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Druid77
27th Jun 2005, 17:48
I originally posted this on the Flight Testing Forum, I had one response from Sycamore and he suggested cross posting to here because some of you guys frequently discuss obscure perf. data and seem to have long memories about the subject and origins. Any info would be appreciated
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Is anybody familiar with the "D" and "X" method of take-off performance prediction and if so can you explain what the "D and X" values actually represent?

I understand that it is a reasonably old method of take off prediction not widely used nowadays but I have failed to find any references either on the web or in text books. I am happy with using the method to predict the take-off performance but I don't have a physical understanding of what the parameters D and X actually represent. (I think X is V1/Vr ratio but what on earth (in real terms) is D?)

A quick summary (or some useful references) would be much appreciated

Thanks

Alex Whittingham
27th Jun 2005, 18:44
I think this is VC10 terminology and, as I was never forced to fly the beast, I don't know a huge amount about it. My guess would be that "D" is a variable that can eventually be turned into a MTOM. You would expect two values of "D", one for all engines, one for engine out and you would take the most limiting to derive your MTOM. I've got some old books in the office, if you don't get a decent reply I'll look through them tomorrow and see if I can answer your question properly and give you some references, The HQ38 Group Performance course notes circa 1976 would be one.

john_tullamarine
27th Jun 2005, 21:48
An older style of graphical presentation in the AFM - common UK format and less so for the US. Doubt that you would find any references outside the dusty archives of various aerodynamics offices among the various manufacturers ...

Long time since I have looked at a sample and none in the office at present.

Never really worried about what the D/X represented specifically but I will dig a couple of AFMs out of the dust and have a think about it for you.

Recollection in the back of the mind is that we are looking at OEI V1/VR (or V1 unbalanced/balanced) ratios and corrected TODR against corrected ASDR .. often referred to as a "web" carpet or similar presentation.

Either way the presentation made it fairly easy to do unbalanced length calculations and run a V1/VR optimisation exercise.

A fairly typical Brit presentation was to have separate AEO/BFL/OEI charts ... a bit of a pile of paper but, once the regressions had been done and the data plugged into the computer, gave the potential for a very accurate (judged against the paper AFM) RTOW calculation.

Give me a day or two .. assuming I can find an old manual somewhere in the filing cabinets ? HS/BAe manuals used this presentation a lot as I recall ...

Old Smokey
4th Jul 2005, 13:08
Away from home and my own pile of dusty archive material, but in agreement with John_Tullamarine that it's largely a British way of data representation.

'D' was simply a given runway distance ( be it ASDA, TODA etc) corrected for Slope and Wind Component to an equivalent still air / zero slope distance, which could then be applied to standard data for comparison with Pressure Height and Temperature.

Although typically British, much the same is done with American data, but the 'D' tag is not applied.

As for John_T's comments regarding the "typical Brit presentation to have separate AEO/BFL/OEI charts", yes indeed, much more accurate analysis and optimisation was possible.

Sorry, 'X' doesn't come to mind, awaiting return to home's dusty tomes.

Regards,

Old Smokey

john_tullamarine
4th Jul 2005, 20:24
I still haven't had time to dig out the older AFMs .. but the D-X generally referred to the carpet which was the intersection of corrected TODR against ASDR which was overlaid with either V1/VR or V1/V1b data.

Either this or the separated data presentation could be used with the same convenience to run up computer simulations. The D-X presentation simply allowed a single chart to be used instead of a number of separate charts.

Alex Whittingham
5th Jul 2005, 07:28
...and not a million miles out from the L1011 graph that derived V1/Vr ratio and engine out D from corrected ASDA and TORA/TODA. Anyone who did the old UK CAA performance exams will remember it fondly as figure 11 in CAP 385.:}

I can't find my VC10 stuff - maybe BEagle can help?

411A
5th Jul 2005, 07:38
I recall this info presented in a couple of ex-G registered L10's that I operated years ago...and it was missing from the FAA AFM.
Likewise with two specific ex-QF B707's that I also flew.
I would suspect the man, DP Davies had a hand in all this.
The data was very well presented, as I recall, especially in the L10 manual.
Nothing wrong with the FAA mind you...just the Brits did it a bit ah, differently.:}

Home James!
5th Jul 2005, 13:08
Guys!

My first post. Shortly I am going to see DORIS. The Department of Research and Information Systems at RAF Hendon.

I am there to look up family facts and figures with regard to Sqn Ldr D O Finlay, the world class athlete and Spitfire pilot who was so sadly to die after the war in sad circumstances.

So whilst I am there I am sure Donald would approve if I asked the librarians to see whichever AP's you think would convey more info. on D and X.

It would be a pleasure and I am eager to see what is meant by "The Old Country" although it's a phrase not on the tip of the tongue of my generation.

And I look forward to contributing what I can to a forum of knowledge and experience and absorbing to the best of my ability.

Just ask and I'll look it up. By the way, where is RAF Hendon, apart from being in London I mean?