BATTERY / MODEL / Flight-Time Calculations

BATTERY / MODEL / Flight-Time Calculations

A few weeks ago I bought a second-hand EDF Jet… a Freewing F-86, the 12-blade 80mm EDF “Performance” version. … and since then, I have been pondering the relationship of battery to flight endurance.

One of our long-time members, JD Devereaux, spent quite a while helping me to understand the formulas I am about to describe to you, here…

Here’s how I understand what JD was trying to show me…

To calculate a potential Flight-Time, you factor-in the type of model you have… the weight of the model… and the battery you intend to use.

Here we go!

WATTS required to fly your model…

Weight of your airplane  x  the Watt Factor = Watts Required for your Model.

The Watt Factor is a variable that you decide upon … ranges from 90 to 130

90 for a simple, light weight Floater model (Seniorita) …
110 for the average aerobatic sport plane…
130 for a high-performance model like an EDF Jet

You can choose any number between 90 to 130 for your model.

So a Two Pound Sport Model x 110 Watt Factor = 220 watts required to fly that model.

———

The Battery Potential…

Basic formula here is Volts x Amps = Watts

Think about your battery… let’s say it’s a 3S 3300mAh LiPo (3300mAh = 3.3 amps)

a 3S = 11.1 volts … so … 11.1 x 3.3 Amps =  36.63 Watts of Battery Potential

——–

So…  how many minutes of flight-time do you expect to get from your battery on that 2-pound airplane?

220 Watts required by your model… divided by your Battery Potential 36.63 = a FACTOR of 6.001

(That “Factor” is the C-Rating required at Max Power)… but you don’t need to think about that…

60 minutes in an hour divided by 6.001 = 9.99 minutes of expected flight time…. for that model using that battery.


Apprentice trainer in flight!On the Apprentice, I get 12 minutes using that 3S 3300mAh LiPo battery… I don’t know the exact weight of the Apprentice… and I don’t know if I should have used that 110 model factor… maybe it IS a Floater and I should have used a factor of only 90.

2 pounds x Factor 90 = 180 … 180/36.63 battery potential = 4.91 … 60 min / 4.91 = 12.21 minutes flight time.

“Perfect!” … and I DO try to land in 12-minutes so as to leave the battery at about 30% power remaining. … so, in the real world, really, I COULD fly 30% longer (a total of 15.5 minutes?) and so kill that battery, completely…

Freewing F-86 EDF jet!Let’s do it, again…

For my F-86 EDF Jet…

Weight = 8-pounds at a Watt Factor of 130 = 1040 Watts Required for that model.

My Battery is a  6S 4500Mah … 6s = 22.2 V x 4.5Amps = 99.9 Watts Battery Potential

1040 / 99.9 = 10.41  … 60 / 10.41 = 5.76 minutes of Flight

——–

But “in the Real World”… I get only about THREE minutes on my Jet…

… JD says “Well… these numbers are theoretical requirements, they are only approximate”…

… so… it is STILL all VooDoo!
——–

Instead of 130 FACTOR for a Jet, Let’s try using 180 …

180 x 8 pounds = 1440 Watts required for that model.

1440/ 99.9 battery Potential = 14.41 C-Factor … 60 minutes / 14.41 =  4.16 Minutes … STILL longer than MY flight time…

——–

Try again… this time using 210 as my Model Factor… (I know… JD says Range is from 90 to 130… but I need a BIGGER number to make the formula reflect a real world 3 minutes flight time…)

Model Aircraft Factor of 210 x 8 pounds = 1680 Watts Required

1680 / 99.9 Battery Potential = 16.82 C Factor … 60 min / 16.82 = 3.57 minutes flight time… STILL 33% More than I actually get when I fly that model.

Of Course, I AM trying to land the jet before the battery reaches “ZERO” … so I COULD actually fly for another fat 30-seconds to kill that battery, completely… maybe more… !

——–

So… Yessss??

My conclusion: … A.) The formulas are REALLY only general (TOO Theoretical)… or B.) my Jet is drawing WAY too much power from the battery… or C.) I need to use a model factor greater than 130 in my calculations…
… or, maybe… all three!

———

I can see that JD’s formulas do not recognize the propeller (or fan unit), the Electric Motor or the ESC that you are using. …

He says the numbers are “theoretical” … So, in a perfect world, my battery set-up in that EDF jet would give me 5.75 minutes of flight-time. … if… THAT is the exact weight of the model, the exact strength of my perfect battery, a perfect motor and propeller, and a perfect air-frame…a beautiful, smooth, perfect model….

… but in the real world I only get THREE minutes (about HALF of the calculated amount).   So I gotta figure my model is not “theoretically” smooth, clean, and perfect. … I gotta figure my battery is not 100% efficient. … I gotta figure my motor, ESC, and fan-unit are NOT perfect (NOT delivering 100% efficiency)… and with all of those “nots”, my number is almost 50% off of the theoretical perfection calculation.

… and yet for that Apprentice calculation, the numbers were exactly correct… maybe a little TOO good. … Maybe the Apprentice IS the perfect airplane, per this theory.

———

ALL OF THAT being said (Sorry to make this be so LONG!) … FOR my EDF Jet, I gotta use this formula to determine my Flight time and then, knowing that I am trying to land before I kill the battery, and knowing that my model is NOT perfect, I gotta cut the calculated result IN HALF to get a number I can trust.

VooDoo… Yes?

9 thoughts on “BATTERY / MODEL / Flight-Time Calculations”

  1. Howdy Kingman Eric,
    I found your “Flight Times Calculations” article interesting. I mostly agree with your watts-per-pound figures and have another method of determining flight time, though it requires the use of a hobby Wattmeter or clamp-on DC ampmeter.

    Battery potential formula is:
    Capacity in Amp hours X 60 minutes = capacity in Amp minutes

    Using the Apprentice as an example, 3.3 Ah X 60 = 198 Amp min.

    Now use the Wattmeter to measure how many amps your power system draws. The Apprentice with stock propeller draws approximately 20 amps at full throttle, so…
    198 Amp min. divided by 20 Amps = 9.9 min @ Full Throttle
    Of course an Apprentice is not flown at full throttle the entire flight so “throttle management” can provide flight times of 15 to 20 minutes depending on how aggressive the plane is flown.

    On the other hand, your F86 draws approximately 75 Amps, so…
    4500 mAh = 4.5 Ah X 60 min. = 270 Amp min.
    and 270 Amp min. divided by 75 Amps = 3.9 min. @ Full Throttle,
    which can be stretched with throttle management…but who wants to do that with a jet?!!

    What makes a Wattmeter worth having is being able to try different props and cell counts in your plane while measuring the current (amps) so as not to exceed the limits of the motor or ESC, and of course it calculates the watts accordingly.

    Your club’s website is excellent!

    Wayne Wilson…..Quartzsite Desert Flyers

    1. Wow… your numbers came out very close to “my reality”! … Thanks for that info! On my F-86, I need to be “on the ground” at the end of three minutes with that 6S 4500mAh 60C battery. I take-off at full throttle and then try to fly at a fat half-throttle for cruise-speed and I add a burst of throttle for any climbing maneuvers (or for a flat-out full-speed pass down the runway!)… chopping back to that half-throttle ASAP after each blast … I have ordered a 6S 6000 35C… I am hoping the smaller C value will allow the battery to discharge more slowly? … so am hoping for longer flight-time at 6000mAh 35C.

      1. A battery with a low “C” rating in a setup that requires high power will result in a over-heated battery resulting in “puffing” and risk of fire. Most EDF jets require high C ratings that enable the motor/fan to turn at their designed RPM. When I used a 35C battery in my 4 cell jet the battery got too hot to hold in your hand; the 50C battery was just warm to the touch. The flight times were identical for both batteries.

        1. Well… THAT’S disappointing… I DO have two 6000mAh 6s 35C batteries on order… hoping for longer flight-time by using lower C-rating… I DID figure the battery might get warmer… hot? … I will run it “on the bench” for a test run to see how hot they get … or could just go for it and see if my F-86 becomes a flaming T-33 “Shooting Star”!

          1. The owners manual for that F-86 (very Chinese) says that model requires at least a 6S 3700 25C … THAT’d get me a minute of flight time!

  2. In your first calculation under the heading The Battery Potential you show the voltage as 11.1 when in fact a fully charged 3 cell is 12.6 volts. One contributing factor when trying to determine battery performance is the internal resistance of the battery. The higher the resistance the hotter the battery will get during discharge. Many good battery chargers will show the internal resistance during recharge. The lower the resistance the better the battery will perform. As batteries age the resistance usually increases. Two identical batteries can have two different current discharge rates, which will determine the amount of flight time from each battery. Throttle management also greatly affects flight times.
    The most important thing to remember is to never fly a battery down to the level of 3.7 volts per cell. That can cause damage to the battery, as well as getting to the cut-off point of most speed controllers, where motor power will be lost.
    Also, remember to keep your batteries warm in cold weather, and out of the sun in hot weather. Leaving them in the sun can raise the cell voltage way beyond the safe voltage of 4.2 per cell which can lead to a LiPo fire.

  3. Our member, Bruno Russo, replied via email…
    Hi Eric,
    Good article, thanks for taking the time.

    There are a few reasons your calculations are basically theoretical but not working in real life.

    1. Most battery manufacturers don’t tell the truth about true capacity or true max c ratings. I target for 50% of max c ratings to keep packs from swelling. IF a packs says 50C it probably won’t swell if kept around 25C.

    2. There is a factor that drops the voltage and wattage because the max power and current is continually dropping over the duration of a flight. Online calculator: Battery discharge time depending on load

    3. The life of a pack decreases over time and worsens when not discharged back to storage voltage. (about 3.7-3.8V/cell for li-po)

    Here are a few other sites to help.
    Battery run-time calculator from PowerStream, The “How long will a battery last calculator”

    RC LiPo Battery Power & Configuration Calculator

    Calculate Flight Time of LiPo Battery

  4. Here is probably more information than you want to know about LiPo batteries and their Internal Resistance properties. Personally when my batteries do not perform as they did when new, or the charge times become excessive, then it’s time to use them for target practice! I have been using LiPo batteries for the past 12-13 years and during that time I have never “Puffed” a battery or had a battery fire…knock on wood!

    https://www.rchelicopterfun.com/lipo-internal-resistance.html
    https://oscarliang.com/when-retire-lipo-battery/
    https://www.youtube.com/watch?v=wP0MhOcPmQY

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