So i am almost there with my quad. It will be ready by a week or so, wanted to know how will i get to know how much time will my copter fly before battery needs to be recharged.

Here are the stuffs:

ZIPPY Flightmax 4000mAh 3S1P 20C
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=7634

hexTronik 24gram Brushless Outrunner 1300kv ( with 9x5 props generating 425~ to 510 gms or thrust )
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=2069

Hobbyking SS Series 18-20A ESC
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=6457

btw i have a separate small batter for my controller.

Rhino 610mAh 2S 7.4v 20C Lipoly Pack
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=7305

Well, you can use only 80% of your lipo's capacity before you start to damage it. So, try to use only 60% and keep that 20% as a safe margin. Which means you want to use around 2400 mAh. Start flying and measure your voltages after 2 minutes.

This is a chart which relates voltage to remaining capacity. It's not perfectly accurate but it's good enough.
4.00V--84%
3.96---77%
3.93---70%
3.90---63%
3.86---56%
3.83---48%
3.80---43%
3.76---35%
3.73---27%
3.70---21%
3.67---14%

Measure the time it takes to get to a specified limit and with that, you should be able to calculate flight time but with the same style of flight.

EDIT: Let's assume your motor draws 10A under full load even though it says 7.5A. Now, you've got 4 so let's say you're drawing 40A total from the battery. Let's keep the limit of discharge to 60% where 60% of your lipo is 2400 mAh, i.e. it can sustain a 2.4A draw for one hour. But at 40A, you should get around 3 minutes only. But that is with over estimating amp draw. Your best bet would be to use the method I mentioned above.

@satyagupta , a little off topic but with a 4000mah 3s + 610mah 2S; your quad seems to be too heavy for the motor you have selected. Also running 9x5 on that motor with a 3s seems little unrealistic. I could be wrong though 

@sublikestar even i thought the same, but i have back ups too:

i have a 4 x backup motors too, again i have a 8x4 props which is recomm for my motor.

btw my quad weight is 900g to 1000g total thrust generated from 4 motor is 1700 gm to 2000 gm which is good for my quad..

some where i got to know that weight/thrust should be between 1.5 - 2.0

To calculate flight time, you must either need a wattmeter (to read amps draw) or a digital charger (which gives amount of charge units fed into the battery).

Using Wattmeter

Plug the wattmeter between the battery and the esc. Now give half throttle and check the amps drawn. Lets say it is 10 amps.

Battery capacity = 4000mah

You would ideally want to use 75% of that = .75x4000 = 3000mah = 3Ah

Now, flight time = 3/10 = .3 hrs = .3x60 = 18 mins at half throttle.

Using Digital Charger

Charge your battery fully. Now run your quad at half throttle for exactly 1 mins. Again charge your battery and check the number of units required for full charge. Lets say it is 200.

Flight time = 3000mah/200 = 15 mins.

Hey Satya, did you measure the weight of your quad on a digital scale or did you just add up the individual weights of the components? Because the above methods can give you surprisingly different results.

And you have to consider thrust/weight and not weight/thrust.

When you consider T/W it should not be for max thrust. What I mean is, you shouldn't use the value of max thrust of motor(s) while calculating T/W unless you want your quad to lift-off at max throttle. (It is mentioned in the kk board manual that you should get a stable hover at about mid stick i.e 1/3 to 2/3 throttle )

And when they say 400+g of thrust, you should take it as 400g max. So (for 1/3 to 2/3 throttle) at 150g thrust for each motor, you get a total thrust of 600g only.

So, a T/W of 1.5 is a minimum for lift-off and not for flight. (For my quad, I'm using motors with 1000+g of thrust)

Have you checked how much heat one of your motors gives off after running continuously at max throttle for at least a minute with a 9x5 prop? If you haven't, please consider doing it. (fix motor on a bench and use a watt-meter if available)



 

So for a quad with the weight of 1000g~ for lift off i need 1500gm of thrust atleast?? I have one more motor which gives 600gm of thrust...

And ya i have considered Thrust/Weight my bad there..

So if we need 1.5 T/W ratio then for quads weighting 1000g we would need a set of motor which produce 1600gm at 1/3 throttle

which implies 400gm per motor should be its 1/3rd of the total thrust. So this means i need motor which has thrust 1200gm+

am i right???

Now how do i reduce the weight of the quad?? most weight is taken by my battery 336gm now shall i change it?? if yes then which one should i buy??

No, 1600g at 1/2 throttle or more for 1000g quad.

What I was trying to say is that you should have take-off between zero throttle to 1/3 throttle (or between zero and 1/2 at most).

I'd say you should use motors with 700+g of thrust at least. I wouldn't recommend changing the battery. 

Hi,

Try this link for approx Calculation of Flight time....

http://www.ecalc.ch/xcoptercalc_e.htm?ecalc

@swapnil...

I dont have that capability motor... ab kya karun??

This links dont have a motor manufacture for hexTronik

O.k just try the setup you have. If it doesn't work then you can make the required changes.

Any suggestion for replacements for motors??

Hey bro you don't need to spend money on different electronics.
Total weight of quad(including battery and everything) should be half of the total thrust. Your quad should hover at 50% of less throttle. Why? Because gyro needs some room to compansate, it can be done by increasing or decreasing speed of the motors. If your quad hovers at say 60% or more throttle, there will be no enough room for gyro and you will struggle to keep it in the air.
If something is in your hand then it's quad weight, decrease it as much as you can without affecting its strength. Use fibre glass material for center plates and make some holes so it will be light enough. Use 1/2 or 3/4 aluminium channels, cut extra part of bolts used for frame and bring overall weight down.

Plus one to that!

Hmm i have used 4x 12 inch aluminium rods for the arms.. And 2x5mmx12cmx12cm plys for the base..

will try to hole upp the arms and cut some unwanted area off the plys and check.. btw any good and cheap LHS to buy carbon fiber or any such stuffs from?

Thanks guys.. I will start shredding extras from my quad and try to pull it well below 800 G wish me luck

This is good bro but with a 3S lipo per cell we get 3.80 as start volt.. not 4.00 V :-/