A strange one....

[forthbridge]

Hi Guys

Now, I know this is connected to something I really shouldn't be doing, but anyway, bear with me here.....

I have, once, managed to fly EGQL - EGYP with the C1K - with literally vapour on touchdown... anyway, I've been trying to replicate this with better flying (for me)

I've extended the range (allbeit with illegal reserves) of several of my 'favoured' A/C with decent results (EG even with real weather I can, for instance reach a given airport with fuel within 2-3% each time)...

Now, to the C1K.... I have had this machine up at FL500 several times over the UK (As part of my long range calculations) and it flies very well, with no tendency to drop or stall. This is an upper limit, but FL450 is 'well' capable. I have just repeated this before typing.

Now, to the Falklands flight. I make an immediate turn onto 200-ish degrees (Course is 222 from EGQL) and commence a maximum climb (290Kts). The A/C gets up to FL350-70 without issue, and I then use time compression. After each 10% of fuel is burned off, I drop to normal speed and initiate a climb until I lose three or four knots off the cruising speed. I then let more fuel burn off and repeat the cycle (the aim being to maintain speed and increase height). I always use normal speed for this for obvious reasons. Now here's the query (at last I hear you all say)...

Above FL415, I keep getting what looks like a 'Dutch Roll' - but the A/C doesn't swing it's tail out, it's just the wings which wag up and down. No matter how light it is, I cannot avoid this - now, I am usually within about 2000 miles of the Falklands when I'm ready to go to this height, but always need to drop to FL390, then coax her back up to FL400/405. **But**, If, for instance I take off from Leuchars and head in any direction, I can extablish FL500 with no issues (except time to get there).

Anyone have a clue why this is?

Incidentally, currently, at around 1800 and 900 miles out (from the Falklands) in real weather I have achieved positive fuel remaining (IE DMs flight computer gives me either equal or up to 100 miles positive range compared to distance out) - although this gets radically altered if (when) the wind changes - but I am working on further refinement of the cruise climb to steal even more range.

On the last attempt, I was forced to rendesvouz with an (invisible) tanker from EGYP at 500 miles out, as fuel had got to the 'no chance' range..... :o

So, ignoring the daft bits, why do the A/C behave differently at altitide depending where you are? (My uneducated reasoning is that at altitude the weather is faiyly uniform?)

[cstorey]

Well, our two real world VC10 pilots will give you an up to date answer , but my recollection was that the VC 10 was always subject to directional instability and therefore Dutch Roll and that this was the reason for the requirement for effectively permanent yaw damping . I anticipate also that the tendency to roll may also vary with longitudinal trim which of course will vary with weight , since the relationship of the centre of pressure and the CG and the roll centre will also vary in complex form as weight, AofA, altitude , mach no and temperature vary . I don't know how realistic the model is on fuel temps and density , but bear in mind that these will vary considerably between a short climb to high altitude, and a very long cold soak of the type you are describing, and this may have something to do with the differences you note

[forthbridge]

Well, I took her up again, and went straight to FL390, and coaxed her to FL410 around Portugal. With 2000 miles to go, I was at FL 434, this time with no rolling tendency. Ground speed went a little high at 501kts, but no overspeed warnings, and I had 270 miles in hand according to the computer. Of course, that is still not quite enough, since even on an idle descent fuel use will rocket on finals, so I hit F2 a couple of times and she was happily hitting 450kts ground speed and range in hand, unfortunately, I went to the kitchen having put it back to 16x speed, and when I came back to the PC, the A/C was plummeting towards the sea - presumably some turbulence had upset her.....

Next time........! ;-)

[cstorey]

Ah, forthbridge, you seem to be using GS as some kind of reference, and this may be the problem. At these levels, mach no is critical . Again, I do not know how realistic the model is , but at these sort of altitudes in real life you tread a very very narrow path in the flight envelope between stable flight and an upset. You do not say what mach no you are at, but I feel sure that you will be close to Mmo. Many swept wing jet aircraft exhibit strong downward pitching moments as compressibility effects come into play, and these effects are compounded by even light turbulence. Really, you are pressing the aeroplane far beyond its capabilities, and you certainly cannot leave it unattended in these circumstances

[forthbridge]

Hiya

I must admit, I tend to just achive overspeed and then knock it down to just below the warning figure, so I tend not to look at the Mach number, but when I have it's high .7s, sometimes early .8s....

As you say there's virtually no leeway and you very much feel you are on a knife-edge when above FL410 - anything but planned moves (with the AP) will probably result in an immediate catastrophe, although most turbulence is coped with reasonably - I have a suspicion the wind wheeled around onto the nose and killed momentum a little (As there had been a very satisfactory tail wind) and I had probably been too conservative with the thrust settings, and the A/C just ran out of puff- it had entered a deep stall and was plummeting tail first, at first I thought the fuel had gone, but there was still 18% load.

Strangely, GPS direct is 7048NM - whereas if I head to Ascension and fly by, on a direct course from there, I can save 50NM - not much but it could be enough. I shall need to have a look and see if there's any way I can shorten a few more miles off the total.

[SkippyBing]

I have a suspicion the wind wheeled around onto the nose and killed momentum a little (As there had been a very satisfactory tail wind)

Err... that would only affect ground speed, not airspeed which is the critical factor in flying, ground speed being relevant to navigation.
As cstorey says at those levels Mach is the governing factor, as an extreme example I think the U-2 only has a five knot band between stalling and exceeding it's critical Mach No at operating altitudes. Are you using the autopilot to hold a set Mach No? Not sure of the exact figures for a VC-10 but I'm guessing somewhere around 0.82 is about right.

Re the distance, I'm not totally sure how the built in GPS works things out, but I don't think it properly implements a great circle route. The quickest/easiest way is to use the ruler in Google Earth, this will draw the actual shortest distance between two points, i.e. a great circle, then make a note of the places it flies over/near. Over the sort of distances you're talking about you can't fly one heading, technically it's continually changing although effectively you can spilt the route up into say 5 sections and alter course every few hours, again use G Earth to chose suitable waypoints along your route and then use them in FS.

[forthbridge]

HI Skip,

Thanks fot that routeing advice, I'll have a look at that later. I tend to not use speed hold, (especially when using compression) as it usually forces the A/C lower which is not what's needed for range. Airspeed could be 250kts at FL390, but 237kts at FL400 will give slightly better range - I monitor the groundspeed as that's what the chinagraph shows and at times allows you to be more precise - for instance If I just look at airspeed, it can sometimes 'hold' at the same speed then suddenly drop to near stalling whereas the G/S creeps up or down so I can slow and stop the climb and stabilise it more easily.

Re the headwind, even with ALT hold, the A/C will climb slightly if the wind drops or changes direction - IIRC there were about 67kts up the tail, which was dropping and shifting now and again, (Which slowed progress) - but if direction changed, it can pitch the A/C a little, and at high compression it's not long before the A/C gets too high and stalls.... I was surprosed a little, as at FL415, eve with some moderate turb it was reasonably stable at 16x speed heading down the Brazilian coastline......

Now for a little route planning.... :flying:

[SkippyBing]

Jim,

Can you not set an altitude and Mach hold? Not sure what the set up is in the -10.

From some research I'm doing on another aircraft, which may be applicable to your situation, the climb is done at a set IAS until it equals a certain Mach no at which point you then climb at that Mach. You then set your cruise at another Mach no, at the flight levels you're talking about that's the limiting factor on the airframe, not IAS which won't be that useful apart from avoiding the stall.
I see what you're trying to do by monitoring ground speed in the climb now, do you check the wind speed/direction at altitude before you launch? You'd need the performance graphs for the -10 to tell what the optimum cruise height is, but generally higher is better.

[forthbridge]

Hi Skip

I tend to use the (DM recommended) climb speed of 290KIAS, and I maintain this by reducing the climb rate as required to maintain speed.
Wind speeds I never check (for this trip) as over the total distance it changes dramatically, however my first aim is always to get to FL380, on course and M.78-80. I've seen the Machmeter read .84 before any overspeed warning, but generally it's anywhere between .73 and .82 - the problems arise when the wind (which does) whips around - or calms. You can have a 99kt tailwind one minute and then calm the next which wreaks havoc on range - and a quick whip up of wind and suddenly you're overspeeding. There are times the machmeter can look like a peak level meter on a valve amplifier when turbulence gets up.

At the moment I am within about 200 miles of doing this flight non stop, I've done it once before, but thinking back I am sure I flew some oddball route near to Ascension and not the 'computers' way....

[SkippyBing]

Jim

I'd say the best way to do it is by ignoring the computers route and planning a proper great circle one then. I did once know how to do this properly, although it does seem slightly irrelevant when you operate an aircraft with 240Nm range!