AP 970 & AP 1208 Design Requirements

More invention

Its Type Record shows that there was much more to the design of the Tiger Moth than is suggested in a previous post.

AP 970 exists today (and has just been extensively revised) as Defence Standard 00-970.

Thank you for identifying that AP 970 persists as DS 00-970; it seems that this AP is the longest lasting official publication, from 1924 to 2014, so far. It would be good to get a bibliography of AP 970 with an index to topics across all the revisions through the years. It is a very practical document that shows what designers were thinking through the evolution of flight.

In respect of Tiger Moths, from Janic Geelen’s opera citare : ” It seems incredible that the highly successful Tiger Moth, the ultimate development of the Moth, should have evolved through such a trial and error process. The drawings were not even started until everybody was satisfied that the mock up was mathematically correct.”

To know this “trial and error process” get the most excellent book ! I think that perhaps what was happening was naked invention at the hands of competent and confident men. What can be found more reliably in the world are disciplined followers, and this, largely for good, is what constitutes the modern aerospace industry. To admit that a successful design arose from a form of chaos is anathema.

Bent ribs

I presume that you are not saying that all Tigers had these angled ribs?

On this subject however, the Hart Trainer had 5 degs difference in sweepback on the upper wing – did this go through life with angled ribs?

My understanding is that all Tiger Moths have angled (to airflow) wing ribs, being a legacy of the decision to adapt existing Moth Trainer wings as described. In this respect existing wing assembly jigs could be utilised across a range of aircraft, a practical thing to do in the middle of the Depression.

All Hart Family biplanes have wing ribs at 90 degrees to airflow, being designed from the outset with swept back wings. Only the ailerons have ribs angled to the airflow.

Its Type Record shows that there was much more to the design of the Tiger Moth than is suggested in a previous post.

AP 970 exists today (and has just been extensively revised) as Defence Standard 00-970.

A streetwise designer would however be aware that not following AvP970 would mean no government orders for the resulting design. Which is not to say that such didn’t happen, but not without considerable negotiation, considered justification and (likely) at least local redesign. AvP970 was not timid and didactic bureaucracy but a distillation of successful practice based on understanding things that didn’t work and why. To your list of contributors I would certainly add the RAE and perhaps the NPL.

My understanding is that the Tiger Moth was redesigned on paper – the proverbial napkin indeed (or was it table cloth?). There’d be no need to do it physically: why saw wings off a perfectly good aircraft when modification to the root fittings would do the job. The redesign also included moving the upper wing forward and hence longer inter-wing struts – not just an afternoon’s job with a saw. I presume that you are not saying that all Tigers had these angled ribs?

On this subject however, the Hart Trainer had 5 degs difference in sweepback on the upper wing – did this go through life with angled ribs?

Yes and No

The Tiger Moth was not ‘unregulated’ – it used AP970 as its design code.

I would not disagree with you but would reposition the reply in the context of timing and the reality of invention. Relying on Janic Geelen’s “Magnificent Enterprise : Moths, Majors and Minors” the 1934 DH82 Tiger Moth was an adaptation of the 1931 DH60 Moth Trainer. As aircraft designed for RAF use they had to comply with AP970, but developed from previous civilian Moth designs that would only be subject to good manufacturing practice, or lessons learnt. It is reassuring to see that even Geoffrey deHavilland had designs with wings that would break off. (DH 52)

I would see Geoffrey deHavilland, through the Society of British Aircraft Constructors and Royal Aeronautical Society, as both a contributing author to AP970 and working within a parallel feedback loop encompassing the factory floor, competitors, RAE and pure invention. For the Tiger Moth they seemed to have got stuck into a Moth trainer with a handsaw and some inspired eyeball engineering.

Having wing ribs that run at an angle to airflow over the wing is an inefficient design that takes 10mph from performance, but is proof that in one inspired session they just cut off Moth Trainer wings then stuck them back on with sweepback. There was no timid and didactic reference to AP970, and a streetwise designer would not author any government rulebook that was too limiting to the fluid conduct of survival in the prewar aircraft business.

Yes

In this respect was AP 970 the ‘bible’ for 1930’s aircraft design ?

Thank you for bringing this back from the dead ! Interesting to see the intersection between live design and the AP970 series.

Since the original post I have sighted a late 50’s AP970 dealing with the jet age and have come to the understanding that this sequence of APs did act as the minimum design requirement guideline for aircraft, both military and, by virtue of stressing data, civilian. What ultimately happened to the AP970 series postwar I do not know, as the main interest was prewar. Each AP970 catches in amber the design thought of the period, from timber truss to strip steel construction in the 20- 30’s to monocoque aluminium structures from 1935 onwards. The 1950’s AP 970 is a totally different beast to the 1930’s AP 970.

The thirty year old aircraft stressman of 1935 is now 110 years old so there will be few who could definately contradict or support any statement made on this page as to the role of AP 970 in 1935. I see the document as the effort of a group of design peers who sought to share hard earned lessons via a commercially independent medium as much as provide some rational context and filters for government buying decisions.

Everything is joined by a thread. Yesterday I sat over the wing of a Boeing and watched as slats extended for the landing sequence and thought of Frederick Handley Page and the H-P slot, the innovation of 1929. Hidden in the guts of AP970 1935 is a section dealing with additional strength allowances for wing spars adjacent to slats, self evident upon reflection, but probably more obtuse if I had not been made directly aware of it as a design factor. I wonder whose wings crumpled back in 1928 to let me know this when the invention was being developed. I wonder how many will stall and spin in tomorrow without knowing about the same questions and answers held in old yellow books. I asked an Aerospace graduate about Handley Page and watched as in the card index of the mind, between Pink and planking, there came back a blank. Perhaps they should teach Aerospace students about some of the heroes of the past. Folk who were less about rules, more about exploration.

There is an old HP company history which shows the first assembly works in a leaky old shed with a canvas drape to stop the rain leaking through the roof. I think it is a most encouraging thing to show to the young designer who will carry me into lunar orbit in 2035.

The Tiger Moth was not ‘unregulated’ – it used AP970 as its design code.

Found something re history of AP 970 in Profile Publications Aircraft Profile 256 Vickers Wellesley about the history of AP 970 “Unfortunately, the Vickers M.1/30 (S1641) first flown on January 11, 1933 by Chief Test Pilot J, (“Mutt”) Summers – was written off on November 23, 1933, because of structural overload during a high-speed dive. The cause of this accident to S1641 was eventually traced to a distorted airplane incident jack. One important result of this failure was a complete reassessment and revision of the tailplane stressing formulae of AP. 970 the official Air Publication 970 Handbook of Design Calculations and Requirements following detailed investigations of tailplane stiffness factors at the Royal Aircraft Establishment (ARE) Farnborough (Hampshire)”

[QUOTE=powerandpassion;2089893]I am trying to work out the topography of 1930s British aircraft design rules and would appreciate if anyone could fill in any gaps.

Things that go bump in the night….

By the way if anyone wants to talk 1919 – 1939 metallurgy I would be pleased to uncork a bottle and settle in…