Module 3 Concepts Flashcards
What are the 4 forces on an aircraft? Where do they originate?
Lift: wings
Drag: Skin, form, interference, induced
Weight: inherent in the aircraft
Thrust: developed by the power-plant
Describe the NACA naming system for aerofoils?
1: Max camber as percentage of chord
2: Position in 10ths of chord of max camber
3/4: Max thickness as a percentage of chord
What does NACA stand for?
National advisory committee for aeronautics
Describe the component so airflow over a finite wing?
Chord wise direction prominent
Vertical direction
Span wise to create vortices
What effect does aspect ration have on the airflow around a wing? What about an infinite wing?
- It will significantly reduce the span-wise component of airflow
- An infinite wing will have only vertical and chord-wise components of airflow.
What is the difference between an elliptical wing and an infinite wing?
-Infinite wing is an idealised wing whereas elliptical wing does not have a wingtip
What are the conditions of using Bernoulli’s equation to describe the production of lift?
- Fluid is incompressible
- Can only be used along the streamline (1D)
- Fluid must be non-viscous
- Fluid is in level flow
- Non-separated
What are the conditions to use Newton’s Momentum/ Impulse theory to describe the production of lift?
- Must be 2D airflow
- Non-viscous fluid
- Non-separated flow
What are the disadvantages of the Partial differential equations?
- They are very hard and require a deep understanding of applied mathematics to solve.
- Must know the initial conditions
Describe the circulation theory of lift
There is an upwash ahead of the aerofoil and downwash behind it. The airflow speed over the aerofoil is faster than free stream, and the flow speed below the wing is lower than free stream.
The free stream is horizontal and uniform and that the vertical components called upwash and downwash form a circulation. The aerofoil is assumed to be thin, symmetrical and infinite.
A force’s created perpendicular to the velocity and circulation.
Describe how to find the direction of circulation?
Using your right hand:
- Let fingers follow the direction of the circulation g particles.
- Stick your thumb out and it will show the direction of the circulation
Describe how to find the direction fo the force on the object due to circulation?
Using your right hand:
- Set thumb, index and muffle finger for a 3D axis
- Index finger points direction of particle motion
- Middle finger indicates the direction of the circulation
- Thumb points in direction of the force on the object.
What are the conditions required to use the circulation theory of lift?
- Thin, symmetrical aerofoil
- No separation
- Air particles leave the training edge uniformly downwards
The assumption Cl =2piAoA is true for a symmetrical aerofoil. What must be done for a cambered aerofoil?
The Cl at 0* added and a correction factor of k incorporated (Cl=2kpi*AoA + Cl0)
-Note: AoA is in radians
How does the thickness affect the circulation around an aerofoil?
Circulation around a thick aerofoil is greater than around a think aerofoil
What direction does drag act in?
In the same direction as the relative airflow
What is skin drag?
The drag caused by the viscosity of a fluid in the boundary layer. When a fluid particles changes speed rapidly, there will be a large amount of viscous friction.
What factors affect the coefficient of skin drag?
- Reynolds Number
- Surface characteristics
What factors affect the total skin drag?
- Airspeed (changes Re, also v^2)
- Density
- Surface area
- Surface quality
What happens when airflow separates from an object?
- Vortices are formed
- A low pressure is formed in the centre of vortices
- From drag is thus created due to the pressure differential
What will affect the intensity of the vortices that are formed when airflow separates? how doe they affect it
Reynolds number
Higher Re will form more intense vortices
Shape of the object
When sudden changes in shape happen the likelihood of the flow separating increases.
How does increasing TAS affect form drag?
- It will increase directly as a result
- It will increase the Re
- It will increase the intensity of flow separation due to increasing Re
What are all the components of Total Drag? Drag tree?
Parasite:
- Form (Profile)
- Skin (Profile)
- Interference
Induced
What is interference drag?
The drag created as airflows around different parts of the aircraft interact. They can cause local pressure differences, turbulence and separated flow. The airflows can not follow the original flow pattern.
When is interference drag especially relevant? why?
Transonic aircraft. The shockwave created can cause shock drag.
If turbulent wake separates from the aircraft and re-impacts that aircraft, what can happen?
It can produce noise and vibrations
How can separate turbulent wake interfere with each other?
- Can constructively or destructively interfere
- Can reflect or deflect of one another
- Can cause a strong oscillation and cause significant energy dissipation from the main stream flow
How can interference drag be reduced?
- Using fairings to create a joint zone
- Blending
- Filleting
- Smooth and gradual change in shape between two components
What things will affect the coefficient of interference drag?
- Shape of the aircraft body
- Airspeed
- Type of manoeuvre the aircraft is in
How is the coefficient of interference drag usually found?
Model tests or full scale testing
Previous design data experience
Good numerical simulation
How does an increase in angle of attack affect parasite drag?
It does NOT directly affect it!
- Increasing AoA can cause earlier separation, increasing form drag
- AoA may increase in manoeuvres, potentially causing a change in flow regime, chasing skin friction drag
How are trailing edge vortices formed?
Due to the inward span-wise component when the airflows over the wing, and the outward span-wise component under the wing meet they will interact and form vortices due to the relative direction of each flow and the viscosity of air.
How are wingtip vortices formed?
- Airflow under the wing has a higher pressure than free stream
- Airflow over the wing has a lower pressure
- Pressure different between top and bottom generate rotating motion as air spills over the wingtip
- Wingtip airflow is given a torque, therefore forming a strong vortex.
What happens to pressure in the middle of a vortex?
It is lower than the pressure outside the vortex
What is induced drag? How does it occur?
The drag caused by the component of lift acting against the direction of motion of the aircraft.
- TE and wingtip vortices induce downwash
- Change direction of relative airflow, decreasing it relative to unaffected airflow
- This reduces effective angle of attack
- Lift will be reduced.
- As lift is 90˚ to RAF the lift vector rearwards
- Lift vector split into vertical (true) and rearwards component
- Rearwards compote is induced drag, acting in same direction as unaffected airflow
- Due to reduced total lift AoA must increase to maintain level, and so induced drag will increase further.
How does Aspect ratio affect the induced drag?
- High AR wing has more 2D flow so vortices are weaker
- Weaker vortices will have less downwash
- Less downwash mean less rearward tilt on the lift vector
- Less rearward tilt means less horizontal component of lift so less induced drag
How does changing AoA change induced drag?
-Increasing AoA will increase induced drag as the coefficient of induced drag is a function of Cl, which is proportional to the AoA.
When calculating CL, what is the unit of angle of attack?
Radians
What is the speed for maximum L/D ratio? Where is it in relation of Vmp?
Minimum drag speed (Vmd)
Vmd is always higher than Vmp (Vmd occurs when line from origin of Pr curve is tangental)
What does the best L/D ratio give? Where does it normally occur?
Shows most effective AoA
Usually occurs around 4˚
What is thickness of an aerofoil? What is considered thin, medium and thick?
-Thickness is the ratio of thickness to chord, the relative thickness
Thin < 7%
Medium 7% - 15%
Thick > 15%
What happens to circulation, Induced drag, skin drag and CL around a thin aerofoil?
- Less circulation, less downwash, weaker vortices, so:
- Lower Cd(induced)
- Lower CL
- Lower stall AoA
- Lower skin drag, due to lower wing area
How does thickness affect boundary layer separation and stall?
-Increased thickness will cause the condition of adverse pressure gradient to occur later on a thick aerofoil than a thin one set at the same AoA. A thick aerofoil will have a higher critical angle, and higher CL(max)
How does a higher aspect ratio affect the coefficient of lift? Why?
CL will increase as the amount of induced drag is decreased and the rearward tilt is less.
How does increasing aspect ratio affect the critical angle attack?
The geometric stall angle will be decreased as the relative airflow is affected less by downwash and vortices and so the effective AoA is less affected.
What is a downside of having a high aspect ratio?
It requires high structural strength
How does camber affect circulation around a wing? Induced Drag? CL?
- Circulation around cambered wing is greater than symmetrical
- Higher circulation leads to higher CL
- Higher CL will lead to a high Cd(induced) and therefore D(induced)
How does increasing camber affect the stalling angle of attack? Why? What about the stall speed?
- The stall speed will be lower as the CL is higher for a given AoA so less velocity is required to maintain level
- The stall AoA will decrease because camber (especially forward camber) will create an adverse pressure gradient that acts further forward, causing BL separation earlier than on a symmetrical aerofoil.
What is the zero-lift pitching moment?
The nose down pitching moment felt the a cambered aerofoil is at its zero lift angle of attack.
What are some wing shapes that reduce the formation of vortices?
- Elliptical
- Tapered
Describe the features so the laminar flow aerofoil? explain what they achieve?
- Thin leading edge: Keeps airflow laminar, gentle even change in pressure over aerofoil
- Further aft maximum camber: adverse pressure gradient region is located further aft (delaying BL separation)
What are some devices that prevent or delay BL separation?
Blowing method Sucking method Vortex generators LE slot Artificial feathers
What are some wingtip/wing devices to reduce vortices?
- Wing fence
- Winglet
- Wingtip tank
- Modified wingtip
- Raked wingtip
- Washout
Describe how ground effect changes the characteristics of the wing?
- Less wingtip vortices
- Less induced drag
- Nose pitch down moment
- False low reading on airspeed
