BGA Bronze C

1. BGA Bronze C Principles of Flight Portsmouth Naval GC J Hale October 2009

2. Forces in Balance Aerodynamic force is due to airflow over the wings. Weight always acts in the direction of the centre of the earth, usually considered as downwards! In steady straight flight the total Aerodynamic forces are equal to the total Weight forces ~

3. Definitions. Chord & Camber Chord Line is the longest straight line that can be drawn from the leading edge to the trailing edge. Mean Camber Lineis the curve lying halfway between the upper and lower surfaces.~

4. Definitions. Angle of Attack The Angle of Attack is the angle between the Chord line & the direction of the Relative airflow Note: The direction of the Relative airflow is due to the direction of motion of the wing. The next slide explains: ~

5. Definitions. Angle of Attack To understand relative airflow, consider these examples: At first sight they all appear to have different angles of attack In fact they all have the SAME angle of attack, the difference is that they are travelling in difference directions (V) as shown by the Red arrows, this makes the relative airflows come from different directions. ~

6. Generation of Aerodynamic Force Air over the wing is accelerated by wing shape. Its pressure drops. Air under the wing is slowed down. Its pressure increases. The pressure difference results in forces acting to pull the wing upwards Note name of point where airflow is brought to standstill ~

7. Centre of Pressure Aerodynamic forces are produced across the entire wing surfaces But for simplicity these forces can be represented by a single force called the Total Reaction. This acts through a point on the Chord called the Centre of Pressure (C o P) The average C o P position is about 1/3 of the chord back from the leading edge of the wing. Generally 80% or more of the aerodynamic force is generated from the upper surface of the wing.~

8. Centre of Gravity (C of G) In the same way that aerodynamic forces occur all over the wing surfaces, so does every part of the glider have some weight. For simplicity these weights can be represented by a single (Total) WEIGHT, acting on a point in the glider called the CENTRE OF GRAVITY. It is the point where the glider would perfectly balance if it where put on a PIVOT Heavy pilots move the C of G forward & vice versa C of G is to WEIGHT, as Centre of Pressure is to TOTAL REACTION

9. Total Reaction & Lift / Drag The Total Reaction can be divided into 2 forces at right angles to each other, Lift & Drag. Lift is at right angles to the airflow direction, and Dragis along it. ~

10. Lift & Drag in Steady Flight TOTAL REACTION is equal & opposite to WEIGHT The forward component of LIFT is equal and opposite to DRAG It is the forward component of lift which makes the glider go forward

11. Control of Lift For our purposes, the amount of lift produced by a wing is controlled in 2 ways: Angle of Attack Airspeed Increasing either or both of these will increase lift ~

12. Controlling Lift by Angle of Attack For a fixed airspeed, lift increases linearly with angle of attack until the stall angle is reached Then the lift reduces smoothly to about 80% of the maximum lift as the angle of attack is further increased ~

13. Controlling Lift by Airspeed For a fixed angle of attack the lift increases with increased airflow Lift increases rapidly obeying a square law E.g. 3 times the speed will create 9 times more lift. (3x3=9) Vne (NEVER EXCEED) is to prevent damage from large aerodynamic forces ~

14. Angle of Attack (AoA) v Speed In steady flight the weight is constant, and must be balanced by the lift Flying fast, needs lower AoA. Flying slowly, needs high AoA. For any speed, there is an corresponding AoA that will provide the right amount of lift to balance the weight. ~

15. Stall As the AoA is increased, the C o P moves forward, because air at the rear of the upper surface becomes more separated. The Stall Angle is reached when the air can no longer follow the top wing surface The air separates at the Stall Angle, and causes turbulence below it When this happens the lift produced by the top surface reduces, and the drag increases substantially. Note as the AoA increases from zero to Stall angle, the CoP moves forward, then rapidly backwards at the stall ~

16. Stall. Summary • For a particular aircraft configuration • the stalling AoA remains constant • An aerofoil surface will always stall at it’s stalling AoA, regardless of airspeed • So an aircraft at 100knots will stall if a • manoeuvre violent enough to increase it’s • AoA to the stall angle is performed • Stalls happen more readily at low airspeeds because the AoA is higher, and thus closer to the stall angle ~

17. Drag………. Two Main Types (Lift) INDUCED DRAG Caused directly by lift generation All other is called PROFILE DRAG (Sometimes Parasitic) DRAG acts in the opposite direction to travel, at right angles to LIFT. Now let’s look at each of these types of drag: ~

18. Profile Drag (Form) Flat Plate Objects moving through air have to push it aside. This causes high pressure at the front and low pressure at the rear leading to a force called form drag. Cylinder: 50% of Flat Plate Reduce by streamlining, and /or reducing frontal area. ~ Streamlined: 10% Of Flat Plate

19. Profile Drag (Skin Friction, Viscous) Drag due to the ‘Stickiness’ (Viscosity) of the air. Within the boundary layer the air layers are increasing speed towards the free flow speed, and causing friction, drag. Friction brings the air actually in contact with surface to a standstill. This drag is very significant on high speed a/c resulting in substantial amounts of heat generated. E.g. Aircraft skins get very hot at Mach2 ~

20. Profile Drag (Interference, Leakage) INTERFERENCE: Collision of two airstreams causing eddy currents and hence drag E.g. Can be caused by gaps between two objects. Each causing their own airflows & then mixing causing turbulence. Smooth filleting between objects reduces Interference drag. Wing tape. LEAKAGE: If the wing joints with the fuselage are not sealed, high pressure air underneath the wing will leak to meet the lower pressure air above. This causes turbulence and drag, and also reduces lift by disturbing the upper surface airflow. Note: Profile Drag increases as the square of the airspeed. For example, a glider travelling at 120 knots has NINE times as much profile drag as it does at 40 knots. ~

21. Lift Induced Drag Low pressure above the wings causes some airflow from tips towards fuselage At the tips, vortices are formed as high pressure from below feeds the low pressure above High pressure below wings causes some airflow from fuselage towards tips The next slide shows this effect looking down on the wing…~

22. Lift Induced Drag Tip and Trailing edge vortices are shed downstream of the airflow Vortices require force to make them, which results as drag on the wing Lift Induced drag reduces with the square of the airspeed ~

23. Total Drag versus Airspeed Adding together the Profile & Induced drags gives the Total Drag This can be shown graphically Note how Profile drag increases with speed & Induced drag reduces with speed Increasing speed from the stall causes the Total Drag to reduce & then increase. There is a speed where the drag is a minimum. ~

24. Lift Induced Drag LIFT INDUCED DRAG CAN BE MINIMISED AS FOLLOWS: 1) Using High Aspect ratio (AR) wing. AR = Span / Mean Chord. 2) Winglets diffuse the tip vortices & therefore reduce drag. 3) Fly with a low AoA (i.e. fast). 4) Fly with minimum weight, so the lift (& hence induced drag) required is minimum. (Water ballast is weight). ~

25. Laminar / Turbulent ( Boundary Layer) Two distinct types of flow in the boundary layer. Laminar, where the air molecules slide smoothly over each other Turbulent, dominated by irregular motion. This has 5 to 10 times more drag than laminar Laminar flow is desirable to minimise skin friction! ~

26. Laminar / Turbulent. Transition Point Definition: TRANSITION POINT is where Laminar flow becomes Turbulent ~

27. Why Turbulent? Air is: Higher pressure Lower speed Air is: Low pressure High speed Air struggles to flow from a low to a high pressure, and becomes unstable making it easy for the wings surface to trip it into a turbulent airflow. ~

28. Why Turbulent? Boundary layer can also be tripped into turbulent flow by any discontinuities in the surface of the wing Leading edge boundary layer is about 1mm thick, & any similar sized object there will be large enough to cause localised transition to occur Bugs, rain, ice, dust etc make a big difference to drag! ~

29. Speeds to fly • Why is Speed to Fly so important? If we measure the sink rate of a glider when it is flown at some different airspeeds we could make a table: ~

30. POLAR CURVE Four gliders all the same except for their colour Line them all up a the same height and watch them fly for one minute.. Notice the distances flown & heights lost for each glider ~

31. POLAR CURVE Connecting the gliders with a smooth curve, & changing the distance scales to speed scales, plots the polar curve for this glider. It shows the sink rate of the glider for any given airspeed. ~

32. POLAR CURVE If all the gliders started at 320’ above the ground. One minute later the red glider would be on the ground. ~

33. POLAR CURVE Continue the descent of the others until the blue and yellow gliders also reach the surface. Some points emerge…. ~

34. POLAR CURVE Firstly, the yellow glider goes farther than any other for the same 320 feet of height loss. ~

35. POLAR CURVE The blue glider touches down about the same time, but has not gone as far. ~

36. POLAR CURVE The green glider is still in the air, about to land on the red one! Two of these gliders represent significant points on the polar curve. ~

37. Polar Curve. Minimum sink. The green glider has the lowest sink rate of any. It will stay in the air for the longest time, but will not go very far. The glide path crosses the curve at its highest point. There is no other speed that can give less sink. Flying at Min Sink speed gives max time in the air ~

38. Polar Curve. Best Distance. The yellow glider has the best glide ratio of all. It will glide the furthest distance in still air from a given height. A line from the origin (0, 0 point) of the graph, that is tangent to (just touches) the polar curve represents the glide path of the yellow glider. If air never moved, there would be no more to know about polar curves. Min Sink speed for max time in air, and Best Glide speed for longest distance. However, air frequently moves both vertically and horizontally. When it goes up faster than the glider’s sink rate within it, we can climb. When it goes down we need to modify the best glide speed to optimize the flight through the sinking air. ~

39. Best Glide.. flying through SINK Consider Red & Yellow gliders flown at the same speeds as before crossing a patch of air sinking at 3 knots. Both gliders now have an additional 3 knots down. Even though the red one is sinking faster than yellow, it is flying faster, and emerges higher than the yellow glider! Watch… With 3 knots of sinking air and the Red & Yellow glider flown at same speeds as before, the Red glider now has the best Glide ratio. ~

40. Construction on Polar for SINK Draw the tangent from 3 knots of sinking air mass to the curve to find best glide in sink In this case the total sink rate for the glider is the sum of it’s sink rate in the air mass (3.2kts), and the sink of the air mass (3kts) itself. Total = 6.2knts ~

41. Construction on Polar for LIFT Here the air mass is going up at 2.5 kts, this line shows the best speed to fly Note that the sink rate of the glider within the air mass is 1.7kts The climb rate of the glider relative to the ground is 2.5 – 1.7 = 0.8knt ~

42. Construction on Polar for HEADWIND Here there is a 15kt headwind, so the line is drawn as shown. It then shows that the best speed to fly has increased from 42kts (no wind) to about 52kts ~

43. Construction on Polar for Tailwind With a tailwind the line is drawn as shown, and now the best speed to fly has reduced. ~

44. Construction for Headwind & Sink With sink & headwind the line is drawn as shown. This example shows a 9knt headwind with 1.5knts of air mass sink. Of course you don’t calculate this in flight. In absence of a flight computer a rule of thumb is "Add about half the estimated value of a headwind, and subtract about half the value of a tailwind from the Speed to Fly determined for lift or sink." Basic idea is to stay in lift or tailwinds by slowing down, and to run away from headwinds & sink by speeding up ~

45. Speed to fly ring on Variometer The speed ring is adjustable, but for our current definition of Speed to Fly it should be set with the arrow on the ring pointing to "0" on the vario. Then when the vario points to any rate of descent it is also pointing to the corresponding Speed to Fly - the one that will produce the flattest glide under those conditions. The example shows Speed to Fly varying from 42 kts in still air to 62 kts in 3 kts sink. Vario shows the best speed to fly for total sink (glider + air mass). (Wind is not taken account of by the Vario) ~

46. Effects of Adding Weight Added weight moves the polar curve to the right & downwards. Best Glide ratio is the same for all weights, but at higher weights it happens at higher speeds. Hence water ballast in strong conditions. ~

47. Control • Next we are going to look at how the aircraft’s shape in terms of its wing, tailplane and fin surfaces coupled with the control surfaces affect its control. ~

48. Control. Aircraft Axis Pitch is a rotation about the Lateral axis Roll is a rotation about the Longitudinal axis Yaw is a rotation about the Normal axis The axis are mutually at right angles and pass through the C of G. They are fixed relative to the aircraft, regardless of aircraft attitude ~

49. Control. Flight Surfaces The fin provides directional stability. The tailplane provides longitudinal stability and control. ~

50. Control. Control Surfaces Glider control surfaces ~