Part III Flashcards
What is the general rule of thumb about computer time and Re when simulating
PU time = Re^3
Storage requirement = Re^9/4
What is Breguet Range cruise distance equation
R = V/c * L/D * ln( Winitial / Wfinal)
What is payload fraction
payload/ total lift off weight
Was is useful payload fraction
payload + fuel / MTOW
What is fuel fraction estimation equation
Wf/Wo = 1.06(1-Wx/Wo)
Define stability
ability of aircraft to return to its original equilibrium position
Define control
ability of the pilot to produce moments about the various axis
Define static stability
Refers to the tendency of the aircraft to counter a disturbance
Dynamic stability
is concerned with how fast the aircraft returns to trim condition after a disturbance
For cambered aerofoils, if centre of pressure moves, what happens to the pitching moment at the aerodynamic centre
stays the same
What equation represents static stability
x / c
where:
x - distance between CG and AC
c - chord length
What is a canard
when the tail wing is in front of the main wing
List 2 benefits of canards
- Canard surface operates at high CL and almost makes aircraft ‘stall-proof’
- Can sometimes have larger range for CG
List 2 disadvantages of canards
- Wing operates in downwash from canard and therefore reducing its efficiency
- Difficult to apply flaps to wings
What is a flying wing design
an aircraft design with no tail (lowest possible drag)
Benefits of a flying wing design
- Removes fuselage therefore improves aerodynamic efficiency
- synergistic effect with relaxed static stability
- low visibility on radar
- distribute load easier
What is drag crisis
In the transition period between laminar and turbulent flow, drag drops
What is the reason for the drag crisis
When turbulence rapidly mixes rapidly-moving external air into the boundary layer, increasing the range over which it adheres to the surface. This makes the trailing low pressure wake smaller and thus reduces drag
What can be done to a surface to ensure drag crisis happen more
roughening the surface i.e. golf ball
What is the wall law
velocity of turbulent flow ∝ log(distance to the wall)
only works when distance < 20% of flow height
What are streaks and what happens when they meander
Turbulent flows vortices form near a wall and are stretched by the mean flow - these are streaks
When they meander, this causes friction and drag to increase
equation for streak size, spacing and length
size = a+ = ua/v (10y)
spacing = s+ = us/v (100y)
length = l+ = ul/v (1000y)
where:
u = friction velocity at wall
y = distance to wall
v = local kinematic viscocity
What are riblets
a series of parallel grooves on a surface that can reduce drag and make a wall more streamlined
they are used to channel vortex streaks created near the wall
List advantages of open circuit tunnels
- less expensive to build
2. pollutants are purged (smoke)
List disadvantages of open circuit tunnels
- more expensive to run
- affects design of surrounding environment
- more noisy
List advantages of closed circuit tunnels
- some energy is recovered
- shielded from weather
- quality of flow easily controlled
List disadvantages of closed circuit tunnels
- more expensive to build
- air inside constantly heated
- full of dust and debris
- noisy
Give equation for energy ratio of wind tunnel
Jet energy / sum of circuit losses
qUA / efficiency * motor power
How to manage turbulence in wind tunnel
via honeycomb and screens
What is hot wire anemometry used for
used to calculate pressure and velocity of airflow
how:
- heated wire
- gives off heat via convection to airflow
- energy required to reheat is directly proportional to airflow
Advantages of hot wire anemometry
- cheap
2. accurate
Disadvantages of hot wire anemometry
- effects airflow
- can’t measure high intensity turbulence
- wind direction has no effect
- very fragile
- can ‘burn out’
What can be used to measure velocity and pressure in multi-phase situations
hot film probe
more expensive and worse frequency response then hot wire anemometry however
What is LDV
Laser Doppler Velocemetry
- use doppler effect to measure velocity/pressure
What is PIV
Particle Image velocemetry
- have particles in fluid that light up under a laser
- post-process images to find velocity/pressure
- can create 2D/3D velocity fields
What equation shows that flight is stable
dCmp / dCl < 0
