Subject: Re: Why does a glider with a straw body and paper wings that form loops fly?

Date: Sun Jun 21 22:26:48 1998
Posted by Ricky J. Sethi
Position: PhD

Hi Karen,

That's a great project! And you're right; it does seem like a "wingless wonder". These wingless gliders do, as you suspect, work by Bernoulli's theorem. There are many sites on the net about paper airplanes and the bernoulli effect (and how airplanes, in general, fly), so I'll keep that discussion to a minimum. Instead, I'll address how to apply Bernoulli's principle to the wingless glider and then I'll provide some links that can help with this activity. Okay, on to the theory...

As you probably know, Bernoulli's principle is another expression of the conservation of energy. In this case, the energy of a fluid, like moving air. It states that the energy from the velocity of the fluid and the energy from the pressure of the fluid are a constant. If one goes down, the other must go up. If the velocity of the fluid slows down, it's pressure must go up. Similarly, if the velocity of the fluid goes up, it's pressure must go down.

Now, in an idealized wing (airfoil), the upper surface is slightly curved while the lower surface is straight, as shown here. When the airstream is incident on the leading edge of the airfoil, it bifurcates, or splits. Part of it goes up and the other part goes down. The part that goes down hits the lower surface of the wing. This causes it to slow down. And this is where Bernoulli's principle comes into play. Since the velocity of the fluid (in this case, the airstream) has gone down, it's pressure must go up.

But this is just half of the picture. The part of the airstream that goes up and over the top surface of the wing actually speeds up. Since this part speeds up, according to Bernoulli's principle, it's pressure must go down. So you end up with a wing that has unbalanced forces: higher pressure on the bottom of the wing (from the slower air) and lower pressure on the top of the wing (from the faster moving air). Net result: lift! The plane flies!

Of course, this is just one way to think of what's going on. You can look at it from a Newtonian perspective, too; sometimes, this approach is more intuitive but assumes some basic physics. You can find a great explanation from both perspectives at Prof. Bloomfield's excellent site.

Luckily for us, we don't need to worry about the intuitive roadblocks to understanding the physics on top of the wing. The reason for this is that the wings of a paper airplane are usually too thin to allow for any appreciable drop in pressure on top of the wing. Due to their design, the pressure on the top of the paper airplane's wing is usually at or slightly below atmospheric pressure. So the wings of a paper airplane work slightly differently from "regular" wings.

Although the wings of the paper airplane don't have the prominent "bump" necessary for an airfoil, the bottom of the wing is still a hindrance to the flow of the airstream. As the stream hits the lower part of the paper airplane's wing, it slows down. And, as we know from our old friend Bernoulli, whenever a fluid slows down, it's pressure goes up. So the pressure on the bottom of the paper airplane's wing STILL goes up and this is enough to provide the necessary lift to sustain the paper airplane in flight.

Of course, the better the design of the wing, the more it acts like an airfoil, and the more the top part contributes to providing greater lift. Okay, all well and good, you say, but what about the "wingless glider"??? Well, you can think of the wingless glider as just a regular plane with the wings folded up! The wingless glider thus behaves almost identically to a regular paper airplane. Here, the air flowing over the bottom-edge of the top-part-of-the-loop behaves like our wing, providing the necessary lift. Of course, there are other, more complicated, effects at work here, but this is basically what's going on.

In addition, there are other design necessities. Like any good plane, the "wingless" glider has a center of gravity. For optimum efficiency, the main loop should be situated right above this. The two loop design is thus a little bulkier, in this regard. After I saw your question, I couldn't resist building my own "wingless wonders" and I love them! I must admit, I hadn't heard of them before now but better late than never, eh? I played around with the design a bit (multiple loops; different placement of loops; etc.), and I found that a single loop, right above the center of gravity provided optimal flight. Of course, actual mileage may vary and you might discover that another approach is even better.

Okay, before I get to the links, I'd like to touch briefly on a couple of pedagogical issues (since you'll be using this for a lab activity). To make the connection between the loops and "regular" wings, you might want to have the students unfold the loops, and tape them (as wings) to the same spots where they had previously taped the loops. The gliders should fly almost the same, depending on the location of the loops/wings and other design issues. Once you've established the correlation between the loops and wings, you can then use some of the standard activities to demonstrate Bernoulli's principle and provide an intuitive understanding of it. I found an excellent activity here; this page is designed from the point of view of the teacher so it should help with your activity, too.

Finally, there is this excellent site maintained by a talented group of 8 kids, ranging from ages 7-15. Their site is absolutely wonderful and they really put a lot of time and effort into actually doing the experiments and explaining the science behind it. They also have a "wingless glider" page where they talk about different "wingless" gliders that they've built; they've got pictures and diagrams to help design the gliders, too. I highly recommend giving them a look.

I hope that wasn't too long-winded. Teachers have a very demanding job that they don't get paid anywhere near enough for. If you need any further help in this, please don't hesitate to drop me a line at science@zentropy.com. Also, if I was unclear about any part of this, I'd be more than happy to go into it in more detail. Good luck with your project!

Best regards,

Rick.

And now, the long promised links: