When does the Bow Wave appear?

[RowleyUK]

M1.0
M1.3

Any help would be nice!

[redsnail]

This is tricky really because are you talking about the mach wave or the specific bow wave that appears to differentiate from the other shock wave at the trailing edge? (that trailing edge shock is the one that moves back from the point on the wing with the greatest acceleration as the aeroplane accelerates. Mcrit)

The mach wave will appear at 1.0M. On a "normal" wing not really designed for supersonic travel, it will attach at about 1.3M.

I am sure the aerodynamicists will rush to your help.

[Jagbag]

Hope to give a convincing argument though its been a long time since I went SS!
Briefly then,
Firstly the formation of the bow wave:

The aerodynamic pressure at the leading edge (LE) is highest. The pressure and temperature reduce moving up stream ahead of the LE till a point is reached where they are akin to the free stream static values. Between this point and the LE speed of sound is higher than free stream numbers due compression. Also this high pressure and temp bubble is what deflects the airflow above and below the aerofoil. Result no bow wave

Consider a wing section at M=1.0, the disturbances i.e.Pressure waves are able to pass up stream only if the their propagation speed is higher than M 1.0 (which is not the case),The disturbances are contained. This results in a weak shock wave well ahead of the LE (in theory infinitely ahead).

Now the M Free Stream > 1.0, The pressure waves will bunch up and form the bow shock wave. It is the forward limit of influence of the wing.

As the aircraft continues to accelerate the bow shock wave attaches itself to the leading edge, this is called the detachment Mach No. which varies from M 1.41 to M 2.00 depending on the deflection angle.

Hence I would say that the answer is closer to M 1.3!
:)

Can give further specific clarifications if you want them.

[Flypuppy]

What does the feedback answer sheet say it is? ;)

[Bagelman]

Just had a look at my notes and they say bow wave first appears at about M1.1 when a/c starts to catch it up. I was taught PoF by the legendary John Dixon and the last time I saw him he was thinking about going to work for the CAA as an examiner. So if its for your JAA's then I'd go for M1.1!

[Goose1976]

As far as exam purposes go- according to a very reliable groundschool the bow wave is M1.3

Hope this helps

[Dick Whittingham]

Writing as the representative of one normally very reliable groundschool, I would say that the bow shockwave forms at around M1.0 freestream, but stands off from the aircraft or wing leading edge because of the rise in temperature mentioned above. At some higher M the bow wave will move in and make contact with the LE as a normal shockwave (Mdet) then begin to become oblique as the freestream M increases. This is a simplification, as always.

Mdet gets its perverse name from the early days of supersonic research when the only way to get a supersonic wind tunnel was to pump up a huge tank of air then suddenly pull the plug, letting the air past the model at high M. As the air slowed down, the bow shock would straighten up and then detach itself from the model. Hence Mdet and not Matt.

Dick W

[Bagelman]

DW thanks for explaining how Mdet got its name, I always wondered!

Cheers
:D

[RowleyUK]

The feedback q's i am aware of are the these two:

When does the Bow Wave Form?
M1

When does the Bow Wave attach?
M1.3

The question i had in my exam is neither of these!!!
They did the usual trick of changing the wording!!!

The question in the exam was:

When does the Bow wave APPEAR?

Now,does the word Appear relate to the word Form?

just another typical CAA play on words scenario!Its not what you know its just whether you can give the meaning of every word in the Latin dictionary!

I personally would say that it appears when it forms and would go for M1, which i did do,but i failed the exam!!

I get different answers off different people and there is no way for me to find out what is the right answer!! Without naming neames this is what i have been told-


A school in Cov believe the Bow Wave forms at M1 and attaches at M1.3

A school 50miles south of them just down the M40/M42 have their feedback which states the bow wave forms and attaches at M1.3!!!

So what is the answer?


oh and btw,John Dixon is an examiner now, but what chance have i got of asking him? I would say about the square root of F.A.!!You cant even speak to anyone unless you know their DD number, but thats another issue!

[Jagbag]

Your frustration is understandable, however I think all of us in aviation (especially the guys at the controls) have to face similar situations very often... which makes flying that much more interesting and challenging, anyway..

No one seems to be able to point the correct answer because apparently the question is incorrectly framed. A better way may have been keeping the choices as M1 and M>1 (or M1.1). In that case I would go for the latter.

In the meanwhile I have referred to AC Kermode (my best friend) and he says M1.1 for Bow Wave approaching from front.

Hope that helps, cheer up!

[RowleyUK]

Oxford reckon it Forms,appears and attaches at Mach 1.3 so thats what im goin with!!!

thanks for the halp guys and girls

[mikmohr]

Formation of the bow wave in the first instance cannot happen unless air is travelling (relative to the a/c) at at least M1 and then meets air being slowed by the presence of the aircraft structure.

If the aircraft continues to accelerate, the size of the region of slower air (by slower I mean <M1) infront of the stagnation point, is reduced. Everything else is determined by the shape and size of the leading edge of the structure. Small sharp structure leading edges will cause the bow wave to be initially weak, forming relatively close to the LE, and attaching quickly beyond M1 (maybe at M1.1 for example), and quickly become oblique. Large blunt LEs will cause the bow wave to be bow wave to form relatively further from the LE, a stronger shock wave, possibly never attaching (below maximum speeds for the a/c of course, the bow wave will allways eventually attach), and part of the wave will remain oblique (again dependant on structural speed limits).

For the JAA, as with the AoA that are taught as representative angles (-4, 0, 4 and 16), an attachement number of around 1.3 is suitable for most med to large jet transport a/c.

Final point on attachment Mach No.. This is the same as detachment Mach No. in magnitude, but each is used respectively for accelerating through the transonic range, to describe the M when the bow wave attaches, and decceleration for the detachment of the wave. The JAA have asked questions talking about accelerating up through the transonic range wrt detachement. This is because it is the same number in the end, and because (as far as they seem concerned) anything that disorientates the students must be a good thing!

Please email me for anything else

Mik Moore
Oxford
[email protected]

:)

[P T Flea]

It is not quite as simple as bow waves forming and attaching at certain speeds.

The answer can be found in shock tables. When a bow shock forms (or a stood off shock - occuring in front of the intake) in will not attach to the surface if the surface is too blunt. There is no solution in the shock tables for above around 30 degrees, i.e shocks will not attach to surfaces above those angles.

It's really hard to explain, but look it up and you will see what I am going on about.