Dartmoor Gliding News - Saturday 16th August 2025

The key to today was the wind. There was a strong 15knot+ easterly with forecast winds of 30knots+ at flying heights. Unfortunately this is well outside the limits for visitor flying so we reluctantly cancelled today's bookings.

The atmospheric soundings did not look at all right for the formation of wave but in these conditions a "hydraulic jump" is always possible. What is this I hear you ask? Well when a fluid ( the air in our case ) reaches a critical speed, after coming down from being lifted over Dartmoor perhaps, then a single standing wave builds up. If the windspeed is above the critical value the standing wave forms but also produces chaotic flow downwind. 

An AI search gave this description

A hydraulic jump is a phenomenon in open channel flow where a fast-moving, supercritical flow transitions to a slower, subcritical flow, causing a sudden rise in water depth. This transition is characterized by energy dissipation and increased turbulence, often observed at the bottom of spillways or in steep mountain streams. 

If you would like to read more try this

https://en.wikipedia.org/wiki/Hydraulic_jump

or perhaps this video

Anyway what actually happened. An initial K13 flight seemed to indicate conditions calm enough for solo flying The second flight ended prematurely with a cable break. The next flight showed just how turbulent the atmosphere really was and flying was suspended with a hope that it might calm down later.

Getting ready.

Keegan and Rick before the first flight

Off they go
Note the cap cloud over the moor

A working party decided that filling in the potholes on the track would be a useful endeavour and after some initial troubles with the tractor hydraulics made good progress with this.

The gang

Rick like machines of all descriptions

Follow that tractor

John is obviously an experienced ground worker.
He leans on his shovel with real style

I decided that the slow puncture of the Puchacz front tyre should be investigated. This led into some difficult engineering challenges as the tapered cotter pins were seized in after what must be many years in position. My solution to this, after finding a suitable extractor, was to apply Scratch to the issue with his big hammer. Luckily no puncture to the inner tube was discovered and, after cleaning out a quantity of rubber swarf from the tyre and replacing the valve, the wheel was refitted with the cotter pins treated with copper grease to aid future removal.

Me wondering why I ever started this job

Another team effort

After a late lunch it was decided that conditions had calmed down enough to try flying again. INdeed the conditions were manageable and it would be useful for the assembled pilots to experience the conditions with Rick in the K13. The last flight of the day was Malcolm Wilton-Jones in the Twin Astir. This is his take on the day.

I had planned to leave by 5 and it was now 4:30 so a quick call home gave me a short extension so a quick brief from Rick regarding the conditions and off I went. The launch was OK until the speed started to increase rapidly so I released at about 1,200' agl and climbed to burn off the excess speed but it felt like I was being rocket assisted and I reached around 2,000' in no time - a check of the attached chart shows that at one point I was climbing at 22.2 kts. Clearly I was in the rotor as it was exceedingly rough at times but the strongest lift was relatively smooth. I searched back and forth trying to find some silky smooth wave but could not locate any, although I was rapidly gaining height and soon reached 4,633' asl, about 3,800' agl after 15 minutes, well above the clouds forming over the airfield. I had promised to land around 5pm local and the club gliders had been put away so it was brakes out and into a very high circuit to land after 18 minutes.

Malcolm's flight

An interesting and productive day.

Steve