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Terminology / Nomenclature

Terminology / Nomenclature

A simple question for the pro's of this forum.
Are the term flaps used exclusively for devices on the trailing edge of an airfoil,or can flaps also be found on the leading edge.All my prior experience and education had led me to use the term flaps when being applied on the trailing edge,with the name slats being used to those devices attached to the leading edge.Recently,on another blog I got in a discussion,as to this issue,he produced several references that made me believe I had been misusing these terms for many years,or has the terminology changed and I was a sleep ?

RE: Terminology / Nomenclature

I think the proper term is slats, but "L/E flaps" is common

another day in paradise, or is paradise one day closer ?

RE: Terminology / Nomenclature

There is a class of flaps called Kreuger flaps which are leading edge devices.

Keep em' Flying
//Fight Corrosion!

RE: Terminology / Nomenclature

Yes,by formal education is dated late 60's and early 70's.We called trailing edge devices flaps.called leading edge devices slats.The Kreuger flaps you speak was from what I recalled designed by the Germans and I had always seen them applied in the rear,or trailing edge,high lift devices.However there are several publications out on the internet from what appears to be very reputable sources talking about leading edge flaps; ie Stanford.Was just wondering what was the accepted practice in this day and time,the blogger that quizzed me on the subject appeared to have a most shallow knowledge of the matter as if it was all acquired from Google,in the last half hour or so.thus my question.

RE: Terminology / Nomenclature

doesn't the B747 have kreuger "flaps" on the L/E ?

another day in paradise, or is paradise one day closer ?

RE: Terminology / Nomenclature

Certain OEMs also define flaps on the lead edge. For example, B737-300 SRM section 57-43-02 is titled "wing leading edge slats and flaps structure", where the flaps are inboard of the nacelles and are indeed the Kreuger type.

In my mind, the classification is more about operation. Flaps are devices that increase the camber and therefore the CL by rotation of a portion of the profile. Slats accomplish the increase camber by translating a portion of the profile. Although I guess this is not always true as a "gurney flap" is completely stationary.

Keep em' Flying
//Fight Corrosion!

RE: Terminology / Nomenclature

Well,with all do respect,I can truthfully say I don't recall ant lift enhancing device on the LE being called a flap.they were all known as slats,and in all honesty few devices along the LE helped lift,the rage then was boundary layer control.Most if not all lift extenders were done along the TE.this is what I recall from a BS degree at a small,but prominent,at one time AE college.The terminology seems to have changed,as I did not practice in the industry,but was forced to take work outside of aeronautics,thus I lost track of what was popular in the vernacular.I see after discussing the subject that the basic vocabulary has changed somewhat,apparently like other engineering fields it has failed to agree upon a common vocabulary.I read a great deal of older aeronautical information,both "NACA" vintage,along with what I consider to be a fairly recent crop of AE text books from well known universities,none to my knowledge have been using this classification,so thanks much for your input.

RE: Terminology / Nomenclature

There are also ailerons and spoilers to consider.

RE: Terminology / Nomenclature

Slats form "channels" with respect to the top surface of a wing to speed up airflow to delay boundary layer separation in high angle of attack situations. Flaps change the camber of a wing and increase the wing area for low speed cases such as landing.


RE: Terminology / Nomenclature

Andries,your thoughts and definitions most closely follow what I had been taught and observed in my career.
This is what I was taught in college,and had witnessed in industry,it's also what other practicing engineers believed.
I have to go with those definitions,and they seem much more in line with past and current theory.Whoever popularized these terms I refer to I can't determine.It's almost as if it's one person saying something,then seeing it in print,on the internet,no less,and that terminology being taken as the Gospel.Sort of create your own terminology.Glad I still have a pretty good pile of books around,and still burn my money by continuing my collection.We and society are going to be in one Hell of a shape if we never do have a massive electromagnetic wave war and all this electronic data and knowledge gets destroyed.

RE: Terminology / Nomenclature

I thought slats (as well as flaps) both work to increase camber of the wing. Folwer flaps have the additional effect of increasing wing area (as they translate).

another day in paradise, or is paradise one day closer ?

RE: Terminology / Nomenclature

Yes, in my opinion both slats and flaps can increase wing camber depending on how exactly they translate / rotate. And both can form channels for the airflow as well, like the design of slotted flaps.

I think flaps were named as such potentially because of their deployment motion - that is, they "flap". So it doesn't really matter if they are installed on the leading or trailing edge.

I understand this is a discussion of common nomenclature. But a quick look at the NASA NTRS pulls up some matches of reports of NACA tests talking about "leading edge flaps" as early as 1947, and maybe earlier. Also, Sighard Hoerner's monumental textbook "Fluid Dynamic Lift", Chapter 6 Section 2, is devoted to leading edge or nose flaps.

It seems there is history for this term.

Keep em' Flying
//Fight Corrosion!

RE: Terminology / Nomenclature


The F-16 and FA-18 have leading edge FLAPS. They are hinged off of the leading edge spar and work in concert with the trailing edge ailerons and flaps. The LE flaps alter camber and are capable of independent movement relative to each other, depending on flap and aileron control commands. They not only alter camber for lift enhancement, but add roll-rate when differentially activated... which is necessary with very heavy wing-store loads [high roll inertia]. In addition these devices have a secondary but very important function: when roll is commanded using ailerons or flaperons together there is a very serious torque on the wing-box; the LE Flaps generate opposite torque-loads [relative to the trailing edge control surfaces] providing a counter-torque to the wing box. The LE Flap counter-torque is essential for minimizing wing aeroelastic twist that could add very high/runaway/destructive torque-loads to the wing-box. Essentially the 'active' LE flaps, independently controlled by the computers, allow the jets to see MUCH higher wing [lift/roll/dynamic] loads than static structure would permit.

Wing leading edge SLATS are like traditional flaps in that they function 'in concert' [same movement, LH RH wings] and are exclusively for lift enhancement. All slats that I'm aware of, actually push-away from the wing-box... which has a second stub leading edge sculpted under the slat... which does 3 things: (a) increases wing-area as the slat moves forwards; (b) forms an aerodynamic gap between the SLAT and the wing structure that forces entrapped airflow [thru the gap] over the wing upper surface to the trailing edge; and (c) in concert with the wing trailing edge FLAPS [which usually increase wing-area aft-ward while increasing camber and torque] tend to balance wing-box torque loads. This would be typical for most new transport acft.

NOTE. The SLATS added wing leading edges of older fighters [F-4, F-100, etc]... where flaps/ailerons are hinged off the rear spar... and were intended [added] for airflow control to enhance fighter lift/roll maneuverability for low-to-high subsonic dog-fight speeds [enhance lift and delay stall when in high G pitching/rolling maneuvers].

NOTE. SOME older Boeing transports [707, 727] have both 'plain leading edge flaps' and SLATS. The plain LE Flaps hinge-down from the under the leading edge to increase camber [primarily in the 'thick' LE area of the inner 1/3 of the wing LE span]... and have the classic extendable/flow-gap SLATS on the outer ~2/3's of the wing LE span. for enhanced lift and roll-controllability.

Some STOL aircraft have SLATS for all the purposes mentioned... but especially for enhancing lift and roll control... so they can safely fly at ridiculously low airspeeds.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", forum]

RE: Terminology / Nomenclature

Thanks All,
Learned more here than Ae 344,along with practical aspects,with regards to structures.This discussion also allowed me to revisit Chapter 6 "Leading Edge High-Lift Devices",by Hoerner & Borst,it's all here,Sorry for such a common question.Must have been asleep during those lectures,in school.

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