1. Technical Information - RC Flying Electric - General
1.1. 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.

1.2. Electric Power - Flight Time Estimator

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.

1.3. 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.