de Havilland Comet Design changes 1

[jpaero]

I am trying to learn the specific design related issues with the de Havilland Comet. I have read the investigation report which spoke of:
1. Underestimation of stresses at the corners
2. Overloading the fatigue specimen before the fatigue test itself
3. Testing only the fwd fuse and effect of end restraints
etc

What I am interested however is the specific changes that were made to the design to make it airworthy again. Possible amends I could think of (based on my limited knowledge) were:

1. Reduction of general stress levels by making the skins thicker
2. Better materials in terms of crack resistance/growth (fracture toughness?)
3. Changing some of the cutout profiles to reduce the Kts.
4. Adding stiffners around windows to allowing stress away from the cutouts

But I was hoping for some definitive answers rather than guessing. An ideal answer would be one which compares the old and new design almost side by side. While a lot seems to written about what led to the accident, I couldn't find much by the way of specific design changes that were incorporated to make it airworthy again. Any pointers in that direction would be great! I will also welcome less than ideal answers .

 

[RoarkS]

I am still intrigued by the engines being mounted at wing root and that being a possible source of excessive vibration into the airframe. I do a lot of shock and vibe testing, and find it funny that when the test is running, that the specific profile actually makes the lab sound like you're in the cabin of said aircraft flying... but yeah some of the zones are significantly more "violent" than the cabin. I got to see a comet up close up at the museum of flight in Everett a few years back. I always wondered how much extra weight went into making the doughnuts in the spar vs just hanging pods.

 

[rb1957]

there ware several examples, mostly early jets.

Mounting the engines submerged in the wings reduces drag (at a time when engine power was limited), or so you'd think. I'm sure the vibration question is "easily" answered with isolators (a well understood technology, though sure back in the day some tuning would've been needed).

mounting engines in pods/nacelles has other issues (drag, gound strike, limiting wing position, etc). Also, I believe, at the time the impact on the wing aerodynamics was not well understood (and eventually understood as a change in lift curve slope and not a shift of the wing lift curve, or so I understand). As a plus, for podded engines, I expect the airflow into the engine is much smoother ... but nothing that can't be fixed is splitter plates; and of course the exhaust is far from the rest of the structure (again, nothing that can't be fixed is angling the exhaust away from the fuselage or mixing cold air, like we do to reduce the IR signature). And, as a plus/minus, the engine chordwise position is critical to wing flutter and other aero-elastic and dynamic cases. And I'd've thought that podded engines would have needed the same vibration isolation (of course, the wing isolates the fuselage and passengers to a large extent). Of course, rotor burst is significantly different.

another day in paradise, or is paradise one day closer ?

 

[WKTaylor]

T-37 'Tweety Bird' is a classis examples of light MIL-Jet with engines between wing spars where the inlet and exhaust passed thru the spars. It was quit successful @ ~7500# GTW.

Straight wings.
[2] J-69 Engines between spars.
MLG between spars, inboard bays adjacent to engines
Fuel cells in all the 'other' wings cavities... and between the engines in the fuselage [main tank]
Crew just forward of the front spar.
Light weight sheet metal cockpit/nose and aft fuselage sections with the stabilizers...

Classic examples of 'span-loading' design as taught is Aero Design 101.

When the T-37 was called to go to war... IE modified to be killing machine A-37s Dragonfly...

The [2] engines were replaced with J-85s [non-afterburning].
[2] 100 gallon wing tip tanks were installed along with pressure refueling/AR probe.
[4] underwing store stations were added to EACH ~10% wider-chord wing.
All fuel cells were upgraded to self-sealing; and ballistic felt-padding was added to cockpit side walls
A 7.62 mini gun was installed in the RH nose compartment and an ammo-feed drum was in the LH nose compartment.
Some comm/camera equipment was added to the aft fuselage 'hell-hole' compartment.
Ejection seats were upgraded for better survivability.
The main landing and nose gear struts and tires were beefed-up... upgraded... to accommodate the new GTW 14500#
Fuel and weapons added-up to almost ~8000# resulting in a GTW TO weight of 14000 [later increased to 14500#]

Amazing what benefits were derived by span-loading!!

NOTE the widely separated engines made for greater safety in case an engine failed and for IR-missile tracking [reduced/split heat signature].

These jets were originally intended for relatively short life of 4500-Hrs of total flight time... but with structural mods could make 7500-hrs safely

I have some structural illustrations, somewhere... I'll post a few IF I can find them.

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[LiftDivergence]

Does this count? (HP.80 Victor)

Speaking of interesting engine mount configurations (NASA QRSA)...

Keep em' Flying
//Fight Corrosion!

 

[WKTaylor]

A-37B Cutaways...

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[Compositepro]

I had the privilege to fly the T-37 for 30 minutes and do barrel rolls when in Air Force ROTC. We got to practice in a simulator the day before the actual flight.

 

[rb1957]

Ah ... the DHC-5 Buffalo QSTOL blown flap prototype. The (internal) cross ducting was "insane" ! but then so was the STOL performance.

another day in paradise, or is paradise one day closer ?

 

[WKTaylor]

Comet 1 and 4 cutaways... for grins....

Comet 1 most were permanently grounded [not economical to mod/repair]... square widows

Comet 4 [used extensively for commercial and military service]... rounded/ellipse windows...

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[WKTaylor]

and for giggles... YC-14 and YC-15 prototypes

Boeing YC-14... radical STOL configuration...

VS the more conventional MAC-DAC YC-15... Mini C-17 wanna-be...

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[rb1957]

and there was a Russian transport with blown flaps (a la C-14 as I remember)

another day in paradise, or is paradise one day closer ?