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Why Slats And Flower Flaps?  
User currently offlineNikrc From United States of America, joined Oct 2004, 4 posts, RR: 0
Posted (9 years 5 months 6 days 13 hours ago) and read 6792 times:

Why do big airliners use complex flap systems like slats and flower flaps, wouldn't it save a lot of weight and therefore fuel if it didn't have all the extra hydraulics equipment on board? Older airliners like the DC-8 and new bizjets like the gulfstream 550 have no slats and a very simple flap structure, so why not the 747 or A340? Does small decrease in speed the slats produce really pay off?

-Nik

12 replies: All unread, jump to last
 
User currently offlineCedarjet From United Kingdom, joined May 1999, 8093 posts, RR: 54
Reply 1, posted (9 years 5 months 6 days 13 hours ago) and read 6782 times:

They're called "Fowler Flaps". Briefly, flaps and slats are all "high lift devices", basically they are extended during flight regimes which involve flying slowly, ie take off and landing. They lower the aircraft's stall speed by expanding the surface area of the wing. They retract for high speed cruise, where extra lift isn't needed but a smaller wing will create less drag.

Don't have time to describe the different types of high lift device ie (trailing edge) Fowler flaps, blown flaps, slotted flaps / (leading edge) slats, droops etc. Anyone else feel like having a go?



fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz
User currently offlinePilotpip From United States of America, joined Sep 2003, 3150 posts, RR: 11
Reply 2, posted (9 years 5 months 6 days 11 hours ago) and read 6701 times:

Gulfstreams have never had leading edge devices. Unless you count the new G100 and 200, but those weren't Gulfstream designs.

Most transport category aircraft have some sort of high-lift devices incorporated into the wing. The purpose of these devices is to change the camber of the wing to allow more lift at a lower speed. This aids in low speed maneuverability and allows the wing to be 'cleaned up' into a much more efficient shape for cruise. To design a wing that has good slow speed charateristics would mean less sweep, and a wider chord. It also allows for a steeper angle of decent at a lower airspeed. Conversely, a wider chord will create more drag. This would ultimately make the wing less efficient at cruise. With the complex high-lift devices you get the best of both worlds.

I don't remember the exact speed, but the UA 232 crash demonstrated how much those devices are needed. The touchdown speed on that aircraft was something like 290 knots and it was nearly at stall. None of the leading edge devices or flaps were deployed as a result of the hydraulic failure after the fan severed all three lines. A normal approach speed for that aircaft would probably be around 130 knots.

Aviation is a game of trade offs. STOL aircraft used in wilderness flying have wings with large chords. As a result they aren't very fast but have great short field performance. At the other end of the spectrum, the global flier that Steve Fossett recently flew around the world in was so efficient that it needed the assistance of drogue chutes to descend back to Salina. In order to gain something, you're going to loose something.



DMI
User currently offlinePositiveClimb From Germany, joined Jun 2004, 214 posts, RR: 0
Reply 3, posted (9 years 5 months 6 days 11 hours ago) and read 6682 times:

Hi Nikrc!
I'll try to shed some light on different flap systems and what they are good for.

As Cedarjet already mentioned, slats and flaps are "high lift devices". The lift provided by a wing follows Bernoulli's law:
A = rho/2 * v² * ca* S
in this formula A is the lift provided, rho is the density of the air, v is the speed of the airflow around the wing, ca is the lift coefficient (dependent on the actual profile and the angle of attack) and S is the wing area.

Having a look at the formula it becomes clear that the lift provided by the wing is the bigger the faster the aircraft travels and the bigger the wing surface F is.

Given the situation on takeoff, the plane should actually climb (at least, that's what it is supposed to do, right?), therefore the force of the lift must be bigger than the gravity force.

So, coming back to the topic: To increase lift in such a situation (where the speed is rather low (as it is on landing), you could

a) increase the lift coefficient
or
b) increase wing area

That's exactly what flaps are supposed to do.

There are different flap systems:
plain flaps: almost looking like rudder surfaces and often found on smaller aircraft. They do not enlarge the wing surface, but increase the curvature of the wing, and therefore the lift coefficient ca.

split flaps: they only affect the lower side of the wing, whereas the upper side remains untouched. They are a little more efficient than plain flaps, as a small part of the airflow from under the wing is directed through a gap to the upper side of the wing surface. so it reduces the danger of a stall. On the other hand, this system produces more drag than the plain flaps

fowler flaps (as a special form of slotted flaps): Fowler flaps do both, they increase the curvature of the wing and therefore the lift coefficient ca, but they also increase the wing area, they are slotted for the same reason as there is a gap in the splitted flaps:to let some air flow from the lower to the upper side of the wing and therefore to reduce the danger of a stall of the airflow on the upper side.

Problem is, as you mentioned, that these forms of flaps are very complicated. The reason they are used is simple. Big aircrafts need very much lift for takeoff and landing (regarding that the takeoff/landing speed of a B747 is not THAT different of the speeds of a much smaller B737). Therefore, often bigger aircraft have much more complex flap/slat systems.

Hope that Helps

Regards,
Fabian/PositiveClimb

[Edited 2005-03-27 20:49:15]


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User currently offlineAeroWeanie From United States of America, joined Dec 2004, 1608 posts, RR: 52
Reply 4, posted (9 years 5 months 6 days 10 hours ago) and read 6640 times:
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PositiveClimb:

Thats not Bernoulli's law! Thats the equation for lift coefficient!


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 5, posted (9 years 5 months 4 days 21 hours ago) and read 6421 times:

The S in the lift equation is really just a reference area. We don't change it when calculating the lift with flaps deployed, there's just a delta to the lift coefficient. We also use the same reference area when calculating the lift from stabilizer etc, just with a suitable lift coefficient. As was stated, many kinds of flaps don't increase the wing area at all. I think it would be more accurate to say that flaps change the camber (curvature) of the wing, thus changing the lift coefficient CL (ca in the lift equation as given above).

Split flaps don't let air pass through from the lower surface to the upper surface. They're a flap where an area on the trailing edge of the wing can split from the rest of the wing and fold down. They're common on WWII era aircraft. The deflection can be quite significant, but the larger deflections provide very little lift incrase if any. The main purpose of the larger deflections is for the flaps to serve as an airbrake on the approach - something split flaps are suited for as they are very draggy.

Slotted flaps are the ones that let air pass through from the lower surface of the wing to reenergize the boundary layer over the flap and thus enable higher flap deflections without the airflow over the flap stalling.

To answer the original posters question yet again, in another way: Aircraft flaps are optimized for the aircraft to be as economical as possible. Yes, you have to carry the weight of the flap actuators and mechanism around but in return, you can lift off with more payload (more revenue) from the runways available and still have an efficient wing in cruise (less cost and longer range).

The main benefit of slats is a better pitch attitude with flaps deployed.

Regards,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 6, posted (9 years 5 months 4 days 14 hours ago) and read 6373 times:

Not valid to compare a Gulfstream with an airliner. It is like comparing a Ferrari with a bus. If the gulfstream wings and engines were used on an airliner it would have to carry 90-120 passengers, Gulfstream carries maybe 10.

What would V1 be for a no-flap takeoff? How much concrete would be required for the plane to:

  • Accelerate to that speed.

  • Lose and engine and accelerate for two more seconds on one engine.

  • Stop in the remaining runway, using only wheel brakes.


  • We don't have runways like that.

    How about landing speeds? We all do no-flap / no-slat landings in the simulator on checkrides. This is a maneuver right up there with the rejected takeoff for drama. It is a very serious procedure. I recall landing speeds being up around 180 knots for even the most docile airplanes.

    So if you keep in mind that the brake energy required to stop the plane increases with the square of the speed, we'd be going through brakes, tires, and possibly perimeter fences with great frequency.

    Flap and slat mechanisms starting to look better?

    Then there is the safety factor. As it is, if someone got it wrong and the plane cannot be stopped in the remaining runway - say it gets into reverted-rubber hydroplaning; would you rather, as a passenger, go off the end of the runway at ten knots or a hundred?

    I like high lift devices.



    Happiness is not seeing another trite Ste. Maarten photo all week long.
    User currently offlineLeanOfPeak From United States of America, joined Oct 2004, 509 posts, RR: 1
    Reply 7, posted (9 years 5 months 4 days 11 hours ago) and read 6320 times:

    I see a lot of nice, well-supported arguments here. But, cutting it down to basics:

    The airplane needs to take off and land in a certain amount of runway. If the wing were sized to allow this without high-lift devices, cruise speed would be very low.

    In order to compromise between high cruise speed and acceptable takeoff and landing performance, high lift devices, which improve the performance of the aircraft in those two disparate regimes when properly deployed or retracted, will have to do until someone devises another option.


    User currently offlineQantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 25
    Reply 8, posted (9 years 5 months 4 days 7 hours ago) and read 6290 times:

    Quoting LeanOfPeak (Reply 7):
    The airplane needs to take off and land in a certain amount of runway. If the wing were sized to allow this without high-lift devices, cruise speed would be very low.

    It's not so much an issue of size as it is of wing camber and airfoil design, as FredT mentioned above. Aircraft layouts optimized for low-speed flight usually comprise a relatively large amount of camber and an airfoil design geared towards high-Clmax. This is almost the polar opposite of layouts optimized for high-speed flight - high-speed flight regimes demand a relatively low-camber wing and a rather flat, low-Clmax supercritical airfoil. Because of this divide, Clmax-increasing and camber-increasing devices (such as flaps and slats) are used to adapt normally high-subsonic aircraft to slower flight regimes.

    Cheers,
    QantasA332


    User currently offlineSonicKidatBWI From , joined Dec 1969, posts, RR:
    Reply 9, posted (9 years 4 months 4 weeks 1 day ago) and read 6160 times:

    Quoting Pilotpip (Reply 2):
    I don't remember the exact speed, but the UA 232 crash demonstrated how much those devices are needed. The touchdown speed on that aircraft was something like 290 knots and it was nearly at stall. None of the leading edge devices or flaps were deployed as a result of the hydraulic failure after the fan severed all three lines.

    The speed at the point of impact was 250mph. I never heard anyone mention a stall when describing the intricate details of this crash. The plane was actually flying too fast during the landing attempt and that was actually the precipitating factor in the events that lead up to the crash. The pilots actually tried to decrease the speed during landing by pulling back the throttles on the remaining two engines as a last ditch effort to decrease speed but it was too little, too late.


    User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2546 posts, RR: 24
    Reply 10, posted (9 years 4 months 3 weeks 6 days 5 hours ago) and read 6070 times:

    Generally speaking, a leading edge slat or flap allows a wing to be operated at a higher angle of attack without stalling, thus providing more lift. A trailing edge flap gives you more lift for a given angle of attack, without affecting stall angle of attack. If you can get enough lift increase from a trailing edge device, there's no need to fit a leading edge device, which will add weight and cost. The BAe 146, Fokker F.28, Fokker 100 and Fokker 70 are examples of current jet airliners which have no leading edge slats or flaps.

    Trailing edge flaps alone might give you an excessively nose down pitch angle on approach, so slats might be used to balance this effect, as FredT hinted in reply 5.

    Manufacturers only fit multi-slotted flaps if they are really necessary. For example the 747SP had a much simpler trailing edge flap system than the 747-100, even though it was a derivative design, because the complexity was not justified.



    The glass isn't half empty, or half full, it's twice as big as it needs to be.
    User currently offlineAirWillie6475 From United States of America, joined Jan 2005, 2448 posts, RR: 1
    Reply 11, posted (9 years 4 months 3 weeks 6 days ago) and read 6057 times:

    Nikrc if what you are saying is used for big airplanes such as a 747 then there will be some crazy final approaches. The 747 would be having 170 knot approaches and landing like a CRJ200. Flaps are used to increase the rate of descent without an increase in speed.

    User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2546 posts, RR: 24
    Reply 12, posted (9 years 4 months 3 weeks 5 days 19 hours ago) and read 6049 times:

    Quoting AirWillie6475 (Reply 11):
    Flaps are used to increase the rate of descent without an increase in speed.

    That sounds more like the function of the speedbrake  Smile In light aircraft, with wings designed for low speed flight, adding drag is the main reason for deploying flaps. But this discussion is about airliner high lift devices, especially slats.

    High lift devices, such as flaps and slats allow an aircraft to approach at slow, safe airspeeds by adding significant extra lift to wings which are designed for efficient high speed flight. The extra drag provided by landing flap settings is important in controlling airspeed on approach, but not the main reason for deploying them.



    The glass isn't half empty, or half full, it's twice as big as it needs to be.
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