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By Rodger K. Tenney

IT takes about three years of constant practice for a pilot to become a good sky writer. That's almost as long as a course in college. Jim Rose, a Chicago pilot, has been sky writing for six years; so he probably could qualify for a sky writing Ph.D.

"Sky writing is hard on your ship," Jim declares. "The pilot gets a lot of smoke and fumes, but when he lands he can get most of them out of his system. Your ship soaks up oily vapors that in time make it a 'flying tinder box.' So if you have any fire trouble, the plane is almost certain to burn."

Rose uses a chemical in his smoke making oil that minimizes fire danger; but the hazard is still there if flame does happen to touch the plane. Almost any light lubricating oil may be used for making sky writing smoke. The oil in Rose's ship is stored in a tank in the front cockpit. From here it is piped into the oversize exhaust pipe, with the flow controlled by a valve in the pilot's cockpit. Smoke for one letter in a word requires from one to one and one-half gallons of oil; but the tank on Jim's plane has a sixty gallon capacity. This is to enable him to make more than one sign without having to land to re-fill his oil tank.

The sky writers job takes him up to altitudes where air is thin and where the ordinary airplane motor would spit and sputter under the strain of the performance required of it. For sky writing purposes, a ship must have twice the power of regular planes without having excessive weight. Rose's plane has a 500 horse power motor that gives it plenty of speed and climbing power.

"When I make a sign, I usually go up to an altitude of between 15.000 and 18,000 feet," Jim said. "A new plane I am developing, however, has an oxygen tank and can be used at 20,000 to 23,000 feet."

High altitudes for sky writing are required because here there is less likelihood of air turbulence. Steady winds don't cause trouble, but cross currents of air ruin the smoke letters. Blow smoke into an electric fan, and it travels in one direction. But place another fan at right angles to the first fan, and the smoke whirls around and is quickly scattered. Cross currents of air in the sky do somewhat the same thing to words written by a sky writer.

"I can put up a sign in a 100 mile an hour wind, if the wind is blowing in one direction," Jim explained.

Anyone attempting to guess the length of letters in words written by a sky writer probably would find serious errors in his calculations. Against the infinity of the sky, the letters appear relatively small. Actually, they" are about 3,500 feet in length, or more than three-fifths of a mile. Width of the letters usually is about 200 feet.

"The pilot, of course, must he able to figure the length of the letters while he is flying. To do this, I use a stop watch," Jim said. "Knowing the speed of my plane, I can determine, by timing, just how far to fly in each direction to get the proper size letter. Spacing each letter is largely a matter of guesswork and experience."

AND making these calculations is done while the pilot is writing backward! Write several words on a piece of paper. Hold the paper up to a mirror. The first left hand letter is on the right, in the mirror. And it is that letter, on the right, where the sky writer begins. But when you see the completed sign it appears as it would if written in the customary manner on paper.

When he climbs to the altitude he wants, and finds a strata of air that's free from conflicting air currents, the pilot does not immediately start his writing.

"Before I turn on the smoke," Jim said, "I line up my ship with streets below. This is necessary so I can keep the letters aligned. At 18,000 feet, city streets, of course, look merely like single intersecting lines. So I put the back edge of my wings parallel with one of these streets. Each time I start a new letter, I line up the wings again. This works all right unless I am over a town with a lot of diagonal streets. In a case like this, I get my original line up and trust mostly to luck that the rest of the letters will be aligned properly.

"Over water it often is possible to make the first letter and use the shadow of that one in arranging the position of the other letters."

After making his first letter, the pilot puts his ship up fifty feet before starting the second. Then with each succeeding letter, he goes up the same distance. This is to avoid hitting the letters with propeller "wash" which would scatter the smoke and ruin the sign. It is somewhat on the order of a Chinese puzzle, for the pilot must not cross his own trail from start to finish.

"With all of our care and preparations, we do make mistakes," Jim confesses. "For instance, take the pilot who was hanging a sign for an air show. He started off well and had a well made sign when he finished, except for one detail. Instead of 'air show' he had written 'air sow.'"