HFH Ch2: Aerodynamics of Flight Flashcards
The inner 25% of the rotor blade is referred to as the _____ region. This region operates above its maximum AOA, also referred to as _____, causing drag & slowing rotation of the blade.
Stall; Stall angle/Critical AOA
AOA is usually _____ at higher airspeeds and _____ at lower airspeeds.
Lower; higher
The _____ region produces the forces needed to turn the blades during autorotation.
Driving
During autorotation in forward flight, all 3 regions of the blade move _____ (outboard/inboard) along the blade span on the retreating blade where AOA is _____ (higher/lower).
Outboard; higher
This is because inflow of air is going up through the rotor disk rather than down.
If the engine fails, the _____ unit automatically disengages the engine from the main rotor. Allowing the main rotor to rotate freely.
Freewheeling
The driven region of the rotor disk produces some lift, but that lift is offset by _____.
Drag
The overall result is a deceleration in the rotation of the blade.
_____ pitch must be increased in order to maintain altitude & airspeed while turning.
Collective
Collective pitch controls the AOI and along with other factors, determines the overall AOA in the rotor disk.
_____ + _____ = AOA
Resultant relative wind; chord line
AOA can change with no change in blade pitch angle or AOI.
What is the difference between centripetal & centrifugal force?
Centripetal force “pulls” or acts toward the center of rotation - a real force
Centrifugal force “pulls” or acts away from the center - considered a pseudo force
Total aerodynamic force (TAF) in the _____ region is inclined slightly forward of the axis of rotation.
Driving region
This forward incline produces a continual acceleration force during autorotation
How do pilots avoid retreating blade stall?
By not exceeding the never-exceed speed (VNE)
Contacting the ground with skids during sideward flight will likely result in a _____.
Dynamic rollover
Why do transverse flow effect vibrations happen close to the same airspeed as ETL?
This is where the greatest lift differential exists between the front & rear portions of the rotor. Just below ETL on takeoff & after passing through ETL on landing- between airspeeds 12-15 knots
During vertical autorotation, the rotor disk is divided into what 3 regions?
Driven, driving, & stall
During translating tendency, the helicopter wants drift slightly to the _____.
Right
As the relative wind speed of the advancing blade increases, the blade _____ (gains/loses) lift & begins to flap _____ (up/down).
Gains; upThis upflap creates a downward flow of air & has the same effect as increasing the induced flow velocity, & decreasing the AOA.
Describe Ground Effect
Aka In Ground Effect (IGE) Increased efficiency of the rotor disk caused by interference of the airflow when near the ground. Because of this interference, induced flow is reduced, increasing AOA and lift efficiency.
What changes the blade’s AOA differentially around the rotor disk & creates a differential lift?
Cyclic feathering
This controls the rotor disk attitude & is the means to control rearward tilt of the rotor (blowback).
What 4 conditions cause retreating blade stall?
- High airspeeds (exceeding VNE)
- High collective pitch angle
- Excessive forward cyclic
- Low Nr (rotor RPM) 3 highs, 1 low
Transverse flow effect _____ (increases/decreases) the AOA at the front disk area causing the blade to _____ (flap up/flap down).
Increase; flap up
A reduction in AOA at the aft causes the blade to flap down. Increasing the chance for blowback.
As induced flow increases, resultant relative wind becomes less _____ (horizontal/vertical). Causing AOA to _____ (increase/decrease).
Horizontal; decrease
Because AOA is determined by measuring the difference between the chord line & the resultant relative wind, as the resultant relative wind becomes less horizontal, AOA decreases.
While transitioning to forward flight around 16 to 24 knots, the helicopter goes through _____.
Effective translational lift (ETL)
The propeller region, or _____ region, consists of 30% of the blade radius.
Driven
How does Newton’s Third Law apply to flight?
The air deflected downward also produces an upward (lifting) reaction.
_____ occurs when the tail rotor becomes more aerodynamically efficient during the transition from hover to forward flight.
Translational thrust
The autorotative region, or _____ region, lies 25-75% of the blade radius.
Driving
The _____ changes the AOI of the rotor blades which changes their AOA.
Collective
Changing the AOA increases drag on blades, power delivered by engine must increase to maintain rotor speed
As the helicopter accelerates in forward flight, induced flow drops to near 0 at the forward disk area & increases in the aft. These differences between the fore & aft portions of the rotor disk are called _____.
Transverse flow effect
Define camber
Curvature of the airfoil
What are the 3 types of rotor systems?
Rigid, semi-rigid, fully articulated
During ETL, the rotor disk outruns the recirculation of old vortices & begins to work in relatively undisturbed air. What 3 ways does this affect airflow?
“HRI”
1. Airflow becomes more horizontal
2. Reduction in induced flow & drag
3. Increase in AOA & lift (The additional lift available at this speed is referred to as ETL)
1 causes 2, 2 causes 3
The tendency for the nose to rise or pitch up is caused by the combined effects of what?
Dissymmetry of lift & transverse flow effect (This is pitch up is also called blowback)
How do you counteract transverse flow effect?
Cyclic input to the left
Rotational relative wind combines with induced flow to form what?
Resultant relative wind
What are the 2 ways to recover from retreating blade stall?
- Lower collective - This decreases AOA & airspeed, increasing NR2. Reduce severity of flight maneuver & adjust flight controls to normal flight
What are signs of retreating blade stall?
- Vibrations
- Vertical bounce
- Flight control feedback
- Pitch up & roll to retreating side“VVFP”
Main rotor blades _____ & _____ automatically to equalize lift across the rotor disk.
Flap & feather
The combination of what 2 things acting on the retreating blade limit the maximum forward airspeed of a helicopter?
Blade flapping & slow relative wind
What’s the difference between a rigid, semi-rigid, and fully articulated rotor system?
Rigid: feather
Semi-rigid: flap & feather - via teetering hinge
Fully articulated: each blade individually flaps, feathers, & lead & lags (“blade hunting”)
Each blade has a _____ AOA due to induced flow.
Decreased (Induced flow = downwash)
What describes the unequal lift between the advancing & retreating halves of the rotor disk caused by the different wind flow velocity across each half?
Dissymmetry of lift (The advancing side produces more lift than the retreating)
In the absence of induced flow, AOA & AOI are _____ (the same, AOA is greater, AOI is greater).
The same
AOA = resultant relative wind + chord line
AOI = rotor hub + chord line
The speed of a rotating mass varies proportionately with the _____ of the radius.
Square
Changes in angular velocity, known as angular acceleration & deceleration, take place as the mass of a rotating body is moved closer to or farther away from the axis of rotation. Think figure skater performing a spin on ice skates.
Improved rotor efficiency resulting from directional flight is what?
Translational lift
The tail rotor also becomes more aerodynamically efficient during the transition from hover to forward flight (translational thrust).
As the relative wind speed of the retreating blade decreases, the blade _____ (gains/loses) lift and begins to flap _____ (up/down)
Loses; down
This downflap creates an upward flow of air & has the same effect as decreasing the induced flow velocity, increasing the AOA
How do you calculate the velocity of the air meeting the advancing or retreating blade?
Rotational velocity of the blade plus or minus wind velocity resulting from forward air speed.Ex. The blade tip speed of a helicopter is 400 knots. If the helicopter is moving forward at 100 knots, the relative windspeed on the advancing side is 500 knots. On the retreating side is only 300 knots. This difference in speed causes a dissymmetry of lift.
Which type of drag remains relatively constant throughout the range of speeds?
Profile
The airflow meeting each blade varies continuously as the blade rotates. The highest velocity of airflow occurs over the _____ (3/9 o’clock position) or the _____ (advancing/retreating) blade. Making the lowest occur where?
Highest: 3 o’clock; advancing blade
Lowest: 9 o’clock; retreating blade
In forward flight, air flows opposite the aircraft’s flightpath. The velocity of this air flow _____ (is greater than / less than / equals) the helicopter’s forward speed.
Equals
Symptoms of vortex ring state
- Low frequency airframe vibrations- Uncommanded pitch, roll, & yaw- Increase in collective lever makes condition worse (settle further)
The amount of torque is _____ (directly/indirectly) related to the amount of engine power being used to turn the main rotor disk.
Directly
What is gyroscopic precession?
The resultant action or deflection of a spinning object when a force is applied to that object. This action occurs 90 degrees in the direction of rotation from the point where the force is applied.
_____ is the state of flight where the main rotor disk is being turned by the action of air moving up through the rotor disk rather than engine power?
Autorotation
Define chord line
Straight line intersecting leading & trailing edges of the airfoil.
Because the helicopter’s body has mass & is suspended from a single point (rotor mast head), it tends to act much like a _____.
Pendulum
The fuselage is free to oscillate either longitudinally or laterally. This pendular action can be exaggerated by overcontrolling flight controls
What does the law of conservation of angular momentum state?
The value of angular momentum of a rotating body doesn’t change unless an external force is applied. It’s also referred as the Coriolis Effect.
Above what degree of bank does increase in gross weight soar?
30 degrees
(30 degrees = 16% increase in gross weight)
What is the most limiting factor for highspeed helicopter flight?
Retreating blade stall
As lift on the blades is increased, what two major forces are acting at the same time?
Centrifugal force (outward) & lift (upward)
These forces assume a conical path instead of remaining perpendicular to the mast. This is also known as coning.
Blade twist provides _____ angles at the root where velocity is lower & _____ pitch angles near the tips where velocity is higher.
Higher; lower
Flightpath velocity refers to the _____ & _____ of the airfoil passing through the air.
Speed; direction
Flightpath velocity is equal to rotational velocity for helicopter rotor blades.
In OGE, induced flow velocities are _____ (increased/decreased), resulting in _____ (increased/decreased) AOA.
Increased; decreased
A higher pitch angle is required to maintain the same AOA in a OGE hover. The increased AOA creates more drag & requires more power to hover OGE than IGE
What describes the drifting of the right of the helicopter due to main rotor thrust & tail rotor compensation of the main rotor thrust?
Translating tendency
What is torque?
A measure of the force that can cause an object to rotate about an axis
Which control input is used to eliminate drift in the horizontal plane?
Cyclic
What is the solidity ratio?
The ratio of the total rotor blade area (combined area of all main rotor blades) to the total rotor disk area.
What is the primary means of compensating for dissymmetry of lift?
Flapping
Higher airspeeds induce blade flapping
What is parasite drag?
Drag caused by non-lifting components of the helicopter (cabin, rotor mast, tail, landing gear, etc.).
Most of the changes in AOA come from change in _____ & _____.
Airspeed & rate of climb/descent; not the pilot’s direct flight control inputs.
What changes the blade’s AOA differentially around the rotor disk & creates a differential lift?
Cyclic feathering
This controls the rotor disk attitude & is the means to control rearward tilt of the rotor (blowback).
Any increase beyond the ____ AOA produces a stall & rapid decrease in lift.
Critical
As the AOA is increased further, it becomes more difficult for air to flow smoothly across the top of the airfoil. At this point, the airflow begins to separate from the airfoil & enters a burbling or turbulent pattern.
The _____ is the angle between the chord line of a main or tail rotor blade & the rotor hub.
Angle Of Incidence (AOI)
This is also known as blade pitch angle & controlled by collective & cyclic inputs.
How does a vortex ring state happen?
Caused when the helicopter descends at a speed slower than ETL and greater than 300fpm descent with power applied. As it descends into it’s own downwash, the tip vortices are significantly enlarged. The helicopter then descends at a rate that exceeds normal induced flow at the inner portion of the rotor disk, and upwards flow of air into the rotor disk turns into itself into a secondary set of vortices that are beginning to engulf the rotor disk.
What are Newton’s Laws of Motion?
- Law of inertia- with no net force acting upon it, an object at rest tends to stay at rest, & an object in motion tends to stay in motion.
- The acceleration of an object is proportional to the force acting on it & inversely proportional to its mass. F = m x a - the greater the mass of an object, the more force required to accelerate (move) it.
- For every action, there’s an equal & opposite reaction
Increasing induced flow velocities, _____ (increases/decreases) AOA.
Decreases
The angle between the chord line & resultant relative wind gets smaller because of the larger downwash (induced flow).
What type of surfaces reduce maximum ground effect?
Tall grass, trees, bushes, rough terrain, & water
Define chord
The length of the chord line from the leading edge to the trailing edge.
As a helicopter gains airspeed, the addition of forward velocity results in _____ induced flow velocity.
Decreased
What serves as the reference plane for development of lift, drag, & total aerodynamic force (TAF) vectors on the airfoil?
Resultant relative wind
Rotational relative wind modified by induced flow is _____.
Resultant relative wind
What is the airflow relative to an airfoil & created by movement of an airfoil through the air?
Rotational relative wind aka tip-path plane
What part of relative wind is caused by the blades turning plus movement of the helicopter through the air?
Horizontal
What part of relative wind is caused by the air being forced down through the rotor blades plus any movement caused by the helicopter climbing or descending?
Vertical
Blade twist serves what 2 main functions?
- Alleviates blade stress
- Distributes lifting force more evenly along the blade.
Blade twist is important because of the differing rotational relative wind values along the blade (faster at tip, slower at hub).
What are advantages of a nonsymmetrical airfoil?
- More lift produced at given AOA
- Improved lift-to-drag ratio
- Better stall characteristics
What are disadvantages of a nonsymmetrical airfoil?
- Center of pressure travels up to 20% of chord line (creates undesirable torque on structure)
- Greater production costs
What is center of pressure (CP)?
CP is where all aerodynamic forces act on the chord line. As angle of attack increases, the center of pressure moves forward.
What is center of gravity (CG)?
- CG is the point which all mass is considered to be concentrated. - Force of gravity acts on the total mass of an object vertically downward.
True or False?
Nonsymmetrical airfoils can produce useful lift at 0 AOA.
True
What describes the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe?
Venturi Effect
Because the mean camber line & chord line are the same on a symmetrical airfoil, it produces how much lift at 0 AOA?
0
The downward flow of air through the rotor disk is known as _____.
Induced flowInduced = caused from producing lift
Define mean camber line
A line drawn halfway between the upper & lower surfaces of the airfoil.
Define airfoil
Any surface that produces more lift than drag.
What is the low point on the total drag curve that shows the airspeed at which drag is minimized?
L/DmaxThis is the point where the lift (L) - to drag (D) ratio is greatest.
Which type of drag is most affected by speed increase?
ParasiteThis drag varies with the square of the velocity; doubling airspeeds increases parasite drag four times.
During lift, what causes the vortices that form behind the blade?
The high-pressure airflow area beneath the blade joins the low-pressure airflow area above the blade at the trailing edge & at the tips.
Define drag
Rearward, retarding force caused by disruption of airflow by the wing, rotor, fuselage, & other protruding objects.
What is induced drag?
Drag that’s generated by the airflow circulation around the rotor blades as lift is created.”Lift induced drag.”
What causes skin friction?
Surface roughness
Form drag results from what?
Turbulent wake caused by the separation of airflow from the surface of a structure.
What two elements make up profile drag?
Form drag & skin friction
What causes profile drag?
Frictional resistance of the blades passing through the air.
What does the solidity ratio measure?
The potential for rotor disk to provide thrust & lift.
What aerodynamic force is generated by the rotation of the main rotor disk and can be forward, rearward, sideward, or vertical?
Thrust
In addition to increased bank angle, what can also increase load factors?
Rough or turbulent air
Define load factor
Actual load on rotor blades at any time divided by gross weight.Aka “G” forces or gravitational loads
Define gross weight
Helicopter weight & it’s contents
What are Newton’s Laws of Motion?
- Law of inertia- with no net force acting upon it, an object at rest tends to stay at rest, & an object in motion tends to stay in motion.2. The acceleration of an object is proportional to the force acting on it & inversely proportional to its mass. F = m x a - the greater the mass of an object, the more force required to accelerate (move) it.3. For every action, there’s an equal & opposite reaction
What does Bernoulli’s principle describe?
The relationship between internal fluid pressure & fluid velocity. Increase in pressure = decrease in velocity & vice versa (to maintain constant flow rate)
_____ opposes drag & _____ opposes lift.
Thrust; weight
Gravity acting on the mass of an object creates a force called what?
Weight
Define thrust
The force produced by the power plant / rotor & acts parallel to the longitudinal axis.
Lift is usually measured _____ to the air flowing around the airfoil.
Perpendicular
The force created by air moving over an object or moving through the air is _____.
Aerodynamic force
When dynamic pressure increases, static pressure _____.
DecreasesTotal pressure = dynamic pressure + static pressure
Every object in the atmosphere is surrounded by a gas that exerts a static force of _____ lbs per square foot at sea level.
2,116
Rotor efficiency is increased by ground effect to a height of _____ rotor diameter.
1This is measured from the ground to the rotor disk.
During IGE, induced flow velocities are _____ (increased/decreased), _____ (increasing/decreasing) the AOA.
Decreased; increasingUltimately reducing the blade pitch angle & induced drag; requiring less power from the engine.
What is the lift formula?
L = 1/2 (p) (V)^2(S)(CL)
p = air density
V = rotor RPM (doubling speed = quadrupling lift)
S = surface area
CL = Coefficient of lift
What is blade pitch angle?
The angle between chord line and the reference plane containing the rotor hub
What is pendular action?
Since the fuselage of the helicopter with a single main rotor is suspended from a single point and has considerable mass, it is free to oscillate either longitudinally or laterally in the same was as a pendulum. This pendular action can be exaggerated by over controlling.
What is coning?
As a vertical takeoff is made, both centrifugal force and lift assume a conical path instead of remaining in the plane perpendicular to the mast.
What is coriolis effect?
When a rotor blade flaps upward, the center of mass of that blade moves closer to the axis of rotation and blade acceleration takes place in order to conserve angular momentum.
What is blade flapping?
The ability of the rotor blade to move up or down in a vertical direction. Lift, acting upward and parallel to the mast, causes the blade to flap upwards. Blades may flap independently or in unison to help compensate for the dissymmetry of lift.
What is dissymmetry of lift?
When the helicopter moves through the air, the relative airflow through the rotor disc is greater on the advancing side than on the retreating side, due to the forward speed of the helicopter. Because of this difference in relative wind speed, the advancing blade creates more lift than the retreating side.
What is effective translational lift?
The additional lift obtained when entering forward flight, due to increased efficiency of the rotor system. As the helicopter accelerates through this speed, the rotor moves out of it’s vortices and is in relatively undisturbed air. The airflow is more horizontal, which reduces induced flow and drag with a corresponding increase in AOA and lift.
What is transverse flow effect?
As the helicopter accelerates in forward flight, induced flow drops to near zero at the forward disc area and increases at the aft disc area. This increases the angle of attack at the front disc area causing the rotor blade to flap up, and reduces angle of attack at the aft disc area causing the blade to flap down. The result is a tendancy for the helicopter to roll slightly to the right as it accelerates through approximately 20 knots.
