Aerodynamics (AA1290) Flashcards
What is center of gravity (Cg)?
-“3 axes intersect and all weight is concentrated”
-the point where the roll, yaw, and pitch axes intersect and from which all axes movements are measured
-the point where all weight is considered to be concentrated
What is aerodynamic center (AC)?
-“changes in lift occur”
-the point along the chord line of the wing, approx. 23-27% from the leading edge, around which the wing forces act through and all changes in lift effectively occur
What is Angle of Attack?
Angle between the wing’s chord line and the relative wind
What are the 4 primary forces acting on an aircraft?
Lift, Weight, Thrust, and Drag
How does weight act on the aircraft?
Weight acts through the aircraft’s CG towards the Earth (always perpendicular to the surface regardless of attitude)
What is lift and how does it act?
-Lift opposes the downward force of weight and acts perpendicular to the flight path
-Lift can be in any direction (not always upwards) based on aircraft attitude
-Wings generate most of the lift, but the horizontal stab and fuselage also generate some
What is drag and how is it formed?
-Drag is the rearward force that opposes thrust
-Drag results from the disruption of airflow and the production of lift
What is thrust?
-thrust is the acceleration and reaction force described in Newton’s 3rd law
-forward force that opposes drag
-assume thrust acts parallel to the longitudinal axis in the T-38C
What is aerodynamic force (AF)?
the combined effect of lift and drag in a single net force
Where does aerodynamic force come from and where does it usually point?
-aerodynamic force is the result of pressure and friction distribution over the wing
-direction and size varies, but generally points up and to the rear of the aircraft
When the aircraft is in stable, unaccelerated flight, the forces are?
balanced
What happens to the 4 forces in a turn?
-Lift: is being shared in a horizontal and vertical component (which decreases the amount available to oppose weight)
-Weight: remains the same
-Drag: ultimately increases b/c AoA must be increased to compensate for the decreasing vertical lift
-Thrust: must increase proportional to the turn to overcome the additional drag
What is the only force that does not change in size or direction when a climb is initiated?
Weight
How does the 4 forces get affected in a climb?
In a stabilized climb, lift is less than weight and thrust must increase to overcome
W: weight is now not only acting downward, but has a rearward component with drag (WD)
L: lift is still equal to the downward force of weight (WL)
T: Thrust must increase to overcome the addition of Wd
The steeper the climb angle, the (more/less) lift is required to maintain balanced flight.
Less lift.
It is the thrust force that increases to support the drag portion of weight (WD) that is not being supported by lift (L = WL)
What are the two components of weight in a power-on descent?
WL and WT (the thrust component of weight)
How are the 4 forces balanced in a power-on descent?
W: 2 components; WL and WT
L: is still opposing WL
D: equals T and WT
T: MUST be less than drag
In a power-on descent, ___ must be less than ____
thrust is less than drag (b/c D = T + WT)
In a wings-level, constant airspeed climb, lift is ____ weight
less than
In the flow of incompressible fluid, the sum of static pressure and dynamic pressure is ____
constant (which is why pressure and velocity are inversely related)
What are the two key factors that the pilot can control in the lift equation?
Velocity and Angle of Attack
What is the simplified “pilot’s lift equation”?
Lift = V^2 x AoA
What is stall angle of attack?
the AoA associated with the lift limit of the wing at a given airspeed (beyond this point results in a decrease in CL)
The aircraft always stalls at the same ___ regardless of ____
AoA, regardless of airspeed
What is stall speed (Vs)?
the minimum airspeed under a given set of conditions that the aircraft will stall (this speed varies)
What is the minimum airspeed to maintain straight-and-level flight?
1-G stall speed
How does weight vary stall speed?
The greater the weight, the greater lift required to maintain level flight, therefore STALL SPEED INCREASES
(same reason why approach speeds increase with weight)
How does flap configuration change stall speed?
Lowering the flaps DECREASES stall speed
How does bank angle affect stall speed?
Increasing bank angle = INCREASING STALL SPEED
How does altitude affect stall speed?
an increase in altitude results in a higher stall TAS, however there is very little (if any) change to the stall CAS
How does stall speed change with and without thrust?
Stall speed with full thrust is somewhat lower than idle power stall speed
In general, what effect does an increase in AOA have on lift if CAS is kept constant?
Lift increases
What effect does lowering the flaps have on the 1-G level flight stall speed of the T-38?
decreases the stall speed
What effect does increasing bank angle have on the level flight stall speed of the T-38?
Increases the stall speed
What is parasite drag?
drag not associated with the production of lift
What are the three main types of parasite drag?
Interference, skin friction, and form drag
What is form drag?
parasite drag generated by the aircraft due to its shape
What is interference drag?
parasite drag from the intersection of airstreams that cause eddy currents, turbulences, or restricts smooth airflow
What is skin friction drag?
parasite drag that is the aerodynamic resistance due to the contact of moving air over the aircraft surface (creates a boundary layer of essentially motionless air)
At transonic speeds and above, a fourth type of parasite drag called ____ is encountered
wave drag
Parasite drag varies with ____
velocity; as velocity increases so does parasite drag
What is induced drag?
drag caused by the production of lift
Induced drag varies with ____
AOA (NOT airspeed)…the induced drag for a 4-G maneuver at 400 KIAS is greater than the same maneuver at 3-G b/c of the greater AOA
How does induced drag change with weight?
the heavier the aircraft, the greater lift is required, therefore induced drag also increases
How does induced drag change with bank angle?
Induced drag increases with bank angle
What is total drag?
The sum of parasite and induced drag
What is L/D max in the T-38C?
230 KCAS plus 1 KCAS for every 100 pounds of fuel
An increase in level-flight airspeed causes induced drag to ____
decrease
What is the relationship between parasite drag and aircraft velocity? (Linear, exponential, negative, etc)
Parasite drag increases exponentially with velocity (CAS)
How does the parasite drag curve change by lowering the gear?
Lowering the gear increased parasite drag which creates a steeper curve with an increase in airspeed
How does the drag due to thrust curve change with increasing weight?
an increase in weight causes an increase in L/D Max CAS. Total drag at the corresponding L/D Max also increases. Thus, we expect the left side of the total drag curve to shift upwards
What is thrust required (TR)? What must it equal?
the thrust needed to maintain steady, unaccelerated level flight. Thrust required must equal total drag (DT)
What is thrust available (TA)
the thrust the engines produce at a given throttle setting, velocity, and density
What is assumed about the available thrust of a turbojet?
it is assumed to be a straight line
What happens to thrust available with temperature?
As the temperature increases, the TA decreases
How does thrust available vary with altitude?
Increasing altitude has less dense air, which decreases the mass flow rate and TA.
The change in pressure altitude has a greater effect on air density than temperature (which is why performance decreases with altitude)
What does absolute ceiling mean?
the point at which the aircraft reaches an altitude where the maximum TA equals the minimum TR
Where on the graph is the maximum speed the aircraft can obtain in level flight?
the intersection of the TR curve and maximum TA line
What is the lock point?
the minimum airspeed at which an airspeed can maintain level flight for that power setting
Where is the lock point on the TA vs TR chart?
the first intersection of TA and TR
Where does the maximum amount of excess thrust occur at?
L/D Max (the lowest point on the TR curve)
What is the region of normal command vs. the region of reverse command?
normal command - greater than L/D max where increasing power setting increases airspeed
reverse command - less than L/D max where a higher power setting is required to fly a slower speed
Which combination of pressure altitude and temperature produces the most thrust avaliable?
Low pressure altitude and low temperature
How does a low AR wing compare to a high AR wing?
a low AR wing cannot produce as high a CL as a high AR wing (a low AR wing also has a much higher stall speed)
How does drag compare on a low AR wing vs. a high AR wing?
a low AR wing produces more induced drag at low airspeeds and less parasite drag at high airspeeds (the T-38C)
How does wing sweep affect Cl?
a swept wing generates less CL than a straighter wing
the longer chord and spanwise flow is a shallower lift curve slope and a lower CL max
How does stall angle of attack change with a swept wing?
a swept wing is less sensitive to changes in AoA because of its shallower curve slope (you can reach a higher AoA before stall occurs)
however, at slow airspeed a higher AoA is required for the same CL b/c lift production is less efficient with swept wings
What is one of the main reasons for sweeping a wing?
Reduce parasite drag at higher airspeeds (by decreasing the frontal area but still maintaining the wing surface area)
However, b/c induced drag is inversely proportional to AR, a swept wing generates more induced drag at low airspeeds
What kind of wing does the T-38 have?
symmetric, low AR, tapered, and swept-back
Where does the T-38 wing first stall at?
at the wingtips first (unlike a straight cambered wing)
A low aspect ratio wing produces ___ parasite drag at high speed as compared to a high aspect ration wing with the same surface area
less parasite at high speeds
In general, sweeping a wing aft will ____ the CL for a given AoA
decrease
Unlike a conventional straight cambered wing which generally stalls at the ____ first, the T-38’s sweptback symmetrical wing begins to stall at the ___
wing root; wingtip
What did the T-38 incorporate to control wave drag?
Area Rule
-drag at high speeds is a function of the aircraft’s cross-section area…gradual changes allow air to flow smoothly around the aircraft with the least amount of disturbance
What does the T-38 have to overcome the increased load forces on the control surfaces at high-speeds?
Irreversible flight controls (meaning the pilot has no direct connection from the flight controls)
What kind of all-moveable control surface does the T-38 have to deal with shock-induced separation?
Horizontal Stabilator
What design features improve the T-38’s high speed flight performance?
Area rule, irreversible flight controls, and a large horizontal stabilator
What is subsonic, transonic, and supersonic flight?
Subsonic: airflow at a speed less than the speed of sound
Transonic: mixed airflow that is part subsonic and part supersonic
Supersonic: airflow at a speed greater than the speed of sound
What is the speed of sound? Is it constant?
-the rate at which pressure waves propagate through air
-speed of sound is not constant, it depends primarily on temperature
What is the key factor which determines the speed of sound?
Temperature
What is Mach number?
used to measure the speed of an aircraft; the ratio of an aircraft’s true airspeed to the speed of sound
