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

[Observer1940]

I have acquired an old Armstrong Whitworth Aircraft Ltd book and in the part about fuselages it states regarding the Whitley:-

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

Are there any aircraft engineers out there, as I would have thought that an aircraft must meet, or exceed 100% of its design load to be airworthy especially when it would be required for AWO (all weather operations)?

Although I understand the design loading is higher than the design load for level flight, due to the necessity to be able to dive the aircraft, pull the aircraft out of a dive, make manoeuvres and turn, it is clear that the Whitley failed on test at "98% of the design load."

Also you have weather forces, icing and turbulence to contend with too.

Any comments?

Mark

[Bill Walker]

In general, this is not a good thing. however, reports like this are quite common. We need more information on the test and the follow up before we declare the Whitley a bad aircraft. In my experience, test results like this can be refined by more detailed analysis of the real design load, and the test setup. I had a wing spar fail at apparently 98% load back in the 1970s. A detailed review and recalculation of the test setup, minus some time saving shortcuts used in the initial test design, convinced us the spar had actually failed at 101%.

Also, failures like this can lead to minor redesigns of the failed area, and then further testing or even just analysis to prove the structure airworthy. They can also lead to minor changes in the flight envelope (like speeds and g-loads) until a fix is completed on the whole fleet. Again we can't condemn this aircraft without knowing if this sort of subsequent activity took place.

One also needs to know the purpose of the test. Was this part of the original qualification testing? Was this an early development article, with the test results being used to refine the design? Was this a high time in-service airframe? There are a number of possibilities, all with different implications of this failure.

[Observer1940]

Bill

Thanks, for those interesting comments you make, especially your comment, about when an aircraft weakness was discovered, it was then likely subsequently modified.

However, a file in AVIA 46 produced at our National Archives Kew, for the Air Ministry Whitley Aircraft Contracts was originally PA to Archives for permanent preservation, but susequently weeded and then PA to Archives twice again, with the numbered Minutes (Index Sheet) relating to correspondence of the early Whitley aircraft contract life struck out with "destroyed" written in, with the corresponding file parts removed.

A question regarding the Whitley was asked in 1940 to the makers Armstrong Whitworth about the dive speed restrictions on the Whitley.

I have seen a genuine copy of that 1940 reply from Armstrong Whitworth stating that the restrictions were due to the rear of the Whitley aircraft and applicable to all Whitley Marks with the Merlin engine. Regarding the restrictions it states:- "The limiting strength consideration is the rear fuselage in bend, probably just aft of the spar."

I have got some photographs of a Whitley V which has managed to force land, but the rear elevator and rudders have completely collapsed and the empennage damaged.

Mark

[Bill Walker]

Mark, any airplane will break if you push it hard enough. The description of the AW's concern (fuselage bending just aft of the spar) doesn't match your description of rudder, elevator and empenage damage. These were likely two unrelated cases.

It is possible that AW decided it was quicker and cheaper to limit diving speed than to reinforce the rear fuselage after the test failure. Given the pressures of the time, the RAF appears to have agreed. This goes on a lot, even today. The "design load" of any component is based on some worst case combination of speed, power, g-loading, etc. The combination is sort of arbitrary, and delivering an airplane with some design case at 98% of what it started out at may not be a bad thing. If everybody involved had taken the time and money to strengthen the rear fusleage to get back to the original design load, there is still no protection for anyone who goes over the design load conditions in actual service. Something else would have broken, somewhere else.

[AndreasB]

98% is obviously not great, but it's not a desaster either, and without knowing if any measures were taken afterwards to bring it to 100% or above (such as with the Airbus A380 that failed the wing load test at 147% instead of 150%, or 98% of test requirement), it's impossible to say whether this had any impact in the real world. Even without mitigating measures it might not have had any impact, since you would need to know first what design load actually was (and in the era of pre-computing, I believe engineers were far more generous with margins than they are today). For example, (making this up) design load could have been 200% of maximum expected load in a dive at maximum angle. In which case 98% is still more than fine.

Please note that none of the above is supposed to argue that the Whitley was a great plane.

All the best

Andreas

[Observer1940]

Hello Bill and Andreas

A Whitley aircraft was twice as likely to be lost on ops than any other Bomber Command aircraft, simply by taking avoiding action.

Twice as many Whitleys were lost over the German searchlight belt according to a 1942 Op Research Section report.

I managed to get a copy from the AHB several years ago (still withheld in the UK), but the Canadians have put the report online along with a summary, click on the link for a pdf copy, which you can save.

http://www.canadianmilitaryhistory.c...operations.pdf

The RAF Whitley was twice as likely to crash due to simply taking avoiding action, diving, turning, or a combination of both, so there must have been something peculiar to the Whitley aircraft itself, in the design?

Mark

[Juha]

Or simply because Whitley was less manoeuvrable than the others and clearly slower than Hampden, it was easier target to AA guns and to night fighters which operated inside those S/L zones. And thanks for the interesting link!

[Graham Boak]

Fascinating note, certainly, but in no way does it justify the phrase "twice as likely to crash simply taking avoiding action", as it says nothing about crashing nor about taking avoiding action! Nor, given the lack of individual detail about the losses, could it. It states that a Whitley is twice as likely as any other type to be shot down in these circumstances, but offers no explanation for this.

I think Juha has stated likelier reasons for these losses. Had there been any specific weakness in the handling or structure of the Whitley this would have shown itself in other ways, and evidence would be available from other sources - raised losses in training or general operations, for example.

It might be interesting to find when the corkscrew became a standard evasion manoeuvre, for the Whitley was something of a old lady for such a violent manoeuvre, but the result is likelier to have been a somewhat stately corkscrew rather than design load exceedance.

[Observer1940]

Paragraph 3 (1st part). “It seems probable that the lack of manoeuvrability of the Whitley and the consequently greater difficulties of getting out of searchlights when once illuminated contribute very largely to the greater losses of Whitley bombers as compared with other types. Whitleys on the other hand appear quite suitable for attacking targets which do not involve crossing the searchlight belts.”

Paragraph 3 (2nd part). “When the Whitley aircraft do not cross a searchlight belt, their losses are about the same as the losses for other aircraft or less than the average for other types.”

So if it was that Whitleys were slow to get out of the way and more got shot down as you suggest, over these searchlight belt areas, then why did this higher statistic of Whitley losses (compared with other bomber types) not follow on other parts of the route to and from the target (where all bomber types were exposed to AA)?

I felt the report distinctly gave the impression that the greater losses of Whitleys than other bomber types was due to evasive action in getting out of searchlights.

1. I do not feel that the report is saying that more Whitleys were victims to A.A., but either way it is still flying limitations, due to design considerations.
or
2. That Pilots did make an attempt to turn quicker than the flying limitations permitted and the Whitley dived away and became a statistic.

A Test Pilot and several Whitley Pilots according to a 1941 report obviously did get back to report parts of the main plane skin torn away.

The Test Pilot turned the Whitley which stalled in the turn, then dived away. However the Test Pilot managed to regain control and land. The damaged Whitley main plane is pictured and you will notice that the structural bracing in the Whitley wing has begun to break apart.

Some of the RAF crews who found themselves in a severe dive and got back, managed to get the elevator to shift and regain control by applying trim tab.

However, when this obviously failed and the tabs / elevator were found damaged in a crashed aircraft, the Pilot was apparently sometimes blamed for having too much trim tab.

In November 1941 Bomber Command commissioned an RAE Engineering Report into diving speed limitations. The Whitley design only permitted a maximum dive of 240 ASI, but Pilots did not like to exceed 200 to 220 ASI.

She may have given the impression of a fine old lady, but Pilots who got back were reporting that the Whitley V was impossible to pull out of the dive, requiring strengthened Pilots cabin floor, strengthened control column floor bracket and strengthened control column pivot pin and other structural mods to the main plane, fuselage and tail. It was clear that huge pressures must have been exerted on the control column.

Other Whitleys were seen/heard to turn and dive, or dive to the ground at varying angles in the UK and that is all we know about the last moments.

Mark

[Juha]

Point IV is advaisable in many planes. Using powerful trim tab to get out from high speed dive easily overloaded a plane.

For natural reasons AA guns and night fighters early in the war were concentrated to S/L zones, not much idea simply get the plane into S/L cone and note "there it goes". Especially for night fighters operating at S/L zones time was paramount. They must see the target, position themselves for the attack, attack and shot the bomber down before it departed from the zone to the dark night.