HFH Ch7 Helicopter Performance

Question 1

What are the 4 major factors that affect a helicopter’s performance?

Answer 1

Density altitude, weight, wind, and surface condition (pavement, water, grass, snow)

Question 2

There’s approximately a _____ % reduction in performance in high humid conditions compared to dry air at the same altitude & temperature.

Answer 2

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

Question 3

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.

Answer 3

Gross weight

At higher gross weights, the increased power required to hover produces more torque, requiring more antitorque from the tail rotor.

Question 4

_____ occurs whether the relative airflow is caused by the helicopter movement or by the wind.

Answer 4

Translational lift

This is the improved rotor efficiency resulting from directional flight.

Question 5

_____ are an important consideration and the most desirable as they contribute to the greatest increase in performance.

Answer 5

Headwinds
As wind speed increases, translational lift increases, resulting in less power required

Question 6

Strong crosswinds & tailwinds may require the use of more _____ to maintain directional control.

Answer 6

Tail rotor thrust

Increased tail rotor thrust absorbs power from the engine, resulting in less power available to the main rotor to produce lift

Question 7

When taking off into a headwind, _____ is achieved earlier, resulting in more lift & a steeper climb angle.

Answer 7

Effective translational lift

Question 8

When taking off with a tailwind, more _____ is required to accelerate through translational lift.

Answer 8

Distance

Question 9

Dead man’s curve refers to what?

Answer 9

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.

Question 10

The H/W diagram defines the height & airspeed combination that will allow a _____.

Answer 10

Safe landing in the result of engine failure

Question 11

The time required, and therefore, altitude necessary to attain a steady autorotative descent, is dependent on the _____ of the helicopter and the _____.

Answer 11

Weight; density altitude

Question 12

_____ occurs whether the relative airflow is caused by the helicopter movement or by the wind.

Answer 12

Translational lift

This is the improved rotor efficiency resulting from directional flight

Question 13

What are the four forces acting on the helicopter?

Answer 13

Thrust, Drag, Weight, Lift

Question 14

What are the three types of drag?

Answer 14

Profile drag, induced drag, parasite drag - PIP

Question 15

What is profile drag?

Answer 15

Frictional resistance of the blades passing through the air

Question 16

What is induced drag?

Answer 16

The component of lift that is acting in a rearward direction generated by the airflow around the rotor blade as it creates lift.

Question 17

What is parasite drag?

Answer 17

Drag caused by the non-lifting components of the helicopter such as the cabin, rotor mast, tail, and landing gear.

Question 18

What is induced flow?

Answer 18

The component of air flowing down through the rotor system that is created by the production of lift

Question 19

What is the resultant relative wind?

Answer 19

Relative wind modified by induced flow

Question 20

What is the angle of attack?

Answer 20

Angle measured between the resultant relative wind and the chord line of the rotor.

Question 21

What is the angle of incidence?

Answer 21

The angle between the chord line of a blade and the rotor hub. Also known as blade pitch angle.

Question 22

Is the angle of incidence mechanical or aerodynamic?

Answer 22

Mechanical

Question 23

Is the angle of attack aerodynamic or mechanical?

Answer 23

Aerodynamic

Question 24

What changes the angle of incidence?

Answer 24

Collective input and cyclic feathering

Question 25

What is the translating tendency?

Answer 25

The tendency of the helicopter to move in the direction of the tail rotor thrust in hovering flight.

Question 26

How is the translating tendency counteracted?

Answer 26

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

Question 27

What is the danger of rearward flight?

Answer 27

The horizontal stabilizer can press the tail downward resulting in tail strike if the helicopter is moved into the wind.

Question 28

What is coning?

Answer 28

An upward sweep of blade angle as a result of lift outperforming centrifugal force

Question 29

What is the danger if the rotor RPM is allowed to go too low?

Answer 29

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

Question 30

What is the Coriolis Effect?

Answer 30

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.

Question 31

What is gyroscopic precession?

Answer 31

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.

Question 32

What is dissymmetry of lift?

Answer 32

The differential (unequal) lift between advancing and retreating halves of the rotor disk caused by the different wind flow velocity across each half

Question 33

What is retreating blade stall?

Answer 33

When the critical angle of attack is reached on the retreating blade.

Question 34

What is evidence of retreating blade stall?

Answer 34

Nose pitch up, vibration, and a rolling tendency usually to the left.

Question 35

What is translational lift?

Answer 35

Improved rotor efficiency resulting from directional flight

Question 36

What is effective translational lift (ETL)?

Answer 36

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

Question 37

What is the transverse flow effect?

Answer 37

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.

Question 38

How does air density affect helicopter performance?

Answer 38

• efficiency of the rotor blades to create lift
• power output of the engine
• drag produced by increased AOA

Question 39

What is MCP?

Answer 39

Max Continuous Power

Question 40

What is a “5 minute takeoff rating”

Answer 40

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.

Question 41

What is the correct procedure after pulling the maximum rated takeoff power for 5 minutes?

Answer 41

Lower power to MCP, monitor engine instruments for higher than normal temps or pressures.

Question 42

What effect does wind have on aircraft performance?

Answer 42

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.

Question 43

Over what types of surfaces will ground effect be most pronounced?

Answer 43

No wind over firm, smooth surface. Grasses, rough terrain, revetments, and water surfaces alter the airflow pattern, causing an increase in rotor tip vortices.

Question 44

What is a major factor in determining Vne?

Answer 44

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

Question 45

Why does an OGE hover require more power than an IGE hover in the same conditions?

Answer 45

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

Question 46

What is the difference between limit manifold pressure and max continuous power?

Answer 46

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

Question 47

Why is the H/V digaram used only for the takeoff profile, and not for landing?

Answer 47

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

Question 48

What are the 3 different types of LTE?

Answer 48

Main Rotor Disk Interference - winds at velocities of 10-30 kts from left front cause the main rotor vorte to be blow ninto 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

Question 49

Why are airplane pilots considered high risk when flying helicopters?

Answer 49

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

Question 50

What is the danger of exceeding manifold pressure limits?

Answer 50

• 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
• 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. Main Rotor blade and/or a drive system failure could occur if the helicopter is repeatedly flown above its approved manifold pressure limit.

HFH

Question 51

When is translating tendency most significant?

Answer 51

In a hover when the tail rotor thrust need is the greatest

HFH

Question 52

What are some of the indications of retreating blade stall?

Answer 52

• Low frequency vibration
• pitch up
• roll in the direction of retreating blade

HFH

Question 53

What is the recovery procedure for retreating blade stall?

Answer 53

1. Lower collective
2. Aft cyclic

HFH