time in minutes = .06*1800/11 = 9.8 minutes

Caveats & Comments:

We cannot emphasize enough, the importance of considering the LiPo battery pack, ESC, brushless motor, propeller, wiring and connectors etc as a system. The components that make up your power system must all work together in such a way that current, voltage and RPM does not exceed the operating limit or efficiency threshold on any particular component. If you overload anything, it will heat up, performance will suffer and the life expectancy of the component(s) will drop dramatically. The system as a whole must also be suitable for the model, it makes little sense to deploy a power system intended for a 36 inch span model aircraft weighing 16 ounces into a 72 inch model aircraft weighing 7 pounds. The power system may work just fine in and of itself but it must be suitable for the model it is being installed into, in order to produce satisfactory flight performance.

We are using Current Draw measured on the ground to derive flight times. This is a bit counter intuitive. In almost all configurations and in almost all models, the Current Draw in the air will be less than that measured on the ground during a static test. Flight times therefore are being estimated conservatively here.

Ideally, Current Draw should be measured at the temperature and altitude expected during flight. Temperature and altitude affect air density. Air density decreases (thins) as temperature and altitude increase. Thinner air will affect performance of the propeller and this in turn affects the Current Draw. Hence try to measure the current draw at the temperature and altitude you expect to fly at. Don't worry about small variations in altitude like a few hundred feet, but flying at 5000 feet will produce a different value for Current Draw than flying at sea level and this should be accounted for when estimating flight times.

You will notice that battery voltage is not used in the equation. Battery voltage is very important to the correct operation of the motor and ESC and has an effect on RPM and the selection of an efficient propeller but once a voltage has been selected that is compatible with the model and the various power system components, the voltage thereafter manifests itself in the magnitude of the Current Draw.  

If your battery is not fully charged or is in bad shape you will get significantly shorter flight times.

If the discharge rate approaches or exceeds the battery discharge maximum recommendation you will get significantly shorter flight times and are heading towards imminent battery failure.

Acknowledgments:

We believe in giving credit where credit is due and would like to thank Don Dombrowski of House of Balsa Inc (www.houseofbalsa.com) for providing much of the information in this article.