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Tuesday, 31 October 1967

Mr Hayden (OXLEY, QUEENSLAND) asked the Minister for Air, upon notice:

1.   Have trials with the F111A aircraft shown that it has a tendency to stall at its highest altitudes and speeds.

2.   If so, what are the causes of this stall, and is progress being made in overcoming this defect.

Mr Howson (FAWKNER, VICTORIA) (Minister Assisting the Treasurer) - The answers to the honourable member's questions are as follows:

1.   It is not clear whether the question relates to aircraft stall which is the literal interpretation of the question or relates to the compressor stall to which much publicity has recently been given. It is therefore intended to cover both aspects in the replies hereunder:

(i)   Aircraft Stall. All aircraft will still at high altitudes and high speeds when the condition is reached where the wings will no longer support the 'apparent weight' of the aircraft. The 'apparent weight' of the aircraft increases with the 'g' (gravity) loading which the pilot applies during manoeuvring. There is no evidence the F111A is not satisfying the specification in respect of this effect, for the conditions under which the aircraft was designed.

(ii)   Compressor Stall. The F111A did suffer from compressor stalls leading to engine surge under certain conditions.

2 -

(i)   Aircraft Stall. Answered as part of Question (i).

(ii)   Compressor Stall -

(a)   The compressor may be considered as a collection of little wings. Under certain air flow conditions these 'wings' in any axial compressor engine can stall and cause engine surge. Air flow conditions under which the compressor can stall are dependent on internal design of the engine and the aerodynamics of the intake of the aircraft. The intake must present a smooth airflow to the face of the engine in both subsonic and supersonic flight, the aerodynamics conditions of which are quite different. In the original configuration of the F111A the air flow conditions at the intake of the engine were not being kept smooth throughout the speed and altitudes at which the aircraft had to fly.

(b)   Considerable progress has been made towards solving this problem by modifications which reduce the engine's sensitivity to irregular air flow, and by changes to the aerodynamics of the intake. The aircraft now operates successfully over the majority of the flight conditions for which it was designed. Development is still continuing to cover the small area of flight conditions where engine surge can occur. In the light of the information currently available, this area does not include the flight conditions in which the majority of the RAAF F111C operations will be carried out.

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