SkyWest Interview Gouge (High Alt Aerodynamics) Questions with correct Answers

SkyWest Interview Gouge (High Alt Aerodynamics) Questions with correct Answers How does stalling speed change with altitude? Stall speed INCREASES with higher altitude Why does stalling speed increase with altitude? Thinner air More airflow needed to generate same amount of lift What is Mach Speed? Speed in relation to speed of sound What is relative Mach Number? Ratio of the speed of aircraft to the speed of sound. Mach 0.8 means the plane is flying at 0.8 times the local speed of sound at that ambient temperature What is critical Mach? The airspeed at which air ANYWHERE around the aircraft reaches the speed of sound. Airflow above the wings can accelerate and reach the speed of sound What is high altitude Mach Buffet? When airflow goes supersonic over your wing and then slows down to subsonic towards the training edge of the wing. Causes a shock wave and turbulent air behind the wing which causes a buffet. What is Mach Tuck? As an aircraft approaches the speed of sound its wing center of pressure moves aft. As it moves aft, it creates a pitch down moment which there by increases airspeed and worsens situation. Can be unrecoverable and exceed the aircraft limitations. Center of lift moves farther aft the more you speed up What is MMO? Max Mach Number Just below critical Mach (supersonic flight) What is the coffin corner? The region where the aircrafts stall speed is very close to the aircrafts MMO (Max Mach speed) by the aircrafts max ceiling. Any slower, the aircraft will stall. Any faster the aircraft will exceed MMO and critical Mach. What happens to your stall speed as you climb? Why? Stall speed increases with altitude. Air becomes less dense and wings need more airflow to generate same amount of lift Does the speed of sound change based off of temperature? Yes. Colder temps cause speed of sound to decrease. As you climb up to higher altitudes with colder temps, speed of sound decreases. What happens if you keep climbing without leveling out? You will reach the "coffin corner". As you climb, stall speed and MMO will eventually reach the same point. Why do jets have swept wings? Swept wings allow us to delay the supersonic flow and raise the critical Mach Number. Tricking wing to think it's flying slower than it actually is What are the two components of airflow that is created with a swept wing? Chordwise - travels perpendicular to leading edge Spanwise - moves parallel to the leading edge Does the chordwise flow accelerate? Yes. Chordwise is less than the total amount of airflow, we are only accelerating part of the air! Allows us to fly faster without supersonic flow What does the spanwise flow do? Spanwise component stacks up. Makes wingtip feel like it's flying at a slower airspeed then it actually is. On a sweptwing, what part of the wing stalls first? Why? Wingtip. The spanwise flown"builds" up which makes it feel like it's at a slower airspeed. When wings begin to stall, you lose aileron effectiveness What's the cons of a sweptback design? Flying at slower airspeeds (bad low speed performance) What is used to allow sweptback wings to be more efficient at lower altitudes? Trailing edge flaps Leading edge slats Leading edge slots What does an impending stall feel like on a sweptback airplane? Reduced control effectiveness Feels mushy How does a swept wing react in low vs high speed flight? Low speed - Less lift, wingtip stalls first, reduced control effectiveness High speed - lower drag at higher speeds, more efficient What's the advantages of a straight wing vs a swept wing? Straight wings fly better at slower airspeeds Designed to stall at the wing root first (better aileron control approaching stalls) What does a wing look like from the side What is a boundary layer? It is the layer of air adjacent to the airfoil surface which adheres to the wing due to surface friction. What is laminar flow? The smooth, uninterrupted flow of air over the wings, fuselage and other parts. important factor in flight What is Reynolds number? predicts whether flow is laminar or turbulent If you are in a yawing and rolling motion, what condition are you in? Dutch roll What is Dutch roll? Tendency for sweptback wings to roll whenever it yaws, and yaw whenever it rolls. When the plane rolls in one direction, it yaws in the other direction What causes Dutch roll? When the aircraft yaws, the wing that yaws forward receives more chordwise airflow which increases the lift of that wing which increases induced drag and yaws the nose towards the wing. Then the other wing yaws forward, increasing the lift/drag, yawing the nose back. Condition repeats over and over How do you recover from Dutch roll? Yaw damper. If damper is inop, follow procedure for that aircraft. Avoid turbulence and slow down Most modern jets will fly themselves out of a Dutch roll How does a yaw damper work? Sensors automatically apply small rudder inputs to counteract Dutch roll How many ailerons do swept wings have? There are outboard and inboard ailerons