question for pilots...

[blume357]

I know there are too many variables for this but was just thinking about it.
I'm here at the beach and I hear a jet flying nearby... so I'm trying to find it and finally do... it is about a mile maybe more out and the first thing I notice is it has to be going twice as fast as the commercial jets or cargo planes I see coming in on occasion... then I realize the sound I'm hearing is coming from about a 1/4 to a half a mile behind it..... seems it was going pretty fast.

years ago wife and I were sailing on the New Bern river in N.C. near Cherry Point Marine base and fighter jets flying over and by were pretty normal... I once, just once, happened to glance back and saw one out over the ocean that was going so fast if I had blinked I would have missed it.... it was like catching sight of a shooting star...

 

[bobski]

so whats the question?

 

[Jeepnik]

The sound isn’t actually “behind” the plane. It’s just that light travels so much faster than sound. Think fireworks or thunder. So you see it before you hear it. Even when an aircraft is supersonic the sound is still with the aircraft when made. The aircraft can’t hear sound behind it because it’s outrunning it.

 

[any ruger]

Live in northern Mn, The 148 Fighter wing Flys out or Duluth Mn, I worked in the taconite mine north of Hibbing Mn. Early afternoon I could hear jet aircraft came out of the truck grange to see 2 F 16s banking in front of the shop. I could read some of the markings on their helmits. That was the best 40 sec airshow I have ever seen. It was private only about 8 or 10 of the workers saw it.

 

[Fox Mike]

Doppler effect also enters in to the sound

 

[Jeepnik]

Doppler effect also enters in to the sound

If folks have no understanding of the difference in light and sound travel speeds, I sure as heck wasn’t going to bring that into it.

 

[blume357]

My actual question was if you could estimate the distance the jet is away from you as well as the difference of where the sound appears to be coming from behind the jet then that should allow you to figure out the approximate speed of it. And can any of you fly boys do that?

 

[Fox Mike]

My actual question was if you could estimate the distance the jet is away from you as well as the difference of where the sound appears to be coming from behind the jet then that should allow you to figure out the approximate speed of it. And can any of you fly boys do that?

You would need to know the exact altitude and the angle of travel in relation to you for starters then, by the time you actually heard the sound, the airplane would no loner be where it was when you heard it. Again I throw in the Doppler effect.

 

[Jeepnik]

During WWII before radar was common large sound detectors were used as an earlier warning for incoming aircraft. While it could give bearing to target, sometimes, range and altitude determination were even less accurate. Still, with multiple detectors picking up the same sound a very crude triangulation was possible.

Obviously once radar became commonplace these sound detectors were replaced.

 

[Dan in MI]

My actual question was if you could estimate the distance the jet is away from you as well as the difference of where the sound appears to be coming from behind the jet then that should allow you to figure out the approximate speed of it. And can any of you fly boys do that?

Using the same method of determining a storms distance. See the flash count in seconds, multiply seconds by 1000 gives approximate distance to storm. The 1000 is ballpark as it change with pressure and altitude.

For your plane scenario do the same thing, but now the plane has moved at x speed for the y seconds it took for the sound to get you. Now your initial calculation is z number of feet off by the delay. (storms are easy usually under 50 mph) Repeat and do some trig to try and get a speed. With such long distances using "guesstimation" time and distance you are asking a lot.

 

[Dave P.]

My actual question was if you could estimate the distance the jet is away from you as well as the difference of where the sound appears to be coming from behind the jet then that should allow you to figure out the approximate speed of it. And can any of you fly boys do that?

The answer is yes if you include the words estimate and approximate. As far as "fly boys"...meh. Should be more like "physics boys"
The question really has nothing to do with flying other than framing a basic physics type question. As already pointed out you would
need air density, wind speed and direction, angle and direction the aircraft was travelling relative to your position, and good number for it's distance
to make a calculation to it's speed as well as a good estimate of where your ears heard the sound vs. the planes location.
Plenty of variables that need some precision to do the math.

 

[bobski]

+1. think thunder and lightning. same same.

cause

effect

 

[blume357]

First understand I'm on vacation and so have too much time on my hands and other parts and was thinking too much... now that I think about it... I would wonder if someone here or otherwise who had spent a good number of years watching jets fly by at various distances... thus by the type of air ship would know the approximate distance then just from repeated observation and also actually knowledge of the speed of the jet and then putting in how far behind the sound is could after a couple of years and a few hundred to a thousand observations be able to say the speed of the jet just by sight and sound. I'm willing to bet they could.... the closest I've ever come to this was 47 years ago in Navy ROTC sitting in class and getting glimpses of different military aircraft and so being taught to identify them just from their silhouette ....... this was in a different time and the instructor to keep us guys alert would actually throw in a picture from Playboy or Penthouse about every 10th to 20th frame. Sure could not do that today.

 

[Pierow]

Yes, approximate and estimated is about as close as you can get. The 1000' per second rule is a good estimate. When I'm shooting competition fireworks shells I'm usually within a 1000' from the mortars and I pause recording, then hit record as soon as I see the light coming from the launch. Usually there is a small delay as the quick match travels down the shell to the lift charge. The result is I catch the entire shell's performance from launch to burst utilizing the "light travels faster than sound" principle.

 

[Pierow]

Blume said ".......after a couple of years and a few hundred to a thousand observations be able to say the speed of the jet just by sight and sound. I'm willing to bet they could............"

I agree. No different then how professional baseball players, even with their backs turned, can tell where the ball is traveling just by the sound of the ball hitting the bat.

 

[Johnnu2]

I forgot what the question was....

J.

 

[larry8]

Me too, but the jet was going at 550 mph.

 

[Fox Mike]

Blume; go online to "Flightradar24". Zoom in near your location. Locate the airplane you are interested in. Click on it and it will show airspeed, altitude, course heading, and other facts.