Dragging heavy objects over rough ground or choppy water is an age-old drudgery.
Humans have invented many ways to make this simple process easier. We have
enlisted the help of stronger animals like horses, and later we created
fuel burning engines to replace work animals. We invented the wheel, refined
it, and used it in many clever ways. We built boats and powered them with
oars, sails, and then with engines. Even more amazing is the fact that we
created vehicles which fly! Vehicles which travel on land may be moved forward
by pushing backward on the surface of the roadway and boats may be propelled
by pushing backward on the water they float in. To fly, one of the technical
problems which had to be solved was propulsion; what can you push against?
The spinning "air screw" or propeller was developed to push against
the obvious - air. Not only did this technical innovation allow engine powered
flight, but it provided another possibility for moving terrestrial vehicles
as well. One of the important drawbacks of this mode of propulsion has been
its general inefficiency.
A Story:
In support of bio-diversity you are assembling a research team to search
the globe for rare and endangered species. To make this exploration possible,
you look for a vehicle which will be helpful in undeveloped wilderness.
You soon discover that standard "all-terrain" vehicles are limited
because they depend on solid ground to push themselves along. This earth-pushing
propulsion technique is not useful for crossing wetlands, or rivers, or
snow fields. It doesn't seem like a good technique for crossing from tree-top
to tree-top on cables at the top of the rain forest. You recall that the
problem of powering aircraft was solved by pushing on air. Perhaps this
technology will meet your needs. You build a model to test some of your
ideas
Questions:
How does the push of air effect the motion of the aircart?
Is this a constant push or does it depend on the speed of the aircart?
What transportation problems could an aircart solve?
What are its drawbacks?
How efficient is it?
What's its maximum speed?
What about starting and stopping?
Can it carry a load?
How about hills?
Performance Criteria
Before designing an aircart it is important to establish performance criteria
The aircart must move on its own power (3 volts from two AA batteries)
Several things should be maximized. Two essential considerations are:
the aircart's top speed (terminal velocity)
its ability to speed up (accelerate)
Also important are:
load carrying capability
hill-climbing ability.
You may think of others.
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You know that many things will effect the performance of your aircart.
The quality of the electric motor, the size and pitch of the propeller,
and the charge of the batteries will all influence the motion of the aircart.
The size and weight of the aircart are important, and the quality of construction
makes big a difference. Because friction is such an important consideration,
pay attention to the wheels. The aircart is a system in which all parts
must work well together.
The design presented here has been tested and works well. There are many
ways to design a worse aircart and a few ways to build a better one. Can
you improve the design?
