Solar Helicopter Design and Test

This project explores my dream to make a solar powered helicopter.

I have been thinking this is feasible for a long time, using special lightweight solar panels and light weight engine.

I began to explore solar aeroplane flight a long time ago, and at this time I think a very small model helicopter than runs completely from solar energy is possible.

I use the flexible thin film solar panels to provide both the helicopter blade and the solar energy - this is where we can acheive big efficicency. In most solar installations, efficicency is very important, because often there is only small energy available, so the 'load' must also be small.

- Flexible solar panels from Powerfilm, SP4.2-37 x 10. I used this type:

https://www.digikey.com/product-detail/en/powerfil...

- Small micro servo converted to continuous rotation ( contains suitable pager motor and gearbox ), e.g. I used this one:

https://hobbyking.com/en_us/hk-282a-single-screw-u...

- Very lightweight hookup wire

Tools include:

- Pliers, small screwdriver, soldering iron, hot glue gun

As listed in the Parts list, I used these thin film solar panels to both generate power, and to form the shape of the propellor blades. They weigh only 0.87 grams each.
I also selected a ultra micro servo because for such a small weight ( just 1.7grams after changes inside it) it provides good speed and torque and includes the gearbox. A gearbox is needed to reduce the high speed pager motor in the servo down to a speed which is practical and has the torque necessary to turn the propellor blades.

With some small changes, the servo can change to a continouous revolving motor.

Usually servos need controller to move to a position within its range of movement ( usually 180 degress )

You can follow many other tutorials about this, so I will add a link and discuss briefly here.

Briefly: On the ULTRA-micro servo that I used, I simply opened it up, removed the circuit board, connected fine wires to the engine directly, and removed the 'stop lugs' which stop the rotation exceeding 180 degrees. These must be removed before connection power otherwise it will damage the servo gearbox.
Connect your new wires to the motor carefully using your soldering iron, and feed them back through the cable hole in the servo, then close it all up and re-tighten the screws.

The panels I've selected are flexible ( so we can form them into a shape of the propellor blades ) and they are very lightweight so we can keep the helicopter very efficient.

The panels produce 4.2 volts and 22milliamp of power each in full midday sun. We will need to caclulate how many we need - see further details below.

Because the motor is operational from between 2.7 to 6 volts, and the panel produces up to 6 volts, if we need more power by adding solar panels, they must be connected in Parallel, not series. If we connect in series the voltage will increase above the capability of the motor, possibly causing damage.

Connecting in parallel means connecting the negative to negative and positive to positive on successive panels. An example is shown in the picture.
In this instructable, I use the hookup wire to pull up the edges to form a propellor edge. The upper edge will capture the wind as it spins around (clockwise in this example ) and that will produce lift.

There is a big scientific explanation about how propellors work - they are similar to a wing, producing lift due to two forces. 1. Their wing angle, or 'angle of attack', pushing against the wind, and 2, the Bernolli effect, due to the pressure difference between above and below the airfoil. This propellor airfoil, or wing shape, has a different angle of attack along the radius of the blade due to the different speed that it hits the air.

Soldering the solar panel connections must be done using the soldering iron, firstly melting a hole through the layer of plastic over the contacts and make contact with the metal connection strip on each end of the solar panel. A small length of wire is used between each panel to make the connection electrically and mechanically solid. If you have not used a soldering iron before, please check on youtube for some tutorials on correct usage.

Here you will see how I connected the wire from the distant ends of the solar panel. Note the orientation of the solar panels.

We then connect the wires from the engine of the servo to the negative and positive ends of the solar panels. One wire goes to the join in the middle, the other goes to the distant ends.

In our first test, the servo did not turn.

We checked the voltage 'under load' and found that the servo was 'pulling down the voltage'
This means that the servo could not start turning because as it drew current, the solar panel could not provide the current needed and this causes the voltage to drop in the solar panel.

When the voltage is pulled down on a solar panel, it will no longer produce power efficently. For best use of a solar panel, the load should only draw enough current according to the specifications of the solar panel.
We checked the motor specifications with a search on google and found that a typical current requirement for these ultra micro servos is 0.2 Amps. Our two solar panels will only produce 0.04 amps, so we will need more solar panels !

Our servo specifications indicated that under normal conditions, it will turn 60 degrees in 0.08 seconds.

With the power of maths, we can calulate the RPM of our motor/gearbox as:
360 degrees for one turn , therefore 360/60=6 6 turns of 0.08 seconds to make one revolution..
6x0.08=0.48 seconds to make one revolution
1/0.48=2.08 revolutions in one second, therefore 125 revolutions per minute. (RPM)

Thats under normal conditions of power supply.

As explained above, two solar panels could not turn the engine.

We added two more solar panels in parallel and glued the centre of our solar propellor to the top of the servo using hot glue.

We attached the smallest control horn that came with the servo to the bottom of the servo crank, and on that we used sticky tape to attach a piece of paper. This will give our engine and propellor something to push against.

You can see the results in the video of our test:
Our helicopter has turning blades made possible by using only solar energy !!

But is it enough to lift our helicopter?

Our total weight of our solar helicopter is 7.1 grams. Thats beautifully light for this ambitious target of solar powered helicopter.

The solar panels which make up just 3.24 grams or 45% of the total weight, and they do not make enough power for lift. If we increase the number of solar panels, then our solar system will produce more power, but it will also weigh more, so we will need more power, etc etc.

We need to do some iterative serious mathematical calculations. Thankfully there is an online tool to assist us called www.ecalc.ch

We use their helicopter calculator and spent some time trying various combinations of power supply, weight, etc.
We paid for one months subscriptions (just $1.99) and used some of their standard devices to start us off.
They didnt have our servo motor, but had a motor which weighed and performed similarly, so I used that, then selected custom to make our 'no load current' a little lower to 0.2 amp because I think our servo is efficient, and also to offset the limit on the propellor width. I set the maximum propellor width of PConst of 2, because we will be wider due to our 35mm solar panel width. We wont be very efficient compared to a commercially made propellor, so I selected a Tconst of 0.8

In my earlier attempts, it stated the helicopter will not hover ! Disaster. But I persevered and came up with some values that caused a hover capability with just 1.09 thrust to weight ratio at sea level ( It must be be above 1 for our helicopter to fly ! ) - there are other values shown in the attached ecalc Results, that must also fit.

Success !

This result shows that solar powered helicopter lift is achievable !

- we need a 4 bladed propellor, extending out to a diameter of 780mm which contains 10 solar panels.
This increases our total weight to 11 grams. That is the lightest solution that can lift our helicopter payload (its motor).

We will build our next prototype using 1mm carbon fibre rod to support the large propellor and publish our results in a follow up Instructable.