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It's well known that the Concorde had no flaps or slats. The lack of high-lift devices considerably reduced lift at low speeds, requiring the Concorde to take off and land at higher speeds and angles of attack than if it had been equipped with flaps and slats. As well as greatly increasing drag (and thus fuel consumption) during takeoff and landing, this greatly increased the force carried by the main landing gear and the speed at which its tyres had to spin; as a result, if and when a tyre blew out (due to, for instance, a piece of debris left by the previous airplane to use the runway), the results would be much worse than with a tyre blowout on a subsonic airliner.

Interestingly, a "Concorde B" was planned for Concorde 217 and onwards, which would have been equipped with slats (although still no flaps); this, along with slightly larger wings and more powerful engines, would have allowed the Concorde B to dispense with afterburners, considerably increasing its fuel efficiency and range. Unfortunately, production ended with Concorde 216, the aircraft immediately preceding the first Concorde B, so no Concorde Bs were ever built.

Why didn't the Concorde, as built, have flaps or slats?

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    $\begingroup$ If you have not come across it yet this website has a lot of great Concorde info. $\endgroup$ – Dave May 6 '18 at 20:24
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    $\begingroup$ because it doesn't have a tail elevator. when you deploy flaps the plane pitches down and there's no way to counter act that. $\endgroup$ – user3528438 May 6 '18 at 20:35
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    $\begingroup$ Concorde B was proposed to have variable leading edge droop, but no slats. A delta wing with slats was used on the F-4 Phantom II, but very few other delta wings. $\endgroup$ – Peter Kämpf May 6 '18 at 22:23
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Why no flaps?

Flaps change the pitching moment of a wing. After all, they add lift over the full chord, so the sum of the increased lift attacks at about mid-chord, which is a quarter chord aft of the regular lift. If there is no separate tail surface to compensate for the pitching moment caused by that extra lift, the aircraft will quickly pitch nose-down and crash.

Next, flaps change camber and, therefore, are destabilizing the wing. Without a stabilizing tail, a cambered delta configuration will become unstable. The only camber which is helpful for delta wings is at the leading edge and must be compensated by a little trailing-edge up deflection of the control surfaces. Positive camber near the trailing edge is destabilizing and can only be tolerated on a flying wing with artificial stability augmentation.

Why no slats?

Slats are helpful for delaying flow separation to higher angles of attack and allow a wing to create more lift. To understand their effect, it is not enough to consider what they do to the flow around the wing, but also the effect of the wing on the slat needs to be understood. A slat is like a small wing flying just ahead, and therefore in the upwash, of the main wing. The wing will induce a very high lift on the slat and in turn see its suction peak around the leading edge greatly reduced, which helps in keeping the flow downstream attached. The comparison plot below should illustrate this effect nicely:

Figure 36 from A. M. O. Smith's paper "High Lift Aerodynamics"

Figure 36 from A. M. O. Smith's paper "High Lift Aerodynamics"

But a delta wing at high angle of attack does not have attached flow on its upper side. It makes use of flow separation at the leading edge which creates a powerful vortex over the upper wing. This is called vortex lift. So for take-off and landing, deploying leading edge devices would help Concorde only a little - they are most helpful in the region just before vortex lift kicks in. This would be for subsonic cruise, for which the original Concorde wing was completely unsuitable. Adding camber at the leading edge would had increased subsonic L/D a lot, so the subsonic cruise segments (like all flight above land) and flight in holding patterns would had been much more efficient. That was not considered initially, and the lower complexity of an un-slatted wing was preferred.

With Concorde B, a span increase and the addition of variable leading edge droop (no slats!) were proposed. The picture below is taken from the original web site which has been the source for the site you linked to in your question.

Concorde B aerodynamic improvements

Concorde B aerodynamic improvements (picture source)

If you now look how the L/D would had been improved by this, it becomes obvious were the leading edge devices helped most. The L/D improvement at take-off and landing, by the way, can be mostly attributed to the span increase.

Concorde A and B aerodynamic efficiency comparison

Concorde A and B aerodynamic efficiency comparison (picture source)

This information is missing from the site you linked to, but is essential for understanding why Concorde B had variable droop added: It should help to extend range beyond the Paris-New York connection and allow more efficient subsonic flight (especially the hold at 250 kts).

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    $\begingroup$ Related - similar issues with the delta configuration Saab Viggen. That aircraft did have flaps, largely owing to the extreme STOL requirements, and they were mounted on the forward canards. $\endgroup$ – J... May 7 '18 at 11:27
  • $\begingroup$ The Concorde engineers had a trick up their sleeves, by making the plane tail heavy during takeoff and landing by moving the fuel aft, this forced the elevons into a downward-ish position, which increases the wing camber (not sure how to fit it in the answer, but it's worth adding IMO). Source: concordesst.com/fuelsys.html $\endgroup$ – ymb1 May 19 '18 at 15:18
  • $\begingroup$ @ymb1: Yes, they flew with relaxed static stability. Only for a brief time, so the increased pilot workload was considered acceptable. Since the A310 all Airbus planes do the same, but for the whole flight and with stability augmentation, so the workload is kept low. $\endgroup$ – Peter Kämpf May 19 '18 at 16:26
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    $\begingroup$ @Sean: No, not at all. This is common knowledge since the Twenties - no need for Concorde to try out relaxed stability first. But Concorde had the technology first to make relaxed stability possible. $\endgroup$ – Peter Kämpf Dec 31 '18 at 12:32
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    $\begingroup$ @Sean: Yes, I agree as far as the certification bureaucracy is concerned. The engineers always knew what works, but to get it certified is a different story. The prestige and magnitude of the Concorde development helped a lot the first time, and having this precedent helped Airbus the second time. $\endgroup$ – Peter Kämpf Jan 1 at 0:21
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Because with a delta wing the trailing edge has the elevators and it's too far aft to have a flap. They depend on wing area and the ability to operate at much higher AOAs than straight wings to get the speeds down.

As for slats, they don't increase Clmax all that much, only a little bit from the increase in chord in drooping the leading edge. The main function of slats is to function as a convergent nozzle to inject a sheet of higher velocity air along the top of the wing, to increase the stalling AOA to allow the wing to develop higher lift by operating at higher AOAs. A normal wing with a stalling AOA of around 15 degrees will have a stalling AOA of around 25 deg with slats extended.

Delta's already operate at very high AOAs when slow because of the giant vortex generated by the steeply swept leading edge which delays the stall. Adding a slat doesn't provide enough additional benefit. I can't think of any delta wing a/c with slatted LEs although maybe someone knows of some. Leading edge flaps yes, but not slats.

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    $\begingroup$ No, slats don't "inject high velocity air" anywhere. Best to look at them as a separate wing flying in close formation with the proper wing. Source: This paper by A. M. O. Smith, McDonnell-Douglas. See section 5.3 for a few sobering words on that issue. $\endgroup$ – Peter Kämpf May 6 '18 at 20:56
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    $\begingroup$ Thanks for the reference. Very interesting stuff. I found on Page 518 where he quotes NASA material that says exactly what I said above and explains why he disputes it. I wonder if this is an aerodynamic debate that is really settled or are there still aerodynamicists arguing over it today? $\endgroup$ – John K May 6 '18 at 22:54
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    $\begingroup$ The NASA stuff is wrong. Bernoulli should be proof enough. $\endgroup$ – Peter Kämpf May 6 '18 at 23:14

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