Aerodynamics And Maneuvering Flight Flashcards
Total Aerodynamic Force
As air flows around an airfoil, a pressure differential develops between the upper and lower surfaces. The differential, combined with air resistance to passage of the airfoil, creates a force on the airfoil. This is known as TAF. TAF acts at the center of pressure on the airfoil and is normally inclined up and rear. TAF, sometimes called resultant force, may be divided into two components, lift and drag.
Airflow During A Hover IGE
Rotor efficiency is increased by ground effect to a height of about one rotor diameter (measured from the ground to the rotor disk) for most helicopters. This increase in AOA requires a reduced blade pitch angle. This reduces the power required to hover IGE.
Airflow During a Hover OGE
The benefit of placing the helicopter near the ground is lost above IGE altitude. Above this altitude, the power required to hover remains nearly constant, given similar conditions (such as wind). Induced flow velocity is increased causing a decrease in AOA. A higher blade pitch angle is required to maintain the same AOA as in IGE hover.
Translating Tendency
During hovering flight, the counterclockwise rotating, single-rotor helicopter has a tendency to drift laterally to the right. The translating tendency results from right lateral tail-rotor thrust exerted to compensate for main rotor torque (main rotor turning in a counterclockwise direction). The aviator must compensate for this right translating tendency of the helicopter by tilting the main rotor disk to the left.
Transverse Flow
In forward flight, air passing through the rear portion of the rotor disk has a greater downwash angle than air passing through the forward portion. This is due to the fact the greater the distance air flows over the rotor disk, the longer the disk has to work on it and the greater the deflection on the aft portion. Downward flow at the rear of the rotor disk causes a reduced AOA, resulting in less lift. The front portion of the disk produces an increased AOA and more lift because airflow is more horizontal. These differences in lift between the fore and aft portions of the rotor disk are called transverse flow effect. This effect causes unequal drag in the fore and aft portions of the rotor disk and results in vibration easily recognizable by the aviator. It occurs between 10 and 20 knots. Transverse flow effect is most noticeable during takeoff and, to a lesser degree, during deceleration for landing. Gyroscopic precession causes the effects to be manifested 90 degrees in the direction of rotation, resulting in a right rolling motion.
Dissymmetry Of Lift
Dissymmetry of lift is the differential (unequal) lift between advancing and retreating halves of the rotor disk caused by the different wind flow velocity across each half. This difference in lift would cause the helicopter to be uncontrollable in any situation other than hovering in a calm wind. In forward flight, two factors in the lift equation, blade area and air density, are the same for the advancing and retreating blades. Airfoil shape is fixed for a given blade, and air density cannot be affected; the only remaining variables are blade speed and AOA. Rotor RPM controls blade speed. Because rotor RPM must remain relatively constant, blade speed also remains relatively constant. This leaves AOA as the one variable remaining that can compensate for dissymmetry of lift. This is accomplished through blade flapping and/or cyclic feathering.
Retreating Blade Stall
In forward flight, decreasing velocity of airflow on the retreating blade demands a higher AOA to generate the same lift as the advancing blade. When forward speed increases, the no-lift areas of the retreating blade grow larger, placing an even greater demand for production of lift on a progressively smaller section of the retreating blade. This smaller section of blade demands a higher AOA until the tip of the blade (area of the highest AOA) stalls. Tip stall causes vibration and buffeting which spread inboard and aggravate the situation while the aircraft may roll left and nose pitches up.
Conditions Producting Stall
- High blade loading (high gross weight).
- Low rotor RPM.
- High- density altitude.
- High G-maneuvers.
- Turbulent air.
Recovering From Retreating Blade Stall
- Reduce collective.
- Reduce airspeed.
- Descend to a lower altitude (if possible).
- Increase rotor RPM to normal limits.
- Reduce the severity of the maneuver.
Compressibility
At high speeds greater pressure changes occur causing compression of air which results in significant changes to air density. This compressible flow occurs when there is a transonic or supersonic flow of air across the airfoil.
Critical Mach number is the highest blade speed without supersonic airflow. As the critical Mach number is exceeded, an area of supersonic airflow is created. A normal shock wave then forms the boundary between supersonic and subsonic flow on the aft portion of the airfoil surface.
As the shock waves move toward the trailing edge of the airfoil, the aerodynamic center begins to move away from its normal location of 25 percent chord. By the time the shock wave has reached the trailing edge of the airfoil, the aerodynamic center has retreated to the 50 percent chord. This causes the leading edge of the airfoil to be deflected down, which may result in structural failure of the blade (skin deformation or separation).
Dynamic Rollover
A helicopter is susceptible to a lateral-rolling tendency called dynamic rollover.
Three conditions are required for dynamic rollover—pivot point, rolling motion, and exceed critical angle.
Human Factors:
- Inattention.
- Inexperience.
- Failure to take timely corrective action.
- Inappropriate control input
- Loss of visual reference.
Settling With Power
Settling with power is a condition of powered flight in which the helicopter settles in its own downwash. This condition may also be referred to as vortex ring state. If descent is so rapid, induced flow at the inner portion of the blades is upward rather than downward. If this rate of descent exists with insufficient power to slow or stop the descent, it will enter the vortex ring state.
Required For Settling With Power:
- A vertical or near-vertical descent of at least 300 feet per minute (FPM). Actual critical rate depends on gross weight, rotor RPM, density altitude, and other pertinent factors.
- Slow forward airspeed (less than ETL).
- Rotor system must be using 20 to 100 percent of the available engine power with insufficient power remaining to arrest the descent.
Conditions Required For Settling With Power
- A vertical or near-vertical descent of at least 300 feet per minute (FPM). Actual critical rate depends on gross weight, rotor RPM, density altitude, and other pertinent factors.
- Slow forward airspeed (less than ETL).
- Rotor system must be using 20 to 100 percent of the available engine power with insufficient power remaining to arrest the descent. Low rotor RPM could aggravate this.
Conditions Conducive To Settling With Power
- Steep approach at a high rate of descent.
- Downwind approach.
- Formation flight approach (where settling with power could be caused by turbulence of preceding aircraft).
- Hovering above the maximum hover ceiling.
- Not maintaining constant altitude control during an OGE hover.
- During masking/unmasking.
Bucket Speed
Bucket speed is the airspeed range providing the best power margin for maneuvering flight. Using the cruise chart for current conditions, enter at 50 percent of maximum torque available, go up to gross weight, over to the lowest and highest airspeed intersecting the aircraft gross weight, and note speeds between which there is the greatest power margin for maneuvering flight.
