Aircraft power plant type

Aircraft Engine Types

The Heat Engine

• Converts chemical energy (fuel) into heat energy.

• Heat energy is then converted into mechanical energy.

• The heat energy is released at a point in the cycle where the pressure is high, relative to atmospheric.

Divided into groups or types depending upon:

• The working fluid used.

• The means of compression.

• The Propulsive working fluid.

Types Of Heat Engines

Turbojet

• Means of compression: Turbine-driven compressor

• Engine working fluid: Fuel/air mixture

• Propulsive working fluid: Fuel/air mixture

Turboprop

• Means of compression: Turbine-driven compressor

• Engine working fluid: Fuel/air mixture

• Propulsive working fluid: Ambient Air

Ramjet

· Means of compression: Ram compression

· Engine working fluid: Fuel/air mixture

· Propulsive working fluid: Fuel/air mixture

Pulse-Jet

• Means of compression: Compression due to combustion
• Engine working Fluid: Fuel/air mixture

· Propulsive working Fluid: Fuel/air mixture

Reciprocating

• Means of compression: Reciprocating action of pistons

• Engine working fluid: Fuel/air mixture

• Propulsive working fluid: Ambient air

Engine Requirements

• Efficiency

• Power and Weight: If the specific weight of an engine is decreased, the performance of the aircraft will increase.

• Reciprocating engines produce approximately 1 HP for each pound of weight.

• Fuel Economy

• The basic parameter for describing the fuel economy of aircraft engines is specific fuel consumption.

• Specific fuel consumption for reciprocating engines is the fuel flow (lbs/hr) divided by brake horsepower.

• Durability and Reliability

• Durability is the amount of engine life obtained while maintaining the desired reliability.

• Reliability and durability are built into the engine by the manufacture.

• Continued reliability is determined by the maintenance, overhaul, and operating personnel

• Operating Flexibility

• The ability of an engine to run smoothly and give desired performance at all speeds from idling to full-power.

• The engine must also function efficiently through all variations in atmospheric conditions.

• Compactness

• To effect proper streamlining and balancing of an aircraft, the shape and size of the engine must be compact.

• In a single engine aircraft, the shape and size of the engine will affect the view of the pilot.

Engine Requirements

• Powerplant Selection

Reciprocating Engine

• For aircraft whose cruising speeds will not exceed 250 MPH the reciprocating engine is the usual choice.

• Chosen for its excellent efficiency.

• Turbocharged or supercharged for high

altitude use.

— Turbo-use exhaust

— Super-use accessory drive

Turboprop Engine

• For cruising speeds from 180 to 350 MPH the turboprop engine performs better.

• Develops more power per pound then reciprocating.

• Operate most economically

at high altitudes.

Turbojet/Turbofan Engines

• Intended to cruise from high subsonic speeds up to Mach 2.0.

• Operates most efficiently at high altitudes.

• Less instrumentation and

controls required.

Types Of Reciprocating Engines

In-Line Engines

• Generally has even number of cylinders.

• Liquid or air cooled.

• Has only one crankshaft.

• Small Frontal area, better adapted to streamlining.

• When mounted inverted, it offers the added advantages of a shorter landing gear.

• High weight to horsepower ratio.

V-type Engines

• Cylinders are arranged in two in-line banks generally set 30-60° apart.

• Even number of cylinders and are liquid or air cooled.

Radial Engines

• Consists of a row, or rows, of cylinders arranged radially about a center crankcase.

• The number of cylinders composing a row may be either three, five, seven, or nine.

• Proven to be very rugged and dependable.

• High horsepower.

Rotary-Radial

• Used during World War I by all of the warring nations.

• Cylinders mounted radially around a small crankcase and rotate with the propeller.

• Torque and gyro effect made aircraft difficult to control.

• Problems with carburetion, lubrication, and exhaust.

Opposed Or O-type Engines

• Two banks of cylinders opposite each other with crankshaft in the center.

• Liquid or air cooled, air cooled version used predominantly in aviation.

Propeller

The propeller is a rotating airfoil, subject to induced drag, stalls, and other aerodynamic principles that apply to any airfoil. It provides the necessary thrust to pull, or in some cases push, the airplane through the air.

The engine power is used to rotate the propeller, which in turn generates thrust very similar to the manner in which a wing produces lift. The amount of thrust produced depends on the shape of the airfoil, the angle of attack of the propeller blade, and the r.p.m. of the engine. The propeller itself is twisted so the blade angle changes from hub to tip. The greatest angle of incidence, or the highest pitch, is at the hub while the smallest pitch is at the tip.

Figure 3: Changes in propeller blade angle from hub to tip.

The reason for the twist is to produce uniform lift from the hub to the tip. As the blade rotates, there is a difference in the actual speed of the various portions of the blade. The tip of the blade travels faster than that part near the hub, because the tip travels a greater distance than the hub in the same length of time.

Changing the angle of incidence (pitch) from the hub to the tip to correspond with the speed produces uniform lift throughout the length of the blade. If the propeller blade was designed with the same angle of incidence throughout its entire length, it would be inefficient, because as airspeed increases in flight, the portion near the hub would have a negative angle of attack while the blade tip would be stalled.

Figure 4: Relationship of travel distance and speed of various portions of propeller blade.

Small airplanes are equipped with either one of two types of propellers. One is the fixed-pitch, and the other is the controllable-pitch.

Materials used in Aircraft

Materials used in Aircraft

Any brief history of aircraft materials must mention timber as one of the first materials used to make a powered, manned flight. The Wright Flyer consisted mainly of Sitka spruce and bamboo glued and screwed together to form a canvas-covered frame. Wooden aircraft were very successful in the early years of flight but by the end of World War I their days were numbered. Today, timber is only suitable for comparatively small aircraft. As aircraft became larger, materials with better specific strength (strength to weight ratios) became necessary. Today aircraft consist largely of aluminium alloys with steel, titanium alloys and polymer composites forming the smaller proportion. The balance of materials does depend on the type of aircraft as military fighter planes have much higher proportions of composites and titanium alloys.

Aircraft need to be made of lightweight materials to increase payload and save in fuel consumption. The more passengers a plane can carry, the more profit an airline company can make.

Whilst pure aluminium has low specific gravity, good corrosion resistance and excellent thermal and electrical conductivity it is too weak and ductile to be used on its own. In 1906 Dr Alfred Wilma, a German metallurgist, discovered that aluminium alloyed with copper and heat treated correctly could be made far stronger. The alloy of aluminium with 4% copper is called Duralumin and the heat treatment process is called precipitation hardening. These alloys have typically low specific gravity (around 2.7) and high strength (450 MPa). They are limited by a maximum service temperature of about 660°C. Since then, other heat treatable aluminium alloys have been developed for aircraft use. These include a range of complex aluminium-zinc alloys which develop the highest strength of any aluminium alloy. These alloys have led to modern aircraft design where the skin of the fuselage and wings are stressed aluminium alloy members which reduces the overall weight.

The aluminium alloys mentioned above have the disadvantage of not being as corrosion resistant as pure aluminium so a thin layer of pure aluminium is often pressure welded to both sides of the alloy. This material is called Alclad.

Titanium, though very expensive, is used where high strength is needed in load bearing applications such as landing gear and engine mounting brackets.

Steel is used where strength is needed in restricted spaces, for example in the carriageways. Alloy steels can be heat treated to give very high mechanical properties and take up less volume, which is very important, as there is not very much “free space”. It is used sparingly though as it is heavy and suffers from increased brittleness (low energy to cause fracture) at the low temperatures found at very high altitudes.

Lithium is also used as an alloying element for improved properties.

One property that is appropriate for selecting materials for aircraft use is their specific strength. The following list indicates the approximate specific strength for some materials:

• Aluminium 30 Mpa
• Mild Steel 51 MPa
• Duralumin (heat treated) 150 MPa
• Al-Zn (heat treated) 220 Mpa
• Titanium alloy (heat treated) 270 MPa

Modern aircraft are extremely efficient and fast flying machines. The advances in materials technology accounts for the economy and reduction in weight while speed comes from development of the aircraft’s shape and its engines.

Advances in polymer composites are making them increasingly more popular. They have very attractive low density and high mechanical properties. Composites consist of fibres of glass, carbon, Kevlar or boron reinforced in an epoxy resin matrix. They are replacing some of the aluminium alloys in commercial aircraft and find even greater applications in military aircraft such as the Eurofighter.

Typical Aircraft wing and fuselage Structure

Typical Aircraft wing and fuselage Structure

According to the current Title 14 of the Code of Federal Regulations (14 CFR) part 1, Definitions and Abbreviations, an aircraft is a device that is used, or intended to be used, for flight. Categories of aircraft for certification of airmen include airplane, rotorcraft, lighter-than-air, powered-lift, and glider. Part 1 also defines airplane as an engine-driven, fixed-wing aircraft heavier than air that is supported in flight by the dynamic reaction of air against its wings. This webpage provides a brief introduction to the airplane and its major components.

Major components

Although airplanes are designed for a variety of purposes, most of them have the same major components.

The overall characteristics are largely determined by the original design objectives. Most airplane structures include a fuselage, wings, an empennage, landing gear, and a powerplant.

Figure 1: Airplane components.

Fuselage

The fuselage includes the cabin and/or cockpit, which contains seats for the occupants and the controls for the airplane. In addition, the fuselage may also provide room for cargo and attachment points for the other major airplane components. Some aircraft utilize an open truss structure. The truss-type fuselage is constructed of steel or aluminum tubing. Strength and rigidity is achieved by welding the tubing together into a series of triangular shapes, called trusses.

Figure 2: The Warren truss.

Construction of the Warren truss features longerons, as well as diagonal and vertical web members. To reduce weight, small airplanes generally utilize aluminum alloy tubing, which may be riveted or bolted into one piece with cross-bracing members.

As technology progressed, aircraft designers began to enclose the truss members to streamline the airplane and improve performance. This was originally accomplished with cloth fabric, which eventually gave way to lightweight metals such as aluminum. In some cases, the outside skin can support all or a major portion of the flight loads. Most modern aircraft use a form of this stressed skin structure known as monocoque or semimonocoque construction.

The monocoque design uses stressed skin to support almost all imposed loads. This structure can be very strong but cannot tolerate dents or deformation of the surface. This characteristic is easily demonstrated by a thin aluminum beverage can. You can exert considerable force to the ends of the can without causing any damage.

However, if the side of the can is dented only slightly, the can will collapse easily. The true monocoque construction mainly consists of the skin, formers, and bulkheads. The formers and bulkheads provide shape for the fuselage.

Figure 3: Monocoque fuselage design.

Since no bracing members are present, the skin must be strong enough to keep the fuselage rigid. Thus, a significant problem involved in monocoque construction is maintaining enough strength while keeping the weight within allowable limits. Due to the limitations of the monocoque design, a semi-monocoque structure is used on many of today´s aircraft.

The semi-monocoque system uses a substructure to which the airplane´s skin is attached. The substructure, which consists of bulkheads and/or formers of various sizes and stringers, reinforces the stressed skin by taking some of the bending stress from the fuselage.

Figure 4: Semi-monocoque construction.

The main section of the fuselage also includes wing attachment points and a firewall.

On single-engine airplanes, the engine is usually attached to the front of the fuselage. There is a fireproof partition between the rear of the engine and the cockpit or cabin to protect the pilot and passengers from accidental engine fires. This partition is called a firewall and is usually made of heat-resistant material such as stainless steel.

Wings

The wings are airfoils attached to each side of the fuselage and are the main lifting surfaces that support the airplane in flight. There are numerous wing designs, sizes, and shapes used by the various manufacturers.

Each fulfills a certain need with respect to the expected performance for the particular airplane.

How the wing produces lift is explained in aerodynamics pages.

Wings may be attached at the top, middle, or lower portion of the fuselage. These designs are referred to as high-, mid-, and low-wing, respectively. The number of wings can also vary. Airplanes with a single set of wings are referred to as monoplanes, while those with two sets are called biplanes.

Figure 5: Monoplane and biplane.

Many high-wing airplanes have external braces, or wing struts, which transmit the flight and landing loads through the struts to the main fuselage structure. Since the wing struts are usually attached approximately halfway out on the wing, this type of wing structure is called semi-cantilever. A few high-wing and most low-wing airplanes have a full cantilever wing designed to carry the loads without external struts.

The principal structural parts of the wing are spars, ribs, and stringers.

Figure 6: Wing components.

These are reinforced by trusses, I-beams, tubing, or other devices, including the skin. The wing ribs determine the shape and thickness of the wing (airfoil). In most modern airplanes, the fuel tanks either are an integral part of the wing´s structure, or consist of flexible containers mounted inside of the wing.

Attached to the rear, or trailing, edges of the wings are two types of control surfaces referred to as ailerons and flaps. Ailerons extend from about the midpoint of each wing outward toward the tip and move in opposite directions to create aerodynamic forces that cause the airplane to roll. Flaps extend outward from the fuselage to near the midpoint of each wing. The flaps are normally flush with the wing´s surface during cruising flight. When extended, the flaps move simultaneously downward to increase the lifting force of the wing for takeoffs and landings.

Types of Aircraft Construction

Types of Aircraft Construction

The construction of aircraft fuselages evolved from the early wood truss structural arrangements to monocoque shell structures to the current semimonocoque shell structures.

Truss Structure
The main drawback of truss structure is its lack of a streamlined shape. In this construction method, lengths of tubing, called longerons, are welded in place to form a well-braced framework. Vertical and horizontal struts are welded to the longerons and give the structure a square or rectangular shape when viewed from the end. Additional struts are needed to resist stress that can come from any direction. Stringers and bulkheads, or formers, are added to shape the fuselage and support the covering.

As technology progressed, aircraft designers began to enclose the truss members to streamline the airplane and improve performance. This was originally accomplished with cloth fabric, which eventually gave way to lightweight metals such as aluminum. In some cases, the outside skin can support all or a major portion of the flight loads. Most modern aircraft use a form of this stressed skin structure known as monocoque or semimonocoque construction. [Figure 2-14]

Monocoque
Monocoque construction uses stressed skin to support almost all loads much like an aluminum beverage can. Although very strong, monocoque construction is not highly tolerant to deformation of the surface. For example, an aluminum beverage can supports considerable forces at the ends of the can, but if the side of the can is deformed slightly while supporting a load, it collapses easily.
Because most twisting and bending stresses are carried by the external skin rather than by an open framework, the need for internal bracing was eliminated or reduced, saving weight and maximizing space. One of the notable and innovative methods for using monocoque construction was employed by Jack Northrop. In 1918, he devised a new way to construct a monocoque fuselage used for the Lockheed S-1 Racer. The technique utilized two molded plywood half-shells that were glued together around wooden hoops or stringers. To construct the half shells, rather than gluing many strips of plywood over a form, three large sets of spruce strips were soaked with glue and laid in a semi-circular concrete mold that looked like a bathtub. Then, under a tightly clamped lid, a rubber balloon was inflated in the cavity to press the plywood against the mold. Twenty-four hours later, the smooth half-shell was ready to be joined to another to create the fuselage. The two halves were each less than a quarter inch thick. Although employed in the early aviation period, monocoque construction would not reemerge for several decades due to the complexities involved. Every day examples of monocoque construction can be found in automobile manufacturing where the unibody is considered standard in manufacturing.

Semimonocoque
Semimonocoque construction, partial or one-half, uses a substructure to which the airplane’s skin is attached. The substructure, which consists of bulkheads and/or formers of various sizes and stringers, reinforces the stressed skin by taking some of the bending stress from the fuselage. The main section of the fuselage also includes wing attachment points and a firewall. On single-engine airplanes, the engine is usually attached to the front of the fuselage. There is a fireproof partition between the rear of the engine and the flight deck or cabin to protect the pilot and passengers from accidental engine fires. This partition is called a firewall and is usually made of heat-resistant material such as stainless steel. However, a new emerging process of construction is the integration of composites or aircraft made entirely of composites.

Aerofoil characteristics-lift, drag curves

Lift and Drag Curves

As the amount of lift varies with the angle of attack, so too does the drag. Hence drag is the price we pay for lift. Thus, although it is desirable to obtain as much lift as possible from a wing, this cannot

be done without increasing the drag. It is therefore necessary to find the best compromise.

The lift and drag of an airfoil depend not only on the angle of attack, but also upon:

The shape of the airfoil

The plan area of the airfoil (or wing area)—S.

The square of the velocity (or true airspeed)—V².

The density of the air—.

Hence the lift (L) and drag (D) of an airfoil can be expressed as follows:-

The symbols C_L and C_D represent the lift coefficient and drag coefficient respectively. They depend on the shape of the airfoil and will alter with changes in the angle of attack and other wing appurtenances.

The lift-drag ratio is used to express the relation between lift and drag and is obtained by dividing the lift coefficient by the drag coefficient, C_L / C_D.

The characteristics of any particular airfoil section can conveniently be represented by graphs showing the amount of lift and drag obtained at various angles of attack, the lift-drag ratio, and the movement of the center of pressure.

Notice that the lift curve (C_L-yellow curve) reaches its maximum for this particular wing section at 18 degrees angle of attack, and then rapidly decreases. 18 degrees angle of attack is therefore the stalling angle.

The drag curve (C_D-blue curve) increases very rapidly from 14 degrees angle of attack and completely overcomes the lift curve at 22 degrees angle of attack.

The lift-drag ratio (L/D-green curve) reaches its maximum at 0 degrees angle of attack, meaning that at this angle we obtain the most lift for the least amount of drag.

The Center of Pressure(CP-red cross) moves gradually forward until 12 degrees angle of attack is reached, and from 18 degrees commences to move back.

The graphs shown above represent data for one airfoil or wing cross-section.  Different airfoils with different camber and airfoil thickness produce different looking curves.  These curves are obtained normally by experimental work using wind tunnel tests.  Many tests were run by the NACA, the predecessor to the NASA, and a booklet, NACA Report 824 (1945) was created to give the relation of coefficient of lift, coefficient of drag, pressure and moment coefficients (not shown above) for many different types of airfoil.  We find that the coefficients of lift, drag and moment depend upon the angle of attack, the mach number and the Reynolds number.

For subsonic speeds, normal airfoils have a linear relationship between angle of attack and coefficient of lift until just before stall occurs (the airfoil or wing experiences a loss of lift).  For higher speeds, when the mach number is higher than 0.3 (mach number is the velocity of the aircraft divided by the velocity of the sound), then the coefficient of lift is

C_L = C_Lo / (1 – M²)^1/2

where

C_Lo= the coefficient of lift at low speed
M  = the mach number in the free stream

This correction for the mach number effect is based on Glauert who proposed it in 1928. Von Karman and Tsien proposed a more complicated equation (not shown here). Note that the coefficient of lift at low speed, C_Lo, is the value that is normally obtained experimentally.  The above equation holds true even for mach number values less than 0.3, but the effect on the coefficient of lift is minimal.
Finally, we said that the lift coefficient is also dependent on Reynolds number,  R_N, where

R_N = rVd/m

Here, the Greek letter, r, represents the density of the fluid–air; V is the velocity of the free-stream airflow; d is the characteristic length of the airfoil and, the denominator, given by the Greek symbol, m, represents the fluid viscosity.  Actually, the Reynolds number determines the type of flow (whether laminar or turbulent), which, in turn, determines where the flow separates from the airfoil or wing.  This, in turn, affects the lift, drag and moment coefficients, as explained above. We note that as Reynolds number increases, the maximum lift coefficient increases. But this does not occur indefinitely; when flows become very turbulent, the maximum lift coefficient begins to drop and so does the overall lift coefficient

Center of pressure

Center of pressure:

The upper surface has pressures distributed which produce the upper surface lift.

The lower surface has pressures distributed which produce the lower surface force. Net lift produced by the airfoil is the difference between lift on the upper surface and the force on the lower surface. Net lift is effectively concentrated at a point on the chord called the Center Of Pressure.

Aerodynamic Center:

The aerodynamic force is applied at a location 1/4 from the leading edge of a rectangular wing
at subsonic speed, the magnitude of the aerodynamic moment remains nearly
constant even when the angle of attack changes.This location is called the wing’s Aerodynamic Centre AC. (At supersonic speed, the aerodynamic centre is near 1/2 of the chord).