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Wednesday, September 19, 2007


In the art of model gliding there is always something new to learn and I wish to share some things I learnt recently, they may come in usefull.
On the recent trip to Volksrust some of our pilots remarked that the air seemed to lack power(not sure if this is the right word) plenty of wind, but when it subsided everyone came down.
It was a noticeably smokey and hazy sky ,as opposed to the last trip 3 weeks prior when air was cold(dense) clear and with vapour trials behind the jets.
There are a few theories , some air flows around the slope, some goes over and accelerates , early morning and late afternoon there are local anabatic and catabatic flows(warm air rising from the valley late afternoon and cold air sinking in the mornings).
With the above lesson in mind one also has to factor in thermal activity as the air warms up.
As the above relates to two incedents that occured over the Big 5 weekend let me explain, during a lull in the wind William from Durban launched his scale Discus and all it did was sink, lower and lower, after a while a thermal came through and he climbed back to ridge height, assuming all was now back on track he continued to fly on the slope front and again sank down , the thermal cycle was about 5-6 minutes apart and he had to outland long before the next lift arrives, so slope lift was replaced by thermal cycle lift.
Some time later the wind appeared to have picked up and I prepared to launch the Ventus,
Evan was doing the honours and I instructed him to wait until the wind picked up (air that is rushing in to a thermal that has already gone by). The glider was launched right at this time and fell away narrowly missing the rocks , it just seemed to sink and even when trimmed continued downwards. I decided to fly away from the slope as this was all sink (at this time I was already starting outlanding plans) .

In hindsight this air was being pulled back away from the slope to an oncomming thermal ,I flew in a straight line outwards and at minus 61mtr hit a good thermal , three turns to slope height and four more saw the glider 120 mtrs above the 200mtr radio mast (a total gain of 381 mtrs in 4 minutes) have a look at the data logged .

This flight ended after some high speed passes on the slope front and then a low slow ,again in this sink cycle resulting in a loss of aileron control, the aircraft came down into the scrub next to some picnickers (I should not have flown so close to the slope and they should not have been camped out in the flight line area) as Piet wisely put it 2 x wrongs dont make a right.
A lucky escape and a big lesson, there is a lot to learn about thermalling and slope air currents and some days they are both present take time to study the conditions.
Its a long story but I think it makes sense.

PS I have my own Jart plans, watch this space.

PPS better still why dont you also get some, they're free

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