BAC 1-11 temperature limiter and engine performance...

Macs · Post by **Macs** » 13 Oct 2006, 13:54

MALTBY D wrote:You've stumbled on a peculiarity of the model.
If you have the pumps, valves & crossfeed set normally, setting the centre tank level down to zero will always result in a fuel cut off in FS.

The fuel system works quite realistically on the 1-11 model, but it's an illusion.
The real thing only ever feeds the engines directly from the wing tanks, but pumps refill the wings from the centre tank.
So under normal running, although the engines take fuel directly from the wing tanks, only the centre tank empties.
To simulate this, the model will actually take fuel directly from the centre tank if it has any fuel. So, suddenly empty the centre tank & the engines cut in FS.
There is some XML code that secretly switches the FS fuel feed from the centre tank when it's near empty. But when the level is suddenly set to zero it can't switch the supply quickly enough to avoid the engines cutting out. It's only a microsecond of fuel starvation, but that's all it takes.

So, if you want to empty the centre tank it's best to leave a small amount (2%) in there & let it drain itself.
Under normal running there's never any need to open the crossfeed when the centre tank runs dry.
Or, if you're on the ground you can just start up again without touching the fuel system.
Nearly every 1-11 flight I do with the CBFS va starts with a fuel cut off.

DM

Ah, OK,that explains it. I was wondering what the crossfeed had to do with it... You know that begs the question; How was fuel load handled in real life? I just even everythng out (same amount on a ll three). I don't think this will matter as the plane will automatically handle tank selection, but just curious what an ops manual would say about this on the real plane. Form what I can see, it really should not matter as the fuel will always be drawn from teh wings... Anyway, thnaks for the explanation Dave

regards,
Macs

Post by **DaveB** » 13 Oct 2006, 14:48

Hi Macs,

I can't find a definative answer to this except that should you need to dump fuel, the centre tank can be totally emptied if required. Therefore, it's probably safe to assume that you can, if and when required, run using only the wing tanks leaving little or nothing in the centre tank.

I know that certain aircraft need to have so much fuel in a centre tank (I think the Trident is one) but no mention of 'required' loading for the 1-11's

ATB

DaveB :tab:

Post by **MALTBY D** » 13 Oct 2006, 16:08

Normally the centre tank would only hold fuel when the wing tanks were full.
Each wing tank feeds its own engine & the centre tank's job is to keep the wings topped up.

The fuel system panel does operate like real on the model.
The arrangement of pumps & valves allows for any tank to be isolated.
You can stop the centre tank feeding the left or right wings (or both).
Both engines can take their fuel from one side by opening the crossfeed & turning off the pumps on the side you don't want to use.
There was no facility for pumping from one wing tank to the other, so any fuel imbalance was sorted by crossfeeding for a while.
The APU only took fuel from the left tank, so you can get a small imbalance from that.

The fuel schematic on the overhead panel gives a very good representation of the system. Makes a lot more sense than some other aircraft.

Any automatic switching of fuel source is only the model's way of simulating the real system.
In FS the engines are secretly taking fuel direct from the centre tank (not possible on the real aircraft). When the centre tank is near empty, the model switches to wing tanks instead.
That's why you'll get an engine cut off if you remove all the fuel from the centre tank. The model was using that tank & so FS shuts down the engines before the model gets a chance to switch over to wing tanks.

DM

Nigel H-J · Post by **Nigel H-J** » 13 Oct 2006, 16:26

Thanks for all the technical information Dave, wondered why my engines had flamed out when selecting fuel after having first selected the 1-11 though one way I did find out to bypass that problem was to have the ignitors switched on when the engines are running then when selecting zero fuel for the centre tank least they spool up immediately instead of flaming out.

Incidentally I always keep the ignitors switched on during flight as a precaution.....Is this best practise?

Finally, I have to echo Macs plea although I did not realise you had incorporated a failure mode into the 1-11......Please don't get rid of it, sounds a very interesting feature. So interesting - I am going to give it a go!!!!

cstorey · Post by **cstorey** » 13 Oct 2006, 19:45

My recollection of Speys is now probably faulty, but I recall there was a 5 minute full power limitation, and not the much shorter period which appears in the model. In general the controls on RR engines were more sophisticated than the American equivalents of that era - the Conways in the 1960s had a thing called Scope which IIRC was the equivalent of an automatic temperature limiter such as you have devised for the 1-11 - I would be interested to hear from Tonks whether it is still used on his Conways

Chris

Post by **MALTBY D** » 13 Oct 2006, 20:58

Yes and I'd expect you could over heat a Spey for ages before it died.
I may increase the time on it.
Having said that, if you overheated it for 90 secs it could possibly put it out of service permanently. Dunno really, it's certainly not a good thing.
May be a bit like the situation with emergency landings, the airframe can handle it safely but major parts may be knackered beyond repair afterwards.

The VC10 has 'Top Temp' switches on the engineers panel.

DM

Macs · Post by **Macs** » 13 Oct 2006, 23:01

MALTBY D wrote:Yes and I'd expect you could over heat a Spey for ages before it died.
I may increase the time on it.
Having said that, if you overheated it for 90 secs it could possibly put it out of service permanently. Dunno really, it's certainly not a good thing.
May be a bit like the situation with emergency landings, the airframe can handle it safely but major parts may be knackered beyond repair afterwards.

The VC10 has 'Top Temp' switches on the engineers panel.

DM

David,
Adding a minute or so would not hurt, I don't belive

It would certainly help in cases like the one described by me on the thread.
I just did a flight from Glasgow to Heathrow, and I might add, these flights on VATSIM in the UK and European reagion are some of my favourite ones.They have some of the best controllers and it really gives it a realistic feel to it. I flew it with a 1-11 of course in BA livery (My fav repaint on the 1-11) and managed to conform to speed and height restrictions

, no sweat with a 1-11 :dance: , it is a very stable platform.
I did wonder though, how was the 1-11 flown in real life? It seems very difficult to use engine RPM settings (N2) as the spool up time can be long and unpredictable, as well as fuel flow or other engine parameters that can be used to your advantage to manage speed and altitude on other planes. My technique involves looking at the trend of the IAS and reacting accordingly. luckily in the 1-11, things don't change as fast, but sometimes it can be a wild goose chase

I have kind of memorized some settings of RPM value, but it is hard to be precise in controlling this. Anyway, just curious as to how it was done in real life and maybe how some of you manage this. Please forgive me if I am getting a bit off- topic :redface:

regards,
Macs

flugkapitan · Post by **flugkapitan** » 14 Oct 2006, 00:06

David,

Thanks for the excellent explanation of the fuel system of the 1-11. Also the posts from the others make very interesting reading! It is amazing the pool of knowledge on this site.

Regards,
Scott
KJMS (or thereabouts)

Post by **MALTBY D** » 14 Oct 2006, 01:20

Sorry Nigel, forgot about the ignitor switches.
It won't make any difference on the model if you leave the ignitors on all the time.
I try to turn them on for takeoff & landing (as per real checklist) but often forget. If I do remember, they get turned off after flaps up on takeoff.

All relight does on the model is trigger a bit of XML code to spark them back up if the RPM drops below 50%, ie. RPM is under the normal running speed.
I'd never thought of leaving them on actually. It would have been a good move while my little daughter was fascinated by the fuel level on my yoke. Mr Thicky or what!
She's now more interested in the trim switches though & there's no cure for that.

------------------

Macs, that's an interesting question about how to fly the 1-11.
I'm not sure how to answer it though, it's like second nature now & I just do what's needed without too much thought.
I'll be watching N2 RPM & IAS maily, but on the latest version of the model I'm using EGT more as well.
The one thing I rarely use is fuel flow, which I know is considered the main man in some American aircraft. Seems alien to me.
The only time I'm checking fuel flow is in the cruise when I'm hoping to be at around 1100 kg/h.
The new version of the model has a faster N2 response, but even on the older model I never found it any trouble. I guess whatever you're used to is best.

This is roughly what I do with it:-

Takeoff - click on the Thrust Index gauges to get the correct yellow number (usually 140-154) & set takeoff thrust so that the Index gauges read at least 100%. 'Top Temp' is ON to avoid overheating.
Flap 16 deg, trim 3 deg nose up.

Initial climb - keep takeoff power, flaps 8 at 160, up after 180.

Climb - Reduce power to 94% N2 RPM or whatever RPM to keep EGT in the green. Allow EGT in yellow if not enough power to climb.
A/P On, HDG mode On, use A/P pitch wheels to adjust the climb rate to control speed. Keep 94% N2 RPM or green EGT.
After speed restrictions, lower climb rate using A/P pitch wheels, accelerate up to 300kt, then adjust climb rate to keep 300kt, every so often adjusting A/P pitch wheels & throttles to keep 300kt & 94% N2 RPM.
300kt until mach 0.72, then climb at 0.72 instead.

Cruise - Usually Mach 0.72, 0.74 is ok if it's not bumpy. 35000ft is max height due to cabin pressure differential limit.

Descent - My descent is usually a 3 degree profile all the way, starting off at around 2500 fpm, keeping to m0.72 or 300kt. Descent rate reduces with ground speed to keep a 3 degree slope, VS rate controlled by the A/P pitch wheels.
Reduce rate to slow for speed restrictions, under 10000 flown with throttles at idle, air brakes if needed.

Approach - Once flaps in use it's usually 84-86% N2 RPM all the time.
Flying level with 16 deg flap, 84-86% should give about 170-180kt.
If on ILS, select 25 deg flap when the GSI arrow has moved half a dot down from the top. Select 45 deg when the GSI arrow is about half a dot above centre. This should leave you on the the glide slope at Vat speed.

Landing - Fly down the glide slope at Vat & it should only need very minor adjustments to RPM.
Keep power on during a very small flare, power off when under 300 fpm & touch down.
Spoilers out, reverse on, brakes at 80kt.

righthandseat · Post by **righthandseat** » 14 Oct 2006, 11:57

Great topic and posts particularly from DM. I can now see why I'm not quite there with my piloting skills!

I endorse flugkapitan's comments on the knowledge here

. Thats why I joined after taking a look.