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Why did the Avro Vulcan use a delta wing?

The B52, from the same era, used a swept wing, and had a higher maximum speed.

In the answer to What are the advantages and disadvantages of a delta wing compared to a swept wing?, Peter Kämpf explains that a swept wing is ideal for subsonic airliners.

No one else has built a subsonic delta-wing aircraft since, military or civilian. Why did the designers of the Vulcan make this unusual choice for a subsonic aircraft?

enter image description here

By Ministry of Defence [OGL 3], via Wikimedia Commons

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    $\begingroup$ Because it's one of the coolest shapes to ever fly. $\endgroup$ – Simon Nov 19 '16 at 15:18
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    $\begingroup$ "Because it's one of the coolest shapes to ever fly" - and AFAIK nobody ever did a barrel roll in a B-52. The second Vulcan off the production line did one at its first Farnborough airshow in 1955. youtube.com/watch?v=GPuTgcrA2Zs $\endgroup$ – alephzero Nov 19 '16 at 22:47
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    $\begingroup$ @alephzero In 1955, Tex Johnston did a barrel roll in a Boeing 707. It didn't fit the image his bosses wanted to project, however. $\endgroup$ – David K Dec 4 '16 at 17:55
  • $\begingroup$ @DavidK: No, but it did net Boeing a lot of orders for the 707. Which was Johnston's intention - when his superiors asked him what he thought he was doing, he famously replied "Selling airplanes". And it worked. $\endgroup$ – Sean Jun 1 '18 at 15:58
  • $\begingroup$ For a smaller radar cross-section, obviously. $\endgroup$ – 0xdd Apr 12 at 14:33
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I would not say that no other bomber ever had delta wings. Convair B-58 Hustler and Mirage IV both have delta wings. They are both supersonic though.

The B-52 did not have to worry too much about MTOW and empty weight since SAC already had bases with long runways capable of really heavy aircraft. That's thanks to B-52's predecessor B-36. The B-36 was the heaviest land based aircraft of its time and ground pressure was a very real concern when basing that beast. Britain did not have such bases. The Vulcans predecessor, the Avro Lincoln, could take of from grass fields.

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  • $\begingroup$ Thank-you! Google Books links are too lengthy to put here but The Avro Vulcan: A History puts take-off speed at 130 kt, and A Century of Air Warfare With Nine (IX) Squadron, RAF puts runway requirement at 5,000 ft. For the B-47, I found landing speeds of 180 kt and runway length (without chute) of 11,000 feet. As I understand it, delta wings allow for higher angles of attack thus accounting for the lower speeds. And you've explained the difference in requirements between the RAF and the USAF. Makes sense to me! $\endgroup$ – Crosbie Apr 8 '17 at 2:11
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It was obvious to the design team that conventional aircraft could not satisfy the required specification; knowing little about high-speed flight and unable to glean much from the Royal Aircraft Establishment or the US, the team at Avro investigated German Second World War swept wing research.

The team estimated that an otherwise conventional aircraft, with a swept wing of 45°, would have doubled the weight requirement. Realizing that swept wings increase longitudinal stability, the team deleted the tail (empennage) and the supporting fuselage, it thus became a swept-back flying wing with only a rudimentary forward fuselage and a fin (vertical stabilizer) at each wingtip.

The estimated weight was now only 50% over the requirement; a delta shape resulted from reducing the wingspan and maintaining the wing area by filling in the space between the wingtips, which enabled the specification to be met.

Wikipedia

In short, the team didn't know much about high-speed flight (blame Wikipedia). And to reduce the weight, they eliminated the tail-plane, shortened the span, and filled in the space between the wingtips. Achieving their weight target.

For the same area, a wide wing is lighter to attach, as it requires smaller spread spars.

So, for weight reduction, a delta wing wins against a conventional swept-back wing aircraft. In all, great engineering solution to a big problem, with one downside—

—higher fuel burn at those speeds, limiting the range.

The B-52's were known for loitering in the air for extended durations. Their efficient design requires fewer rendezvous' with the tanker. Unlike the famous Vulcan train.

Some numbers (in kg and km):

        empty weight     MTOW      payload+fuel      range
B-52      83,250        220,000      136,750         7,210
Vulcan    37,144         77,111       39,967         4,171

enter image description here

Maintaining the same wing area; saving weight by reducing the heavy structural elements—the tail-plane, big spars, and wing-span. (Eyeballed the scale, only illustrative.)

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  • $\begingroup$ Thanks for the answer! I've up-voted, but I haven't accepted. If the Delta wing of the Vulcan is a great engineering solution, why did no other bomber design adopt it? Weight is a major design goal for all aircraft, not just the Vulcan. $\endgroup$ – Crosbie Nov 19 '16 at 16:12
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    $\begingroup$ Is it simply that no-one else ever built a non-stealth sub-sonic bomber again? $\endgroup$ – Crosbie Nov 19 '16 at 16:14
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    $\begingroup$ @Crosbie To adopt a particular solution, you first need a certain problem. As you see, B-52 weights 2 times more. Solution was not replicated because nobody else asked for a "bomber twice as light as current technology allows it to be". $\endgroup$ – Agent_L Nov 19 '16 at 17:50
  • $\begingroup$ @Crosbie, probably because the B52 demonstrated that for the same payload the generals could have nearly double the range if they didn't worry so much about empty weight. $\endgroup$ – FreeMan Nov 22 '16 at 13:10
  • $\begingroup$ The numbers listed here for the B-52 are questionable. The Air Force fact B-52 fact-sheet lists the MTOW as 219,600 kg. That's for the B-52H model, but I doubt the earlier models were significantly less. That's an empty weight of 38% of the maximum take off weight. Using the numbers given here for the Vulcan gives us 48%. So the swept wing B-52 gives us a better fuel + payload fraction. $\endgroup$ – Crosbie Nov 22 '16 at 20:16
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Vulcan was built to a specification which gave it a top speed capable of outrunning the fighters of that time. The specs from wiki make it too slow to do that. The Mk 10 used the same engines adopted much later for the Concord. Rumour had it that the last ships off the line were capable of 900mph. In the fifties six Vulcans did an exersize. They flew over the pole and attacked six major US cities. They were not intercepted. After the initial report in the newspapers a D notice made sure that the incident was hushed up. It did appear as a front page feature in the AVRO news, A monthly company newsletter. Much later the Valkerie was developed in the US. This compensated the nose dive tendency by adding a canard wing on the nose.Later again the B1 was a similar shape. The latest idea is now the blended wing design which like the Vulcan is capable of carrying 50% more than an aircraft with a fuselage. Just in case you missed the point. The initial research was done in the AVRO factory in Chadderton. Tests in a huge tank found the wings suffered fatigue and fell off. The solution was found when it was suggested that a line of perforations along the breakage line would solve the problem. The toilet paper never ripped along the perforations. Solution Honeycomb sections. Saved lots of weight and was far stronger.

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    $\begingroup$ That toilet paper may not rip along the perforations has nothing to do with the perforations themselves, but with the fact that any paper rips easier fiber-wise. Because of production reasons, a roll of toilet paper has the fiber direction 'wrong'... $\endgroup$ – xxavier Oct 28 '18 at 10:11
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    $\begingroup$ also, would you mind providing sources for your statements? $\endgroup$ – Federico Oct 28 '18 at 11:58
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The Vulcan was capable of supersonic flight. The B52 never was designed for that. The Vulcan shows heavy influence of 1940s high speed designs (The Me 163 Komet), but as can clearly be seen by the MTOW chart, suffers from delta blues as far as lifting capacity and range. Deltas simply cannot lift as much as conventional wings.

They did, however, provide an easier path, particularly in those times, to supersonic flight. By generating lift with vortex flow, a thin, flat wing can be employed, side stepping Mach airflow compression issues (rocket fins routinely go supersonic). Deltas enjoy a wide flight speed envelope and are found in applications from hang gliders to many supersonic aircraft. They are very strong and easy to make.

However, we do not see them in subsonic transport or bombers because they are not as efficient in generating lift for their drag. Blended wing/body, same thing. For long range efficiency, the wing itself does its job, everything else belongs in a streamlined tube.

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  • $\begingroup$ "The Vulcan was capable of supersonic flight" do you have any sources for this? Beyond the one instance of overspeed, which may or may not have led to brief supersonic flight, the Vulcan was not designed for it. A further clue is that there were subsequent proposals to develop a supersonic Vulcan (Avro Type 732), meaning the team was aware of the limitations of the existing platform. $\endgroup$ – AEhere May 8 at 9:47
  • $\begingroup$ From wikipedia: "A cruising speed of 500 knots (580 mph; 930 km/h) at heights between 35,000 ft (11,000 m) and 50,000 ft (15,000 m) was specified." and "Maximum speed Mach .95 indicated" but please point out the correct paragraph if I´m missing something. On the contrary, your source states "The Avro 732 was a 1956 proposal for a supersonic development of the Vulcan [...]" $\endgroup$ – AEhere May 8 at 19:09
  • $\begingroup$ I will try, for now (I'm at work) the F 86 Sabre Jet was also built to be high subsonic, but could go supersonic in a dive. This was a handy trick, as other early jets could not do this. Legend has it a Sabre broke the sound barrier the same day as it was done, under controlled (and much safer) conditions by the X-1. The boom, from a dive, was much louder. As mentioned, rockets routinely were already breaking Mach 1. The Vulcan, with its hugely stable delta, could as well (without an all moving elevator or MCAS). $\endgroup$ – Robert DiGiovanni May 8 at 19:27
  • $\begingroup$ The Vulcan was also difficult to see on radar. This may have been the primary driving force in its planform. But I would not doubt it's designers dreamed of supersonic flight as well. $\endgroup$ – Robert DiGiovanni May 16 at 12:33

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