Transonic Boom

[67Wing]

Is it possible to drop a boom at sub-sonic speeds? The usual excuse goes, "I was at M.97 in a dive but pulled a bit hard so might have 'thrown-off' a boom but I didn't go supersonic, honestly." It was a well known 'fact' in the US that it was possible to drop a full sonic boom by pulling G at transonic speed without actually exceeding M1.0. I always felt it was bull**** and just a convenient excuse for blowing a high speed dive recovery that went super-sonic. I expect the Phoon boys will have discussed this one a bit as the jet's very slippery in a descent.

[BEagle]

Yes it is.

Whilst chasing some Jaguars at low level on an exercise years ago, I suddenly noted a twitch on the pressure altimeter. "Transonic Jump", thought I and throttled back immediately whilst muttering "DON'T PULL" to myself.

Probably knocked the osprey off its nest - sorry!

When the Hawk first appeared over Wales, it seems that selecting airbrake at high IMN would pop a boom - there had been a few unexplained Viet Taff complaints.

[ShyTorque]

Beagle, Yes I had that happen in the Bulldog, too. :E

;)

[Rigger1]

It's very easy to make certain parts of the airframe go supersonic whilst the majority isn't, it depends on the localised airflow, and everyone of those bits will give off a small boom. If the bit is big enough, yes it can give of a boom that could be heard on the ground whilst the aircraft is still sub sonic.

[A2QFI]

I was No2 in a pair of Jaguars that caused considerable damage to airfield buildings, (windows blown in, ceilings collapsed etc). We were below 1000' and straight and level and as it was a Jaguar I don't think we can have been quite supersonic!

[67Wing]

I don't doubt that you could get all sorts of jumps on pressure instruments in the transonic region but I don't see that this mean you've either gone supersonic or dropped a boom. Likewise, pressure waves will form on various parts of the aircraft where the air is supersonic, typically over the wings but well before a sonic boom is caused - some of these cause Mach buffet but not a sonic boom. I expect the mach whiskers over the wings grow when G is pulled but I don't see how these can cause a boom. I've also heard of the 'don't put the airbrake out at high IMN or you will cause a boom', brigade. I've always thought this more BS and just another excuse for failing to notice the jet's gone through M1.0 and dropped one. Who can prove me wrong?

Beagle's 'Yes it is' doesn't do it for me. Rigger1's thoughts are more interesting but I don't think a sonic boom is just a large bit of localised supersonic air. It's a shockwave caused by he first bit of supersonic material to hit the air and forms at the leading edges.

There are two aspects here. Air made supersonic around the airframe, this can happen at quite slow IMN. A Tornado in 25^ can be made to mach buffet quite dramatically at VNE (M0.85 plus a tiny bit!), a Jaguar won't do it at all due to the decent fixed sweep. The other aspect is stuff moving through the air at supersonic speed. I contend that it is objects at supersonic speed that cause a boom and not supersonic air about the aircraft. If this is the case, then pulling g, opening airbrakes and all the other stuff you hear that might, 'cause a shock-wave to detach' are urban myths.

So, if you dropped a boom the aircraft exceeded M1.0 - or did it ??!!!? :)

[Rigger1]

It's a shockwave caused by the first bit of supersonic material to hit the air and forms at the leading edges.


Totally agree, however what makes the material supersonic ..... the speed of the local airflow going over it, round it etc. Therefore it will cause a shockwave.... well thats what I was always taught and what I remember from running supersonic wind tunnels. But I may be wrong, it was all a long time ago.....

[Lightning Mate]

It is perfectly feasible to throw a shockwave when the free stream Mach number (MFS) is below 1.0.

It is important to remember that a wing shockwave forms where air is decelerated from supersonic to subsonic speed. Increased AoA results in increased acceleration over the wing upper surface. It is therefore possible to "throw" a shockwave when MFS is below 1 at high g. I know, I have done it during displays at 8g.

A2QFI

Sorry mate, but I have had a Jaguar at 690knots/M1.05 at low level - it was clean of course. The Lightning would do it with no trouble at all. PM me if you wish.

Oh, and to all Jaguar bashers, it was very quick if you knew how to do it.

Orrabest to you all,

Lighning Mate (six years and 900 hours) (and Jaguar, almost eight and 1850 hours).

..... don't give me a hard time, I'm now old and fragile.....

[67Wing]

I accept that wing loading affects Mcrit, the speed at which supersonic air first appears over the wing. I'm also happy that shock waves occur where the air decelerates to subsonic. However, is all we have is anecdotes that a boom can be dropped at subsonic speed by pulling G. If that's the case, there should be a relationship between G and IMN that results in a full sonic boom? I'm still waving the BS flag but would be happy to be convinced otherwise. I've spent plenty of time just below the mach and supersonic at low-level in F-111s and never succeeded in dropping a SUBsonic boom despite all sorts of wacky manoeuvring. I know I'm fighting received wisdom here but we've all combated at close to M1.0 with no evidence of booming. Where are our aerodynamicists? I'm in danger of losing a beer here!

[67Wing]

Just found this:
Analyzing Sonic Boom Footprints of Military Jets with GIS (http://proceedings.esri.com/library/userconf/proc01/professional/papers/pap284/p284.htm)
It seems that G might change the strength of a boom when supersonic. I'm going firm on 2 pints saying you cannot drop a boom subsonic by pulling g or fiddling with airbrakes. If you have already gone supersonic, it might be possible that the sonic boom does not reach the ground and pulling G might cause it to do so. But that wasn't the question.

[artyhug]

Hmmm, well I could very very vaguely be accused of once being slightly better informed than most over aerodynamics but the following is more thoughts than guaranteed facts because it is a very long time since I studied bernoullis at graduate level.

My understanding then, and reinforced by subsequent CFS A2 groundschool and the like is that most if not all the above is true, to a point.

Yes Mcrit gives localised supersonic flow over an airframe, yes altering configuration will alter Mcrit as will manouevring.

Any airflow at or above Mcrit will form a shockwave however the mere existence of a shockwave does not imply the existence of an associated 'boom' at ground level.

As airflow speeds increase beyond Mcrit the shockwave will strengthen and move, generally forward, until Mdet when it will be attached to the leading edge of the wing and you will without a doubt be dropping a 'boom'.

So in my opinion, yes it is physically possible to 'drop a boom' subsonic. Is it likely? My arse......

[VinRouge]

Do we get supersonic booms off Airliners?

http://www.solanum.org/shock_0.jpg


http://www.solanum.org/shock_1.jpg



Thought not. Methinks talking pants. Is it possible at speeds close to full supersonic turbulent pressure variations round the aircraft cause the Machmeter to under-read?

[trap one]

A not that long time ago when the Lakenheath wing was converting from F111's to F15E's had an F15E drop a boom over the coastline. When the aircraw reviewed the tape, the Mach and TAS/IAS were very closely checked and it was confirmed that the Speed remained Sub Sonic. However, at the time of the boom the aircraft was manouvering hard and it was admitted that the wingtip could have gone Super Sonic and thus USAFE were opening theirselves up for any claims for damage. There were no claims just moans about the boom.

[67Wing]

Ah ha! This is what I'm talking about. The wing tip went supersonic eh! :ugh:
I just don't understand why this doesn't happen more often. The link I posted earlier shows how a sonic boom can be intensified or its propagation modified by manoeuvre but only when supersonic. The only 'evidence' of subsonic booms seems to be when someone is trying to dig themselves out of the poo. Assuming they took the film from the correct aircraft, and the film was independently verified, I would vote for instrument error. Not that anyone would fib.