Relative Wind

AERODYNAMICS

RELATIVE WIND TOTAL AERODYNAMIC FORCE AIRFLOW AT A HOVER TRANSLATING TENDENCY DISSYMMETRY OF LIFT TRANSVERSE FLOW AIRFLOW PATTERNS IN FORWARD FLIGHT SETTLING WITH POWER AERODYNAMICS OF AUTOROTATION LOSS OF TAILROTOR EFFECTIVENESS DYNAMIC ROLLOVER RETREATING BLADE STALL TYPES OF DRAG

RELATIVE WIND

Ref. FM 1-203

Def: The movement or flow of air molecules with respect to an object. Flow of air -

Hover - Airfoil is subject to rotational relative wind and induced flow. The resultant of these factors is resultant relative wind.

TOTAL AERODYNAMIC FORCE

Ref- FM l-203

Def: As an airfoil passes through the air a pressure differential develops around the airfoil. This differential combined with the resistance of the airfoil passing through the air creates a total aerodynamic force which acts on the airfoil. LIFT: Force created by pressure differential. (Always perpendicular to the resultant relative wind.

DRAG: Resistance force created. Parallel to resultant wind.

1) Induced - Drag created through lift production (results from downward motion imparted to air and vortexes)

2) Parasite - Total drag of aircraft. Created by non-lift producing devices/parts of the aircraft.

Profile = Friction of airfoil passing through the air.

TOTAL AERODYNAMIC FORCE: Resultant of lift and drag.

AIRFLOW AT A HOVER

Ref: FM 1-203

Description: At a hover airflow is downward through the rotor system and then outward as it comes in contact with the ground.

In Ground Effect: Airflow -

1) Reduced rotor tip vortexes 2) Increased induced flow

Out of ground effect:

a) Height- Approximately 1 1/4 rotor diameter b) Airflow- 1) Increased rotor tip vortexes 2) Increased induced flow

Results. Increased power requirement when out of ground effect.

NOTE: Change the angle of incidence to allow for an equal angle of attack for IGE - VS - OGE.

TRANSLATING TENDANCY

Ref: FM 1-203

Def: Tendency for a single rotor helicopter to drift laterally in the direction of the tail rotor thrust. The tendency results from right lateral tail rotor thrust that is exerted to compensate for main rotor torque.

PREVENTIONS: Helicopter design usually includes one or more features that help the aviator compensate for translating tendency.

1) Primary is pilot input through the cyclic control.

2) Flight control rigging may be designed so that the rotor disk is tilted slightly left when cyclic is centered.

3) Main transmission flay be tilted slightly left when fuselage is level.

4) Collective control may be designed so that the rotor tilts left as collective is increased to hover helicopter.

OH-58 Transmission is tilted: a) 1.5 degrees left b) 5 degrees forward

DISSYMMETRY OF LIFT

Ref: 1-203 Def. The difference in lift between the advancing and retreating portions of the rotor disk, caused by a differential of airflow velocity across the advancing and retreating blades.

From a hover in a no wind condition, where rotational relative wind is constant.

To the introduction of 100 kts fps forward airspeed which has the effect of adding to the advancing side and subtracting from the retreating side.

a) Blade Flapping (explain)

causing Blowback:

b) Cyclic Feathering (explain)

TRANSVERSE FLOW

Ref: FM 1-203

Def: Difference of airflow across the front and rear halves of the rotor disk.

Recognition: Increased vibrations just below ETL on takeoff and after passing through ETL on landings.

Cause: Induced flow is greater in the rear half of the disk than the front because as the air moves over the disk it is deflected downward at greater velocities This increases the induced flow over the aft and reduces induced flow in the front (more horizontal). Effects: The small induced flow on the front section results in an increased angle of attack. The greater induced flow on the aft section results in a decreased angle of attack.

The greater lift in the forward section causes the blade to flap up over the nose and due to phase lag results in upward displacement over the left side. The reduced lift in the aft section causes the blade to flap down over the tail due to phase lag results in a downward displacement over the right side. This tilts the disk to the right.

Compensation. Left cyclic will change the angle of incidence and angle of attack on the disk and prevent the tilt to the right. This left cyclic will continue until higher forward speeds are reached and transverse flow effect is reduced.

AIRFLOW PATTERNS IN FORWARD FLIGHT

Ref: FM 1-2O3

Description of Increased Efficiency:

1) Reduction of vortices 2) More horizontal airflow

Translational Lift: Increased efficiency with increased relative wind velocity.

ETL: 1) Define 2) Speed Range 3) Notable Effects 4) Tailrotor Efficiency/effects

Airflow Diagrams: 1) At hover

2) Increased velocity SETTLING WITH POWER

Ref: FM 1-203

Def: Is a condition in powered flight in which the aircraft settles in it's own downwash.

Three things needed to get into settling with power:

1) Low airspeed 2) 20 to 100% of available engine power applied 3) A 300 foot per minute or greater rate of descent, with insufficient power remaining to retard the sink rate

Induced flow at a hover:

Induced flow prior to vortex ring state:

Vortex ring state:

Corrective Actions:

1) Early stages - Increase power before entering vortex ring state.

2) Latter stages - Reduce collective (Altitude permitting) and increase airspeed.

NOTE: Angle of descent below 30 degrees at any airspeed should keep you out of settling with power.

AERODYNAMICS OF AUTOROTATION

Ref: FM 1-203 Def: The action of turning a rotor system by airflow and not by engine power. The airflow is produced by movement through the air.

Vertical Autorotation:

Rotor Regions

1) Driven Region - (A) The outboard 30% also known as the propeller region.

2) Driving Region - (B) The next inboard 45% also known as the autorotative region.

3) Stall Region - (C) The inboard most 25%.

Aerodynamics:

1) Driven Region (A)

a) Total aerodynamic force is aft of the center of rotation. b) This increases drag and produces a decelerative force on the rotor.

2) Point of Equilibrium (B & D)

a) Total aerodynamic force is aligned with the axis at rotation. b) Lift and drag are produced but provide neither acceleration or deceleration.

3) Driving Region (C)

a) Total aerodynamic force is inclined slightly forward of the axis of rotation. b) Provides an accelerating force over the entire blade.

4) Stall Region (E)

a) Blades are working above the stall angle of attack increasing drag. b) TAF is aft of the axis causing deceleration of the blade. Rotor Regions: All remain about the same but shift slightly to the retreating side of the rotor.

Stages:There are 3 stages of autorotation:

a) Entry b) Steady State c) Deceleration

Aerodynamics: 1) Entry. Level powered flight: The total aerodynamic force is inclined slightly aft, engine power overcomes this decelerative action.

2) Steady State: Upflow through the rotor changes the airflow which increases the angle of attack.

a) TAF is inclined forward to equilibrium. b) Best glide and rotor RPM will establish the steady state.

3) Deceleration: Rotor disk is tilted aft which increases the angle of attack and increases the total aerodynamic force and provides the necessary energy to drive the rotor for cushioning.

LOSS OF TAILROTOR EFFECTIVENESS Ref: -10 LTE Tape

Def: Occurrence of an un-commanded rapid right yaw, which if not reacted to quickly could cause loss of aircraft control. A) Regions of relative wind azimuths:

1) Weathercock Stabilitv: 120 degrees to 240 degrees a) Effects - Weather vane b) Corrective Actions

2) Vortex Ring State: 210 degrees to 330 degrees a) Normal airflow pattern b) Early vortex generation c) Late vortex stage d) Effects - pitch, roll, yaw excursions e) corrective actions

3) Disc Vortex: 280 degrees to 330 degrees a) Airflow patterns at 10 to 30 knots b) Effects - rapid, sudden right yaw c) Corrective Actions

4) Overlap Areas a) 210 - 240 degrees wx cock stability/vortex ring b) 280 - 330 degrees vortex ring/disk vortex

B) Contributing Factors: 1) High gross weight/DA- Effects 2) Low indicated airspeed: Effects 3) Power droop: Effects 4) Right downwind turns: Effects

C) Preventive or Corrective Measures: 1)Develop Feel - 2)Know winds - 3)Make turns to left if possible - 4)Avoid overcontrolling - 5)Fly Aircraft -

D) Caution in the -10:

E) Emergency Procedure:

1. WARNING

DYNAMIC ROLLOVER Ref: FM 1-203 -10 Def: If an OH-58A/C exceeds a critical angle with one skid on the ground acting as a pivot point, a rolling moment, and thrust approximately equal to weight, it will rollover, regardless of any further control inputs made.

Factors That Reduce Critical/Recovery Angle: (cyclic limits travel for stabilized position) 1)Crosswind -cyclic into the wind for stabilized position. 2)Lateral Offset CG -near pivot point. 3)right Skid Down -As pivot point due to tail rotor thrust 4)Left pedal Inputs -Increase thrust to right (If not trimmed out the helicopter can rollover in 2 seconds. 5)High roll rate -Reduces recognition/reaction time.

Corrective Actions: Use of collective is more effective by reducing thrust. The cyclic is less effective and sluggish when the helicopter has one skid on the ground. Make a moderate collective pitch change to control the roll. NOTE: Collective should be used before reaching 5-8 degrees above true horizon and could have a 2 degree angle of overshoot with a normal roll rate.

NOTE:To assist in prevention observe -10 limits of no more than 8 degree slopes and use caution on slopes greater than 5 degrees.

RETREATING BLADE STALL

A phenomenon resulting from the retreating blade not producing enought lift due to slow relative wind or burbling due to excessive angle of attack in compensating for dissymmetry of lift. this causes a change in relative wind, which causes a stall. since the blade tip moves downward faster than any other portion of the blade, the change in relative wind, and thus the stall originates at the blade tip and moves inboard. critical angle of attack 15*-20*. factors: high gross weight, high density altitude, turbulence, high “g” maneuvers, low rotor rpm. signs: abnormal vibration, pitch up, and a roll to the stalled blade side. corrective action: reduce power, reduce airspeed, check trim, reduse severity of the maneuver, increase rpm.

TYPES OF DRAG

DRAG IS THE FORCE THAT RETARDS THE MOTION OF AN AIRCRAFT OR AIRFOIL THROUGH THE AIR. 1. INDUCED: RESULT FROM THE PRODUCTION OF LIFT (1) BLADE TIP VORTICES, (2) INDUCED FLOW- CAUSES LIFT AND TOTAL AERODYNAMIC FORCE TO TILT REWARD ON THE AIRFOIL 2. PARASITE: DRAG FROM AIR MOVING OVER NON-LIFT PRODUCING SURFACES. 3. PROFILE: THE EQUIVALENT OF PARASITE DRAG BUT ACTING UPON THE ROTOR BLADE ITSELF WHILE IT IS PRODUCING LIFT MOVING THROUGH THE AIR. 4. TOTAL: THE SUM OF INDUCED, PARASITE AND PROFILE DRAG.