Aerodynamics Flashcards
Describe Settling with Power
a condition of powered flight in which the helicopter settles in its own downwash (aka vortex ring state)
What conditions must exist simultaneously for settling with power to occur
- a vertical or near-vertical descent of at least 300 ft per minute. 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
What flight conditions are conductive 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
How do you recover from settling with power
one or a combination of:
- during the initial stage, a large application of collective pitch may arrest rapid descent. If done carelessly or too late, collective increase can aggravate the situation resulting in more turbulence and an increased rate of decent
- in single-rotor helicopters, aviators can accomplish recovery by applying cyclic to gain airspeed and arrest upward induced flow of air and or/by lowering the collective. Normally, gaining airspeed in the preferred method as less altitude is lost
Describe Dynamic Roll Over
A helicopter is susceptible to a lateral-rolling tendency. Can occur on level ground as well as during a slope or crossing landing and takeoff.
What conditions are required for dynamic roll over
pivot point, rolling motion, exceed critical angle
Describe static rollover angle and dynamic angle
- helicopter has a static rollover angle that if exceeded will cause the aircraft to rollover. Based on CG and pivot point angle where CG is located over pivot point
- when a rolling motion is present the dynamic rollover angle is introduced and called the critical angle. Varies based on rate of rolling motion
Describe rollover on level ground
- during takeoff from level ground if one skid or wheel is stuck on the ground. The stuck portion becomes the pivot point. A smooth and moderate collective reduction is recommended lowering the aircraft back to the ground until the stuck skid or wheel is free.
Describe rolling downslope
occurs when the steepness of the slope causes the helicopter to tilt beyond the lateral cyclic control limits
Describe rolling upslope
occurs when the aviator applies to much cyclic into the slope to hold the skid/wheel firmly on the slope
Describe physical and human factors of dynamic rollover
physical factors:
- main rotor thrust
- CG
- tail-rotor thrust
- crosswind component
- ground surface
- sloped landing area
- presence of a low fuel condition which might cause the CG to move upward
human factors:
- inattention
- inexperience
- failure to take timely corrective action
- inappropriate control input
- loss of visual reference
Describe Retreating Blade Stall
the retreading blade of a helicopter will eventually stall in forward flight. Limits the high speed of a rotor wing aircraft. In forward flight decreasing velocity of airflow on the retreating blade demands 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
Describe conditions that produce blade stall
- high blade loading (high gross weight)
- low rotor RPM
- high density altitude
- high G-maneuvers
- Turbulent air
Describe how to recover from 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
Describe Compressibility
at low airspeeds, air is incompressible. Incompressible airflow is similar to the flow of water, hydraulic fluid, or any other incompressible fluid. At low speeds air experiences relatively small changes in pressure with little change in density. In high speeds greater pressure changes occur causing compression of air which results in significant changes to air density. Compressible flow occurs when there is a transonic or supersonic flow of air across the airfoil
