Aspect Ratio generally is related to the amount of lift that a kite produces compared to its drag; it is defined mathematically as the span of the kite squared divided by area (i.e. its length divided by its width). Sometimes AR is quoted in terms of projected area, when the kite is actually flying and the canopy is curved vs lying flat on the ground.
Kites have what's termed low, medium & high aspect ratio design planforms or features. 'Low aspect' foils are very 'forgiving at the edges of the wind window' to prevent luffing or stalling and falling, & are generally easier to turn without such occurrences. They are wider proportionate to length, have fewer & more voluminous cells and aren't nearly not as 'slim' per se as high aspect foils (which at the extreme can have as many as 40 cells compared to perhaps only 8-12 for the absolute entry level, and anywhere from 12-26 cells in the intermediate aspect range). Kites in the lower aspect range are more forgiving in most conditions, & being far less expensive, are well-suited to the less aggressive/competitive flyer, while at the other end of the scale, higher aspect kites are faster, have more 'forward' directional pull qualities & the higher the aspect planform, get significantly trickier to fly & turn.
The highest aspect foils are the dedicated race kites, developed through sophisticated R&D, attention to detail, internal construction subtleties, bridle type, and line thickness, eg. These are kites which most definitely demand constant rider attention, vigilant concentration & of course the skill to be effectively competitive in any event. So choice of kite design depends entirely upon a rider's purpose.
Racing events are extremely popular, especially in parts of Europe where contestants primarily use 3-wheeled buggies in wide open spaces, such as on the extensive firm surfaced sand beaches found in many coastal regions. The British Isles for one is famous for these havens.
The image below shows the plan view of two different kites. The top one has a lower AR than the bottom one, a high AR. In simple terms the longer and narrower the kite, the higher the aspect ratio will be, and conversely so.
Aspect ratio (AR) in part determines the performance characteristics of a kite. Some people believe that the higher the aspect ratio of the kite, the more performance it gives. This can be argued in theory, but can also be misleading. Aspect ratio is a strong determinant of the amount of drag on the wing; the deeper the sail the more drag. This is especially true in aerodynamic theory and when dealing with an extremely efficient airfoil. Kites are not as sophisticated as a high speed airfoil, and therefore aspect ratio is not necessarily the main source of drag for kites. For example, the thicker the gauge (diameter) of the flying lines used, the greater the amount of drag created.
…a few general conclusions on aspect ratio:
1. High AR is absolutely the way to go for larger kites because it increases the speed of the wing. This makes larger kites turn faster and are therefore easier to control. Most kites in a certain model range will typically have decreasing AR as their area or size decreases.
2. High AR for small kites is more a hindrance than a help if you are a novice pilot. It can make smaller kites unstable and likely to over-fly the wind window.
3. Low AR kites are much easier to re-launch than high AR as they can more easily roll over. The high AR kites can tend to get stuck on their leading edges.
The profile a kite has also plays a huge part on the overall performance. A thin profile generally makes a kite that flies fast, hence good for racekites.
A thicker profile gives a slower kite, however typically provides more lift. The shape of the profile is also very important; two profiles with the same height however, with very different aerodynamic shapes ― will behave very differently.
The image above shows two different profiles, the first, allegedly from a Flexifoil Blade and the second from a speed foil. They are very similar in size, however very different in shape so will behave very differently.