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I have this idea or a very curious question in my mind. I am no where near professional though, but i would like it to be answered.

We all know how wind turbines can be used to generate electricity. So is it possible to create a quadcoptor that will start with some minimal power by small battery but in time will sustain and keep its system on by self generating electricity and keep on rotating its rotors on its own without other external supply?

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  • $\begingroup$ How you want to generate this electricity? There are already some solutions with photovoltaics on the market, but this systems are far away from being a "good" quadrocopter $\endgroup$
    – TobiasK
    Oct 10 '14 at 7:15
  • $\begingroup$ well, that is what i am asking.. is it possible to generate electricity, by the rotation of rotors?? $\endgroup$
    – monk
    Oct 10 '14 at 7:27
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    $\begingroup$ Definetly not!! Because you just cannot let a motor rotate and get more energy out of it with a generator. This would a perpetual motion machine. This is fortunately impossible. $\endgroup$
    – TobiasK
    Oct 10 '14 at 7:35
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    $\begingroup$ I think monk means using other rotors than the ones used for lift. I would say it is not possible, because in a turbine you use the wind speed relative to the ground to generate electricity. In a quadcopter there is no fixed ground and you would have to expend power to stay fixed relative to the ground. But then again there are things like this car: wired.com/2012/07/wind-powered-car-upwind showing that (my) intuition cannot be always trusted :-) $\endgroup$
    – cube
    Oct 10 '14 at 8:33
  • $\begingroup$ Well you can say this works under certain circumstances, but you will need an energyflow into the system. So you need a windy day or whatever, nothing that works in general. $\endgroup$
    – TobiasK
    Oct 10 '14 at 14:40
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A quadcopter flies because the propellers put the air into motion, which generates a lift. This relies on the air flow around the propellers being relatively unobstructed. As you might imagine it would not fly if you obstructed the propellers with pieces of cardboard for instance.

Wind turbines generate electricity from the wind. If you measured the wind speed before and after the turbine you would see that the wind speed has decreased after passing through the turbine. As such they do obstruct the air flow.

Let's imagine a system with a quadcopter propeller and a wind turbine just underneath it. Because the turbine reduces the wind speed coming out of the whole system, it also reduces the quadcopter lift. To make things worse, the conservation of energy tells us that in an ideal system you can only collect as much energy as what is required to create the lift you lost. In a real system you collect less energy than the lift you lost. So not only would you reduce the lift, you would also collect less energy than what is required to produce the wasted lift.

So I'm afraid the answer is a no. Sorry.

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Generating electricity by rotation of rotors

Yes, electric power generators and electric motors are often similar enough to do both tasks -- converting electrical energy to mechanical energy at one time (such as on takeoff), and converting mechanical energy to electrical energy at some other time (such as during landing).

Any system that takes (mechanical) energy out of the wind and converts it to electrical energy is a wind turbine. Pretty much all commercial wind turbines are mounted on top of a rigid tower.

Generating electricity in the air without a rigid tower

A few airborne wind turbines have been demonstrated. They convert some of the energy of the wind into electrical energy, and simultaneously use the wind (or balloons) to stay aloft.

All airborne wind turbines that I've seen so far have one or two steel cables tethering them to a winch on the ground.

Airborne wind turbines can potentially produce more electric power at lower cost than wind turbines on a tower. The cost is lower because a kilometer of steel cable that can support a few tons of tension from the wind is much simpler and lower cost than a 220 meter tower that can resist a ton of bending force from the wind. The electric power is (potentially) higher because airborne wind turbines can fly higher than a tower, where the wind usually blows faster (has more mechanical energy) than it does closer to the ground.

Other parts of airborne wind turbines are much more complicated than putting wind turbines on a tower.

Generating electricity in the air without any connection to the ground

Many airplanes have a ram air turbine that can generate electricity in the air.

While one might think that the first law of thermodynamics (conservation of energy) might make it impossible to generate more electrical energy with a ram air turbine than the energy is consumed keeping the airplane in the air, a careful analysis shows that there's no physical law that makes it impossible to generate electricity in the air without any connection to the ground.

However, it seems to me that it would be more difficult than keeping a glider in the air, which is already tricky enough.

In principle, gliders can stay aloft indefinitely by extracting energy from the wind (typically from updrafts). In practice Charles Atger managed to keep a glider in the air 56 hours 15 minutes in 1952. The FAI stopped recognizing new glider flight-time endurance records after that due to safety concerns. Occasionally "cloud suck" updrafts put far more energy into a glider than is safe. In principle a ram air turbine could convert some of that excess energy into electrical energy.

Quadrotors generating electric power

EDIT: In response to comments that mathematically "prove" that quadrotors use up more electric power just staying airborne than they could possibly generate, let me point out that

"If it happens, it must be possible."

See the 4-rotor (and therefore quadrotor, right?) flying machine photographs in Google X Buys Airborne Wind Turbine Company Makani Power; Makani technology, Makani photos, etc.

In particular,

Wing 7 generating power

"A composite image of Wing 7 generating power autonomously during crosswind flight. Photo by Andrea Dunlap, composite work by Anita Tirapelle." -- (Wing 7 generating power)

Wing 7 on the ground

(Wing 7 on the ground)

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  • $\begingroup$ A lot of interesting information about other types of aircrafts than quadcopters. You might want to remind the OP that it would not apply to multicopters or helis. $\endgroup$
    – marcv81
    Oct 11 '14 at 17:26
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    $\begingroup$ I suspect that "cloud suck" applies to every kind of aircraft, including quadcopters. Why doesn't it apply to multicopters or helis? $\endgroup$
    – David Cary
    Oct 13 '14 at 2:39
  • $\begingroup$ Cloud suck does apply. Let's do the maths. A typical quadcopter weights 1kg (with battery), has a 3300mAh 11.1V battery (3S), and flies for 15 minutes. 3300mAh at 11.1V is 36.6Wh = 131.8kJ. The same potential energy for a 1kg mass represents a 13.4km gain in altitude. Assuming a 100% energy efficiency in your wind turbine you'd have to gain 13.4km from cloud suck in 15 minutes to keep the same charge in your battery. Well built turbines (not hobby grade) have a 50% efficiency, so plan on 26.8km. You don't get that much cloud suck every day! $\endgroup$
    – marcv81
    Oct 13 '14 at 14:32
  • $\begingroup$ In comparison gliders don't need to spend any energy to carry on gliding. Quadcopters and helicopters need to spend energy just for the privilege of staying airborne, hence the different treatment. $\endgroup$
    – marcv81
    Oct 13 '14 at 14:39
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I have thought of that too, I am not very sure but I feel we can improve the battery life of a quadcopter by doing the following:

  1. We can have a secondary winding on the quadcopter motors.
  2. As the motor is powered to rotate even the secondary winding rotates too hence cuts magnetic field hence creating electricity (we need to make sure that the secondary winding is on primary winding but they shouldn't be connected).
  3. Later somehow we can drain the energy produced by the secondary coil from the motors using some wires and slip rings.
  4. The energy could either be used for led lighting on the quadcopter or for charging the battery, hence increasing the battery life.
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