With the Red Bull Air Race as competitive as ever, and now with teams using the same Lycoming engines and Hartzell propellers, it’s down to the engineers to find the speed that the pilots need to win. But although aerodynamics is theoretically a science, there’s art in its application, and just like in Formula 1 motor racing, all the teams have differing opinions on aero and different setups on their machines. In common with F1, its the wings that give us an insight into their thinking.
All 14 Master Class teams have different approaches to that most critical area of the wing; the tip. Three have taken their tip designs to the extreme, sporting rakish shark-like fins called “winglets”. They look fast. They look cool. But with the competition at the top of the championship so close between Matt Hall, who’s used them all season, and Paul Bonhomme who has always gone without, we asked our resident aerospace engineer; do winglets actually do anything?
Winglets usually have an aerofoil shape and are an extension of the wing. A well designed winglet can make a wing more efficient – see the boxout for the fundamentals.
Back to Basics – Wings and Lift
Whilst we can’t see it, air behaves like a fluid, and we can most certainly feel and see it in action. Much like water, air moves with a speed and imparts a force on everything it flows into. It imparts this force in one of two ways: pressure distribution, and something called shear distribution.
To explain pressure distribution, imagine blowing up a balloon. Once inflated it will resist you squeezing on it. This is pressure acting on the walls of the balloon – higher-pressure air inside the balloon wants to get to the lower pressure ambient air outside and you feel this force as you squeeze.
For shear distribution, imagine sliding a knife through a block of butter. The knife is thin, and is moving a small amount of butter out of the way, but you still have to push fairly hard. The sides of the knife are dragging against your freshly cut butter, and trying to slow you down. This is shear force. Generally speaking with light aircraft, this force is quite small, and can almost be ignored. You’ll also hear it called “parasitic drag”.
Moving air over a wing creates low pressure above the wing surface. Just like your balloon, the higher pressure under the wing really wants to get to the lower pressure above the wing – just like the balloon, the wing is in the way and experiences this force pushing against it. This is a major part of the force called “Lift”.
Unfortunately in nature there’s no such thing as a free lunch – to create the low pressure we need, we have to force air to change direction and flow around the wing. This creates drag, which slows the aircraft down. Whenever we want more lift, we have to pay for it with drag. When aerodynamicists talk about making the wing more efficient, what they really mean is paying less drag for their lift. With less drag holding the aircraft back, it will go faster.
So how do winglets help? Well, having created low pressure above the wing we now have a problem. Unlike in a balloon, the high pressure air below isn’t sealed in, and it desperately wants to leak around the tips into the low pressure area. As the air swirls around the tip, its momentum keeps it spinning in what is known as a vortex.
There’s a bit of aerodynamic magic happening here. The fact that we make a swirling vortex off the wing tells us there’s some energy here, energy that we’re putting into the air – energy being wasted! If you mount a winglet at the end of the wing in the right way, you can take advantage of this high-energy air and create a force which opposes the drag experienced when the wing makes lift.
It turns out that if we design it carefully, we can recover some significant loses. The air doesn’t curl over quite as aggressively, we loose less lift and make less drag. Our wing becomes more efficient.
Excellent – I’m half way to a free lunch with my wing. How does this help me win a race?
There are two principal benefits to having some well-designed winglets: Less lift-induced drag in a straight line, and the ability to pull-G with less drag.
For Air Race courses with long straights, you want to be hauling as fast as you possibly can in those straight lines. When the engine is already maxed out, and the pilot has flown precisely, you’re left with reducing your aerodynamic drag to eek out the last few knots of speed. Well, as we discussed, with a well-designed winglet, you can make enough lift to keep your aircraft in the air and create less drag while you’re at it. This allows you to fly a little bit faster for the same engine horsepower.
The other aspect where winglets can be a big help is experiencing less drag as you pull ‘G’ – gravitational force; at some points as much as 10G. Ten times the force of gravity means the aircraft weighs ten times as much and suddenly you need ten times more lift; lift that has to be paid for with – you guessed it – drag.
At this point things can get really unpleasant; as the drag increases, the aircraft may start to slow down, which in turn means the wing has to work even harder to generate lift, creating even more drag, the aircraft gets even slower, and down we go. A decent winglet design gets you better value lift, giving you an edge in the corners and losing less speed than your competitors.
Sounds great – so why doesn’t everyone have winglets?
Well, there’s still no such thing as a free lunch, and no such thing as free lift.
While winglets do generally make a wing more efficient, they do it best for a particular combination of speed and angle to the air (angle of attack). For the speeds that Air Race aircraft fly at, often a well-designed wing will be more efficient in a straight-line than an equivalent wing fitted with a winglet. For courses with lots of long straights, you might be better off leaving your winglets in the shed.
As with all things though, its the pilots that have to fly the aircraft and winglets will affect handling, and as a result their confidence. Following the old adage of “if it aint broke don’t fix it” you’ll be unlikely to find Paul Bonhomme changing his aircraft design while he’s still ahead in the points. If Matt Hall and his winglets have their way, that might not be for much longer.