What is the principle of thrust
you accelerate air backwards to accelerate yourself forward
- you move a lift producing aerofoil up and down
When we have a propellor what force is the engine mostly circumventing
It needs to circumvent the drag force that is a result of the relative wind
What is a problem with a symmetric aerofoil
- there is no net lift although there is a net thrust
- downstroke produces lift and thrust
- upstroke produces negative lift and thrust
Why are wings typically an asymmetrical aerofoil
- they can generate more lift for the same angle of attack
2. but can still generate negative lift during the upstroke ( but not as much)
How does flapping flight occur naturally in birds
- the bird wing’s are twisted such that the tips have a greater angle of attack compared to the section closer to the body of the wing
- the portion closer to the wing tip is responsible for contributing to the thrust
- portions closer to the body is responsible for the lift
- during the power stroke the wing tip twists and angling the distal wing downwards and directs the lift vector forward as thrust
- during the recovery stroke, the proximal part of the wing continues to generate lift
What is the Strouhal number
- relates to vortex shedding from the aerofoils
- St= f * A / v ( f= frequency of the wing beats, A : whole vertical distance travelled by wingtip, velocity = airspeed)
- ideally want a low Strouhal number
How does form drag and induced drag change with air speed
form drag increases with airspeed
induced drag decreases with airspeed
Why does induced drag decrease with air speed
induced drag is caused by the downwash behind it at low speeds we need a greater angle of attack for more lift so we have greater vortices
but at high speeds we need not adopt that high an angle of attack ( because lift is prop. to speed) and so there is less vortices and less downwash
Draw how a drag vs airspeed graph would look like for total drag, induced drag and form drag
L 23 slide 22
Draw how a power vs air speed graph will look like with power required for flight. on the graph identify where it is cheapest to stay airborne and where is the cheapest to cover distance and where is the fastest flying speed
L 23 pg 23
How do we obtain the power required for flight
multiply the velocity of that flight with the drag experienced.
What is winglet theory
- large wingtip vortices produced by planar wings create a strong downwash behind the trailing edge of the wing and this pushes the effective relative wind downwards and the lift vector rearwards increasing the induced drag
- smaller wingtip vortices (curved tip) can spread the vortex vertically and reduce the downwash and the induced drag– increases the wing’s effective aspect ratio without increasing the wingspan
What happens to the total drag when the winglets are clipped
- the total drag increases since the induced drag increases
What is a bastard wing/alula
It is the protrusion on the leading edge that allows birds to operate at lower airspeed and higher angles of attack needed to sustain lift : wing continues to produce lift (as stalling is delayed)
What are covert feathers
Are wings covering the top surface of the wing that form the trailing edge of the wings
- turbulence begins at the trailing edge of the wing and can extend forwards == stalling
- covert feathers function as eddy flaps preventing the separated flow rom extending up to the top of the aerofoil
Draw the difference between wings with covert feathers and wings without covert feather for a coefficient of lift vs angle of attack graph
L 25 slide 13
Why do we want covert feathers?
good for slow flight AT high angle of attack ( because in slow flights we need greater angles of attack to generate more lift to sustain the animal with this it can cause stalling)
What is hovering flight
ability to remain airborne in a fluid with negligible mean relative velocity ( ie. stay up in air without moving relative to surrounding air)
How do hummingbirds maintain hovering flight
They produce a resultant lift force by flapping their wings horizontally to give a upward resultant force - RECIPROCATING PATTERN. the drag force from the downstroke and upstroke cancel out such that there seems to be a net upward lift
- upward thrust is produced both upstroke and downstroke
- air mass is accelerated down by the downstroke of the wind
Why is the muscle needed for upstroke for hovering animals smaller than the muscle needed for the power upstroke
recovery stroke does not need to support the bird’s weight so the muscle is comparatively small
