Aero ELO's

HELO AERODYNAMICS ELO’s

=CHAPTER 1=

Define the following terms:

 * Scalar quantity: Describes only magnitude (time, temp or volume)
 * Vector: Describes magnitude and direction (velocity, acceleration or force)
 * Force (F): Vector measure of push or pull exerted on a body (F = ma)
 * Mass (m): Quantity of molecular material that comprise an object
 * Volume: Amount of space occupied by an object
 * Density (p or rho): Mass per unit volume (p = m/v)
 * Weight: Force with which a mass is attracted toward earth by gravity
 * Moment (M): -Moment is a vector quantity equal to a (F) force times the (d) distance from the point of rotation on a line that is perpendicular to the applied force vector. (M = Fd) - (Q) Torque is another word for a moment created by a force
 * Work (W): Done when a force acts on a body, scalar quantity equal to the force (F) times the distance of displacement (s)
 * Power (P): Rate of doing work, i.e. work done per unit of time.
 * 1) P = F(s)/t = W/t
 * 2) P = F(s/t) = FV
 * 3) P = (Fd)/t = Q * RPM
 * Energy (TE): Scalar measure of a body’s capacity to do work (TE = KE + PE)
 * Potential energy (PE): Ability of a body to do work because of its position or state (PE = weight x height = mgh)
 * Kinetic energy (KE): Ability of a body to do work because of its motion (KE = _ mV_)

Define equilibrium flight:

 * Equilibrium flight is the absence of acceleration, either linear or angular. It exists when the sum of all the forces and the sum of all moments around the center of gravity equal zero.
 * Trimmed flight exists when the sum of the moments around the center of gravity equal zero, therefore if you are in Equilibrium flight you are in trimmed flight, but reverse may not be true.

Define the following:

 * Static pressure (Ps): - The force that each particle exerts on those around it.
 * - Atmospheric static pressure decreases with an increase in altitude
 * Air density (p): - Total mass of air particles per unit of volume.
 * - Air density decreases with an increase in altitude
 * Temp: - Average kinetic energy of air particles
 * - Temp decreases with an increase in altitude at 2ºC (3.57ºF) per 1000 ft up to 36,000 ft MSL
 * - 36,000 feet MSL constant at -56.5ºC (Isothermal layer)
 * Lapse rate: Temp decreases with an increase in altitude at 2ºC (3.57ºF) per 1000 ft up to 36,000 ft MSL
 * Humidity: - Amount of water vapor in the air
 * - Humidity increase, air density decreases
 * - Density of air at 100% humidity is 4% less than @ 0% relative humidity.
 * Air viscosity: - Air resistance to flow and shearing
 * - Air viscosity increases with an increase in temperature

Define Pressure Altitude:
That altitude in the standard atmosphere which corresponds to a particular static air pressure. (“High to low look out below” or “hot to cold look out below”)
 * - Lapse rate is 1000 feet of PA for each 1 inch of Hg
 * - QNH = Alt corrected to standard sea level
 * - QNE = pressure Alt (29.92)
 * - QFE =Airfield setting to read height above ground

Define Density Altitude:
Pressure Alt corrected for temp and humidity deviations from the standard atmosphere.
 * DA = PA + [(T ambient – T std@ Alt) x 120]
 * Increase in humidity decreases air density, therefore increasing density Alt

Newtons 3 laws of motion:

 * 1) The Law of Equilibrium – “A body at rest tends to remain at rest and a body in motion tends to remain in motion at a constant velocity unless acted upon by some unbalanced force”
 * 2) The Law of Acceleration – “ An unbalanced force acting on a body produces an acceleration in the direction of the force that is directly proportional to the force and inversely proportional to the mass of the body” (F = ma)
 * 3) The Law of Interaction – “For every action, there is an equal and opposite reaction”

Relationship between humidity and air density:
As humidity increases air density decreases.

Relationship between temp and air viscosity: Air viscosity increases with an increase in temp.

Main gases of the air:
78% Nitrogen, 21% Oxygen, 1% other gases

Effects of pressure, temp and humidity on the density of air related to General Gas Law

 * Pressure increase, density increases 	(air molecules closer together, more per unit volume)
 * Temp increases, density decreases	(air moves faster, fewer per unit volume)
 * Humidity increases, density decreases 	(water molecules lighter then air molecules, less dense)

Relationship between helo performance and density altitude:
Increase in density altitude decreases aircraft performance. As density Alt increases power available decreases and power required increases.

Describe standard atmosphere and purpose:
It is a set of reference conditions giving avg values of air properties as a function of Alt, to help discuss aircraft performance

Relationship between temp, pressure, air density, local speed of sound and altitude within standard atmosphere: Local speed of sound is the rate at which sound travels through a particular air mass.

 * Primarily dependent on temperature
 * As temp increases speed of sound increases

Effect of temp and humidity on density altitude:
(WARNING: AERO INSTRUCTORS HAVE CONFIRMED THAT THE 10% RULE FOR HUMIDITY IS USED IN CALCULATING ANSWERS ON THE TEST. DO NOT USE THE 40% RULE TO DETERMINE THE HUMIDITY CORRECTION) =CHAPTER 2=
 * As temp increases, density Alt increases
 * As humidity increases, density Alt increases (every 10% increase in RH, DA increases 100 ft)

Describe how the Momentum Theory can be used to model helicopter aerodynamics.
The Momentum Theory makes use of Newton’s Laws to explain how helicopters stay in the air. It uses primarily the 2nd Law (Acceleration F=ma) to explain how the rotation of the main rotor accelerates a given mass of air from well above the rotor disc to a downward velocity (rotorwash) below the rotor disc. The acceleration of this air mass requires that a force be imparted on it, the reaction, by Newton’s 3rd Law (Action/Reaction) of which is thrust. See fig. 2-10

Describe how the Blade Element Theory can be used to model helicopter aerodynamics.
The Blade Element Theory is based on a cross-sectional view of a rotor blade usually in reference to a section near the tip of the blade looking in towards the hub. It makes use of the lift equation which states

Where,
 * ∆L=1/2_(_r)2c1c∆r
 * ∆L = Change in Lift
 * _ = Density of Air
 * _ = Angular Velocity
 * r = Radius of point from hub                         *Fortunately, you don’t need to know this
 * c1 = Differential Coefficient of Lift                but it’s probably been a while since physics
 * c = Chord
 * ∆r = Thickness of segment

The big thing with the Blade Element Theory is on fig. 2-22 Blade Element Diagram. You DO need to know this. Know the diagram, love the diagram, be able to draw the diagram. Most other analysis will be based on the manipulation of this diagram.

Definitions:
Induced Drag – The horizontal component of lift (parallel to the tip-path-plane) attributed to a downward induced velocity and the energy spent in the creation of trailing vortices.
 * Aerodynamic Center – point along the chord line about which all of the changes in lift are considered to take effect.
 * Airfoil – A structure designed to produce lift as it moves through the air.
 * Angle of Attack – The angle at which a body, such as an airfoil or fuselage, or a system of bodies, such as a helicopter rotor, meets a flow. Usually expressed as the acute angle between the chord line of an airfoil and the resultant relative wind.
 * Blade Pitch / Angle of Incidence –
 * Fixed Airfoils (wings, horizontal and vertical fins, stabilizers): the acute angle between the chord line of the airfoil and a selected reference plane, usually the longitudinal axis of the aircraft.
 * Rotating Airfoils (helicopters main and tail rotors, propellers): the acute angle between the chord line of the airfoil and the tip-path plane.
 * Twisted Airfoils : the root chord is commonly chosen to measure the angle of incidence. Angle of incidence is normally called pitch angle for main rotor, tail rotor and propeller blades.
 * Chord Line – A straight line intersecting the leading and trailing edges of an airfoil.
 * Induced Velocity – The speed and direction of the induced downward flow of air resulting from the passage of an airfoil.
 * In-Plane Drag – The summation of all decelerating forces in the plane of rotation (induced drag + horizontal component of profile drag).
 * Lift – The component of the total aerodynamic force (thrust on a blade element), which is perpendicular to the relative wind.
 * Linear Velocity – (Rotational velocity +/- translational velocity) Horizontal component of relative wind.
 * Profile Drag – Drag developed by moving rotor blades through the air. Comprised of skin friction, form drag and wave drag.
 * Relative Wind – The resultant vector of linear velocity and induced velocity as depicted on the blade element diagram.
 * Rotational Velocity – The component of relative wind produced by the rotation of rotor blades.
 * Rotor Disc – The area of the circle inscribed by the blade tips in the tip path plane.
 * Thrust – Rotor thrust is the vector sum of forces produced in the rotor system and is used to overcome the weight of the helicopter.
 * Tip Path Plane – The planed defined by the path inscribed by the tips of the main rotor blade as they rotate.
 * Translational Velocity – The component of relative wind determined by the blade’s orientation to the relative wind acting on the whole aircraft. i.e. The advancing blade sees a higher translational velocity than the retreating blade in forward flight.

Draw the Blade Element Diagram and label the components listed above.
Blade element diagram

State the relationships between relative wind and angle of attack
Angle of attack (AOA) is the angle formed by the airfoil chordline and the resultant relative wind. Since the resultant relative wind is a result of the induced velocity and linear velocity, it shifts up or down with respect to the rotational plane. If the resultant relative wind shifts up, the AOA decreases. If the resultant wind shifts down, the AOA increases. This interaction will in turn affect the amount of lift generated by the airfoil.

State the relationships between angle of incidence and angle of attack.
Angle of incidence is the angle between the chordline and the rotational plane (TPP). In the absence of induced velocity, it is the same as AOA (such as in a fixed wing A/C). With the addition of induced flow, the relative wind changes such that it is no longer defined by just the chordline and TPP. This by definition changes the AOA such that it no longer coincides with the angle of incidence. However, since the chordline is still a factor in determining both angles, if the blade pitch is changed, both angles will change.

State the relationships between induced velocity, linear velocity and relative wind.
Relative wind is the resultant vector of the two other velocities. Linear velocity is the horizontal component resulting from the movement of the airfoil through the air while induced velocity is the result of downward flow of the air the airfoil is passing through. This downward flow can be caused by either upward flapping of the blade, induced flow caused by air being pulled down towards the rotor or angling of the airstream due to rotor disc orientation.

State the relationship between rotational velocity, translational velocity and linear velocity
Rotational velocity refers to the horizontal component of relative wind on the airfoil which is the result of the rotation of the main rotor. Essentially, it is the horizontal component of relative wind in a no-wind hover. Translational velocity is the wind correction made to rotational velocity once the helicopter is in motion. It varies based on the position of the airfoil with respect to the helicopter. i.e. the advancing blade will experience higher translational velocity than the retreating blade. Linear velocity is the combination of the rotational and translational velocities. When the helicopter is in motion it is the horizontal component of resultant relative wind.

Recall the differences between symmetrical and non-symmetrical airfoils
Symmetrical Airfoils – 	Symmetrical airfoils have symmetrical distance from the chordline to the upper or lower surface of the airfoil. This means the camber line and chordline are the same. The center of upper and lower pressures and aerodynamic center in symmetrical airfoils are co-located. The sum of moments is therefore zero for any given AOA.

Non-Symmetrical Airfoils – 	Non-symmetrical airfoils have different upper an lower surface designs. Depending on the camber positive/negative, the distance from the chordline is different from the upper to lower surface. The camber line is therefore above or below the chordline. Non-symmetric airfoils offer higher lift at lower AOA’s. They do not, however, have co-located centers of pressure and therefore have a pitching moment associated with them about the aerodynamic center. This causes torsional stress along the length of the blade.

Given basic aerodynamic equations (lift, drag, etc), identify the variables in each, and state the relationships between the variables.
Lift Equation L = 1/2 V2 S CL

Where L   = lift force CL  = (C "sub L") Coefficient of lift 1/2 = .5 x  (rho) = density of the air (in slugs per cubic foot) S   = surface area (in square feet) V2  = velocity squared (in feet per second)

Drag Equation
 * D = _ _SV2CD
 * Where D = Drag Force
 * CD = Coefficient of Drag
 * _ _ (rho) = Density of the Air (slugs/ft3)
 * S = Surface Area (ft2)
 * V = Relative Wind Velocity (ft/sec)

The important point in addressing lift and drag equations is not the math involved, but knowing what the pilot has control over. Given the conventional helicopter design the pilot has control over CL or CD by changing the AOA. The pilot also has some control over the relative wind velocity by means of NR control. NR is noted to have greater effect on V than airspeed change. See sect. 225 on page 2-36.

Describe the forces produced by the main rotor system and tail rotor, and their effects on the helicopter
The main rotor system exerts Total Rotor Thrust on the helicopter. This thrust tilts from the vertical axis to provide two component forces for forward flight. These forces it’s vertical and horizontal components. The vertical component acts against the weight of the helicopter while the horizontal component to counter drag and accelerate the helicopter in desired directions.

The tail rotor system exerts force in the horizontal plane at the end of the tail boom primarily to counteract main rotor torque effect. In slow flight or hover, it can be used to control heading. It also causes translating tendency when not countered by left cyclic in a hover.

=CHAPTER 3=

Describe feathering and its role as well as cyclic and collective pitch control:

 * Feathering is the rotation of the blade about its spanwise axis, permits changes in blade pitch angle
 * Collective Fx – changes all blades the same pitch
 * Cyclic Fx – changes pitch on blades according to azimuth position with the opposite blades changing pitch equally but in opposite direction

Describe flapping and how it is accomplished in each system (3 types):

 * Flapping is the upward and downward rotation of just a few degrees of a rotor blade about the horizontal hinge during rotation about the mast. It occurs from aerodynamic forces generated by cyclic changes to pitch or from dissymmetry of lift or transverse flow effect.
 * Semi-rigid, Flapping occurs around the trunnion bearing (horizontal teetering)
 * Fully articulated, flapping occurs around horizontal hinge pins mounted at the head
 * Rigid, flapping primarily through the flexibility of the hub and some in the blade

Recall the definition of the physical and aerodynamic forces acting on the main rotor head, including rotor thrust, drag, centrifugal force, inertia and coning:

 * Centrifugal force is the outward force created by the rotation of the main rotor head. The large centrifugal force is what allows the weight of the helo to be supported with the flexible rotor blades.
 * Coning is the upward displacement of the main rotor blades due to lift, the amount of coning depends on weight, G-force and RPM
 * Inertia governs how an aircraft reacts to directional control inputs to the flight control system, high inertia rotor system will tend to allow a pilot greater reaction time before rotor speeds decays, but is slower to regain lost RPM.

State the relationship between center of gravity and mechanical axis:

 * The center of gravity should be in line with the mechanical axis but realistically cannot always be. When not in line the cyclic must be displaced to compensate for the unbalanced CG condition.

Describe how geometric imbalance affects rotational blade movement (lead/lag), and how it is compensated for or eliminated in different rotor systems:

 * When the rotor disc is tilted the radius of the CG of the rotor blades is changed.
 * In order to maintain a constant angular momentum, the rotational velocity must change whenever the radius changes
 * When the disc is tilted the radius of the down blade is increased and therefore the rotational velocity for that blade must decrease (lag)
 * The radius for the up blade is decreased therefore the rotational velocity must increase (lead)
 * A Fully articulated rotor head uses vertical hinge pins to compensate for geometric imbalance by allowing the blades to move independently in the horizontal plane (Lead and Lag) as necessary to compensate for the changing radii, a rigid rotor head allows flexing.
 * The semi rigid rotor head use underslinging to allow the blades to increase/decrease radii equally so no lead/lag is necessary.

Differentiate between and characterize the three types of rotor systems in use today in Navy/Marine Corps

 * Fully articulated rotor systems uses more then two blades, lead/lag is possible using vertical hinge pins, Horizontal hinge pins allow flapping.  Movement of each blade is independent of each other
 * Rigid rotor systems are flexible and the hub itself bends and twists in order to provide flapping, lead/lag and pitch change.
 * Semi-Rigid rotor systems use two rotor blades and incorporate a horizontal hinge pin (trunnion bearing) for flapping only, do not use lead/lag hinges. Use underslinging to compensate for geometric imbalance (keeps both blades Center of masses equal in distance from center of rotation during flapping

Describe effects of main rotor torque on the movement of the fuselage about the vertical axis and methods of countering it:

 * Main rotor torque causes the fuselage to try to rotate clockwise
 * This moment is compensated by placing an anti-torque tail rotor a certain distance from the center of gravity of the helo to create a moment
 * Can also use size, tip speed, airfoil, number of blades, spacing, direction of rotation, cant, and whether to use a tractor or pusher tail rotor

State the means by which an aircraft with more then one main rotor maintains directional control:

 * It solves the problem of torque by maintaining a balance of the opposing moments
 * They still have to meet the demands of induced power, profile power, and accessory loads, but do not expend power through a tail rotor

Recall the definition of geometric twist and state why it is used in helo design:

 * Geometric twist is the twisting of the rotor blade to help more equally distributes the Aerodynamic force along the blade. It is needed because more lift is present at the tip than the root because there is more linear flow at the tip because the tip is traveling much faster than the root.  The pitch of the blade is reduced from the root outward to the tips.

Recall the definition of compressibility and methods used in helo design:
* sweeping the leading edge of the rotor blades back (reduces velocity the blade tips see) * varying the airfoil thickness along the span (Change airfoil to take advantage of increase of V)  * varying the airfoil section along the span. (Change airfoil take advantage of increase of speed)
 * Compressibility occurs when velocity over the blade tips exceed the speed of sound.
 * Some solutions for compressibility are:

=CHAPTER 4=

Recall the definition of static stability and dynamic stability.

 * Static stability – the initial tendency of an object to return to equilibrium (or trimmed condition) after it has been disturbed
 * Dynamic stability – the tendency of an object to return to its state of equilibrium (or trimmed condition) over time.
 * Maneuverability and stability typically exclude one another

Describe the contributions of speed stability and angle of attack stability to longitudinal stability in forward flight.
Airspeed stability is more or less the effect that an increase or decrease in airspeed has upon the longitudinal axis. With an increase in airspeed, the helicopter experiences blowback, which attempts to realign the longitudinal axis to equilibrium, or level, and slow back down.

Conversely, with a decrease in airspeed, the nose will pitch forward, trying to get the helicopter to speed back up to where it previously was.

Both of these effects are examples of positive static stability about the longitudinal axis

Angle of attack stability on the other hand, exhibits negative static stability about the longitudinal axis, meaning the initial tendency of the axis, once disturbed, is not to return to equilibrium.

If the rotor disc experiences a change in vertical flow, the effects are felt on all blades, and the advancing blade begins to generate more lift. The rotor will continue to move away from its point of equilibrium, and the aircraft will pitch up.

Recall the definitions of control, controllability, control authority, and control sensitivity
Control Sensitivity – The moment generation capability per unit of displacement (i.e., 1 inch cyclic movement in the cockpit translates into xxx in the rotors)
 * Control – (same thing as maneuverability) is the application of forces or moments to a helicopter to achieve or maintain a desired condition of flight.
 * Controllability – The helicopter’s ability to accept pilot inputs and turn them into the desired responses
 * Control Authority – The maximum moment that can be generated on the aircraft by the cockpit controls

Recall the definition of and state the relationships between center of gravity, mechanical axis, and virtual axis
Virtual Axis – The total rotor thrust perpendicular to the tip path plane (TPP). Control Axis – An axis which extends perpendicularly from the swashplates. Mechanical Axis – The shaft axis which is an extension of the main rotor mast.


 * There are certain limitations imposed by these axes, and if the center of gravity (CG) is placed outside these bounds, control authority can be lost.
 * Think of the control axis as being the first to move forward when you put in forward cyclic. The virtual axis follows it, initially wanting to go further forward, but ending up a little behind due to blowback. The mechanical axis, following the CG, eventually attempts to realign itself with these axes, providing your pitch in a certain direction.

Describe the effect of the location of a helicopter’s center of gravity (CG) on control.

 * The CG is considered the balancing point for weight and balance purposes. Not exceeding CG limitations is critical to maintaining positive control of the helicopter. Theoretically, you should be able to suspend the aircraft by a single cable attached to the CG.
 * In general, a CG forward of the aircraft’s aerodynamic center will create stability, and a CG aft of the aerodynamic center will be a negative contributor to stability.
 * A CG offset from center requires cyclic application in opposite direction
 * A lateral CG reduces lateral cyclic authority in the opposite direction.

Identify causes for departure from controlled flight.

 * A CG outside the helicopter’s limitations
 * Aircraft damage
 * Over-controlling
 * Atmospheric limitations
 * Mechanical failure

Recall the definition of torque effect.

 * As the rotor blades rotate in a counterclockwise direction, it imparts a force on the fuselage that wants to rotate it (fuselage) in the opposite direction (clockwise). An anti-torque system becomes necessary to counter this effect. Examples include a conventional tail rotor (TH57), multiple/opposing rotors (CH46), NOTAR (no tail rotor) systems, and fenestron (fin in fan…HH65).
 * All compensate by keeping the fuselage from rotating in the opposite direction of the rotors.
 * At very low altitudes, like in taxi, the pedals are used to turn.
 * At higher airspeeds, the pedals are used to keep the ball trimmed.

4.8 State the means by which we control the helicopter about the vertical axis.

 * At slow airspeeds, we use inputs to the torque pedals as rotate the aircraft and turn it in our desired direction. At higher airspeeds, we use the same anti-torque pedals to keep the helicopter in trimmed flight.
 * Applying force to the left pedal increases the pitch on the tail rotor blades and takes away power which would otherwise be available to the main rotor. It requires more power than right pedal input.
 * Right pedal input decreases the pitch on the tail rotor blades and basically allows the helicopter to rotate clockwise like it wants to.

4.9 Describe the problems created by the use of tail rotor system to counteract torque.
Two main problems:
 * 1) In a hover, right drift will result from the horizontal force the tail rotor imparts on the air. Left cyclic will be necessary to counteract this force, which is what creates the TH57’s tendency to take off and land with the left skid down.
 * 2) The tail rotor becomes subject to the effects that the main rotor imparts on the airflow in almost all flight regimes. In a hover, it is experiencing vortices and downwash around the tips. In forward flight, depending on the helicopter’s attitude, it can also be disturbed by airflow through the main rotor system. This will depend on its position relative to the TPP and relative wind.

State two means by which tail rotor loading is reduced in forward flight.

 * 1) The vertical stabilizer. It is shaped like an airfoil and can also produce lift in a direction opposite the rotation of the rotor blades.
 * 2) Weathervaning of the tail boom. If the back end is kicked out for some reason during forward flight, the relative wind will act as like a moment arm and push the tail boom back to center.


 * Both of these characteristics free up more power for the main rotor to use.

4.11 Recall the definition of aircraft trim and explain its importance.

 * An aircraft is said to be trimmed if all moments in pitch, roll, and yaw are equal to zero. That doesn’t mean it can’t be moving. It just has to do so at a constant rate…one without acceleration.
 * It’s important in that it is used to create as much of a hands off condition as possible. This fatigues pilots less and allows them to divide their attention among other matters.

=CHAPTER 5=

Describe how power req to hover is effected by changes in gross weight, alt, density alt, and rotor speed.
- As weight increases, power required increases. As altitude increases, power required increases. As density alt increases, power required increases. As rotor speed increases, power required increases.

Use a chart to determine airspeed for max range and max endurance/rate of climb in accordance with NATOPS.
- See corresponding chart in NATOPS
 * Remember to use power required charts for power-on situations, and Total Drag charts for power-off.

Describe effective wind on maximum range airspeed
- With a headwind it will have a higher airspeed but max range will decrease and with a tail wind it will decrease the airspeed but increase max range. It’s the point where the ratio of fuel flow to velocity is at a min. value.

Explain how a turboshaft engine provides power to a rotor system
- A typical turboshaft engine pulls air in thru the inlet, compresses it, adds thermal energy thru combustion and converts that thermal energy to mechanical energy thru turbines. One set of turbines drives the compression and the other set of turbines provides power to the drive train (transmission, tail rotor)

Describe the physical properties that limit engine performance

 * fuel flow limitation (cold)-as temp decreases, air density increases, so fuel flow must increase to maintain the fuel/air ratio. However, the amount of fuel in the fuel nozzles has a limit (fuel flow can only increase so much)


 * Turbing temperature limit--the materials that make the turbine have temperature and stress limits


 * Ng-Gas generator limitations--As OAT increases, air density decreases, meaning the turbine has to rotate faster.


 * Component rating degradation--transmission materials have physical limits


 * humidity/moisture--cause a decrease in air density, but reduces engine temperature. The effect of humidity is Negligible.


 * Torque limits--drive train limits


 * airspeed effects--airspeed increases flow rate into the engine, but because helos are slow, airspeed's effects are negligible.

Use a chart to convert torque to shaft horsepower in accordance with NATOPS
- See corresponding chart in NATOPS Figure 23-4

Use a chart to determine power (torque) available and power required to hover in ground effect and out of ground effect in accordance with NATOPS.
- See corresponding chart in NATOPS Figure 24-1

Use a chart to determine distance required to clear an obstacle on takeoff IAW NATOPS
- See corresponding chart in NATOPS Figure 24-2

Describe conditions which can cause an aircraft’s power required to exceed its power available and recovery procedures.
When rotor RPM is drooping and the tail rotor is most sensitive to its own decreased RPM
 * In a climb
 * Termination of a steep approach
 * For all of these, avoid unnecessary maneuvering. An uncommanded descent will occur when the power required exceeds the available power.  Aggravating factors include high g-loading, high gross weight, high DA, rapid maneuvering, slow spool up time, loss of wind effect, loss of wind direction, and loss of ground effect.

State the environmental factor which most significantly affects power available

 * Air density, because less dense air requires that the engine work harder to produce the same mass flow rate through the engine.

Explain how compressor blade erosion and salt encrustation will affect engine performance

 * They erode the compressor blades, which results in a decreased surface area of the blades, and in turn they will push less air, reducing compression and power available. Salt encrustation changes the shape of the compressor blade, and the new shape will push air through less efficiently.  The airflow will be poorly distributed, and may result in a stall.

State the effect the tail rotor will have on power available to the main rotor
The tail rotor uses 5-15% of total power available, leaving 85-95% available for the main rotor. The tail rotor will use more power for the same reasons as shown in 9.

List five causes of compressor stalls

 * 1. Reduced or turbulent axial flow/IGV malfunction
 * 2. Rapid acceleration (throttle control)
 * 3. Rapid deceleration (throttle control)
 * 4. Excessive axial air flow
 * 5. Bleed air valve malfunction.

Explain why compressor bleed air valves are needed for starting compressors
- Prior to engine start, the pressure across the compressor (atmospheric pressure) is identical. During engine start, the compressor is trying to force a lot of air into a small space, so the compressor must do work on the air in order to compress it. Some of the air must be bled off until the necessary compression ratio exists to force the air through the compressor for proper use in the engine.

Describe what causes blowback in a compressor
- If a loss in compression occurs, a breakdown in the flow inertia will result. Backflow from the combustor occurs as the higher pressure air in the combustor moves towards the lower pressure air in the compressor. This will cause a loss of engine power and turbine cooling air, with engine overtemps. Blowback could cause flames to come out of the engine inlet.

Using a blade element diagram, indicate how changes in rotational velocity, RPM, and axial airflow will affect the performance of the compressor.

 * I refuse to make a blade element diagram for a compressor blade, it is stupid. (The SNA who was responsible for Chapter 5 [4407] is apparently a Blue Falcon pilot)
 * A decrease in the velocity of the air approaching the blades, when they are at a constant rotational speed, increases the angle of attack on the blades. When the blades are at a high RPM, they are at an increased AOA and are more likely to stall.  When at a low RPM, more likely to choke, and not produce anything since not enough air is flowing through.  When there is excessive axial flow, or decrease in RPM, the compressor will choke due to lower AOA.

=CHAPTER 6=

Describe a vortex, how it is formed, and how it affects induced velocity and the efficiency of the rotor system

 * A vortex is the spiral flow that is produced due to the lift differential created when an airfoil is producing lift. There is a lower pressure above the airfoil, and a higher pressure beneath, therefore causing the air to flow from the high pressure to low pressure region causing vortices.
 * The increased downwash that is associated with vortices produces induced drag. In a hover IGE this induced drag is decreased because of lower induced velocities, causing greater rotor efficiency.  When moving out of ground effect, the blades see a larger induced velocity, in effect causing a larger induced drag.  All in all, an increase in induced velocity decreases efficiency of a rotor system, especially at a hover.

List effects of AOA, aspect ratio, weight, and disk loading on vortex strength
- Vortex strength increases with an increase in weight, AOA, and disk loading. Vortices decrease with an increase in aspect ratio.

Recall the definition of ground effect and describe how it affects power required
- Ground effect is the increased efficiency (decreasing power requirement) of the rotor system of the helicopter beginning at approximately one rotor diameter above the surface and increasing as the aircraft approaches the ground. The aerodynamic effect can be largely attributed to reduction of the induced velocity because the ground interrupts the airflow beneath the helicopter. The ground also interrupts the formation of tip vortices, reducing their contribution to induced flow. Decrease in induced velocity increases angle of attack, providing an increase in lift with a corresponding reduction in blade pitch setting/power setting.

Recall the definition of translational lift as related to wind conditions in hovering flight, and describe how it affects power required.
- Translational lift is the increased efficiency of the rotor system in the production of lift by increasing horizontal airflow, due to the decrease in induced flow and vortices. It occurs while in a hover with a wind from any direction.

Using a blade element diagram show how the AOA is affected by ground effect and translational lift
- The induced velocity is decreased in ground effect, and when translational lift is increased, which means the AOA increases.

On a conventional helicopter how does tail rotor thrust affect hover attitude
- The tail rotor thrust is to the right, and is located below the main rotor which is tilted left to compensate. This creates a couple that rolls the helicopter to the left and results in a left skid low attitude in a hover. To compensate, the tail rotor may be placed closer to the horizontal plane of the main rotor. This equalizes the moment arm of the system.

Explain the effects of wind on a helo during hovering turns
- Tail wind may degrade engine performance, and may lead to an excessive tail low attitude. Maintain an awareness at all times of wind direction to avoid LTE due to weathervaning, VRS, main rotor vortex, and AOA reduction

=CHAPTER 7=

Identify on a rotor disk the areas of high AOA and low AOA.
AOA varies due to dissymmetry of lift, control inputs and tip vortices.
 * 1. AOA lowest at 90 degree position
 * 2. AOA highest in the vicinity of 270 degree position
 * 3. Region near hub on retreating side has reverse flow
 * 4. AOA is not symmetric between the fore and aft portions of the disk

Recall the definition of translational lift, state the phenomena which cause it, and describe its effect on power required.
Translational lift is the increase in lift due to the reduction of vortices and induced velocity as a result of the increased horizontal airflow through the rotor system. Power required decreases due to increased rotor efficiency.

State the effect of dissymmetry of lift on a rotor system and explain how the effect is overcome.
In forward flight the advancing blade sees an increase in linear velocity, which increases AOA and lift. The retreating blade sees less linear velocity, decreasing AOA and lift. This effect is overcome by permitting flapping in the blade. The advancing blade flaps up which increases induced velocity and decreases AOA. The retreating blade flaps down which decreases induced velocity and increases AOA.

Describe the effect of phase lag on helicopter control.
Maximum displacement of the rotor surface occurs 90° after applied force. Therefore control input/Max AOA must be put in at the 12 o’clock in order to have the rotor displace up at the 9 o’clock and roll right. Control inputs are mechanically translated 90° prior in the TH-57 so the TPP tilts the same direction as the cyclic.

Describe transverse flow effect and its cause
Air passing through the rear portion of the rotor disk has a greater downwash angle than air passing through forward portion, resulting in reduced AOA on the rear of the rotor disk. This is due to both acceleration of the air mass by forward portion and the coning angle of the blade in the rear. The resultant right tilt of the TPP causes a right drift as airspeed increases, and vibrations that are noticeable from 10-20 knots.

Recall the definition of blowback, and describe its effect on helicopter attitude and airspeed.
The tendency for the rotor head to tilt aft due to dissymmetry of lift and flapping. As airspeed increases dissymmetry of lift increases, causing longitudinal flapping and an aft tilt of the TPP, pitching the nose up which requires a constant nose-down trimming process. The opposite response is also true as airspeed decreases.

Describe the typical velocity distribution on a rotor disk for a rotor system in forward flight
See Figure 7-23 on 7-23

Recall the definition of retreating blade stall, its cause, contributing factors, effects on flight and corrective action.
The point at which the downward flapping of the retreating blade causes the blade to exceed critical AOA.
 * Effects: Vibrations, Increased power required, Pitch up and possibly roll

Contributing Factors: Corrective Action:
 * 1. Low rotor RPM
 * 2. High Gross Weight
 * 3. High Density Altitude
 * 4. G-loading
 * 5. High Airspeed
 * 1. Increase Rotor Rotor RPM
 * 2. Reduce Gross Weight
 * 3. Descend
 * 4. Reduce G-loading
 * 5. Reduce Airspeed

Recall the definition of compressibility, its cause and the effects on flight, and corrective action
When the tip section of the advancing blade exceeds the critical Mach number for the rotor blade section. Effects:
 * 1. Increase the power required to maintain rotor RPM
 * 2. Vibration and rotor roughness
 * 3. Cyclic shake
 * 4. An undesirable structural twisting of the blade shift the aerodynamic center rearward

Corrective Action:
 * Decrease blade pitch
 * Decrease rotor Rpm
 * Decrease severity of maneuver
 * Decrease airspeed

Identify the minimum and maximum single engine airspeeds from a given helicopter performance chart.
Review figure 7-44

Recall the definition of bucket airspeed and explain its usefulness
Bucket airspeed is the point of maximum endurance. This is the point of lowest power required, the lowest fuel flow, and max rate of climb due to greatest excess power.

=CHAPTER 8=

List the four flow states of a rotor system in a descent.

 * 1. Normal thrusting state - This condition exists from hover to descent rates up to approximately 70% of the ideal hovering induced velocity.
 * 2. Vortex ring state - an uncontrolled rate of descent caused by the helicopter rotor encountering disturbed air as it settles into its own downwash
 * 3. Autorotative state -
 * 4. Windmill state.

Recall the definition of the vortex ring state, conditions for occurrence, and recovery procedures.
Vortex Ring State- is an uncontrolled rate of descent caused by the helicopter rotor encountering disturbed air as it settles into its own downwash. Entry- Recovery-
 * Descent with some power (generate induced flow)
 * Descent rate of 0.7-1.25 induced velocity (to descend with the vortices)
 * Low airspeed (to avoid outrunning the vortices)
 * Shed ring through non-vertical motion, fwd or lateral
 * Decrease collective
 * Enter autorotative state (Altitude permitting)

Recall the definition of autorotation, pro-autorotative force, and anti-rotative force.
Autorotation- Descending flight of a helicopter without engine power where the air approaching from below the rotor disc (upward induced flow) keeps the rotor blades turning at an operational speed. Pro-autorotative force- in unpowered flight, the accelerating horizontal component of the total aerodynamic force vector in the region where it is tilted forward of vertical/ axial (driving) region. Anti-autorotational force- In autorotational flight, the decelerating horizontal component of the aerodynamic force along the driven and no-lift regions.

Draw and label a blade element diagram for autorotation.
(please be a shipmate and add to this gouge on Marinegouge.com)

State the three phases required to transition from powered to un-powered flight.

 * Entry
 * Steady State Descent
 * Deceleration and Touchdown

State the effects of a flare in autorotation.
Tilting the entire rotor disc aft increases both upward induced velocity and angle of attack on the entire rotor, and tilting the overall thrust vector aft. This increased thrust reduces both forward speed and decent rate temporarily. In addition, the overall increase in AOA increases the size of the auto region on the rotor disc by displacing some of the propeller region. This increases driving force and speeds up the rotor. At the same time, increased lift on all blades causes increased coning, which causes and increase in RPM due to conservation of angular momentum. Finally, the nose-up attitude exposes more fuselage area to forwards flow, increasing drag and slowing the helicopter down.

State the variables that affect autorotative descent. (DR.TAG)

 * Density Altitude
 * RPM
 * Trim
 * Airspeed
 * Gross Weight

Identify maximum glide airspeed on a power vs. airspeed chart.
It is found at a point tangent to the power required curve from a line extending from the origin.

List the four flow regions along a rotor blade in autorotative flight.

 * Stall Region- AOA is greater than AOA for stall due to the low rotational velocity (Drag > Lift).
 * Autorotative Region (Driving) - In-Plane thrust is higher than in In-Plane drag, provides lifting and driving forces.
 * Propeller Region- (Driven)/ dragging Region- In-Plane drag is higher than In-Plane thrust, produces lift with high drag.
 * Reverse Flow Region (Fwd Flight) - region that exists in fwd flight autorotations due to forward speed overtaking rotational velocity.

State the purpose of the height-velocity diagram.
To identify the portions of the flight envelope from which a safe landing can be made in the event of a sudden engine failure.

Identify safe and unsafe areas of an H-V diagram, describing reasons for operation in safe areas and effects of gross weight, density altitude and rotor speed (Nr) on it.

 * Unsafe areas:
 * Low airspeed/ high Altitude region
 * High airspeed/ low Altitude region
 * Safe Area: everywhere else.
 * Effects: same as always high gross weight and high density altitude is bad. Low Nr is bad (drooping)

=CHAPTER 9=

Recall the definition of load factor and describe its relationship to maneuvering flight

 * Load factor is the ratio of total lift to the aircraft's weight. It is equivalent to the number of times the earth's gravitational pull felt by the aircraft and pilot, so is often referred to as "G".  The rotor system must supply thrust equal to apparent weight throughout maneuvering to maintain altitude.
 * The more angle of bank in a turn, the more load factor which mean more thrust needed to maintain altitude.
 * Collective adjustments provide sustained maneuvering capability while cyclic adjustments provide transient ‘G’ capability (Gs bleed off).

Explain which factors define the flight envelope and are driving factors in the Development of the V-N Diagram
risk of some permanent damage. If exceeded damage may occur.
 * Limit Load Factor – is the greatest load factor a structure can sustain without any
 * Ultimate Load Factor- the max load factor an a/c can sustain without structural failure. If exceeded something will break. It is equal to 150% of the limit load factor.
 * Lift Limit- This is the max load factor at a given speed.
 * Limit Airspeed- (aka Vne/redline) Highest airspeed an a/c is allowed to fly. Speeds in excess can cause structural damage. It is determined by Critical Mach Number (Mcrit), airframe temp, excessive structural loads or controllability limits.
 * V-aft- Max rearward speed allowed.
 * Manuevering Speed- (aka corner airspeed, Va) Speed at which full deflection of controls can be applied without overstressing the a/c.
 * Gust loading- refers to the increase in G loads due to vertical wind gusts.

Definition of mast bumping, causes and recovery procedures

 * The rotor head tilts and contacts the mast. It happens during low G maneuvers when the rotor loses its control power and excessive flapping occurs.
 * Causes- Turn up/shutdown with controls not centered, Slope operations, gusty winds, Low G maneuvers, abrupt cyclic inputs, hard landings.
 * Recovery- 1)Use cyclic pitch to reload the rotor 2) once positive Gs are established use other controls as necessary to recover from any unusual attitude

Describe helicopter vibrations and their sources. (low, medium and high)

 * Main source of vibrations come from main rotor .1P, 2P, 3P vibes are all related   to main rotor frequency or a multiple of that frequency.
 * Tail shake- tail rotor receives turbulent air from the main rotor @ same frequency and resonance is felt throughout a/c.
 * External Loads- Load oscillates @ same frequency and vibrations affect flight.

Recall the definition of main rotor vortex interference with the tail rotor

 * Usually happens in a right turn. The tail rotor gets into the path of the main rotor vortex and the AOA of the tail rotor is effectively increased increasing thrust. The pilot is required to add right pedal to reduce thrust to maintain same rate of turn. As the vortex passes, the AOA is reduced and a right yaw ensues. The pilot must then control this with left pedal.

Recall the definition of weathercock instability

 * Occurs in the region of 120° to 240° or from the 4 o’clock to the 8 o’clock position.
 * Winds attempts to weathervane the nose of the a/c into the wind. The wind catches the vertical fin and the fuselage.

Recall the definition of the tail rotor vortex ring state

 * Occurs in the region from 210° to 330° or from 7o’clock to 11 o’clock.
 * Winds cause the tail vortices in this region to be recirculated into the rotor causing an uncommanded yaw in either direction.

Recall the definition of ground resonance, the causes and effects, and the recovery procedures

 * Ground resonance- Associated with fully articulated systems, it is a destructive oscillation caused when the helo is in contact with the ground and one or more rotor blades is displaced due to wind gusts, sudden control movement, or hard landing. CG spirals violently outward.

(Landing)- get airborne, select different touchdown point
 * Recovery- 	(in the runup)- secure eng, rotor brake, wheel brakes

Recall the definition dynamic rollover, its causes and recovery procedures

 * The lateral rolling of the helo onto its side due to exceeding the static rollover angle or the critical rollover angle for a critical roll rate, regardless of cyclic corrections. It must have a ground pivot point.
 * Static rollover angle- angle at which the helo would tip over if placed on an incline.
 * Critical rollover angle- the AOB which the pilot’s control power is insufficient to arrest velocity around a pivot point. Point of no return.
 * Lateral control limit- Angle the rotor disk makes with the mast when cyclic is displaced full throw.
 * Control power- is the effectiveness of the cyclic control in achieving changes in fuselage control.
 * Moment of Inertia- an objects resistance to change in angular momentum.

Causes- 	Pivot around a fixed point, operating on a slope, pilot technique, whiteout/brownout, CG shifts (loads coming loose), tail rotor thrust, main rotor design(fully articulated/rigid are more prone), crosswinds, main rotor thrust(with power comes right yaw), ship motion

Recovery-
 * 1) Reduce collective. Weight is the only force that will produce a moment that will prevent rollover up until static rollover angle.
 * 2) Lateral cyclic to opposite side