Yesterday, Sunday June 4, 2023, millions of people in Washington DC and its surrounding suburbs heard a terrific bang in the air which rattled houses and cars. It was the sound of a sonic boom made by a fighter aircraft, which had taken off from Andrews Air Force Base to respond to a Cessna Citation overflying the area. For many who are not accustomed to living near a military base, it was their first exposure to the effects of supersonic flight.
The Speed of Sound
When you study for the Commercial pilot exam, you’re learn about concepts related to transonic and supersonic flight. There are a few questions on this topic in the Commercial Pilot Knowledge test, so let’s take a moment to do a very quick and simple overview of transonic and supersonic flight.
Sound travels through the air at certain speeds. When we’re sitting in a small room talking to our friends, it can seem like the sound travels to our ears instantaneously, but in reality the speed of sound isn’t all that fast.
An aircraft on the ground will emit sound from its engine running in all directions around it. Much of the noise will get absorbed by the ground, but a lot of it will spread out in the air. In standard conditions at sea level, where the temperature is 15 degrees Celsius, sound travels at about 667 knots or 760 miles per hour. This is the speed the sound of our engine spreads out from where we are, all around. At this speed, it will take an hour for the sound to reach someone 667 nautical miles away, although the sound likely isn’t loud enough to be heard from that distance.
As we climb, the temperature drops. Changes in temperature affect the speed of sound. Let’s say at our higher altitude the temperature is only 9 degrees, in dry air sound will travel at 654 knots, a bit slower than at sea level.
More importantly, we’re no longer stationary up here. We’re flying at 100 knots. This means that we’re able to somewhat catch up with the sound our engine is emitting as we fly through the air. The result is that the sound waves in front of our aircraft become more compressed, the closer to the speed of sound we fly.
The Mach Number
We can express how close we are to the speed of sound, and thereby how much compression there is in our own soundwaves, by taking a ratio of our true airspeed, or the speed of the airflow over our aircraft, to the speed of sound in the air we’re flying in. This ratio is called the Mach number.
So assuming our indicated and true airspeeds are the same here, at 100 knots, with a speed of sound of 654 knots, we are flying at Mach .15. Not very close to the speed of sound.
In smaller piston aircraft, we don’t often worry about our Mach number. It just isn’t possible for us to fly fast enough to have the compression of our sound waves become an issue. We’re limited by our aircraft’s never exceed speeds.
Up in the flight levels, the air is much colder. Let’s say it’s minus 45° up here, that gives us a local speed of sound of 588 knots, significantly slower than down below. Now, we’ll swap out our small general aviation aircraft for an airliner, capable of much faster true airspeeds.
Now, as we fly faster, we worry much less about exceeding the aircraft’s maximum true airspeed, because what happens before reaching those speeds is we begin to catch up with our own sound waves. They’re much more compressed up there than before.
Let’s say we’re flying a true airspeed of 470 knots, our Mach number given the speed of sound is .80. We’re much closer to the speed of sound.
Critical Mach Number
Why do we need to worry about that compression of air in front of us as we approach the speed of sound, Mach 1? Let’s say if we go any faster than this speed of Mach .80, the speed of the airflow over the wings gets high enough to actually exceed the speed of sound. At that specific point above the wings, we’ll have caught up with those sound waves and they’ll compress into a boundary between the supersonic air above the wings, and the still subsonic air around the rest of the aircraft. Like any interaction of air at different speeds and directions, this will create a drag effect on the aircraft, affecting performance and its ability to maintain speed. In addition, as this boundary moves aft over the control surfaces of the wings and tail, it will cause issues with controllability in this transonic flight.
What we’ve done is exceeded what is called our critical Mach number. This is the fastest we can fly without experiencing supersonic airflow over some part of the aircraft. This is why airliners and other fast movers will use the Mach number up at altitude to express speed. It represents a limit much better than true or indicated airspeed can.
The Sonic Boom: Breaking the Sound Barrier
So what happens to our aircraft when we exceed the speed of sound? With the exception of the no longer operational Concorde, airliners don’t go faster than Mach 1, but fighter jets like our friend over Washington DC yesterday routinely hit those speeds and beyond.
A fighter jet is loud. Even when only one of them is airborne at a very high altitude, the noise is easily audible from the ground, and sounds louder than a normal jet aircraft. Still, it’s no where near the power of the sound of a sonic boom.
The sound the engine makes propagates out in waves, as we said. This is what we hear from the ground. When the aircraft exceeds the speed of sound, all of these sound waves are compressed, rather than hear the continuous whine of the jet engine, we hear nothing, followed by a very loud and powerful bang that shakes houses and sets off car alarms. This is why civilian airliners are barred from exceeding Mach 1, and military aircraft are only authorized to do so in an emergency like the situation with the Cessna Citation could have become.
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