Questions about F-14 Video.

Factual Questions
1

I was watching the following short video about the F-14 Tomcat that is being retired:

F-14 Tomcat

Several questions. . .

I assume that in that the video is showing the Tomcat breaking the sound barrier in front of the viewing area. We see a visible cloud forming around the plane. What is the cloud, water vapor? Why does it form? What are the basic physics of what’s going on (in pedestrian terms)?

I have experienced sonic booms at long range and they are quite bone rattling. The spectators are not wearing ear muff ear protection. Wouldn’t a sonic boom that close possibly break ear drums or otherwise be inadvisable?

2

In very basic terms, the pressure drops because of the shock wave from the sonic boom. This causes the temperature of the air to drop as well. If the temperature drops below the dewpoint of the air, then moisture will condense out, forming a cloud.

It doesn’t always happen - the air has to be relatively moist so that the dewpoint can be reached as the temperature drops.

3

Wikipedia has more.

4

Going supersonic isn’t necessary to create a shock collar. And since it was in close proximity to people, you can guarantee the aircraft wasn’t going supersonic.

As the plane moves though the air, air in some places around the plane is accelerated in a way that it moves past the wings even faster than other parts of the stream. This is easier to picture if you think like an aerodynamicist, and consider the plane to be still and the air to be rushing past it (like in a wind tunnel). At certain points around the aircraft, the air will be going faster than in the “freestream,” which is the air moving far from the plane.

This acceleration can be substantial. At speeds near, but below, Mach 1, some of the air near the wings can actually are accelerated to greater than Mach 1. The aircraft might be moving at Mach 0.8 relative to the air as a whole, yet experience regions of airflow at, say, Mach 1.1 over the wings.

If some of the air gets going supersonic, it must shock itself down to subsonic at some point. Such a shock wave is the only mechanism by which supersonic flow can transition to subsonic flow. This shock forms somewhere over the middle of the wing (and/or fuselage), and is responsible for significant pressure changes in the air. These pressure changes cause the condensation you see associated with a shock wave if it’s humid enough.

The speed regime where this mix of supersonic and subsonic flow occurs around the aircraft is called transonic. You can tell the F-14 is only going transonic because the shock location is revealed to be in the middle of the aircraft. If it were emanating from the nose, that would be a sign that the plane was moving at supersonic speeds.

Now, it’s important to remember that this transonic shock is a very weak shock. It’s enough to condense moisture out of the air, but not to cause a sonic boom. That’s why it’s not a problem for the spectators. The bow shock ahead of a supersonic airplane is much much stronger, and its effects reach much farther from the source. The pressure spike associated with a shock like that is what causes sonic booms and broken windows.

In addition, a transonic shock isn’t the only thing that can condense the vapor out of the air. If it’s really, really humid, the drop in pressure associated with making lift can be enough to make clouds over the tops of the wings. This is more apparent during a high-G pull or other high angle-of-attack maneuver, when there’s some serious suction above the wing. This type of condensation is much less distinct, and more fluffy-cloud like. The shock-based condensation in Colophon’s link is a product of the humidity being “just right,” so that condensation occurs when the shock causes a pressure drop, but not at some lower threshold.

5

Had an F-22 go transonic right next to me at an airshow and there was a partial break in the sound barrier (sounded more like a snap than a boom). The vapor cones were forming all over the plane.

The same effect can be seen in the formation of tornados. Every tornado I’ve seen associated with a leading edge storm springs into view like popcorn. Pretty neat if you’re not in it’s path.

6

Here is a previous thread on the “cloud” that forms. It contains a number of excellent links.

7

Thanks, Dopers. I can always learn something from this board. Therefore, I will beg the next questions:

I accept aerodave’s explanation for the phenomenon. So the F-14 was transonic rather than supersonic . . .

What is the difference in speed between transonic and supersonic? My understanding is that supersonic is not absolute. It depends on variables such as air pressure and temperature. Therefore, can the pilot accurately and intentionally fly at a transonic speed making sure that the plane does not go supersonic in order to avoid the dreaded “sonic boom”?

Was the video just a lucky capture of the phenomenon or is something like this able to be replicated by doing the calculations ahead of time?

The relatively poor quality of the video seems to indicate that someone had the camera at the right place at the right time. Could the pilot have made a slight speed error, created a sonic boom and blown the spectators into a life of sign language?

Thanks in advance!

8

“Slight” error? No. It would be hard to make the speed of any sizable portion of the airflow over a jet less than the net airspeed of the jet. It is, on the other hand, very easy to make it higher: just divert the airflow at an angle and it has to move faster to “catch up with” (may the Gods of aerodynamics forgive me!) the air that wasn’t diverted. Phrased slightly more accurately, a plane flying at a constant speed represents an impediment to direct airflow, so the airflow much follow a longer path without leaving “a hole in the air” behind the jet. To fill that hole, it has to hurry a bit, relative to the air away from the jet. (I’m taking the wind-tunnel view: pretend the jet is stationary but experiencing to a high velocity wind)

if you recall your geometry, you probably see that a relatively small diversion (e.g. ~ 10 degrees from normal) could make the air follow a 10% longer path – and at some points in the airflow the diversion is likely to be even greater. if you are flying close to local Mach 1 (after factoring in what ever combination of local barometric pressure, temperature, altitude, humidity, and other atmospheric conditions, you wish), parts of your airflow will be exceeding Mach 1 (but may/may not fully produce what we call a “sonic boom”, depending on whether the waves superimpose over time in a steady state) what’s 10% of Mach 1 at see level – well, let’s call it 65-70 mph. That is NOT a small error in speed when you are flying, much less when doing a flyby of a surface ship!

Further, the Prandtl-Glauert discontinuity does not require a pupersonic transition. You may, if you wish, think of it as the kind of discontinuity or pressure wave that becomes a sonic boom, when Mach 1 is exceeded , because at that speed, the shock waves can’t "get out of the way (Mach 1 is the speed at which air “gets out of the way” locally) and therefore builds (superimposes, piles up) into a new phenomenon .

All in all, you can create these clouds at a net airspeed of 10-20% or more below local Mach 1, or 67-150 mph below shock wave speeds. If you were that poor a pilot under controlled conditions, you wouldn’t be allowed to do that close a flyby at anything NEAR Mach 1. Conversely, over open water, you would have little difficulty finding “just the right” local speed: start at a crude guess of mach 0.8, and slowly speed up until you got “clouds”. It’s only take a minute or three. and you’d get a very good empirical estimate of the local “magic speed”, so you could turn around and safely give everyone on deck a show.

And then there’s always the wacky idea of "being careful "

But dammit Jim, I’m a doctor (and a sleepy one at that). not an aerodynamicist

9

Make that “All in all, you can create these clouds at a net airspeed of 10-20% or more below local Mach 1, or 67-150 mph below sonic boom speeds”

I also negected to mention that you could use your flight controls/maneuvering to induce a P-G at lower safer speeds (flaps settings, sharp climbs or turns, etc.)

10

Are the shock collars very stressful on the aircraft? in this video (starting at the 20 second mark) a jet (an F-14 according to the caption) does a flyby past a ship, forms a shock collar near its wingtips, then suddenly explodes a few seconds later.

11

Is this the transition from *having * to having just *had * explosive diorrhea?

Sorry, but someone was going to do it, so why not me?

12

KP already gave you a good answer about the margin involved. I just wanted to say that you’re correct that the speed at which things become supersonic is not constant. But it’s not unpredictable, either. The speed of sound is a very simple function of temperature. It actually increases as the square root of temperature (expressed on an absoute scale, like Kelvin or Rankine, of course).

Moreover, a pilot will find it very easy to maintain a speed somewhat lower than the local speed of sound if he wants to. That’s because it’s easier to make an instrument to read Mach number than it is airspeed in knots. You don’t actually need to know what the speed of sound is at the moment to know you’re going 80% of it, or twice it. That makes it very easy to constuct a Mach guage using only a common pressure sensor (a Pitot-static probe). But giving your speed in knots or mph is actually a lot harder, because you have to include the input of a temperature sensor, and do a little math inside your instrument.

But that’s fine, because in transonic and supersonic flight, the Mach number is really more important than your absolute speed for almost everything related to performance and control. It matters more that you’re going Mach 0.75 than it does that you’re going 500 knots. Well, from an aerodynamic standpoint, anytway…getting to the destination on time requires knowing that velocity.