7
$\begingroup$

The two illustrations below show the typical spoiler location and the lift distribution of a wing. For comparison a third picture shows the location of spoilers on a glider.

It appears the airliner spoiler is located so it destroys very little lift but would really only contribute to drag - is this really the primary contribution spoilers have to the aerodynamics of an airliner or is there more happening here?

enter image description here
Typical location and size of airliner spoilers. - source


enter image description here
Typical lift distribution of wing. Red references have been added to show typical airliner spoiler. - source


enter image description here
For comparison, a glider spoiler located at the center of lift - source

$\endgroup$
1
  • $\begingroup$ The first picture looks like a deployed trailing edge fowler type flap to me (drops down and back along a track), not a spoiler. The spoiler would be the flat plates in front of the spoiler, it is not deployed in the picture. $\endgroup$ – CrossRoads Jul 30 '18 at 16:48
4
$\begingroup$

You modified the lift distribution diagram by adding spoilers without showing how that modifies the lift of the wing. The trailing edge of a wing controls how much lift is produced, whether you call it flow turning, circulation, or whatever.

Your diagram implies that lift is produced regardless trailing edge position. If this were true ailerons would have no effect since they only occupy the trailing quarter of the typical wing. Look at the camber line of the wing in your diagram with and without spoilers and you'll see. Flow does not turn with spoilers deployed, so you lose lift and gain a lot of drag.

enter image description here

source

Extending the wing flaps increases the camber or curvature of the wing, raising the maximum lift coefficient or the upper limit to the lift a wing can generate.

A spoiler of the variety shown in the first diagram works like a split flap upside down, reducing camber and maximum lift coefficient.

enter image description here inverted enter image description here source

$\endgroup$
11
  • $\begingroup$ The diagram you are showing is a symmetrical vs cambered airfoil, that is not an accurate depiction of the aerodynamics of this question. This illustration suggests interaction of flaps but not spoilers. Spoilers do not change the wing geometry, flaps do. $\endgroup$ – jwzumwalt Jul 29 '18 at 19:54
  • $\begingroup$ The deployment of spoilers decambers the airfoil as the effective trailing edge of the wing moves to some position between the tip of the flap and the tip of the spoiler. The amount of decamber depends on the shape of the wing and the extension of the spoiler, so it may very well amount to neutral camber. $\endgroup$ – Pilothead Jul 29 '18 at 20:06
  • 2
    $\begingroup$ @jwzumwalt, the thing is that due to its inertia, the air can only navigate so sharp corner. To get from over the spoiler to the trailing edge, it would have to turn sharper than it can, so instead it flows off upward and a pocket of stagnant (and stagnant means ambient, so higher, pressure) forms behind the spoiler. This is the same principle as stall—the spoilers effectively force a stall. $\endgroup$ – Jan Hudec Jul 29 '18 at 20:30
  • 1
    $\begingroup$ @jwzumwalt The front 90% depends on the last 10%. You don't get the isobars shown without the airflow deflection at the trailing edge. Again, ailerons don't work otherwise. $\endgroup$ – Pilothead Jul 29 '18 at 20:39
  • 1
    $\begingroup$ @jwzumwalt, keep in mind that due to principle of action and reaction, the total change of momentum of the airflow is equal to the lift generated. If the air does not leave the trailing edge downward, the sum of the forces it applied to the wing was not upward (it might generate some lift in the forward part, but it generates more negative lift over the rear part, creating some upward pitching moment, but no total lift). $\endgroup$ – Jan Hudec Jul 29 '18 at 20:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.