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Electric Power - Flight Performance Estimator

Question: I'm interested in getting into Electric RC Flying. I've checked out a couple of the local hobby shops, thumbed through a bunch of the RC magazines and browsed the web. I'm learning but I find some of the techno talk rather confusing and I'm more than a bit skeptical about some of the advertised claims. I've been around the block over the years and understand that nobody is going to advertise that their model flies "like a sick pig". Fair enough but sheesh... it would sure be nice to get real information instead of a load of bunkercarb! A friend of mine bought one of the low cost "foamies". The box art claimed the moon but the darned thing would hardly stay in the air... never mind actually fly!

I'd like to find a way of sifting BS from fact that does not involve smashing up models in order to find that they fly like a truck. Is there a way to estimate flight performance from some of the specs?  

Answer: Yes. There is an estimating equation that works from basic specifications to calculate the Flight Performance Index for an electric RC model. It's pretty rough and ready but it really helps sort out what is probably real from what is patent nonsense. Here is a simple way to calculate the approximate flight performance of 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:

Performance Index  = Power supplied to the motor [watts] / Flying Weight of the model [ounces]

where:

    • Performance Index is a number ranging from 1 to 6 as calculated above and described below.   
    • Power supplied to the motor [watts]. Use an AstroFlight Whatt Meter or Medusa Flight Power Analyzer.
    • Flying Weight of the model [ounces] complete with motor, ESC, battery etc... everything that goes into the air.

To determine the Power supplied to the motor [watts] you will need to measure the power in [watts] flowing from the LiPo battery to the brushless motor 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 value for Power [watts]. If you have a volt meter across the battery leads and a good quality medium current DC amp meter in one line between the LiPo and the ESC you can calculate the Power [watts] = Voltage [V] * Current [Amps]. Be careful to avoid the propeller at all times. The easiest way to measure Power [watts] is to use an AstroFlight Whatt Meter or a Medusa Power Analyzer. These are both great tools for the electric flight enthusiast!

Example:

  • Power supplied to the brushless motor  = 100 Watts
  • Flying Weight of the model (complete) = 25 ounces
  • Flight Performance Index = Power[watts] / Weight[ounces] = 100/25 = 4

Interpretation:

The Flight Performance Index is a number between 1 and 6 for most models. In fact the index seldom is valued below 1 or above 6 but both are theoretically possible.

Here's the decode to interpret the Power Index:

  • 1 = Here doggy doggy! Woof woof! This simply won't fly. You'll be able to throw a brick further than you can fly this puppy!
  • 2 = Not an Eagle and not for beginners but in the hands of skilled RC pilot at sea level it will probably fly.  Hand launches pretty well a must. Ground takeoffs very tricky if not impossible. Best to not attempt to fly in the thin air of a high and/or hot location.
  • 3 = Fun to Fly. Ground takeoffs the norm. OK for beginners and up. No ball of fire but it flies well. Seldom induces panic attacks. Reduced flight performance at high altitude locations but it should still fly OK even on a hot day.
  • 4 = Sport Model. Ground takeoffs are a piece of cake. Flies well and does all the basic maneuvers. Seriously good fun and will put a smile on most faces! Relatively tolerant of thinner air found at high altitude locations and hot summer days.
  • 5 = Warbirds and hot Sport flying. Goes like stink. Too hot for beginners. Will provide a nice blend of thrills and chills. You'll be gasping for breath  long before the thin air at high and/or hot locations bogs this fast flyer down.
  • 6 = 3D capable with the right model in the right hands. Bores holes in the sky. Best to inform the Air Force. Lay on the Depends.

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. 

We are using Power [watts] measured on the ground to derive the Flight Performance Index. This is a bit counter intuitive. In almost all configurations and in almost all models, the Power [watts] in the air will be less than that measured on the ground during a static test. The Performance Index therefore is being estimated somewhat optimistically here although we have factored some of that into the interpretation of the index number itself.

Ideally, Power [watts] 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 Power of the motor. Hence try to measure the Power 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 of the Flight Performance Index than flying at sea level and this should be accounted for when estimating actual performance.

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 manifests itself as a contributor to Power [watts].

If your battery is not fully charged or is in bad shape you will get actual performance results that are lower than indicated by the Flight Performance Index.

If the discharge rate approaches or exceeds the battery discharge maximum recommendation you will get significant reductions in actual performance 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.

Article ID: 4759