Saturday, May 26, 2007


When fabricating a carb-heat box one of the trickier bits is the axle on which the flap or air-vane pivots. Most start with a round rod then file or mill it flat on one side to accept the flap. The axle now assumes a D-shape and the flap is attached to the flat part of the D with rivets or screws. A few use steel and weld flap to the shaft, trading the heavier weight for the greater ease of fabrication.

To actuate carb-heat we must cause the air-vane to move. An interesting point, often overlooked by the novice, is that the flap or air-vane need not swing through a 90 degree arc to let hot air into the box. If the hot air source is fed into the side of the box perpendicular to the normal air flow, any angle greater than 45 degrees is sufficient for the carb to draw-in 100% heated air.

The usual method of causing the air-vane to move is to attach a lever to one end of the shaft to which the air-vane is attached. The tricky bit is to do so in a manner secure enough to risk your life upon. The most popular method appears to be to continue the D-shape to the end of the rod and to make a matching D-shaped hole in your lever. At his point the novice is liable to give up because producing an accurate D-shaped hole with hand-tools can be rather difficult. Some convert the D-shape of the axle to a square shape, since it’s fairly easy to square a round hole. Of course, if they started with quarter-inch rod they end up with a rather small square - - just .177" on a side; about 11/64". With a trunion that small, if the rod is aluminum or brass you can’t put much stress on the lever, which is why a lot of guys go to a steel rod - - and weld the flapper to it. Or end up using a larger, heavier rod.

I’ve a hunch you could fill a fair sized book with the variations on this theme. And I know a few guys who have done exactly that, bouncing back and forth between the need for an air-vane that pivots smoothly, a lever that provides fail-safe actuation and the need for the thing to be fabricated with basic hand tools. The results are often blindingly complex, with stepped shafts, ball-bearings, adjustable linkages with ball-end fittings and... Most homebuilders fail to notice that in creating such marvels they’ve given up flying for designing.

The method I use is so crude it makes real engineers cringe, but it’s reliable, inexpensive, low in weight and easy to fabricate. While the 'experts' are peering down several yards of nose, talking about 'trophy points' and 'state of the art,' you're actually flying.

I make the axle out of square rod rather than round. The basic idea here is that a square rod will rotate smoothly in a round hole having a diameter equal to the diagonal dimension of the rod. This method works because the rod only has to rotate about a quarter-turn to do its job; it doesn’t have to rotate fast nor often. For a rod that is one-quarter inch on a side, a size ‘T’ drill makes a hole that is a pretty good fit. To make the square hole in the lever you simply begin with a hole 1/4" in diameter and use a rat-tailed file to give it four corners. Since you have the square rod as a gauge it’s pretty hard to go wrong. A washer and cotter key then serves to retain the lever.

There's a bit more to the actual fabrication of course, details specific to the carburetor, method of mounting the engine to a particular airframe and so forth. The purpose of this post is to make you aware of the principle - - that a square shaft can rotate smoothly in a round hole.

Need I mention that this method works equally well for cabin heat or controlling the flow of air through an oil cooler? Probably not :-)


PS -- This method isn't new. I first saw it in the 1940's, on a rather ugly flintlock rifle manufactured in the back-woods of Pennsylvania in the 18th century. The fact the rifle was still shooting 190 years later served as a nice lesson in the practical value of low-tech simplicity.