Aerodynamics.2 Flashcards
Ground effect alters…
Wing up wash, downwash, wing tip vortices
Reduction in wing tip vortices due to ground effect alters…
Spanwise lift distribution and reduces induced drag from AOA. Lower AOA for same CL
As to thrust, ground effect causes…
Less thrust needed for speed due to reduced induced drag. It can also cause change in local pressure at the static source producing lower indication of airspeed and altitude
Percentage of drag reduction based on wing distance to the ground
Equal to wing height - 1.4%
1/4 wing height - 23.5%
1/10 wing height - 47.6%
Ground effect during take off…
- require an increase in AOA to maintain the same CL
- Experience an increase in induced drag and thrust required
- Experience a decrease in stability and a nose up change
- Experience a reduction in static source pressure and increase in indicated airspeed
What is a Moment
Measure of aircraft’s tendency to rotate about its CG. Equal to the product of the force applied and the distance at which the force is applied. Moment arm is the distance from a reference point to the applied force.
Stability is…
Inherent quality of aircraft to correct for conditions that disturb it’s equilibrium
Static stability is…
Initial response when disturbed from given pitch, yaw, or bank
Positive, negative, neutral
Dynamic stability is…
Aircraft response over time when disturbed from a given pitch, yaw, or bank
Positive, neutral, negative
Maneuverability
Quality permitting easily maneuvers and withstand stresses. Governed by weight, inertia, size, location of controls, structural strength, powerplant
Controllability
Respond to pilot’s control, flight path and attitude. Regardless of stability characteristics
Longitudinal stability
Quality that make aircraft stable about its lateral axis
- location of wing with respect to CG
- location of horizontal tail surface with respect to CG
- Area or size of tail surfaces
Center of lift tendency to change…
Its fore and aft positions.with a change in AOA. Tends to move forward with AOA increase, backward with decrease
Typical location of Center of Lift = Center of pressure
Behind CG to make aircraft slightly nose heavy. Requires horizontal stabilizer at slight negative AOA to balance
Longitudinal stability in flight…
Downwash of wing pushes onto horizontal stabilizer even if level. Decreased speed, decreased pressure on stabilizer, nose dips forward and picks up speed, pushes stabilizer down again
Thrust line for longitudinal stability…
Above CG pulling plane slightly down when accelerating
Lateral stability design factors
Dihedral (wing tips higher than roots), sweepback, keep effect, weight distribution
Sweepback for longitudinal stability
1) Move center of pressure towards rear
2) When yawing, forward wing perpendicular to airflow, airspeed increase, more drag than back wing, pulls wing back, plane back to original path
Dutch roll
Lateral/directional oscillation, usually dies out automatically
Spiral instability
Strong directional stability as compared to dihedral effect - detail unclear
Can be easily corrected.
Tricky when intense spiral, pulling elevator makes spiral tighter, airspeed faster
Wing planform 3 ratios
Aspect - wing span to wing chord
Taper - decrease from root to tip in thickness or chord, decrease drag, increase lift
Sweepback - rearward slant
Changing aspect ratio
Increase (increase span and weight) with constant velocity will decrease drag, improve climbing
Decrease causes increase in drag
Turning
Vertical component
Horizontal component
Centrifugal force
Total lift
Level turn requires
Increase in thrust due to increase in induced drag due to increased angle of bank which causes reduction of lift. Required thrust proportional to angle of bank
Slipping turn
Banked too much for the ROT, horizontal lift greater than centrifugal force
Decrease the bank or increasing the ROT
Skidding turn
Excess of centrifugal force. ROT too great for angle of bank
Need to reduce ROT, or increase bank
Stabilized climb requires
Thrust equal to drag plus percentage of weight. Aircraft uses excess thrust to maintain climb
Wing designed to stall…
Wing root first, to keep aileron effective
Wing twist design with root at higher AOA or using stall strips
Unstalling
CL is aft CG, nose dips after stall reducing AOA
Stall AOA
Constant for a particular aircraft independent of airspeed, weight, etc
Between 16-20•
Wing icing impacts…
Disrupts boundary layer
Increases drag
Reduces lift
Propeller: pitch vs blade angle
Blade: between chord line and plane of rotation
74-48 = 74in long, 48in effective pitch
Propeller thrust
Equals mass of air handled multiplied by slipstream velocity minus aircraft velocity. Thrust about 80% of torque, 20% lost in friction and slippage
Propeller slip
Difference between geometric pitch of propeller and effective pitch
Geometric pitch based on no slippage
Twisted propeller
Outer part travels faster, different AOA
Twisted to change blade angle in proportion to differences in speed of rotation along length of prop, keeping thrust equal
Constant speed prop
Take off: low blade angle, AOA small, smaller mass of air, engine at high rpm
After liftoff, higher pitch, keep AOA small efficient, increase mass of air per revolution
After climb, reduce power, increase blade angle
Torque 4 elements
From engine and prop
Corkscrewing of slipstream
Gyroscopic prop action
Asymmetric loading of prop
Torque reaction
Reaction to action of prop spin, cause roll tendency, left yawing on ground Counter with wing or engine offset Depends on Size and hp of engine Size and rpm of prop Size of plane Ground surface condition
Corkscrew effect
Rotating slipstream, sideward force on tail, yawing to the left, rolling to the right
Gyroscopic action
Precession is resulting action of a spinning rotor when a deflection force is applied. Resulting force takes effect 90• Ahead of and in direction of rotation
Asymmetric loading, P factor
High AOA flying, Prop down more bite/speed/lift than prop up causing left yawing around vertical axis (helicopter Example)
Load factor important for two reasons
Overload aircraft structure
Increase stall speed
Different load factors
Gust load factor
Maneuvering load factor
Load limit factor
Ultimate load
Load factor in steep turns
Exponential increase after 45•
60• = 2G
80• = 5.76G
Stalling speed increases…
In proportion to square root of load factor
50knots regular stalling speed
100knots at 4G
Design maneuvering speed VA
Move single flight control one time full deflection for one axis of rotation only in smooth air without risk of damage
Entered in AFM/POH
Vg Diagram
Velocity vs load factor
Each aircraft has its own
Lines of maximum lift capability - stalls above that line
Intersection of positive limit load factor and line if max positive lift capability - minimum airspeed for limit load
ROT formula
= (1,091 x tangent of bank angle)/knots
Radius of turn formula
R = v2 / (11.26 x tangent of bank angle)
Or
R = (speed fps x 360/ROT)/Pi/2
CG position influences…
Lift and AOA and force on the tail
Forward CG stalls at…
Higher speeds due to increased wing loading
Aft CG aircraft cruises…
Faster because of reduced drag due to smaller AOA, less downward deflection of stabilizer
CG moved rearward…
Less stabile due to decrease in AOA, wing contribution to stabilize decreases until neutral stability, then unstable
CG moved forward…
Increases need for greater elevator pressure, may not be able to opposes nose-down pitching
Aircraft speed regimes
Subsonic - below 0.75 M
Transonic - 0.75-1.20
Supersonic - 1.20-5.00
Hypersonic - above 5
Critical Mach number
Speed at which some part of airflow reached M 1.0
Drag divergence
5-10% above Mach Crit compressibility starts causing drag rise impacting buffet, trim, stability, control effectiveness
Max operating speed limit
Vmo = lower altitudes, structural loads and flutter Mmo = higher altitudes, compressibility, flutter
KIAS, KCAS, KTAS calculation?
??
Boundary layers
Laminar
Turbulent
Separation
Shock wave or compression wave
Boundary between undisturbed air and region of compressed air
Supersonic airstream passing through normal shock wave
Airstream is slowed to subsonic
Airflow behind shockwave does not change
Static pressure and density of airstream behind wave is greatly increased
Energy of airstream greatly reduced
Wave drag
Shock wave causes drag due to dense high pressure region behind wave
Drag from airflow separation
Mach tuck
CP move aft, diving moment is produced, if it moves forward, a nose-up movement
Reason for T tail
Sweepback for Mach
Delays onset of compressibility effects
Increase in critical Mach number, force divergence mach number
Force divergence Mach number
Number producing a sharp change in coefficient of drag
Exceed critical Mach number by 5-10%
Sweepback disadvantages
Stalls at wing tips rather than roots
Boundary layer flows spanwise
Causes CL to move forward causing nose to rise
Aggravated by T tail
Stick pusher and stick shaker
Push stick forward to prevent stall
Shaker at 5-7% above stall speed
Mach buffet occurs…
High altitudes
Heavy weights
G loading
Variable incidence horizontal stabilizer
For jet needing large pitch trim changes
Larger than elevator, leaving elevator with full range of motion
