Jets and Props

Peter waxes eloquent about jets, props and turbofans, which, Peter explains, are both.

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Some time ago I wrote about the relationship between thrust and horsepower, and the question of why one is used to describe the output of pure reaction engines and the other that of engines driving propellers. I argued that the reason was historical. The great majority of engines-steam, gasoline or what have you-have been designed to bring about some sort of mechanical rotation. The force driving a rotating shaft, multiplied by the speed of rotation, provides a straightforward way of measuring the rate at which a machine can do work, which we call its horsepower.

How horses got involved in all this is another matter. A human athlete is capable of brief bursts of about one horsepower, and continuous output at about a third of one. I would have thought that a normal horse, being much larger than a human being and having eaten fewer Twinkies, should be capable of putting out much more than one horsepower; but the Oxford English Dictionary avers that horses possess only three-fourths of a horsepower. I invite readers to contemplate this koan-like paradox at their leisure. At any rate, we apparently owe the term horsepower, which came into use around the start of the 19th century, to James Watt. Watt developed the first practical steam engine, and in modern times his name has replaced the horsepower as a measure of engine power wherever the metric system is used, which is to say, almost everywhere in the world except here. The watt is a relatively puny thing, however-without umbrage, I hope, to the excellent Mr. Watt-and it takes about 750 of them to make a single horsepower.

What I did not touch upon before, however, is why jets perform differently from props, and why they are the preferred powerplants for fast aircraft.

When you bolt a propeller to a rotating shaft, be it driven by a reciprocating engine or a turbine or what you will, you have a device for producing thrust-a reaction engine, just like a jet. The propeller sucks air in from ahead and shoots it out behind, and the momentum imparted to the air that passes through the propeller finds its equal and opposite reaction in the forward impulse imparted to the airplane. The propeller, however, has a very large diameter in comparison to the amount of power it absorbs. The increment of velocity in the slipstream may be surprisingly small-a few knots-because the amount of air passing through the propeller is so large. A propeller seven feet in diameter advancing at 200 knots chews its way through more than 13,000 cubic feet of air per second. Since sea level air weighs about eight-hundredths of a pound per cubic foot, that's 1,000 pounds of air per second. To overcome the drag of a clean small airplane at 200 knots-about 400 pounds-requires accelerating 1,000 pounds of air by very little.

The efficiency of propellers is high-as much as 85 to 90 percent of the power put into them ends up doing useful work-because the velocity of the slipstream is so low compared with the velocity of the surrounding air. As a general rule, when producing thrust by accelerating gas backward, it is more efficient to add a little speed to a lot of gas than a lot of speed to a little. Jets, at the opposite end of the spectrum, impart a lot of velocity to a small cylinder of exhaust, and are less efficient.

Ordinary propellers work well up to 400 knots or so. At higher speeds their effectiveness declines because their blades must be set at ever greater pitch angles. The coarser the blade pitch, the less lift the propeller generates parallel to the line of flight, and the more parallel to the plane of propeller disc. In the extreme case of a propeller turning very slowly while the airplane advances rapidly?a feathered propeller, for instance?the blade lift ceases to provide any thrust component at all. The lift component parallel to the prop disc merely resists rotation; it eats up power without contributing anything to the forward motion of the airplane.

One solution to this problem would be to spin the prop faster, so that its blades could have a flatter pitch. But blade drag rises rapidly as the tips go transonic. We might solve this problem by reducing the propeller's radius; but then there isn't enough blade area to generate a large amount of thrust. The final consequence of this convergence of limitations is that conventional propellers are not a good choice for high-subsonic airplanes, and certainly not for supersonic ones. "Unducted fans"? fanlike propellers with eight or more broad, thin, steeply pitched blades with swept tips?can push the speed boundary for propellers up a bit, achieving acceptable efficiencies at up to Mach .85; but after a period of enthusiastic endorsements, they do not seem to have found favor with manufacturers.

A pure turbojet engine produces thrust by a different mechanism. The sequence of events within a jet engine?compression, combustion and exhaust through a power turbine?is familiar enough, but obscures its basic character. Really, a turbojet is like a rocket: a slowly exploding bomb with a hole at one end.

The thrust of a jet engine is regulated primarily by the amount of fuel fed to it. The burning fuel heats air, which expands greatly and rushes out the tailpipe at near-sonic speed. Unlike a reciprocating engine, a jet engine takes in several times more air than is needed for combustion. The unburned portion merely absorbs heat and adds to the mass being ejected.

In a bypass engine, which nearly all subsonic engines are these days, a fraction of the inflowing air, often quite a large one, is not heated at all. A high-bypass engine, though it is called a jet, is really a hybrid, and can more accurately be thought of as a ducted turboprop. The duct slows the incoming flow and shifts the efficient speed range of the fan upward.

In a vacuum, the thrust of a rocket is constant regardless of its speed. In air, some thrust is lost to the back-pressure of the surrounding air, but this loss diminishes as the rocket accelerates and the speed difference between the exhaust plume and the surrounding air shrinks. The same thing happens with a jet; the faster the airplane moves, the less difference there is between the velocity of the exhaust and that of the surrounding air, and the higher the efficiency of the powerplant.

The thrust of a jet engine is relatively constant for a given fuel flow at a given altitude, regardless of the speed. But, like the power output of an unturbocharged reciprocating engine, the thrust of a jet diminishes with altitude in proportion to the density of the air. At 40,000 feet it may be only a sixth of what it is at sea level.

A reciprocating engine can be turbocharged to deliver its rated cruise power at 30 or even 40 thousand feet. Jets cannot be turbocharged; a jet is already a kind of self-propelled turbocharger. But jets, because they are mechanically simple and need not be massively constructed, can be made to surpass the output of any recip at any altitude, and still be lighter in weight (though not necessarily in the combined weight of engine and fuel), simpler to operate and more reliable.

The turbine has two great advantages over the reciprocating engine as a powerplant for fast aircraft. One is that it does not suffer the same loss of efficiency with increasing speed, and particularly with the approach of Mach 1, that propellers do; as it goes faster it becomes more efficient, not less. The other is that very powerful, highly turbocharged reciprocating engines are extremely complicated mechanical monstrosities, in part because any single cylinder can only be so big, and so you have to add more and more of them to get more power. On the other hand, large jet engines are essentially similar to small ones; they do not suffer penalties of scale. If you need more thrust at a high altitude, you just make the engine bigger. When all is said and done, thrust plus altitude equals speed, and jets are better at both.

Extra! Read All About It! My old homebuilt, Melmoth, carried on its instrument panel, in front of the passenger's seat, a small metal placard saying: PASSENGER WARNING. This aircraft is amateur-built and does not comply with federal safety regulations for standard aircraft.

Such a placard is required for amateur-built airplanes. I once pointed out to an inspector-back when there were more inspectors than homebuilts, they used to come out and look your airplane over every year?that its wording was not strictly correct. It should have said may not comply, since for all anyone knew my airplane might have complied with, or even exceeded, all federal requirements for certification. The resourceful inspector replied that the airplane was in fact not certified, and therefore did not comply with regulations for standard aircraft. Out of such quibbles was the ecclesiastical history of the Middle Ages made.

Now that I am attempting to license a second homebuilt, a process only slightly less stressful than getting through Checkpoint Charlie to visit relatives in East Berlin during the Cold War, I have had occasion to reacquaint myself with that particular piece of signage. Consulting the FAA's Advisory Circular 20-27E, Certification and Operation of Amateur-Built Aircraft, I find that this placard (in which, by the way, the word "warning" has been replaced by the more innocuous-sounding "notice") must not only be easily visible and legible to passengers, but must also be in letters "at least 3/8 inches in height."

That's 38-point type, and requires (depending on how you format it, of course) a placard of at least three by seven inches. Some homebuilts may not even have enough empty space on their panels for such a placard, which would take up the space of two large instruments or six small ones.

Looking in the Aircraft Spruce catalogue, to which I suspect many homebuilders in search of a suitable ready-made placard would turn, I find that the one offered, in spite of containing an extra, non-mandated "the" and using the superseded "warning" language, measures a mere 1.25 by 3.75 inches. I feel fairly certain that the lettering on it cannot possibly comply with the requirements of the circular. I regret to have to say it, but it would seem that virtually every amateur-built aircraft in the United States is now operating in violation of FAA guidelines and should be grounded until a suitable new placard can be installed.

There is a bright side to all this, however. For aircraft in which space for the display of the full-size placard is not available, a moveable, Velcro-backed placard could be devised that could be temporarily affixed to the panel in such a way as to eclipse only those instruments not currently in use. Its benefits for partial-panel practice would be immense.