Question re max diving speed of the Bristol Beaufort [Archive]

A37575

5th May 2010, 00:24

The story below is what prompted my question. The RAAF lost many Beauforts during training and in the Pacific war. Some were due to enemy action; others were thought to have been caused by carbon monoxide problems incapacitating the pilots. Some were thought to have been caused by elevator flutter and now as you can read below, others may have been caused by runway props and subsequent engine disintegration.
The following extract was a cut and paste extract second hand (emailed from a friend of mine) from a book called "The Beaufort Story" by an engineer Geoffrey Venn-Brown.

There are a few instances of perhaps slight exaggeration such as the claimed 50,000 hours flying hours of the Beaufort test pilot mentioned and that made me wonder about the rest of the story. Plus the 420 knots diving speed had me wondering, too.

He writes...

"...however I think I have come across another major reason for some of the early crashes, as follows:
"A very serious problem appeared, however, shortly after the Beaufort was put into service. Several incidents occurred when an aircraft would be seen to burst into flames when it dived, and then plunge into the sea with the total loss both of the plane and all the members of its crew. Deeply concerned, the Air Board embarked on an urgent investigation in an anxious effort to discover the cause, but it turned out that a long time passed before the explanation emerged, and sadly, more airmen met their deaths, in the meantime.

To begin its enquiry, Air Board focussed its attention at first on the structural integrity of the plane's wings and airframe, examining both the basic design, and the quality of materials used in the Australian manufacturing process. However this approach threw no light on the problem. Next, they put the new Twin Wasp engines from Commonwealth Aircraft Corporation under scrutiny, but again no clues emerged.

At this point a small party of three officials from Air Board made a special trip from Melbourne to Sydney in order to visit the factory of De Havilland Propeller Division, only for the purpose, they said apologetically, of informing us about the problem, and of asking us to make a check of our propellers "merely as a matter of routine". At the time it was perfectly obvious that no one attending that meeting including myself, at that stage held even the slightest suspicion that the propellers were responsible for the baffling misbehaviour under inquiry.

At that time my own role at De Havilland was Experimental Engineer for the Propeller Division, and in this capacity I became responsible for undertaking the investigation personally. In spite of my belief at the time that it was inconceivable that any feature of our propellers could be responsible for the reported disasters, I resolved nevertheless to probe every aspect of our manufacturing process thoroughly, and in meticulous detail.

I applied myself to the task immediately, but found nothing amiss at the manufacturing level. With the feeling now that I was doing nothing better than performing a formality, I then made a close study of the basic parameters of the propeller design itself, including both the mechanical features and the aerodynamics of its operation.

By reference to the propeller's characteristic design curves, and with the aid of additional calculations of my own, I was greatly astonished to discover that the accumulation of manufacturing tolerances could in extreme marginal cases actually result in a condition where the forces generated by counterweights (exerted upon the propeller blades for the express purpose of limiting the extent of their maximum. rotational speed) would be inadequate The effect of this would be that the propeller blades would become dynamically locked in fully fine pitch, leading to the condition of a "runaway" (overspeeding) engine. Catastrophic events would then develop within seconds.

I realised with a shock that now I had uncovered the real (though completely unexpected) cause of the problem. Even so, there remained the puzzle about the reports from observers that all the stricken aircraft had been in flames as they dived and crashed. The onset of overspeeding engines could be expected to have dangerous consequences, but what caused the fires? I asked myself (At that moment I did not know that not far down the track I was soon discover the answer to this question at first hand and under alarming circumstances). At the time however, it was satisfaction enough for me to have established that the problem was due to the inadequacy of the counterweights. I was convinced now that it was urgently necessary for these to be modified, by increasing their weight, in order for them to operate effectively.

Although Hamilton Standard Propellers produced amid their product options a range of counterweights provided with a range of different caps of varying weights, on checking the available information I found that the one used for the propeller they had recommended was actually the heaviest of the series. However, clearly it could not continue to be used, and it appeared essential to develop a new design in order to make a heavier version.

Naturally I reported these findings without delay, but to my dismay I was immediately confronted by uncompromising opposition: the counterweight caps were made from steel forgings, and my proposal was rejected out of hand on the grounds that in order to modify the design new forging dies would have to be manufactured. Not only would this require additional expenditure, but far more seriously, the exercise would cause delay to the production program. As the progress of the war against the Japanese invasion at that time had reached a very critical stage, any suggestion of delay was regarded as out of the question.

Young and inexperienced as I was, I did not know how to handle this situation. Lacking the confidence to make a big issue out of it, and conscious of the fact that my hypothesis was based largely on academic considerations and was as yet unsupported by hard evidence, I went back into my shell.

However my conscience was in turmoil: I felt extremely troubled and guilty at my failure to secure the corrective action which I felt confident would save more airmen from a horrible death. After fretting unhappily about the situation for about a week, I finally decided that I was simply not prepared to allow the matter to stand still ' at the first opportunity, and without informing anyone else, I sought out Tommy Young (the Test Pilot for Beauforts at Mascot who was at that time recognised as Australia's senior and most experienced pilot, with his record of well over 50,000 hours of flying experience with the original ANA commercial airline) and appealed to him for support. (I had flown in Beauforts on some other investigations with Tommy previously, on several occasions, and we had come to know each other well).

Having first explained my theory, I asked if he would help me get hold of some hard evidence in order to nail the problem down, thus placing me on firmer ground and enabling me to insist that the matter be raised again.. He listened intently, making very little comment, but agreeing without hesitation to include a special flight procedure (which I set out for him in detail) in his routine test flights on each new Beaufort off the production line, with the aim of exposing any suspicious propeller behaviour which might point to inadequacy of the counterweight caps, and thus lend credence to my belief

One sunny afternoon a week or so later, Tommy called me by phone and announced that he had just finished testing that day's Beaufort, and thought it might be the example I was seeking. If I could come to Mascot without delay, he suggested, there would be just enough time left for us to make another flight in it together.

I rushed from my office in Doody St Alexandria forthwith and made my way to the Government Aircraft Factory flight hangar at Mascot, to see Tommy standing there impatiently, waiting until AID (Aircraft Inspection Directorate) inspectors had completed the mandatory pre flight check.

"Hurry up!" he called across the flight hangar as soon as he saw me enter, "There's just enough time to get another flight in to day" In this atmosphere of pressure from the much older man I did not feel that I could enter into a conversation to rehearse with him the procedure I had in my mind, and struggling awkwardly into our parachutes we clambered up into the cockpit, settled down into our seats and taxied out at a fast clip to the end of the runway; it was already well into the afternoon, and the sun would soon begin to sink. Taking off immediately and climbing steeply, we quickly reached the modest height of only 650Oft, where, without warning and to my horror, Tommy pushed the nose of the plane over abruptly into a steep dive under full cruise power.

(In aflash I realised instantly that I had made a hideous mistake! Clearly I had failed to communicate with him properly when I gave him the preliminary briefing recently about the flight procedure which I envisaged for this test flight. It was obvious now, however, that I had carelessly omitted any reference to height through at the time it was crystal clear in my own mind that before performing the test it would be prudent to climb up to a relatively high altitude say around II000 feet, in order to have at least some chance to escape disaster should we find ourselves faced with the same conditions that had led to those unlucky Air Force crews losing their lives. I cursed myself for a fool now for this blunder but of course there was no way now to undo the error!)

The events of the next few minutes were dramatic. Instantly both engines screamed far beyond the maximum speed of 3050rpm, allowed for take off. In my anxiety to make a report of events, I began at first an attempt to record instrument readings on the notepad strapped to my thigh, but this ambition was crushed immediately by the utter chaos now unfolding around me. I observed that the starboard engine had quickly built up to a terrifying 4I00 rpm; but from this point on I was much too distracted to register what the other engine was doing or what other instruments were indicating and in any case it hardly seemed to be of any consequence now! The airspeed indicator indicated that we were already exceeding the maximum permissible diving speed of 430 knots and the cockpit was filled with ear splitting noise coming from both the overspeeding engines and the banshee screaming from the tips of propeller blades which were now slicing through the air at speeds far exceeding the sound barrier.

A sharp report like a rifle shot penetrating the general bedlam signalled that the external radio mast had snapped off (but we discovered this connection only after we made our landing) and in the same instant I became conscious of a fierce draft coming up vertically from beneath my seat, (the consequence, as we discovered later, of a large panel being ripped away from the exterior skin of the fuselage. It disappeared somewhere in the sky over the Botany area.)

Now I was starting to brace myself for what appeared to be the inevitable final disaster. At 3000 feet we were hurtling almost vertically downwards towards the ground now, and travelling well above the aircraft's maximum permitted diving speed. Shortly after this I glimpsed the altimeter needle, unwinding at an alarming speed, pass through II00 feet, and through the transparent window of the bomb aimer's position in the nose I could now clearly see small details of a tranquil-looking grassy paddock directly below us.

The ground was looming up fast now, a mere thousand feet below My gaze fastened on the patch where we seemed certain to strike in only seconds more, when I felt my body gripped by violent pressure and I blacked out.

Consciousness returned, accompanied in the first brief moments by a curtain of red haze drawn across my sight, which then cleared swiftly to reveal a welcome vista of flawless blue sky extending across the peaceful expanse of heaven. My puzzled disbelief at this sight was instantly replaced by an overwhelmingly thankful wave of relief.. I realised then that Tommy had somehow performed an impossible feat, risking and incredibly winning a desperate gamble by brutally pulling the plane out of the hideous dive at what surely had to be the very last possible split second.

Vibrating roughly and moving erratically (not a word being uttered by its shaken occupants) the plane flew uneasily back over the few miles to Mascot, where it bumped and lurched to a halt after a rough and unruly landing, gouging a deep furrow in the grass just off to the side of the runway. (That scar remained there for many years afterwards. whenever I saw it I was reminded again of the fateful events which led to its formation).

The stillness and quiet were palpable when the engines were finally shut down. After a brief silence I regained my Wits and flew at Tommy :angrily: "Why on earth didn't you throttle back the engines the instant they started to overspeed ! ? You should have been prepared for this you knew it was likely to happen!" (It was with some difficulty that I restrained my impulse to rebuke him also for forcing us into the dive prematurely from an unsafe altitude, for I felt really angry about that too. Fortunately, however, my conscience held me in check, reminding me that I myself was at least partly responsible for this error).

A normally mild man, Tommy swivelled in his seat, turned right around to face me with a glare and barked " Obviously you don't understand much about aeroplane engines do you? ! You just don't realise he went on," that if I had throttled back the way you have just said, the sudden drop in gas pressure on top of the pistons would almost certainly have caused the connecting rods to break and the bits of these would have shattered the cylinders ".

Those few words pierced my consciousness like a blinding flash of light, and left me speechless. Suddenly, and dramatically, the elusive solution of the puzzle of the burning Beauforts was revealed to me :-

As an experienced veteran pilot, Tommy was well aware that catastrophe was likely to result from handling engines incorrectly in a situation like the one we had just survived. We had escaped certain disaster only thanks to his experience and wisdom. By contrast, though, the unfortunate RAAF pilots who had suffered fiery death were all relative novices, with only brief experience of flying (most of them had had considerably less than 50 hours flying Beauforts). With understandable panic when their engines over speeded as they went into their dives, these raw pilots reacted instinctively by throttling back their engines (as indeed, in my own ignorance, I would have done myself in similar circumstances). Their mistake caused internal damage to their engines, and this resulted in the fires and fatal consequences
. .
As anti climax to the afternoon's drama I drove myself shakily back to De Havillands in the fading light of the late afternoon, and rushed up the stairs to Major Murray Jones' office, where I burst into his room un announced to find him still at work, though by now twilight had already fallen, and no other staff were to be seen.

Totally forgetting my manners, and dispensing with all preamble, I strode up to his desk, pounded on its top with my fist and blurted out breathlessly - "We… have...to….change…….those……..counterweight caps…..we MUST change those caps !"

Fine man that he was, and realising that I was close to hysteria, Murray Jones ignored my uncontrolled behaviour, spoke to me quietly and finally succeeded in calming me down. As I regained some composure and managed to unfold my story, he listened intently, while his expression grew more and more grave, until finally, without further comment, he said he could understand the problem clearly now, and would certainly assert his influence to ensure that the necessary changes would be authorised by the relevant authorities and implemented forthwith.

He proved as good as his word; immediately following this disturbing episode I received instructions to re design the counterweight caps

Actually the task turned out to be somewhat more difficult than I had anticipated when it became clear that there was only very restricted space available to allow larger counterweights to swing freely past the profile of the "barrell" (the steel shell which enclosed the mechanism of the propeller hub). Also, the limitations made it impossible to devise a new cap large enough to contain the larger quantity of lead required to attain the necessary weight. I was aided in my search for a solution to this problem when I consulted my erstwhile mentor at Sydney University, Professor Eastaugh, who suggested that I might consider using the alloy known as type metal, (which was heavier than the lead used for counterweight caps by Hamilton Propeller) , owing to its content of the metal antimony. This turned out to be a satisfactory answer to the problem.

Following revision of the design, the work of altering the tooling used for manufacturing the drop forging components was put in hand, and within the short space of nine weeks since that eventful flight with Tommy Young, all the necessary remedial action had been completed, including modification of the propellers which had already been made in Australia by DeHavilland for Beauforts.

Ever since that traumatic day I have remembered the debt which I will always owe to Tommy Young, and my gratitude for his ability and presence of mind. Apart from the escape from a potentially fatal crash, it was a further testimony to his airmanship that he managed to get the battered Beaufort back to the airfield and land it safely with no further mishaps; in addition to other damage the fuselage was found to be twisted out of alignment by the savage forces imposed on the structure by the vicious recovery from the dive, so much so that the entire aircraft (including its engines) was declared unrepairable, condemned and sent to the scrap heap."

BY ENGINEER GEOFF VENN-BROWN "THE BEAUFORT STORY"