Reader Questions on Work and Machines

The following are questions from readers on work and machines. You can see what concerns other people, and perhaps get some ideas yourself.

On science experiment:

My name is Aric Ohana, and I am doing a science experiment. My science experiment may sound complex and maybe even to some impossible, but I would still like to try. I am going to try to make remote control aircraft that runs on its motion in the air, kind of sounds like perpetual motion machine, but I am not intending it to stay in the air forever.

The plane is most probably going to be powered by a 7.2 volt engine and it will have a 7.2 volt NICd battery pack. When the plane is flying with just the battery pack it can only stay airborne for about 5-10 minutes. The plane does not run at constant 7.2 volts when in flight. It may only need 6-7 volts to take off and about 4 volts to keep itself in the air.

As a plane flies in the air it creates wind resistance, if you were to use a another prop to collect the wind that is passing over the plane and turn it into energy, like a miniature wind generator. It took some thought in how to make successful wind generator, but then I thought why not just another small motor, like the one used to run the engine. The plane will fly about 25-35 mph and if I use a well modified prop to be able to turn the engine fast enough only to produce 3-5 volt.

If I am able to produce 3 volts then I will be able to keep the plane in the air longer, but if I can create 5 volts then the plane will stay in the air for as long as I want it to. I know when you transfer energy you loose some, I was wondering if you would be able to give me some insight on the transfer of energy and your opinion of my project.

With all gratitude Aric Ohana

Answer

Aric — You have a good idea, but there are a few things to consider. The problem with a battery powered model airplane is that weight of the batteries compared to the amount of power that the electric motor can generate. Most remote control airplanes have a powerful gasoline engine. They have a battery pack to power the radio controls, but the gas engine keeps the plane in the air. So, that is problem number 1: can the motor fly the plane with a battery in it? It sounds like you have figured out that part, but you still must see if it will work.

The second idea has the problem that you mentioned: losses. The friction involved and the inefficiency of the propeller and converting motion to electricity would result in not enough electricity being generated to power your motor. The propeller would only catch 10% of the air to turn it. The generator only converts 25-40% of the energy to turn it into electricity. So, you would lose way too much for it to work.

Your idea is very similar to what they do in electric cars they are planning to produce. The battery powers the car, but when the car goes down hill or slows down, the motor generates energy to charge the battery up. The difference here is that there is not a weight problem as in a airplane, and rolling is more efficient than using a propeller.

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