A while after writing this article I encountered another reference to flight by birds suggesting that, in addition to other ‘slope soaring’ methods, they use dynamic soaring using the technique below rather than the more gradual wind gradient thought to extend for some height above the waves.

Soaring Without Lift - Nigel Page - December 2001

That is, without lift as in 'rising air' and not as in 'force produced by air moving over a wing'. Impossible? Thousands of sea birds can't all be wrong, or appear to defy the laws of physics by playing Uri Geller type tricks on us!

Dynamic soaring is not, as some pilots think, the same as soaring in hill lift. i.e. Rising air forced upwards over the windward side of a ridge. In dynamic soaring energy is obtained by flying to and fro between two air masses which are moving at different speeds. Sounds tricky? Nobody said it was going to be easy! Until recently I had heard of no one managing to soar a glider continuously in this manner, although some sailplane pilots are thought to have stretched their glides a bit using this technique. Believe it or not the people who have cracked the problem are model glider pilots, some of whom gave a very impressive demonstration on Rushup Edge, a Derbyshire paragliding and hang-gliding site, on Saturday 8th December. Lets have a look at what's going on.

First we need two airmasses moving at significantly different speeds adjacent to each other. The model pilots find this condition on the downwind side of a sharp ridge. Apparently Rushup edge is a known good site for this. 

At this point most pilots break into a cold sweat (Any who don't should be given a wide berth!). What about rotor and sink? There certainly may be some rotor but the modellers have to choose conditions which minimise turbulence and give a laminar flow. In general this will be cold, stable conditions. I'm not going to go into lapse rates and all that stuff here, it gives me a headache. Put as simply as possible, air which is stable does not want to go up and down. It tends to flow around hills rather than over them and can 'fetch up' in front of a ridge. It will also leave more 'dead' or slow moving air behind ridges and in the bottom of valleys. Laminar flow means that the air tends to flow as if it is in layers sliding over one another with very little mixing, or turbulance, between them.

This drawing is probably not a very accurate depiction of what is going on, I can't see the air! On the Saturday I measured a wind speed of 10 to 12mph at the top of the ridge which would normally enable paragliders to soar easily on the windward side. On this occasion, because the air was stable, very little air was rising in 'front' of the hill and any paraglider who launched went down.

To obtain energy ('lift' is not the correct term to use for dynamic soaring!) the model is flown on the downwind side of the hill into wind from the slower moving air into the faster air. The model then turns downwind which gives it an acceleration and flies back into into the slow air considerably faster than it left it. Another turn back into wind and the extra airspeed is used to fly it back into wind and up into the faster air again for another go.

The actual pattern preferred by the modellers is a sort of a sloping oval with the time spent in the fast air kept to a minimum. It needs a lot of piloting skill.

So, apart from needing the other halves of our brains removed, why can't we do it in sailplanes, paragliders or hang-gliders? The actual acquision of kinetic energy occurs only during the turn in the fast moving air. This energy has to be enough to carry the glider around the rest of the 'circuit' and our gliders are just not efficient enough. Some sailplanes might be if they could find a big enough area of profound 'shear' to fly in.

A few types of sea birds are thought to use a combination of dynamic lift from wind gradient and 'hill' lift from big waves to soar continuously over the sea by flying a complicated zig-zag pattern. I have watched birds soaring east facing Bempton cliffs in a westerly and assumed they were simply flying in a big rotor. Now I'm not so sure that's the whole story. The model pilots have certainly broken some new ground in sorting out dynamic soaring. There is much we can learn from them.

If you want to find out more about how the model pilots do it then have a look at www.dyanmic-soaring.co.uk .

Copyright © Nigel Page - March 2003