Supersonic DC8 - Could today's airliners do it?
Discussion
Hi All,
I've always been a fan of the DC8 over & above the 707 because although a little later into production it always struck me as a stronger better engineered aircraft with a handling advantage (less dutch roll).An ex-DC8 pilot I know (admittedly you'd expect bias)felt is was the better aircraft & acknowledged that the comparison was hotly debated at the time.
From wikipedia :
"By 2002, of the 1032 707s and 720s manufactured for commercial use, just 80 remained in service — though many of those 707s were converted for USAF use, either in service or for spare parts. Of the 556 DC-8s made, around 200 were still in commercial service in 2002, including about 25 50-Series, 82 of the stretched 60-Series, and 96 out of the 110 re-engined 70-Series. Most of the surviving DC-8s are now used as freighters. As of May 2009, 97 DC-8s were in service following UPS's decision to retire their remaining fleet of 44."
The strength & efficency of the airframe was proven by it's deliberate supersonic flight (in a dive) on 23rd August 1961:
www.dc-8jet.com/0-dc8-sst-flight.htm
I wonder how many of today's airliners could survive a supersonic dive without damage or overstress & then continue on in airline service as CF-CPG did with Canadian Pacific Airlines?
I've always been a fan of the DC8 over & above the 707 because although a little later into production it always struck me as a stronger better engineered aircraft with a handling advantage (less dutch roll).An ex-DC8 pilot I know (admittedly you'd expect bias)felt is was the better aircraft & acknowledged that the comparison was hotly debated at the time.
From wikipedia :
"By 2002, of the 1032 707s and 720s manufactured for commercial use, just 80 remained in service — though many of those 707s were converted for USAF use, either in service or for spare parts. Of the 556 DC-8s made, around 200 were still in commercial service in 2002, including about 25 50-Series, 82 of the stretched 60-Series, and 96 out of the 110 re-engined 70-Series. Most of the surviving DC-8s are now used as freighters. As of May 2009, 97 DC-8s were in service following UPS's decision to retire their remaining fleet of 44."
The strength & efficency of the airframe was proven by it's deliberate supersonic flight (in a dive) on 23rd August 1961:
www.dc-8jet.com/0-dc8-sst-flight.htm
I wonder how many of today's airliners could survive a supersonic dive without damage or overstress & then continue on in airline service as CF-CPG did with Canadian Pacific Airlines?
john_p said:
Impressive that the DC-8 went supersonic, but did that mean it was over-engineered? So as a result, over the lifetime of the aircraft, wasting a lot of fuel..
Pssibly, but the Comet disasters of the early 1950s showed what could happen if a design was under-engineered. Douglas would have erred on the side of caution.Eric Mc said:
The Comet disasters of the early 1950s showed what could happen if a design was under-engineered.
I wouldn't say that the Comet was under-engineered, merely that the original had a design flaw owing to a lack of knowledge about high cycle fatigue of metals.Indeed, if anything, the Comet was over-engineered as evidenced by the 40+ years service of the Nimrod, especially given that these a/c operated in a corrosive (salt water), high fatigue environment (low level). Even more so given that the Nimrods had their Main Spars 'shaved' (to remove corrosion) in the late 1980's and yet still flew for a further 20+ years!
Ginetta G15 Girl said:
Eric Mc said:
The Comet disasters of the early 1950s showed what could happen if a design was under-engineered.
I wouldn't say that the Comet was under-engineered, merely that the original had a design flaw owing to a lack of knowledge about high cycle fatigue of metals.Indeed, if anything, the Comet was over-engineered as evidenced by the 40+ years service of the Nimrod, especially given that these a/c operated in a corrosive (salt water), high fatigue environment (low level). Even more so given that the Nimrods had their Main Spars 'shaved' (to remove corrosion) in the late 1980's and yet still flew for a further 20+ years!
I would highly recommend Bill Gunston's book called "Plane Speaking" where he gives his assesment of the fragility of some of the airframes coming out of De Havilland.
The convair 990 was pretty quick as well and could cruise in level flight at 0.96 odd mach. Looks fast aswell 
In flutter trials the 787 was tested to 0.97 (in a 32 degree dive) and the airbus a380 was tested to 0.96 mach. Both would go supersonic once I reckon but only shortly before the wings making a revised flight plan independantly of the fuselage...
In flutter trials the 787 was tested to 0.97 (in a 32 degree dive) and the airbus a380 was tested to 0.96 mach. Both would go supersonic once I reckon but only shortly before the wings making a revised flight plan independantly of the fuselage...
Eric Mc said:
john_p said:
Impressive that the DC-8 went supersonic, but did that mean it was over-engineered? So as a result, over the lifetime of the aircraft, wasting a lot of fuel..
Pssibly, but the Comet disasters of the early 1950s showed what could happen if a design was under-engineered. Douglas would have erred on the side of caution.Critical Mach no. is the problem. Exceed this and Mach buffet follows with much shaking and possible detaching of important parts, not to mention drag suddenly going through the roof. In general, the greater the wing-sweep the faster the jet can go before reaching critical Mach.
Control surface flutter (ailerons etc) can also be a problem at speed, and if resonant or divergent usually also results in the relevant part eventually detaching from the aircraft.
The bottom line is that designers work out the speed they want the aircraft to cruise at, and then make sure the critical Mach number is slightly higher. There's no reason to design-in a greater margin between cruise and never-exceed Mach number as it'll not be needed unless in the rare case of a 'jet upset' such as the 747 a while back that survived rolling almost inverted and pulling through, during which the passengers and crew briefly joined the Mach 1 club. I imagine the queue for the toilets was pretty long after that one.
Control surface flutter (ailerons etc) can also be a problem at speed, and if resonant or divergent usually also results in the relevant part eventually detaching from the aircraft.
The bottom line is that designers work out the speed they want the aircraft to cruise at, and then make sure the critical Mach number is slightly higher. There's no reason to design-in a greater margin between cruise and never-exceed Mach number as it'll not be needed unless in the rare case of a 'jet upset' such as the 747 a while back that survived rolling almost inverted and pulling through, during which the passengers and crew briefly joined the Mach 1 club. I imagine the queue for the toilets was pretty long after that one.
Ginetta G15 Girl said:
Eric Mc said:
Other manufacturers (such as Vickers) who saw what De Havilland were doing with the Comet 1 were very concerned about certain aspects of the design
That'll be Vickers of the cracking Valiant main spars then. 
Vickers built tough old birds.
Eric Mc said:
Ginetta G15 Girl said:
Eric Mc said:
Other manufacturers (such as Vickers) who saw what De Havilland were doing with the Comet 1 were very concerned about certain aspects of the design
That'll be Vickers of the cracking Valiant main spars then. Vickers built tough old birds.
Eric Mc said:
That's a bit unfair.
I'm afraid I'm going to have to disagree with you Eric. It is often claimed that it was the switch to low level that caused the Valiant main spar fatigue, but that's just not true. After the 1964 Gaydon incident with the snapped main spar, the entire fleet was grounded and inspected. Fatigue cracks were found in tanker aircraft that had spent no time at low level. Indeed a trials aircraft, with a mere 5 flying hrs, was inspected and found to have cracking in its main spar. Inspections were also done on spare spars, which had never been fitted to an a/c and they too exhibited the same fatigue problems.It appears that the lightweight alloys used for the main spar had an inherent problem with their metallurgy, crystallising when formed and fatiguing with age, whether fitted to an aircraft or not.
As early as 1957, during the separation trials of the Super Sprite RATO system, the second Valiant prototype developed a fatigue crack in the bottom main spar cap resulting in the spar breaking.
So, ISTM that Vickers (and possibly the MoD) knew there was a problem but kept it quiet (in much the same way that BAe and the MoD were culpably negligent in hiding the inherent AAR problems in the Nimrod MR2P which resulted in the loss of XV230 and her crew).
Edited by Ginetta G15 Girl on Wednesday 27th October 19:42
Ginetta G15 Girl said:
Eric Mc said:
That's a bit unfair.
I'm afraid I'm going to have to disagree with you Eric. It is often claimed that it was the switch to low level that caused the Valiant main spar fatigue, but that's just not true. After the 1964 Gaydon incident with the snapped main spar, the entire fleet was grounded and inspected. Fatigue cracks were found in tanker aircraft that had spent no time at low level. Indeed a trials aircraft, with a mere 5 flying hrs, was inspected and found to have cracking in its main spar. Inspections were also done on spare spars, which had never been fitted to an a/c and they too exhibited the same fatigue problems.It appears that the lightweight alloys used for the main spar had an inherent problem with their metallurgy, crystallising when formed and fatiguing with age, whether fitted to an aircraft or not.
As early as 1957, during the separation trials of the Super Sprite RATO system, the second Valiant prototype developed a fatigue crack in the bottom main spar cap resulting in the spar breaking.
So, ISTM that Vickers (and possibly the MoD) knew there was a problem but kept it quiet (in much the same way that BAe and the MoD were culpably negligent in hiding the inherent AAR problems in the Nimrod MR2P which resulted in the loss of XV230 and her crew).
Edited by Ginetta G15 Girl on Wednesday 27th October 19:42
Whatever the truth is, I have often felt that the fleet could have been rescued if a bit of time, effort and money had been put into fixing the problem, but the beginning of the RAF's long downward spiral to its present sad state began with the scrapping of the Valiants.
I find it difficult to believe that a 747 could reach a genuine Mach 1.0 (as opposed to an indicated M 1.0) without catastrophic structural failure. Aerodynamically there's a world of difference between Mach 1.0 and M 0.96. The Douglas DC8 supersonic test was conducted according to rigorous scientific standards and as far as am aware it is the only airliner other than Concorde & the TU-144) to have been deliberately flown supersonic & to have this performance independently verified by independent external instrumentation.I believe a Cessna citation X was later taken beyond M 1.0 during pre-certification testing. Anecdotal instances of IAS beyond M 1.0 in all manner of aircraft have been common place, but with pitot position & compressibility error these have frequently been over-readings. I'm aware that the China Airlines Flight 006 747 SP which was alleged to have exceed M 1.0 in an upset on 19 Feb 1985 (again not scientifically verified) was heavily damaged. Likewise the Flight 841 727 incident of 4 April 1979 produced Flight Data Recorder readings of M 1.0 - but this is hardly an independent verification & the aircraft was substantially damaged in the spiral dive.
Eric Mc said:
Oh well, every book and article I've read on the demise of the Valiant must have been wrong then.
GGG's about right. The switch to low level undoubtedly accelerated the discovery of a *big* problem but Vickers already knew there was *some* problem before then. Eric Morgan's book on the Valiant has some brief coverage of the spar fatigue issue and mentions that inspections were already underway before the in-flight failure; Viscounts were similarly affected (and many were re-sparred as a result - much simpler to do so on the Viscount as opposed to the Valiant); XD816 was resparred by Vickers to test the process (but was too expensive).In recent years there seems to be a fashion of blaming the alloy used (R.R.77), but the same alloy was used in many other aircraft which did not suffer from the same issues (Canberra for instance) so it seems to be an over-simplification to blame the alloy alone.
DamienB said:
Eric Mc said:
Oh well, every book and article I've read on the demise of the Valiant must have been wrong then.
GGG's about right. The switch to low level undoubtedly accelerated the discovery of a *big* problem but Vickers already knew there was *some* problem before then. Eric Morgan's book on the Valiant has some brief coverage of the spar fatigue issue and mentions that inspections were already underway before the in-flight failure; Viscounts were similarly affected (and many were re-sparred as a result - much simpler to do so on the Viscount as opposed to the Valiant); XD816 was resparred by Vickers to test the process (but was too expensive).In recent years there seems to be a fashion of blaming the alloy used (R.R.77), but the same alloy was used in many other aircraft which did not suffer from the same issues (Canberra for instance) so it seems to be an over-simplification to blame the alloy alone.
I wonder at what stage in the process of the conversion of the Valiant B1s into K1 tankers was the spar problem discovered. Was it known about BEFORE the tanker conversion programme began?
How many Valiants were lost due to spar failure?
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