After doing so many calculation I am stuck at

how can I increase the flying time and without compromising the maximum height

Steps considered

1. Reducing weight

2. Low Kv or high Kv motors

3. smaller prop or bigger prop

4. prop pitch

5. Hgher capacity battery (but it adds weight)

6. lower internal resistance of battery

7. anything else.

Suggestions needed

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Thats about what I came up with too.  I decided to go with:

motors: 750kv motors rated 20a MAX, 15v / 12a,15v continuous- in a 28mmx30mm package

battery: 5000mah 4S Li-Po 14.8V @ 25C

props: 10-6 three blade

esc: turnigy plush 25a

I found this combo should give my machine the best run times and thrust:weight ratio.  At 1400gms and almost 1000gms thrust per motor (4Kg total thrust MAX) should provide about 2.8G's of acceleration @MAX.  Thrust to weight ratio is very important. 

The way I figure it,I should see hover @ about 35% throttle or about 65W per motor.  MAX acceleration should consume almost 300W per motor which puts flight time into the five minutes range.  Using lowest possible power for hover gives longest flight time.  Since thrust depends on actual power (mechanical- ie, minus all efficiency losses) delivered to the props, using higher voltage lowers the current draw to give the same power output.  Batteries are rated in Ah (or mAh) and therefore flight time is a function of current draw.  Trading voltage for amperage in P=VI gives longer flight times.  Raise voltage, use slower motors and more aggressive props.  Lower resistance everywhere (batt, esc, wiring) is crucial.

I think what you need is one large propeller, instead of several small inefficient ones.  Turn them much slower than you currently are. Might need gear reduction.  Since you only have one propeller, you obviously would only need one large, high voltage (8S-12S), efficient motor and speed controller.  But since you only have one propeller, you'll need to be able to vector the thrust so that you have control.  While you're at it, use a one-way bearing so that if the motor stops for any reason, you can still auto-gyro down to the ground instead of falling.

 

Oh wait.  That's describing a traditional heli. ;)  :p

Better efficiency is the key :

Here is a non limitative list :

- lower weight

- better battery capacity / weight ratio

- lighter and more rigid propeller (wood or carbon), good balancing

- variable pitch propellers

- high efficiency motors and ESCs, high voltage to avoid iron loss.

- optimal frame / motors / propellers matching

- advanced brushless motor control (vector control) with regenerative braking

- smooth and optimal control of motors to avoid energy loss

- smooth driving !

I think I am stuck with the equation of low Kv but large props.

Does someone tell what is the motor, Kv and prop size for MD4-200

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