Are we getting away from the original "issue"?
The assertion that "a metal covered wing is stronger than a fabric one"
is a bit of a "red herring". So long as the original strength was
suitable for the purpose intended, extra strength is like fuel carried
in excess of the appropriate "minimum"...unless an exceptional event
occurs the extra weight added by metal wings reduces operational
efficiency and the airfoil change adds frontal area (drag).
Also, there are persons who should be physically restrained from
handling tools...let's call them "mechanically challenged". Any
mechanic with much experience has seen the work of such persons on
Ercoupes before. It would seem well within the realm of possibility
that there are some metal covered wings "out there" that are less safe,
in terms of strength today, than the "average" fabric-covered wing.
State of the art glider wings are incredibly strong (i.e. for the
purpose intended). If "fashion" were a determining factor these would
be metal today too.
The DC-3 cruises at over four times the speed of the Ercoupe and its
contemporaries. Mentioning the 747 (in terms of sheer size) and the
F-18 (in terms of being designed for combat) merely makes a total of
three airplanes inappropriate to compare with Ercoupes. Quoting from
the Service Manual: "In its outward appearance the Ercoupe resembles a
commercial or military airplane. But in servicing it cannot be
treated in the same light as heavier aircraft, for the metals
used...are of much lighter gauge. ...these solidly designed
light-weight Ercoupeparts are relatively inexpensive to manufacture..."
There is no doubt that aircraft design made great leaps between the
late thirties, when the Ercoupe was designed, and the end of World War
II. Even so, neither Fred Weick nor Erco apparently believed it
necessary or desirable to change the wing design from a fabric covered
one to a metal covered one. The difference between a safe wing from
one with a fatal flaw is likely independent of how it is covered, and
most usually evident in hindsight.
It may well be that it was easier for Forney, Alon and Mooney to use
metal wing covering because between 1946 and 1956 the pendulum of
skilled labor swung from doing fabric to metal for covering wings. Why
Monday morning quarterback the issue some 40-60 years later?
It would be my considered opinion that either fabric or metal wing
covering on an Ercoupe is intended to and does contribute to the
structural capability of a complete wing under normal stresses of
flight. I would not speculate as to how much less strength an
uncovered wing has in comparison to a covered one. Obviously both lift
and structural integrity are equally essential to go and remain aloft.
Regards,
WRB
--
On Sep 10, 2009, at 14:54, Hartmut Beil wrote:
Guys,
Why is it so hard to admit a simple technical fact?
If cloth covered wings are so superior, why are they so out of
fashion? Every load carrying aircraft is using a rigid wing covering.
Would you accept a cloth covered 747?
Or can you picture a F-18 with cloth covered surfaces?
Even the DC-3 has metalized wings.
There must be a reason for that.
One praised characteristic of fabric is that it withstands hail. That
is because it flexes. Metal wings are riveted to the ribs, thus
creating pretty stiff boxes that as a box transfer the forces on the
wing directly to the ribs and spars. If that is a good thing maybe
discussed, but one has to admit the different features of each design.
One thing is clear to me. A fabric wing should be carefully recovered
by a knowledgeable person.
As an amateur, I would not be able to decide between a safe wing and
a wing that could kill you.
http://www.ntsb.gov/ntsb/GenPDF.asp?id=FTW89DPJ02&rpt=fi
(for those that can't open the accident file:
N99636 Ercoupe (eng & Research Corp.) / 415C, 2 fatally injured,
INSPECTION REVEALED THAT THE FABRIC OF BOTH WINGS HAD BEEN INSTALLED
IMPROPERLY, AND PART OF THE FABRIC ON THE LEFT
WING HAD COME LOOSE IN FLIGHT.)
As previously stated, finding someone that does the covering right
might be a challenge sometimes.
On the other hand it is easy to find someone that can do a great job
with sheet metal and rivets.
Hartmut
To: [email protected]
From: [email protected]
Date: Thu, 10 Sep 2009 13:54:13 -0500
Subject: Re: {Disarmed} [ercoupe-tech] Metal Wings are stronger.....
I would suggest that any engineering evaluation use the correct
approach in evaluating "stronger".
Adding thin sheets to a wing structure already designed and capable of
coping with the normal stresses of flight (plus a considerable safety
factor)would add little, if any, to the overall load carrying capacity
of an Ercoupe wing in terms of the additional stress said could carry
before it failed under load. The materials and procedures utilized by
the various STCs presently installed or available does not transform
the Ercoupe wing designed for grade A cotton covering into a monocoque
structure as would seem to have been inferred.
"Monocoque" construction is defined as "an aircraft structure in which
the outer covering skin carries all or a major portion of the
stresses".
The internal structural components, metal skin thickness, rivet type,
size, and spacing of a stressed skin wing design would be quite
different, and likely lighter.
Consider also these questions:
1. What practical purpose is served by increasing wing strength
beyond regulatory design requirements?
2. How much has the load capability of Ercoupe wing design increased
when covered with modern synthetic fabrics of higher tensional
strength abd durability than the original grade A cotton?
Regards,
WRB
--
On Sep 10, 2009, at 13:05, Art Langston wrote:
Chris, could you please share the engineering data with us and cite
the documentation where we can read it?
When finished, I believe the Polyfiber may actually be stronger than
the aluminum. I've de-skinned Ercoupe wings, and the metal is very
thin. Polyfiber is tough.
Thanks
Art N2666H
Chris wrote:A metal wing is stronger than a fabric one. The metal
distributes the loads over the surface. Would anyone want to consider
a fabric covered fuselage?
Chris
Santa Cruz
99674
‘--o-O-o--’
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