# Why are delta wings not commonly used in commercial airliners?

Delta wings are most commonly seen on military aircraft. I understand that delta wings are ideal for supersonic flight, and provide high maneuverability.

Having high maneuverability and speed are good characteristics for commercial airliners. But, most of the commercial airliners do not use delta wings (Except for Concorde and Tu-144).

• Are there any specific reasons for this?
• Are there any regulations that prevent using delta wings in commercial airliners?
• Does it cost more in maintenance?
• delta wings aren't really optimal for subsonic performance, and most commercial jets don't need amazing manoeuvrability – shortstheory May 20 '14 at 3:47
• Having High Maneuverability is not a good characteristic for a commercial airliner, as maneuverability and stability are competing interests. Airliners want stability, and do not need maneuverability as they generally don't bank more than a few degrees, and make large, gentle, well-planned turns. – abelenky May 21 '14 at 19:12
• Similarly, speed is not an important characteristic for a commercial airliner. The main focus is on efficiency. Airliners today actually fly slower than they did a decade or more ago. But they are more fuel efficient, and therefore cost less to operate and more profitable. – abelenky May 21 '14 at 19:14
• @abelenky: speed is good (to an extent). Aircraft have an optimal cruise speed, and cruising below that is inefficient. – shortstheory May 22 '14 at 9:23
• Delta wings are commonly seen on airliners. They look like this. :) – reirab Mar 27 '15 at 15:08

One of the most prominent and useful aspects of the delta wing is the fact that it increases an airplane's critical Mach number (the speed at which some air passing over any part of an aircraft - usually the wing - reaches Mach 1.0).

By increasing the critical Mach number, it makes supersonic flight much more feasible on delta wings than on conventional wings.

Delta wings are particularly happy even at a high AoA (angle of attack). This is because of the reaction lift generated by the wing, and the vortex created at the leading edge of the wing. Hence, airplanes with delta wings have a much higher tolerance to stalling. This makes it easier to attain higher vertical speed, also known as "rate of climb" safely.

But on the flip side:

One of the downsides of deltas is that they create more drag, because of the bigger surface area, but they do have a bigger internal volume (for fuel, for example) which could lead to a smaller fuselage, or more space in the fuselage for other things. Another disadvantage is higher landing and take off speeds.

The last point is particularly important. The Concorde's already small number of usable airports was reduced because the runways needed to be long enough to accommodate for the 160 KIAS landing speeds (for comparison, typical turbofan-based airliners lands at 130-140 KIAS).

As a result of the higher takeoff/landing speeds, the cost of maintenance of vital components such as the brakes and tyres is higher. Moreover, the high landing speeds can even lead to greater stress on the airframe. So, indirectly delta wings do contribute to a higher cost of maintenance.

Other disadvantages of delta wings I can think of is the lack of a flaps/slats system. Delta wings don't require flaps as they can simply reduce their airspeed by increasing the AoA, but the lack of flaps reduces their subsonic efficiency, and increases the takeoff distance and takeoff velocity. For instance, the Concorde's delta wings produced very little lift during takeoff prior to rotation, necessitating high takeoff speeds of nearly 220 knots!

As an answer to your second question, I could not find anything about regulations relating to the use of delta wings on airliners.

All in all, delta wings work well for niche purposes such as supersonic aircraft, but their disadvantages and poor operating economics make them impractical for subsonic flight.

However, as you can see here, the now-scrapped Boeing Sonic Cruiser attempted to make the best of both worlds by combining a conventional wing with a delta wing, to form a wide, "cranked-delta" wing design.

• Several good points, but your statement that a high angle of attack allows a higher climb speed is wrong. A higher climb speed is achieved with more engine power. – Peter Kämpf May 20 '14 at 12:16
• Hmm, thanks for the comment. I meant to say that delta wings are less prone to stalling than conventional wings, and hence high AoA's are possible; I'll make the required edit to make it more clear. – shortstheory May 20 '14 at 13:33
• Well, not quite. Yes, they will stall at a higher AoA, but stall they will. And if they have a straight leading edge (as opposed to a cranked one with less sweep outboard), they will produce a strong pitch-up moment around and beyond stall, making recovery very hard. – Peter Kämpf May 20 '14 at 19:47
• @shortstheory What's stopping designers from giving delta wings flaps and slats? IIRC modern delta-wing fighters (I think especially the fly-by-wire ones) like the Mirage 2000 and newer models have slats and flaperons. My knowledge is very limited, but remember reading somewhere that a lifting canard helps reduce the landing and takeoff speeds of delta-wing designs. – markvgti Jun 14 '14 at 9:51
• @markvgti, I'm sorry I didn't clarify a bit more. Indeed, many delta-wing based airplanes have flaps and slats, but the original Concorde did not (IIRC the never-produced Concorde B model did have a flaps/slats system). Most airplanes with delta wings don't need flaps/slats, so I suppose engineers don't install it because of weight issues. – shortstheory Jun 15 '14 at 3:32

Generally, having as little sweep as possible is the best way to achieve high lift at low drag. Also, regardless of flight speed, a subsonic leading edge gives the best lift/drag ratio. If the sweep of the leading edge is higher than the angle of the Mach cone in supersonic flow, the flow around the leading edge is still like in subsonic flow, and this creates suction, thereby reducing pressure drag. If the cruise speed is Mach 2, the sweep must be bigger than 60 degrees, and with that sweep the lift at slow speed would suffer. Lift goes down with the cosine of the sweep angle at the same angle of attack.
Also, at high angle of attack the high sweep angle produces a vortex on the upper surface which reduces pressure (= increases lift) and prevents flow separation (= allows for higher angle of attack). Having more chord inboard helps to utilize this lift better and gives you more space for the wing spar and fuel, even with the low relative thickness that supersonic flow requires.

A subsonic airliner wing needs only moderate sweep to reduce Mach effects even with relatively thick airfoils (14% t/c versus 4% to 6% for supersonic airfoils), which gives you a lot of internal volume for fuel, the wing spar and high-lift devices. Going to a delta wing would help with gust sensitivity in low-level flight (that is why you see delta wings on subsonic aircraft at all), but it would increase the surface area and, consequently, friction drag.

FAR Part 25 would be the relevant regulation, and it (thankfully) leaves it to the applicant how conformance is achieved. The reason why you see no delta wing on subsonic airliners is purely technical.

If you take all together, the moderately swept-back cantilever wing with high-lift devices is the best way to fly Mach 0.8, and the delta wing is the best for flight at Mach 2.

• swept-back cantilever wing with high-lift devices is the best way to fly Mach 0.8 uhm, you mean that the devices are there, not that are used to fly at 0.8M, right? – Federico May 20 '14 at 12:32
• @Federico: Yes, you only need high-lift devices at low speed, but need to carry them around for the full flight. And storing them away is non-trivial. Having a relatively thick wing helps immensely with that. The delta wing has the high-lift device built-in (high leading edge sweep + high inboard chord). A swing-wing combines both, but adds a lot of weight, reducing payload. – Peter Kämpf May 20 '14 at 12:55
• "Going to a delta wing would help with gust sensitivity in low-level flight" But what about microbursts/downdrafts? Surely those broad, great surface area wings would "absorb" more downwind and thus be pushed downwards? This would be bad anywhere, let alone in low-level flight. – DrZ214 Jul 26 '15 at 3:46
• @DrZ214: Wings don't "absorb" wind. The delta wing is less affected by the angle of attack change caused by a vertical wind speed change than a straight wing. But the real danger in microbursts at low level is the change in horizontal speed, and here both wing types will react in the same way. – Peter Kämpf Jul 26 '15 at 4:59

The primary design driver of a commercial airliner wing is efficiency, and second to that is speed. While delta wings are good at high speeds, they are not very efficient. Also, while an airliner should certainly be maneuverable, they do not need to "do a barrel roll" or make 9G turns light a fighter jet.

It was mentioned that delta wings have a large surface area. This helps to distribute the weight of the aircraft more, which leads to a low wing loading. This is one factor in the higher maneuverability of the delta wing. Compare the wing loading of the F-15 at 73.1 lb/ft2 with the wing loading of a 737-800 at 98.7 lb/ft2. The delta wing allows the F-15 to have lower wing loading and thus much more maneuverability. The design also keeps this large wing area to a low wingspan, which means that high-G maneuvers cause lower stress on the wing. Airliners have plenty of maneuverability with their current configuration.

This large area and high speed performance is at the expense of efficiency. The lift to drag ratio (L/D) of an F-15 is around 10, while commercial jets get over 16 (see this table for some examples). This shows the huge advantage that a non-delta wing has at subsonic speeds. Modern aerodynamics have allowed commercial airliners to reach high subsonic speeds (Mach 0.8-0.9) while remaining very efficient. While fighter jets use aerial refueling to compensate for burning more fuel, this would be a bit unrealistic for a commercial aircraft.

Low speed performance is also a factor. Both wings are able to use flaps and slats, but landing speeds for fighter jets are generally higher than commercial aircraft. Being better suited for high-speed flight, delta wings are less effective at low speeds.

As far as maintenance is concerned, there are fewer differences. Since the wingspan would be lower, this would put less load on the wings. Reducing the load could be turned into a maintenance advantage, but there are many more factors in determining maintenance cost.

• Tex Johnson disagrees that airliners don't need to do barrel rolls. – cpast Mar 1 '15 at 16:16
• @cpast Well his point is that you can do it, and it is impressive. But you can sell airplanes without it, and the people in charge don't like it (for some reason). Also, that picture is one of my desktop images. : ) – fooot Mar 2 '15 at 0:13