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!

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