Sunday, March 19, 2006

Building an Intelligent Sailplane

The Intelligent Sailplane
March 19, 2006

This week, I read a very interesting item on detecting thermals on the website:

It talks about hand-launched model gliders and how to detect pockets of rising warm air into which you wish to launch your glider. It looks like young glider-builders get up to 60 seconds afloat for their model aircraft using these techniques. An interesting statement I read was that in the Northern Hemisphere, if you look up and see some evidence of a thermal, you will notice that the thermal is swirling clockwise! I have seen birds such as kites flying in lazy circles for half an hour or more at a stretch. I assume that they are riding a thermal, and fly along with the swirling wind and not against it! Stick your head out of the window and find out if they do clockwise circles.

The challenge

The question I wish to pose is this: Can we build a model sailplane controlled by a microprocessor on board, programmed to exploit any available thermals and stay up as long as possible?

This raises questions about identifying thermals. I will visualize one way of doing this and describe it. You may be able to think of better ones, or who knows, you may decide to build something like the sailplane I describe.

Detecting Thermals:

This could involve random exploration of airspace around the model craft while it is gliding. It should have sensors to detect increase in temperature as it flies into a warm column or bubble of air. This might mean a raise of 1 degree C over 5 to 10 meters of travel. Assuming that the glide speed is about 15 meters/second,
This would mean detecting 1 degree C change over about half a second.

Secondly, the craft should ideally have some sensor to give it information of its own altitude, and signs of any leftward acceleration. I assume that a glider being carried in a clockwise direction (as seen from the ground) will sense a constant leftward acceleration. The altitude sensor should tell the craft when it is going down fast and when it is being carried up by a thermal.

Can you use an altimeter, which works by sensing air pressure, for this purpose? I don’t know. Can we get inexpensive accelerometers to sense the leftward acceleration reliably? Again, I don’t know.

Assume that the craft is in a swirling column of air, being carried round in a circle of about 100 meters diameter, and that is gliding at a speed of about 15 Meters/second. What would be the leftward acceleration? How many “g”? It is best I leave this to you to calculate! The accelerometer you look for should be capable of sensing this reasonably well.

With all this gadgetry, I guess you might be able to build a computer-controlled sailplane that will exploit thermals.

Learning from Kite-like birds

I assume that they use a little bit of effort from their muscles to get the most out of the thermal. Understanding how they do this might help you use limited amounts of engine power (electrical drive, powered by solar cells on the wings?) to help your glider stay afloat longer. In any case, you have to develop techniques that will use input from the sensors to identify what should be done: Use the ailerons and rudder to make a gentle turn? Use the elevators to get the most of being carried around by the thermals? The secrets of the birds could tell us something about this.


Before you rush off to buy the parts for your project, I would suggest that you consult a physics teacher or a meteorologist. At least, do an Internet search to see if you can get information about the frequency and strength of thermals and when & where they occur. Then perhaps you could make a computer simulation of a cubic KM of air showing roughly the structure and statistical properties of your atmospheric model.

The next step would be to create a simulation of the controlling program you want to fly. Combined with the simulated atmospheric model, this simulated craft will let you figure out many issues, unrelated to your model-aircraft building skills! It may also teach a bit of atmospheric physics! The simulation itself might be a science fair project, or a research assignment.

I did an Internet search for “gliding simulators” and found two relevant sites: and

The first one describes a glider simulator which runs on a PC and the second one gives useful information.


You might find a shortcut to the objective, by building a radio controlled model sailplane equipped with sensors that help you on the ground to use your head and steer the craft. It might be wise to do this first. This may be an alternative to simulation and hunting for atmospheric models. In fact it might help you earn a degree by doing research to create suitable atmospheric models. I suspect that useful atmospheric models for this purpose may not be available ready-made.

Have fun!

I dream of an autonomous model sailplane which would keep circling overhead my city and help us watch the traffic jams below! And, we do have plenty of the traffic jams!

Srinivasan Ramani

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