| What Is Aerodynamics? |
|
How Do Airplanes Fly? What is the most important feature of an airplane, the one that really makes it possible to fly? Is the most important thing the propeller or jet engine, which gives it speed? The jet fuel? Or how about the wings? If you guessed the wings, you are right. But there is more to it than that. It is the shape of the wings that makes it possible for a plane to fly. We're not talking about the length of the wings, although that is important. If you sawed straight through the wing and looked at it, you would see a shape like this: Picture of how an airfoil works This shape is very important—it makes it possible for a plane to lift off the ground. This is the shape of an "airfoil." Notice the features of the airfoil: a curved top surface and a flatter bottom one. This is the secret of heavier-than-air flight. What is it about an airfoil that makes it so special? Air flows over the top, curved surface of the airfoil faster than under the bottom, flat surface. This creates a difference in air pressure between the top and bottom of the airfoil-shaped wing. The air above the wing is at a lower pressure than the air under it, and the higher pressure pushing up on the bottom of the wing overcomes the lower pressure, pushing it down. When an airplane is moving fast enough down the runway so that the upward pressure is greater than the force of gravity, the airplane lifts off. Of course, the plane could not reach high speed without a jet engine (or a propeller) and fuel, so these are important components too. And if the weight of a plane is too great it might never go fast enough to leave the ground. But without the key shape of the wing—the airfoil—none of these things would matter. Here is a simulation of how an airfoil works. How Do Helicopters Fly? A helicopter has a rotor—pretty much a rotating wing—on top and another rotor in its tail. The rotors are made in the shape of—you guessed it—airfoils. When the helicopter's engine turns the main rotors at high speed counterclockwise (looking from above), the pressure on the bottom side of the rotor is greater than the pressure on the top, making the helicopter lift off straight up into the air. However, even though they share the same basic way of getting off the ground, helicopters are much more complicated than airplanes. Planes can go forward, up and down, and left and right. Helicopters can do the same, while adding the ability to go straight up, backwards, or just hover in midair, going nowhere. The key to these extra abilities is that the helicopter pilot can change the angle of the rotor blades. The steeper the angle of the blades, the greater the amount of lift, and the faster the helicopter rises. Illustration of a helicopter To go forward or backward, right or left, the pilot tilts the blades in the direction he wants to move. It is interesting to note that the same engine that spins the main rotors counterclockwise also tries to spin the helicopter body clockwise—every action has an equal and opposite reaction, just as Isaac Newton said. The tail rotor produces a force to counter the tendency of the helicopter body to turn, thus stabilizing it. How Do Blimps Fly? Blimps are filled with helium, a gas that is lighter than air. You may have seen helium balloons at a parade or a party. What happens when you let go of the string of a helium balloon? It rises into the sky. Similarly, helium provides the lift in a blimp. Once in the air, small engines turn propellers that control the speed and direction of the blimp. How Do Hot-air Balloons Fly? Just like a blimp, a hot-air balloon uses a gas that is lighter than air to achieve lift. In this case, the lighter-than-air gas is simply hot air. The pilot of a hot-air balloon controls a flame located at the bottom opening of the balloon. By turning on the flame and heating the air inside the balloon, he can make it rise. By turning off the flame and letting the air inside cool, he can make it descend. Unfortunately, a pilot has little control over the direction his balloon moves across the sky, because there are no motors or propellers on a hot-air balloon. To move in a certain direction, the pilot has to move the balloon up or down to catch a wind current moving in the desired direction. How Do Rockets Fly? A rocket relies on high-pressure exhaust to propel it into the sky. It is a case of one of Isaac Newton's laws again—for every action there is an equal and opposite reaction. High-pressure exhaust gases blasting downward cause the rocket to blast upward—a blastoff! The burning of fuel (called combustion) makes the exhaust gases. Many different types of fuels are used in rockets. The most common fuels used by NASA are liquid oxygen and liquid hydrogen. These fuels are held in separate tanks located on the booster rocket. When mixed together, hydrogen and oxygen burn to produce enormous amounts of energy, as you have seen if you ever watched a launch from the Kennedy Space Center. The tremendous flames that shoot out the bottom of a rocket when the controller says "ignition" provide the energy to "blast off." Sometimes solid-rocket fuels are used instead of liquid ones, but the principle is the same—high-energy combustion of fuels launches a rocket into space. How Does a Spacecraft Break Free of Earth's Gravitational Pull? If you toss a baseball straight up into the air, it keeps rising until Earth's gravitational pull overcomes the force you put into the throw, and it falls back to the ground. However, if you could toss it hard enough to reach a speed of 7 miles per second, or 25,000 miles per hour, the ball will keep going into outer space. This is known as the "escape velocity." At that speed, the force of gravity can never be stronger than the force causing the ball to rise, so it will escape the gravitational bonds of Earth. Of course, the best baseball pitchers can only throw at a velocity of 95-100 miles per hour, which is why you don't see baseballs in orbit. Rockets, however, can reach escape velocity, so we can send spacecraft beyond Earth's grasp to explore the other planets in our solar system. What is Aerodynamics? How does a plane control its height? What affects a planes aerodynamics? How have planes changed over the years? How does a plane take off? How does a plane keep on route? Aerodynamics is the study of how gases interact with moving bodies. Because the gas that we encounter most is air, aerodynamics is primarily concerned with the forces of drag and lift, which are caused by air passing over and around solid bodies. Engineers apply the principles of aerodynamics to the designs of many different things, including buildings, bridges and even soccer balls; however, of primary concern is the aerodynamics of aircraft and automobiles. Aerodynamics comes into play in the study of flight and the science of building and operating an aircraft, which is called aeronautics. Aeronautical engineers use the fundamentals of aerodynamics to design aircraft that fly through the Earth's atmosphere. Aerodynamics • What are disadvantages of wing sweep? • What is Dutch roll? • Span wise flow • Chord wise flow • Where will shockwaves first occur on the wing? • At what point on an aircraft is the local airflow the fastest? • How does sweepback affect Mcrit? • What are the advantages and disadvantages of a thin wing? • What are the advantages and disadvantages of sweepback? • Why are wings swept? • Why doesn’t the B777 have winglets? • Define angle of incidence. • Define AoA... • Define pitch attitude... • What is the mean camber line? • Define chord. • Positive camber, negative camber, symmetrical airfoil definitions. • If you decrease AoA how does that affect induced drag? • Define 'Mach number'... • How do you get zero lift in a positive camber wing? • Define Equilibrium • Draw the forces acting on an aircraft in a descent. • Draw the forces acting on an aircraft in a climb. • Why do some aircraft have winglets? • How is range increased when flying into a headwind? • What is the difference between Max Range Cruise [MRC] and Long Range Cruise [LRC]? • What is the Critical Drag-rise Mach number [Mcdr]? • Define Mach Number Detachment [Mdet] • Explain Compressibility Mach Number [Mcomp]... • What is Mach Number? • How do you get zero lift in a positive camber wing? • If you decrease AoA how does that affect induced drag? • Explain positive camber, negative camber and symmetrical airfoil definitions. • What is the mean camber line? • Define chord • Define chord line • Define angle of incidence. • Define angle of attack • Define pitch attitude • What is the Mean Camber Line? • Define relative wind • Explain IAS, EAS and TAS • What limits an aircraft climb performance? • How is drag defined? • Define Thrust • Define aerodynamic lift • What are the forces acting on an aircraft in flight? • What is the Free Stream Mach number [Mfs] • Explain Local Mach Mumber [Ml] • Define Critical Mac Number [Mcrit] • What is the definition of Subsonic, Transonic, and Supersonic? What are disadvantages of wing sweep? Sweep reduces a wings coefficient of lift, increases stall speed, thus increasing take off and landing speeds and reducing field performance. Swept wings tend to stall first at the wingtips, which in turn causes the C of P to move forward producing a nose up pitching moment. This can lead to a deep stall, particularly in rear engined, T tailed aircraft What is Dutch roll? Dutch roll is a coupled lateral-directional oscillation, which is usually dynamically stable but is objectionable because of the oscillatory nature. The damping of this oscillatory mode may be weak or strong depending on the properties of the aircraft. The response of the aircraft to a disturbance from equilibrium is a combined rolling-yawing oscillation in which the rolling motion is phased to precede the yawing motion. Generally, Dutch roll will occur when the dihedral effect is large when compared to the static directional stability. Span wise flow Travels from the root to the tip and produces no lift. Chord wise flow The airflow over the wing that is perpendicular [at a right angle to the leading edge of the wing]. The airflow is accelerated over the wing and produces lift. Where will shockwaves first occur on the wing? At the point of maximum camber, usually at the wing root an a swept wing. At what point on an aircraft is the local airflow the fastest? At the point of greatest curvature. [Top of the 747's hump] How does sweepback affect Mcrit? By sweeping a wing significantly the velocity vector normal to the leading edge is made less than the chord wise resultant, thus MCRIT is increased. What are the advantages and disadvantages of a thin wing? Advantages: •Raise Mcrit to a higher value Buffet, drag rise and control and stability problems are all deferred to a higher Mach number and when they do occur, they are less severe than on a thicker wing. Wave drag is proportional to thickness/chord [T/C] ratio Disadvantages •Structural weaknesses [rigidity/strength] •Limited storage capacity [fuel and undercarriage] •Poor low-speed aerodynamic characteristics [low CL, high Vs] and may also be prone to leading edge stall [sudden, no buffet] What are the advantages and disadvantages of sweepback? Advantages: •Sweep increases spiral stability, as does dihedral. •Raise Mcrit to a higher value - Mcrit = Mcrit [straight]__ Cosine sweep angle Eg. Mcrit straight = 0.8 [Now with sweep angle 30º] Mcrit [swept] = 0.8___ Cos 30 = 0.8_ 0.87 = 0.92 [in theory, a little less in practice] This increase in Mcrit means buffet, drag rise and control and stability problems are all deferred to a higher Mach number and when they do occur, they are less severe than on a similar straight wing. Disadvantages: •Poor oscillatory stability •Poor lift at low airspeeds [flatter lift curve] •Less lift for a given airspeed/AoA [higher stall speed] •High AoA at stall •Tendency for the tips to stall first causing a pitch up at stall •Steep deck angle on approach •A swept wing has a high Vimd [min drag speed] requiring a large acceleration after rotation to achieve Vy •Excessive lateral stability [can result in Dutch roll] •Aero elastic effects such as aileron reversal, reduced tip AoA under G-loading which may cause a pitch up and tightening turn •Limited visibility [of the wing] from the cockpit Why are wings swept? As the air passes over the wing accelerates to near sonic speeds, shock waves form and compressibility effects become apparent; the drag increases, buffeting is felt and changes in lift and C of P occur. The speed at which these compressibility effects first become apparent is the Critical Mach number [MCRIT]. By sweeping a wing significantly the velocity vector normal to the leading edge is made less than the chord wise resultant. As the wing is only responsive to the velocity vector to the normal leading edge, for a given Mach number the effective chord wise velocity is reduced (in effect the wing is persuaded to believe it is flying slower than it actually is). This means the airspeed can be increased before the effective chord wise component becomes sonic and thus the critical Mach number is raised. [HTBJ] Why doesn’t the B777 have winglets? The B777 uses the most aerodynamically efficient wing ever developed for sub-sonic commercial aircraft, allowing it to climb quickly, cruise at higher levels and higher speeds than comparable aircraft. [FTBJ B777] Define angle of incidence. Angle between longitudinal axis and the chord line of the wing. Define AoA... Angle between the relative wind and the chord line of the wing. Define pitch attitude... The angle between longitudinal axis and the horizon. What is the mean camber line? Line drawn halfway between the upper and lower surfaces of a wing. Define chord. Measure of the width of the wing. Positive camber, negative camber, symmetrical airfoil definitions. •Positive camber: mean camber line is above chord line •Negative camber: mean camber line is below chord line •Symmetrical airfoil: mean camber line coincides with chord line If you decrease AoA how does that affect induced drag? If you decrease AoA that will increase velocity which means that induced drag will decrease. Define 'Mach number'... Mach number is the ratio of the speed of an object or flow to the local speed of sound, under the same conditions. How do you get zero lift in a positive camber wing? You must go to a negative AOA. Define Equilibrium Sum of the forces is equals zero. Sum of the moments is equals zero. Aircraft is moving in a straight line at a constant velocity. Draw the forces acting on an aircraft in a descent. Draw the forces acting on an aircraft in a climb. Why do some aircraft have winglets? Tip devices have become a popular technique to increase the aerodynamic performances of lifting wings, short and slender alike. The idea behind all wingtip devices is to diffuse the strong vortices released at the tip and optimise the span-wise lift distribution, while maintaining the additional moments on the wing within certain limits. Investigations and experiments, indicated that the use of vertical lifting surfaces placed at the wing tips produce a beneficial effect on both lift and drag characteristics. This is found at the cost of increased bending moment. The increase in root bending moment is found to be lower than for an equivalent tip extension. Winglet sections can be airfoils with their own design. At the tip, due to the pressure differential between the upper and lower surfaces, there is a significant span wise component to the airflow. On the lower surface, the span wise component of flow is outwards, away from the wing root, and on the upper surface, the span wise component tends to be towards the root. Lift is defined as acting perpendicularly to the flow local of the airfoil and the surface plan form, then with a bit of cunning engineering, the lift on a vertical surface at the wing tip, in a flow with a span wise component toward the root such as occurs on the upper wing surface, could be directed "forward" - in the direction of flight, - and "inward" - toward the wing root. The forward component of lift manifests itself as a reduction in total aircraft drag. Of course, the benefit is reduced somewhat by the component of winglet drag acting aft, but nonetheless, the net result is a reduction in total aircraft drag. And as mentioned elsewhere, winglets will indeed reduce the strength of the shed vortices in the tip region, but only as a consequence of the generation of a lift force on the winglet. For a given angle of attack, installation of winglets can also increase lift, but since aircraft mass is approximately unchanged, the aircraft would have to fly at a decreased angle of attack to maintain the same lift as in the pre-winglet case - which further decreases drag. Winglets can be used to produce extra lift, besides lower drag. The winglets must be mounted on the rear part of the wing (region of lowest pressure), to minimize interference effects. Drag reduction rates are of the order of 5 %. Winglets are applied in the latest generation of Boeing 747, MD 11, Airbus, and most executive jets and many sailplanes. Data available for the Boeing 747-400 indicate that without winglets the aircraft. suffers about 2.5 % drag losses, which corresponds to +9.5 tons at take-off. How is range increased when flying into a headwind? In a headwind maximum range is achieved by flying faster than 1.32 Vimd to minimise exposure to the headwind. What is the difference between Max Range Cruise [MRC] and Long Range Cruise [LRC]? MRC: The speed at which, for a given weight and altitude, the maximum fuel mileage is obtained. It is difficult to establish and maintain stable cruise conditions at max range speeds. 1.32 Vimd constant speed with variable AoA [dependent on weight]. LRC: Speed slightly faster than MRC at a constant AoA [slightly faster than Vimd] As weight decreases, speed needs to decrease to maintain AoA Reducing speed necessitates reducing thrust, though because best SFC for a given engine occurs at a particular design RPM, you must climb In the graph below a drag curve has been re-labelled 'Fuel Flow vs. Velocity'. In order to better see the origin of this graph the parasite drag and induced drag curves have been drawn in. Maximum Endurance Previously we defined SE as 1/FF. in other words an aircraft achieves more endurance when FF is smaller. Therefore, it is obvious that maximum endurance occurs at the bottom of the FF curve as shown above. Since the above FF curve is exactly the same shape as the Drag curve, the lowest fuel consumption would correspond to the speed for minimum drag, [also known as L/D max AoA]. Maximum Range As discussed previously, the speed for maximum specific range, in zero wind, will occur where the tangent line drawn from the origin just touches the curve [as shown below]. It is worth noting that maximum range always occurs at a higher speed than maximum endurance. More correctly best range always occurs at a smaller angle of attack than best endurance. It is critical to remember that best range and best endurance both occur at specific angles of attack, regardless of weight. Effect of wind on Range A headwind will decrease the range and a tailwind will increase the range. This is only common sense. However, with a headwind the aircraft must fly faster. In other words at a smaller angle of attack. The tailwind graph below shows that theoretically the pilot should slow down [fly at a greater angle of attack] with a tailwind. Note: the tailwind or headwind tangent line is drawn with the headwind or tailwind added. This ensures that the tangent [FF/V] of the line has the correct magnitude. As a rule of thumb the pilot should speed up by half the headwind velocity. You can see from the above graph that this is a reasonable approximation. Effect of Weight on Endurance and Range Previously we examined how weight changes affected the total drag curve. You must remember that only the induced drag changes with weight. In the diagram below the green curve is the original drag curve. The red curve is the total drag after some fuel is consumed [weight reduced]. You can see from the above graph that SE improves with lower weight. In other words the aircraft can fly for longer if it is lighter. However the aircraft must fly slower at the reduced weight. As proven earlier in our discussion of gliding however, the same L/D max angle of attack applies in both cases. If you draw the tangent line in from the origin to both the green and red curves, you can quickly see that SR also improves at lighter weights. Just as with endurance the aircraft must fly slower as the weight is decreased. However, it should remain at the same angle of attack. In summary, there is an optimum angle of attack for endurance. There is another optimum angle of attack (smaller) for range. The aircraft should always be operated at the correct angle of attack, which means that airspeed must be reduced as weight decreases (other factors being equal.) Effect of Altitude on Range and Endurance (Jet) The graph below shows how the drag curves and Fuel Flow vs. Velocity curves change with altitude. As the aircraft climbs into the less dense air the parasite drag decreases, but the induced drag increases. As a result the total drag curve moves to the right. Remember that the drag curve is exactly the same shape as the FF vs. Velocity graph for a jet. As you can see in the graphs below there should be no change in the maximum endurance of the aircraft with altitude. However, the required endurance speed will increase. As before maximum endurance always occurs at L/D max. [I.e. always the same angle of attack] On web page 8 we will discuss the effect of engine and propeller efficiency. For a jet range is significantly affected by altitude. As you can see in the graph below, as the aircraft climbs higher the max SR [V/FF] keeps getting better and better. Therefore, the jet aircraft should always be operated at high altitude unless there is a very strong headwind. Jet Aircraft range/endurance summary The TSFC of the jet engine improves up to the altitude for the coldest air temperature. In the ISA this is the tropopause [TSFC holds constant in the stratosphere]. Endurance will increase with altitude as long as temperature decreases with altitude. Maximum endurance will therefore occur at the tropopause. Range will increase with altitude up to the altitude at which Mach effects arise (see Cruise Control.) Endurance does not increase in the stratosphere, but it does not decline either. Therefore, pilots should not descend when holding. Wind will be a factor. But, due to the powerful benefit of altitude a jet will often get better range at altitude even with a moderate headwind. Jet engine fuel consumption Both jet and propeller engines consume fuel at a certain rate [FF] Jet engines convert the fuel-flow directly into thrust Specific Fuel Consumption Specific Fuel Consumption is a measure of the fuel consumed by an engine. There are two types of specific fuel consumption: 1. Thrust Specific Fuel Consumption [TSFC] 2. Power Specific Fuel Consumption [SFC] TSFC is defined as fuel-low per pound of thrust produced [FF/Thrust] SFC is defined as fuel-flow per horsepower produced [FF/HP] Fuel-flow should be measured in units of pounds of fuel per hour, rather than gallons per hour. This is because the chemical energy in the fuel is a function of the mass of the fuel. A gallon of fuel expands or contracts with temperature. Therefore, a gallon of cold fuel contains more energy than a gallon of warm fuel. The units of TSFC and SFC will be: TSFC = lb per hr/thrust lb SFC = lb per hr/HP Converting the Drag vs. Velocity Curve A perfectly accurate conversion of the drag curve into a Fuel-flow vs. Velocity graph must take variations in engine and propeller efficiency into account. However, we will find it easier to break the process into two steps. We will therefore conduct a simple aerodynamic analysis first, in which we will assume that: •TSFC is constant for a jet •SFC is constant for piston and turbo-prop engines FF vs. Velocity for a Jet We will start by converting a Drag vs. Velocity curve into a Fuel-flow vs. Velocity curve for a jet aircraft. This will be very easy because the TSFC is a constant. Remember the definition of TSFC: TSFC = FF/Thrust We will assume that: TSFC = FF/Drag (i.e. we assume thrust = drag) Therefore: FF = TSFC x Drag What is the Critical Drag-rise Mach number [Mcdr]? Mcdr is that free stream Mach number at which, because of compressibility effects the drag co-efficient at a specified angle of attack, has risen by 20% of its low subsonic value. Define Mach Number Detachment [Mdet] Mdet is that free stream Mach number at which the bow wave becomes attached to the leading edge. Explain Compressibility Mach Number [Mcomp]... Mcomp is that free stream Mach number at which, because of compressibility effects, control of an aircraft becomes difficult and beyond which loss of control is probable. What is Mach Number? Mach number is the ratio of the speed of an object or flow to the local speed of sound, under the same conditions. How do you get zero lift in a positive camber wing? You must go to a negative AOA. If you decrease AoA how does that affect induced drag? If you decrease AoA that will increase velocity which means that induced drag will decrease. Explain positive camber, negative camber and symmetrical airfoil definitions. •Positive camber: mean camber line is above chord line •Negative camber: mean camber line is below chord line •Symmetrical airfoil: mean camber line coincides with chord line What is the mean camber line? A line drawn halfway between the upper and lower surfaces of a wing. Define chord This is the measure of the width of the wing. Define chord line Infinitely long line drawn through the trailing edge and leading edge of airfoil (wing). Define angle of incidence. Angle between longitudinal axis and the chord line of the wing. Define angle of attack Angle between the relative wind and the chord line of the wing. Define pitch attitude The angle between longitudinal axis and the horizon. What is the Mean Camber Line? It's a line drawn between the upper and lower surfaces of the airfoil. Define relative wind Airflow the airplane experiences as moves through the air. Equal in magnitude and opposite in direction to the flight path. Explain IAS, EAS and TAS IAS: Indicated airspeed EAS: Equivalent airspeed-IAS corrected for position and compressibility errors TAS: True airspeed, EAS corrected for atmospheric conditions What limits an aircraft climb performance? The amount of 'excess thrust' available. How is drag defined? Drag acts along the dragline of the aircraft Drag = CD½?V²S Induced drag is the by product of the production of lift Define Thrust Thrust acts along the average centreline of the engines. Thrust = Mass air flow x [Vj-V], or [mass x acceleration] Define aerodynamic lift Lift acts through the centre of pressure and acts perpendicular to the relative airflow. Lift = CL½?V²S CL: Co-efficient of lift [Lifting ability for a particular wing design at a given AoA] ?: Rho represents the value of density [If density doubles, lift doubles] V: Velocity or TAS of the air flowing around the wing [If velocity doubles, lift quadruples] S: Surface area of the wing [If wing area doubles lift doubles] What are the forces acting on an aircraft in flight? Drag, thrust, lift and weight. If thrust is greater than drag the aircraft will accelerate. If lift and weight are the same, an aircraft will maintain a steady, level attitude.If the aircraft is in a turn, lift is reduced due to the reduction of effective wingspan. The weight of the aircraft though remains the same. To maintain altitude when in a turn, speed and/or angle of attack has to be increased. What is the Free Stream Mach number [Mfs] Mfs is the Mach number of the flow sufficiently remote from the aircraft to be unaffected by it. Explain Local Mach Mumber [Ml] Ml is the ratio of the speed of the flow at a specified point to the speed of sound at the same point Define Critical Mac Number [Mcrit] MCRIT is that free stream Mach number at which the highest local Mach number reaches Mach 1. What is the definition of Subsonic, Transonic, and Supersonic? Subsonic All flow everywhere on the aircraft is less than the speed of sound Transonic Some flow is subsonic and some is supersonic Supersonic All flow everywhere on the aircraft is supersonic |