the best thing

balsa wood airplane
Originally uploaded by Ray G.
It's with a heavy heart that I announce Part VIII in The World’s Best Mechanical Engineer Explains It All for You series, with our Special Guest Star: The World's Best Mechanical Engineer.

Why the heavy heart? This is the last of Joe-Henry’s questions for the World’s Best Mechanical Engineer. If we’re going to keep this thing going we’re going to need more questions, gentle reader. There’s a comment box below – please, if you value the dispersion of mechanical engineering know-how across the blogosphere: Use it.[1]

Of course, on the upside, the World's Best Mechanical Engineer has promised to ask Joe-Henry a few questions once this series wraps up.

Joe-Henry,
The last unanswered question you asked was:

Q. What's the best thing about being a mechanical engineer?

A. We'll jump straight to the experiment on this one. You will be an aerospace engineer for a day. I will be a marketing guy for a day (ick).

As your marketing director, I tell you that I want you to design a balsa wood airplane with a 12 inch wingspan. This is just how it works in real life except the marketing director will say something vague and nebulous like, "I want an airplane just like the Competition's -- only better". It's a good thing I'm your marketing guy.

You don't have to build an airplane if you don't want too. You can build something else instead, like a boat. But my experience is that if the marketing guy asks you to build an airplane and you build a boat instead, they get very upset, even if it IS a great boat. You'd be surprised how often this happens.

So hop in the Galaxy and go to the hobby shop with your Mom.

• You'll need a sheet of 1/8" thick balsa wood (usually 4" wide by 36" or 3" wide by 36").



• Also buy a stick of balsa 1/4" x 1/2" or 1/4"x1/4". They'll probably be 36" long also.



• Get some Elmer’s glue and some modeling clay.



• You'll need a little bit of sandpaper, but your Mom's nail file will also work if you use it when she's not looking.



• Also have your Mom buy a small x-acto knife if she doesn't have one already.



• Also buy 3 Mountain Dews (4 if your Mom wants one).

You should have enough materials to build two planes. Your Mom may complain that this is like $15 worth of stuff to build a balsa wood airplane. Assure her that prototypes are always expensive -- the production model will be much cheaper (she doesn't know this, she's not an engineer).

When you get home, measure out a 6" length of the 1/8" balsa wood. With a magic marker, lightly draw the shape you want your left wing to be on the balsa wood. YOU get to decide the wing shape because YOU are the aerospace engineer. Don't get too wild with the first design, save the more advanced designs for the second airplane. Have your Mom cut out the wing with the Exact-o knife. Lay it on top of the balsa wood sheet and trace it with a magic marker. Have your Mom cut it out, this will be your right wing.

Use the sandpaper to round the front edges of the wings off. Then use the sandpaper to sand the rear edge of the wing to a point (sand off the top rear corner of the wing). You are trying to give the wing a little bit of an airfoil shape. See the picture of the rounded front wing edge and the tapered rear wing edge.

Round the front edge and taper the rear edge of the wing.

This is the point where most science teachers would talk about the Bernoulli Principle being the explanation for why airplane wings lift the plane. But it turns out that's not really the main reason. It's just that Bernoulli's public relations department did a really good job convincing people that this was the main reason. In reality when flying the wing is tilted just a little bit back. The force of the air hitting the bottom of the wing pushes up on the wing.

The tail will be one piece. Make it about 1/3 the length of the wingspan (12"/3= 4"). Again: You get to decide the shape. Now make the rudder 2" long. The wood grain on the rudder should run vertically so it will be strong. As always, You get to decide the shape.

Now you will glue the wings together. You made a separate left and right so that you could put the wings at an angle compared to each other. This is called a dihedral angle, and it makes the airplane much more stable. Hit your Mom up for a stack of coins. Lay one wing on a sheet of paper. Put some Elmer's glue on the mating surface of the other wing, then use the stack of coins to prop the wing tip up off of the surface of the table about 2.5".

Prop the wing with coins to create the dihedral angle.

On another sheet of paper, glue your rudder to the top of your tail. Use the unopened Mountain Dew cans to hold the rudder in a vertical position. (See pic).

Use Mountain Dew cans, the Engineer's friend, to hold the tail square.

Pop open the other two Mountain Dews and take a break while the glue dries.

Cut a 12" long piece of the 1/4" thick balsa stick to be the fuselage of the airplane. Glue the entire tail assembly to the rear of the fuselage. Use coins to make the tail level.

After the tail has dried, glue the wings to the fuselage a couple of inches behind the tip of the plane. (See picture). Hit your mom up for a fistful of coins to use to hold the wings and tail even while the glue is drying. R&D costs money.

Use coins to level plane wings and tail.

The last step is to balance your plane using modeling clay. The plane should balance on a pencil placed under the middle of the airplane wing. Add modeling clay to the nose or tail of the plane to make it balance under the center of the wing (see picture). You'll probably have to add weight to the nose.

Plane should balance under the center of the wing.

Now take your plane outside and gently throw it against the wind.

STOP! The anticipation the second before the plane leaves your fingertips, and the satisfaction when it flies.... those feelings are the best things about being a mechanical engineer.

If the plane dives into the ground, move weight to the tail or remove weight from the nose. If the plane stalls (climbs quickly, almost stops in the air, then dives towards the ground), add weight to the nose. If it banks to the right, add clay to the left wingtip. If it banks to the left, add clay to the right wingtip.

A college professor once told me that design is an iterative process. Iterative means you do something more than once, and each time you do it you get closer to the right answer. Build another plane with the other half of your wood. Change all the things you didn't like about the first design. Experiment with your wing design. Put Jefferson Airplane's White Rabbit on the stereo. Dig being an engineer.

Your pal,
B1-67er

Ed.: aka The World’s Best Mechanical Engineer

Also in this series:

• faves from the world's best mechanical engineer
• get it in gear
• busta dew
• holy hydraulics, batman!
• solenoid spectacular
• springs & things
• the world's best mechanical engineer explains it all for you

[1] I should add, the World's Best Mechanical Engineer is under no contractual obligation to answer any more mechanical engineering questions -- heaven knows if he even has time -- but we won't know until we try now, will we? Comments. Below. All yours.