Will it sonic boom?

mat

Well-Known Member
A plane flies at 650 mph. There is a tailwind of 150 mph. The plane isn't supersonic in the air mass, but it is to someone on the ground. It is going 800 mph when mach 1 is 760 at the ground. There is no shockwave on the aircraft, but can there be some kind of sonic boom on the ground?

I'm guessing the sound you would normally hear over time instead arrives at once. This is different because there is no shockwave associated with it?
 

Hacker15e

Dunning–Kruger Observer
A plane flies at 650 mph. There is a tailwind of 150 mph. The plane isn't supersonic in the air mass, but it is to someone on the ground. It is going 800 mph when mach 1 is 760 at the ground. There is no shockwave on the aircraft, but can there be some kind of sonic boom on the ground?

I'm guessing the sound you would normally hear over time instead arrives at once. This is different because there is no shockwave associated with it?
IMN has zero to do with groundspeed.
 

Screaming_Emu

Great and Unmatched Wisdom
Nope. A sonic boom is caused by the compression of the airflow (shockwave) as the airplane breaks the sound barrier. If the airspeed is subsonic, there are no shockwaves, and therefore no sonic boom.

I saw a ground speed in the back of a 747 that was well over 700 MPH once. I'm sure the people on the ground heard nothing.

Think of it in an extreme manner. Pretend it is a glider caught in an 800mph tailwind, but with zero airspeed. While the airplane is hauling ass, there is no airflow over the wings, making it impossible for shockwaves to form.
 

ZapBrannigan

Old School
Coolest thing I ever heard were the twin sonic booms from the space shuttle on reentry. The vehicle was so large that it apparently generated a couple of shockwaves. Not sure from where on the vehicle they came... Maybe Wong leading edge and tail leading edge?

Anybody know, Did Concorde make 2 booms as well?
 

mat

Well-Known Member
Oh, I understood most of that. So, it's possible to be subsonic and be traveling faster than sound(relative..). Got it. If the airplane were making noise loud enough to be heard from the ground, those sound waves would all be behind the airplane(relative to the ground). It seems like it would concentrate a lot of sound into a bow wave. I thought this may resemble a boom, but apparently it's not even close. I was confusing sonic booms with sound waves, they are totally different.
 

z987k

Well-Known Member
Oh, I understood most of that. So, it's possible to be subsonic and be traveling faster than sound(relative..). Got it. If the airplane were making noise loud enough to be heard from the ground, those sound waves would all be behind the airplane(relative to the ground). It seems like it would concentrate a lot of sound into a bow wave. I thought this may resemble a boom, but apparently it's not even close. I was confusing sonic booms with sound waves, they are totally different.
you're still missing the understanding of how this works. When I haven't been drinking I'll (or maybe someone else) will explain things.
 

///AMG

Well-Known Member
The sonic boom comes from the shock wave the aircraft makes while flying through the air at supersonic speed. This is relative to the air, not the ground. Think of it like IAS/CAS compared to groundspeed. You might have a 100 kt tailwind, but you will still stall a Cessna at 30 kts indicated (or whatever the stall speed is for a Cessna, I forget now) even though you have a ground speed of 130 kts in that case. Same for mach number. It's about how fast you are moving through the chunk of air in front of you/around you, irrespective of how fast that air mass is moving over the ground itself.
 

SteveCostello

My member is well-known.
It's about how fast you are moving through the chunk of air in front of you/around you, irrespective of how fast that air mass is moving over the ground itself.
Indeed. Just because of the way we are wired to think about how speed works, I think people have a hard time separating ground speed from air speed. The two are basically unrelated, really.

I remember being relatively young (12-ish) and going to my first NFL game in Buffalo, NY. It was the perfect day for football... cold, miserable, sleeting. We were way up in nose bleeds with drunks all around us, fighting, tossing each other around. Great time. I was bewildered by the banner tow plane above us, though. It was likely a C172, and at times it was essentially hovering. It wasn't long after that game I found out why, and ever since then, it's always been amusing to me the number of people that think a plane is most certainly going to crash if it isn't actually streaking through the sky.

By the same token, as has been mentioned, you could be moving at well over 768mph over ground. Doesn't matter a wink... it's all relative to your movement through the air around you.

This ties into that question about an aircraft on a conveyor belt (plane on a conveyor belt moving at whatever speed) that, no matter what people are told, do not believe. It's ALL about the air over the wings, and nothing about the plane over the ground.
 

fish314

Well-Known Member
Oh, I understood most of that. So, it's possible to be subsonic and be traveling faster than sound(relative..). Got it. If the airplane were making noise loud enough to be heard from the ground, those sound waves would all be behind the airplane(relative to the ground). It seems like it would concentrate a lot of sound into a bow wave. I thought this may resemble a boom, but apparently it's not even close. I was confusing sonic booms with sound waves, they are totally different.

I think the point that you are confusing has to do with what the sound coming from the plane does after it leaves the plane. Let's hit that for a little bit:

Ok, we've already covered the fact that a sonic boom is really a shock wave caused from the fact that the airplane is traveling faster than the speed of sound relative to the air. Why does this even happen? (... it will be a little bit illuminating when we talk about sound waves later...) Well, when an airplane (or anything else for that matter) is moving through the air at sub-sonic speeds the air has to move out of the way of the plane as the plane flows through the air. The air molecules and the "plane" molecules can't occupy the same space at the same time, right?

So, here comes the plane, and the air molecules nearest to the surface of the plane hit the plane and bounce off. As they bounce back off the plane, they hit other air molecules in front of the plane that have yet to be disturbed by the on-coming plane. Those air molecules in turn hit OTHER air molecules even more in front of the plane, et cetera, et cetera. So, in sub-sonic speeds, air molecules way out in front of the plane are getting pushed out of the way of the plane well before the plane gets there (it's still tiny fractions of a second... but it's BEFORE the plane gets there, none the less).

That signal, or that wave if you prefer, that results from air molecules bumping into other air molecules is a sound wave. All sound is just a bunch of air molecules bumping into each other, and then in turn those molecules bumping into OTHER air molecules, and on and on. But there is a maximum speed at which that can happen... the speed of sound.

If a plane is going faster than the speed of sound, the signal "transmitted" from air molecule to air molecule to the air in front of the plane takes longer to get there than the plane ITSELF does. So the air in front of the plane doesn't have time to get out of the way, and instead of flowing around the plane the air molecules right next to the plane get smashed (or compressed) between the air in front of the plane and the surface of the plane... this is the shock wave, or as it is sometimes called, the compression wave, that causes the sonic boom you hear. At this wave, a whole host of the properties of the air are forced to change over a very small distance, including the temperature, pressure and density of the air as the air gets "squished." It's a violent process.

Now, what about when we are subsonic? First and foremost, subsonic and supersonic can ONLY mean relative to the surrounding air... which will become clearer in a few seconds. So, regardless of a headwind or tailwind, we are subsonic relative to the surrounding air. Our plane's surface pushes the air molecules right next to the plane out of the way, and this signal is transmitted as sound through the air. Those signals (SOUND) travel... at the speed of sound, of course. For lack of a better way to write this, once the sound is in the air, it doesn't care what happened to the plane that made the sound. It isn't like they are "tied" together, with the plane "dragging" the sound. They are not. So... they can't get "bunched up" or "concentrated" or anything. They move outward from the plane, through the air, and in every direction including forward. Even if a plane's true airspeed plus the wind equates to a speed higher than the local speed of sound on the ground, sound waves from the plane will reach the ground before the plane passes overhead.
 

USMCmech

Well-Known Member
Coolest thing I ever heard were the twin sonic booms from the space shuttle on reentry. The vehicle was so large that it apparently generated a couple of shockwaves. Not sure from where on the vehicle they came... Maybe Wong leading edge and tail leading edge?
Actually the space shuttle made 3 sonic booms. The nose, the wings, and the vertical stab. The shockwave from the wings and vertical stab were so close together that they sound as one.
 

Minuteman

“Dongola”
You know, in Star Trek the concept of Stardates and space-time was created to explain the local differences as information propagated at "only" the speed of light but ships were travelling faster at Warp speeds. Have a couple of daiquiris and think about that.

(Derg make the Liberal Nerdfest trophy already.)
 

thevideographer

Well-Known Member
You know, in Star Trek the concept of Stardates and space-time was created to explain the local differences as information propagated at "only" the speed of light but ships were travelling faster at Warp speeds. Have a couple of daiquiris and think about that.

(Derg make the Liberal Nerdfest trophy already.)
Not sure what you mean. Ships traveling at warp speed are stationary compared to their local space so there are no temporal effects due to relativity.
 
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