A question about speedbrakes?

[TotalBeginner]

Can anyone tell me why some speedbrakes, such as those used on the Cessna Citation have holes in them?

http://www.eliteaircenter.com/graphics/64BKPhotos/BaronSpeedbrakes.jpg

Are they purely for creating interference drag, or do they have some other purpose? Wouldn't a flat panel have more aerodynamic effect?

[parabellum]

Ready to be corrected but that doesn't look like a speed brake to me and the wing doesn't look like a Citation wing to me, isn't that the rear end of a piston or tubo prop engine on the left of picture?

[TotalBeginner]

It's fitted to a Baron, it was the only picture I could find...

If you look closely here, you can see the lower speedbrake just foward of the flap. It's very similar to the one above.

http://cdn-www.!!!!!!!!!!!!!!/aviation-photos/photos/1/6/3/0584361.jpg

So why the holes?

[parabellum]

Look again TB, there is no way the speed brake shown in the Citation picture, below and forward of the flap, could eventually stow above the flap as in your original picture! Mechanically impossible I would say!
(Not to mention the four screws that seem to attach the well holed piece of metal to the wing in your original picture, with no hinge or extend mechanism visible).

[TotalBeginner]

I have absolutely no idea what you're talking about! I posted the original picture because it shows the kind of speedbrake that my question referes to. Look at the citation picture again (closely). You can just see the top of the upper speedbrake above the wing. No, it doesn't stow above the wing because there are two. One on the surface and one on the underside! :rolleyes:

Back to the original question...

[parabellum]

Sorry TB, I have obviously not fully understood your original question, I thought you were suggesting that the holed piece of metal in Pic 1 was an actual speed brake.:O

[TotalBeginner]

:ugh::ugh::ugh:

The holed piece of metal in pic 1 IS a speedbrake. It's a modification fitted to a Beech Baron!!! I posted it because it looks very similar to the speedbrakes used on the Cessna Citation, for which I could not find a picture! Maybe this question is more suited to "Biz Jets, Ag FLying, GA etc." Mods?

[DBate]

The holed piece of metal in pic 1 IS a speedbrake.Not sure about the picture of the Citation, but concerning your first picture I doubt that it is really a speedbrake.

As parabellum mentioned, there are four screws attaching that piece to the wing, with no hinge or other mechanism to extend that piece visible. Looks more like some kind of step to me.

Anyway, waiting to be corrected...

[Avtrician]

The upper picture seems to be some sort of vortex generator that comes in to play when the flaps are lowered, probably to compensate for the engine.

I must agree with the other posters in that it doesnt apear to be a speed brake, which is a device that is presented to the ariflow, in a manner which causes drag thus slowing the aircraft.


As to Speed brakes them selves, a speed brake the is flatish (Like a board) can cause an aircracft to pitch up or down, depending on its position. By placing a regular system of holes in the surface, this pitching effect is reduced, but also has the effect of creating greater turbulance behind the brake causing more drag therefore greater slowing performance.

[TotalBeginner]

Why on earth would they bolt a piece of metal to a wing that looks just like a speedbrake? I know, because it's a speedbrake!

http://www.aso.com/aircraft/109444/ext-4.jpg

I think avtrician has answered my question. The holes reduce the speedbrake's spoiler effect, while maintaining efficiency as a speedbrake?

[Captain Dilbert]

I have no idea what the answer is but at least I do understand the question; why are some speed brakes perforated? Come on, someone must have a theory, or shall we all just keep debating which picture shows what? :ok:

[parabellum]

OK TB, I'll save you further head banging! Now that you show the picture of the extended speed brake it is very obvious that the four screws attach to the operating mechanism and yes, it really is a speed brake!!!
Sorry about that, I'll stick to aircraft I know in future!;)

[CJ Driver]

I suspect that the holes in the speedbrakes are a way of reducing weight, in which the overall plate effect (due to the size of the holes and the turbulence introduced) is as good as a solid panel, but with only half the weight.

[TotalBeginner]

I was thinking that maybe the holes reduce the force required to extend the panel into the airflow, and hence the size and weight of the actuator required to operate them?

[EatMyShorts!]

Hi TB,

why don't you send your question to Cessna? I am sure they will answer that question.

[DBate]

Why on earth would they bolt a piece of metal to a wing that looks just like a speedbrake? I know, because it's a speedbrake!


I learn something new everyday. Sorry about that!

Regards,
DBate

[Request FL510]

TotalBeginner,

Found this on Wikipedia, and it explains the difference between spoilers and speed brakes very nicely. Personally I still do not understand the use of speed brakes on on pistons (except the slick Mooneys maybe)...

Here goes (curousy of Wikipedia)....

In aeronautics (http://en.wikipedia.org/wiki/Aeronautics) a spoiler (sometimes called a lift dumper) is a device intended to reduce lift (http://en.wikipedia.org/wiki/Lift_(force)) in an aircraft. Spoilers are plates on the top surface of a wing which can be extended upward into the airflow (http://en.wikipedia.org/w/index.php?title=Airflow&action=edit&redlink=1) and spoil it. By doing so, the spoiler creates a carefully controlled stall (http://en.wikipedia.org/wiki/Stall_(flight)) over the portion of the wing behind it, greatly reducing the lift of that wing section. Spoilers differ from airbrakes (http://en.wikipedia.org/wiki/Air_brake_(aircraft)) in that airbrakes are designed to increase drag while making little change to lift, while spoilers greatly reduce lift while making only a moderate increase in drag.

Spoilers are used by gliders (http://en.wikipedia.org/wiki/Glider) to control their rate of descent and thus achieve a controlled landing at a desired spot. An increased rate of descent could also be achieved by lowering the nose of an aircraft, but this would result in an excessive landing speed. However spoilers enable the approach to be made at a safe speed for landing.

Airliners (http://en.wikipedia.org/wiki/Airliner) too are usually fitted with spoilers. Spoilers are sometimes used when descending from cruise altitudes to assist the aircraft in descending to lower altitudes without picking up speed. Their use is often limited, however, as turbulent airflow which develops behind them causes noticeable noise and vibration, which may cause discomfort to extra-sensitive passengers.

The spoilers may also be differentially operated to provide roll control. On landing (http://en.wikipedia.org/wiki/Landing), however, the spoilers are nearly always used at full effect to assist in slowing the aircraft. The increase in form drag (http://en.wikipedia.org/wiki/Form_drag) created by the spoilers directly assists the braking effect. However, the real gain comes as the spoilers cause a dramatic loss of lift and hence the weight of the aircraft is transferred from the wings to the undercarriage, allowing the wheels to be mechanically braked with much less chance of skidding.

[Fakawi]

During the dacades there have been quite a few Airplanes using drag devices with holes in them.
My best guess would be that it has to do with flutter.
You can try this at home. Make a small parachute from cloth or similar material.
Then drop it. You will see it starts to oscilate as it descends.
Now cut a hole in the center and repeat. The swinging should have stopped. If not hole propably too small.
I would suspect a similar phenomenon here with the "boards"

[TotalBeginner]

Personally I still do not understand the use of speed brakes on on pistons (except the slick Mooneys maybe)...

I think the idea on piston aircraft is to allow you to descend without shock-cooling the engines.

[JRBarrett]

As another user pointed out, the main use of a speed brake on a Baron, or similar piston-powered aircraft, is to permit a rapid descent while still carrying a reasonable amount of power - to prevent over-cooling the engine(s).

The Mooney is a bit of a different beast, in that the aircraft is so "slick" that it can approach or exceed VNE very easily in a descent if the pilot isn't paying close attention. The speed brake helps to prevent that.

In any case, I belive that the main reason for the holes is to reduce the aerodynamic forces on the speed brake panel when it is extended - which allows for the use of a small, lightweight electrically-driven actuator and less need for heavy-duty supporting structure for the panel and actuator.

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.

Jim Barrett

[SNS3Guppy]

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.


The original poster was headed in the right direction. A speed brake has one purpose; to create drag. It may be used in several situations (landing vs. cruise, for example) and have different names for each purpose (ground spoilers, vs. speed brakes vs. aileron augmentation, etc)...but a speed brake on a light airplane generally has just one purpose; create drag.

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.

gallery image 39 (http://www.aircraftresourcecenter.com/AWA2/1001-1100/walk1029-T-38-Kubes/39.shtm)

The same can be said of the BAE-146 speed brake, as seen here:

Photos: BAE Systems Avro 146-RJ85 Aircraft Pictures | !!!!!!!!!!!!!! (http://www.!!!!!!!!!!!!!!/open.file/0167829/M/)

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 | !!!!!!!!!!!!!! (http://www.!!!!!!!!!!!!!!/open.file/0530903/M/)

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 | !!!!!!!!!!!!!! (http://www.!!!!!!!!!!!!!!/open.file/0687502/M/)

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.

[Captain Dilbert]

Great stuff 3G:D

[TotalBeginner]

Great stuff 3G

I agree! Very informative, thanks :ok:

[Bruce Roy]

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.