PRE-TEST Module 2 The Principles of Flight Units /60 points

Transcription

1 PRE-TEST Module 2 The Principles of Flight Units /60 points 1 Answer the following questions. (20 p.) moving the plane (4) upward / forward. Opposed to that is 1. What are the names of the four forces acting on an drag, caused by air resistance. If lift is (5) more / less than airplane? weight, the plane will rise. If thrust is more than drag, the plane will (6) go up / accelerate. 2. Which are the two natural forces? Airplane wings are designed to take advantage of lift. They are shaped so that air has to travel (7) faster / slower over the 3. And the two artificial forces? top of the wing than underneath it. The reason for this is explained in (8) Bernoulli s Principle / Venturi Tube, which 4. What is an airfoil? states that an increase in the velocity of any fluid results in (9) a decrease / an increase in pressure. When the air has to 5. What are the front and back edges of an airfoil travel over the (10) top / bottom of the airplane wing, it must also travel faster, which results in (11) lower / higher respectively called? pressure. The (12) shorter / longer distance under the wings results in higher pressure, causing the airplane to move 6. What is an angle of attack? upward. 7. What does Bernoulli s Principle state? 8. What device can you use to demonstrate Bernoulli s Principle? 9. What is low speed associated with? 4 Label the pictures. 1. b. (18 p.) 10. What generates lift? a. c. 2 Write the English for the following Italian words. (10 p.) 1. portanza spinta/trazione velocità bordo d attacco traiettoria di volo angolo di incidenza portata (di fluido) impatto flusso molecola a. 3. d. b. e. f. g. h. c. d. (4 p.) (8 p.) 3 Choose the correct alternative. (12 p.) There are four main forces involved in flight. Lift is caused by the variation in air (1) pressure / speed when air flows under and over an airplane s wing. It is in opposition to (2) thrust / weight, or the force of gravity, pulling downward. (3) Thrust / Lift is caused by the action of the propellers d. a. velocity pressure b. pressure e.. velocity f. c. velocity pressure (6 p.)

2 PRE-TEST Module 2 The Principles of Flight Units /40 points 1 Complete the following diagram which refers to the c an upward lift force at a zero degree angle of attack. factors influencing lift and drag. (5 p.) d an equal reaction both upward and downward when at a zero degree angle of attack. Air 3.Which of the following statements is correct? Airfoil... a The total reaction of a wing acts at 90 to the... relative airfoil. b The camber of a wing is the imaginary line between the leading and trailing edges. c The angle of attack is the angle between the chord line and the relative wind. d All the above statements are correct. Angle of... Factors influencing lift and drag Area of the... Air... 2 Fill in the blanks choosing from the following words. In some cases you will find terms whose meaning is the same as the one in your text. (5 p.) body density inclination square surface Lift depends on the (1) of the air, the (2) of the velocity, the air s viscosity and compressibility, the (3) area over which the air flows, the shape of the (4), and the body s (5) to the flow. 3 Which is right: a, b, c or d? (8 p.) 1.A symmetrical airfoil section which is moved through the air at a zero angle of attack will a cause the air passing over the top of the airfoil to travel faster than the air passing below it. b produce low pressure above the wing and high pressure below the wing. c cause an acceleration of the airflow passing round it and the speed of the airflow above and below the wing will be the same. d produce high pressure above the wing and low pressure below the wing. 2.An asymmetric airfoil, passing through the air at sufficient speed, will create a an upward lift force, but only if the angle of attack is more than zero. b a negative lift force when placed at an angle of attack of zero degrees. 4.If the angle of attack of an airfoil in a given airflow is increased, the a lift and drag will decrease. b lift will increase and drag will decrease. c drag will increase and the lift will decrease. d lift and drag will increase. 5.Induced drag is caused by the a high-pressure area over the wings moving outwards to the tips. b air below the wing moving outwards towards the tips. c turbulence produced by the joining of the wing root to the aircraft fuselage. d frictional effect of the air passing over the wing surface. 6.In relation to induced drag, which of the following is true? a It increases with increase of airspeed. b It remains constant regardless of variation in airspeed. c If the speed decreases and level flight is maintained, the induced drag will increase. d Induced drag is very low at high angles of attack. 7.The term lift/drag ratio a can be used to illustrate the aerodynamic efficiency of the wing. b can be defined as the ratio of the amount of lift to drag produced from an airfoil at different angles of attack. c is obtained by comparing the amount of lift and drag produced from an airfoil at different angles of attack. d All the above responses are correct. 8.A wing of high aspect ratio will a be less efficient than one with a low aspect ratio. b produce a larger drag force at any angle of attack than that of a low aspect ratio wing. c be more efficient than one with a low aspect ratio. d have a long chord and a short span.

3 PRE-TEST Module 2 The Principles of Flight Units /40 points 4 Fill in the blanks choosing from the following words. When engineers (7)... a new airplane, the (22 p.) size and (8)... of the wings are a very aspect ratio design drag (2) engines faster flows gliders higher increase less (2) lifts narrow pressure reduce same shape (2) thrust wing The aspect ratio of wings As air (1) over and under a wing, we know from our study of lift that the air flowing over the top flows (2) than the air that flows under the (3).... We also know from Bernoulli s Principle that the air that flows faster applies less to the surface it is flowing over. Therefore, since the air flowing over the top of a wing has less (4)... (because it is flowing faster), the air pressure on top is (5)... than on the bottom of the wing. The higher air pressure on the bottom (6)... the wing. important issue. Wings provide the majority of the lift for the airplane, but they also cause (9).... Remember that drag is a force that opposes the (10). force. Engineers are always trying to find ways to (11) lift and (12)... drag caused by the wings. Experiments have shown that a wing built with a (13) aspect ratio tends to create (14) drag than a wing built with a smaller (15), even when their area remains the (16). Maybe you ve wondered why sailplanes and (17) have long, (18) wings. Since they don t have (19) to provide thrust, their wing (20) helps to provide the greatest amount of lift with the least amount of (21)....

4 Module 2 Something more about THE PRINCIPLES OF FLIGHT IN-DEPTH ANALYSIS TEXTS A The lift formula Here is the standard equation for calculating lift using a lift coefficient: 1 L = C l ρ V 2 2 A L = lift Cl = lift coefficient ρ (rho) = air density V = air velocity A = wing area As an example, let s calculate the lift of an airplane with a wingspan of 40 ft and a chord length of 4 ft (wing area = 160 sq ft), moving at a speed of 100 mph (161 kph) at sea level (that s 147 ft, or 45 m, per second). Let s assume that the wing has a constant cross-section using an NACA 1408 airfoil shape, and that the plane is flying so that the angle of attack of the wing is 4 degrees. We know that: A = 160 sq ft ρ = slugs/cu ft (at sea level on a standard day) V = 147 fps Cl = 0.55 (lift coefficient for NACA 1408 airfoil at 4 degrees AOA). So let s calculate the lift: Lift = x = 2,260 lbs B What forces act on an airplane? There are four forces acting on an airplane in flight: lift, weight, thrust and drag. Lift... Drag Thrust Weight We already know that lift comes from the wings. What about the other forces? Weight is a force caused by gravity. You feel gravity every time you jump. Gravity is what pulls you and everything else back down to the ground. This pulling down by gravity is what causes you to have a weight that you measure when you step on a bathroom scale. An airplane s weight also pulls downward on it directly opposite to the lift force that is pulling the airplane up. It s as if the lift and weight forces are playing a game of tug-of-war. One pulls in one direction and the other pulls in the opposite direction. For level flight, lift and weight must balance each other out.

5 Module 2 The Principles of Flight Thrust, caused by the airplane s engines, is the force that moves the airplane forward. If an airplane did not keep moving, air would stop moving over and under the wings. Without this movement of air, the wings could not create lift and the airplane would start to fall back to the ground. Drag is the force that tries to slow down a moving object. Most airplanes are made more streamlined, or aerodynamic, to reduce the amount of drag they feel. A streamlined airplane has smooth surfaces and no bumpy sections, causing as little resistance to the air as possible. Just like lift and weight are opposite forces, thrust and drag are opposites to each other too. For an airplane to keep flying, its thrust must be greater than its drag. C How does an airplane fly? Most of us have probably seen or felt wind blowing things like flags or kites around. But how is it possible that heavy airplanes (some weighing almost half a million pounds, or more than 200,000 kilograms) are able to be supported by air high above the ground? The answer may sound strange at first, but it s actually the air that is pushing the airplane s wings and the rest of it up. The air under the wings pushes up more than the air on top of the wings pushes down. This pushing by the air is called air pressure. We can think of air pressure as air pressing down or up against something else. On a windy day, you can actually feel the air pressure push against your body. The wings of an airplane feel a similar pushing, but there happens to be a bigger pressure under the wings and a smaller pressure on the top. What s really amazing is why this happens. The special shape of an airplane s wing makes the air moving around it behave in a certain way. If we look at an airplane s wing from the side, we can see that the wing is a special shape called an airfoil. An airfoil is curved on the top and flat on the bottom, causing some of the air to go over the top and the rest of the air to go along the bottom. This shape looks simple, but it is the main reason why airplanes can fly at all. Because of the airfoil s curved shape, the air under the airfoil moves at a slower speed than the air going over the top. A scientist named Daniel Bernoulli ( ) demonstrated that slow-moving fluid (liquid or gas) causes high pressure, and fast-moving fluid causes low pressure. Because there is a higher pressure pushing on the underside of the wing than on the top, the bottom pressure wins out overall and the wings (and the airplane attached to it) are pushed up, making the entire thing fly. You can actually see this for yourself. If you take a strip of paper and blow over the top of it, the paper will rise (as shown in the picture). What happened? You lowered the pressure that was pushing down on the top of the paper, causing the pressure on the bottom side of the paper to push the paper strip up. The same thing happens when air pushes on the bottom side of an airplane s wing. The pressure that is pushing the airplane up creates a force called lift in the upwards direction.

6 Module 2 The Principles of Flight LINKS (Four forces) (Airfoil features and Bernoulli s Principle) (Factors affecting lift and drag) (Induced drag coefficient and aspect ratio) VIDEOS YouTube Forces that act on an aircraft in flight YouTube Bernoulli s Principle (Magnus effect on a soccer ball) Drag (Physics) Explanation of drag by NASA