What Is The Coffin Corner In Aviation?
By: Jake Hardiman (Simple Flying)
A geeky bit of trivia that has absolutely nothing to do with politics.
S E E D E D C O N T E N T
The language of the aviation industry is one that is full of fascinating but complex terms. While some terms are obvious enough for people to be able to work out their meanings from the word(s) alone, others aren't so explicit. One such term that, at face value, doesn't seem to have an explicit meaning is 'coffin corner.' Let's take a look at what this refers to.
What is the coffin corner?
The 'coffin corner' refers to the intersection of a given aircraft's stall speed and critical Mach number. While stall speed is enough of a self-explanatory term, critical Mach number may be a less well-known term. Skybrary reports that it refers to the lowest Mach number at which the airflow over any part of the aircraft reaches the speed of sound.
An aircraft's altitude at a given time defines the speed at which these two variables intersect. The margin between them will decrease as a plane climbs towards this defining altitude. Bold Method reports that the real name for this is the 'Q corner.' The reason for this is the fact that Q is recognized as an official abbreviation for dynamic pressure.
However, the phenomenon has also taken on the name 'coffin corner' due to the triangular region of a flight envelope chart where stall speed and critical Mach number are close together. More morbidly, it also refers to the danger of death that can arise as a result.
What can happen?
When an aircraft is in the coffin corner, it can be more difficult to maintain stable flight. This is because reducing speed can cause the plane to stall, whereas increasing speed can reduce its lift. Even the slightest of movements can prove dangerous, as Skybrary explains:
" In the most critical case, simply turning the aircraft could result in exceeding both limits simultaneously. In a turn, the inside wing slows down, whereas the outside wing increases speed. Likewise, encountering turbulence could result in a 'beyond limits' change in airspeed ."
In any case, both a stall and a loss of lift can cause the aircraft to fall from its existing altitude. Such situations can be extremely perilous to an aircraft as the g-forces encountered when falling can cause structural failures, leading the plane to break apart.
Case study: Air France flight 447
Air France flight 447 crashed in the Atlantic Ocean in June 2009 due to a high altitude stall while flying from Rio de Janeiro to Paris. All 228 passengers and crew perished in the tragic accident, which was the deadliest involving both Air France and the Airbus A330.
The crash has been linked to the coffin corner by publications such as Scientific American. This is because the accident happened after ice crystals obstructed the aircraft's pitot tubes. This led to inconsistencies in speed readings and a failure to report stall conditions.
Amid confusion in the cockpit due to the incorrect readings, the flight's crew pitched the aircraft's nose upward to negate a roll caused by turbulence. This caused the aircraft to deviate from its cruising altitude of 35,000 feet, climbing to FL380. At this point, it lost lift and experienced the aerodynamic stall that caused the tragic accident.
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An airfoil can lose lift by going too fast as well as too slow. The same thing can happen with a wind turbine blade.
The Lockheed F-104 Starfighter was notorious for having a insidious coffin corner due to it's high speed and it's short stubby wings that generated insufficient lift in certain flight regimes. A lot of pilots were lost because of it, especially the West German Bundesluftwaffe (Air Force).
102s, 104s and 105s were most often described as lead sleds.
GW Bush had several landings bad enough to retire at least one airframe, possibly two.
The learning curve of flight has been dramatic.
Then there are those strange and wonderful people who think
landing a jet aircraft on a slow moving pitching and yawing aircraft carrier
is a great idea...
All prototypes of the above listed aircraft were initially designed without the "area rule" principle to the fuselage. This was especially evident in the Convair F-102 Delta Dagger, the second of the "Century Series" of jet fighters. The prototype failed to meet the manufacturer's performance specifications. When the Area Rule principle, which applied to pinching the fuselage midway down the fuselage, also known as the "Coke Bottle" configuration, it increased performance dramatically. The same principle was used in the Lockheed F-104 Starfighter and the Republic F-105 Thunderchief. Convair used the improvements in the F-102 to ultimately redesign it and produced it's successor, the F-106 Delta Dart.
Weren't those stiletto designs influenced by the X-15 program? I think the X-15 design emphasized reduction of turbulent drag caused by the wing which was incorporated into the stiletto design military aircraft. The X-15 overcame the Q corner with limited maneuverability and thrust to transition from aircraft to projectile. The space shuttle did the reverse, I suppose, transitioning from a projectile to an aircraft.
If I recall correctly, the first problem to address was how to go fast. Maneuverability suffered until that problem was worked out.
The F-102 Delta Dagger and F-104 Starfighter were designed to be interceptors and not fighters. If I recall correctly, the SAC goal was to build a mach 3 aircraft to intercept the Russkies outside the US and nuke 'em at altitude. Nike Hercules was the last line.
The F-105 Thud was designed to be a nuclear attack aircraft. That's back when the military was still attempting to use nukes as tactical weapons. The idea was to go straight in at low altitude and high speed to avoid ground defense. The Thud really wasn't designed for a ground support role; the Thud was intended to be a manned cruise missile.
The F-104 G was specifically a redesign of the Starfighter at the request of and for the the German Luftwaffe... It was turned into a fighter bomber that could be used as an interceptor...
The F-104 as designed by Lockheed (Kelly Johnson) to be an interceptor was a dream to fly, I got that from a friend on a modeling forum who spent 20 years in the cockpit of F-4's... It's US Air Force nickname was the "Zipper" and when there was one available on the airbase, they were usually reserved for the senior pilots or generals... The fighter bomber variants also had a nickname, the "Widowmaker" because of wing loading which made it a tricky aircraft to fly, you had to pay attention to it at all times, get careless and it would bite you... Chuck Yeager had to eject from one once during the evaluations of it for the airforce... he said it was a dream to fly, EXCEPT when at high altitude/speed attempting manuevers, they could easily flip into a flat spin which was unrecoverable in the F-104... In it's normal fight regime, the F-104 was a easy plane to fly with no bad habits.. in the "G" configuration especially when loaded you had to pay attention to it at all times...
The US Air Force didn't possess a single "G" model, ALL of Lockheed's "G" model production went to European Air Forces...
Area Rule as an idea was first patented in Germany in March of 1944 ...
Junkers patent drawing showing it....
The US History comes postwar...
Wallace D. Hayes, a pioneer of supersonic flight, developed the transonic area rule in publications beginning in 1947 with his Ph.D. thesis at the California Institute of Technology.
Richard T. Whitcomb, after whom the rule is named, independently discovered this rule in 1952, while working at the NACA. While using the new Eight-Foot High-Speed Tunnel, a wind tunnel with performance up to Mach 0.95 at NACA's Langley Research Center, he was surprised by the increase in drag due to shock wave formation. The shocks could be seen using Schlieren photography, but the reason they were being created at speeds far below the speed of sound, sometimes as low as Mach 0.70, remained a mystery.
In late 1951, the lab hosted a talk by Adolf Busemann, a famous German aerodynamicist who had moved to Langley after World War II. He talked about the behavior of airflow around an airplane as its speed approached the critical Mach number, when air no longer behaved as an incompressible fluid. Whereas engineers were used to thinking of air flowing smoothly around the body of the aircraft, at high speeds it simply did not have time to "get out of the way", and instead started to flow as if it were rigid pipes of flow, a concept Busemann referred to as "streampipes", as opposed to streamlines, and jokingly suggested that engineers had to consider themselves "pipefitters".
Several days later Whitcomb had a "Eureka" moment. The reason for the high drag was that the "pipes" of air were interfering with each other in three dimensions. One does not simply consider the air flowing over a 2D cross-section of the aircraft as others could in the past; now they also had to consider the air to the "sides" of the aircraft which would also interact with these streampipes. Whitcomb realized that the shaping had to apply to the aircraft as a whole, rather than just to the fuselage. That meant that the extra cross-sectional area of the wings and tail had to be accounted for in the overall shaping, and that the fuselage should actually be narrowed where they meet to more closely match the ideal.
The area rule was made available to the U.S. aircraft industry on a secret basis for military programs from 1952 and it was reported in 1957 for civilian programs...
The Grumman F-11 Tiger was the first aircraft to fly with it and had been designed using the area rule from the outset. The Convair F-102 Delta Dagger was the second.. It had to be redesigned as it had been unable to reach Mach 1 although its design speed was Mach 1.2. The expectation that it would reach design speed had been based on over optimistic wind-tunnel drag predictions. Once the Area Rule was incorporated in the design it easily reached it's design speed....
Actually, you have it the other way around as the X-3's first flight was three years before the X-15's. Data from the Stiletto wing design was used on the X-15 wing shape.
Yeah, I forgot about the X-3. It's a little long ago and far away for me. I was just a pie eyed kid with a Popular Mechanics subscription when all this stuff was going on. It's fun to sorta, kinda remember.
I did remember the SAC attempt at a mach 3 interceptor was the XF-108 Rapier. That was part of North American's XB-70 program. I don't think the Rapier ever got off the drawing board. The airframe designs exceeded engine capability.
Most USAF fighter pilots stated the Thud was at it's fastest down low in the weeds after dropping all it's ordinance and egressing out of the target area on afterburner. The North Vietnamese air force could not catch it. Most were shot down before or during attack run ins, but rarely after.
That's what the Thunderchief was designed to do. The F-105 was designed to fly in weeds at high speed to deliver one tactical nuclear weapon stored internally. The Thud wasn't designed to carry a heavy load of munitions in a ground attack and support role.
May not have been designed for it, but did so in a exemplary manner! 14,000 lbs of onboard ordinance was nothing to sneeze at.
Wow, I guess not too many people interested in aeronautics on NT.
Yeah, I am interested in aeronautics, it's just that this is an FYI article the answer is already known and aviation designers account for it.
The aircraft I have flown are well within the larger area of the "coffin corner" and therefore is not much of a concern, doesn't mean it doesn't happen. One instance was a local pilot who owned a Cessna Cardinal 172 he decided his son and he would fly across the country from Mich. to Cali. so his son could attend a university out there. The 172 he had he outfitted it with a 140 engine so it was underpowered and take offs from our grass strip was always a nail biter, but we had escape routes at each end to allow for the gain in altitude without encountering the powerlines at each end. As he went west he didn't account for the increasing altitude of the airstrips and he attempted to take off on a high altitude strip that ran uphill, what he did on our strip is he would before rotation he would pop the craft off the ground to reduce the friction of the grass so he did the same there but the runway kept coming up at him so he pulled back more and stalled it and the crash killed both of them.
There is something to be said for seat of the pants flying experience..., It pays to know what your aircraft can actually do...
That's one of the things that made pilots like Yeager, Crossfield, and others of that era so great! Don't have that anymore, everything as all computerized these days.