Single-surface wings are incredible. They weigh nearly nothing, fit in to your hand luggage, and yet are still real paragliders, capable of soaring and even cross-country flights. Understandably, many pilots are considering buying one. This post exists to help you make that decision.
All wings are compromises. Traditionally the compromise has been between performance and passive safety as measured by certification. Single-surface (SS) wings are equally compromises, trading some disadvantages for other advantages over classical double-surface (DS) wings. Let's go through these.
Weight. SS wings are much lighter - but not half the weight, as you might have expected. There's no under-surface, but there is still a lot of fabric structure, more line, and the same risers as a DS wing. Consequently, you typically save about 40% of weight - significant, but not 50% - compared to a DS wing.
When considering weight, you need to consider the total weight of your gear, which includes your harness, reserve, helmet, flight clothing, and food. Dropping the wing weight from a 2.2kg DS to a 1.2kg SS sounds like at 45% saving, but is much less interesting if your total flying gear weight drops from 5.4kg to 4.5kg - that's just 17%.
Pack volume. Fabric is not heavy (typically 27g/m^2 for a lightweight cloth) but it does take volume. SS wings have much less fabric, and you can squeeze the air out of both ends, so they pack much smaller. You can pack them into 4-6 litres of space (that's a tiny backpack), easily half the volume required for a DS wing. When they're this small, there's no excuse for not having the wing with you on every trip.
Price. The cost of building a paraglider is the composition of many things. Fabric is expensive (which SS wings have less of), but so is line and construction complexity (which SS wings have the same or more of). Overall, like weight, expect an SS wing to be cheaper than a DS wing, but not by as much as you might initially expect.
Fragility. SS wings are built to be light, and feel very fragile in your hands. Yet, they still pass the same 8G load tests as normal wings. Made of light materials, you need to be more careful not to snag lines or fabric on rocks or bushes, but here they are no different to their DS cousins.
Inflation. SS wings love to fly. Like, they just wanna be up there above your head all the time, even in the lightest zephyr. This is fantastic in zero or light winds (and even backwinds) but becomes a problem in stronger winds. Specifically, DS wings are easy to pin down on the ground using the brakes or D-risers. SS wings are always trying to leap up into the air. Great if you're looking to fly from a tight spot, but if you're launching in wind you're gonna be fighting the wing all the time until you get off. Similarly, for landing in wind this is a problem: the only way to "kill" the wing can be to run downwind past it, gathering the wing up as you go. Doable on a solo wing but almost impossible on a tandem.
Launch. They're immediately above your head and inflated, but they're not lifting you yet. You still have to run, The runway needed to take off is perhaps slightly longer than a DS wing.
Calm-air glide. Here, they're pretty OK. They don't glide as well as DS wings, but modern SS wings get a comfortable 7:1 or so in still air.
Speed. First-generation SS wings were very slow (e.g. 26km/h, with no accelerator). Modern SS wings claim trim speeds closer to DS wings and have speed systems offering a few extra km/h. However, if you're considering a SS wing you should measure the trim and accelerated speeds yourself.
Turbulence. This is where the compromise happens. Many things are different.
Firstly, SS wings move around a lot. They don't contain the mass of air that DS wings do. The air in a DS wing easily weighs a few kilograms, SS wings have no air in them. SS wings move initially very quickly, and then stop. This makes the glider feel very nervous: they jump around, feeling like they're gonna dive too far, but then they just stop. This takes some getting used to. They can be more stressful to fly than CCC competition wings.
These damped responses make SS wings great wings to throw around: you can wingover and spiral as much as you want, but it takes perfect timing to get large-amplitude movements, and as soon as you stop, the wing immediately loses all energy and resumes normal straight flight.
Secondly, SS wings lose all performance in turbulence, period. Rough air is where glider performance differences are most evident, and here SS wings are the worst of the bunch. Once it gets even a little bit bumpy, SS wings perform significantly worse than a school EN A wing, At times, it can feel like you're descending vertically with no forward speed - and there's nothing you can do about it.
This is really important for your flight planning. If you launch a SS wing, you need to have a landing field within a 2-3:1 glide angle, unless you're very sure of the weather conditions.
Sink rate. SS gliders have higher sink rates (especially in thermals which are by their nature turbulent) and have less efficient turns, even in smooth dynamic lift. You get more feedback from the wing's nervousness, which compensates to a certain extent, but you're still going to climb less well than a school wing. Don't expect to be top of the stack, and you'll almost definitely be the first to sink out and land.
Landing. Older SS wings have very poor flares. You need to pull on the brakes fractions of a second before you touch down, and even then you get a hard and fast landing. Modern SS wings claim much better flare characteristics, but you should test the wing yourself before buying it.
In conclusion, SS wings are awesome. If you want a wing that you can take with you everywhere to fly in good conditions, then get one. They're not without compromise though. The range of conditions in which SS wings are safe and fun to fly is significantly smaller than the conditions for a lightweight DS wing. An SS wing is not a replacement for a DS wing, it is a complement to it.