Chap B: Basic Aerodynamics Flashcards
mass flow rate equation
ρAV (kg/s)
ρ - density
A - cross-section area
V - Flow velocity
what is the maximum velocity for compressibility to be neglected?
100m/s (M = 0.3)
what happens when compressibility is accounted for?
density, ρ, changes
types of flow and its explanation
steady flow - no change in parameters at any given time
unsteady flow - parameters at point of consideration are constantly changing
streamline definition
imaginary line joining all particles moving along the same path, there is no flow across streamlines (no exchange)
what is a stream tube?
a bundle of streamlines for a stream tube (no exchange of particles)
how is velocity related to area in continuity equation?
as cross-section area decreases, flow velocity increases
Total energy equation
ET = P∀ + 1/2 mairV2 + mairgz
P - static pressure
∀ - volume of air
V - speed of air
z - height above reference datum
which factor is the same between reality view and wind tunnel view?
airspeed & Vrelative
magnitudes are the same just that airspeed for real. view is against drag whereas Vrelative in WT view is same direction as drag
what does Bernoulli’s equation neglect?
it assumes that there is no fluid friction (energy lost due to friction is ignored)
What is static pressure?
static pressure is pressure exerted by still air at a point (not moving)
what is dynamic pressure?
pressure exerted by moving air (1/2 ρV2
what is stagnation pressure?
flow is brought to rest (Pdyn = 0)
so Pstagnation = Ptotal = Pstatic
how to measure stagnation pressure?
stagnation tube or pitot tube
dynamic pressure formula using difference of pressure
1/2 ρV2 = Pstagnation - Pstatic
why would using difference of pressure to calculate velocity be inaccurate?
1) as it requires density, and it cannot be measured during flight. And using ISA table is not accurate it assumes SL density is 1.225kg/m3
2) compressibility effect
what does pitot tube and static tube measure?
and what does pitot-static tube measure?
pitot tube - total pressure
static tube - static pressure
pitot-static tube measures the differential pressure (ΔP) to calculate airspeed
what is indicated airspeed?
IAS -measured by pressure difference and flight altitude density
what is equivalent airspeed?
EAS - airspeed using SL density, using Pdyn at altitude and bring it down to SL to get speed at SL.
what is true airspeed?
TAS - obtained with accurate density at flight altitude, when flow is incompressible, TAS = IAS
what is calibrated airspeed?
CAS - airspeed obtained by SL pressure and temp. (P0 & T0)
approx speed only, as speed increases, it is less accurate
types of error in airspeed measurements
1) density error - EAS is using SL density to calculate airspeed (EAS always < TAS when flying at altitude other than SL)
2) compressibility error - density changes for speed > M0.3
3) instrument error - not calibrated properly
4) position error
types of position error in airspeed measurements
1) location & orientation of the pitot-static tube
2) pitot tube may not sense full stagnation pressure at high angle incline
3) static port will not sense correct pressure when landing gears/flaps are extended / covered in ice
what is VIAS equal to in pressure formula?
VIAS = V
V in 1/2 ρV2
what does subscript s in Ps and P stands for in airspeed calculations?
Ps: Stagnation pressure
P: static pressure
s means stagnation
what is calibrated airspeed corrected for?
instrument & position errors
compressibility error at high speeds & altitudes
what is ground speed?
speed of aircraft relative to ground (sum of TAS & wind speed)
what affects ground speed?
headwinds (lowers ground speed)
tailwinds (increases ground speed)
what is boundary layer?
molecules adjacent to surface tend to stick to it when viscous fluid flow past objects (V = 0)
molecules further away from surface has more velocity
what is laminar flow?
aka streamline flow,
flow parameters may vary from point to point but remains constant wrt time at their respective points
no exchange of particles between streamlines
assumed to be frictionless & steady
what is turbulent flow?
breakdown of flow, randomness of motion of each particle. For high speed flow & friction
difference between laminar & turbulent flow
laminar:
- low thickness
- uniform flow
- low velocity next to surface
- gradual velocity change
- low skin friction
turbulent:
- greater thickness
- random flow
- higher velocity next to surface
- sharp velocity change
- high skin friction
what is transition point?
a point where flow changes from laminar to turbulent
range of Reynolds number for air
laminar flow: <= half a million
transition flow: 500000 to 10 million
turbulent flow: >= 10 million
what causes flow separation?
adverse pressure gradient leads to separation and lost of lift as fluid is flowing to a region of higher pressure
difference between laminar and turbulent flow separation
laminar flow separation:
less energy in boundary layer, early separation
turbulent flow separation:
greater energy in boundary layer, separation further aft
what is magnus effect?
generation of lift due to imbalanced pressure on both sides of a spinning object
flow separation may occur when?
flow slows down and static pressure increases
why do golf balls with dimples that act as turbulators travel further?
dimples induced turbulent boundary layer
turbulent boundary layer has more kinetic energy than laminar to go against adverse pressure gradient
lesser chance of flow separation which increases total drag
how to calculate CAS when TAS, a0, Palt is given?
use TAS formula to find Pstag, sub Pstag into CAS equation
lift, drag, and L/D graphs against AOA
lift against AOA: increasing until stalling angle, then decreases a bit
drag against AOA: exponential graph, starts above origin
L/D against AOA: increases sharply until max point, before going down to stalling angle near the end
define reynolds’ number
ratio of forces of inertia which is force of motion to force of resistance.
reynolds’ number equation variables
ρ - fluid density
V - flow speed
d - distance from LE to location
µ - viscosity
