Leaf-Blower Hovercraft and the Scientific Method

Factual Questions
1

What principles of science/physics/engineering can be MEASURED AND DEMONSTRATED using a leaf-blower powered hovercraft and simple components from home depot? Background:

I am currently working with my 4th grade son building a hovercraft for his science fair project. While he chose a hovercraft because he though t it would be cool to drive around the neighborhood, I chose it because it was cool to drive around the neighborhood and there are some simple physical principles that can be explored using the hovercraft as an example. My idea is things that I can 1) measure simply and 2) compare to theoretical values. So far, I have come up with the following:

  1. Using a water manometer constructed by a plastic u-tube full of water, measure the pressure under the hovercraft, and demonstrate that the pressure times the area of the plenum is equal to the weight of the hovercraft and contents.
  2. Hang the lead blower from the ceiling and use a fish scale to measure the thrust (Blower #2 which is used for propulsion, not lift). Compare this to drag calculated from the measured maximum speed of the hovercraft. Alternatively, I could use a pulley and string attached to the blower to lift a weight vertically, clearly demonstrating the amount of thrust in a very understandable, graphic form. This may be better than the spring scale.

What other simple principles can be measured and compared to the theoretical results? I am a very hands-on engineer with a fully equipped shop, but I want to keep it simple so that I can believe my son actually understands what it going on. For example, I would like to measure the mass air flow and velocity in order to calculate thrust, and compare that to the measured thrust, but I cannot figure out how to do this simply. What other simple things am I missing?

If you are interested, the hovercraft is constructed using a 4’ diameter plywood disk. The plenum constructed using heavy plastic placed on the bottom of the disk. The plenum is inflated using a gas-powered leaf blower, with a second blower from my neighbor being used to provide thrust for movement. This is the concept anyway, I know thing have a way of changing when the rubber meets to road.

You know, overall this post make me feel dirty like I am asking you to do my homework.

2

Like mine at HOVERCRAFT: A DIY SCIENCE FAIR PROJECT, VACUUM CLEANER POWERED, ULTRA-SIMPLE HOVER CRAFT ?

You could also measure the maximum leafblower pressure when the air
outlet is plugged. Then multiply this pressure by the area of the wood
disk, which gives you the maximum load to be lifted (which is an amazingly
large value!)

Nah, you already did the “homework.” You’re just asking for extra credit questions to answer.

3

[QUOTE=bbeaty]
You could also measure the maximum leafblower pressure when the air
outlet is plugged. Then multiply this pressure by the area of the wood
disk, which gives you the maximum load to be lifted (which is an amazingly
large value!)
QUOTE]

Great idea. I had not though of this one. This is exactly the kind of thing I was looking for.

4

[QUOTE=bbeaty]
You could also measure the maximum leafblower pressure when the air outlet is plugged. Then multiply this pressure by the area of the wood disk, which gives you the maximum load to be lifted (which is an amazingly large value!)

[QUOTE]

Great idea. I had not though of this one. This is exactly the kind of thing I was looking for.

5

If you want to find air velocity, you can bodge together a homemade Pitot tube (see this page, and a very good description here, and do a search for ‘Pitot’) using your water manometer and a drinking straw.

You could also demonstrate power efficency be relating the fuel consumption (google ’ “btu/lb” petrol’) to the thrust generated.

Oh, and chekc out the BBC’s Juckyard Challenge special of hovercrafts here.