Every three or four years when the airlines need to consider the purchase of new airplanes, in addition to the considerations they must make on passenger load configurations and baggage capacity, they must consider the question of performance versus efficiency.
Today’s airplanes have a choice of two basic types of engines, those that turn propellers or rotors and those that produce a direct high velocity discharge that generates thrust or forward momentum. While the engines that turn propellers is usually less in cost and operate on more efficient fuel sources, the direct thrust jet engines can’t be beat when it comes to power and reliability. So because of this fact alone today’s modern airplanes are generally equipped with direct thrust producing jet engines.
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"Thrust Producing Engines: A Discussion of Performance vs Efficiency".
At this point in the decision making process, all other things being equal, the airline is faced with the decision of whether to purchase the airplane with a high performance jet engine that can propel the aircraft at supersonic speeds or to settle for a more fuel efficient jet engine that will operate a little slower? Since the government has set limits on the speed in which an aircraft can travel over land due to the potential to generate a sonic boom when the airplane travels faster than the speed of sound and generates a shock wave the consideration of how fast the airplane can travel is moot. Also due to the fact that all airport arrivals and departures are carefully timed all travel routes and distances are set to allow even the slowest airplane to make the time necessary to arrive at their gate when scheduled. If they were to arrive too early they would not have any place to unload and would create a traffic jam on the tarmac and be in the way of airplanes trying to take off. (14 CFR 91.817 Subpart 1)
So the real concern is what can they do to get the engine that will operate with the most efficient thrust to weight ratio leaving enough power and lift for emergency short take off and landings? The original jet engines were turbojet engines which were very inefficient for flight slower than he speed of sound. Today’s modern jet engines are much more efficient because they use a high bypass turbo fan which offers better fuel efficiency than even some of the older turbo prop rotating engines and doesn’t reduce the performance of the aircraft. Since all jet engines are considered as reaction engines, generating forward thrust by rearward exhausting of gas or air, the key questions are how efficiently does the engine use its fuel source and how tightly does it compact its exhaust through the propelling nozzle? The engine that does this the most efficiently and weighs the least is the one the airlines are looking to fit on the airplanes they purchase.
There have been various design changes within this field but the results have not caused substantial changed the performance verses efficiency debate but rather more focus on the thrust to weight ratio index. The engines that produce the highest propulsive efficiency are the ones that airlines look to for their short haul continental flights because they have a longer cycle between required maintenance and their per pound fuel consumption at subsonic speeds is a lot less than say the turbojet engines used in military aircraft with an after burner or those found on the former Concord for trans-Atlantic flights.
It is a very tricky decision that each airline must make when they buy the new planes. If we look at the real simple comparison and realize that E=FxD (Energy = Force x Distance) we can see that as long as most airlines use the hub method and fly more flights shorter distances with heavier loads the engine design that will give them the best performance value for their money is the turbo fan jet engine with the high bypass fan that produces over 58,000fp of thrust. It saves money on fuel and is ultra efficient but it has enough emergency power for a short field take off and landing allowing the airline to fly into smaller regional airports for additional revenue and passenger satisfaction.
- US Government Federal Flight Regulations (2005), Title 14: Aeronautics and Space, PART 91—GENERAL OPERATING AND FLIGHT RULES, Subpart I—Operating Noise Limits
