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AOE 2104 Introduction to Aerospace Engineering

AOE 2104 Introduction to Aerospace Engineering. Lecture 9 Stability and Control. Stability and Control. Stability. Control. Static. Dynamic. Coordinate Systems Control Surfaces Coordinated Turn. Longitudinal. Lateral. Directional. Criteria. Adverse Yaw. Adverse Roll.

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AOE 2104 Introduction to Aerospace Engineering

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  1. AOE 2104 Introduction to Aerospace Engineering Lecture 9 Stability and Control

  2. Stability and Control Stability Control Static Dynamic Coordinate Systems Control Surfaces Coordinated Turn Longitudinal Lateral Directional Criteria Adverse Yaw Adverse Roll

  3. Aircraft Control

  4. Control perspective – body axis system Coordinate Systems • The movement of the center of mass can be in three directions: x, y, or z. These directions can either be related to the body of the airplane (body axis system) or to the relative wind (wind axis system) .

  5. Aerodynamic perspective – wind axis system Note: Neither of the above mentioned axis systems are necessary perpendicular or parallel to the ground. Virginia Tech

  6. Degrees of freedom (motion) Six degrees of freedom • Axial • Normal • Transverse • Roll • Pitch • Yaw

  7. Aircraft Control – Roll, Pitch, and Yaw Nomenclature! Axial: A Normal: N Side: Y Roll: L Pitch: M Yaw: N

  8. Aircraft Control – Aft, Port, Starboard, Fore • Often in Stability and Control, people refer to front, rear, left and right as port, fore, aft, and starboard. Aft Rear = Port Left = Starboard Right = Fore Front =

  9. Aircraft Control – Control Surfaces • How do you control or induce the 6 motions we described earlier? • Other methods than can be used: • Vectored Thrust • Aerodynamic Forces If ailerons are designed to control the roll motion, why are they located that far on the wing? Why do they deflect in opposite directions?

  10. Aircraft Control – Ailerons and Rolling Moment Ailerons deflect differentially (one moves up while the other one moves down). The increased camber on one of the wing results in increased lift on that wing, resulting in the roll motion. Why do you need a vertical stabilizer?

  11. Utah State Utah State Aircraft Control – Roll Control through Wing Twist We saw earlier the benefits of wing twist… Instead of deflecting a flap-like control surface, studies have looked into having variable twist wings, on which ailerons are called twisterons. http://www.centennialofflight.gov/wbh/wr_experience/1903b/html/warp.htm#

  12. Active Aeroelastic Wing Courtesy of NASA Courtesy of Benini and Marques Aircraft Control – Motion Control Advanced Technology Morphing Wings change their shape to reach optimal flying efficiency.

  13. Aircraft Control – Ailerons and Pure Control Reaction The resulting geometry differential can sometimes lead to a drag differential, that will end up in yawing moment. Of course, if the desire is to turn, the induced yaw may be just fine.

  14. Aircraft Control – Rudder and Yawing Moment The rudder is essentially a flap on the vertical stabilizer. It is used to control the motion about the aircraft’s vertical axis. How ? When the rudder is deflected, the increased camber on the verticaltailleads to an increase in lift causing the yawing moment.

  15. Higher Velocity Lower Velocity Aircraft Control – Rudder and Pure Control Reaction How ??? Rotating the aircraft around its vertical axis results in a velocity differential between the two wings, resulting in a lift differential, leading to adverse rolling.

  16. Aircraft Control – Rudder-Aileron Cross-Control Occasionally a pilot will “cross-control” an airplane to keep the wings level in yaw. This requires applying the rudder in one direction while using the ailerons in the other direction to prevent the rolling tendency that comes from yaw. This can be a very effective way to descend quickly without building up a lot of speed. It can also be a technique used to keep the airplane aligned with a runway while making a landing approach in a cross wind. Cross-Wind Cross-wind force

  17. Aircraft Control – Elevators and Pitching Moment The elevators are essentially flaps on the horizontal stabilizer. They are used to control the motion about the aircraft’s transverse axis (axis “passing through the wings”). How ? When the elevators are deflected, the increased camber on the horizontaltail leads to an increase in lift causing the pitching moment. Why do you need a horizontal stabilizer?

  18. Aircraft Control – Elevators and Pure Control Reaction How ??? No adverse motion, other than drag. Elevators are used to increase or decrease the lifting capability or lift coefficient of the wing. They are used, along with the engine power or thrust setting, to enable the plane to climb or descend at the desired rate and to help provide the extra lift needed in a turn. Why would you need extra lift in a turn?

  19. Aircraft Control – Coordinated Turn Any vehicle that is turning must produce a force along the turn radius in the direction of the turn to overcome the centripetal acceleration. You could use rudder to produce side force…But wings have much greater area, and can therefore produce much greater force! The wing is rolled into the turn and part of the lift produced by the wing is used to provide the force to pull the plane through the turn. But, by using some of the lift to make the turn there is no longer enough lift to balance the weight! The result is that to keep everything balanced or coordinated in a turn all the controls on the plane must be involved: In a coordinated turn, altitude and turning radius are constant. • The rudder is used to begin the turn. • 2.The ailerons are used to give the needed amount of roll. • 3.The elevator is used to increase the angle of attack and, thus, the lift. • 4.Finally, power or thrust is increased to overcome the added drag.

  20. Aircraft Stability

  21. Static versus Dynamic Stability Static-vehicles initial tendency following a disturbance Dynamic- time history of the vehicles motion after its responds to its static stability Aircraft Stability – Static Stability

  22. Aircraft Stability – Static Stability Stable Neutral Unstable

  23. LONGITUDINAL STABILITY Assume lift acts behind CG and the gust increases AoA, what happens? Now assume lift acts in front of CG, what happens? What if lift acts at CG? For a stable aircraft, one that corrects itself, we want the lift to act behind the CG.

  24. STABLE OR UNSTABLE? LONGITUDINALLY STABLE! Remember, by convention positive pitching moment = nose up.

  25. STABLE OR UNSTABLE LONGITUDINALLY UNSTABLE!

  26. CONVENTIONAL TAIL SURFACE For stability, we will always have a nose down pitching moment. How do we keep the plane from flipping nose down onto its back? One way is to use a horizontal stabilizer. Since the horizontal stabilizer is aft of the aerodynamic center, to counteract the nose up motion induced by the lift, we need a down force from the stabilizer. This means we want the stabilizer at a negative AoA.

  27. Advantage of CONVENTIONAL TAIL SURFACE An increase in angle of attack on the wing will be accompanied by a decrease in the negative angle of attack of the horizontal stabilizer, reducing the download on the tail so that the α of the wing decreases This wing / stabilizer arrangement is also self correcting. Why? .

  28. DIRECTIONAL STABILITY Airplanes achieve yaw stability the same way an arrow does. The feathers (really small fins) at the aft end of an arrow aerodynamically correct any tendency to yaw by producing a yaw-countering side force. The fins on a missile work the same way and an airplane does the same with a vertical stabilizer. The rudder allows to overcome this inherent stability when yaw is desired.

  29. LATERAL STABILITY • Roll stability is more complicated. • We want stable roll, however we want to be able to roll on demand (for maneuvers like coordinated turn for example). • Two ways to reach stable roll: • High Wings • Wing Dihedral

  30. Lateral Stability – High Wings Pendulum effect = stable roll. Higher Pressure

  31. LATERAL STABILITY AND SWEPT WING

  32. LATERAL CONTROL AND LARGE WINGS At high speeds, large wings (and therefore large ailerons) tend to have complex dynamic motions that reduce their efficiency. Using spoilers to create a lift differential is usually then preferred.

  33. Lateral Stability – Roll Induced by propellers How do we fix this?

  34. Pulling it all together…

  35. Assignments: Read the relevant sections of Chapter 7 Homework 6: Due 10 November Timeline for the rest of the semester: (based on your midterm feedback) Nov 3-Stability and Control Nov 5-Aircraft and Spacecraft Propulsion Nov 10-Helicopter Aero-special guest presentation Nov 12-structures quiz Nov 17-performance and stability quiz Nov 19-intro to space Nov 24/26-break Dec 1-orbital mechanics Dec 3-take home space quiz due Dec 8-review for final

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