Why do commercial aircraft need "speed brakes" and how do they differ from spoilers?

Why does an airliner or other similarly sizeable aircraft need “speed brakes” to slow it down when existing air resistance at, say, 450 mph should be more than enough to do the job? Why not just throttle back the engines rather than deploy speed brakes? Intuitively, I can vaguely grasp some control/stabililty issues at work, but that’s about as far as that goes.

Also, how do speed brakes differ from spoilers?

And a last question: What is the purpose of that of tiny (matchbox- sized) metallic fins that stick above the surface of airline wings in a long row?

I’m sure one of the pilots like Broomstick or Pilot141 will be along shortly, but the short answer is because air resistance isn’t enough to do the job. Aircraft are very aerodynamic.

First of all, it’s not just big aircraft that need these - the two-person glider I rode in last year also had speed brakes and spoilers, and that was a vehicle smaller and lighter than your average sedan.

Quartz had it right - airplanes are very aerodynamic. They want to fly. Landing isn’t just about slowing down your speed, it’s about getting closer to the ground in a controlled manner, and sometimes that means you want to descend faster, not slower.

But I feel compelled to point out that, on landing, the big jets are NOT going 450 mph - more like 150 - 200 mph, if I recall correctly (if I don’t, I’m sure one of our big iron guys will correct me). So there isn’t as much air resistance as you originally thought.

It’s counter-intuitive to the non-flyer, who frequently worries about falling out of the sky, but sometimes it can be surprisingly hard to get down.

There’s a certain minimum speed below which you can’t drop or the airplane stops flying. However, going forward at any speed generates lift. The aircraft that need spoilers are those where an adequate speed to keep you in the air generates sufficient lift to prevent you from descending at a proper rate of speed to land at most available runways. If a Boeing or Aibus didn’t have all that stuff popping out of the wings they’d still land - but they’d need a MUCH longer approach and runway.

Spoilers spoil the lift. You keep going forward at a speed that generates sufficient lift for control and continued flying, but you delibrately mess up the aerodynamics of the wing so with less lift you descend more steeply and thus don’t take forever to get down and can use a typical sized runway. Speed brakes - well, they slow you down by adding drag, to keep you from exceeding certain critical airspeeds. (I’ve never used them myself, hence, I’m a bit fuzzy on when, how, and why)

Spoilers spoil the lift generation of the wing. Speed brakes add drag to slow you down.

Truthfully, spoilers also add some drag, and I think speed brakes affect lift, but those are side effects of the primary function.

I’m not quite sure what you’re referring to - that might be vortex generators to improve slow-speed handling, or it might be part of the speed brake system, or it might be something else.

When one of the big jet flyers happens by he might be able to help you out with that, plus correct any errors I may have made since the big jets aren’t my speciality.

That’s it, speed brakes increase drag, spoilers kill lift, and both have their uses.

The little fins are vortex generators, which help keep the boundary layer attached to the upper surface, and delay stall until higher angles of attack, which reduces the plane’s stall (minimum) airspeed. Some planes, notably the BAC 1-11, have “stall fences” running most of the wing chord, which inhibit transverse flow during incipient stall and have a similar effect.

We might have learned of the need for speed brakes and spoilers on aircraft by experience. The little, light glider, for example, always has to land without power and sometimes in a rather tight place. When the pilot gets low to the ground over the place he needs to land he has to land right now. So the spoilers kill the lift and get the craft on the ground.

As to large planes, a co-worker of mine was once on a project for the FAA. In the course of this project he got a pass to ride in the cockpit of airliners and this was back in the day of the early 707’s. He said that on his first ride the trip was to St. Louis and upon arrival there they were held at 20000 ft. for a short time and then told to descend, enter the traffic pattern and assigned a place in the landing sequence. They needed to descend rapidly in order to follow the traffic instructions and the 707 didn’t descend all that well. During the descent the pilot kept muttering about “this damned glider won’t get down.” Without spoilers and speed brakes he had a hard time getting to the lower assigned altitude without building up so much speed that it would have taken excessive time to lose it in order to be at the correct pattern altitude and speed to maintain a place in the landing sequence. Such things as spoilers and speed brakes make the job a lot easier.

And, of course, the land available for runway length is limited and expensive. Some airplanes need all the speed brakes, wheel brakes, thrust reversers and all such devices in order to ensure that they can stop safely under all conditions and at all the suitable airports.

Airline pilot here.

What they said.

Strictly speaking, speed brakes simply increase drag, while spoilers also spoil lift. Fighters often have speedbrakes, but never spoilers. Most airliners have spoilers, although a few (eg Fokker F28/F50 and Avro/BAE-146) have tail mounted speedbrakes instead of / in addition to wing-mounted spoilers. Some Googling for photos will pay dividends if you’re interested.

In common pilot talk the terms are often used interchangeably although they really shouldn’t be.

Are some flaps mounted on the leading edge of the wing, while some are more traditionally located aft? I flew an Airbus 320 a couple days ago and IIRC the flaps (or some kind of wing surface feature got cranked into position prior to take off.

Can anyone point to a website that depicts flaps, spoilers, air brakes, thrust reversers, ailerons, etc.?

Yeah. I knew that. Thanks.

Not sure when speed brakes are deployed, but I thought it possible at much higher speed.

Gosh, by the time I get here the OP has been answered – even down to the necessity of a glider to land on target the first time!

Some aircraft have deployable ‘slats’ on the leading edges of their wings. I don’t know if ‘slat’ is the right word. On jets, these are deployed by the pilot. On some WWII aircraft, they deployed automatically when airspeed dropped below a certain point (held in by air pressure?). The idea is to increase the curvature of the airfoil to provide more lift so that the aircraft can fly at a slower speed. Trailing edge flaps do the same thing, but usually turn into ‘speed brakes’ (isn’t that redundant?) beyond a certain angle. For example, the Cessna 172 I used to fly slowed down well with 10° or 20° of flaps, but still flew quite nicely. At 40° of ‘barn door’ hanging down, it was like tossing out an anchor. (My instructor blew it once, and took off with 40° flaps deployed. We got airborne, but it took a lot of power!) Anyway, the leading edge slats/flaps are for generating more lift at a lower airspeed. Some aircraft have slots on their wings or horizontal stabilizer – basically the same thing, except they are fixed and there is a space between the ‘slat’ and the wing proper. I think this helps with the boundary layer in a similar way as the vortex generatos do.

Of course, I’m a helicopter pilot; so take that for what it’s worth. (How can you fixed-wingers trust a wing you can’t see working? :eek: :wink: :smiley: )

One time I was checking out in the Cessna 182 with the guy I rented airplanes from. I overshot the landing approach and went around and afterwards he showed me a method for making sure you never screwed up a dead-engine forced landing in a Cessna.

Make sure you are overshooting so that you are high enough to make the field no matter what. Lower full flaps and dive at any speed up to maximum flaps down speed so as to come in over the downwind end of the field. The plane comes down like an elevator. When you get close to the ground break the glide and level out. The high drag of the full flaps quickly kills off the excess speed and the plane lands. Works like a charm.