Attitude Changes

AIR EXERCISE: Attitude Changes

AIM:

To teach how to change power settings and attitude and show the attitude-speed relationship. To develop co-ordination and to learn to control power and attitude.

THEORY:

Attitude is the orientation of the aircraft on its three (longitudinal, vertical and lateral) axes relative to the earth’s surface and the aircraft’s motion through the air. It is a picture that the pilot sees outside the aircraft (towards the horizon) which, if kept constant, relates to a certain airspeed.

Power is the amount of work being done by the engine in a given time.

Power changes are accomplished by raising or lowering the collective. In response to these movements the pitch on both blades changes by the same amount and the Manifold Air Pressure (MAP) gauge indicates the amount of power change; the larger the collective movement the greater the corresponding change on the MAP gauge.

Power, airspeed and attitude are all closely related.

Over pitching:

As one approaches the full throttle limit (when the governor has the throttle already in the fully open position), one must be careful not to raise the collective beyond the corresponding MAP limit or an over pitching situation will occur. High pitch angles result in high rotor drag, and therefore more power will be required to maintain rotor RPM. If in normal powered flight pitch is increased without a corresponding increase in power, the rotor RPM will decrease and the blades will cone upwards. This will result in a decrease in disc area and rotor RPM, therefore more pitch will be required to maintain the rotor thrust and consequently, rotor RPM will decrease further as drag increases.

Depending on the type of helicopter and its design limitations of its transmission, over pitching at full power may damage the transmission by over-torque.

Corrective action for over pitching:

Over pitching at full power is a particularly dangerous situation because the only corrective action is to lower the collective to reduce the pitch angle, thus incurring a loss in height.

Flapback:

The tendency of the rotor disc to tilt aft(or forward) in forward flight as a result of flapping.

It is more pronounced with higher forward speeds.

As forward speed increases more forward cyclic is required to counter the effects in increasing flapback.

Dissymmetry of lift:

The unequal lift across the rotor disc resulting from the difference in the velocity of the air over the advancing half and retreating blade half of the rotor disc area.

 

When the helicopter moves through the air, the relative airflow through the main rotor disc is different on the advancing side than on the retreating side. The relative wind encountered by the advancing blade is increased by the forward speed of the helicopter, while the relative wind speed acting on the retreating blade is reduced by the helicopter’s forward airspeed.

Therefore, as a result of the relative wind speed, the advancing blade side of the rotor disc produces more lift than the retreating blade side.

If this condition was allowed to exist, a helicopter with a counter clockwise main rotor blade rotation would roll to the left because of the difference in lift.

In reality, the main rotor blades flap and feather automatically to equalise lift across the rotor disc.

A semi rigid rotor system (two blades) utilises a teetering hinge, which allows the blades to flap as a unit.

When one blade flaps up, the other flaps down.

As the rotor blade reaches the advancing side of the rotor disc (A), it reaches its maximum flapping up velocity. When the blade flaps upward, the angle between the chord line and the resultant relative wind decreases. This decreases the angle of attack which reduces the amount of lift produced by the blade.

At position (C) the rotor blade is now at its maximum flapping down velocity. Due to the flapping down, the angle between the chord line and the resultant relative wind increases. This increases the angle of attack and thus the amount of lift produced by the blade.

The combination of blade flapping and slow relative wind acting on the retreating blade normally limits the maximum forward speed of a helicopter. At a high forward speed, the retreating blade stalls because of a high angle of attack and the slower relative blade speed. This situation is called retreating blade stall and is evidenced by a nose pitch up, vibration, and a rolling tendency — usually to the left in helicopters with counter clockwise blade rotation.

You can avoid retreating blade stall by not exceeding the never-exceed speed. This speed is designated VNE and is usually indicated on a placard and marked on the airspeed indicator by a red line.

During aerodynamic flapping of the rotor blades as they compensate for dissymmetry of lift; the advancing blade achieves maximum up flapping displacement over the nose and maximum down flapping displacement over the tail. This causes the tip-path plane to tilt to the rear and is referred to as flapback. The rotor disc was originally oriented with the front down following the initial cyclic input, but as airspeed is gained and flapping eliminates dissymmetry of lift, the front of the disc comes up, and the back of the disc goes down. This reorientation of the rotor disc changes the direction in which total rotor thrust acts so that the helicopter’s forward speed decreases as the nose is pitching up but, can be corrected with cyclic input.

Scanning:

This is a technique used to interpret the aircrafts instruments in an orderly and quick fashion to enable more accurate flying. The importance of scanning must be fully understood. An improper scan can cause a misinterpretation of the instruments as well as a lack of scanning may cause a fixation on one instrument which results in the others becoming erratic.

The most important instruments at this stage of the flying are:

  • Balance – string
  • Airspeed
  • Altitude

Get very familiar as to where these instruments are situated to enable quick reference.

Remember 95% of helicopter flying should be done looking outside of the cockpit (towards the horizon), the other 5% is used for your scanning.

Example:

An airspeed change is required whilst maintaining your height.

  • Adjust the attitude with reference to the horizon, outside the helicopter. Don’t forget the adjustments needed on the collective and pedal to maintain height and direction(balance).
  • Wait
  • Scan Airspeed: is it correct
  • Look outside
  • Scan Balance (woolometer): is it correct
  • Look outside
  • Scan Height: is it correct
  • Look outside

If one of the scans shows an inaccurate balance for example, then continue looking outside, make the correction and continue the scan until all your instruments show you what you desire.

I normally let the student do this exercise whilst not looking at any instruments. I call out the max/min MAP that I want him to stop, on the collective. Normally, between 15 and 23”

It is important to realise that if one of the instruments are not what you would like it to read to then look outside and make the adjustment – do not fixate on that instrument until it is correct because most likely other instruments will then be incorrect.

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