Transonic Flight Flashcards
What is the transonic flight regime?
- Speed range in which the transition between subsonic and supersonic flow occurs.
- Characterized by the development and movement of local shock waves.
Between what Mach numbers is the transonic regime?
Mcrit at the lower end and Mdet at the upper end
When does positive pressure occur on the lower surface of the wing?
When there is excessive Angle of Attack (AoA)
When does separation of the boundary layer occur for transonic flow?
Immediately behind the shock
What are characteristics of laminar boundary layer?
- Thin
- Slippery
- Low energy
- Cannot negotiate an adverse pressure gradient
What are the characteristics of a turbulent boundary layer?
- Thick
- Draggy
- High energy
What are the characteristics of the separation point?
- Boundary layer detaches
- Wing lift coefficient reduces
- Drag increases
What is the formula for lift?
L = 0.5ClrhoV^2S
Is this formula accurate for subsonic, high subsonic and supersonic airspeeds?
Subsonic
What is the formula not accurate for high subsonic and supersonic airspeeds?
- At high subsonic airspeeds, compressibility begins to take effect and the amount of lift produces is no longer proportional to V^2
- At at supersonic speeds the amount of lift is influenced by upwash and formation and movement of shockwaves.
What occurs to wing upwash as speed increases?
Pressure waves do not propogate as far upwind, so incoming airflow has less warning of the approaching wing and upwash is less.
At low subsonic speeds, describe the upwash on the wing?
- At low subsonic speeds, upwash ahead of LE gives an angle of attack AoA and related Cl
- As flow becomes compressible, upwash occurs closer to the LE which creates a higher effective AoA and subsquent higher Cl
What occurs to upwash at high subsonic speeds?
- At high subsonic compressible flow, inertia of the airflow begins to have an effect by reducing the warning and upwash.
- Airflow deviates less, creates less effective AoA and reducing Cl
What occurs to upwash at supersonic flow?
- At speeds above Mdet(superosnic flow), free stream airflow experiences no warning of the approaching wing and is not deviated
- AoA and Cl are low
What occurs on the wing at Mfs = 0.75 or Mcrit? What occurs on top surface and lower surface? What happens to movement of Cp? What happens to value of Cl?
- Some airflow over the wing approaches M = 1.0, and Mach whiskers begin to form on to surfaces
- No shockwave is formed yet as Mach whiskers hold it off
- Airflow below the wing is also accelerated but not to sonic speeds
- Cp moves forward to around 25% of chord
- Cl increases to about 160% of low speed value for same AoA
What occurs on the wing as Mfs increases to 0.81? Is there a shockwave on bottom surface of wing? How does the lamda foot form? What happens to Cp movement? What happens to Cl compared to low speed value for same AoA?
- A shockwave first begins to form around 70% of chord on upper surface
- Flow not yet sufficient to form shock on bottom surface
- Pressure drop ahead of shockwave is larger
- Adverse pressure gradient behind the shock thickens the boundary layer and forms turbulent flow
- Lamba foot is formed as pressure waves move further forward in boundary layer
- Cp moves rearward to about 30% of chord - nose down pitching moment is created
- Cl nearly doubles and is at its highest point of low speed value for same AoA
What occurs on wing at Mach = 0.89 on upper and lower surfaces? Where is lower edge shock positioned? What happens to Cl value? What happens to Cp movement?
- Upper surface shockwaves remains in place
- Flow on bottom surface has created shockwave on lower surface, which positions at trailing edge due to weaker adverse pressure gradient
- Cl reduces to around 70% of low speed value due to lower pressure on bottom of wing
- Cp moves forward to around 15% chord
What occurs at M = 0.98 on upper and lower surfaces? What is pressure differential like now? What occurs to Cp movement? What occurs to Cl movement?
- Speed of flow is sufficient that both shocks on upper and lower surfaces are at the trailing edge.
- Turbulent flow is significantly reduces and uniform pressure differential is established between upper and lower surface
- Cp is around 45% of chord rearwards
- Cl is around 110% of lower speed value
What occurs at speed greater than Mdet to upper and lower shocks? Pressure distribution over the wing? Cp position? Cl compared to low speed value?
- Bow shockwave has formed at is attached to leading edge
- Upper and lower shockwaves are more oblique and are established on trailing edge
- Uniform pressure distribution is maintained and Cp is around 50% of chord
- Cl stabilizes to 70% of low speed value
For SUBSONIC flow, what are the two types of drag?
1) Zero lift drag
2) Lift Dependent drag (induced drag)
What is zero lift drag and what are the different types?
- Drag due to geometry and structure of aircraft
- Surface Friction, Form and interference drag
What is Lift Dependent drag (induced drag)? What are two types?
- By product of creating lift
- Induces and Trim drag
What are the two types of drag at transonic/supersonic speeds?
Wave Drag which is broken down into Energy Drag and Boundary Layer Separation Drag
What is energy drag and how is it formed? How does energy drag increase with Mfs?
- Energy drag results from the non adiabatic temperature rise that occurs as air passes through a shockwave
- The energy that is added for this temp change comes from aircraft’s kinetic energy which results in drag on the aircraft
- Increases with Mfs
What is boundary layer separation drag and when does it occur? How does it change with increases in Mfs?
- Boundary layer separation drag occurs with the movement of the shockwave
- The thickened boundary layer caused by lambda foot separates from the base of the shockwave and causes a thick turbulent wake
- This turbulence results in increase in drag
- As Mfs increases, the shockwaves move to the TE and lamba foot is reduced which reduces boundary layer separation drag
What occurs to Cd as Mfs increases? Draw the graph of Cd vs Mfs?
- Increases past Mcrit to peak where boundary layer separation drag is greatest
- Reduces to steady state value past Mach 1 as boundary layer drag is no more because of energy drag
What is Mach tuck and what are the two main causes?
- Mach tuck is the nose down pitch experienced by aircraft as they increase in the transonic region.
- Caused by rearward movement of Cp during Transonic regime
- Rearward of Cp reduces downwash on the tail plane which decreases the balancing force and causes the nose to pitch down.
What is wing drop and when can it occur?
- Can occur at Mcrit
- Shockwaves occur at different point on each wing due to minor differences in construction, surface and variation in lift and drag between wings can cause wind drop.
What is transonic/supersonic buffet?
- Turbulent shock induced separation can contact rear control surfaces and cause buffet throughout the airframe.
What can occur to control effectiveness when shockwaves form?
Can be reduced due to:
- Operating in area of turbulent flow and lift is reduced
- Any change that occurs in supersonic flow cannot propagate forward
- Aircraft controls can form their shockwaves which reduce lift and drag over them
What special control surfaces are used for high speed aircraft?
1) All moving tail planes (Stabilators)
2) Combined ailerons and elevators
3) Multiple fins and rudders
What can be fitted to overcome Mach tuck?
Mach trimmers
What occurs to longitudinal stability with the rearward movement of Cp? What happens to maneuverability ?
- Lift Weight couple is increased which increases longitudinal stability
- Maneuverability is reduced and can be fixed by using larger control surcfaces
What is often used to counteract wing drop?
- More complex flight control computers
What occurs to lateral stability at supersonic speeds?
- Less laterally stable due to reduce in Cl on the wing and thus correcting forces are less.
- Mach cones form on the wingtips which define an area of reduced effectiveness.
- Advanced flight control systems can be used to counteract this effect
What occurs to directional stability at supersonic airspeeds? What is used to counteract this?
- Differential shockwaves can create a yawing moment on the aircraft.
- Yaw dampers can be used to counteract this
What is Dutch Roll? What can overcome this?
Airplane is yawing and rolling at the same time when lateral stability exceeds directional stability
- Can be counteracted by increasing directional stability or with a yaw damper
