Flight duration calculations

Hello,

I’m just trying to decide on a safe flight duration for an eventual, but hopefully uneventful, first flight. I’m guessing once I’ve had a few flights I can monitor how much I’m putting back into the battery and adjust the timer accordingly.

I’ve used one of those Watt Meter things and get the following:

Battery Voltage (4S): 14.7
Voltage Minimum: 13.23
Peak Amps: 33.40
Peak Watts: 443.8
That's 7.23 Watts per minute or 0.1205 watts per second

So with a 2700 mAh Battery at full throttle I would have… scratches head and wishes he paid attention in school…

Okay first of all 80 % of 2700mAh is 2160mAh
That gives me (2.16A x 14.7V) 31.752 Watts

So 31.752W divided by 0.1205W gives me 264 seconds, which is 4 minutes 24 seconds.

I appreciate that the battery voltage sag will increase as the battery is discharged, but at the same time I know I’m unlikely to be at full throttle all the time, if at all, so I’d be grateful if someone could just double check my sums and let me know if, for the first flights, a timer setting of 4min 30 would be sensible.

Many thanks,

Chris

The sums seem OK. However, you can simplify things and ignore voltage and watts. Here's the sum I use:

flight time = (battery capacity x 0.06) / current

Capacity is in mAh and current in amps. The constant is derived from the fact there are 60 minutes in an hour and 1000mA in an A (60/1000 = 0.06)

Using your figures I get:

(2700 x 0.06) / 33.4 = 4.85 minutes or 4:51

80% of this is 3.88 minutes or 3:53

If you know you won't be flying at full throttle, you could set the timer for 4 minutes for the first flight.

BTW, make sure you time your first flight and make a note of the capacity recharged into the battery. You can then calculate your theoretical maximum flight time:

theoretical flight time = (battery capacity / recharge capacity) x flight time

Final post on the matter...

The reason for the discrepancy in our times is possibly due to your assumption that you are using 0.1205 watts per second. Watts is a measure of power and 1 Watt is 1 joule per second. So you can't technically have watts per second

Chris

If this is for your 48" Extra, you will not be needing full throttle, on 4s it will be a rocket. The ones I flew would do 5 1/5 to 6 minutes of hard flying. The stage you are currently at you should get that easily.

The trouble with calculations is that you are guessing how much throttle you will be using, so the result is a guess. The safest way as Mike suggests is to start with a short time, recharge and assess, Then increase the time to suit until you find the correct one. 4 Mins 30 will be very safe.

Hi Mike,

Thanks for the equation for estimating the theoretical flight time, far easier than what I came up with, I was dredging up some rather rusty memories from my school days. I also obviously have a misunderstanding of what exactly a watt is, so think I'll brush up on my electronics.

Hi Jon, yep this is for my 48" Extra, full throttle in the back garden and it's got a bit of pull, and as you say I'm unlikely to be using all that potential power for a long time, but good to know that if and when I can fly it hard it'll still be good for 5-6minutes.

I'll set the timer to 4 mins and time the actual flight and see what I'm putting back in and adjust from there.

Thank you both for your help,

Chris