Destruction due to turbulence when below Va - how?

Howdy!

In article 01PLc.162096$XM6.47385@attbi_s53,
William W. Plummer wrote:
Capt.Doug wrote:
[snip]

Other planes break apart in different ways. The T-34 has been in the news
quite a bit lately because of wings falling off. It appears that the tail
isn't breaking. The cause is attributed to metal fatigue from repeated large

No. I've seen discussion that suggests the problem is operator error. Rolling
pullups can, apparently, generate excessive g-loads with remarkable ease.

[snip]

Metal fatigue, cracks and construction defects are not caused by
turbulence although turbulence may be the straw that breaks the camel's
back when those problems exist.

IIRC the Convair Electra was the first plane that metal fatigue was
determined to be the cause of its wings coming off. And, it took
years. What caused the fatigue? Gyroscopic motion of the wings.

Not quite. Damage to engine mounts would lead to whirl mode oscillation of
the engines, which would resonate with the wing, inducing flutter, leading
to wing coming off.

yours,
Michael


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"William W. Plummer" wrote

Metal fatigue, cracks and construction defects are not caused by
turbulence although turbulence may be the straw that breaks the
camel's back when those problems exist.

IIRC the Convair Electra was the first plane that metal fatigue was
determined to be the cause of its wings coming off. And, it took
years. What caused the fatigue? Gyroscopic motion of the wings.

Turbulence most certainly is the cause of fatigue in the wing
structure of aircraft. I would suggest that you read FAR Part 25
to understand how the nature of the Standard Atmosphere and the
cruise speed is used in designing a "fatigue life" in transport
category aircraft.

"Convair Electra"??? No way...my Electra time was flown in a
Lockheed Electra..L-188 and it's US Navy derivitive the P-3 Orion.

The civilian L-188s failed early in their service life, Braniff's
Flight 542 crashed in 1959, only two months after it's delivery
from Lockheed. The Northwest Flight 710 crashed in 1960 after
only one year in service.

It was not "gyroscopic motion of the wings" nor "fatigue".......

"On May 12, 1960, Lockeed President Bob Gross announced that both
airliners broke-up due to an undampened propeller whirl mode that
produced destructive flutter of the wing."

This from the "Great Airliners Series, Volume Five, Lockheed 188
Electra" by David G. Powers.

You disappoint me William.

Bob Moore
Air Florida L-188 1973
VP-46 P-3B 1965-67

The design limit is not really lower for negative G. Remember that you are
starting at +1G so, if the limits are +3.8G and -1.8G, the limits are both
2.0g away from steady state flight.

Mike
MU-2

"Peter" wrote in message
...

I think I understand the reasoning behind Va, the max maneuvering
speed, being that the wing will stall (and thus dispose of the
loading) before it breaks. This is why Va falls as the weight falls,
because at any given IAS a higher weight takes the aircraft closer to
stall already.

So, how is it possible to have aircraft destruction due to weather,
e.g. flying into a strong updraught in a CB, if flying below Va?

A DOWNdraught would do it more easily because most aircraft designs
have a lower design limit for negative G.


Peter.
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Mike Rapoport wrote:
The design limit is not really lower for negative G. Remember that you are
starting at +1G so, if the limits are +3.8G and -1.8G, the limits are both
2.0g away from steady state flight.

Mike has the idea. You need to look at a V-g/V-n diagram for your
aircraft to see where the load limits lie for a given airspeed. This is
how Eckalbar explains it.

"Peter" wrote in message
...

I think I understand the reasoning behind Va, the max maneuvering
speed, being that the wing will stall (and thus dispose of the
loading) before it breaks. This is why Va falls as the weight falls,
because at any given IAS a higher weight takes the aircraft closer to
stall already.

There was a long thread in January, "Va and turbulent air
penetration speed" which you ought to be able to find
on Google.

The short answer is that flying below Va (even at max
weight) doesn't guarantee that the wing will stall before
exceeding the load factor. Va is defined in terms of
what control surfaces can handle, not when the wings
will fall off; see for example FAR 23.423 and 23.335.

Tony Cox wrote:

"Peter" wrote in message
...

I think I understand the reasoning behind Va, the max maneuvering
speed, being that the wing will stall (and thus dispose of the
loading) before it breaks. This is why Va falls as the weight falls,
because at any given IAS a higher weight takes the aircraft closer to
stall already.

There was a long thread in January, "Va and turbulent air
penetration speed" which you ought to be able to find
on Google.

Can I suggest that avoiding turbulent air is simply the most pragmatic
approach?

It's broken enough airliners for starters !


Graham