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Old 18th May 2001, 11:41
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john_tullamarine
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CB .. My comments related only to the big VG out front ... that one is a big no-no for despatch with it missing. It has a significant effect on the flight test stall speed. It was one of these which was missing on the aircraft to which I referred previously.

Whether people want to call the big VGs chines, fins, or whatever is immaterial. Their purpose is as vortex generators.. the shed vortex can usually be seen in humid ambient conditions at higher alpha and curls over the leading edge and back along the wing upper surface quite effectively. The smaller VGs are, presumably, for local leading edge/nacelle/pylon flow control.

On another note, Zeke makes comment regarding nacelle lift. Possibly he/she is thinking of the normal (ie perpendicular) force seen at either the propeller disc in props or the front of the nacelle in jets.

This is due to the change in direction of airflow as the disc or inlet is traversed. The flow, viewed from the side, appears somewhat similar to the upwash/downwash pattern past a wing, and has the same result - a significant vertical force. This force has material implications in respect of static stability - for instance, on piston-turboprop conversions, the prop disc usually is relocated out near the nose of the aircraft to allow the CG to be kept under control with the lighter engine. However, this then causes a large destabilising vertical force out forward during, eg, missed approach as the power/thrust is increased. As a result, it is common on piston/turboprop conversions to require installation of a variable downspring arrangement (usually called something in the nature of a stability augmentation system) to overcome a potentially very nasty unstable pitch up tendency due to the normal force referred to above.

[This message has been edited by john_tullamarine (edited 19 May 2001).]