Technical Talk

[Engine 711]

First engine start since 2016 tonight and, like everything else, full of legacy-headaches from 1966. Despite our best care and efforts we had a rather hot-start with much fire requiring extra effort to blow the fire out, rein in the JPT then shut down and dry-crank to get the heat out of the engine. Not a conspicuous success.
The main issues were de-inhibiting the engine, as it doesn't like to burn a mix of Aeroshell 1 and kero, possible mis-adjustment of the control runs such that we didn't achieve idle, and therefore sufficient mass airflow to pull the heat away in the prescribed startup time and the shocking design of the combustor drain tank such that its pipework is higher than the tank so it doesn't actually drain. Whose stupid idea was that? At least the inhibitor is out of there now having achieved a successful acceleration to idle - eventually.
We also remain unsure on the possible influence of our second LP boost pump on fuel scheduling on startup but at least we can simply disable that.

What was an amazing success and a great team effort was getting the boat out of the workshop with inches to spare, into the yard then back again. OK - so it took a little fiddling about as it was our first try but we did it with no damage except a few frayed tempers and a late finish.

Someone remind me why we do this?

Oh dear.... Sounds like it was rather too exciting.

A way to 'de-inhibit' is to do some wet motors (or cranks) - like a start, so fuel on, but with no ignition. BUT this assumes your drains work well...! ANd you need a least 1 dry motor (or crank) - so just starter only - to 'blow' any fuel left in the engine out of the back. If in doubt, do 2 or more. All of which hammers your Start system - which you don't want to really do. Bugger....

Cannot see how the 2nd Boost Pump/Tank can affect starting - BUT its a HydroMech system, so anything's possible....

Why...? Because the end result is worth it.

[Renegadenemo]

A way to 'de-inhibit' is to do some wet motors (or cranks) - like a start, so fuel on, but with no ignition. BUT this assumes your drains work well...! ANd you need a least 1 dry motor (or crank) - so just starter only - to 'blow' any fuel left in the engine out of the back. If in doubt, do 2 or more. All of which hammers your Start system - which you don't want to really do. Bugger....

Cannot see how the 2nd Boost Pump/Tank can affect starting - BUT its a HydroMech system, so anything's possible....

We did two wet cranks then two dry cranks, leaving an hour between the last two but the drains were rigged in 1966 configuration and simply didn't drain. We've fixed that now.

The fuel system was set up by AEC but to the 101 spec, which uses a different pump that generates servo pressure instantly. On our 1959 pump the servo pressure is generated centrifugally via drillings in the rotor so it comes up very slowly. Because of this we had to go very light on the spring in the pressurising valve that schedules fuel on startup. When we jumped the LP boost pressure from 10 to 20 psi we overfuelled. Seems incredible but the delivery pressure has to climb through 20psi on its way to idle so the appearance is that we're affected by the second pump. That question is yet to be satisfactorily answered.
In the meantime we had some good engine runs yesterday to test the drains and the hyd system. We had a failure with the hyd when the accumulator let go but we can run without that. We still have the water brake so long as the engine keeps turning, it's only in the event of a flameout that we lose the brake.
We had some failed starts later traced to the batteries getting low and not allowing sufficient time for the igniters to clean themselves. Our throttle linkage gave some setting up issues with a high idle and not hitting 100% but we've pretty much bottomed that and we had a minor fuel leak. Lots of practice in handling the boat on the cradle in and out of the workshop so our crew training is already well advanced. Bute is raring to go with the red carpet fully rolled out but not a peep from Coniston. Hopefully they're on it too.

[Engine 711]

You are speaking 'Derby', if you call them Cranks...... Two Dry is cool, after any Wet - thats on the cautious side.

Servo pressure was (AFAIK) generated by an orifice of drilling in the Pump body, which bled Delivery pressure off. Servo was generally about 100psi less the Delivery. Looking as the Bristol Siddeley Orpheus 10001 ECU Fuel System Diagram (as you do....), the Servo drilling/orifice is visible just to the right of the HM Governor Diaphragm on the pump (where the Yellow & Orange colors meet). LP Fuel Pressure is usually the 'reference pressure' that everything bleeds back too - altering it can change everything - so I am not that surprised the change in LP pressure severely messed up starting.

Otherwise - all good.... But nothing from Coniston...? Not surprised.

[Renegadenemo]

Looking as the Bristol Siddeley Orpheus 10001 ECU Fuel System Diagram (as you do....), the Servo drilling/orifice is visible just to the right of the HM Governor Diaphragm on the pump (where the Yellow & Orange colors meet).

Now if we had that type of pump we'd be OK. Ours predates the HM governor and generates servo by basically centrifuging fuel through drillings in the pump rotor. It's therefore dependent on rotor speed rather tapping into delivery pressure. Seeing as the servo strokes the pump it's easy to see why we had to go light on all the settings to get the engine to start. we have a couple of Orph' 101s with the later pump and the one we have run and instrumented uses double the servo pressure and generates it instantly.

[Engine 711]

Now if we had that type of pump we'd be OK. Ours predates the HM governor and generates servo by basically centrifuging fuel through drillings in the pump rotor. It's therefore dependent on rotor speed rather tapping into delivery pressure. Seeing as the servo strokes the pump it's easy to see why we had to go light on all the settings to get the engine to start. we have a couple of Orph' 101s with the later pump and the one we have run and instrumented uses double the servo pressure and generates it instantly.

OK,so..... why not.... Change the Pump.....? Better to start the Orph you have - safely....?

[Renegadenemo]

Because it's the original one that pumped fuel on the 4th January 1967

[Engine 711]

Because it's the original one that pumped fuel on the 4th January 1967

Ah.....

[Renegadenemo]

The whole fuel system with the exception of the AFRC is original. The AFRC came from a crashed Gnat and was lying in the cellar at Shaftmoor Lane. K7's fuel system didn't have one. It had a primitive anti-surge control, one of only two ever made, and described in its own test report as a 'lash up'.

[Engine 711]

The whole fuel system with the exception of the AFRC is original. The AFRC came from a crashed Gnat and was lying in the cellar at Shaftmoor Lane. K7's fuel system didn't have one. It had a primitive anti-surge control, one of only two ever made, and described in its own test report as a 'lash up'.

Ah.... again. My knowledge of the Orf is almost zero.....

The Orff in K7 was - both times - a 700 series engine. The 2nd engine was 711 - so - there must have been 11 of them, at least.....? And 711 definitely came out of a scrap Gnat that DMC bought, for parts & spares.

[Mike Bull]

Gnat XM691 was the first of 14 pre-production prototypes of what became the Gnat T.1, so it was far from a standard example of the type.