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Electric Power - Performance at High Altitudes

Question: I live at 5000 feet above sea level. What should I do to get my electric power system to perform at high altitudes as well as it does at sea level?

Answer: It's tough to get the same performance at 5000 or 8000 feet as you do at sea level but we do have a technique that will help you come close.

First of all it's important to understand a few things:

1) Unlike an internal combustion engine (i.e. glow or gas), an electric motor does not consume oxygen and hence could care less about oxygen or anything else that affects combustion.

2) An electric motor system that runs well at sea level will underperform at higher altitudes not because the air has less oxygen but because the air is thinner. Hot weather does the same thing... the air gets thinner. Hot and high together can really gang up and take a chunk out of the performance.

In order to get your electric motor system to perform in thinner air the same way it does at sea level, you may actually have to change the motor, speed control, battery pack and prop but before you go reaching for your credit card here is a technique that attacks the problem from the lowest cost component first.

1) Measure the watts that your power system consumes at sea level or other relatively low altitude location. Use a Medusa Power Analyzer or AstroFlight Whatt meter.

2) Better yet, if possible, use the Medusa Power Analyzer PRO thrust meter to measure the thrust generated and the watts consumed.

3) Now do the same measurements (1 & 2) at the higher altitude location. You will notice that your system consumes less watts and if you are able to measure thrust you will notice that you get less thrust at higher altitude than you do at lower altitude. This is because the air is thinner (less dense) at higher altitudes.

4) In order to get the thrust back up to where it was at lower altitudes you need the prop to move more of the thinner air. To do this, select a higher pitch and/or larger diameter prop for use at higher altitudes. Test again and try to find a prop that gives you the same thrust and/or watt values as you were getting on the original prop at lower altitude. If you can get the same thrust at 5000 feet as you got at sea level for example, you will have similar flight performance. Thrust is really the key here and it's best to work with thrust numbers but watts are a good relative indicator of thrust. Not perfect but still helpful!

Be cautious when testing to ensure that the motor, ESC and battery do not overheat with the new prop. Some compromising may be necessary to get good performance that does not overheat components.

In a perfect world, you would carefully select a specific motor, ESC, battery and prop to get the ideal combination for high altitude performance when installed in a particular model. Yup... perfect is best... but for those of us who can settle for a bit less than ideal and have limited budgets, changing the prop to generate similar thrust in high, hot & thinner air as you get in low, cool & thicker air is a good low cost way to go.    

 

VMA-U160X V-Stik 60 ARF - Electric Power Conversion

For those of you who may be wondering about flying a VMAR V-Stik 60 ARF using Electric Power here is an equipment report from a modeler who has electrified his V-Stik 60 ARF as follows:

  • AXI Outrunner 4120/18
  • 5 Cell LiPo
  • APC 13 x 8.5
  • Reports brisk reliable performance with this power system in the V-Stik 60 ARF

Please note that this report comes from a modeler flying near Durango, CO at approximately 7000 feet above sea level. A lower pitch and/or smaller diameter prop could be used at lower elevations. See the article included below for more information re adjusting for altitude when flying electric.


Article ID: 4768