1. Advanced, High Speed AerodynamicS Flashcards
What is Mach number
The ratio of an aircraft true air speed to the local speed of sound
What are the main factors affecting the coefficient of lift
Angle of attack, cleanliness of wing surface, wing shape in plan form and cross section, local speed of sound
What is the critical Mach number (MCRIT)
The free stream Mach number at which Sonic flow is reached at any point on the aircraft
Speed of sound at sea level Isa conditions
661kt (38.94√15+273)
How does the speed of sound change with an increase in altitude
Reducing speed of sound with reducing temperature increases Mach number
What is detachment Mac number of an aircraft
The mach free stream at which the bow shockwave attaches to the leading edge of the wing
What is the only factor that affects the speed of sound through the air
Temperature. LSS proportional to T
What is the relationship between local speed of sound and temperature? Proportional or in proportional?
Proportional
How is air modified as it flows through a contracting nozzle at supersonic speed
Velocity reduces, temperature, pressure and density all increase
How does true airspeed vary in the climb at a constant Mach number in Isa conditions
Constant Mac, reducing true airspeed.
What are speed transition and the crossover altitude
Crossover altitude is the altitude at which the process of speed transition occurs, where reference to speed moves from IAS to MaCH number in The climb, and from MACH number to IAS in The descent
What are the points A B and C on the Drag curve
MCRIT, Mcdr, M1.0 (max Drag)
What is the lambda foot?
The shape of the base of the wings upper shockwave caused by high pressure in the shockwave creeping forward into the subsonic boundary layer.
Where does MCDR occur?
At a slightly higher Mach number than MCRIT, when the upper shock Wave forms, increasing coefficient of drag significantly
What does the formation of a definite shockwave on the upper surface of the wing indicate?
That Mcrit has been exceeded.
Advantages of a supercritical aerofoil section?
Delayed Mcrit due to reduced acceleration on top surface.
Increased Gap between Mcrit and Mcdr due to delayed shock wave formation
Increased high-speed cruise efficiency and still relatively high coefficient of lift Max for slow flight
Advantages of a swept wing
Higher MFS Mcrit
Delays shockwave formation, Mcdr
Disadvantages of swept wing
High stall angle of attack
High nose attitude due to increased induced drag (downward vortices)
High drag at high angle of attack
Higher chance of wing drop stall (tip stalling)
Forward movement of centre of pressure as tips stall, causing nose up pitch.
Nose up pitch can deepen, causing deep stall on t-tail aircraft
What design features result in a higher Mcrit
Sweepback, supercritical wing, low thickness to chord ratio
Design characteristics of a supercritical aerofoil section?
Flat top of wing, reflex Camber in rear third of wing, relatively high thickness to chord ratio to provide space for carriage of fuel.
Advantages of a low thickness to chord ratio wing
Coefficien of drag is less at stall, Mcrit and Mcdr both delayed (Faster TAS)
What is mach tuck
Entering transonic phase of flight, as shockwave form, further and further rearwards, centre pressure moves aft, thus creating a nose pitch down moment.
How does the ad bridge centre of pressure of a swept wing aircraft move at the point of stall?
Due to wing tips stalling first, centre of pressure moves forwards, creating a nose up moment, further deepening the stall
Advantages of wing sweep
Higher Mac free stream for Mcrit (V1 vector less than IAS, delayed Mcdr- meaning shockwave formation is delayed
Disadvantages of wing sweep
Low coefficient of lift Max, low coefficient of lift at any angle of attack, high angles of attack to produce lift, therefore greater drag to produce given coefficient of lift, Dutch roll, greater chance of wing drop stall due to tip stalling, deep stall from reduction of downwash on tail plane
How is air affected as it passes through a shockwave?
Above the wing is a converging duct, so velocity of air reduces at supersonic, raising temperature density and pressure
Design features that lead to a greater Mcrit
Low thickness to chord ratio wing, supercritical wing, wing sweepback
