Jul
13
2010

Inversions

Here in Idaho, specifically the Treasure Valley, we have to deal with inversions. Here is some info I found on ParaglidingForum.com that might help. Copied here in case something happens to the original.

Posted: Wed Feb 27, 2008 3:55 pm    Post subject: Techniques for exploiting weak lift under an inversion
Hi All,

Over the winter we get a lot of stable, high pressure days here in the Alps. I’ve been trying different strategies for using the weak and broken lift. Here are some of my thoughts, I’d be very interested to learn from you lot as well!

In strong conditions you use pretty much just two modes: either cranked over in the core or straight line speed to the next climb. In weak lift, however, I find myself using a much wider range of techniques, from tight turns in small cores to wide lazy turns in weaker areas, and all speeds from minimum sink to best glide (often a bit of speed bar).

On stable days there are usually inversions at several levels. It can be hard to impossible to break through an inversion above you, but it’s very easy to fall through an inversion below. This means that once you drop below a given level you can be stuck beneath it for a long time. So it’s best to stay high! The many levels of inversion are easily visible.

Photo: Quentin King

Flying on a stable day in Annecy. Several inversion layers are visible. Photo: Quentin King.

AIR MOVEMENTS

Between the inversions you still get thermals, but on a very small scale. The ground below the lower inversion cannot contribute to thermals so the total heating source area is very small: it’s just the ground above the lower inversion. Similarly, thermals do not rise very far before they stop at the inversion above so they do not have time to form larger bubbles. This model explains both why thermals are small, turbulent just below the inversion (where they break up) and also why it’s easy to bounce along just below an inversion. At the inversion level you have the maximum area of thermal source (ground) beneath you generating thermals. If you’re just above an inversion then you effectively have no thermal source area beneath you and there is little lift available.

This is in fact the usual cell structure with both rising and sinking air. On a “normal” day this cell occupies the entire surface boundary layer with air warming at ground level and rising all the way to the top of the convection (cloud tops) several thousand metres higher. On inverted days you get a complete cell between inversions. The cells can be only a couple of hundred metres high, or less.

TACTICS

You’ll find the most reliable lift above the thermal source, i.e. close to the hill. Sometimes complete thermal bubbles release but more often I find myself tucking in close to the hill and soaring the slope breeze. I find the slope breeze is often strong enough to allow me to maintain altitude or even climb up to the inversion above. However, the slope breeze is fairly constant, light lift and does not always have enough momentum to break through the inversion. If you want to break through then you have to leave the comfort of the slope breeze and try to find a strong bubble that has broken free, clear from the hill.

In weak lift small differences in sink rate can make a big difference, certainly in climb rate relative to other pilots and sometimes the difference between staying up and sinking out. You get your best sink rate flying at minimum sink in a straight line. Turning increases your sink rate! Therefore the best strategy is to do as few turns as possible. This generally means that long beats up and down the ridge and in the slope breeze are the most effective. Try to make your turns when you hit a stronger area of lift and tuck back into the slope breeze as soon as possible after making your turn.

Photo: Quentin King

Tucking in close to stay up. Photo: Quentin King.

Every now and then you will hit stronger lift and you have to make an immediate decision as to whether it’s worth trying to circle in it. The stronger lift tends to come in small bubbles so if you wait too long you will fly out of it and it will be gone by the time you’ve turned around and you’ll end up losing altitude overall. Generally I react in two different ways. If I don’t think it’s worth turning then I will still slow down to spend more time in the lift, but I won’t turn. If I do think it’s a very good bubble, given that I’m already flying close to min sink, rather then applying inside brake I let up quickly on the outside brake. This gives an immediate flat-ish turn on the spot. As the wing turns through ninety degrees I damp the wing as it starts to dive by re-applying outside brake and shift my weight to the inside. This technique gets you circling in the right place immediately and avoids flying out of the lift.

Once in the small bubble you have to turn tightly to stay in it! It’s more important to stay in the bubble as it breaks through the inversion above than it is to maximise your climb relative to the bubble itself. So, crank the wing over and hang on! This also loads the wing more and makes it less likely to collapse if your bubble breaks up at the inversion instead of breaking through.

STRATEGY

The general strategy is to stay up by conserving altitude and seizing opportunities to break through the layer above whenever possible.

Before even taking off you should look at other pilots and see how high they are getting. If they’re all below take off then it’s probably worth waiting for a while. If they’re above take off then go for it!

Photo: Quentin King

Pilots in the air show the various inversion layers. On this day the maximum altitude was about 1400m, 500m above take off. Photo: Quentin King.

Once in the air use the slope breeze to explore the local area and try to find which areas are working best. It’s even more important than normal to find the strongest lift in the area because it’ll be only in these places that the thermals will be strong enough to break through the inversion above. Once you’ve identified the best areas then hang around there waiting for your ticket out. The thermal will be small, but the cycles will be quick so you shouldn’t have to wait long.

Finally, when you reach the maximum altitude for the day (you can judge this by looking at the other pilots) then go exploring. You’ll be just below an inversion so you’ll have the maximum possible area of thermal source below you so you should be able to bounce along without needing to turn much, just slowing down in the lift and speeding up in the sink should keep you at a roughly constant altitude. It’s not worth wasting time trying to get higher: you won’t.

On a normal day you’ll climb high in a thermal then do a long glide. On a stable day you’ll climb a bit, glide a bit, then climb a bit again. The climbs won’t be strong, indeed you’ll have to be very patient at times, but the sink won’t be strong either so your glides will be better. On some long alpine ridges you can fly several tens of kilometres in a thin layer between two inversions. Anything other than very short transitions are difficult because you lose too much height and end up stuck below lower inversions.

Anyhow, that’s a bit of a brain dump from my last few flights in Annecy and Chamonix. Please tell me what your secrets for making the most of weak/inverted days are!

Cheers,
Tom

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