A question about speedbrakes?
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Location: USA
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On large turbojet aircraft, where the spoiler panels serve the dual purpose of dumping lift AND augmenting the ailerons in controlling roll, the panels have to be solid - and that requires a very powerful, hydraulically-driven actuator, as the aerodynamic forces on a solid panel can be quite high.
Spoilers kill lift. Speed brakes create drag.
Drag isn't just a matter of flat plate area; the acreage of the speed brake that's exposed to the relative wind. Interference drag is an important part of the equation. The speed brake on the T-38, for example, isn't perforated, and is reasonably beefy by comparison to those under discussion (as it's exposed to much higher flight loads), but it's not just the area that's important. It's the disruption of the airflow around it and behind it. It creates low pressure behind it, opposed to the higher pressure ahead of it, and the difference in pressure by definition is drag...it's not so much the high pressure of exposing the flat plate to the slipstream that's important there...it's what the speed brake does downstream, and the differential between the two that's created which is important.
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The same can be said of the BAE-146 speed brake, as seen here:
Photos: BAE Systems Avro 146-RJ85 Aircraft Pictures | Airliners.net
Yes, it's the increased flat plate area helping slow it down, but it's much more the interference drag created by putting low pressure behind the airplane that accounts for the drag. You can see this for yourself by setting a tennis ball on the table and pressing on each side of it with your fingers. Take your fingers away from one side of the ball...take away pressure, and the ball moves in that direction. What you've simulated is drag; the fingers doing the pushing is part of the picture...that's what we envison when we stick our hand out the car window on the highway and it's blown back...drag...dynamic pressure pushing the ball or our hand. But the truth is that what drag really is, and how it works with the speed brake, is by the lack of pressure pushing the other way. That's what a speed brake is and does, and how it creates drag. Ever ride a motorcycle behind a big truck on the freeway? Feel like you're getting sucked into the draft behind the trailer? Lower pressure there, as opposed to what's in front of the truck does what wind does to a speed brake, what your fingers do to the tennis ball; that's drag. It's as much what's not there, as what is, in determining the aerodynamic force on the airplane.
Photos: North American NA-265 Sabreliner 40 Aircraft Pictures | Airliners.net
I used to fly a sabreliner 60. It had an unusual speed brake aft of the nosegear. It was a true speedbrake, with no other purpose (no aileron augmentation, didn't kill lift on landing, etc). It was very effective. It had a slot down the middle, and it's purpose was to create interference drag. It didn't affect the handling of the airplane; it was designed to keep the airplane flyable but create a lot of drag (and noise, as it turned out); it wasn't just the size of the speed brake, but the shape and the reaction it created downstream. One might surmise that not having that slot in the speedbrake would have left more area exposed to the relative wind...but that's really not necessarily the goal of a speed brake.
So far as perforated speed brakes, perhaps the most well known dive brakes were on the Douglas Dauntless, as seen here:
Photos: Douglas SBD Dauntless Aircraft Pictures | Airliners.net
Another shot of a long forgotten system, very different form the Dauntless, but still using a perforated interference system was on the Bristol 188. The A-6 originally had perforated speedbrakes, but these were deactivated and the airplane later used split ailerons to accomplish the same purpose. The F9F Cougar also used perforated speed brakes. The 1937 DeHavilland Moth Minor incorporated a perforated speed brake too.
Perforation reduces buffeting downrange of the speedbrake, and reduces it's interference with flying surfaces or the fuselage or wing or horizontal stab. Remember, the speed brake is there to create drag, but not undesirable flight characteristics. A perforated brake doesn't create nearly the airflow disruption, pitch change, or load on the surrounding and supporting structure that a solid brake might create. The number and placement of holes are important considerations, and part of the design. Holes permit a lighter structure that takes less of an airload, reducing not only the weight of the brake assembly but the force required to actuate it and the structure around it that must support the load. Remember that much of the time, that speed brake isn't anything but dead weight.
Another way to think about it is to think about the propeller. When a propeller is no longer being driven by the engine, but is instead being driven by the slipstream ("windmilling"), the drag produced by that propeller exceeds that of a big plywood disc the same diameter as the propeller. In other words, you've got more drag going on, absorbed on multiple levels in the engine, and in the airframe, than if you made a big disc out of wood or metal the same diameter as the propeller arc and hung it on the front of the airplane. The drag of that little thin propeller...many don't realize it. That's a big part of why higher performance and multi engine airplanes feather the propeller...a windmilling prop is a LOT of drag.
With that in mind, you can see that a speed brake which is properly placed and properly designed doesn't need to be solid. Sometimes it is, sometimes it isn't, depending on the design and intent of the speedbrake. Obviously where a speed brake is used for other purposes such as spoiling lift on the ground and augmenting ailerons (or taking their place, as in the Mitsubishi MU-2), there are more considerations than simply creating drag. For a speed brake, many different designs have been used, including perforated panels and various shapes. Whatever their design, they're often a useful method for controlling descent rate and angle, as well as slowing quickly when there's a need.
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Great discussion, and my 50c worth. I´m not too sure about the holes being there to relieve the aerodynamic force on the actuator. If that were the case then a smaller panel with no holes would be easier and cheaper to manufacture. It seems that most panels that you see on fuselages, under wings or towards the rear of the wings often look like swiss cheese. So I reckon its something to do with the difference between a spoiler (intended to kill lift) and an air brake (purely to increase drag). A fine example of lift spoilers is with the Dash 8-100. On touchdown with a sequence of main and nose-wheel compression, the roll spoilers and ground spoilers all pop up and the wing instantly takes on the flight characteristics of a steel girder. It won´t fly again until the spoilers are stowed. Also pulling a nice big hand-full of reverse will also kill a lot of lift as the props act like a pair of 14´ spoilers. The spoilers do not and cannot deploy in flight at all, except for the outers assisting roll (clever dem DH people eh). However in flight if you want drag, close the power levers and push the props up to max and all your standing passengers and crew will soon be joining you in the cockpit! Not to mention the noise...
Back a while when flying a pair of TIO-540´s (Piper Chieftain) I found myself planning descents around the finickity and expensive engines and would certainly have found airbrake panels very handy.
Something also worth thinking about is the terminology. Boeing and Gulfstream call their wing mounted panels ¨speedbrakes¨ and BAe call their device on the tail, an ¨air brake¨. The Boeing and GLF panels revert to spoiler mode on touch down where the 146 has a separate set of panels on the top of the wing (not to used in flight) that are deployed on landing. Once again the steel girder theory. I think that BAe (for once) are more correct in their terminology.
Back a while when flying a pair of TIO-540´s (Piper Chieftain) I found myself planning descents around the finickity and expensive engines and would certainly have found airbrake panels very handy.
Something also worth thinking about is the terminology. Boeing and Gulfstream call their wing mounted panels ¨speedbrakes¨ and BAe call their device on the tail, an ¨air brake¨. The Boeing and GLF panels revert to spoiler mode on touch down where the 146 has a separate set of panels on the top of the wing (not to used in flight) that are deployed on landing. Once again the steel girder theory. I think that BAe (for once) are more correct in their terminology.
