There was a recent accident in Ireland and among a lot of other information the final report contains the following statement
"Shortly after the sighting by Witness B, it is likely that the aircraft began a downwind turn to the right. This turn is significant, as the aircraft would be manoeuvring from a high indicated speed/low groundspeed condition when heading into wind, to a low indicated speed when turned downwind. If the turn is made quickly, the effect of inertia may not allow the aircraft’s speed to increase sufficiently to maintain flight. This will result in a substantial loss of lift across the wing precipitating an aerodynamic stall"
Does this make sense ? Perhaps Peter Garrison would like to comment - there is a lot of discussion on the web site http://www.flyinginireland.com/forum/viewtopic.php?f=1&t=3665&start=30.
All Comments
I must concur that the verbiage is quite wordy, but I think I understand the basic reason why a stall would occur ...
The relative wind (with respect to the leading edge of the wing) would be somewhat higher while in the crosswind leg of an approach. Obviously, if the aircraft is in the pattern and approach configuration, the airspeed is going to be "relatively" closer to the stall speed.
Once turning downwind, the relative wind across the wing is going to decrease due to the wind basically pushing the aircraft from behind. Additionally, if the turn is made abruptly, the G forces associated with the turn are going to be higher (i.e. - a 45 degree turn results in 2X the weight component thus requiring additional lift to maintain level flight). The decrease in the relative wind along with the required additional lift may very well put the aircraft into a stall zone.
The only possible explanation I can come up with on the inertia point is that if the turn is performed in an uncoordinated fashion, the aircraft could be "sliding" through the turn. In this type of configuration, it would obviously take longer to get the aircraft back into coordinated flight thus providing an additional window of opportunity for the conditions in the previous paragraph to manifest themselves further and produce a stall.
I am definitely not saying this is correct ... just an idea ...
Clear Skies,
Dave
Completely incorrect. There is no inertial change in the aircraft when turning upwind or down wind, unless there is a shear in the wind or the indicated speed of the aircraft changes. Inertial forces imposed on the plane are relative to the air mass alone, and are in no way affected by a steady wind once off the ground. Turning downwind, if you can see the ground, makes it appear as if you are flying faster. In reality, you are flying faster only relative to the ground, but not relative to the air mass you are flying in, and in the end that is all that matters.
Agreed. The airplane (while in the air) does not know if you are turning up wind, down wind, or cross wind.
My wife and I departed Marysville KS with winds 30G50 right down the runway. Other than allowing for the gusts on take off with an extra 10 knots, the plane behaved normally as we turned cross wind and down wind and at pattern altitude the wind was a LOT more than 30G50 as we had over a 100 knot tail wind at 1500 feet.
Indicated airspeed is indicated airspeed- it does not know or care which leg you are flying. The GPS may indicate slower or faster on various legs, but only airspeed dictates if you have enough lift to keep the wings flying. Whoever was quoted is apparently not a pilot- but may be trying to sound like one.
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