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Flaps - How They Work

Thu Mar 02, 2000 9:37 pm

I need a little help with how flaps work.

I know they can help slow you down while desending. I am also fairly certain that becasue of the design of wings on the planes flaps are needed to keep the plane in control at lower speeds, like when landing.

My question is, how do the flaps slow the plane down or help it stay in control?

RE: Flaps - How They Work

Thu Mar 02, 2000 9:45 pm

Basically, designers face a certain problem with airplanes. At high altitude, during cruise flight, you want to fly as fast as possible while when landing you want to fly as slow as possible. Therefore, at high altitudes you need less wing area than at low altitude flight and the net result is that at low altitudes, the wing area is extended by the flaps and slats etc.

They do not slow the aircraft down, or at least that is not the main intention. That is done by the air brakes. Flaps generate lift and therefore they ALLOW the pilot to fly slower.

PS: An interesting fact about flaps: did you know that when landing some big planes (e.g. 747) have so much wing area that airflow at (above) the slats (the equivalent of flaps on the frontal part of the wing) exceeds MACH 1? Amazing, isn't it.
(All this information is based on our aircraft performance lectures)
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RE: Flaps - How They Work (a Little Fun)

Thu Mar 02, 2000 11:38 pm

Flaps are one of the most ingenious parts of the plane, and to most people, how they work is an utter mystery. The problem lies in that from the cabin, you can't really see what is really going on with the flaps when extended.

Most people think that it is a way to extend the wing so that it provides more lift (and drag because of the extra area) both granting more control during landings, and more lift during takeoff. That's baloney. Here's what's really going on.

The flaps are actually just support structures for a few cages. The cages hang from the flaps, and house a family of ducks, and we all know that ducks are the absolute best fliers in the world. On takeoff, these ducks use their highly efficient wings to give the plane a bit of extra lift at slow speed. You'd be surprised just how well mother nature does compared to the best of the best from Seattle and Toulouse. Nothing beats a duck for getting a plane in the air. Unfortunately, it's cold up in the air, and the ducks would freeze to death if not sheltered. So, shortly after takeoff, the flaps are retracted pulling the ducks in to a heated holding area where they are fed and kept away from the bitter cold.
Upon descent, the flaps are engaged again, and the ducks come out to aid the pilot in controlling the plane. You see, airplane wings don't do so well at low speeds as they do at higher speeds. No worries, because the duck is also the most graceful flier in the world. The ducks guide the plane to the runway at very high precision and flare at just the right time to give you a smooth landing. Again, to protect the ducks from the elements like poor air quality on the ground, the flaps may be retracted again as the plane taxis to the gate bringing the ducks out of harms way.

So, you can attribute the marvelous flying behavior of today's modern jetliners to the remarkable flying skills of the duck.  

(Don't believe a word of this. It is just for fun.  )
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RE: Flaps - How They Work

Fri Mar 03, 2000 1:10 am

Ahem! Anyway,most flaps out there are the "Fowler flap" design. The first quarter to third of travel during deployment is aft and only slightly down (more or less paralell to the upper camber of the wing). This generates lift more than drag because it effectively increases the wing chord area and hence makes more lift. As the flaps travel further,they start curving more sharply downward to full travel. This provides the largest wing chord area but also the most drag,which is beneficial because you wnt your landing speed to be as slow as possible,while at takeoff you're increasing speed and care more about lift than drag. Methods employed are: 1) Hydraulically driven gearbox with power transferred to a series of jackscrews to move the flaps along curved tracks by the roller bearing equipped flaps. 2) Hydraulic (linear) actuators moving flaps hinged in a manner that the movement effect is similar to method 1. 3) Hydraulically diven rotary actuators (via gearboxes) that move a set of linkage that position the flaps. As I said earlier,the net affect is nearly the same: the flaps move out and then down. There's much more to it than that as flaps are often double or triple slotted to,shall I say,"tune" airflow over them the most efficiently at various settings.

Flaps In Concert With Other Control Surfaces

Fri Mar 03, 2000 1:45 am

Here's a question:

One time a few years ago I flew CVG-DCA on a DL 757. We were approaching DCA from the west, having flown over IAD, and we had turned south just before reaching the Potomac. Then we turned back to the north (over Mt. Vernon) to land on RWY 01.

I was sitting well behind the wing and couldn't help but being astonished at the fact that, on the final turn over Mt. Vernon, we were descending and banking sharply to the left with full trailing-edge flaps and fully extended spoilers. I thought, "How is this possible? Wouldn't the spoilers dump all of the lift - especially at landing speeds?"

As I exited the aircraft I asked the captain about it and he replied that there is so much lift generated by the 757 wings he wished he had spoilers nearly twice the normal size! He said the 757 just wants to keep on flying; it's rather resistant to being put down on the ground.

Some time later, I flew on an AA 727 and sat next to a pilot from FedEx. We were talking about control surfaces and he told me that on the 727, spoiler extension is not possible when flaps are extended. He explained that there is an automatic lockout of the flaps until spoilers are retracted, saying that the 727 would basically drop out of the sky if one were to deploy spoilers with flaps extended.

Is this true? If so, it amazes me that the 727 and 757 should be so different. I realize that the 757 has a much newer wing design but wouldn't the basic aerodynamic principles, which govern whether an airplane flies or drops out of the sky, apply across both designs?

I will appreciate your comments...thanks.

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Flaps And Spoilers

Fri Mar 03, 2000 2:19 am

The 757(don't know about 727) can have full flaps and full speedbrakes extended at the same time. There is a cockpit warning below a certain altitude that warns the pilot about speedbrakes still deployed. The result of which if not corrected will more than likely lead to a tail strike on landing due to the high power and pitch settings required to maintain profile. (AA YYZ circ.'95) The Airbus 320 has a different design whereby speedbrakes will automatically retract when flaps are selected full.
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RE: Flaps In Concert With Other Control Surfaces

Fri Mar 03, 2000 2:36 am

Why would it amaze you? All planes are not the same in every area,and different planes were built to optimize different criteria durig there heydays. Another thing of note;727 outboard ailerons are also locked out when the flaps are not up. Since I'm most familiar with Boeing narrowbody products I can tell you that,while "spoilers" may be "spoilers" in the academic sense,their actual purpose in life varies depending on the phase of flight. Certain spoilers on the wing are used for roll control to assist the aileron on the wing that deflects up beyond a certain travel. Certain spoilers on most A/C are used as speedbrakes during flight. Use of spoilers under those aforementioned conditions is typically at less travel than during the actual "spoiler" use after touchdown--whereby all spoilers on all wings will deploy maximum deflection (typically 45 degrees). The authority for how spoilers are used is largely an automatic function. Speedbrakes can be selected by a lever on the cockpit quadrant,while in roll controll mode is linked automatically to aileron input via a "spoiler mixer"--an extraordinarily complicated beast. Ground spoilers can be selected manually using full travel of the aforementioned level or automatically when the anti-skid wheel speed transducers sense wheel rotation on touchdown. Fokkers have what they call 'Liftdumpers' (funny huh?) and are either up or down. Only used after touchdown. I did not metion in my previous post that leading edge slats and or flaps (Krueger flaps) are automatically coordinated with the trailing edge flaps to extend when and how far depending on what flap setting is selected.
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RE: Flaps - How They Work

Fri Mar 03, 2000 3:10 am

A very good question and some equally as very good answers, very informative, for which I thank all of you posters. This is the kind of subject that makes this Forum so good.There are times when I despair when I see some of the absolute rubbish taking up space here (such as all the postings in "Oh- The humanity",) most of which have absolutely no bearing on the subject of Aviation. I know there is such a thing as free speech, but I wish those other subjects would be aired on the appropriate forums where they belong. I'm hoping to see more postings on this subject of flaps before it's taken off. Keep them coming. Thank you all. Peter.

To "nkp S2"

Fri Mar 03, 2000 7:12 am

Thanks for your post. Actually I was aware of the various uses of "spoilers" and the varying degrees of delfection for roll control vs. speed braking or lift dumping but I appreciate your explanation.

I have one question for you, not to split hairs, but simply to be clear. You said, "Another thing of note;727 outboard ailerons are also locked out when the flaps are not up."

I had understood that the outboad aileron on a 727 wing is locked out with the flaps up (retracted). Did you mean to say the same thing? My understanding is that travel is restored, progressively, to the 727 outboard aileron as the trailing edge (Fowler) flaps are progressively extended until full travel is restored to the aileron at some point. Please let me know if I am off base here.


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RE: Flaps - How They Work

Fri Mar 03, 2000 7:41 am

Since we are on the subject of flight controls- Does anyone know the reason for the sequence of the leading edge slats on the 727? #2 and #3 come down before #1 and #4, etc. Does it have something to do with the outboard aileron, or is it related to hydraulic demand?
This plane is built to withstand anything... except a bad pilot.

RE: Flaps - How They Work

Fri Mar 03, 2000 11:48 am

First of all...DLX - very cute!   I will undoubtably think about this next time I fly. And to the rest of you, thank you for helping me to understand how the flaps work. I always wonder about the various workings of airliners when I fly.
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RE: Flaps - How They Work

Fri Mar 03, 2000 12:54 pm

With flaps 2 on the 727 only slats 2,3,6,7 deploy.

The outbord ailerons become active anytime the flaps are set at flaps 2 or beyond. It is not progressive with flap deployment. They are either engaged or disengaged. When flaps 5 are selected the remaining leading edge devices including all leading edge flaps deploy.

The roll control spoilers becom active anytime the flaps are in the down position and the control wheel is deflected more than 10 degrees.
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RE: To "nkp S2"

Fri Mar 03, 2000 1:10 pm

Shoulda' said flaps up,as in no outbd ailerons with flaps up,or as someone said,flaps 2 units or greater for them to be active. Despite proofreading,I never seem to notice my errors till they're posted.
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RE: Flaps - How They Work

Fri Mar 03, 2000 2:47 pm

I have noticed the term "travel" used on a couple of posts. In the context that the term has been used I can sort of figure out what it means (in this sense), but would like a definition from those who have used it.

Thanks guys - interesting topic,

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RE: Flaps - How They Work

Fri Mar 03, 2000 2:59 pm

Travel = movement.