Multi-element airfoils are used in a variety of aerodynamic devices from airplanes to bicycles. Let's take a brief tour through the applications of this versatile airfoil configuration.
Multi-element airfoils are typically closely coupled airfoils consisting of 2 or more elements. There are two basic types of additional elements:
- Slats are deployed in front of the main airfoil element
- Flaps are deployed behind the main airfoil element
Slats tend to delay stall where increases in the angle of attack no longer result in incremental increases in lift. Flaps tend to provide increased lift at a given angle of attack compared to the lone main airfoil element. Additional airfoil elements tend to significantly increase the drag of a configuration.
The most efficient means to generate lift and minimize drag is a single airfoil as used during cruise conditions on airplanes. However, airplanes also have to land and take off at much lower (safer) speeds, so to provide lift at these lower speeds they deploy slats and flaps, known as high-lift devices. They essentially transform the main airfoil into a multi-element airfoil (sometimes consisting of 6 elements) which produces increased lift at these lower speeds, with the penalty of increased drag.
You'll notice that to compensate for the increased drag during landing, airplanes increase their engine thrust relative to their cruise condition – that's most of the extra noise you hear during landing.
In motor racing, such as Formula 1, there are limited volumes in which to position airfoils, especially at the rear. It would be difficult to fit a single optimal airfoil within the rear F1 rule mandated box. Instead, fitting multi-element airfoils is a better compromise in generating the required downforce.
Downforce in F1 is always more important than drag, so the extra drag associated with multi-element airfoils is something for the engine developers to counter with more horsepower.
Multi-element airfoils are incorporated in the front forks of some time-trial bicycles resulting in a claimed significant reduction in front wheel drag. It is accepted that airfoil shaped bicycle forks, known as aero forks, reduce drag, but the use of a multi-element airfoil configuration is a novel idea.
The manufacturer of the Opal Concepts JetStream forks explains that the multi-element airfoil configuration draws air away from the top of the front wheel where the spokes are traveling at twice the speed of the bicycle and in so doing reduces the drag created by the spokes. This counterintuitive claim – multi-elements airfoils usually increase drag compared to a single airfoil – was cited as a possible cause of Australian Cadel Evans' improved time-trialing in the 2007 Tour de France. Other techniques to reduce the drag caused by bicycle spokes are to replace them with a solid disk or substitute airfoils in their place.