Aerodynamics is a strange thing. On the one hand familiar, but also mysterious. We’ve all been outside on a windy day or stuck our hand out of a moving car window, so we’re naturally acquainted with the general effects of fast-moving air, but what the air itself is actually doing remains invisible, the realm of wind tunnels and high-powered computer simulations.
Many, many years ago I was one of those slightly annoying children who asked, “Why?”, all of the time, and growing up near a golf course, it wasn’t long before I enquired of my father, “Why are golf balls lumpy?”. Pleased to be furthering his child’s education my father confidently replied, “Son, It makes them go further”. Intrigued, and frankly somewhat suspicious, (I was old enough to know that speedy things like fast cars and aeroplanes were smooth and streamlined, and also that parents were not a reliable source of information… After all, they appeared to believe in both the tooth fairy and Santa!). So I paused, cocked my head to one side, furrowed my brow and launched my second-most-favourite question, “How?”.
Now, I don’t remember the exact response, but I’m 100% sure it didn’t involve the words, “Delaying turbulent boundary layer separation”, although there just may have been a mumbled mention of, “less drag”, immediately followed by dad disappearing behind the newspaper or going off to do something ‘urgent’ in the shed.
Why am I telling you this? Mostly because I want to talk about vortex generators, and they fall into the same category for pilots, as ‘golf ball dimples’ do for golfers: That is to say, most are familiar with them, a good portion know what they do, but far less know how they actually work.
Before we plunge headlong into the details of how vortex generators work, let’s first have a look at what they are and what they do:
VGs (to save ink/pixels I’m going to call them VGs from now on) come in many different shapes and sizes, but In their commonest form are thin, usually triangular, tabs attached perpendicular to a surface and at an angle to the oncoming airflow (see Fig.1). Invariably used in groups, when applied to aerofoils they are usually arranged in pairs along the span set-back from the leading edge.
Figure 1 – Typical vortex generator application
So we know what VGs look like, but what do they do? The obvious answer is, “Exactly what their name suggests”, they generate vortices. Behaving like tiny wings, each VG creates a small amount of lift perpendicular to the oncoming airflow and in the process sheds a trailing vortex downstream from its tip. This explanation is all well and good, but sadly not very enlightening, so a more practical answer is that VGs, “Fix aerodynamic problems”.
You can be pretty confident that VGs were nowhere to be seen in the original designs for almost every aircraft they are now attached to, in fact you can almost guarantee they were added later after something unsavory turned up during flight testing:
As far as possible, aircraft designers like the airflow to stay firmly stuck to the surface of their aeroplanes. Depending on the location, detached flow can result in a multitude of effects, from additional drag and early stall to ineffective control surfaces and stability problems. None of these traits is desirable, but unfortunately detached flows are hard to avoid. As soon as an aerodynamic body starts to narrow, such as at the rear portion of an aerofoil or fuselage the airflow wants to separate from the surface. Gentle tapering of surfaces helps, (giving familiar ‘streamlined’ shapes), but is not always practical and is ineffective at higher angles-of-attack or where surface discontinuities such as at flaps or control surfaces occur.
Hitting a Boundary
Flow separation occurs thanks to the behaviour of the air in a thin layer immediately adjacent to the aircraft’s surface (See Fig.2). Air has some viscosity – it’s not in the same league as honey, but nonetheless it possesses a degree of ‘thickness’ or internal friction. What this means is that when air flows over a surface some molecules stick to it whilst the others rub against each other as they flow past and are slowed down. This area of friction-affected air is called the boundary layer and it starts off very shallow, but thickens as the air travels further along the surface.
Figure 2 – The boundary layer and flow separation
When air flows over a tapered surface such as the rear portion of an aerofoil there is a combined effect of viscosity and also an adverse pressure gradient (the pressure is lowest over the front portion of an aerofoil where most of the lift is produced and then increases as the surface tapers). In this case the air immediately adjacent to the surface experiences both viscous drag and a pressure differential trying to push it back towards the lower pressure area at the nose. This can cause the airflow at the rear of the aerofoil to turn back on itself, reversing direction and so acting like a wedge forcing the oncoming airflow to separate from the surface.
A Quick Fix
If an aircraft design demonstrates flow separation problems the obvious solution is to tweak the aerofoil shape or re-contour the fuselage profile to solve the problem, but if the aircraft is already at the flying prototype stage, or is a one off design, significantly altering the outline of the aircraft will be expensive at best, and at worst completely impractical. This is where VGs come to the rescue. Because they are simply attached to the existing surface, aerodynamic problems can be fixed without the need for re-tooling or major structural changes.
Vortex generators work because sluggish air in the boundary layer is at the root of most separation problems. Correctly dimensioned VGs extend slightly above the boundary layer and create vortices at their tips that grab fast moving air in the free stream and mix it into the boundary layer. The now highly turbulent and energy-rich boundary layer is far more resistant to flow separation and so will follow more sharply tapered surfaces, better negotiate sharp discontinuities caused by deflected control surfaces, and resist aerodynamic stall to higher angles of attack (Fig.3).
Figure 3 – Re-energising the boundary layer
No Such Thing as a Free Lunch
Of course it can’t all be good news or our aircraft would be peppered with VGs. In reality you have to pay somewhere, and for VGs that penalty is drag. Whilst they avoid the large drag increases that come with flow separation, the drag generated by VGs occurs in all flight regimes, and so adds to the total parasitic drag of the aircraft whenever it is flying – even in conditions where flow separation may not actually be a problem.
To control the drag VGs create their dimensions and positioning are critical. If VGs are well proportioned and well positioned then the bulk of the VG (around 80%) will sit inside the boundary layer and the drag penalty incurred will be modest. Make VGs too tall and unnecessary drag will result with no added benefit, too short and they simply won’t work.
In the final analysis, if VGs are taming poor stall behaviour or a loss of control authority at high angles of attack, then a modest increase in drag is a small price to pay. Similarly, curing a fuselage flow separation problem during cruising flight is almost guaranteed to give a net drag reduction and so be well worthwhile. As long as VGs are the right size and in the right place there is very little down-side to them as long as they are correctly applied. I suppose they are quite delicate, which makes them prone to damage, but that’s about it. In fact, for me at least, their biggest drawback is that whenever I see them I immediately ask myself the question, “Is that a clever piece of design, or just a band-aid solution to an unforseen problem.”