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Rigid wings and flex wings

Christian Ciech


    For many years, rigid wings were seen only rarely at the popular hang-gliding areas. But today they are gradually shouldering in amongst the flex wings, introducing new and interesting ideas into the world of hang-gliding. Over two decades have gone by since the appearance of the Manta Fledge, a rigid wing that offered excellent performance and optimum strength, in the 1970s. The Fledge was used a great deal for acrobatics, as a result of its robust structure.

    Today's new rigid wings are soaring machines offering a higher performance than any other type of hang-glider or paraglider, while conserving those characteristics of practicality and functionality essential for all hang-glider pilots.

    Personally speaking, the thing that I appreciate most of all in a rigid-wing hang-glider - apart from its unparalleled performance - is the fact that it makes the search for "lines of thermal energy" more important, and more rewarding. By "lines of thermal energy", I mean the lines along which ascending columns of air develop, enabling the pilot to optimise his speed.

    Modern rigid wings and the correct exploitation of lines of thermal energy make it possible to perform flights that would have been unthinkable, even just three or four years ago, with any other type of hang-glider. In addition, the more efficient the machine that we are using, the lower the effect of our mistakes. I don't mean just that the better the wing, the easier it is to fly further: that's obvious. What I mean is that, considering an equal period of time, we make less mistakes when we fly with a wing of greater efficiency compared with another of lower performance. As a result, we will be able to fly faster and further, not just because the greater efficiency of the hang-glider enables us to, but also because we will have flown better.

    A few years ago, during cross-country flights, pilots often deliberately abandoned a promising cloud street when it took them a few kilometres away from their ideal course. Today, that sort of decision would be increasingly disadvantageous. To provide a concrete example, we can compare two champion wings, a flex wing and a rigid wing.

    Let's pick a pilot at random, say... Manfred Ruhmer (four times world champion, etc.). With his gleaming new Laminar 07, he flies at 75 km/h, or about 20.8 m/s, with a descent rate of 2 m/s - you may think that this is over-optimistic, but it's not far off. He flies into a cloud street where the air is rising at 1 m/s. This means that his glide ratio doubles - because his descent rate drops to 1 m/s from the initial value of 2 m/s - from 10.4 to 20.8. Now, lets consider your brand-new Stratos. At the same speed, your descent rate is 1.5 m/s. To be honest, there's not an enormous difference... but this is just an example! When you enter the same line of rising air, your glide ratio will treble, because your descent rate will be just 0.5 m/s, and your glide ratio increases from 13.9 to 41.7. Never mind, Manfred ...

    As I mentioned previously, I have often decided not to follow a promising cloud street because it would have meant increasing the distance flown. But in recent years, this decision has increasingly proved to be a mistake.

    It is easier - and far more enjoyable - to look for lines of thermal energy using a high-performance wing instead of a lower efficiency craft, because our efforts reap greater rewards. Moreover, in a rigid wing, the ease of control, or rather the force necessary for controlling the craft, is virtually independent of the speed at which you fly, and in any case, the force required is very low. All considered, it should come as no surprise that with a rigid wing, you can soar along cloud streets better than any (and I mean any!) flex wing pilot.

    On a line of thermal energy, you can slow down, keeping the wing at the angle of incidence for maximum efficiency, and maintain perfect control. This is not always possible with a modern flex wing, because when the tension of a variable geometry flex wing is set at its maximum - in order to achieve maximum efficiency - control is harder at lower speeds.

    But the flex wing has a few points in its favour. It responds directly and immediately to your commands, and this gives the feel of it being a part of your body. A flex wing transmits exciting sensations. You feel the air rising faster and faster around you, and you understand the pattern of its movements, metre after metre. From this point of view, your sense of feel alone is sufficient to monitor what is going on. And of course, flex wings are a little more practical in terms of transport and assembly, even though there is not much difference.

    At the present moment, limitations on the development of hang-gliders depend on a number of factors. Economic factors are probably more significant, in this respect, than structural considerations. The hang-glider market is relatively small, and this means that manufacturers cannot invest large sums into research. This sort of investment is costly, particularly in the case of rigid wings. Development continues, of course, but the limited resources available mean that the timescale is longer.

    Other limiting factors depend on the fact that the glider has to be foot-launched and foot-landed. This calls for airfoil sections that permit flight at very low speeds, which are therefore less efficient at high speeds. For the same reason, gliders cannot be too heavy, and so wingspan and wing loading cannot exceed certain limits. A light structure with a high aspect ratio would be far more costly to build.

    I think that the greatest opportunities for development, as regards flex wings, lie in the introduction of certain structural modifications. These would not even require the use of new materials. Such a hypothetically revised structure should ideally offer a different wing-loading distribution, in order to achieve better performance in rising air and in "dolphin flight".

    As regard fixed wings, there is also a lot of room for improvement, but the correct compromise between development and costs has to be identified. Though not a new idea, the market for this type of glider could receive a notable impetus from the production of a machine in which the pilot is enclosed in a profiled cabin. It would have to be highly functional, and not too expensive.

    The optimisation of techniques - and a consequent reduction in costs - for the production of those composite materials increasingly used in hang-gliders will also offer a powerful contribution to the development of both categories.

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