Formula for flight times

[Paul Williams]

Trying to get my head round working out flight times using different capacity batteries. Any body got a simple formula? save me from headache please.

[Pete Rieden]

Too many variables. The most accurate one would be:

Time of flight = end of flight - start of flight

But this isn't very useful. Your shortest possible flight time (motor run time) is simply:

Flight time (minutes) = 60 * Capacity (in mAh) / Full Throttle Current (in mA) 

Or to look at it another way, use the "C" rating. If you're running your setup at a full-throttle rating of 10C then your full-throttle run time is 1/10 of an hour (6 minutes), and if your at 20C then its 1/20 of an hour (3 minutes).

For each model and its pilot's flying style the *actual* flight time will vary depending on throttle use. For example I know that my Formosa has a full-throttle current equivilent to just under 10C, but I generally get flight times of 12-15 minutes of continuous aerobatics. So for general aerobatic flying I use a gerenal guide:

(1/c-rate) * 2 * 60 minutes

For my Trex 450 I have a 2200mAh battery and a hover current of around 8A, so I get:

2200 / 8000 * 60 =around 16 minutes of hovber practice

But I also know that I can easily draw over 25A in "enthusiastic aerobatics", which goves me less than 6 minutes from the same battery.

HTH,

PDR

[Frank Skilbeck]

at full throttle, flight time in mins = 60 x mah/(amps x 1000).

This is not a bad starting point as you don't usually fly at full throttle, so I measure how much charge I put back in and then adjust the flight time accordingly.

[Robert Bromwich]

HI Paul

with a Watt meter test fullpower ampere then use this:

Amp draw at full power / 60 (which is minutes) = amp used in 1 minute

Then use battery amp / amp used in minute = flight time

Eg:

2.2 amp battery

35amp power draw from motor

so :

(power draw)35 / 60minutes = 0.583 milliamp per minute

(battery size)2.2 / 0.583 (milliamp per min) = 3.7 minutes fullpower

 another one:

3.7 amp battery

24 amp powerdraw

so:

24 / 60 = 0.400

then

3.7 / 0.400 = 9.25 minutes

hope this helps

Rob

[Nick Rigg]

Frank mentioned you'll rarely fly all the time at full throttle but it's also worth mentioning that any measurement from your Watt meter will be static - in flight, the propeller will unwind to some extent so the amp draw will reduce further and the potential flight time increase 

[Rick Brayford]

I find the safest method is to tether model to the ground, run at maximum safe amps until battery shows first sign of fading, and set your timer to that. this will ensure a little extra time up your sleeve a landing que or flying at full power all the time.

Skydive

[Pete Rieden]

...having comprehensively cooked your motor, ESC and batteries...

PDR

[Paul Williams]

Cheers guys I have my head round it now,

I agree Rick thats the safest option but I was wanting to do some research before buying set up.

If I double up the battery IE 2 off 3s 2200mah i am right in assuming that i will get 4400mah therefore doubling the flight time?

Or if I go 22v (the system will take it) I will use less amps.

I am trying to get longer flight times then my current 6 mins

[Frank Skilbeck]

"Or if I go 22v (the system will take it) I will use less amps."

 Only if you prop the motor to use half the amps.

[Andy Gates]

Paul

What plane are you flying? 6mins in not very long, could it be you may need a more efficient motor?

Flight times and performance are all about getting the right parts to provide the power required. You need the right motor, the right prop and the right battery pack all in balance so one part is not more stressed than any of the others.

[Johan510]

A LIPO BATTERY WITH CAPACITY 2200mAh will allow:

1 hour flying time (60 min / 1) if you only draw 2,200mA continuous(1C = 1 X capacity of battery)

 6 minutes flying time (60/10) if you draw 22,000mA continuous(10C = 10 X capacity of battery)

Regards,

Johan

[iawnski]

how would you work this out on a glider with a lipo, say 2 aileron servos and 1 rudder and 1 elevator ?

[Tim Mackey]

First off, although theoretically correct it is not actually quite as simple in practice - actual duration will also be affected by winds, and flying style - and of course, doubling the battery capacity will pretty much double its weight and therefore power to weight ratio will change and the model will require more power to fly the same way etc.

Iawnski - assuming no motor and only the radio gear you mention- the only real way to tell will be to fly and find out - however as a good indication, connect an ammeter in series with the battery positive line, and note the current drawn in various conditions EG: servos at idle, servos being moved, and servos stalled  - also with everything switched on simply grasp a control surface such as the elevator and push / pull it hard and see what the meter reads. Average these figures out and you will have an average current draw for normal flight conditions. Lets assume the figure is .25A. If your battery is say 2000 m/ahr ( 2A) this would supply power for approx 8 hours. This of course would put your battery down to "flat" and this is not recommended. Powering your glider radio from a lipo would only be possible by means of a regulator, and this itself will also be consuming power itself.

[iawnski]

well thats me sorted dunno about anyone else .excellent  that timbo well presented

[Paul Williams]

Its a large wing trainer 80". I was using a 1000kv motor on a 10.5" x 6 prop. The set up flew ok but was pulling 25amp on full throttle with 2.5lbs of thrust. All up wieght was 3lbs. The li-po was a 3s 2200. I was getting around 6 mins which works out right using Rob's formula.

I purchased a 400kv motor which will take 22v and wired 2 batteries up. I have tested the set up and its using 13 amps on full which with a 12x8 prop pulls 5ibs of thrust. I have moved the batteries back into the servo bay by making a support shelf to sort the CG and the increased wieght is only 1lb. So the plane now wieghs 4ib all up, with a 5lb thrust. I worked out the flight times to be 20 mins and after flying (in between showers yesterday) I got 17 mins before the esc called fun over. I am well pleased the wind yesterday was 17mph gusts which made it hard going at times so on a calm day I can throttle back much more.

It cost me 28 quid for the motor (giant cod) plus a few quid for props and connectors to wire batteries, The plane is a different beast I cant reconmend this upgrade enough.