Tech Tuesday

As we're all busy people when we're not building K7 (believe it or not we also have day jobs and as we're mostly middle aged or headed there some of those day jobs come with real world responsibilities) we find it easier to disseminate news via Twitter but the 'non twitterati' quite rightly ask for their info by other means and writing a diary post allows for more depth and detail and I always get that job so here goes.

Getting the engine into the hole mostly isn't difficult, we're just not very good at it. If we did it every week we'd be slick as you like but, as mentioned previously, this is only the fourth time we've ever done it and only the second time with the outer skins of the hull riveted on so we have to work from above when all the difficult bits are underneath. Plus, we keep learning new things that affect the process.

As the engine is lowered gradually onto its mounts we have to connect up engine-oil and low fuel pressure switches for the cockpit warning lights. The Jetpipe temperature thermocouples have to go into the jetpipe and the igniters must have their cables run from the exciter units on the hull to spark plugs in the engine. All the engine drains need to be connected to the catch tanks so we don't fill the bilges with oily slop that might get into an ecologically sensitive lake or loch and hydraulic hoses must be attached to the pump. It's a slow, careful process.

But the throttle and fuel supply setup is much more involved so I'll set out a little more detail about those.

Most things are easy to connect, the biggest problem is not forgetting something. It's supremely irritating having to lift the engine up again because you got distracted whilst connecting a boost pump and now the plug is lying forlornly on the bottom of the hull with no means to plug it in any more. Yes, we did that once.

But one thing that is extremely NOT easy is the throttle linkage. the setup was designed to be used on boats, yes it is a boat, but it's controlling a lump of aeroplane and they don't get on very well. The controls run down the right hand side of the hull down at the bottom where you can't work on them with the engine installed then a link turns though 90 degrees, runs under the engine and operates the throttle lever on the engine. That link was cobbled from bits of old rebar and scrap tubes and was so poorly executed that full throttle could not be achieved in any conditions with the added risk of stressing the linkage every time anyone tried.

So we modified it and, as a result, then had to move the actual throttle valve lever on the engine. It's adjustable so that part was simple enough. This then eased one problem but now we were starting from scratch again with regard to setting it up. None of this was solved when we went to Bute. We didn't know of these problems then but we do now so this time we built a gauge to see what the throttle linkage is doing at any given moment and that helped enormously but it wasn't a complete fix.

The main cause of all this is that in 1966 the fitters took out the Beryl engine, squeezed in an Orpheus then simply connected it to the same old set of linkages. This made it all a bit of a bodge and extremely difficult to work with because whereas the throttle linkage connects to the side of a Beryl where you can get at it, on the Orph is buried underneath where you can't. Next there's the fact that the throttle must be set to 34 degrees for starting and idle. Any more and it gets too hot on startup and idles too high, any less and it either won't start or remains sluggish and gets too hot then too. Setup has to be perfect without you actually being able to see it.

When we went to Bute we were miles away with the setup and the idle was around 50% when 36% is considered correct. Now then, we once had to point out to certain uneducated individuals who leapt upon us to call us unsafe or dangerous or irresponsible for running our engines to 65% in the yard that 65% throttle does not equate to 65% thrust. It's 65% RPM and most of the thrust is made in the last 20% but even then our pilots had to be mindful that K7, once up on the plane, would waft along at that throttle setting with a reluctance to slow down.

Another small historical item we unearthed in this exercise is that the physical impossibility of achieving full throttle applied in 66/67 too. Donald never had full throttle travel and on Bute neither did we. It made no difference to us but it did to him and evidence suggests that his power and fuelling problems were resolved by increasing the stroke of the main engine pump to simply throw kerosene at the problem. It would appear that K7's engine was quite drastically overrated to give the output and instrument readings they expected to see when in reality he was missing 10% of his throttle opening due to a foul condition between the linkage and the lever on the engine.

Our small modification on the linkage gave us full throttle and before the idiots start again saying we're dangerous, etc. If the throttle linkage binds up, the pilot won't feel that under their boot and if it binds up enough times it may well fail and do we want that? Nuff said.

So we sorted all of that then built a gauge to show us what the throttle lever is up to,

The red painted quadrant is exactly 34 degrees across the arc so while one person cycles the HP fuel cock and another shouts out what the gauge is up to, the person who drew the short straw (Al Vernon this time) got to shred his forearms down in the gap between engine and frame putting a sixteenth of a turn at a time on various turnbuckles to adjust the throw of the linkage. It's not adjusted in the video clip but you can see how it goes.

So that was one evolution we've made since Bute. Other things just fell back together. The LP fuel cock is a very simple affair that cuts off fuel to the engine ahead of the HP cock. We once forgot to open it on Bute and suffered the embarrassment of moving about twenty yards before the engine died having not even achieved idle. Note the grey tank held down with a bit of tatty webbing. More of that in a moment.

But the more noteworthy detail in that clip is the big fat fuel hose leaving the LP cock then turning through 90 degrees and disappearing away between the batteries. Here's something you might not have have thought about.

In 1966 the specifications for hydraulic hoses (that's all it is and that's what was used then) were very different. Hoses were less bulky and more pliant. The fittings were smaller too so a hose of that vintage was a much easier beast to bend around corners. The second problem is that the hose is only in a relaxed condition when it's lying flat along the bottom of the hull but there's no way to install it with the engine lowered. It's therefore necessary to tighten one fitting onto the filter beneath the engine whilst guessing at the angle it wants to achieve at the other end once the engine goes down but the result of that is a real fight to start the other end onto the LP fuel cock, which has a magnesium thread on the end of it. See the problem? It requires great care to start that fitting without hurting the thread. The result this time around is that it wasn't properly tightened so it leaked badly when tested and had to be remedied. It's all tight now.

Another hose issue caught us further up the main fuel delivery line.

In the last days of 66, Donald's team was having a lot of perceived fuelling problems as already mentioned. In reality it was most likely that they had one fuelling problem - air entering the fuel line with the engine under load - one instrument issue as the rev counter was mismatched to the engine so it always read low and one mechanical gotcha in that the throttle linkage could never achieve full opening.

But at the time, with a newly engineered craft, pressure to get on with the job, a dwindling budget, disenchanted press snapping at them and terrible working conditions they quite understandably diagnosed this as a single problem - that their engine wasn't getting enough juice.

In an attempt to sort this out, Ken Norris designed a small extra fuel tank containing a boost pump removed from the donor Gnat and this was placed in the fuel line where previously there had been a filter. There's another filter under the engine so no harm done there. It used to look like this.

(Pic courtesy of Neil Sheppard)

See the filter at the left? The thing with FRAM written on it.

But with the new tank fitted it looked as below. Notice also how the copper pipe for the fire suppression has had to be altered to clear it. We didn't do that.

We have never seen a pic of that tank (we always refer to it as the 'swirl pot') installed in 67 and it's likely it was never photographed so, though we knew there was a tank from anecdotal evidence, we didn't know how it had been mounted or what it looked like until we took the cover off in 2001.

It's had a bit of work since (in fact most of it is new) and we had to choose a colour because there's no way of knowing what that might have been or whether it was even painted, such was the rush to get it installed. Maybe it got a splash of blue.

But when we came to install it again we hit a snag. We were back to the hoses of the day being more bendy and, try as we might, a modern equivalent just wouldn't go without kinking so we settled on making up a rigid pipe.

This solved the kinking hose problem and fixed another one. See the small tank down in the hull that the other end of the pipe is attached to? You can see it much better in the video clip of the LP fuel cock being cycled. It's a six-gallon collector tank with another boost pump in the bottom that supplies fuel at a positive pressure directly to the main engine fuel pump. Well that tank was held in with a couple of bits of rubbish seatbelt webbing so it could flop all over the place taking its boost pump and the LP cock with it. There was never any guarantee that the the LP cock would fully close or open when commanded because the tank could move around so much.

We partly solved this by pulling it down with big-boy cable ties then we put the belts over the top to keep the authentic look then the rigid pipe ensures that it can't move at all because the small tank above is bolted immovably to the bulkhead. All in all a very elegant fix, the only drawback being that loading both ends of the pipe is a really tricky without burring the threads because they're at 90 degrees to each other so the upper tank has to be unbolted and that's a whole other set of skinned knuckles and dropped bolts but it's a small price to pay for the added rigidity.

The only other thing to do was to fill up both small tanks with kero, run the pumps to check for leaks then spool the engine to get fresh fuel to the burners so we know the fire will light when we give it some sparks.

So that's the more interesting (anoracky) points of the throttle and fuel arrangement briefly set out, the hydraulic and electrical systems have their own foibles so I'll write some of that down next and when we get the rest of the air system parts we can do a quick Tech-Tuesday, or maybe a Friday or Monday, on that too so I'll write another short piece soon.

If you just had an interesting little read please forward it to a fellow detail fanatic and don't forget that the workshop crew can usually be found on Twitter @bluebirdk7 and if you want to have a big catch up and haven't seen it already have a look at our YouTube channel.

https://www.youtube.com/@TheBluebirdProject/videos

Bill