Post by LOU on Sep 8th, 2011 at 1:39am
Correct!
There are two questions to answer here...
First, why the mix of Kruger and slats on the 727 & 737?
The Kruger flap is a pretty basic device. It folds forward from below the leading edge & diverts airflow over the top surface of the wing. More airflow flow over the top of the wing equals more lift. Air is trapped behind the flap and acts as a smoothing area keeping the airflow laminar both over and under the wing. At higher angles of attack the flow breaks down where the Kruger is installed stalling that part of the wing first. These type of Kruger flaps are installed inboard on the 727 and 737 with slats outboard to insure that the wing keeps its lift on the outboard part of the wing in much the same way the wing is washed out. You want the tip to stall last so you keep roll control right to the end.
Second, why does the 707 and 747 have only Kruger L E flaps?
In the case of the 707, the early ones had no L E devices. That did not last long as they were runway hogs.
As the 707 progressed, Kruger flaps were added to the wing. The progression of the -B -BA and -BAH saw additional Kruger flaps added. The reason for the Kruger vs the slat on the early Boeing planes was probably in part a cost factor. The Kruger was cheaper to make than the slat. When the 747 came along the designers were very innovative with a mix of Kruger flat flaps on the inboard part of the wing and variable camber Kruger flaps toward the outboard part of the wing. This insured the wing would keep flying at the tip during a stall.
The Kruger flap has the job of increasing lift and then stalling the wing root early to counter the pich up at stall inherent in swept wings. The cambered leading edge of the 747 does the same thing at moderate angles of attack, but hangs in there a bit longer, giving smooth flow and lift to much higher angles. Later models of the 767 and 757 lost the Kruger and went to more of a full span slat.
Lou
There are two questions to answer here...
First, why the mix of Kruger and slats on the 727 & 737?
The Kruger flap is a pretty basic device. It folds forward from below the leading edge & diverts airflow over the top surface of the wing. More airflow flow over the top of the wing equals more lift. Air is trapped behind the flap and acts as a smoothing area keeping the airflow laminar both over and under the wing. At higher angles of attack the flow breaks down where the Kruger is installed stalling that part of the wing first. These type of Kruger flaps are installed inboard on the 727 and 737 with slats outboard to insure that the wing keeps its lift on the outboard part of the wing in much the same way the wing is washed out. You want the tip to stall last so you keep roll control right to the end.
Second, why does the 707 and 747 have only Kruger L E flaps?
In the case of the 707, the early ones had no L E devices. That did not last long as they were runway hogs.
As the 707 progressed, Kruger flaps were added to the wing. The progression of the -B -BA and -BAH saw additional Kruger flaps added. The reason for the Kruger vs the slat on the early Boeing planes was probably in part a cost factor. The Kruger was cheaper to make than the slat. When the 747 came along the designers were very innovative with a mix of Kruger flat flaps on the inboard part of the wing and variable camber Kruger flaps toward the outboard part of the wing. This insured the wing would keep flying at the tip during a stall.
The Kruger flap has the job of increasing lift and then stalling the wing root early to counter the pich up at stall inherent in swept wings. The cambered leading edge of the 747 does the same thing at moderate angles of attack, but hangs in there a bit longer, giving smooth flow and lift to much higher angles. Later models of the 767 and 757 lost the Kruger and went to more of a full span slat.
Lou