Whitley test "produced a failure at 98% of the design load" is that good?

[Bill Walker]

From everything I've read (including this latest information) there is no doubt that the Whitley was an old design, obsolete before the war began. This probably accounts for the large losses. I'm missing the link between this and the original question about failure of a static test.

[drgondog]

A couple of observations. Implied but not stated is that the 'Failure' at 98% was during an Ultimate Load under static conditions. Assumption not fact (for me) is that it was a static wing load test with dead weight sand bags distributed like an assumed loading for a dive pullout or coupled with the fuselage in some way to predict symmetric bending loads on the fuselage for a pullout or even ditching/hard landing..

Nobody in the airframe business was attempting to analyze asymmetric load conditions except for possibly lateral rudder/vertical stabilizer loads during a yaw condition during dive. Offhand, such analysis could yield useful insight to the fuselage failure modes while taking out the empennage loads in bending and torsion at the carry through structure/longerons.

98% sounds pretty damn good to me but I'm sure the structures chief would have preferred 102.

[Bill Walker]

Don't ask me for a reference, but I seem to recall asymmetric loading being discussed at the university level in pre-war papers. (The classic rolling pull-up.) Whether it had made its way into RAF requirements is another question.

In today's terms, ultimate load can be 150% design load or 150% service load. As I said, without knowing more it is hard to draw firm conclusions about this test result.

And I have had more than one boss who would tell you that failure at 102% meant the @#$%& thing was too heavy!

[drgondog]

Bill - I don't doubt that there was a reference to bending analysis of asymmetric loads in text, but curious how the aero loads would have been simulated accurately.

It is relatively straight forward to calculate the sectional moments of inertia for the fuselage/tail station by station but quite a bit more to look at a moment by moment load/fuselage response with no finite element/relaxation methods applied via computer simulation in the 30's (or 40's or 50's or early 60's)...

having said that my practical experience didn't start until late 60's so my comments are to be taken with grain of salt about WWII methodology-

Ditto on the presumed reaction of airframe structures chief when confronted with 2% above design load structural integrity.

And ditto on Limit Load @ 100% Design Gross Weight for the configuration/condition analyzed with 150%. for Ultimate.. Simple rules for steel/aluminum with homogeneous metals - with departure when stress/strain curves were not linear in elastic range.

[Bill Walker]

My exerience started in the late 1960s as well, I'm going by conversations with the old hands. Maybe I'm over simplifying, but dynamic load cases can be treated as quasi-static, with inertia forces added to the free body diagrams. Statically indeterminate loads were handled by hand iterative calculations way back before airplanes, I think early iron boat hulls were analyzed this way. When I started writing Fortran code in 1970 to do this the process was well understood. Computers just replaced rooms full of clerks with slide rules or mechanical calculators. Look for a book called "Slide Rule" by Neville Chamberlin for an interesting description of hand cranked finite element analysis of dirigible structures in the 1920s. Lots of people, using lots of paper and lots of pencils.

I think the limit here may have been in defining the aero loads to apply, as you suggest. If you look at old airworthiness codes, it seems to have been a fairly emprical process. "We know airplanes THIS strong don't break, so make yours just as strong". When that failed, the required loads were upped a little bit in the next amendment. Even today, the fine print in FAR 25 loads state that rolling pullouts may not be covered with great accuracy - so the manufacturers tell you their published g limit is for symetrical loading only. In other words, don't do rolling pull outs in your airliner.

Having said all that, the designer really has no special insight into what is required in an airplane. They have to rely on the customer (like the RAF for the Whitley) asking for the right things. Wrong assumptions can lead to perfectly designed wrong airplanes. It has happened over and over.

[drgondog]

Very well summarized..

Interesting on the Fortran. before I was immersed in Nastran in late sixties - I co-op'ed at NASA and Lockheed. At NASA I programmed the LoW Orbital Degradation model, first on an IBM 620, then 7090 (I couldn't imagine a Bigger Machine at the time) and was introduced to Bessel and Hankel Functions applied to spheres or oblate Spheroids for first time.

Even when building 'real' airframe models with NASTRAN, using RODS as beams, and shear panels (PLATES way too stiff), getting either static or dynamic loads wasn't in the application at the time..

[MarkRS]

"A full scale test at R.A.E. produced a failure at 98% of the design load."

The significance of 98% depends on the definition of "design load". If it meant the highest expected operational load, then they were in trouble. If it meant the test load including a safety factor on the highest expected operational load, then they were fine. 2% was well within the noise of their calculation accuracy. Pre-war, the standard safety factor for metal structures (i.e. steel) was 2x. This was reduced in the war by decree to 1.6x to save materials. I am sure that wooden structures would have had a higher safety factor to take into account the non-homogenuity of wood. This would be one of the advantages of using plywood which can be considered homogeneous, at least in 2 dimensions. So 98% of 2x is still 1.96x, an insignificant degradation. I doubt that there is any corrolation between this report and the combat losses.

Mark

[Observer1940]

Thank you all for your comments.

Found this http://www.flightglobal.com/pdfarchi...0-%201470.html but it is 1949. Whitley spec was 1934. I did find an ARC or RAE article, but cannot find it now.

The Captain and crew of a Whitley V survived a forced landing when their tail beam (beam which also holds the rear wheel) failed at the end of July 1940 and within two weeks another occurred at Boscombe Down. By December 1940 a modified tail beam kit had been designed by AW and was being supplied to 4 Group and Contractors. Any weakness appeared to be in the tail.

Later in WW2 a second Whitley tail beam modification was supplied to supersede the first modified Dec 1940 tail beam.

Mark

[Juha]

Hello Mark
you are surely aware that John Nesbitt-Dufort in his Scramble (London 1970) gives a short description (2 pages) what it was fly Whitley Mk V in ops.

Juha

ADDITION: ans not surprising a more positive analyze by Charles Turner-Hughes (former Chief Test Pilot, AW) in Air Enthusiast Number Nine.

[Observer1940]

Thanks Juha

I have another article about AW Test Pilot Turner-Hughes, but did not know about Air Enthusiast No.9.

The other reference was also unknown to me.

Another photograph in December 1940 Flight shows all of the wing skin virtually stripped clean off a Whitley wing (due it claims) to A.A. fire? But the skin looks too cleanly ripped off to be A.A. fire!

My Grandfather was in one of the tail failure Whitleys, which force landed!

Thanks again, Mark