Definitions and nomenclature Flashcards
What is dimensional analysis?
Using the fact that a physical problem needs to be dimensionally consistent
What is buck-pi and how is it used?
Explain how it is used in practice. A complex physical relation which is dependent on many parameters can be reduced to a relation that is dependent on dimensionless groups.
What is dynamic similarity
If two flows are dynamically similar we can directly compare one with the other. Allows for reduced or englarged prototype testing.
When are flow situations dynamically similar?
- The bodies and solid boundaries are geometrically similar.
- The similarity gourps are the same (pi-groups)
What is the reynolds number?
Re=rhoUL/mu
DensityFreestream Velocitycharacteristic lengths scale/ Dynamic viscosity
The ratio of inertia Forces divided by viscous forces.
Generally important for all types of fluid problems
What is the Froude number?
Fr=U/sqrt(g*L)
The ratio of inertia forces to gravitatonal forces.
Relevant in problems with flow with a free surface
definition of Euler Number
Eu=p/rho*U^2
Ratio of pressure forces to intertia forces
Relevant in problems where pressure or pressure differences are of interest.
definition of Cauchy Number
Ca=rho*U^2/E_v (E_v=Bulkmodulus)
Ratio of inertia forces and compressibility forces.
Flows in which compressibility of fluid is important.
definition of Mach Number
Ma=U/a (a speed of sound)
Ratio of inertia forces and compressibility forces.
Flows in which compressibility of fluid is important.
definition of Strouhal Number
St=omega*l/U
Local intertial forces/Convective inertia forces
For unsteady flow with a characteristic oscillation frequency.
definition of Weber Number
We=rhoU^2L/sigma (sigma surface tension)
Inertia forces/Surface tension forces.
Problems in which surface tension is important.
What are the aerodyanmic forces acting on an object?
- Lift= perpendicular to free stream
- Drag= Parallell to free stream
- Normal force= perpendicular to chord
- Axial force= Parallell to chord.
- Cross wind force (y-direction).
- Weight
- Thrust
What are the aerodynamic moments acting on an object?
Rolling moment (x-axis). Yawing moment (z-axis). Pitching moment (y-axis) Positive when it tends to want to pitch nose up.
definition ofCoefficient of lift?
C_L=L/(1/2rhoU^2*S)
C_l=L/(1/2rhoU^2*c) (lift/meter wing)
definition of Coefficient of drag?
C_D=D/(1/2rhoU^2*S)
C_d=D/(1/2rhoU^2*c)
definition of Moment Coefficient?
C_M=M/(1/2rhoS*c_bar) (c_bar =mean chord)
C_m=M/(1/2rhoc*c_bar)
definition of Coefficient of pressure?
C_p=p-p_inf/(1/2rhoU^2)
Airfoil nomenclature?
Chord, Camberline, Thickness ratio, Wingspan, Aspect Ratio, Incidence
What is Chord?
Distance from leading edge to trailing edge.
What is Camberline?
Take half the distance between upper and lower surface. Measure this point to the chordline. The camberline is the locus of all these points.
What is Thickness ratio?
Divide the distance from the upper surface to the lower surface by the chord
What is meant by Spanwise direction?
The direction of on the wing from tip-to-tip (y)
What is Aspect Ratio?
b^2/S (b- span width) The ratio of span length to chord length.
What is Wingspan
length of the wing, tip-to-tip.
What does NACA stand for?
National advisory committe of aviation
NACA 4-digit series
1st digit: Max camber (percent of chord length)
2nd digit: Location of max camber (tenths of chord)
3&4 digit: Max thickness ratio.
NACA 5-digit series
Ex: NACA LPSTT
L= Single digit indicating max C_L at optimal alpha (CL=0.15L)
P= Max camber point (x=0.05P)
S= if 0 simple, if 1 camber reflexion point exists.
TT= Max thickness ratio in percent of chord.
How is lift generated?
long answer
What is Aerodynamic centre?
The AC is the chordwise position where the pitching moment is independent of aplha
dM/d(alpha)=0
What is Centre of pressure
The centre of pressure, X_cp, is the chordwise position at which the pitching moment is zero.
What is Centre of gravity
The point over which the aircraft would balance.
How should AC, X_cp, CG be arrenged?
We want the moment about the CG to be positive for all positive lifts.
AC behind CG.
Pros and cons of Sweep angle
Angle between wing and centerline normal.
+ delays high mach effects since it reduces the effective free stream velocity
- performance loss at low speeds.
-Tip stalling problem.
Pros and cons of Dihedral angle
Angle between xy-plane and wing. gamma>0 Dihedral wing gamma<0 Anhedral wing \+ Roll stability (fbd why) - Performance loss
Pros and cons of Thickness Ratio
+ Structural reasons
-In general increased C_d,min with increasing thickness
Pros and cons of large Aspect Ratio?
+Higher aerodynamic efficiency
- less manoverable
- Structurally challenging.
Pros and cons of Wing taper
+Reduced weight
-Bad stalling characteristics
+ Reduced tip vortex strength, look at dimensional lift,
reduce c and lift decreases towards wingtip => smaller wing tip vorticies.
Pros and cons of Wing twist
Change of alpha along span.
+ Main reason is to make sure that the flow over the control surfaces dont deattach first. We want to the wing to stall close to the fueselage first
(alpha_root>alpha_tip)
What is the parallell axis-theorem and how can it be applied for moving moment coefficients?
Babar.
M_b=M_a+BAxR
Or with nondimensional coeff.
C_mb=C_ma+((b/c)-(a/c))*C_n
