DEFINE the boundary layer
- That layer of airflow over a surface that demonstrates local airflow retardation due to viscosity.
- It is usually no more than 1mm thick at the leading edge of an airfoil, and grows in thickness as it moves aft over the surface.
- It comprises of laminar flow and turbulent flow.
DESCRIBE the different types of flow within the boundary layer
- Laminar flowNear the leading edge.
- Air moves smoothly along in streamlines.
- Produces very little friction, but is easily separated from the surface
- Turbulent flow
- aft of the laminar boundary.
- airflow is disorganized and irregular
- Produces higher friction drag than a laminar boundary layer but adheres better o the upper surface of the airfoil, delaying boundary layer separation.
DESCRIBE boundary layer separation
- Laminar Boundary layer flows aft and becomes the turbulent boundary layer.
- Air flows aft from the leading edge (high static pressure) of the airfoil towards the point of maximum thickness (low static pressure) resulting in a favorable pressure gradient assisting the boundary layer in adhering to the surface by maintaining its high kinetic energy.
- As the air flows aft from the point of maximum thickness (lower static pressure) toward the trailing edge (high static pressure), it encounters an adverse pressure gradient which impedes the flow of the boundary layer.
- The adverse pressure gradient is strongest at high lift conditions and at high angles of attack in particular.
- If the boundary layer does not have sufficient kinetic energy to overcome the adverse pressure gradient, the lower levels of the boundary layer will stagnate and separate from the surface, and airflow along the surface aft of the separation point will be reversed.
- Aft of the separation point, the low static pressure that produced lift is replaced by a turbulent wake.
DEFINE CL MAX AOA
- The angle of attack beyond which CL begins to decrease.
- An increase in AOA above the CL MAX AOA will result in a decrease in CL.
- AKA the stalling angle of attack or critical AOA.
a condition of flight in which an increase in AOA results in a decrease in CL.
EXPLAIN how a stall occurs
- When AOA is increased above CL MAX AOA.
- The AOA is increased to a point where the separation point progresses forward, and the airflow can not conform to the sharp turn of the wing.
- The only cause of a stall in excessive AOA.
- The point where stall occurs is dependent upon AOA and not velocity.
IDENTIFY the aerodynamic parameters causing a stall
AOA above the CL MAX AOA
COMPARE power-on and power-off stalls
- Power-off - engines are idle.
- Power-on - stall speeds are less than power-off stall speed because at high pitch attitudes, part of the weight of the airplane is actually being supported by the vertical component of the thrust vector.
DESCRIBE the order of losing control effectiveness approaching a stall in the T-6B
Progresses from ailerons to elevator to rudder
EXPLAIN the difference between true and indicated stall speed
- True stall speedAs angle of attack increases, up to CL MAX AOA, true airspeed decreases in level flight.
- Since CL decreases beyond CL MAX, true airspeed cannot decrease any further.
- The minimum airspeed required for level flight occurs at CL MAX AOA.
- Indicated AirspeedBy substituting the stall speed equation into the true airspeed equation and solving for indicated airspeed, we derive the equation for the IAS.
- P0 is constant.
True stall speed will increase as altitude increases, but indicated stall speed will remain the same since P0 is constant.
EXPLAIN the effects of gross weight, altitude, load factor and maneuvering on stall speed, given the stall speed equation
- As airplane weight decreases, stall speed decreases because the amount of lift required decreases.
- Flying at higher altitudes requires flying at higher velocity (TAS) to produce the required lift. Stall speed will increase.
- However, since P0 is constant, indicated stall speed will not change as altitude changes.
- As load factor increases, Stall speed increases.
- As maneuvering increases, stall speed increases.
STATE the purpose of using high lift devices
- High lift devices increase CL at high AOA.
- Primary purpose of High lift devices is to reduce takeoff and landing speeds by reducing stall speed.
- The increase in CL allows a decrease in airspeed.
DESCRIBE how different high lift devices affect the values of CL, CL MAX, and CL MAX AOA
- Boundary Layer Control Devices:
- Causes airflow separation to be delayed to an AOA higher than normal stalling.
- No change to CL.
- Increase CL MAX.
- Increase CL MAX AOA
- Camber Change Devices:
- Used to increase the camber of an airfoil.
- Increase CL.
- Increase CL MAX.
- Decrease CL MAX AOA.
DESCRIBE devices used to control boundary layer separation
- allow the high static pressure air beneath the wing to be accelerated through a nozzle and inject into the boundary layer on the upper surface of the airfoil. The extra kinetic energy allows the turbulent boundary layer to overcome the adverse pressure gradient and adhere to the airfoil at high AOAs.
- Two types of Slots:
- -Fixed Slots
- Gaps located at the leading edge of a wing that allow air to flow from below the wing to the upper surface. Very efficient and causes only a small increase in drag
- -Automatic slots
- Consist of a movable vane (slat) attached to the leading edge of the wing that moves away from the body of the wing to allow airflow from below the wing to reach the upper surface to delay separation.
- The vane used in a slot, especially in an automatic slot. Forms a slot when deployed.
Since slats and slots on their own effect no change in camber, there is no change to CL at low AOA. The higher value of CL MAX is achieve at a higher AOA.
- Vortex Generators
- Small vanes installed on the upper surface of an airfoil to disturb the laminar boundary layer and induce a turbulent boundary layer (which adheres better and delays separation).
DESCRIBE devices used to change the camber of an airfoil
- Flaps are used to increase the camber of the wing in order to increase CL and CL MAX.
- Trailing Edge FlapsThe most common type of high lift device.
- Used to increase the camber by adjusted the trailing edge of a wing.
- 4 types:
- -Plain flap: a simple hinged portion of the trailing edge that is forced down into the airstream to increase the camber.
- -Split flap: a plate deflected from the lower surface of the airfoil. This type of flap creates a lot of drag because of the turbulent air between the wing and deflected surface.
- -Slotted flap: similar to the plain flap, but moves away from the wing to open a narrow slot between the flap and wing for boundary layer control. It may cause a slight increase in wing area, but the increase in lift is insignificant.
- -Fowler flap: used extensively on larger airplanes. When extended, it moves down, increasing the camber, and aft, causing a significant increase in wing area as well as opening one or more slots for boundary layer control. Because of the larger area created, a large twisting moment is developed, requiring a structurally stronger wing and precludes their use on high speed, thin wings.
- Leading Edge Flapschange the wing camber at the leading edge of the airfoil.
- Leading edge plain flaps are similar to trailing edge plain flaps.
- Leading edge slotted flaps are similar to trailing edge slotted flaps, and are sometimes confused with automatic slots.
DESCRIBE methods of stall warning used in the T-6B
The T-6B uses AOA indexer and stick shakers that receive their input from an AOA probe on the left wing. The stick shakers are activated at 15.5 units AOA, followed by air frame buffeting.
DESCRIBE the stall tendency of the general types of wing planforms
Rectangular - strong root tendency
High Taper - strong tip tendency
Swept - strong tip tendency
Elliptical - Even distribution of lift from root to tip and produces minimum induced drag.
- Moderate taper - Even stall progression similar to the elliptical
DESCRIBE the various methods of wing tailoring, including geometric twist, aerodynamic twist, stall strips, and stall fences
- Geometric Twist: A decrease in angle of incidence from wing root to wingtip.
- Aerodynamic Twist: A gradual change in airfoil shape that increases CLMAX AOA to a higher value near the tip than at the root.
- Stall Fences: redirect the airflow along the chord, thereby delaying tip stall and enabling the wing to achieve a higher AOA without stalling.
- Stall Strip: Mounted on the leading edge of the root section to induce a stall at the wing root.