As a slot car moves through the air it is greatly affected by it. The faster the car, the more effect the air has upon it. Air control (wing) cars depend on this for their very survival, but all cars are affected and can take advantage of aerodynamics if their designers are clever and aware!

The nose of the car is perhaps the most important part of the body. The leading edge is responsible for cutting the air and directing it around the car. Air control cars use a tough frontal "blade" called a "diaplane". The name means "di-"(two) "-a-" (separate) "-plane" (layer), or to "split the air into two layers".

As the car cuts through the air, the diaplane scoops up the air in front of the car and causes it to be compressed against the top of the body, pushing it down onto the track. Meanwhile, underneath the car, venturi effect sucks all the air out and so there is low air pressure, which means there is a vacuum sucking the car down onto the track. All these forces can be referred to as "aerodynamic down force". Assuming that we want the car to stick instead of fly, this force is a good thing! The car is a wedge sliding along the ground slipping itself under the air. If air gets under the car, it degrades the down-force. This suggests that the diaplanes leading edge should be as low as possible so as to scoop up the most air, and let the least air get under the body.

So, what goes wrong with diaplanes? Well, if it gets too low it hits the track (that’s bad!) or digs in (that’s REAL bad!). If it gets too high, air leaks under the car and robs it of the vacuum underneath and the pressure on top - taking away the much needed down force that keeps the car in the slot.

What’s worse, when the nose lifts up a little (due to a bump on the track or braid, tilting on a corner, or whatever) the diaplane lifts up even more, and down-pressure is further reduced when it is needed most. The entire car doesn’t rise up, so which end of the car is likely to rise first? If we assume that the down pressure is greatest at the rear where the car is displacing the most air, and the weight is greatest in the rear, then the nose would certainly rise up first. When this happens, and air begins to leak under the car, our wedge action begins to reverse! Now the air is building up underneath, and vacuum is forming on top. The nose lifts more, more air gets underneath, and so on until - you guessed it - we have a wing! (Might as well get clearance from the tower - this car is taking off! ) Orville and Wilbur couldn’t have done much better!

If we could have only held that darned diaplane down when the rest of the car went up!

Now for my crazy invention:

If you’re going to be a true inventor, you have to take the inventor’s oath. Repeat the following aloud three times (with appropriate reverence) before you proceed:

Got that? Are you sufficiently "reality impaired"? Okay, now we can invent!

Lets say that the diaplane was hinged along the front of the car and could float up and down. Sure, it would drag on the track surface (remember your oath and quit being so doggone logical!) but it would also always stay down, wouldn’t it? Okay, now think (it hurts, but do it anyway): How can we make the diaplane move along the track surface and stay just above it - very close to it but not dragging - no matter what the height of the car? Hmmm.....

A good scientist is not afraid to break the rules, right? So - what if the diaplane did touch the track - but in a way that has low friction, is simple, and durable? Like wheels (no, that would be too complicated).... How about skids! Round skids - for minimum contact and friction. Nylon skids - to be good and slick! Round + nylon = fishing line! Pushed through, say, pin-holes in the diaplane. Now your cooking! Before you get too excited, let’s find a napkin (like all good engineers do) and write this down:

The loop of fishing line makes a "skid" that supports the diaplane just off the track surface. No matter what the car does, the diaplane remains at the same height off the track surface at all times. I use 0.030" diameter fish line and space the pin holes about 1/10th inch apart.

Diaplane is free to "droop" when the car is off the track...

When the nose rises, the diaplane will drop down...

When the nose drops down, the diaplane can move up...

Nylon skids? Hinged, droopy diaplanes? What will they think of next!

Anyway, I can report that this trick works VERY well. I cut the leading edge off of the diaplane in a very straight line and then secured it back in place using strapping tape to across the top make a hinge. (Cleaning the surfaces first, with alcohol, makes the tape stick much better!) The diaplane leading edge should be able to move up and down freely. I bought large gauge nylon fishing line and it worked great. The fish line loop has a spring-like action and provides almost no friction against the track surface at all. Adjust the spacing of the diaplane pin holes, or the gauge of the line, to set the size of the loop and therefore the height of the diaplane. And, when the nylon line loop shows wear, just yank the old piece out and shove a new piece in. Cut the line at a sharp angle to form a point on the end and it will push through the pin hole more easily. (I don’t even snip the ends off very close on top. It makes people ask, "What the heck is that?", when they see that my car has "whiskers"!)

The nicest thing I discovered was that I could now take the car from one track to another, each with a different braid depth, and didn’t have to re-pin the body. The diaplane adjusts automatically. Is this trick legal? I’ve never been told that it isn’t, and I’ve never been stopped from using it.

Remember your inventors oath, and repeat it before you begin your next inventing session! Next time, we’ll look at dynamic braking and how electro-magnetic braking works.