Sonic Bang Below Mach 1?

[Pontius Navigator]

Read an article in the paper a few days ago about the 'crack' went you flick a whip. Apparently a sonic bang. The article then mentined that the nose of an aircraft could also go supersonic as the result of a rapid manoeuvre. The implication was that the aircraft itself had not gone supersonic?

Fact or journalism again?

[rej]

As far as I know it is a fact that an aircraft can drop a supersonic bang at subsonic speeds - I understand that the airflow over the frame reach supersonic speeds despite the aircraft flying subsonic.

A few years ago when I was controlling at an area radar unit, a colleague of mine asked a formation of F16s to expidite climb to pass over civil trafic on an upper air route, which they duly obliged (subsonic), but in the process, dropped a 'bang' somewhere over UK land. Ooops

[engineer(retard)]

I believe there was also a case at Lossie of a Bucc doing something similar during an air test one afternoon. I was just dozing before night shift. It cracked a window in the Sgts mess and the lcoal papers reported it. I think that it has something to do with localised airflow over parts of the frame that are subject to rapid changes during manoeuvre and is more likely to occur at higher airspeeds.

[caspertheghost]

It is quite easy to drop sonic booms inadvertantly whilst airtesting or carrying out Vne dives as in the recovery from said manoeuvre the local Mach No over the top surfaces of the ac will exceed 1. It's all to do with Mcrit, or the critical Mach no, which is where the airflow on the top surface of the wing reaches sonic speed before the oh my god I'm even boring myself........:zzz:

[engineer(retard)]

PN

Which paper do you get that compares whipping techniques? :sad:

Regards

Retard

[mbga9pgf]

You definately get shock waves below mach 1, usually starts at around M 0.8 ish depending on wing design, due to flow accelerated over varios parts of the wing. If the aircraft is loaded above 1g, the flow accelerates more and thus shock waves occur at increasingly lower speeds.

Not sure about sonic booms though! (depends on strength of the shock i think)

[Pontius Navigator]

engineer (Retard), I believe it was that Tory tabloid rag that posts reader's questions and later posts tehe rudite replies from people such as yourselves what know all the answers.

[Tarnished]

You can most certainly produce shock waves and hence sonic booms on the ground from an aircraft at subsonic speed. If the local flow about any part of the airframe goes supersonic a shockwave is formed. The intensity of the boom on the ground is influenced by a wide range of factors such as atmospheric breakdown, temperature, density, humidity. The attitude and any manoeuvre of the aircraft can also have the effect of concentrating the boom (focusing it in a turn for example).

I think it fair to say that these sorts of booms are likely to cause little or no disturbance to folks on the ground. For "full blooded" supersonic flight the rules (>30 miles parallel or divergent from land or 10miles going away from land IIRC) should keep the headlines free from scandal.

As Typhoon becomes more widespread I would expect to see a few more unexpected "did the earth move for you" events.

T

[Flying_Anorak]

Is not the angry snarly engine note of a Pratt & Whitney Wasp radial engine in a Harvard (Texan to our Colonial Cousins) the result of the tips of the prop blades going supersonic? Sure I read / heard that somewhere!

[Pontius Navigator]

Tarnished, was that >30 or <30 <vbg>.

Someone dropped one over Lincolnshire last week, Thursday I think, which was reported over an area of 20 miles of so. The local police (Boston) confirmed that it was an aircraft and not a bomb.

I know who I suspect as I saw them do it once over Coningsby while doing ACM at about 6-9k.

[The Rocket]

Is not the angry snarly engine note of a Pratt & Whitney Wasp radial engine in a Harvard (Texan to our Colonial Cousins) the result of the tips of the prop blades going supersonic? Sure I read / heard that somewhere!

I wouldn't have thought so. The reason jet engines were developed in the first place was because propeller driven aircraft were reaching the end of their usefulness as fighters, due to the fact they had hit a ceiling as far as speed was concerned

This is because once the propeller begins to reach the speed of sound (M1) the shockwaves that are formed extend along the blade, disrupting the airflow over it, thus destroying much of the lift (thrust) that the blade was producing. Therefore if the blade tips were supersonic, the aircraft would be producing a much reduced amount of lift, thus making the aircraft terribly inefficient

This is why the majority of prop aircraft (with the obvious exception of early aircraft) have constant speed, variable pitch designs.

[henry crun]

Rocket: I think you will find that the Harvard prop tips were supersonic at max revs.

I understand the TU95 was another with that characteristic.

[buoy15]

Perhaps something to do with the speed of sound, measured as an adopted 'standard' at sea level, with temp and pressures of 15 degreesC and 1030mb.

You can fly subsonic at sea level in a WIG, Hovercraft or Kite-Board

However, any rapid acceleration or manoeuvere at about or above 725kts, at low level, or higher altitudes, in these climate conditions, will create a sonic boom.

[flipflopman RB199]

buoy15,

Sorry to be a pedant,

ISA standard atmosphere is 15 deg C, and 1013.2 mb

The nominal speed of sound at sea level is 331 m/s