Saturday, 13 April 2013

Annual time

April is annual time for G-JONL. The Permit to Fly expires on 5th May each year and so I start working on her ready for the annual inspection early in  April.

My Sportcruiser is three years old now and has done around 220 flying hours. For the first time there were a few minor faults that had to be sorted out in addition to the standard annual maintenance work.

The first problem was that the port exhaust gas temperature (EGT) sensor suddenly stopped working a few weeks ago. Not really a huge problem but the odd values it was registering (100°C one moment, 1100°C the next) were a bit disconcerting, even though I knew there was nothing actually wrong with the engine.

Well it turned out that the sensor had gone open circuit and the Dynon engine management system was greatly confused by this and displayed essentially random temperature data as a result. The problem was easily fixed by fitting a new sensor. I got a second spare, in case the other side decides to fail next.

Still on the port side of the aircraft, the port fuel level indication has always been a bit unreliable. Again this wasn't really a big problem, as I fly 30 minutes per side, so fuel quantities are always more or less the same in each wing. The starboard fuel gauge worked perfectly, so no big deal.

I decided I would try to fix it. Tests suggested that the problem was in the vicinity of the wing root, where there is a connector to simplify removal of the wing. With the wing fairing removed, it turned out to be just possible to get this connector out through the gap between the wing and the fuselage. Sure enough the connector was at fault and was quickly replaced.

Unfortunately, this didn't fix the problem! Much head scratching. Eventually I realised that the next connector in the line,  where I had been measuring the circuit was itself at fault. This was self inflicted - by poking the test meter probe into the connector I had forced the socket pins open and there was no longer a reliable connection. A little work with a tiny watchmaker's screwdriver got the sockets back in good shape and a reliable connection ensued.

It was now time to calibrate the fuel level sensor by slowly adding known amounts of fuel and noting sensor values. Time consuming but straightforward enough. Result: a reliable port fuel tank level indication on the D120 EMS display.

Finally, taking the main landing gear wheel spats off to inspect the landing gear I was surprised to find a broken spat mounting bracket on the starboard side. Apparently this is a known problem and breakage is "just a matter of time". The solution is to weld some gussets into the bends, so I shall get that done on both sides as soon as possible. Meanwhile I can fly without the spats fitted, with only a small reduction in airspeed.

The remainder of the inspection went well and my ever helpful inspector signed everything off yesterday. I completed the test flight yesterday afternoon and all the paperwork is now on its way to the LAA for permit renewal.

Monday, 4 March 2013

Early experiences with the new prop

I've got about ten hours of flying in the log with the new propeller and everything seems to be just fine.

It actually makes the Sportcruiser a quite different aircraft in many respects, not just because it is faster. All aspects of the flight envelope are improved, from take-off run to climb rate and cruise speed, to fuel economy.

The aircraft feels different too. The constant speed of the propeller means that the old tell-tale sounds are now largely absent. Where I would notice a small change in engine pitch and start looking for the cause (perhaps an inadvertent nose up or down) now the engine note is constant and after a while it almost fades into the background.

I find that I tend to cruise at around 100 to 105 knots now, where 85 knots was about the going rate with the old propeller. 15-20 knots may not sound like a lot but it is at least 18% faster and that makes for significantly reduced flight times. The speed doesn't fluctuate as much as it used to either.

The sweet spot seems to be at about 18 litres per hour and 100 to 105 knots. This compares with 16 litres per hour and a cruise speed of 85 knots. So I've increased my cruise speed by around 20% for an increase in fuel consumption of 12%. Not a bad result!

On Saturday I flew up to Fife and back along the east coast, past St Abbs to Boulmer and then along the northern edge of Newcastle's zone before heading back to Carlisle. A total distance of 211nm with a flight time of 2h 15m, giving an average speed of 93.8 knots. Given that there was almost 7000ft of climb over the two legs, that ain't at all bad!

Thursday, 28 February 2013

Reviewing 2012

Somewhat belatedly, I realise that I should have commented on my flying in 2012.

It was my best year ever, with 118 hours total flying, of which all but one hour was in G-JONL. I had several trips down south, including, on one trip, a wonderful soiree around the west country, dropping in on delightful airfields such as Dunkeswell, Bodmin and Perranporth then continuing right on down to Lands End. A few days later I was swanning around the Western Isles of Scotland. So it seems I have been getting around the place a bit!

2012 was also the year that I made my first visit to the LAA rally at Sywell and that enabled me to progress the approvals for my variable pitch propeller. It was great to meet some of the LAA people that I was dealing with on a routine basis whilst building my Sportcruiser.

G-JONL performed flawlessly throughout the entire year, requiring no significant maintenance work and always ready whenever I wanted to fly. It's a real delight to own an aircraft that is available whenever I want it and which is proving to be so reliable.

It may be hard to beat the amount of flying I managed in 2012!

Tuesday, 29 January 2013

Back in the air... perhaps

The revised paperwork from the LAA arrived today, so I am good to go with the new propeller. Sadly the weather is not cooperating, so I think it will be the weekend at the earliest before I can slip the surly bonds once more.

Friday, 25 January 2013

Propeller flight testing completed

Well it took a while, what with the typical wintry weather (why is it that I always seem to be working on my aircraft when it's sub-zero and there's snow on the ground?) but the flight testing is now completed and all the documentation has been sent off to the LAA. E-mail exchanges confirm that the LAA is happy with what they see, so I am now just waiting for the revised Permit to Fly and Aircraft Limitations documents to arrive. Meanwhile, JONL is grounded, not that that is a big issue - I certainly wouldn't be interested in flying in these meteorological conditions!

So what's the result of fitting the new propeller?
  1. Improved take off performance 
  2. Significantly better climb rate
  3. Higher cruise speed
  4. Slightly better "MPG"
The take off performance of JONL was always pretty good but I reckon the new prop knocks a good 50m, possibly more, off the take off run. The acceleration is noticeably more brisk and, of course, the engine RPM is constant at 5800 RPM, which certainly sounds a little different.

Shortly after take off, once a positive rate of climb has been established, it is generally considered good practice to bring the engine revs back to 5600, as that makes the climb out a quieter affair. When I had the fixed pitch (FP) propeller, I had been in the habit of slightly reducing the throttle to achieve the same effect. I haven't yet fully worked out the best combination of engine speed and throttle but I think it will still be entirely reasonable to slightly back off the throttle, whilst setting RPM to 5600.

With the RPM at 5600 the climb rate is significantly improved. The FP propeller gave me around 700 feet per minute (ft/m) climb at MAUW, noticeably tailing off at higher altitude. The new propeller gives climb rates around 900 ft/m and less tailing off at altitude. One up, with relatively little fuel, I've seen climb rates in the 1300 to 1500 ft/m region, although I can't keep that rate of climb going for very long. More usefully, at a very acceptable 500 ft/m climb rate, the air speed is now around 90 knots, whereas with the old propeller I would struggle to make 75-80 knots.

Which brings us neatly onto cruise speed. Now, of course, I have two levers to play with and just like having multiple gears on a mountain bike, there are combinations that work and those that do not. So I have spent some time finding the sweet spot(s). It turns out that there are several.

The following are at 4800RPM and at 2000ft altitude:

  • At what was my old sweet spot cruise, a fuel consumption rate (16 litres/hour (l/hr)) I now get an air speed of around 94 knots for 23" manifold pressure (MAP). This is an improvement on the 85 knots or so I used to get but it's not the best setting. 
  • At 24" MAP my speed has gone up to 100 knots for just one litre per hour higher fuel consumption. Better but still not the best.
  • At 24.7" MAP and 18 litres per hour, I was rather impressed to see 105 knots.
  • At 25" MAP, I now get 108 knots for 20 litres per hour. This is 27% faster than the old FP propeller for more or less the same "MPG" and seems to be the best overall performance.
 In practice, I think that unless I am in a real hurry, 25" MAP is more than I need to use. I think it will work out the sweet spot area lies somewhere between 24.5" and 25" MAP.

I did try other RPM settings. 4900RPM gives very similar airspeed vs. MAP readings but with noticeably higher fuel consumption. 4700RPM appears to give much the same airspeed, MAP and fuel consumption results as 4800RPM but the engine will be working harder, so that's probably not a good place to be. So it looks like 4800RPM is the place to be.

The new VP propeller is giving me around a 25% improvement in airspeed for a 12% increase in fuel consumption compared with the old FP propeller. The entire flight envelope is noticeably more agile.

This was a fairly major modification, certainly the biggest I have undertaken since completing the build, almost three years ago. I think it has been worthwhile!