The AFRC spills servo pressure causing the piston pump to de-stroke until the engine catches up at which point it restores the servo pressure and off you go. But we’d never managed to get ours set up so it wouldn’t over-temp the engine on slam-accelerations'.
Now I don't know about jet engines (but I'm reading the diaries closely) ...but with the over-temp does that mean it's running too lean on this set up and you'd need to 'up' the kero (stroking the piston up slightly?) because you can't or don't want to slow down the turbine pick-up?
Good question, and if it was your average petrol piston engine you'd be spot on but gas turbines use about 70% of their inducted air for cooling (this is how come afterburners can work - because most of the air exiting the engine still contains unburnt oxygen so chucking in extra fuel and lighting another fire will produce extra thrust without frying the turbine.)
With the limited spilling of the AFRC such as we lft it in the summer there was just too much fuel for the airflow in the summer conditions so it all got too hot. In lean-running petrol engines you get erratic flame-paths, compression-ignition and incomplete combustion leading to too much heat but in our case all we have is too much burning fuel and not enough airflow to carry the heat away.
The trick is to introduce enough fuel to accelerate the engine whilst creating sufficient draught to blow the unwanted heat out of the other end until it reaches that glorious self-sustaining 3650rpm
I'm only a plumber from Cannock...
"As to reward, my profession is its own reward;" Sherlock Holmes.
'It ain't what they call you, it's what you answer to.' W.C. Fields.
Thanks for that reply, ok I think get that, (I get why afterburners work too now, I should have realised there'd be unburned oxy to do that, bonus!) you'll drop the fuel a 'gnats' then? Yep, that was counter intuitive to my automotive upbringing. Also, this you say is for your summer conditions, I'm presuming ambient air temp too high (not so much ambient pressure) to get enough cooling. May the settings be ok in these colder days and so get better results in the winter because you can burn more fuel without raising the temp too high? I can see that on start up 20deg is a big drop in temp from summer to winter...Just to clarify,this is just for start up isn't it?.. Cheers, and sorry for being a pain with all the questions but I find this fascinating. Bp.
Basically, the AFRC was spilling in the summer but not hard enough to perform slam-accelerations in the prevailing conditions - you take the engine to 40%, let it stabilise then slam it wide open and what you see on the instruments is an almost instantaneous drop in servo pressure but it comes after you put the extra fuel in and in response to it so you already have extra fuel in the engine. The fall in servo pressure de-strokes the piston pump so it doesn't throw any more in after it and the engine accelerates on what it already has. Once its pressures are back within limits the AFRC stops spilling and the pump strokes up again but by now the engine is spinning fast enough to cope with the extra fuel. There is a rise in JPT associated with all of this.
What we had in the summer with higher ambient temperatures was the AFRC not spilling enough to keep the JPT within limits as all this went on. But when we ran in the winter we had much more in hand so it allowed us to perform some nice clean slam-accelerations. The AFRC doesn't do anything on startup, that's done by the pump and the CCU. (Combined Control Unit)
The early systems were fitted with simple screw adjusters with a locknut and they are ridiculously sensitive. We have one on our 1959 BPC (Barometric Pressure Control). In fact the only way we found to set it up properly was to instrument the pressure we wanted to alter then watch the gauge whilst applying minuscule adjustments, then try to nip up the locknut without anything moving. Now because some of these adjusters are supposed to be tweaked by mechanics in the field, and because you can be a quarter turn off and your engine won't go, the later systems were fitted with a clicker adjuster and that's the opposite extreme. Six million turns later you may see something happening and this is what's fitted to our AFRC so we never quite got it set up. We will, though.
I'm only a plumber from Cannock...
"As to reward, my profession is its own reward;" Sherlock Holmes.
'It ain't what they call you, it's what you answer to.' W.C. Fields.
The link in the subscriber email goes straight to the homepage/pic of the day and when I click on the diary page, the last entry is the one from February
Treat life's problems like your dog would... if you can't eat it or sh*g it, just pee on it and walk away
Sorry guys, you can put that on me. I had it taken down for maintenance. I wrote half of it in a hurry and wasn't happy with it so I'm going to mend it this weekend and put it back on Monday...
I'm only a plumber from Cannock...
"As to reward, my profession is its own reward;" Sherlock Holmes.
'It ain't what they call you, it's what you answer to.' W.C. Fields.
You've finally pushed me into the modern world of multi media communication. This is the first time ever I have written anything other than an email so if I get the etiquette all wrong l offer my apologies. To explain....
I am a newbee observer of the project and started off by watching videos and reading the early diaries, all
I might add while wearing my new BBP hat (well not all the time). However l got side tracked into looking at books about BB and ended up spending much of Christmas reading through different forum threads to see which book to kick off with, which in turn led to more side tracking off into all manner of subjects, all thoroughly enjoyed and mostly understood. Anyway to get to the point having ordered today Leap into legend and Neil's book, l return to catching up on the diaries to find that they have disappeared and the archive button is not working for me. Is there anyway I can get to the early diaries to catch up on my homework?
I would also say that the first version of the Dec 2016 diary tickled me pink. I'm not at all sure that 'a pretty girl intent on saving the planet' is in any way as deliciously descriptive as the first version, 'silly bint in designer heels'. Took me back over 40 years since the last time I heard that particular B word.
Well done to all the team, I hope you realise how much your efforts are appreciated not just by the serious enthusiasts but also to the layman like myself.
