Aircraft Design Flashcards
Aircraft vehicle design, constraint analysis, mission analysis, performance, aerodynamics, propulsion, structures, basics of everything (187 cards)
1
Q
What is dimensional analysis? What do we use it for?
A
Dimensional analysis is used to reduce n-dimensional vbl functions to non-dimensional similarity parameters.
We use similarity parameters to tell us if two non-dim characterizing parameters of aerodynamic flow (Mach and Reyonolds #) are the same for two different flows (rho,v,mu,a) then the Lift coefficient will be the same given two geometrically similar bodies at the same AOA.
2
Q
What is design Cl?
A
Design lift coefficient such that at an ideal angle of attack all flow is tangent to the camber line.
3
Q
What is Mission Analysis?
A
Mission analysis estimates takeoff gross weight given an initial sized thrust loading and wing loading (T/W, W/S). Uses weight fractions to solve for each mission phase relative to fuel burned (or historical data, empty weight, etc…)
4
Q
Draw a free body diagram of an A/C with all angles labeled. (Define Flight path angle, angle of attack, pitch angle, thrust vector)
A
γ - flight path angle = angle between horizontal and V∞
α - angle of attack = angle between AC longitudinal axis and V∞
θ - pitch angle = angle between AC longitudinal axis and horizontal
ϵ - thrust vector = offset of thrust vector w.r.t. to AC longitudinal axis
5
Q
What different types of uncertainty can occur?
A
Uncertainty in estimation, model propagation, interpolation, experimental, bias, variability
6
Q
What are the two categories of uncertainty?
A
Aleatoric - statistical uncertainty based on unknown and uncontrollable factors
Epistemic - systemic uncertainty based on the lack of knowledge, or not full understanding of behaviors, can be improved by learning more
7
Q
Which type of uncertainty can you reduce? and How?
A
Can only reduce epistemic uncertainty by learning more about the behavior. Cannot reduce aleatoric uncertainty, but can design to be insensitive/reduced susceptibility to uncontrollable factors through Robust Design
8
Q
What is the basic process of Mission Analysis?
A
- Start with initial takeoff gross weight estimate (Wto from constraint analysis)
- Run mission profile analysis for total Wfuel/Wto
- Recalculate takeoff gross weight through mission fuel fraction and empty weight regression equations Wto = (Wc+Wp)/(1-Wf/Wto-We/Wto)
- Re-estimate Wto and run an iterative analysis over mission profile weight fractions to convergence
9
Q
What is Constraint Analysis? What does it yield?
A
Constraint analysis is the numeric and graphical process of using performance and vehicle design scaling parameters thrust loading T/W and wing loading W/S to size your vehicle based on constraining mission phases of flight.
Constraint analysis yields a feasible design space and an optimized design point.
10
Q
What are sizing/scaling parameters?
A
Sizing parameters are relationships between both design parameters and aerodynamic properties to size the vehicle with respect to top level characteristics of the vehicle
11
Q
What are the phases of design?
A
Conceptual Design - finish requirements analysis, build fuzzy vehicle configuration, and make initial guesses for weight/size, and look at performance requirements
Preliminary Design - Lock in major design features, begin initial Aero/Prop/Structural analysis, only small changes in minor details moving on.
Detailed Design - final analysis set OK for fabrication/manufacturing, testing, vehicle assembly.
12
Q
What is sizing and synthesis?
A
Sizing and synthesis is the process of creating a parametric model for a concept based on requirements. The concept is being sized, while the engineering disciplines and constraints are synthesized together to impact the final design.
13
Q
What are the four forces of aerodynamic flight?
A
Weight, Lift, Thrust, Drag.
14
Q
What are requirements?
A
Requirements are a set of unambiguous, verifiable, traceable, and necessary statements that represent the needs of the user/stakeholder.
15
Q
Why are requirements important to design?
A
They are important to design because they are the primary driver the design and what the the design performance is measured against.
16
Q
What is the difference between uncertainty and risk?
A
The level of consequences, specifically the quantity of these consequences which propagate.
(uncertainty has consequences, but risk has impactful “danger” associated with the consequences)
17
Q
What is robust design?
A
A learning based systematic approach to optimize a design factor value resulting in low variability.
*Economic designs insensitive to uncontrollable factors
18
Q
What are the main assumptions of constraint analysis? What does each assumption tell us
A
- Body is a point mass
- No moments propagated across the body - Thrust and Drag are aligned with the velocity vector
- No thrust vectoring
- L is perpendicular to V, –> L is perpendicular to T & D - Gravity is a constant over the planet
- W = mg is constant (but changes g for other planets)
19
Q
Define Systems Engineering
A
“overarching discipline to integrate business and engineering aspects to achieve the best overall product/service that meets requirements and does so within budgetary and scheduling constraints”
- Qualitative/quantitative req.
- Design process integration
- Test and analysis of design
- Verification and Validation
20
Q
What are functional requirements? Performance requirements?
A
Functional requirements describe what the product/system should be able to do. Performance requirements describe how well the product/systems performs the function/action required
21
Q
What is verification and validation
A
Verification is the process and testifying that the product/design has met all the requirements set forth. Validation is the process of testifying that the product/design meets the stakeholder’s needs and performs the desired job.
22
Q
Derive the Master Equation using energy balance: (tell me the process)
A
- Energy balance
E = PE + KE = Fds = mgh + 1/2mV^2 = (T-(d+r))ds
2. time derivative to get power and assume mg = W, P = d/dt[(T-(D+R))ds = Wh + 1/2(W/g)V^2]
gives us specific excess power = time derivative of energy height
Ps = [(T-(D+R))]/WV = d/dt(h+(WV^2)/g))
3. Assume: loss factors, drag equations, and L = W
- Solve for Tsl/Wto and arrange in terms of (W/S) fractions
have constant + linear + inverse term
23
Q
What is a streamline?
A
A streamline is an element that is tangential to the local velocity vector for the flow field @ a given instant.
*ds X V = 0
24
Q
Can streamlines cross? Defend why or why not
A
No, to meet the specification of tangential to flow at every point dsXV = 0 at a single point the crossing of two lines would mean a flow element has two directions and two velocity vectors which is impossible unless V = 0.
25
What is Lift?
Lift is a component of resultant force due to aerodynamic forces of pressure and shear distributions. Perpendicular to V∞ and Drag
26
What is Drag?
Drag is a component of resultant force due to aerodynamic forces of pressure and shear distributions. There are components of drag due to both pressure/shear and viscous forces.
27
What is a drag polar?
A drag polar is an expression for the relationship between drag and lift coefficients with view on performance.
CD = CD0 + K1CL^2+K2CL
```
CD = total drag
CD0 = zero lift drag - parasite drag of A/C
CL = total lift coef
K1 = K' + K" - induced drag coef due to lift
K2 = -2K"CLmin - interference drag/skin friction/press drag
```
28
What are the two types of drag?
Pressure drag, drag due to net imbalance of surface pressure acting in the direction of drag
Friction drag, drag due to the net effect of shear stress acting in the drag direction.
29
What is the total drag of an airplane composed of?
CD = CD,e + CD,w + CL^2/(pi*eAR)
| total drag) = (parasite drag) + (wave drag) + (induced drag
30
What is parasite drag?
Profile drag of a complete airplane.
31
What is profile drag?
Profile drag is:
- skin friction drag due to flow separation
- skin friction drag due to frictional shear stress acting on airfoil surface
- pressure drag due to flow separation (form drag) specifically caused by net imbalance of pressure distribution in drag direction when boundary layer separates from airfoil
32
What is induced drag due to lift?
induced drag is developed by a 3D lifting body from the perturbation of flow due to vortex generation over a wing. Produced when not at zero-lift AOA .
33
What is zero-lift drag?
parasite drag of complete aircraft occurring when at zero-lift α(L=0)
34
Can you have lift in inviscid flow?
Technically no, because without viscosity/friction we can't derive shearing flow circulation that satisfy the Kutta Condition which maintains natural correction of flow maintaining attached to the surface until the TE. Were there no condition satisfied, there would be no circulation which would mean there is no Lift
35
How do you get Lift from Viscosity?
1. From viscosity and satisfaction of the Kutta condition we have rotational flow which gives us vorticity.
2. From integration of vorticity, we have circulation
3. From circulation, we have to satisfy the Kutta Joukowski Thrm of infinitesimal L' = rho*v*Gamma. Integrate circulation over the span to get total lift.
36
What is the Kutta Condition?
THRM:
A naturally occurring circulatory correction maintains that flow is steady over an airfoil, this perfect circulation condition corrects the resulting flow such that the flow must leave the trailing edge smoothly.
* Given viscosity (shear and pressure distribution) over an airfoil.
* increases Vtop to push the stagnation pressure pt to the TE and slows Vbottom to pull the stagnation pressure pt to the LE.
37
What is the difference between viscous and inviscid flow?
Viscous flow is a flow with phenomena of mass diffusion, friction, and thermal conduction (as a result of molecular transport)
Inviscid flow is assumed to have no resistance to shear stress/friction.
*has no friction, no mass diffusion, nor thermal conduction.
38
What is Reynolds number? What is the magnitude with respect to viscous/inviscid flow?
Reynolds number is a similarity parameter, relating the ratio of inertial forces to viscous forces in a fluid. It is a measurable parameter between flow boundaries and behaviors.
Re = rho*V∞*c/(u∞)
inviscid flow Re --> ∞
viscous flow 10^4
39
What is the difference between dynamic and kinematic viscosity?
Dynamic viscosity is mu, which is the internal resistance to motion of flow. which accounts for density of flow (u∞ = v*rho)
Kinematic viscosity is the ratio of dynamic viscosity to its density (u/rho)
40
What is the difference between incompressible flow and compressible flow?
Incompressible flow is flow assumed to have a constant density with respect to pressure, but can change due to thermal changes.
*low velocity where V is only variable pressure = 1/2 rho V^2 - no changes in energy
Compressible flow is flow which allows density to vary with pressure. Density/volume varies with pressure changes, means changes in energy as fluid compresses/expands volume
41
What are the assumptions of stead level flight?
Stead Level Flight:
T = D & L = W
* *BECAUSE:
- Const Velocity
- No thrust incidence angle (e = 0)
- V∞, D parallel to horizontal
- no pitch, or flight path angles (theta,gamma = 0)
- turn radius (eq of motion) --> ∞
42
What is the difference between cd and CD? Which is larger/smaller?
Cd is profile drag coefficient = (Df+Dp)/(q∞S) = skin friction+ pressure drag due to separation
CD = total drag coefficient = Cd + CDi
Where Cd = profile drag and CDi is induced drag due to lift over finite wing = CL^2/(pi*e*AR)
Cd will be smaller, there is always more drag on a finite wing due to lift
43
Is the zero lift AOA a(L=0) the same for both infinite and finite wings?
Yes, because the zero lift angle of attack is based on effective angle of attack aeff = a-geometric - a-induced
When L = 0, no induced angle of attack ai from the to induced drag of finite wing, so the effective angle of attack is the same on both wings.
44
What is the difference between Cl and CL? Which is smaller/larger?
Cl is the infinite lifting coefficient w.r.t chord of the airfoil
CL is the total lift over a finite wing w.r.t. planform wing area
Cl will be bigger because an infinite wing has less drag, and therefor more lift. (this means the CL lift curve will have a decreased slope a < a0 (dcl/da))
45
What is the true measure of aerodynamic efficiency - w.r.t. body shape?
L/D
The lift to drag ratio measure aerodynamic efficiency with respect to capable lift for the given wing area and the related drag associate with that wing.
*L/Dmax -> V∞|(L/Dmax) -> alpha(L/Dmax)
46
What is the center of pressure?
The location at which the total resultant forces, L & D, effectively act on the body such that the moments about Xcp are zero.
Essentially the centroid of pressure distribution
47
Why do we not use this for aerodynamic calculations? What do we use?
Xcp is dependent upon angle of attack, so as alpha changes, the location continuously changes. We use Aerodynamic Center
48
What is Aerodynamic Center?
The location at which the resultant forces act for which the moments are independent of angle of attack.
49
What are the two main types of reference area?
Wetted area - total surface of which pressure and shear distributions act
Planform area - projected shadow area
50
What properties vary the thrust if installed engines?
Mach number, altitude, and afterburner operation.
51
What variable do we account for varied installed thrust with? (what is the equation)
```
Thrust Lapse (alpha)
T = alpha*T_sl
```
52
What is the primary variable used to account for weight changes in the aircraft from fuel burn?
Mission weight fuel fractions (beta)
| W = beta*W_to
53
In mission analysis, how do we find the weight fuel fractions from fuel burn?
Using TSFC and known Range, we can derive our change in weight (beta = Wfin/Win) using the Breguet Range Equation.
54
What is centripetal acceleration?
Centripetal acceleration is the radial component of acceleration which the aircraft feels in the direction toward the central pivoting turning point (pulling to the center of the circle).
55
What is centrifugal acceleration?
Centrifugal acceleration is an outward apparent radial force pushing from the central pivoting turning point.
(This force is non-existent for inertial reference frames b.c. there is nothing pushing outward in your turn, whereas centripetal accel is pulled toward the center from gravity/weight)
56
What is the mathematical definition of load factor?
L = nW, n = L/W
* *rem: a_centrip = V^2/R
1. Pythag. Thrm (nW)^2 = Fc^2+(-W)^2
2. n = (Fc/W)^2+ 1)^(1/2)
3. Fc = mac = W/g*V^2/R
4. n = [1+ {V^2/(g*Rc)}^2]^(1/2)
57
What is service cieling?
The altitude at which an aircraft's maximum climb rate has a specific value.
*upper limit for steady flight often set to value 100ft/min - Piston engines, 500ft/min - jet engines
58
What is absolute cieling?
Absolute ceiling is the altitude at which an aircraft has no climb rate left (R/Cmax) = 0
*Can only be reached asymptotically, not practical limit because its at minimum excess power point.
59
What pt on the body do Lift and Drag forces act? what does this location imply?
Center of pressure, it is the point about which the moments on the body due to aerodynamic resultant forces are zero.
60
What is an important coefficient measure of a fluid's compressibility? What does its magnitude imply?
Speed of sound, lower speed of sound = higher compressibility.
61
What is the speed of sound?
The speed of sound is the distance travelled per unit time by a sound wave as it propagates through an elastic medium
a = √γRT
62
What is the primary difference in drag between low AR straight wing, Delta, and Swept wing?
A low AR straight wing reduces supersonic wave drag but has high subsonic induced drag. Delta and Swept wings also reduce wave drag without as high a penalty of subsonic induced drag.
63
What is the main function of a swept wing?
To reduce wave drag at transonic and supersonic speeds.
64
What are impacts of sweeping a wing?
reduced lift coefficients (in comparison to straight wing)
65
Why does a swept wing have less lift than a straight wing?
Due to angled nature, only a portion of free stream velocity is seen perpendicular to chord line. This causes pressure differential from the top to bottom to be less than a normal straight wing
66
What are the limitations of Prandtl's lifting line theory? (When does it not apply)
Low AR wings <4
Swept wings
Compressible flow
67
What is the aerodynamic benefit of a Delta wing?
Creates Vortex Lift.
68
What is vortex lift?
The production of vortex flow over the entire edge of a Delta wings that reattaches over the top surface and produces high energy, high vorticity flow. This reduces surface pressure on the top and increases general pressure differential AKA lift generation.
69
What is a wing-body combo, and how is it treated aerodynamically?
A wing-body combo is the mating of a wing and body within the flow field. It is treated as if the lift on the wing itself including the portion masked by the fuselage.
70
What is the critical Mach number?
Mcrit is the mach number at which there is a local pocket of sonic flow over a body. (supersonic flow has shocks, but are terminated by downstream pressure)
71
What is the drag divergence Mach?
Mdd is the mach number, just barely higher than Mcrit, where shocks begin to separate flow over the body (pressure due to shocks on the body over come the down stream pressure)
72
What is the primary design goal of supercritical airfoils?
Whitcomb designed airfoils to increase the "grace period" between Mcrit and Mdd, meaning airfoils which encouraged the supersonic flow with lower M to remain attached longer. *Results in weaker terminating shocks
73
What is the benefit of supercritical airfoil flow with respect to lift
Supercritical airfoils have relatively flat tops and negative cambered lower tailing edge. This results in lower lift coefficient due to smaller values of negative Cp.
74
What type of drag is supersonic wave drag?
Pressure drag
75
How does a shock wave flow field over wedge airfoil look?
Minf > 1, pinf in free stream, pressure increases over shock wave as velocity drops, and pressure drops over expansion to meet back to free stream.
76
How does a shock wave flow field over flat plate at an angle of attack look?
wave pattern with normal shock on bottom and expansion waves on top. This results in an increased pressure distribution on the bottom, constant over the entire plate. A reduced pressure on top, constant over the entire plate. And the reversed (normal shock on top, expansion on bottom) when leaving the plate to return to free stream.
77
For subsonic aircraft, which type of drag dominates Takeoff? Cruise?
Takeoff: Lift-dependent drag, induced
Cruise: Parasite drag
78
For supersonic aircraft, which type of drag dominates Takeoff? Cruise?
Takeoff: induced drag, parasite drag due to lift (higher than subsonic)
Cruise: Wave drag - combo of zero lift drag and induced
79
What area governs zero-lift parasite drag CD0?
Swet - Wetted area
| Swet ~ 2-8*Splanform
80
What does the tangency pt in drag polar represent?
Max L/D ratio for aircraft
81
Are CL/CD drag polars representative of all speeds?
No, because Cl and Cd change with mach number you will have different drag polars for each mach number.
82
Is there any range of speed which your drag polar will not change with mach number?
Yes, at low subsonic speeds, the differences are small enough and can be ignored.
83
Can you have drag inviscid flow? Why, why not?
You cannot have drag in 2D flow but you CAN have drag in 3D inviscid flow.
Because the drag polar is Cd = Cd0 + Cdi.
- In 2D flow you don't have induced drag, and Cd0 is governed by skin friction and pressure changes due to flow separation (Cd0 = 0 for inviscid flow)
- In 3D flow you have finite wings and induced drag Cdi terms even when Cd0 = 0.
84
What are viscous effects of flow?
Boundary layer - inducing flow separation
| Skin friction
85
Is pressure drag due to flow separation an inviscid flow effect?
No. Because the pressure drag is caused by separating flow, which is caused by viscous forces inducing the separation of the boundary layer it would be a viscous effect.
86
Are shock waves a viscous effect?
No, shock waves are caused by pressure forces. When the static pressure imbalance downstream of the flow is higher than the upstream flow it results in a shock wave. Which then causes wave drag, which is a no-friction drag.
87
Can you have wave drag in inviscid flow?
Yes, wave drag is a form of pressure drag which can occur in inviscid flow.
88
How can you adapt an airfoil to increase your CLMax?
To increase CLMax one needs to maintain attached flow for higher angles of attack. To delay separation, slots/slats can be added to increase the flow's pressure differential suctioning the attached flow through the LE/TE gap
89
Explain the aerodynamic impact of Slots/Slats
Slots - low pressure zone through front gap s.t. flow remain attached @ high flap deflection
Slats - modifies pressure distribution, with low pressure zone through tailing edge gap s.t. flow is suctioned over top surface to delay separation.
90
How can you adapt an airfoil to increase the lift of your airfoil by reducing your zero-lift alpha a(L=0)?
To increase your effective lift of an airfoil, the airfoil needs a more effective surface when at a(L=0). Therefore, by inducing a more negative zero lift angle of attack the airfoil is capable of a greater lift at a smaller angle of attack inherently increasing the lift. To do so, we use flaps/slots/slats/etc.
91
How can you adapt an airfoil to increase your dCl/da (Slope of your lifting curve)?
To increase your lifting curve slope of an airfoil, the lifting surface needs to increase in size. Through area flaps, deployables (Kruger), and increased effective chord line, the relative slope of the lifting curve increases and pivots your line to have higher CL, CLmax, and increased camber (negative a(L=0)).
92
TYSK: What is upwash and downwash?
Upwash is the ascending air over the wings
Downwash is the downward flow of air produced by pressure differential causing wing tip vortices to have downward velocity toward the ground.
93
TYSK: What is fly-by-wire? What benefit does it provide?
Fly-by-wire converts the pilot input to an electrical signal which is interpreted by a flight computer to send control input commands to the control surface deflection.
Safer, allows flight of inherently unstable aircraft configurations (maneuverability in military a/c), saves weight away from cables and hydraulics, finer control signals
94
TYSK: What is viscosity?
Viscosity describes how resistant a fluid is to flow.
95
TYSK: What is wash out angle? Why is it used?
Wash out angle is when the geometric twist of the wing reduces AOA of the airfoil from the root to the tip of the wing. It is used to delay stalling on the wing tips.
96
TYSK: What is a streamline?
Line along which all freestream velocity, V∞, of a particle is tangent to the body
97
TYSK: How does Cl change with Reynolds number?
1. Reynolds number tells you about relative inertial resistance to viscosity
2. As Re increases you are seeing a reduction in viscosity because Re proportional to (1/mu)
3. Reduced viscosity means reduced drag and increased lift.
98
TYSK: When are two flows dynamically similar?
When they have:
- Similar streamlines
- Distributions of V/V∞, rho/rho∞, T/T∞ are equal when plotted in non-dimensional coordinates
99
TYSK: Define two possible forms of wing twist
Geometric twist - when AOA changes over spanwise locations of airfoil distribution, also changes dcl/da for wing
αtip < αroot = wash out
αtip > αroot = wash in
Aerodynamic twist - when the airfoil changes along spanwise locations to change the aerodynamic properties over the wings (different α(L=0)).
100
TYSK: What is equilibrium glide velocity?
V∞ at which the aircraft must fly to achieve a specific L/D at a given height.
101
TYSK: Do you need more lift in steady level flight or steady climbing flight?
*Need more in level flight because climbing flight maintains a portion of W for lift
Steady level flight L = W
Climbing L = Wcos(γ)
102
TYSK: What is the design lift coefficient? When is it used?
Design lift Cl is a theoretical Cl for which α the flow is tangent to the camber line. Used in 5/6 digit airfoils
103
TYSK: What is the relative L/D for an airfoil versus an airplane?
L/D (airfoil) ~ 100
| L/D (airplane) ~ 10-20
104
What are two propulsive quantities that dictate aircraft performance?
Thrust/Power, fuel consumption
105
What information is contained in an engine deck?
Thrust, mach, fuel consumption/efficiency, and altitude of performance
106
What is "Wasted Power" on an aircraft?
The lost kinetic energy associated with thrust generation.
| 1/2(Vj-V∞)^2
107
Units of Power
Power = Force [N] * Velocity [m/s] = Energy [N*m] / time [s]
[kg*m^2/s^3]
108
What is the thrust equation?
The thrust equation is the time rate of change in momentum of flow entering and leaving the engine.
T = mdot*(Vj-V∞) = mass flow rate * (Vout-Vin)
109
What properties govern the thrust equation?
Newton's second law - time rate of change of momentum = force
T = mdot*(Vj-V∞) = mass flow rate * (Vout-Vin)
Says to increase thrust we either increase mass flow rate through inlet area or increase differential between inlet-exit flow velocities.
110
What is the Biot-Savart Law? Assumptions?
Biot Savart Law actually comes from magnetic fields, saying you can calculate the strength of field for a distance from a given current carrying wire. In aerodynamics we can calculate the velocity at a specific distance from the circulatory flow given a vortex filament with strength Γ.
Assumptions:
-Inviscid, incompressible flow, finite wing aero (defined for +/- ∞ filament)
111
Define isobaric, isothermal, isentropic
Isobaric - lines of constant pressure
Isothermal - lines of constant temperature
Isentropic - ideal thermodynamic efficiency - CONST entropy (adiabatic and reversible)
112
Define adiabatic, reversible thermal systems
Adiabatic - no losses in heat transfer (only energy is transferred)
Reversible - no loss in entropy
113
What is entropy and enthalpy?
Entropy (s) - is the measure of heat added to the system reversibly
Enthalpy (h) - total energy + pressure*Volume (perfect gas) h = e + pv
relates total heat content of the system
114
How many laws of thermodynamics are there? What are they?
4 laws, 3 primary and 1 secondary:
0th: if two thermodynamic systems each are in thermal equilibrium with a third one, then they are in thermal equilibrium with each other.
1st: Total energy is conserved
Change in internal energy = net heat + net work
dU = ΔQ + W
(not path dep Energy state)= Path dependent net heat supplied to system) + net work done on the system
2nd: Change in Entropy = net heat added reversibly /Temperature
dS=ΔQ/T
3rd: Abs Zero def 0 [K] for system with zero KE.
115
What theory and its assumptions are used for the distribution of vortices over thin airfoil that satisfies Kutta Joukowski Thrm?
Thin Airfoil Theory: distributes vortex sheet along camber line s.t. streamline flow results in KJ is satisfied γ(TE) = 0
Assumptions:
- Cl = 2πα
- Lift slope = 2π
- cp and ac located at quarter chord
116
What causes an adverse pressure gradient?
Geometric of body - changing flow directions
117
How does a drag polar change with M∞?
It squeezes down and shifts to the right, as induced drag increases exponentially with increased Lift, and lift is increased as speed increases.
118
How does camber change the AC and Cp of an airfoil?
```
Symmetric airfoils:
cp and ac @ c/4, moments = 0
Cambered:
a(L=0) changes, so Cl = 2π(α-α(L=0))
*thin airfoil theory
AC still at c/4 but now finite and moments ≠0
Cp shifts aft as α(L=0) gets negative
```
119
How does thickness affect the Cp and AC of an airfoil?
As you increase thickness you increase Cl, so you adapt AOA to maintain same Cl. Inducing more negative Cma.
*Changes for airfoil families
AC tends to move forward of c/4 with thickness, Cp distribution gets more negative with increased lift, moves Cp further aft.
120
Explain how thickness changes the flow over the airfoil
As thickness increases, airfoil sees induced angle of attack changing the curve. Also, see a smoother Cl and reduced Clmax. Due to the thickness changes, you see the stalling occur later in the flow pushing down from the LE to the TE.
121
What is the Thin Airfoil Theory?
The assumption that an airfoil is thin enough to compress the body down to its camber line, s.t. can represent the body as a streamline and distribute vortex sheet along the camber line for aerodynamic calculations.
122
What are the assumptions of the TAF?
Is only valid for small AOA
- To assume streamlined body, Vn = 0 at all control points on the body by definition.
- This is only true for small angles of attack because as AOA grows, the flow would separate and no longer be a streamline along the body (Vn = 0 @ all control pts)
123
Total Power Equation
P=T*V∞+1/2 mdot*(Vj-V∞ )^2
124
What is the total power efficiency?
ηp=(useful power available)/(total power generated)
125
What are Helmholz Theorems?
1. Vortex filament is line of constant strength vortices
2. Vortex filament cannot end in a fluid - must extend to the boundary or form closed path
3. Strength of vorticity is proportional to the length of the vortex line
126
What aerodynamic properties are explained by Helmholz laws?
Trailing vortices along finite filament - Horseshoe vortex.
Filament can't end at the edge of wing, so vortex flow continues to ∞ trailing the wing tips
127
What is Vstall? What dictates it?
The lowest possible velocity an airplane/airfoil can maintain steady level flight. Its dictated by CLmax.
128
What are benefits and pitfalls of rectangular wings?
Benefits:
Rectangular wings separate at the root first leaving full control over ailerons.
Easier to manufacture
Pitfalls:
Lift distribution < elliptical
lift distribution is rounded square (spanwise)
129
What are the benefits of an elliptical wing?
Benefits:
Has greatest lift distribution (e = 1)
minimizes induced drag
Pitfalls:
Tough to manufactures
No twist or variation, so the whole wing stalls at once
130
Explain vortex flow
Vortex flow is concentric streamline flow where V = constant along each streamline. V is tangent to the flow at each point, Vn=0, and varies inversely w.r.t. radial distance.
```
∇xV = 0 @ every pt but origin (irrotational)
∇∙V = 0 @ every pt (incompressible flow)
```
131
What the definitions of TSFC and PSFC?
TSFC = Thrust specific fuel consumption
-fuel consumed by gas turbine engine in order to generate unit thrust
ct = [1/hr] = (fuel flow [lb/hr])/(Thrust [lb])
PSFC = Power specific fuel consumption =
-fuel consumed by piston engine in order to generate unit shaft power
c = [lb/(hp.hr)] = (fuel flow [lb/hr])/(Thrust [hp])
*ct = cV∞/ηpr
132
How do you get attainable airspeed for a given thrust?
```
Assuming steady level flight, T = D.
Tr = D = q∞S(Cd0+kCl^2)
*break into components of Cl
*Multiply eq. bi V^2, get bi-quadratic
*Solve for V = V(T/W, W/S)
```
133
what two ways reduce TSFC/PSFC?
1. Reduce required thrust Tr and required power Pr
- Solve TRmin for L/Dmax -> CL/CDmax
- Solve PRmin for [(CL^3/2 /CD)]max
2. Design engine cycle for better efficiencies ct, c
134
What is the relationship between change in weight of fuel over time?
dWf/dt is the fuel consumed over unit time for given thrust.
dWf/dt = -TSFC*T
dWf/dt = -PSFC*p
135
What varies installed thrust?
Mach #, Altitude, Afterburner operations
dry - no afterburners
wet - use of afterburners
T/W = α/β{Tsl/Wto}
136
What are the benefits/disadvantages of AR changes?
- High AR: Lower subsonic induced wave drag, higher supersonic wave drag b.c. outside mach cone
- Low AR: high induced subsonic wave drag, low induced supersonic wave drag
137
What are the benefits/disadvantages of sweep changes?
Sweep reduces wave drag in transonic regime, and supersonic speeds
Has lower lift than straight wing because sees potion of freestream velocity over chord
138
What are the benefits/disadvantages of taper ratio changes?
increasing taper from root to chord reduces induced drag by approximating an elliptical lift distribution.
Easier to manufacture than elliptical wing.
It pushes the stalling point out from the root to the tip as you increase the taper ratio, ideal is about 0.3 so you don't lose maneuverability of ailerons and stalling at the tip for ct = 0.
139
What are the impacts from downwash do you see on relative wind and local AOA?
1. Local airfoil sections have induced AOA due to downwash which reduces effective angle of attack from geometric angle of attack. a_eff = a_g-a_ind
2. The relative wind vector is changed to be aligned with local lift vector s.t. lift has a component in V∞ direction, and drag component created = induced drag.
* reduces aeff, increases induced drag Di
140
What do the points of intersection between Pa and Pr, Ta and Tr curves mean?
Indicate maximum speed attainable at that altitude for a given PA=PR and TA=TR.
Power intersection at max shows the points where R/C = 0, there is no excess power, Vmax.
141
TYSK: Why don't we use props for faster A/C if they are such highly efficient systems?
As you increase in speed, the propeller tip speeds increase with rotational velocity - r*ω and see shock waves forming early on.
*Shock waves -> increased wave drag -> increase torque on engine -> reduces RPM -> Pavailable reduces -> T reduces
142
What drives power output in a reciprocating engine?
```
Engine size: cylinder displacement
# engine cycles: #power strokes/min (RPM) - greater power output
Force applied by combustion doing work on piston: higher pressure - more power
```
143
What is taper ratio? What is its effect on the wing with small taper versus large?
Taper ratio = chord tip/chord root
By tapering the wing you can get closer to an elliptical lift distribution for a swept wing
smaller ct/cr:
- lift distribution shifts toward root, reducing bending moment over span of the wing
- can drastically reduce structural weight b.c. sees less loading w/out losses in lift
larger ct/cr:
- as you increase ratio, you see separation move closer to tips and losses of lift or undesirable separation occurs.
- Could lose use of ailerons
144
TYSK: What is the purpose of winglets? (benefits/disadvantages)
Purpose:
Winglets act as fence to block/inhibit high pressure air on bottom from escaping around wing tips to lower pressure on top.
Induce more 2D flow over wing
Benefits:
Reduces wingtip vortices (reduces downwash & induced drag)
Allows wing to generate more lift
Provides lower fuel consumption (longer range, heavier payloads, short TOFL)
Disadvantages:
++ structural complexity, alters W/S, requires stronger structure, increases viscous drag over more are
145
What is advance ratio for propellers?
Advance ratio is a dimensionless performance parameter for prop efficiency.
J = V/(N*D) - N = # blades, D = prop diam
*Can still have zero efficiency when prop on b.c Vinfinity is = 0.
(graph arcs w.r.t. mounted blade angle) We know J does arcs w.r.t. increasing speed and angle of mounted blades. As you increase your angle you increase the lifting flow over each blade, so as the J does is convex curve, you peak higher.
146
Explain variable ROC w.r.t. propeller efficiencies between fixed pitched, constant speed, and two position props (variable pitch)
Fixed pitch prop has fixed blade angle, so the max efficiency is reached at a specific V∞ as a design point (chosen by designer). FP prop has lowest climb rate because lower efficiency and loading on engine slows RPM (power) even if you maintain Lift
Variable pitch prop intends to let user change pitch for increased efficiency at given V∞. Means its improvement on ROC due to constant RPM, but still has high loading on engine torque
Constant speed props have the highest efficiency and bet ROC because of the governor over variable pitch for constant RPM, increasing effectiveness and reduces pilot loading.
147
What are your L/D ratios for prop and jet max range, endurance, and carsons speeds?
```
CL/CDmax: Min TR
*Best jet endurance, best prop range
(CL^3/2 /CD)max: Min Pr
*Best prop endurance
(CL^1/2 / CD)max: Min TR/V∞
* Best jet range, carsons best propeller cruise speed
```
148
Explain drag of an airfoil in subsonic flow
profile drag (Cd) = skin friction drag (Cf) + form drag (Cdp)
-viscous effects
149
What is D'Alembert Paradox?
In incompressible inviscid flow, the assumptions imply the potential flow over an airfoil has zero drag. Because of symmetrical pressure distribution with no viscous forces there are no shear forces and no flow separation.
Instead, D'Alembert recognized that actual flow is viscid and there is flow separation. So, what he hypothesized was that when it separates the integrated pressure distribution is unbalanced producing pressure drag (form drag).
150
Explain 4 digit NACA airfoil names
2412:
digit 1 - '2' max camber in %c
digit 2 - '4' location of max camber in 1/10c from LE
digit 3,4 - '12' max thickness in %c
151
Explain 5 digit NACA airfoil names
23012:
digit 1 - '2' design cl*(3/2) in 1/10c
digit 2,3 - '30' location of max camber*1/2 in 1/10c from LE
digit 4,5 - '12' max thickness in %c
152
Explain 6 series NACA airfoil names
64-212:
digit 1 - '6' series designation - Laminar flow airfoil to reduce skin friction
digit 2 - '4' location of min pressure in 1/10c
digit 3 - '2' design cl in 1/10c
digit 4,5 - '12' max thickness in %c
153
Explain total drag of an airplane
Total Drag (D) = Parasite Drag + Induced Drag + Wave Drag
Parasite - profile drag for total aircraft (includes interference drag)
Induced drag - drag due to lift production
Wave drag - compressibility and viscous effects of M>1
154
What is the effect of wind on range, endurance, climb angle and ROC?
Endurance is simply V∞ for max endurance and not a function of wind (ground speed). Similarly, ROC is the vertical component of flight, which is unaffected by the horizontal Vg.
Range is function ground speed:
- In presence of headwind, airspeed for max range is higher, and lower for tailwind.
Climb Angle:
- in presence of wind, you see increase in climb angle with HW and decrease with TW
* Which is why ROC doesn't change because your speed proportionally changes with angle
155
What are Payload-Range Characteristic ranges?
Harmonic Range - Max payload, less than full fuel, less than MTOW
Max Economic Range - less than max payload, full fuel, MTOW
Ferry Range - 0 payload, full fuel, less than MTOW
156
Give Tsl/Wto, Wto/S, AR for typical jet engine Boeing 747 - 300pax
Tsl/Wto ~.25
Wto/S ~ 150 N/M^2
AR ~ 7
157
What is a pitot tube? How?
A pitot tube measures the velocity of the flow using a static and ram air stagnation pressure differential.
The static pressure measures the random motion of the molecules in the gas.
The ram air measures the pressure at stagnation pressure when V = 0.
This differential allows the flow velocity to be calculated in incompressible flow/bernoulli's eq
158
TYSK: What is supercharging an engine?
In a piston engine, when the air is compressed before going into the engine (to increase p and ρ), with the intent of increasing performance of the engine as if it were at lower altitudes.
*similar for turbocharging, but with turbine
159
What is the coefficient of pressure? Describe pressure distribution on top/bottom of airfoil
Coefficient of pressure is the non-dimensional differential between p at some point and freestream pressure.
cp = (p-p∞)/q∞
top surface has negative cp because p
p∞. Capped between +/- 1, over x/c
160
TYSK: For what type of airfoil is a laminar flow drag bucket seen?
6-series laminar flow airfoils designed to reduce skin friction by encouraging laminar flow longer.
161
Considering L is fixed, is CL for an airplane higher at TO or cruise?
L = 1/2ρ∞V∞^2*CL*S
Cruise - V∞ increases, so CL decreases
TO - V∞ decreases, so CL increases
Takeoff has higher CL
162
Which performance parameters govern climb performance?
Vstall, W/S
(T-D)V∞/W = ROC
163
How does CL vary with V∞?
As V∞ increases, CL decreases quadratically.
| Lift increases with V∞^2, but CL and AOA decrease in parallel.
164
What is energy height?
Energy height is the representative altitude an aircraft would have per unit W were all its kinetic energy to be traded for potential energy.
165
How is energy height represented graphically?
Sky-Maps - Altitude Vs. Mach with iso-lines of energy height curves to which energy must be traded to move between.
166
How does your drag polar change with control surface deflection?
Control surface deflection both increases CL and CD so it shifts up and over to right with df.
167
What is source flow/source panel method? What type of flow is it used for?
2D potential flow approximation method which is used to represent a body with a source and sink to derive the non-lifting potential and stream functions for irrotational flow.
-No circulation, no vorticity -> irrotational flow
168
What is the vortex panel method?
The Vortex panel method is a lifting flow approximation used for 2D thick airfoils (cannot use thin airfoil theory) to represent the body with vortices of varied strength over infinitesimal panels ds over the airfoil.
*More accurate, can derive lift because it includes vorticity and circulation
169
What is the vortex lattice method?
The vortex lattice method discretizes a finite wing into a finite number of panels to derive the lifting effects of circulation over the wing.
* Includes induced drag, but no skin friction drag b.c. 2D
* Can account for sweep, flaps, twist, dihedral, etc...
170
What are the phases of a turbojet engine?
Diffuser, Compressor, Combustor, Turbine, Nozzle
171
Explain the trend each phase of turbojet engine sees w.r.t. Pressure, Temperature, Velocity (of air), and net Force in the system.
Diffuser: Slows down free stream air, which slightly increases P,T from ambient pressure/temp. Increase net force.
Compressor: Does work on air by rotating compressor blades which greatly increases P,T, and begins to speed up Velocity. Increase net force.
Combuster (burner): Mixes fuel into air and burns at relatively constant pressure, further increases T,V,F.
Turbine: burned air expands through turbine, work is extracted by shaft for compressor, decreases P,T,F, and bubbles V.
Nozzle: Final expansion through nozzle to increase Vj, decrease P,T,F.
172
What are the primary factors which constrain level turn performance?
Stall speed, max structural loading 'n', and vehicle max thrust.
173
How do you maximize level turn performance?
Minimize turn radius R, maximize turn rate ω
174
What is the difference between Flutter & Buffeting? (which is a design constraint)
Flutter is a the component vibration due to excitation at the natural frequency of aerodynamic flow and elastic properties on lifting surfaces/vehicle.
- static-aeroelastic phenomena
- a design constraint
Buffeting is an aerodynamic turbulent flow phenomena due to separation behind lifting surface.
- Leads to random fluctuations of pressure forces on body (shaking)
- Causes life cycle fatigue
175
What form of propulsion/energy has no weight change?
Solar Power
176
What for of propulsion/energy has a weight increase?
Hydrogen fuel cells
177
What is the difference between consumable and non-consumable energy? (does it affect traditional sizing/synthesis?
Consumable - weight decreases as energy used (gas)
Non-consumable - weight does not decrease as energy used (batteries)
-It changes weight decomposition approach and weight change differential for fuel fractions/Mission Phase Weight estimate
178
What type of drag is potential flow theory unable to predict?
(potential flow = no circulation --> no viscosity --> no boundary layer)
Drag due to viscous forces - Form/Skin Friction/Induced
179
TYSK: What is a mach angle?
Angle between mach wave and flow direction.
180
TYSK: What is the neutral point? Why is it significant?
Neutral point is the point on A/C where if C.G(@np) the moment will be zero - on cusp between stable/unstable
This is significant because stable aircraft are designed with CG in front of NP.
181
What are the lateral modes of stability? Which are stable/unstable
Dutch Roll Mode - Lightly damped, low freq oscillatory mode [Can be unstable, if large amplitude]
Spiral Mode - Slowly convergent/divergent motion [relatively stable b.c. so slow and easy to correct]
-based on lateral stability (dihedral)
Roll Mode - Highly convergent, heavily damped, non-oscillatory motion [Stable]
182
What are the Longitudinal Modes of Stability?
Short Period Mode - high frequency, highly damped mode [Stable]
Phugoid Mode - low frequency, lightly damped [semi-unstable]
183
What types of aircraft can Longitudinal modes of stability be unstable for?
Aft C.G aircraft - unstable phugoid modes
| [***remember which type, think its fighters]
184
What is Aeroelasticity?
The field of study concerned with interaction between deformation of an elastic structure in an airstream resulting in aerodynamic forces
185
TYSK: What types of aeroelastic effects are there?
Static Aeroelasticity - study of interaction between aerodynamics and elasticity
[Effects: Divergence, control reversal]
Dynamic Aeroelasticity - study of interaction between dynamics, aerodynamics and elasticity
[Effects: Flutter, Buffeting, Transonic Dip]
186
What is the dihedral effect? What type of wing is this used on (high/low)
The use of dihedral/anhedral angle of wing chord like w.r.t. horizontal
- Dihedral (positive - up) increases dynamic stability, reduces maneuverability. Also decreases lift than straight wing. Used on LOW WING
-Anhedral (negative - down) used to reduce dynamic lateral mode "dutch roll" on HIGH WING, induces lower stability but more maneuverability.
187
What are the different types of speeds in Aerodynamics? & definitions
Indicated -
Equivalent -
Calibrated -
True-
