Tuesday, 31 May 2011

Wobbly prop?

I suppose I'm getting itchy fingers. It's 15 months since I finished building JONL and I'm starting to think how I might get my hands dirty again making a few improvements.

One thought that recurs often is the idea of fitting a variable pitch propeller to my steed. This, it is alleged, would transform the aircraft, providing even shorter take off runs and anywhere between 5 and 20 knots of additional speed in the cruise, whilst simultaneously reducing fuel consumption. Quite a feat if all that can be achieved in one go!

The principle is simple enough: at slow speeds a fixed pitch propeller is set too coarse and as a result the engine cannot reach maximum revs and thus, maximum power. By having a finer pitch this problem is fixed and take off becomes a more efficient affair. Conversely, in the cruise the prop pitch needs to be much more coarse so the propeller takes a bigger slice of air per revolution at a given engine speed, resulting in higher air speed. The problem is that these requirements are mutually inconsistent, so a compromise is necessary with a fixed pitch propeller. And compromises aren't good in aviation.

Enter the in-flight variable pitch propeller. This can be adjusted for fine pitch during take off and coarse pitch in the cruise. The propeller and engine can therefore work together to provide the best combination of power and thrust for any given situation. The system is variously called variable pitch, constant speed or, more prosaically, wobbly prop. And I want one!

There are a couple of options that are approved for fitting to the Sportcruiser. One is electrically driven, with a pitch control motor in the propeller spinner, connected via slip rings to the pitch controller. The other variant is perhaps nicer. Here the propeller pitch is driven via hydraulics, with the control motor mounted somewhere convenient, such as the fire wall.

The electrical system is simpler and cheaper but it is also heavier and the weight is in the wrong place, right at the front of the aircraft, which is a nuisance for weight and balance considerations. The hydraulic system has fewer moving parts and much less weight at the nose of the aircraft but is the better part of twice the price.

It's a difficult decision and I'm not planning to rush into it but I am thinking about it quite hard. I reckon there is every chance that this will be a project for when the summer flying season is over.


Yesterday was the first decent day after quite a long run of wet and windy weather. My friend Mike and I had hoped to get up to the Glenforsa fly in last weekend but it was a washout and in the end we didn't event attempt it.

I'd promised to take one of our Air Cadet instructors flying and we decided that a trip to Bagby, near RAF Leeming in North Yorkshire would be a pleasant excursion. The weather on the way out was OK but there was still some low cloud scudding around, so I decided to route via the Hexham gap, towards Newcastle, rather than try to get up to 3500ft VMC to clear the high Pennines on the direct route.

Although all the MATZs were closed (the RAF doesn't do weekends and does bank holidays even less), nearby Topcliffe was active with motor gliding so we had to give that a wide berth, resulting in a rather circuitous approach to Bagby from the north. In all the faffing around I carelessly managed to completely mislay Bagby airfield and ended up more or less overhead before I'd realised what was going on! No problem - a quick reposition to downwind right for 06 and a minute later we were on the ground.

What a thoroughly delightful airfield! Within a few minutes of landing JONL had gathered a group of enthusiastic onlookers and I thoroughly enjoyed chatting with them for a good hour. There is a friendly little clubhouse where you can get coffee and snacks. Much to my surprise I found it impossible to pay a landing fee, even the offer of a contribution to the grass cutting fund was politely declined.

All too soon it was time to start heading back to Carlisle. In the meantime the cloud base had lifted considerably so we routed directly back home, once again giving Topcliffe a wide berth after departure. Although the return journey was shorter, it was slightly into wind and as a result the outbound and return journeys took exactly the same time: 65 minutes, chock to chock.

I'm very taken by Bagby International. It's one of several airfields that I've visited on the east side of the Pennines and all have been remarkably friendly, delightful places to visit. A return trip will have to be scheduled!

Saturday, 7 May 2011

Just bimbling around

I find that I've been somewhat remiss in that I've not updated this Blog for a couple of months! It's not been the best of times for flying to be honest but I have managed a dozen or so local bimbles and the odd land-away.

In April I dropped into Kirkbride as part of the annual flight test for JONL. It was nice to catch up with the gang there after a couple of months of being based at Carlisle.

Early in May I flew to Andreas on the Isle of Man to visit a radio amateur friend. Andreas is on the northern tip of the island and is very convenient for my friend's home. It's also a considerably lower cost option than flying into Ronaldsway, with its mandatory handling and relatively expensive landing fees.

The journey over was a real pleasure, with excellent visibility and virtually no cloud. This enabled me to get up to 5000ft for the water crossing from St Bees Head , which is always comforting in case the engine should suddenly realise it's over water and decide to resign its commission. With a 20:1 glide ratio, I reckon that at 5000ft I was always within glide range of one or other coast, although quite what I would do if I had to return to the 300ft high cliff edge of St Bees Head is not entirely obvious.

A rather soggy Andreas airfield
The return journey was not so pleasant. An unexpectedly vigorous trough produced quite heavy and un-forecast rain on the day of my departure. In the end I decided to leave early as that would let me fly away from the bad weather rather than into it. The combination of rain, low cloud and sea made for one of those odd occasions where the flying was definitely VFR, yet there was no horizon whatsoever. Not for the first time I was glad that I have an IMC rating and flew the water crossing at 2500ft, just below the clouds entirely on instruments.

Once I reached St Bees Head it all became a lot easier. Firstly there was land to see and that created a horizon to fly by. But I was also heading out of the weather and by the time I got back to Carlisle it was dry with high cloud and all the 9's visibility. The excitement wasn't entirely over though, for the north/south runway was closed for the upcoming Radio One Big Weekend and I was obliged to land with a fairly hefty crosswind on 07. Not the prettiest of landings but hey! I could still use the aircraft afterwards, so it was, by the usual definition, an excellent landing!

All the other flights have just been local bimbles so I won't bother you with further waffle on those.

Thursday, 5 May 2011

My first annual

G-JONL is a year old!

Golly, that came round quickly. It seems only yesterday that I was finishing off the build and wondering whether it would actually fly. Now, with some 70 hours in the logbooks it was time to see whether it would be allowed to continue to fly.

Tom, my LAA inspector was, of course, his usual helpful self.A fairly rigorous inspection of flying surfaces, controls, engine and avionics followed but it didn't take Tom long to pronounce JONL fit and well. The necessary paperwork took nearly as long to complete, over a coffee in the airport cafe, naturally.

Next was the flight test. As I now have plenty of hours on the Sportcruiser there was no problem with me flying this myself but an observer was needed to take notes. Step forward Don, a radio amateur friend, with whom I had been on a weekend of boozing interspersed with some radio convention stuff. As he used to fly gliders he wouldn't be fazed by stalls and VNE dives, so off we went.

To be honest the flight test was a complete non-event. The aircraft behaved impeccably and the flight results were almost identical to those that we had recorded on the original flight tests a year earlier. I'm still amazed just how slow you can fly a Sportcruiser before it is anywhere near the stall, especially with full flap!

As part of the flight tests we had to do an aborted landing so we popped over to Kirkbride and did it there. After the baulked landing we did a proper landing and partook of a quick coffee in Mike's caravan before heading back to Carlisle, job done.

I must say, the LAA excelled themselves. The new permit to fly was back with me in well under a week, making it possible for me to do the trip to the Isle of Man a few days later (see separate report).