Aerodynamics Flashcards
What is an aerofoil?
A body that gives a large lift force compared with its drag when in a moving airstream.
What is an aerofoil chord line?
A line from the leading edge to the trailing edge of an aerofoil.
What is a mean camber line?
A line of equidistance from the leading edge to the trailing edge of an aerofoil.
What is the angle of incidence?
The angle between the chord line and a longitudinal datum on the aircraft.
What is the angle of attack?
The angle between the chord line and the relative airflow.
What is wing washout?
Where there is a decrease in the angle at the chord of the wing route to the wingtip.
What is dihedral?
An upwards inclination of the wing.
What is anhedral?
A downwards inclination of the wing.
What is lift?
Lift is the force generated by an aerofoil due to pressure differences above and below when passed through moving air.
What is coefficient of lift?
This is the lifting ability of a wing and is dependent on the surface area of the wing and the chord as well as the angle of attack.
Describe the CoP
CoP is the point where the wings lifting force is produced and is at right angles to the relative air flow.
What happens to the CoP as you increase the AoA?
It moves towards the leading edge of the wing.
Describe aspect ratio?
AR is the ratio of the wingspan to the geometric chord.
During what phase of flight is lift the greatest?
Generally during take off due to optimised AoA at this stage of flight. It is affected by: Flaps Speed of the airflow over the wing Angle of attack Air Density
What is a direct lift control?
Elevator / Stabilator which creates an upwards or downwards balancing force determining the attitude of the aircraft about its lateral axis.
What are high lift devices? (3)
Trailing edge flaps (Fowler Flaps - increase area of the wing) Leading edge flaps (Krueger Flaps - Create a longer wing chord line / camber / area) Slats and slots - Used to re-energise the boundary layer and delay separation
What are the different types of Drag?
Parasite: [Skin / Pressure (form)] known as profile drag / Interference drag. Induced: Generated by lift and is associated with high AoA and wing tip vortices.
Describe the drag curve for a piston / propellor aircraft? (Take this as a straight wing)
Well defined steep parasite drag curve as it’s not designed for high speeds. Well defined steep induced drag curve as it has a high CL at lower airspeeds. Well defined Vmd (stable / unstable speeds) with Lower stalling speed than a jet.
Describe the drag curve for a jet aircraft? (Take this as a swept wing)
Shallower parasite drag curve as it’s designed for higher speeds. Shallower induced drag curve as generates a lower CL at lower airspeeds. Flatter Vmd (stable / unstable speeds) with high stalling speed than a straight wing aircraft.
What are high drag devices?
Trailing edge flap (Drag flap) Spoilers: Flight detent = Speed brake / Ground = lift dumpers Landing gear
What causes wingtip vortices?
Spanwise flow of air on the upper and lower wing surface meeting at the wingtips as turbulence and causing induced drag. This is especially true on a swept wing aircraft.
What is the affect of spanwise flow over a wing?
Increased wingtip vortices Reduced aileron efficiency Reverse spanwise flow contributes to wingtip stall
What are the affects of wingtip vortices?
Induced drag / loss of energy Turbulence which may affect the safety of other aircraft up to 1000 feet below you and up to 9 nm behind you The downwash may affect the longitudinal stability of the aircraft
How to you prevent spanwise flow on a wing, especially a swept wing?
The use of wing fences and vortex generators.
What are the purpose of vortex generators?
To re-energise the boundary layer and delay separation in order to maximise the effectiveness of the control surfaces (especially the ailerons).
What are the purpose of wing fences?
These help prevent reverse spanwise flow on the upper surface from reaching the wingtip and therefore reducing the chances of a wing tip stall.
What are winglets and how do they work?
Winglets are aerodynamic efficient surfaces that are placed at the end of the wing in order to help prevent the mixing of the spanwise flow which causes induced drag. Often the upper and lower spanwise flows are dispensed at different points.
What limits an aircrafts structural weight?
The force generated to balance weight is lift. The direct limit of weight is the amount of lift that can be generated by the aircraft. Design Speed Power Airspeed
What are the affects of excess aircraft weight? (5)
Performance: LDR / TORR increased Range / Speed / ROC & ROD decreased Stall speeds increased Manoeuvrability reduced Safety Margins decreased Wear on Tyres and brakes increased
Describe the CoG?
CoG is the point at which all of the aircrafts weight acts through.
Describe a component arm?
This is the distance from a datum that forms a moment arm. Moment = Force x Distance. If nose up then it’s positive If nose down then it’s negative
How is the lift weight moment balanced?
When The lift weight moment is not perfectly balanced, extra forces are provided by the horizontal tailplane to centre the pitching moment.
Describe the centre of gravity range?
This related to the aircrafts most forward and most rearward CoG position along the aircrafts longitudinal axis that the aircraft is permitted to fly. This is so the horizontal tailplane can generate enough force to balance the aircrafts lift-weight couple.
Why does forward CoG need to be kept within limits? (3)
To ensure the aircraft is not too nose heavy and to ensure the tailplane has enough turning moment to overcome the natural longitudinal stability. To ensure that there is enough controllability without high stick forces at low speeds (rotating / flaring). To ensure minimum horizontal tailplane deflection in order to reduce drag and maintain performance.
Why does aft CoG need to be kept within limits? (3)
To ensure the aircraft is not too “tail heavy” and to ensure the aircraft remains longitudinally stable. To ensure adequate stick force is felt through the control column when pitching. To ensure minimum horizontal tailplane deflection in order to reduce drag and maintain performance.
What are the affects of a CoG being outside its forward limit? (4)
Longitudinal stability is increased which will reduce controllability of the aircraft A large balancing download from the horizontal stabiliser which will: Increase stall speed Increase drag Reduce performance Reduce range Less elevator trim available In flight minimum speeds are restricted due to the high AoA that is required at lower airspeeds
What are the affects of a CoG being outside its aft limit? (5)
Longitudinal stability is reduced which will mean the aircraft is too “tail heavy” Over controllable which means the airframe could be overstressed A large balancing upload from the horizontal stabiliser which will: Reduce stall speed Reduce drag Increase performance Increase range Less elevator trim available In flight minimum speeds are restricted due to the high AoA that is required at lower airspeeds
Why does a forward CoG increase the stall speed of the aircraft?
Having a forward CoG means that the horizontal tailplane will have to generate a higher downforce. This higher downforce increases the aircrafts effective weight. As weight is a function of the stall speed, the heavier the aircraft the higher the stall speed.
Why does a jet aircraft have a large CoG range?
Typically as a large jet aircraft burns a lot of fuel the CoG will change during flight. In order to compensate for that it has a large horizontal tailplane in order to generate the necessary forces required to maintain longitudinal stability.
What causes CoG movement? (3)
Fuel burn - Most common, also because wings are swept it changes the weight distribution Passenger movement High speed - At higher speeds the aircraft adopts a greater nose down attitude.
How does weight affect the aircrafts flight profile descent point?
The heavier the aircraft, the earlier its required descent point. The heavier the aircraft the shallower it’s rate of descent needs to be and therefore it will be covering a greater distance over the ground. Momentum = mass x velocity.
How would you design a high speed aircraft wing?
Swept Thin Minimal camber
How does swept wing increase M crit?
Since the wing is responsive only to the velocity vector normal to the leading edge, the effective chordwise velocity is reduced. Thus airspeed can be increased before the effective chordwise component becomes sonic.
What is the ideal fineness ratio?
About 3:1
How would you optimise the lift design on a high wing? (3)
Trailing edge flaps (Fowler Flaps - increase area of the wing) Leading edge flaps (Krueger Flaps - Create a longer wing chord line / camber / area) Slats and slots - Used to re-energise the boundary layer and delay separation
Advantages of a swept wing? (3)
As it provides less lift, ironically it is less responsive to turbulence / updrafts / downdraft. Higher Mach cruise speeds. Reduced form drag due to reduced camber and thickness. of the wing.
Disadvantages of a swept back wing? (6)
Increase stall speed / Lower CL Max More violent Mach tuck characteristics Reduced lift qualities Tip stall characteristics Possibility of super stall - needs stick pusher Speed instability below VMD