HFH Ch7 Helicopter Performance Flashcards
What are the 4 major factors that affect a helicopter’s performance?
Density altitude, weight, wind, and surface condition (pavement, water, grass, snow)
There’s approximately a _____ % reduction in performance in high humid conditions compared to dry air at the same altitude & temperature.
3-4%- Expect a decrease in hovering & takeoff performance- It can be the cause of a mishap when already operating at the limits of the helicopter
In some helicopters during high altitude operations, the maximum antitorque produced by the tail rotor during a hover may not be sufficient to overcome torque even if the _____ is within limits.
Gross weight
At higher gross weights, the increased power required to hover produces more torque, requiring more antitorque from the tail rotor.
_____ occurs whether the relative airflow is caused by the helicopter movement or by the wind.
Translational lift
This is the improved rotor efficiency resulting from directional flight.
_____ are an important consideration and the most desirable as they contribute to the greatest increase in performance.
Headwinds
As wind speed increases, translational lift increases, resulting in less power required
Strong crosswinds & tailwinds may require the use of more _____ to maintain directional control.
Tail rotor thrust
Increased tail rotor thrust absorbs power from the engine, resulting in less power available to the main rotor to produce lift
When taking off into a headwind, _____ is achieved earlier, resulting in more lift & a steeper climb angle.
Effective translational lift
When taking off with a tailwind, more _____ is required to accelerate through translational lift.
Distance
Dead man’s curve refers to what?
The shaded areas on the Height/Weight (H/W) diagram that don’t allow sufficient time to execute a final flare & recovery during autorotation. Somewhat analogous to base jumping from too low a height- you need to be high enough for your parachute to deploy, fill with air, & slow your decent.
The H/W diagram defines the height & airspeed combination that will allow a _____.
Safe landing in the result of engine failure
The time required, and therefore, altitude necessary to attain a steady autorotative descent, is dependent on the _____ of the helicopter and the _____.
Weight; density altitude
_____ occurs whether the relative airflow is caused by the helicopter movement or by the wind.
Translational lift
This is the improved rotor efficiency resulting from directional flight
What are the four forces acting on the helicopter?
Thrust, Drag, Weight, Lift
What are the three types of drag?
Profile drag, induced drag, parasite drag - PIP
What is profile drag?
Frictional resistance of the blades passing through the air
What is induced drag?
The component of lift that is acting in a rearward direction generated by the airflow around the rotor blade as it creates lift.
What is parasite drag?
Drag caused by the non-lifting components of the helicopter such as the cabin, rotor mast, tail, and landing gear.
What is induced flow?
The component of air flowing down through the rotor system that is created by the production of lift
What is the resultant relative wind?
Relative wind modified by induced flow
What is the angle of attack?
Angle measured between the resultant relative wind and the chord line of the rotor.
What is the angle of incidence?
The angle between the chord line of a blade and the rotor hub. Also known as blade pitch angle.
Is the angle of incidence mechanical or aerodynamic?
Mechanical
Is the angle of attack aerodynamic or mechanical?
Aerodynamic
What changes the angle of incidence?
Collective input and cyclic feathering
What is the translating tendency?
The tendency of the helicopter to move in the direction of the tail rotor thrust in hovering flight.
How is the translating tendency counteracted?
One or more of the following is used:
* Main transmission is mounted at slight angle to the left
* Flight controls can be rigged so the rotor disk is tilted to the left slightly when the cyclic is centered
* Transmission is mounted so the rotor shaft is vertical with respect to the fuselage, the helicopter “hangs” left skid low in the hover
* The fuselage will also be tilted when the tail rotor is below the main rotor disk and supplying antitorque thrust. The fuselage tilt is caused by the imperfect balance of the tail rotor thrust against the main rotor torque in the same plane. The helicopter tilts down due to two separate forces, the main rotor disk tilt to neutralize translating tendency and the lower tail rotor thrust below the plane of the torque action
* In forward flight, the tail rotor continues to push to the right, and the helicopter makes a small angle with the wind when the rotors are level and the slip ball is in the middle. this is called inherent sideslip
HFH 2-15
What is the danger of rearward flight?
The horizontal stabilizer can press the tail downward resulting in tail strike if the helicopter is moved into the wind.
What is coning?
An upward sweep of blade angle as a result of lift outperforming centrifugal force
What is the danger if the rotor RPM is allowed to go too low?
If rotor RPM continues to fall, the rotor will eventually stall, failing to produce enough lift to support the helicopter and causing a rapid descent. The resulting AOA becomes so high that even full down collective will not restore normal airflow
What is the Coriolis Effect?
Also known as the Law of Conservation of Angular Momentum. Rotating body continues to rotate with the same rotational velocity until some external force is applied to change the speed of the rotation.
What is gyroscopic precession?
The resultant action or deflection of a spinning object when a force is applied to this object which occurs approximately 90 degrees in the direction of rotation from the point where the force is applied.
What is dissymmetry of lift?
The differential (unequal) lift between advancing and retreating halves of the rotor disk caused by the different wind flow velocity across each half
What is retreating blade stall?
When the critical angle of attack is reached on the retreating blade.
What is evidence of retreating blade stall?
Nose pitch up, vibration, and a rolling tendency usually to the left.
What is translational lift?
Improved rotor efficiency resulting from directional flight
What is effective translational lift (ETL)?
Between 16 and 24 kts, the rotor system completely outruns the recirculation of old vortices and begins to work in relatively undisturbed air. Flow through the air is more horizontal
What is the transverse flow effect?
As the helicopter accelerates in forward flight, induced flow drops to near zero at the forward disk area and increases at the aft disk area. The result is a tendency for the helicopter to roll slightly to the right as it accelerates through approximately 20 kts.
How does air density affect helicopter performance?
- efficiency of the rotor blades to create lift
- power output of the engine
- drag produced by increased AOA
What is MCP?
Max Continuous Power
What is a “5 minute takeoff rating”
Rated takeoff power, with respect to reciprocating, turboprop, and turboshaft engines, means the approved horsepower that is developed statically under standard sea level conditions, within the engine operating limitations established under part 33, and limited in use to periods of not over 5 minutes.
What is the correct procedure after pulling the maximum rated takeoff power for 5 minutes?
Lower power to MCP, monitor engine instruments for higher than normal temps or pressures.
What effect does wind have on aircraft performance?
If a helicopter is taking off into the wind, the helicopter will reach ETL sooner; if taking off downwind ETL will be reached later, causing a longer ground run.
Over what types of surfaces will ground effect be most pronounced?
No wind over firm, smooth surface. Grasses, rough terrain, revetments, and water surfaces alter the airflow pattern, causing an increase in rotor tip vortices.
What is a major factor in determining Vne?
Retreating blade stall. Symptomized by low frequency vibration, pitch up, and roll in the direction of retreating blade (left in american helicopters). External load can further limit Vne.
HFH
Why does an OGE hover require more power than an IGE hover in the same conditions?
In OGE the airflow aroud the blade tips is no longer restricted, and the blade tip vortices increase with the decrease in outward airflow. As a result, induced drag increases, which means a higher pitch angle, meaning more power is needed to move air down through the rotor.
HFH
What is the difference between limit manifold pressure and max continuous power?
LMP - otherwise known as 5 min takeoff power rating. After 5 min, the collective should be lowered to a max continuous power and all pertinent engine gauges checked for abnormalities
MCP - Max continuous power is the maximum power is the maximum power to be used for any continuous period
POH
Why is the H/V digaram used only for the takeoff profile, and not for landing?
An engine failure accruing in the upper section of the H/V diagram is the most critical in a climb after takeoff. During a climb, a helicopter is operating at higher power settings and blade angle of attack. If descending when in the upper section of the diagram an engine failure is less critical provided a safe landing area.
HFH
What are the 3 different types of LTE?
Main Rotor Disk Interference - winds at velocities of 10-30 kts from left front cause the main rotor vortex to be blown into the tail rotor by the relative wind. The effect causes the tail rotor to operate in an extremely turbulent environment.
Weathercock Instablility - when winds are coming from the rear of the helicopter, the helicopter attempts to weathervane its nose relative to the wind. Unless a resisting pedal input is made, the helicopter begins a slow, uncommanded turn to either the right or left depending on the wind direction. If the pilot allows a yaw rate to develop, it can accelerate rapidly.
Tail Rotor Vortex Ring State - winds coming from the left of the helicopter cause a tail rotor vortex ring state to develop. The result is non-uniform, unsteady flow into the tail rotor. The vortex ring state causes tail rotor thrust variation, which result in yaw deviations. The net effect of the unsteady flow is an oscillation of the tail rotor thrust. Rapid and continuous pedal movements are necessary to compensate for the rapid changes in tail rotor thrust when hovering in a crosswind. Maintaining a precise heading in this region is difficult, but this characteristic presents no significant problem unless corrective action is delayed.
HFH
Why are airplane pilots considered high risk when flying helicopters?
An experienced airplane pilot’s ingrained reactions can be deadly when flying a helicopter. The airplane pilot, when required to react suddenly or under unexpected circumstances, may revert to airplane reactions and commit a fatal error. Under those conditions, his or her hands and feet move purely by reaction without conscious thought.
POH
What is the danger of exceeding manifold pressure limits?
Acute
- Exceeding manifold pressure limits could also result in demanding more power out of the engine than it is able to produce, which would result in a decay in rotor RPM. If the condition is allowed to further increase, rotor stall is likely.
Chronic
- If manifold pressure is excessive for a given RPM, the pressure within the cylinders could be exceeded, thus placing undue stress on the cylinders
- If repeated too frequently, this stress could weaken the cylinder components, and eventually cause engine failure
- Main Rotor blade and/or a drive system failure could occur if the helicopter is repeatedly flown above its approved manifold pressure limit.
HFH
When is translating tendency most significant?
In a hover when the tail rotor thrust need is the greatest
HFH
What are some of the indications of retreating blade stall?
- Low frequency vibration
- pitch up
- roll in the direction of retreating blade
HFH
What is the recovery procedure for retreating blade stall?
- Lower collective
- Aft cyclic
HFH
