8.2 Flashcards
In what region is the air incompressible?
Subsonic region
What does the continuity equation state?
The speed of the airflow is inversely proportional to the area of the cross section of the tube as long as the density remains constant.
What term is used when the diameter increases for the continuity equation?
Diffuser outlet.
What term is used when the diameter decreases for the continuity equation?
Jet outlet.
What is Bernoulli’s principle?
The total pressure is always the sum of static pressure and dynamic pressure.
What happens to the static and dynamic pressure when the valve is open?
Static pressure decreases
Dynamic pressure increases.
How does the dynamic pressure change with a change in speed?
Changes with the square of change in speed.
What results in lift?
Difference in pressure.
What is an up-wash?
When some streamlines approach the profile in a low position slope upwards in front of the wing.
What is down-wash?
When some streamlines of the upper surface flow downwards when leaving.
What is the Magnus effect?
When a body is rotating and it creates an increase of speed on the upper surface and a decrease in speed on lower surface
(Something rotating creating lift)
What is the profile on a wing?
The cross section.
What is the chord line on a wing?
A straight line connecting the leading edge and the trailing edge.
What is the mean chamber line?
A line drawn halfway between the upper and the lower surface of the wing
What is the chamber of the wing?
The displacement of the mean chamber line from the chord line.
(Quantities expressed as a fraction or % of basic chord dimension)
What would a typical low speed profile have a maximum camber loft?
5% located 45% aft of the leading edge.
What is the maximum thickness of a profile as a fraction also known as?
Fineness ratio
What is the flight path velocity?
Speed of the aircraft in a certain direction through the air.
What is the relative wind?
Speed and direction of the air acting on the aircraft which is passing through it.
(Opposite in direction to the flight path velocity)
What is the angle of attack?
The angle between the chord line of the profile and the relative wind.
What is the angle of attack denoted by?
Alpha.
What is the angle of incidence?
Angle between the chord line and the longitudinal axis of the aircraft.
What is the angle of incidence denoted by?
y (gamma)
What is the maximum thickness of low speed profile?
18% of chord line.
Located 30% aft of the leading edge.
What is the wing area?
The projection of outline on the plane of the chord. Includes area of fuselage between the wings.
What is the formula for wing area?
Wing area = wing span (b) x chord of the wing (c)
Wing span x average chord of the wing (C)
How is the taper ratio (lambda) of a wing found?
Tip chord/ root chord
What is the aspect ratio?
Wing span / average chord
Wing span squared / wing area
What are typical aspect ratios?
35 for high performance sail plane
3.5 for jet fighter plane.
What is the sweep angle?
The angle between the line of 25% chords and a line perpendicular to the root chord.
(Positive sweep = backwards)
(Negative sweep = forwards)
What is the dihedral angle of the wing?
Formed between the wing and the horizontal plane passing through the root of the wing.
What is the aerodynamic force?
The resultant of all forces on a profile in an airflow acting on the centre of pressure.
What is the centre of pressure?
The point on which all pressure and forces act.
What does the aerodynamic forces of lift and drag depend on?
Dynamic pressure
Surface area of profile
Shape of profile
Angle of attack
What is the formula for theoretical lift?
Theoretical lift = 1/2 x air density x velocity squared x surface area
Or
Theoretical lift = dynamic pressure x surface area
How do you measure the actual lift?
Using a wind tunnel
What scales are attached to a universal joint in a wind tunnel?
Horizontal scale measuring drag
Vertical scale measuring lift
What is the coefficient of lift?
CL
Account for difference between measure lift and theoretical lift
What is the formula for coefficient of lift?
CL = measured lift / theoretical lift
What is the lift equation?
Coefficient of lift x dynamic pressure x surface area
How is the coefficient of drag measured?
Drag / theoretical drag
When does a stall occur?
When the airflow cannot follow the upper surface of the profile and there is an airflow separation.
What is an advantage of a high maximum lift coefficient?
Aircraft can fly slowly
What is a disadvantage of a high maximum lift coefficient?
The thickness and camber necessary for profiles may produce high drag and low critical Mach number.
What does upper surface frost do to the maximum coefficient of lift and maximum angle of attack?
Reduces it.
Who invented the method of evaluating wind tunnel tests?
Otto Lilienthal
What does a polar diagram plot?
The lift and drag coefficients
What is the lift-drag ratio?
Lift/Drag
When an aircraft is in a steeper climb, what must happen to thrust?
Must be increased
What must the thrust equal to maintain a constant speed on climb?
Not only the drag but also the rear wards acting weight W Sin 0
When climbing, will the lift generated be less or more than the aircraft weight?
Less than.
Must increase thrust
What are the 3 categories of drag?
Induced drag
Parasite drag
Compressible drag
What is induced drag?
The drag on the wing caused by lift.
What is parasite drag?
Not related to lift. Can cause drag caused by distribution of pressure, friction drag caused by skin friction, or interference drag caused by aerodynamic interference.
What is compressible drag?
Caused by shock waves on an aircraft approaching the speed of sound.
What does the turbulence at wing tips form?
Vortices
What do vortices at wing tips cause?
Drag and a loss of lift.
Induced drag
What is a bound vortex?
If a wing span is infinite the circulation around the profile causes an upward wash on leading edge and down wash on trailing edge
On a finite wing span, what vortices do we have?
Wing tip vortices and the bound vortex.
What is the induced drag affected by?
Aspect ratio, wing tip design and aircraft speed.
What aircraft use wing tip fences and winglets?
Small aircraft- wing tip fences
Large aircraft- winglets
What type of drag is form drag?
Parasite drag caused by pressure distribution on body.
What would an airflow with friction cause?
More drag, pressure at front of cylinder is not the same as the pressure behind the cylinder.
How does the shape of the nose affect drag?
Disc form- very high drag
Bullet nose- drag decreases to 20%
Bullet nose with streamline tail- decreases to less than 10%
What does high form drag equal to in form of fiction drag?
High form drag = low friction drag
What does the ratio for friction drag range from?
1 at top and 10 at bottom.
What is the retarded air called?
Boundary layer.
What does retarded airflow mean?
Surface of aircraft is quite rough and velocity of some trapped air particles is reduced to zero.
What are the 2 basic types of boundary layer?
Turbulent boundary layer
Laminar boundary layer
What is the laminar boundary layer?
Immediately downstream of leading edge.
Air particles do not move from one layer to another.
What is turbulent boundary layer?
Downstream of laminar boundary layer.
Laminar flow breaks down and we get turbulent flow.
(Air particles travel from one layer to another and produces energy exchange)
What is the thickness difference between turbulent and laminar boundary?
Turbulent boundary much thicker and produces 3 times more friction drag.
Which boundary layer produces more kinetic energy?
Turbulent boundary layer next to surface and reduces tendency for flow separation.
Why is the area of the laminar boundary kept smooth and clean?
Because small disturbances in the laminar boundary bring it into the turbulent boundary or produce flow separation.
What boundary has the lowest static pressure?
Laminar boundary layer between the leading edge and the point of maximum thickness.
When does the air particle lose energy due to friction?
As it enters the turbulent boundary layer after the point of maximum thickness.
What does the slot do?
Helps to regenerate the flow of air from low pressure to high pressure at the trailing edge.
What does the slot prevent?
Flow separation.
What friction would a profile with a greater low drag laminar region have?
Lower friction.
Known as a laminar profile
What is the interference drag?
The sum of all drags from the wing, strut and engine is greater than the drag of all individual components.
What can interference drag be reduced by?
Fairings.
When does compressible drag only occur?
Transonic and supersonic.
What is compressible drag caused by?
Shock waves on aircraft approaching speed of sound.
What is compressible drag sometimes called?
Wave drag.
What is a Lillenthal diagram?
Graph plotting the co-efficient of lift against of coefficient of drag for each angle of attack.
Best glide ratio can be found.
What is the advantage of a thicker wing?
Raises coefficient of lift at higher angles of attack and raising alpha max.
What are the 3 main categories of parasite drag?
Form, friction and interference.
What is an advantage of a cambered wing?
Increases coefficient of lift at low angles of attack.
What speeds is induced drag highest?
Low speeds
At What speeds does parasitic drag increase?
Parasitic drag increases with increase in speed.
What does a high down wash produce?
Low local angle of attack.
When does an elliptical wing stall?
Stalls at the same time everywhere.
When does a rectangular wing start to stall?
At the root.
Tip last.
What stalls first on a tapered wing?
Tip before the root.
What stalls first on a swept wing?
Tip section first.
Dangerous implications for the lateral control and stability of the aircraft
What wings are used on most aircraft?
Swept wing.
What is geometric twisting?
Designing the wing so that the root stalls before the tip
Angle of incidence greater at root than tip, camber remains constant
What is aerodynamic twisting?
Camber at the root is greater than at the tip.
Angle of incidence is constant.
What is MAC?
Mean aerodynamic chord..
(Chord line through the centre of lift)
(Approx 30% centre of lift)
When is the aircraft in level flight?
When the position of the centre of lift is in the same position of the centre of gravity and no aircraft rotation.
When the position of the centre of the lift moves forward of the position of centre of gravity, what happens to the nose?
Nose up reaction.
When there is flow separation at the root of the wing, where does the centre of lift move to?
Towards the tip and behind the centre of gravity.
Nose down, airspeed increases and angle of attack decreases
What stall does the aircraft recover from the stall without pilot input?
Wing root stall.
What is wing tip stall?
Flow separation at tip of wing, centre of lift moves towards the root and forward of centre of gravity.
(Nose up)
What is a device on smaller aircraft which prevents the wing tip form stalling first?
A stall strip
Mounted at leading edge of root of wing
What is a disadvantage of the stall strip?
Disturbs the lift.
What are used to prevent wing tip stall on some larger aircraft?
Slats
Where are slats located?
Leading edges of wing tips.
What happens at the point of stagnation?
Speed of airflow falls to zero and static pressure equals total pressure.
What is a spanwise flow?
Air particles splitting, some at right angles to leading edge and others follow leading edge
What effect does a spanwise flow have on the boundary layer towards the wing tip?
Thickening it, especially during low speed flight.
What reduces the effect of spanwise flow?
Wing fences.
Or saw tooth leading edge
What does a vortex generator used for?
Improve boundary layer control.
What is a vortex generator?
Small, low aspect ratio wing placed vertically on surface of large wing.
Produces lift.
