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747 flap retraction

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Old 18th Aug 2001, 21:53
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Post 747 flap retraction

Would anyone know why does the 747-400 buffets when flaps are being retracted between flaps 5 and 1, this i noticed is very evident on a heavy weight take off. I reckon it has something to do with the laminar flow over the wing as the leading edges are being retracted. I don't know if the other series have the same problem. Well its not a problem but i'm just curious!
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Old 19th Aug 2001, 13:41
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It is to do with lateral airflow I think...

The other day while doing a walk around, I asked the instructor why we cannot jettison fuel whilst flaps are in transit between 1-5.

He said it was due to the fact the airflow carried the fuel to the fuselage, which can cause fumes in the air conditioning system.

Methinks the two are related.

ODL
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Old 20th Aug 2001, 11:46
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Rage and ODL
The 744 flaps 5 to 1 selection would almost certainly generate significant aerodynamic changes since it combines the largest area change with a removal of approx one third of the leading edge devices.
The 'no jettison with flaps between 1 and 5' rule is due to the juxtaposition of the jettison nozzles and the outboard L E devices. The vortex generation pattern will change markedly during that particular range of flap deployment/retraction.
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