A few days ago Blake Scholl the CEO of BOOM, put a picture out on X of something that looked vaguely like a Klingon ‘ Bird of Prey ‘ from Startrek. The most striking feature of the image were the highly sculpted wings.
A couple of seconds later I realized it was a 3D rendering of the BOOM Overture rearview. The image got me thinking about Supersonic wings, hence this piece.
The Klingon ‘Bird of Prey’ was inspired by the grand daddy of supersonic flight, the NAA ( North American Aviation) XB-70 Valkyrie and the manta ray, the largest of the ray fishes.
This piece attempts a walk through the evolution of wings, delta wings in particular.
Wings
Flight of birds and insects has always held mankind’s attention. Icarus the Greek mythological figure crashed back to Earth as he flew too close to the Sun as his wax wings melted. Indian mythology is full of Vimanas (aircraft).
Leonardo da Vinci sketched ornithopters and gliders. Issac Newton & Daniel Bernouli identified the concept of lift through air pressure differences.
Sir George Cayley is credited with creating the first fixed wing followed by Otto Lilienthal who experimented extensively with fixed wing gliders
And then the Wright Brothers and powered flight in 1903.
Bernouli’s Principle
Bernouli’s Principle states that ‘ as the speed of a fluid (air or water) increases their pressure decreases.
Simply put the total energy in a fluid system remains constant along a streamline (path of a fluid particle).
From the perspective of an airfoil (wing) the curved surface on the top forces air to flow faster over the top of the wing than the bottom.
Using the principle, air pressure on the top of the wing is less than the pressure below and the wing generates lift.
The Supersonic Wing Evolution
Mention a delta wing and our mind’s eye thinks of wings that formed a triangle such as those on Mirage III fighters.
The evolution of the Delta Wing began in Germany towards the end of WW2 first with swept wings and then complete deltas.
The Dassault Mirage was the first delta winged fighter . It had a leading edge sweep of sixty degrees, no horizontal stabilizers . Elevons on the trailing edge provided both pitch and roll control.
The sharp leading edge and thin airfoil minimized drag and the Mirage III could sustain Mach 2 and over. The delta wing provided stable control at trans/supersonic speeds but directional control at low speeds was challenging, required high angles of attack and long runways to roll in and out.
The Mirage III was the first production aircraft to use Vortex Lift (by accident!).
Vortex Lift??
The Evolution of the Delta Wing
All wings produce vortices.
Subsonic wings use planar lift, vortices develop at the wingtips. The vortices induce drag and reduce lift efficiency of the wing. The solution was winglets to mitigate the vortices increasing efficiency. Today most subsonic airliners have them.
What if Vortices are an essential lifting tool of the wing?
Vortices by their very nature are unstable and need to be actively managed. Many early slender deltas were tailless with sharp leading edges that resulted in airflow separation at low speeds and high angles of attack. (Angle of attack is the angle between the airfoil(wing) and relative wind experienced by the aircraft.) The separation created vortices which rolled off the leading edges of the wing and created lift especially at takeoff and landing.
Low speed handling needed better control , i.e vortices needed to be controlled, enter the highly sculpted ogival delta on the Concorde.
But before that we need to speak about the legendary XB-70 Valkyrie, the first aircraft to use all three kinds of primary lift. The types of lift were:
- Planar lift : At sub & transonic speeds
- Compression lift : High supersonic speeds
- Unoptimized vortex lift : Takeoff / landing / maneuvering
We have already spoken about planar lift (Bernouli’s Principle), and Vortex lift (more on this later). But lets now speak about Compression Lift and the XB-70.
The XB-70 Valkyrie Wing
The XB-70 is an aircraft design like no other.
In the mid 1950s as the Cold War heating up the United States needed a bomber that could fly at Mach 3+ ( over three times the speed of sound) and fly at altitudes over 70,000 feet.
Enter the XB-70 Valkyrie!
With a length of 185 feet, wingspan of 105 feet and MTOW (maximum takeoff weight) of 542,000 pounds this aircraft powered by 6 (six!!) General Electric YJ93 engines was massive. Each engine produced 28,000 pounds of thrust with afterburners and 19,000 pounds without. The range of this behemoth was 3,275 nm (nautical miles).
Managing the bulk of this aircraft across the speed range was a challenge that set the ground work for future SSTs (Supersonic Transport).Engineers came up with a triple whammy!
Planar Flow
The XB-70’s compound delta wing with it’s 58 degree sweep angle used planar flow to keep the air attached to the wing at subsonic speeds.
This was possible only at low AoA .As the the XB-70 had a large wing span and area, distribution of lift was even.
The wing had a subtle twist and a thin airfoil to delay early flow separation.
Vortex Lift
The swept back wings automatically generated vortices off their leading edges at high AoA. At low speeds and banking events vortices compensated for lack of high lift devices such as flaps.
The XB-70 was not optimized for vortices.
Compression Lift
During high Mach cruise the final 20 feet of each wing folded down 65 degrees from the horizontal ( a feature the Klingon Bird of Prey was inspired by ) and formed a compression chamber.
The sculpted nose and forebody trapped shockwaves along with the shaped engine intakes and directed the shockwaves between the Klingon wings.
Compression accounted for between 05-40% of total lift at high Mach numbers. The rest of the lift was generated by planar flows plus unoptimized vortex lift (at low subsonic numbers)
The canards on the XB-70 behaved as pitch control surfaces. As a tailess delta wing the XB-70 needed these surfaces to maintain balance at speeds in excess of Mach 2.5 as the nose of the aircraft had a tendency to pitch downwards.
The Valkyrie Learnings
The two prototypes constructed flew a total of 160 hours and 16 minutes over 83 flights. Of this time AV-1 flew a total of 32 minutes over Mach 3 and reached a top speed of Mach 3.06. AV-2 which had several improvements over AV-1 flew a total of 20 minutes at Mach 3.08.
AV-2 was tragically lost on 08 Jun 1966 while flying in formation for a photo shoot. The vortices generated by the unfolded wingtips sucked the F-104 flown by NASA Chief Test Pilot Joe Walker, flipped it upside down over the starboard wing and across the twin vertical stabilizers killing Captain Walker instantly, and Major Carl Cross the copilot on the XB-70. Al White the pilot of AV-2 survived with serious injuries.The first ever warning on the dangers of wing vortices off large aircraft.
While the XB-70 flew for a very limited time between 1964-69 the lessons learned influenced future supersonic travel. A few listed below:
- The XB-70 influenced the selection of Titanium as the material for use on the legendary SR-71 Blackbird/Habu
- Aerodynamic data such as inlet design, shock wave interactions at high altitude & high Mach stability influenced the SR -71. Chines and inlet spike behavior were innovated as a result.
- Human fatigue at high Mach numbers were studied and solutions found
- Building on lessons learned by the XB -70 the Concorde (developed independently) focused on low speed behavior and wing sculpting to control stability at low speeds, take off & landing
- The material limitations of Mach 3 cruise on the XB-70 helped Concorde engineers arrive at a cruise speed of Mach 2
- The XB-70’s boom shaping and compression lift formed the early basis of NASAs QueSST program. The X-59 with its coke bottle shape and boom cheating design is a result of the early learnings from the XB-70
The XB-70 continues to be a supersonic reference point 61 years after its first flight.
The Concorde Wing
If the wing of the XB-70 Valkyrie was cutting edge technology & over the top engineering the Concorde wing comes across as a piece of supersonically sculpted art.
The Concorde’s secret was Vortex black magic.
The delta wing is naturally unstable at low speeds, while highly efficient at Mach 2. Extensive wind tunnel testing resulted in the ogival delta wing.
The slender Concorde ogival delta has a curved leading edge. If the plane is observed from the front, the S is clearly visible on the leading edge. Furthermore the wings droop down towards the wing tips. The droop or dihedral inversion contributed towards improved stability, and stable vortex structure.
This allowed the Concorde to smoothly transition from vortex lift during takeoff to planar lift as it got to transonic & Mach cruise speeds. The wing structure had minimal lifting surfaces and lift was managed by the sculpted wings.
The inboard section of the wings (swept back to 55 degrees) carried more lift at low AoA while the outboard sections improved stability and efficiency.
The Concorde used a state of the art fuel management system to shift its CG (centre of gravity) by pumping fuel between 13 different tanks.
At supersonic cruise the CG was shifted towards the tail (trim tank 11), the forward tanks were used during take off and low speeds, and forward (trim tank 9) during descent.
The entire system was automatic and the CG could be shifted by two meters (6.5 feet). The MAC (mean aerodynamic chord) at 55-59%. To explain, 0% of MAC is the leading edge (the front edge of the wing) and 100% is the trailing edge (the rear edge of the wing). 55-59% means at cruise the Concorde’s CG was just aft of the mid chord of MAC.
This enabled Concorde to be extremely stable at Mach 2, minimize drag and maximize fuel efficiency.
The Concorde used all three types of lift as well. Controlled vortex lift at low speeds (the sculpted wings), planar lift at transonic and supersonic speeds along with compression lift as well. Concorde drew approx 30-40% of its lift at cruise from natural shockwave compression.
The XB-70 and the Concorde, two aircraft, two different deltas, so different yet so similar.
The Overture Wing
The Overture will be operating in a very different speed regime from the Concorde. Where the Concorde operated at Mach 2+ for most it’s flights the overture will be operating at high transonic speeds (Mach 0.8 – 1.2). Aerodynamic needs at these speeds are a combination of subsonic & supersonic speeds.
The Boom website says the sweep of the Overture wings will be more than a 777 but less than Concorde.The 777 is at 31.6 degrees while the Concorde is at 55 degrees. The wing is of course thinner than a 777’s!
Where the Concorde relied on vortex lift at lower speeds the Overture will rely on planar lift(attached airflows).
The Overture will be a hybrid delta wing with horizontal stabilizers and will use a gull wing design. This means the wing arches upwards near the root and angles downwards towards the wingtips. The upwards attitude towards the root increases efficiency at Mach speeds. The outward droop towards the wingtips improves stability at subsonic speeds, takeoff, climb, descent and landing. The trailing edge has been shaped as well.
The podded engines slung below the wings on pylons offer net overall efficiencies, the embedded engines of the Concorde offered better aerodynamic efficiencies at Mach 2 (the max speed of the Overture is Mach 1.7)
Underwing podded engines offer advantages such as ease of maintenance, more space to add acoustic liners for sound damping, allows for modular engine design which in turn allows ease of maintenance. In case of uncontained engine failure engines on pylons under the wings are always safer.
Since the wings are a hybrid between the Concorde’s ogival delta and subsonic wings, the tail is more conventional with a horizontal stabilizer. The stabilizer offers finer pitch and directional control. The placement of the tail plane is outside the airflows over the wings and engines.
The wings of the Overture will provide higher lift than the Concorde at low speeds. The angle of attack at landing and take off will be similar to Concorde ( making the Overture a tall plane as well..the landing gear will be another story).
Since the Overture has a horizontal stabilizer, trim will be handled aerodynamically. There will be no need for a fuel transfer system to manage CG.
Lastly AI( Artificial Intelligence) renders the aircraft intelligent, translates to lighter crew workload.
Exciting times!
Summation
The delta wing has matured over the last 70 years. The Overture represents the next step in the evolution of the supersonic delta wing.
Disclaimer: This article has used images from multiple sources accessed through Google.
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