9 Ways To Get More Lift (Or Lose It) - Lift Augmentation
Ever since I was a kid with a window seat I have been fascinated but the mechanical goings on of the wings. Even now as an airline pilot, if I find myself positioning in the back of the plane I still can't help myself but study the jigsaw of metal and actuators during approach and landing.
I can't help look at a vane or unusually placed fin without wondering what aerodynamic purpose it serves.
Lift augmentation refers to anything that changes the amount of lift on an aerofoil. It allows aircraft designers to build aircraft that have a wide range of operating speeds that otherwise would not be possible. Without lift augmentation the landing speed of a 747 might be nearly 300mph, and there are not that many runways that are long enough for that. Equally if the wing was designed for a reasonable landing speed, it would be unable to cruise at high speed.
Lift augmentation allows you to change the shape of the wing in-flight or influence the airflow at different phases of flight.
1 - Flaps
Flaps are the most frequently used method of lift augmentation. They are panels on the trailing edge of the wing and come in several different flavours. They increase the camber of the wing and sometimes the area too.
The purpose of the 'slots' on certain types of flap is to re-energise the airflow, that is, at the angle of attack where the airflow would normally start to break away from the wing, the slot allows some high energy air from underneath the wing to help keep it 'sticking' to the upper surface.
Flaps are usually on the inboard section of the wing, this is for structural reasons and so as not encourage tip stalls.
From a modelling point of view, plain, slotted and split flaps are probably the easiest to model.
In terms of performance, flaps increase drag as well as lift, some more than others. Fowler flaps increase the wing area as well as camber.
Aircraft will often have stages of flap extension, using less flap for takeoff where we want more lift but less drag and more flap for landing where we want more lift AND drag.
This is a combination of an aileron and a flap. Usually when flaps are extended the ailerons droop down by a few degrees. It can be used in conjunction with an inboard flap, though you should be cautious that you don't encourage tip stalls by dropping the aileron too much.
3. Slats and slots
Slats are just like flaps but on the front of the wing. They are more often seen on high performance jet aircraft due to their structural complexity. On jet aircraft slats are arguably more important than flaps as they give the wing a much higher stalling angle of attack. They also provide less drag than trailing edge flaps.
From a model aircraft point fo a view, a fixed slat, sometimes called a slot is a good option. This does away with the moving parts whilst providing some of the same benefits. This is especially interesting for 3D printing as it offers some construction possibilities for integrated slots. Slots in the leading edge of the wing work in a similar way to slots in fowler flaps, by re-energising the flow over the top of the wing.
I am currently working on some fowler flaps for the Porter wing and think I might add a leading edge slot as well.
4. Leading Edge Flaps
Similar to leading edge slats but possibly easier to model, leading edge flaps extend out from under the front of the wing. The 747 has these 'Kruger flaps' on the inboard section of its wing.
5. Vortex Generators
Vortex generators are little vanes added to the upper surface of a wing.
They are usually added to a wing after the design stage when it isn't performing up to scratch, usually when the stall characteristics are poor. They protrude into the boundary layer and re-energise the airflow to prevent it separating from the wing at high angles of attack. Here is a great article with the maths included, and the video showing their effectiveness.
Vortex generators are something to keep in mind if your wing is behaving badly after the design stage.
6. Wing Fence
Wing fences, also know as boundary layer or potential fences, prevent span wise flow of air, usually fitted to highly swept wings. This span wise flow can make the effective chord wise airspeed at the tip well below the stall speed.
The wing fence literally straightens up the airflow and stops the cumulative span wise flow across the wing.
7. Vortilons and Saw Tooth
Vortilons work in roughly the same way as wing fences in that they disrupt the span wise flow of air over the top of the wings and re-energise the airflow. They do this by generating a high energy vortex that spins back over the upper surface of the wing They are only effective at high angles of attack (which is when they are needed) so are considered a better option to wing fences.
Saw Tooth, works in the same way. Here on the Cirrus they have what they call a cuffed wing, which is a similar. Sometimes this might be used in conjunction with washout.
As the name suggest, these are designed to spoil the airflow over the wing. When thinking about landing they are often called lift dumpers as they deploy on landing to dump any residual lift. In flight they can be used as airbrakes to slow the aircraft down by increasing drag and reducing lift (so the aircraft has to fly a larger angle of attack) and on most large airliners they are the primary roll control on the down-going wing. On the Airbus A320 using only the spoilers for roll control without ailerons is barely any different
From a modelling point of view the spoilers often seen on gliders are of interest, especially if you have a high lift design that doesn't want to land without getting close to the stall.