Sonic booms: not talked about very much, especially in the helicopter industry. However, the speed of sound and the effects of breaking it, has a massive influence on the design, procedures and aerodynamic of both planes and helicopters. So what is a sonic boom, and how does it effect helicopters as well?
For clarity, the picture above is called a vapour cone, and is NOT the same as a sonic boom, but is related to it. More on that a bit further down! Today, we’ll be looking at:
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What is a sonic boom?
So let’s started with the basics shall we? To understand it a little bit better, let’s look at this example. If we throw a pebble in a lake, the waves that get formed around the pebble look something like this:
These waves can be compared to sound (or what we call pressure waves, caused by sound and movement), which move away from the source with the speed of sound, which is 661 kts or Mach 1. All good so far, but watch what happens if we would be able to magically move the pebble through the water? In this case, the waves that are ‘ahead’ of the pebble get closer together, while the waves behind the pebble have more spacing between them:
As we said earlier, the speed at which these pressure waves move away from an object is the speed of sound: Mach 1. So what happens if the object itself is travelling close to, or at the speed of sound? Something weird happens! As these pressure waves move away from the object, the object itself moves at an equal speed and therefore the waves in front of it are not gaining any more distance from it, which looks like this:
This has now resulted in a lot of pressure waves overlapping, creating a an area with a very high pressure, also called a shockwave. As this area moves across where you might be sipping a cup of coffee in a peaceful park, you’ll hear (and often even feel) a really loud bang. This is the energy that is all clumped up in this one overlapping line of pressure waves, also called a sonic ‘boom’. It’s the sound of all these cumulative moments of sound, all in 1 go! The shape of this shockwave is a cone:
This shockwave and high pressure creates an extra obstacle for the aircraft to overcome if it wants to go even faster. This is what the main struggle was when we first started aiming for going supersonic with various aircraft designs. You need enough power to break through the increased drag when you get closer and closer to Mach 1. This is called wave drag, and is a massive downside for aircraft performance at these speeds.
If the aircraft is hypersonic, it could be long gone by the time you hear the sound chasing after it. This used to be something that was sometimes demonstrated at airshows. However, due to a lot broken windows, injuries, and other damage caused, this is almost never done anymore at airshows, depending on where you are in the world.
What is a Vapour Cone?
So what about the picture you saw above, what is that then? This is called a vapour cone. Even before reaching the speed of sound (transonic) in a fighter jet, certain parts of the airframe will accelerate the air around it (such as the wings). This will create a situation where parts of the aircraft actually supersonic (faster than the speed of sound).
This creates a similar picture to what we described above. Pressure waves will overlap, unable to escape at the front. This creates an area of high pressure, followed by an area of extremely low pressure nearer to the tail of the aircraft. This massive drop in pressure reduces the local temperature of the air around the aircraft. If the temperature drops below the dew point, a cloud forms in the shape of the cone described earlier!
It’s not always just 1 big cone, various parts of the aircraft can create their own mini cones, such as shown in this picture:
And it’s not only fighter jet’s that can produce these. This is the Ares I-X rocket that was launched in 2009 producing a vapour cone as well:
This doesn’t always happen if air is dry enough or the dewpoint low enough, as you can see by looking at this amazing picture, taken by talented photographer Camden Thrasher. Even without reaching the dewpoint, these overlapping pressure waves can still be seen! Pretty neat huh?
How does a sonic boom affect aircraft design?
So how does all of this affect aircraft in general? Well, it took us quite a while to figure out how to overcome the massive amounts of pressure, and therefore drag, to bust through these overlapping pressure waves.
The biggest contributor used nowadays is called sweepback, which the Germans started experimenting with back in 1931. The speed over a wing is usually quicker compared to the actual airspeed of the aircraft. This is because wings generally accelerate air in various ways (we’ll cover exactly how in a different article). We want to ‘delay’ the onset of a sonic boom, to go as fast as possible, without having to deal with all the nonsense we went through earlier, that’d be great right?
So how do we delay the speed over the wings to go supersonic? Well, first we need to talk about how airspeed is relevant for a wing. When we measure wing airspeed, we are talking about the flow that is perpendicular to the leading edge. So, if we have conventional wing, the speed and angle measured looks like this:
But if we look at a wing with sweepback, you can now see that the effective airspeed perpendicular to the leading edge, is now less than for a conventional wing. There is a big component of the airspeed vector that now travels along the trailing edge, instead of over it, reducing actual speed going across the wing perpendicularly, and therefore delaying the onset of a sonic boom:
It’s not just the wing shape that’s different, the aerofoil (or airfoil for our fellow Americans) shape can be tuned to deal with supersonic speeds as well! These are called supercritical aerofoils. These essentially minimise the acceleration at the top of the aerofoil to delay the onset of a shockwave, by having a flat top:
How does a sonic boom affect helicopters?
So what about helicopters then? Well, obviously helicopters do not have the ability to go very fast compared to fixed wing aircraft. The biggest contributor to this is the speed of sound. Let’s have a look at an example. Let’s look at a disc in still air:
If we take the AW169 as an example, the disc diameter is 12.12 meters with an RRPM of 338 at 100%. If we want to calculate the blade tip speeds, first we need the amount of distance the blade tips cover in 1 rotation:
They cover this distance 338 times every minute. So to get the total distance per hour:
This is meters per hour, let’s convert to kts:
As you can see, that tip speed is already quite high. The problem now is that when we are increasing our forward airspeed, the advancing tip’s speed is the sum of it’s own speed due to RRPM, and the aircraft’s forward airspeed. A typical cruise speed in the AW169 is around 140 kts, so we get:
There we go.. That’s rather close to Mach 1 at sea level. How close? To convert any speed to Mach number, we devide speed by the local speed of sound (roughly 661 kts at MSL):
Now of course this example is oversimplified. We have just ignored a lot of variables such as compressibility characteristics, blade design, and atmospheric conditions to name a few. But it illustrates perfectly just how close tip speeds get to Mach 1. But this illustrates how badly helicopter Vne is dictated by the speed of sound.
This is also why most tip designs have included tip sweepback in most bigger helicopters with longer blades, as you can see on the AW169 in the picture below. There is also a compromise between RRPM and disc diameter. The bigger the rotor disc is, the lower your RRPM will have to be to avoid supersonic issues.
The speed of sound has dictated how airplanes have been designed for almost a century now. It’s easy to forget sometimes how much helicopters are limited by this as well, so hopefully this was a handy refresher of what the main considerations are. If you want to read more about high speed flight aerodynamics, have a look at Engineering Pilot’s latest article.
Looking for more content featuring helicopter aerodynamics? Check out this article!