Pic of the Day

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KW Mitchell
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Re: Pic of the Day

Post by KW Mitchell » Tue Apr 28, 2009 10:37 am

Bill's pic' of the forward section of the wrecked Orpheus is interesting and illuminating. The relatively minor damage to the first compressor stage does indeed confirm that the engine was throttled back on impact.

My current researches of the subject both biographical and technical - though currently incomplete - lead to the conclusion that removal of the nose-down moment i.e. the force acting downwards at the bows when DC came back off the throttle was the single most important factor contributing to the crash.

All evidence points to the fact that although stability margins were very low at the estimated peak speed of 320mph+ due to tramping, pitching, reduced mass moments (low fuel), aerodynamic asymmetries resulting from spar damage, possible engine gyroscopic effects etc., as long as DC '----had his foot to the boards ----' BB was coping with such exigencies.

There is video evidence to substantiate such - particularly in relation to pitching (which appears to be much underplayed in most analyses in my view) - the links to such I will post, when I have them all to hand.

Of course, once DC let his foot off, the Orpheus' overwhelming thrust moment disappeared and the laws of aerodynamics would not be denied the cataclysm that followed --------!

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Renegadenemo
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Re: Pic of the Day

Post by Renegadenemo » Tue Apr 28, 2009 10:57 am

There's little doubt that losing the thrust was the final straw but why was it lost? Fuel starvation? And if so, what caused that? I remember once having a lengthy chat with an eminent aerodynamicist who'd modelled the whole affair right down to how air rolled over the water surface from the surrounding fells but when asked how he'd modelled Donald losing his nerve and jumping off the throttle it turned out to be something difficult to express mathematically.
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KW Mitchell
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Re: Pic of the Day

Post by KW Mitchell » Tue Apr 28, 2009 11:25 am

Yes - my arguments assume that DC deliberately closed the throttle and, of course, that will be debated in perpetuity as we don't know conclusively.

If we stick with the solid physical evidence that thrust was removed - or lost - then factors such as fuel starvation enter the equation. The flame-out at the end of the first run in '67 lends credence to the notion that all was not right with the Orpheus' fuel supply --------------.

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Renegadenemo
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Re: Pic of the Day

Post by Renegadenemo » Tue Apr 28, 2009 11:28 am

I'm still not completely sold on the engine flameout having ever taken place but safe to say the fuel system was a disaster waiting to happen.
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Mike Bull
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Re: Pic of the Day

Post by Mike Bull » Tue Apr 28, 2009 11:55 am

Whether Donald lifted his foot or the engine spluttered out by it's own means, you'd have thought he'd have said something on his commentary? Or was it all too quick- the engine loses thrust, 'I've got the bows up!', etc. How quickly would the engine have spooled down to a speed where it shows no water damage, as being discussed?

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Renegadenemo
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Re: Pic of the Day

Post by Renegadenemo » Tue Apr 28, 2009 12:13 pm

Those little turbojets spool down very quickly, the rotating assembly doesn't have a great deal of energy so with no drive from the power turbine it'll all come to a standstill in a hurry. Bit like a turbocharger.
I'm only a plumber from Cannock...

"As to reward, my profession is its own reward;" Sherlock Holmes.

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Or the man who’s half a boy.

Sir Arthur Conan Doyle.

KW Mitchell
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Re: Pic of the Day

Post by KW Mitchell » Tue Apr 28, 2009 2:52 pm

Of course there is rotational inertia in the compressor-shaft-turbine assembly which has to be dissipated, but once the fuel supply is cut off there 's a dramatic - and very quick - reduction in pressure inside the engine with a resultant drop in thrust.

The rotational spool-down lags the drop in thrust. In spool-up the opposite occurs - though much more slowly - because thrust requires the pressure to be built-up first. If one attempts to accelerate a jet engine too quickly, the compressor blades can stall leading to a drop in pressure in the engine and dangerously high temperatures in the combustors. The drop in pressure unstalls the compressor blades, the pressure than builds up again and an oscillating - and very damaging condition develops - accompanied by a deafening noise not unlike a chainsaw cutting through wood. In the worst case flame can be emitted from the inlet of the engine - which it most assuredly wasn't designed to do!

Modern jet engines have some very complex engine management systems (EMS) which gradually increase the fuel flow so that this cannot occur and also to minimise fuel wastage as a jet engine is very inefficient when it is changing states from low to high power. You may also have noticed when taking-off in a modern airliner, that there are two stages in this process; from standstill the rpm is advanced to an intermediate level and then increased fairly rapidly eliminating potential compressor stall.The rate at which the pilot opens the throttles is not of great consequence e.g. if he bangs them to the stops, the EMS is the overriding factor and will only allow an increase in rpm consistent with engine performance parameters, rpm, temp's etc..

In the Orpheus the system would be very much more primitive but would still prevent it from trying to accelerate too quickly if DC 'banged the pedal to the floor'.

Also, to return to the conditions under which the engine was operating at BB's point of impact we need to take heed of the very important video/photographic data earlier in the drama i.e. following lift-off. There is no plume of spray emanating from the water surface rearwards of the exit nozzle indicating no efflux from the engine.

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Re: Pic of the Day

Post by DMCK7 Fan » Tue Apr 28, 2009 3:58 pm

KW Mitchell wrote:Also, to return to the conditions under which the engine was operating at BB's point of impact we need to take heed of the very important video/photographic data earlier in the drama i.e. following lift-off. There is no plume of spray emanating from the water surface rearwards of the exit nozzle indicating no efflux from the engine.
If you look closely just before her sponson/ and or nose hits the water there is what looks like a small plume exiting the nozzle again, could this be the engine tryng to re-ignite ?

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Renegadenemo
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Re: Pic of the Day

Post by Renegadenemo » Tue Apr 28, 2009 4:20 pm

The start procedure was manually instigated so it seems very unlikely that Donald would be trying to relight unless, of course, he was already doing so when K7 took off but in that instance he'd have been shouting about it, I'm sure.
I'm only a plumber from Cannock...

"As to reward, my profession is its own reward;" Sherlock Holmes.

I have wrought my simple plan
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To the boy who’s half a man,
Or the man who’s half a boy.

Sir Arthur Conan Doyle.

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Mike Bull
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Re: Pic of the Day

Post by Mike Bull » Tue Apr 28, 2009 4:24 pm

I've never been convinced about the plume of 'relighting' vapour from the jet pipe thing; think about it- K7 was soaking wet, and I think what we're seeing is just a stream of water spray coming off her as she dives forwards.

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