ESDU Challenge: Spoiler on a Wing

It is increased, but not enough to overcome the downward force of the effect of the spoiler.

It's late and I'm tired.

Does it have counter or co rotating vortex generators?

Loses lift....pressure decreases...

Those are not spoilers.

That clearly was one!

Answer: The pressure on the lower surface of the wing drops (relative to the pressure on its top surface).

Spoilers, like their name suggests, disrupt the airfoil of a wing (on the last half of the wing) and "spoils" its lift. They will increase drag of the wing considerably and are used as an aid for landing.

Typically, spoilers are used on sail planes and used during the landing phase to modulate descent rates. Occasionally, advanced students or pilots may use them during tow to altitude to correct for a slack rope condition.

That's it.

Interesting bit about sailplanes... makes complete sense.

As our flight chief would say, "Every takeoff is guaranteed a landing." However, I soon realized, you only get one.

I'm going to assume that speeds are sufficiently slow for consideration to remain strictly non-compressible. I'm also assuming that the spoiler is at or reasonably close to the trailing edge....

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The changes in pressure distribution on the lower surface should be those that result from changes introduced to the flow from the spoiler. Two effects seem like they would be the most important:

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- Slowing flow and building pressure above the wing. This should cause the effective point at which air is separated into either the path above or below the wing to shift higher on the leading edge. Exactly where this point fell, on the upper or lower portion, prior to the spoiler will probably have an important effect, but in general, there is likely to be an increase in pressure at the very front of the lower surface.

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- Reduced resistance to flow/ pressure at just aft of the trailing edge. This will allow air below the rear portion of the wing to move at relatively higher speeds. Relatively lower pressure will act on the rear portion of the lower surface.

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So my vote is that the pressure distribution shifts forward as a result of a a good probability of a small area of higher pressure at the leading edge of the lower surface, but more importantly a reduction in pressure over a much of the aft part of the lower surface.

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Seems fairly safe to predict, decreased lift, increased drag and a torque developed from the wing relative to without the spoiler in the leading edge up, trailing edge down direction.

On sail planes the spoilers are located midway along the chord of the wing, not the trailing edge.

I suppose if the spoiler were mid-chord, the same effects would likely result, just to a less pronounced degree:

- impediment to flow on upper surface should shift the stagnation point up, so that the height where the air stream are divided into what goes above and below is higher on the front of the foil.

- since the spoiler redirects the flow higher than it would have been, or at least causes the flow to separate, this makes the flow into the spaces just aft of the trailing edge far less contentious. This should allow the flow beneath the wing to move faster creating lower pressure, especially progressing towards the rear.

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I'm keeping my vote concerning the lower surface as 'the pressure distribution is shifted forward'. Of course, lower lift, more drag and an torque in the direction of leading edge up, trailing edge down, should all still be the typical response.

"On sail planes the spoilers are located midway along the chord of the wing, not the trailing edge."

On most yes that is the case but not on all sail planes/gliders. For example with the Glasflügel 206 Hornet they are located on the trailing edge of the wing and pivot a large section of the trailing edge so that part rises up above the wing's upper surface while the other half deploys below the lower surface of the wing. The net effect is a slight reduction in the stall speed of about 2 knots and a phenomenal increase in drag without any loss of lift. The increase in drag is so great that even with the stick pushed full forward the aircraft will not go past V_NE. Although I haven't actually tried this for myself, but I'm rather repeating what other pilots have told me about the aircraft.

Something that doesn't reduce lift, increases drag tremendously and is deployed on both sides of the wing, seems like it would be better described as a 'brake' than as a 'spoiler'.

Yes, I think that it really is a brake, not a spoiler.

I have not seen all the sailplanes made, but all the glass ships I have seen have dive brakes and not spoilers.

"all the glass ships I have seen have dive brakes and not spoilers."

Don't you mean the opposite. The Glasflügel 206 Hornet does have air brakes as it reduces the speed while actually slightly increasing the lift produced by the wing, on most gliders that have air brakes that deploy on the upper surface of the wing are actually spoilers as they spoil or disrupt the flow of air over the top of the wing. This has three effects

• Loss of Lift: The disruption of airflow caused by the spoiler means that that section of the wing is no longer creating lift and hence you start to lose altitude.
• Increased Drag: There will be an increase in drag which is proportional to how much of the spoiler is deployed. However as I have said before the loss of lift and conversion of potential energy into kinetic energy can cause a small increase in airspeed.
• Change in Centre of Lift: Unless the spoilers are correctly positioned the loss of lift and increase in drag can cause the lift vector to change which causes a corresponding change in the aircraft's pitch requiring re-trimming of the aircraft.

I haven't flown a modern multi engine jet airliner, but from what I have observed they use the spoilers on the upper surface of the wing to roll and yaw the aircraft simultaneously negating the need for use of rudder and elevator to counter the secondary effects of ailerons¹ and rudder². However, they may have to use some elevator to increase the angle of attack slightly to counter the loss of lift caused by the asymmetric deployment of the spoilers.

It's only when they slowdown that they need to start using ailerons, elevator and rudder, although it's critical that you never use sudden rudder reversals as you WILL (and it has happened) rip the tail fin clear off the aircraft.

Note 1: The secondary effect of aileron causes the aircraft to yaw in the opposite direction that it is banked. This is because the high wing is creating more lift than the low wing and as a result the induced drag created by the high wing will yaw the aircraft in that direction which is the exact opposite of what you want in a balanced turn. To counter this you have to use rudder that yaws the aircraft in the direction you are banking the aircraft.

Note 2: The secondary effect of rudder causes the aircraft to bank in the direction of the yaw caused by the rudder. This is because the wing on the side you are yawing towards to is travelling through the air slower than the outer wing. This results in the inner wing producing less lift and the outer wing more. Now this sounds like what you would want, but the secondary effect of rudder especially on something like a glider with long slender wings banks the aircraft too much and without correction the aircraft will end up in either a spiral dive or stall and spin.

To balance a turn you need to coordinate all three control surfaces, the ailerons to control the angle of bank, the rudder to control the yaw and the elevator to allow for the increased load on the wing due to centripetal forces cause by the turn.

Glider pilots and jet fighter pilots use a very sophisticated instrument to balance all these control inputs and keep the aircraft flying through the air cleanly. It's called a yaw string (see image below) and consists of a piece of wool about 10 cm long that is taped to the middle of the canopy directly in front of the pilots field of view. The idea is to keep the yaw string in line with the centre line of the aircraft at all times. If you do that then you have the aircraft balanced and flying correctly.

Nothing happens to the pressure distribution on the lower side of the wing as there is no physical change to the flow path. However, the pressure on the upper side of the wing as well as the aerodynamic drag will increase due to the flow obstruction of the spoiler. This will reduce the wing lift but the symmetric effect of deploying spoilers equally on both wings will negate any yaw effect. The net effect will be reduced airspeed and increased descent rate, which is why they are used on commercial airliners in landing when close to the runway. Once solid ground contact is made the engine thrust reversers are engaged to further slow the aircraft until the brakes can be safely engaged to achieve taxing speed.

"The net effect will be reduced airspeed and increased descent rate"

No necessarily. It depends on the variation in the lift to drag ratio. If it doesn't increase the drag by the same amount as it has destroyed the lift then the potential energy that is being converted into kinetic energy by the aircraft's descent will result in a slight increase in speed, but it's generally only marginal.

You need dead calm air to test this and on the gliders that I have had a chance to do this on and have normal spoilers that deploy along upper wing along the cord (I usually flew a Glasflügel 206 Hornet) I've noticed a slight increase in airspeed meaning that the loss of lift and subsequent loss of altitude cancels out the increased drag and you end up with a slight gain in airspeed.

If you have ever watched a bomb dropped from an aircraft you will notice that once the bomb stabilizes in the air it starts to overtake the aircraft ultimately impacting the ground in front of the aircraft. That's why the pilots that dropped the bombs on Hiroshima and Nagasaki performed steep high speed turns to get away from the effect of the bomb going off. If they had continued straight ahead the would have ended up flying directly into the rising mushroom cloud which wouldn't be good for the crews future prospects as aviators.

First, a polite (hopefully) interjection on drag and spoilers as drag seems to be cropping up.

Can I start just by setting out the difference between spoilers and air brakes. Air brakes work predominantly by creating drag, spoilers predominantly by reducing the efficiency of a wing. There's not necessarily that much drag from a spoiler.

Spoilers, as the name suggest, "spoil" the airflow over a wing, the top surface since that's where you get the greatest effect, air brakes act as extra drag. By spoiling the airflow over a wing the spoiler reduces the efficiency of the wing and changes the rate of descent of the aircraft while maintaining the same flight angle and airspeed. An air brake enables a more acute flight angle for a given airspeed. The net result relative to the ground can be the same for each.

At the risk of starting a very old argument among glider (sailplane) pilots some feel spoilers are safer as when you return the spoilers into the wing back comes the lift and there's less messing about with the nose angle involved. They will tell you that air brakes are potentially dangerous, particularly for trainee pilots as when you return them you're in a fast accelerating aircraft pointing at the ground.

Advocates of air brakes over spoilers will tell you that spoilers come with an increased threat of stall and that inevitably close to the ground where a stall risk is most dangerous. Get into a potential stall situation with air brakes and you're already part way to a recovery position.

(Glider pilots aim long into the landing area, for obvious reasons and waste the extra height on final.)

Anyway, spoilers spoil, they don't drag very much.

So the question. Interesting. I don't know the answer for sure but what I think is happening is that there is no effect on the underside of the wing where the spoiler is positioned on the top surface. I think that the slower moving air on the underside of the wing will maintain the pressure that it had prior to the deployment of the spoiler while the disturbance of the airflow on the upper surface reduces the low pressure effect on the top of the wing. The relative pressure difference will drop as the pressure on the upper surface rises and there is a consequent reduction or cessation of net lift from that part of the wing 'spoiled' but there is no actual increase in pressure on the underside.

Must look back later to find out if I'm right!

I, too am keenly interested in the answer to this, and other questions posed here.

So far, I'm not sure there are definitive answers to any of them.

The 'answer' was recently posted for the cabin noise reduction challenge question... as you speculated, it isn't really a definitive answer.

We'll have to see about this one and future ESDU CQ's.

I don't feel any smarter now that I know the answer.

Well, I'd like to read more about spillover, but I get...

"The Data Item document you have requested is available only to subscribers or purchasers."

Looks like AH has the first correct answer on this one.

"Looks like AH has the first correct answer "

No surprise there.

I think your right. Only the relative pressure under the airfoil changes not the absolute pressure.

The net result is a loss of lift and the aircraft's descent rate increases.

And yes, there is some increase in drag, but not as much as you tend to get from dive brakes due to typical dive brakes extend from both top and bottom of the wing, where as a spoiler is located on the top portion of the wing.

At least that was how I was taught it from my instructors.

"...what typically happens to the pressure distribution on the lower surface?"

Nothing.