Disadvantages of the airplane structure FW 200

[Robcio]

Recently, interest in aircraft design and applications FW200.
I wonder whether the plane was originally designed for the needs of the Air Force was strong enough for military aircraft.
The literature often encounter information about defects and technical problems encountered during aircraft landings especially. Again, there were problems on the fly
the supporting structure of the aircraft.
I ask for feedback.
Thank you in advance for your help.
Regards, Robert

[G.R.Morrison]

Here are two examples of a typical structural failure, aft of the trailing edge of the wing:
http://wwiimodeller.co.nz/tag/fw200-...roken-in-half/

GRM

[bearoutwest]

Hi Robert,

My “Reader’s Digest Guide” to aircraft structures (heavily simplified version):

Fw200 on the ground:
- in profile supported between the main wheels and the tailwheel
- simplistic load distribution mean most of the weight is between cockpit and trailing edge of wing
(m representing weight)

...........mmmmmmmmmmmmmm***
mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
...................^ .................................................. ....................^

........Main wheel(s) .................................................. ...Tail wheel

* point of highest bending stress on fuselage on the ground (bending the tail up direction)


Fw200 in the air:
- in profile with main wheels and the tailwheel location shown for reference but obviously not used for support)
- simplistic load distribution mean most of the weight is between cockpit and trailing edge of wing
(m representing weight)
(L represents upwards lift)
(D represents downwards lift – tailplanes designed with downward lift for aerodymanic balance/control of the aircraft)

.............LLLLLLLLLLLLLLLLL
..........mmmmmmmmmmmmmm****
mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
....................^............................. .......................................... ^
.................................................. ...................................DDDDD

- forces in play during flight means aircraft is supported off it’s wings
- the tail and rear fuselage is hanging off the end of the wing (with the tailplane surfaces providing downward lift to balance the pitch of the aircraft)

* point of likely highest bending stress (in level flight) on the fuselage (bending the tail down direction)

So, if we ignore the in-flight manoeuvres and weight change due to fuel burn and bomb-drops, etc:
- everytime the Fw200 is loaded up on the ground, the fuselage bends tail up.
- everytime it’s taking off and flying along, the fuselage bends tail down,
- everytime it lands, the fuselage bends tail up (heavily).

This is a classic case of “fatigue failure loading” in tail-wheeled aircraft. All tailwheel aircraft suffer from this – most are strongly designed to be able to withstand 10,000s – 100,000s of flight cycles.

The issue with the Fw200 was the relative slenderness of the fuselage. It was a slender, fast airliner which was re-designated as a bomber/recce/military-transport aircraft. It was more heavily loaded than originally intended or conceived, and used militarily on more flight cycles (while heavily loaded) than originally conceived.

The rear fuselage structure would have been strengthened during military upgrade design, but with the overall slenderness of the design, the weak point was always going to be somewhere aft of the wing trailing edge – be it from flight cycle fatigue, overloading during flight manoeuvres, combat damage, etc (as evident from most of the photos of wrecked Fw200s). The slenderness (lack of height) of the rear fuselage greatly reduces the effectiveness of strengthening the structure against bending loading.

Hope that’s not too confusing.
...geoff

[leonventer]

Nicely done explanation, Geoff -- very clear.

Thanks,
Leon Venter

[drgondog]

Additionally the Fw 200 was built under the guise of a passenger aircraft/airliner. The design load would have been evenly distributed to account for passengers but possibly a smaller region of load for baggage.

Ideally, baggage (and bomb load) would be placed near the airframe cg but I am unfamiliar with the layout of the Fw 200.

While fatigue is a possibility, I doubt that there were enough reversible loads to account for this - particularly if you compare to an airframe like a UH-1 Huey that experiences a full reversible load due to the Pylon and rotor system - which shakes it +- 1.2G nearly forever.

[Robcio]

Thank you for the information.
Indeed, the fuselage was constructed for the purpose of passenger flights. Therefore, do not withstand the conditions of sorties and large overloads. Especially during landing. You can see it in the pictures. The fuselage behind the wing cracks.
Whether similar cases of fracture may have occurred during the flight.
Technical problems have been known for Luftwaffe aircraft still has not been withdrawn from combat duty. Is supposed to be used only until a successor produce such .: He 177, as a temporary measure?
Thank you in advance for your help

[bearoutwest]

Fatigue as a primary source of airframe failure in operational WW2 aircraft? I doubt if any WW2 era aircraft achieved anywhere near normal flight cycle loading – except maybe for the long haul transports C-46, C-47, C-54, during the war years.

My discussion of the cyclic style loading was to highlight the area of concern – i.e. the fuselage just aft of the wing trailing edge on the Fw200. I would have thought that the flight regimes contributing to overall wear and damage would have been (in no particular order of importance, as each individual aircraft would have differed):
- high g-load flight manoeuvres (Fw200s used initially for low level bombing of ships with high-ish g-load pull up at end of bomb-run)
- heavy landings
- cyclic loads due to heavy operational loads
- combat damage (holes in stressed skin would require airframe structure to take a higher portion of load).

As for operational loading of the Fw200:
Civil and military transports:
- passenger seating from aft of cockpit to just after trailing edge of wing
- small mail/baggage compartment in nose, and towards tail of aircraft (remember that passengers and baggage weighed in this era of air transport, so 23kg suitcases and 10kg duty free goods per passenger was not allowed)
- fuel tanks in the wing, possibly temporary fuel tanks in fuselage but this may have only been for the early long range civil air route demonstration flights.

Military recce/bomber versions:
- bomb-bay (small/medium size items) in underfuselage gondola (about the same location as the wing)
- large bombs on wing or back of outboard engine nacelle
- fuel tankage in wings and in fuselage (where civil passenger seating used to be)
- additional operational items – gun turrets, ammunition, etc distributed through out aircraft

As for Robert’s question of replacing the Fw200 with something else:
- Fw200 originally used – quite effectively – as maritime recce/low & medium level anti-shipping bomber. So it reported the convoys, gathered the U-Boats and made it’s own attacks.
- As shipboard AA guns increased in number and as firstly CAM-ships (with the one use Sea Hurricane catapult fighters) and subsequently escort carriers with reusable deck fighters, the Fw200 was used less as an attacking bomber and more as a pure recce aircraft.
- Ju90/290 aircraft would supplement the Fw200s, but there were never enough of these to totally replace the Fw200.
- He177 and Do217 aircraft would be used more as an shipping attack bomber (especially with the Hs293 and Fritz-X controlled guide bombs)
- As a military transport, the Luftwaffe had Ju52 and Fw200 for most of the war, supplemented later by Me323, early He111s and beute-aircraft like the French LeO 451 and Italian Sm82. As I understand it, there was never sufficient transports available to replace the Fw200.

So essentially, the Fw200 soldiered on……….(until shot down or it’s tail fell off). A number of Royal Navy carrier pilots did note the tail coming off the Fw200 while under attack. I don’t recall whether this was directly due to combat manoeuvres overstressing the rear fuselage, whether from combat damage or perhaps a combination. (I will see if I can dig out the relevant incidents, but it may take awhile.)


Regards,
...geoff

[Robcio]

Looking at the pictures of the accident aircraft FW 200 you will see the destruction of the fuselage (fracture) occurring during landing. Such damage can not be seen during a forced landing (without wheels). Strange, then there are huge forces of fatigue and overload. Conclusion: breaking the fuselage is probably due to improper weight distribution and center of gravity of the support points (wheel chassis), the formation of large bending moments.
But is this village is correct?

[bearoutwest]

Important to look at heavy landings with wheels down separately to force landings with wheels up (either crash landings on land or ditching into the sea).

Wheels down means the fuselage is supported on two points – 1st in line with the two main wheels, and 2nd on the tailwheel. The fuselage then bends between the 1st and 2nd. The weakest part of the structure suffering the most bending load is more likely to fail under this circumstance.

Wheels up landing means the fuselage is supported by many points of contact with the ground (or water), so the bending load is different and affects the fuselage less than with wheels down. If the fuselage breaks up, it may be from different reasons.

The weak point on the fuselage occurs not necessarily from improper weight distribution, but rather to the dimensions of the fuselage. The wider/taller sections of the fuselage will have more space to carry load and also have more spacing for more efficient structural design. The narrower sections of fuselage carry less load as they have less space, but will also be less efficient in structural design strength. At some stage the change in fuselage profile causes a weak point. With the Fw200 it was near the trailing edge of the wing.

I don’t know the actual design loads for the Fw200, but let’s make up some numbers to demonstrate what I mean.
- Say a Fw200 carrying 2000kg of bombs and fuel, is capable of managing a 4g pull-out from a glide attack (i.e. 4 times the weight in gravity), with a “proper and correct ” weight distribution. It should be able to do this 10,000 (perhaps 100,000) times without any fatigue damage to the structure. If it has suffered microscopic cracking during this time, then the weakened (cracked) structure might only be able of sustaining a 3g pull-out. The next 4g pull-out might weaken it more…….eventually it fails.
- Say another similarly loaded Fw200, has to pull 5g to pull-out of an attack. As the structure is designed for only 4g, it is now damaged (either cracked or stretched but differently to fatigue cracking). It may not necessarily fail immediately, but it may only now be able to sustain 3g as a maximum pull-out, etc. It will also eventually fail, if it continues to be used on normal operations.
- Say a third similarly loaded Fw200 has to sustain an 8g pull-out while avoiding a fighter attack (i.e. twice the design load). It may well fail immediately, even without any other damage from the fighter attack.

This pull-out load is based on centrifugal force, and bends the aircraft fuselage in the same manner as when it sits on the ground on its wheels. So heavy landings can also add to the damage.

Note that these are not the only forces acting on the fuselage, but are the only ones discussed in my “simplified” example.

[Robcio]

Thank you for the explanation of the issues.