4 - Application of Aerodynamic Forces (Subsonic) Flashcards
What is thickness? What effect does it have on lift and drag?
Small thickness/chord ratio
- low CL, low Cdin
High thickness/chord ratio
- high CL, more downwash, high Cdin
Thickness effect on CL, CD, Stall. Why?
Thin - pressure difference between upper and lower surface is relatively small, downwash is small. Vorticies caused by pressure differential will be smaller so Cdin will be smaller, also has less surface area than a thick aerofoil so Cdskin will be smaller. According to the circulation of lift theory a small downwash indicates lower circulation so the CL will be relatively smaller. Stall will occur at a lower AOA and at a lower CLmax.
Thick - pressure differential greater, more downwash, higher Cdin, higher circulation, higher CL, particularly when AOA is high.
Stall is caused by early BL separation, one condition, adverse pressure gradient, will occur later on the thicker aerofoil than a thinner one if they are set at the same AOA because the larger downwash changes the direction of the RA and decreases the effective AOA of the thicker aerofoil.
What is AR? What is its effect on the aerofoil?
AR = span/chord
Large AR - reduces wingtip/trailing edge vortices, reducing Cdin (but requires high structural strength)
Small AR - higher induced drag, deflection of RA leads to a higher stall aoa
AR effect on CL, CD, Stall. Why?
Low AR - large span wise component (3-d), stronger vortices, more induced drag. Higher stall AOA (reduced effective AOA from larger downwash)
High AR - smaller span wise component (closer to 2-d), weaker vortices, less induced drag.
High AR higher CL than Low AR (for same thickness and camber), Low AR produces stronger downwash, drawing the RA closer to the wing, tilting the lift vector rearward and reducing the effective AOA and the magnitude of the “true lift” produced.
What are the characteristics of a laminar aerofoil? What is their purpose?
Thin leading edge, keeps laminar air flow as far back as possible and ensures a gentle change in air pressure over the aerofoil. Laminar flow produces less skin drag than turbulent as the rate of change of the speed perpendicular to the surface is less.
Point of max camber far back, makes adverse pressure gradient located toward the rear of the aerofoil, delaying BL separation and increasing stall AOA. Low form drag.
What design features can be used to delay or prevent BL separation?
Suction near TE - suck stagnant particles, adds weight to a/c
Blowing away - jet of air released over the wing to push the stagnant air particles toward the TE
Vortex generators/Vortilons - produce micro vortices to transfer more kinetic energy from the main flow to the BL layer. Will increase skin drag but delay BL separation and increase stall aoa.
LE slot - increases the kinetic energy in the BL
MEMs (blowing away)
Shape of wing tips (winglet, tank, fence)
Washout
Multi element aerofoils (high CL devices, movable flaps)
Camber effect on CL, CD, Stall. Why?
Cambered aerofoil will produce a higher CL (CL > 0) at the same aoa as a symmetrical aerofoil (CL = 0)
Cambered - higher circulation at the same aoa as symmetrical so CL higher. Higher CL, lower airspeed required to produce the same amount of lift so stall speed lower. Decrease in stall AOA, with increasing camber the adverse pressure gradient after the max camber becomes stronger, a strong adverse pressure gradient encourages early BL separation (early stall).
Higher camber, more 3-D airflow, more circulation, more downwash, higher Cdin.
How does the coefficient of skin drag change?
Cdskin is the function of Re, but varies with the regime of the boundary flow. In the laminar BL Cdskin decreases with an increase in Re, Cdskin is higher for the turbulent BL than the laminar BL for the same Re. (change u/change y) turbulent > (change u/change y) laminar. Aka higher viscous friction.
Where does min drag occur?
Where parasite drag is equal to induced drag, is also where L/D is maximum
Where is Vmp in relation to Vmd
Lower airspeed, Vmp is not necessarily the point of min drag is its where power to TAS is greatest
What are the types of Pa?
Brake horse power - engine produced power at crankshaft
Shaft horse power - power delivered to the propellor
Thrust horse power - power to produce thrust (THE POWER AVAILABLE)
What is propellor efficiency?
nProp = THP/SHP
How does Pa change with alt for a normally aspirated engine?
produces 50% of full sea level power at 19,000ft
What gives the speed limits for turbojet aircraft?
Ta and Tr
What is the absolute ceiling?
The altitude at which there is no excess power, level flight can only be maintained at one airspeed, and the ROC is nil
What is the service ceiling?
The altitude at which the ROC Vc < 0.5m/s or some other specified rate (<2.5m/s jet)
