Turbine Pilot

An introduction to jet engines

Most pilots formed their first impression of jets and jet flying in much the same way. The corporate and airline pilots were held in awe by those not initiated to the intricacies of turbine flying. Most wondered if they would ever have the expertise to handle one of the complicated jet-powered monsters that the elite operated on a daily basis. It's not clear where the myth that it takes a superhuman pilot to fly jets got started, but it's not true. Also, we've all heard many times over that the bigger the plane, the easier it is to fly and in most cases, that is true — especially with jet-powered airplanes.

Jet engine theory can be summed up in four words: suck, squeeze, bang, and blow. When you get down to it, though, that describes the operation of most aviation engines. The suck of a jet engine can be compared to the intake stroke of the piston. Squeeze parallels the compression stroke, bang is ignition, and blow is exhaust. Piston engines carry out these actions in sequence but a jet engine performs all operations simultaneously. That simultaneous operation makes jet engines free of the vibration that is found in even the smoothest-running piston engine.

Some of the major advantages of jet propulsion are:

• Lower weight and drag allow higher speeds and larger payloads.
• Virtual absence of engine vibration caused by reciprocating parts allows airframes to be lighter.
• Increased efficiency with altitude without the need for complex supercharging systems.
• Cheaper and speedier production. A turbine powerplant has approximately one-quarter the parts of a reciprocating powerplant.
• Increased reliability and lowered maintenance costs because of fewer parts, constant combustion, and absence of reciprocating parts.
• Increased engine efficiency with rising aircraft speed because of the influence of ram pressure increasing the engine mass airflow and exhaust velocity.

All aviation engines work in the same basic way — they accelerate a volume of air backward. That rearward acceleration of the air results in an equal and opposite action on the engine and it is accelerated forward. Since the engine is bolted to the airplane, forward motion is achieved. Engines with propellers attached to the front (or rear, as the case may be) take a relatively large volume of air and accelerate it a moderate amount. Jet engines take a small amount of air and accelerate it by a large amount.

Newton's second law states that a force is equal to the product of a mass and the amount the mass is accelerated. If you take a large mass and accelerate it a small amount, you can achieve the same results as taking a small mass and accelerating it a large amount. The equation is similar to the calculation you do when you work the weight and balance figures for your airplane — the same moment can be achieved using a large weight and a short arm or a small weight and a long arm. No matter how you figure it, 10 times 100 is the same as 100 times 10.

Types of jet engines

Turbine engines come in four varieties: turboshaft, turboprop, turbojet, and turbofan. Turboshaft engines, found on helicopters, and turboprop engines, found on the likes of Beech King Airs and Saab 340s, are jet engines with a rotor or propeller attached. Turbojet and turbofan engines are the two typically referred to as jet engines, although they all work the same way.

Turbojet engines

A turbojet engine is a reaction engine that produces thrust by taking in air, compressing it, adding fuel, and combusting it, creating a flow of hot gases. This flow of gases is used to turn the turbine wheel, and the remaining gases in the tailpipe accelerate into the atmosphere and create the reaction we refer to as thrust. The main sections of a turbojet engine are intake, compression, combustion, and exhaust.

Although most subsonic turbine aircraft in use today employ turbofan engines, the pure turbojet is still in use on the supersonic Concorde and some military craft. Its relative inefficiency, when compared to the turbofan, and the higher noise levels associated with its operation have caused the turbojet to fall into disfavor for commercial use.

Turbofan engines

A turbofan engine is simply a turbojet engine with a fixed-pitch propeller in the front. (Usually. Rear-fan engines are much less efficient and therefore rarely used.) The fan acts as a propeller and gives a relatively small rearward acceleration to a large volume of air. The turbojet engine core gives a relatively large amount of acceleration to a small package of air. This combination engine preserves the low-speed, low-altitude efficiency (when contrasted to turbojet engines) of propeller engines with the high-speed, high-altitude efficiency of jets. That is, with its ducted design the turbofan has turbojet-type cruise speed and altitude capability and yet retains the short-field takeoff capability of the propeller-driven craft.

Turbofans are as much as 30 to 40 percent more fuel efficient than turbojet engines for two reasons. The turbofan wastes a lower amount of kinetic energy from its fan exhaust. This is because the average speed of the combination fan exhaust and turbine core exhaust is closer to the speed of the aircraft. Second, there is a lower amount of kinetic energy left in the atmosphere after the aircraft has passed. Since kinetic energy in the atmosphere is wasted energy, this adds to turbofan efficiency.

The greatest propulsive efficiency is gained by giving the smallest amount of acceleration to the largest parcel of air. That's the efficiency of propellers. A turbofan engine achieves this goal while maintaining high-altitude jet efficiency. This discussion should also lead the reader to the conclusion that high exhaust velocity and low aircraft speed are a highly inefficient operating condition.

Turbofan engines are also quieter than turbojet engines since the lower-velocity bypass air shrouds the high-velocity jet exhaust. That's certainly important in this age of noise-abatement awareness.

Bypass ratio

Turbofan engines are classified according to their bypass ratio as low-, medium-, or high-bypass turbofans. Bypass ratio compares the amount of air that bypasses the jet core of the engine to that routed through the core. It should be noted that, with all turbofan engines, the ratio of thrust provided by the fan to the ratio of thrust provided by the jet core would be approximately the same as the bypass ratio.

In a low-bypass turbofan, the fan and the compressor sections receive and utilize approximately the same amount of air. This, then, would be indicated as a bypass ratio of 1:1. Tactical military turbofans usually have a bypass ratio of 1:1 or less because of the narrow profile requirements of supersonic flight. Medium- or intermediate-bypass turbofans have bypass ratios in the range of 2:1 to 3:1.

High-bypass turbofans boast the lowest fuel consumption of the various turbofan configurations and have bypass ratios in the area of 5:1. The thrust produced by the fans in these engines is 75 percent to 85 percent of the total thrust output of the engine. Very few high-bypass fans are fully ducted (having a cowling enclosing the bypass duct from the front of the engine to the rear) because of the weight penalty involved in a wide-diameter fully ducted design. The high-bypass turbofan design has gained wide popularity on medium to large airliners because of its greater propulsive and thermal efficiency and consequent greater fuel economy.

Jet propulsion advantages

It is widely believed that turbine aircraft consume large amounts of fuel while reciprocating powerplants seem to be more fuel conscious. If the simple comparison of relative gallons or pounds of fuel per hour is used, this misleading conclusion may seem to be true. If the comparison is made, however, between ton/miles of payload per unit of fuel consumed, the jet would prove to be more efficient. If the Boeing 747 could somehow be fitted with piston engines (although it would take 23 of the largest piston engines ever produced to equal the 230,000 lb of static thrust provided by the four Pratt & Whitney JT9D turbofans), the fuel consumption of the combination would be much higher than that of the jet-powered craft. This increased fuel consumption would be largely the result of the increased weight and drag of the piston engines; however, propeller efficiency losses would also contribute to the penalty.

All in all, jets are simple engines, economical to use and easy to fly — even by us mere mortals.

Linda Pendleton, AOPA 525616, is an art director and author for King Schools. She has accumulated more than 10,000 hours in her 27 years of flying and has given more than 4,000 hours of jet instruction.