Technicalities: That Reminds Me

Last week, when I was at the airport working on my airplane, I heard the sound of an airplane taking off and I turned to watch, as I always do, partly from curiosity and partly just in case. This time it was a Long-EZ that was taking off, and it was well above the trees and the hangars when I first saw it. But then its engine seemed to lose power, then regain it, then lose it again. The airplane was about 300 feet above the ground, I think, and not far past the departure end, when the pilot made the decision to turn back. He did a good job, banking steeply, keeping the nose down, rolling smoothly from a 230-degree right turn to a 50-degree left one. When he disappeared behind the hangars, it was evident that he would make the runway comfortably.

My first thought was that he was lucky to be flying one of those Rutan canards, because they won’t stall, and so the major danger of any turnback, a spin departure, was spared him. Also, he had some power; it was intermittent, but it never went away entirely. On the whole, it was a good thing to see — an emergency well and successfully disposed of.

It reminded me of something that happened to me many years ago, in the 1970s, also at this airport. I had made a couple of mufflers for my then airplane, the all-metal first Melmoth. They hung beneath the fuselage, each one a yard-long cylinder with a perforated inner liner, a packing of glass wool and an outer shell. Lengths of flexible stainless-steel ducting connected them to the exhaust pipes. They were made of aluminum, because someone whom I had confidence in had told me that the temperature of exhaust gas leaving the pipe is around 500 degrees. Of course, he had been thinking of an open pipe, not one with several feet of additional tubing downstream.

Whether because of the heat or the vibration or the force of the exhaust stream or all three in concert, I was about 200 feet in the air when I heard a muffled thud and the power fell off drastically. The engine did not sound good, and I was just able to maintain altitude at 90 knots. I knew enough not to try to turn back. The railroad tracks and power lines ahead of me did not look hospitable, and so I decided — not that I had much choice — to try to make it around the pattern.

I was in the middle of the downwind leg when smoke began rising from the cabin floor on the passenger’s side. On base, it filled the cockpit; it was becoming difficult to breathe and see as I turned final, touched down and opened the gull-wing door as I rolled off the runway.

Both mufflers had collapsed internally, completely blocking the exhaust. By luck, however, a circumferential crack had opened in one of the flexible ducts, and so the three cylinders on the right side of the engine continued to operate, though with greatly increased back pressure. The crack, as it happened, was on top of the pipe, and a narrow jet of exhaust gas, like a blowtorch flame, had melted its way through the cabin floor, which, for what I imagined at the time to be sound-deadening purposes, was a laminate of two aluminum sheets with dense foam rubber in between and a carpet on top. The smoke in the cockpit was from the burning rubber.

After that experience, I decided that on balance, earplugs were quite a good deal.

While exhaust gas seldom — though not never — burns a hole in an airplane, it is strongly inclined to leave gray deposits on the paint unless the pipe protrudes quite far. The reason for this persistent phenomenon is something called Coanda Effect, one of those semifamiliar aerodynamic notions, like the Bernoulli Principle, that most pilots only semiunderstand. Coanda Effect is the tendency of a high-velocity jet of fluid to adhere to a nearby surface. It is due to the entrainment of some of the surrounding medium into the jet, which creates a low pressure between the jet and the surface. The low pressure deflects the jet toward the surface, to which it eventually adheres, even following it around corners provided they are of sufficiently large radius. Coanda Effect is often wrongly credited with the adhesion of a stream of water to a curved spoon, but that is really due to surface tension. Coanda Effect is, however, related to the lift generated by wings, especially so-called “blown” wings or flaps, where an internal pressure source, like a jet engine, is used to direct a high-velocity stream over part of the wing’s surface. One imaginative application of the Coanda Effect is in the McDonnell-Douglas Helicopters NOTAR, which replaces the conventional tail rotor with a cylindrical surface across which air is blown, through shutters, by an internal fan to produce a low pressure on one side of the cylinder.

Coanda Effect took its name from a Romanian inventor, Henri Coanda, who, while experimenting with what was essentially a centrifugal compressor driven by a 50 hp piston engine, noticed the tendency of its output stream to adhere to the sides of the airplane that this compressor, or turbo-motor, was supposed to propel. This was in 1910. Reverence for the truth is not always an ornament of the creative mind, and Coanda later claimed to have flown his airplane for a short distance while injecting fuel into the compressor exhaust as a sort of primitive afterburner. Supposedly it crashed and burned, but not before throwing Coanda clear so that half a century later he could anoint himself the inventor of the jet engine. Whether the so-called Coanda-1910 in fact ever flew, or crashed, at all is a matter of dispute among historians, though it is a treasured article of faith for those who like to imagine that medieval monks glided from the parapets of abbeys and that Boeing stole the blended wing-body from Vincent Burnelli.

The type of engine that Coanda later claimed to have invented is now called a motorjet. It occupies a sort of transitional zone between props and jets and was used, during and shortly after World War II in Russia and various European countries, to power several prototypes, the best-known of which, the Caproni-Campini N.1, has sometimes been called — incorrectly — the first jet airplane to fly.

The motorjet principle provides a helpful illustration of the relationship between props and jets. All powered airplanes propel themselves forward by propelling something else backward. The propeller, or “airscrew” as it used to be called, disguises this principle, because it appears to be clawing its way forward through the air; but if you put the propeller into a tube, making it into a ducted fan, the fact that it produces thrust by throwing air backward becomes more obvious. Now, if you replace the propeller in the tube with a series of fans or with a centrifugal compressor, you get more of the same effect: The air downstream from the propeller is compressed and rushes out the open exhaust at high speed, propelling the airplane forward.

You can increase the compression still further by injecting fuel into the exhaust stream and igniting it to expand and accelerate the air. Now you have a sort of afterburning ducted fan. This is the motorjet: The fan is still driven by some kind of auxiliary engine, which was a plain old reciprocating engine in all the historical examples. What would turn the contraption into a pure jet would be the replacement of the driving motor by a turbine in the exhaust.

The motorjet was impractical for many reasons, one of which was that, to attain a high enough compression ratio for moderately efficient operation, a very powerful driving engine was needed — thousands of horsepower for even a medium-size compressor. Between an unprecedentedly big, heavy and enormously complicated reciprocating engine and one or two stages of turbine blading in the exhaust stream to drive the compressor, there was really no contest. And so the brief heyday of the motorjet, which combined the unreliability of the recip with the inefficiency of the jet, came to an end, and with it this peripatetic article.