# What happens to a plane approaching the speed of sound?

• September 15th, 2014, 10:09 AM
MacGyver1968
What happens to a plane approaching the speed of sound?
This might be a topic for physics, but I put it here.

I know that when a plane approaches the speed of sound...it's a pretty bumpy ride, but when you break the sound barrier, things smooth out. So I have a few questions:

1. What causes the increased turbulence when you approach mach 1?

2. Why does that turbulence smooth out after you break mach 1?

3. How has the shape of the plane reduced these effects?

Please explain in "high school" level physics. :)

Extra credit: Please explain to me why a propeller aircraft can't break the sound barrier.
• September 15th, 2014, 10:28 AM
Dywyddyr
Quote:

Originally Posted by MacGyver1968
1. What causes the increased turbulence when you approach mach 1?

Air builds up around the leading edges and other parts, becoming relatively denser and causing higher localised wave drag.
Airflow breaks up (over different parts of the aircraft) and some of it becomes locally supersonic while other parts are still in subsonic flow.

Quote:

2. why do they smooth out after you break mach 1?
Because ALL airflow is then supersonic.

Quote:

3. How has the shape of the plane reduced these effects?
Ooh boy...
Short answer. Make it pointy. Sweep the leading edge of the wing 0, or make the wing (relatively) short and thin 1. Make the cross-sectional area variation smoother (as plotted against length) 2: the closer that plot gets to a streamline body the better the transonic/ supersonic performance.
There's entire books written about how to shape a plane to do this.

0 And, ideally, make sure that all of the aircraft (i.e. wingtips) will be inside the shock cone generated by the nose of the aircraft.
1 That's "relatively" thin: compared to its chord (the front-to-back length at any given point). Eg a thin wing could be three times thicker in actual measurement IF the chord is more than 3 times greater. This is the T/C ration (Thickness/ Chord. 3
2 The reason for so many jets having the wasp-waisted/ coke bottle look in the '60s. See Whitcomb's area rule.
3 The Supermarine Spitfire had a particularly good (i.e. low) T/C ratio (despite its wing actually being thicker than the P-51 Mustang's) and achieved the highest Mach number recorded for a propeller-driven piston-engined aircraft - M=0.92 (although it did completely wreck the engine...) other contemporary fighters - even with more power - had a lower critical Mach number and became uncontrollable/ broke up somewhat short of that speed.

Edit:
Quote:

Please explain to me why a propeller aircraft can't break the sound barrier.
Two main reasons - propeller efficiency drops with increasing speed - so the faster you want to go the more power is required (i.e. over and above any "straight" extrapolation). And to go faster the faster the prop needs to turn - at which point the tips of the prop start to become supersonic A (in fact some aircraft had supersonic prop tips even while stationary - XF-84H and the Harvard/ T-6 Texan also had tips that did more than Mach 1). And then you end up with a prop that's part supersonic and part subsonic: massively increased drag (and a f*cking awful noise!) with concomitant mechanical strength problems and possible problems from multiple shock waves hitting the aircraft structure i.e. could lead to failure of the fuselage due to "hammering" by the shock waves.

A Actually the tip speed is a function of rotational speed AND forward speed of the aircraft, but let's keep it simple.
• September 15th, 2014, 11:58 AM
MacGyver1968
• September 19th, 2014, 12:57 PM
One beer
Bloody hell, Daffy....... An intelligent, informative answer? Are you feeling OK?

What happened to all the normal grumpy: 'evidence?', 'citation required', 'your opinion', etc. that we usually get? You're not mellowing are you? :grin:

OB
• September 19th, 2014, 01:11 PM
Dywyddyr
Quote:

Originally Posted by One beer
Bloody hell, Daffy....... An intelligent, informative answer? Are you feeling OK?
What happened to all the normal grumpy: 'evidence?', 'citation required', 'your opinion', etc. that we usually get? You're not mellowing are you? :grin:
OB

Pfft, it was a sensible straightforward question.
Mac didn't make any ridiculous assertions 1, no wild speculations...

1 On the other hand he didn't send me any beer either.
• September 19th, 2014, 09:38 PM
RedPanda
Quote:

Originally Posted by Dywyddyr
Quote:

Please explain to me why a propeller aircraft can't break the sound barrier.
Two main reasons - propeller efficiency drops with increasing speed - so the faster you want to go the more power is required (i.e. over and above any "straight" extrapolation). And to go faster the faster the prop needs to turn - at which point the tips of the prop start to become supersonic A (in fact some aircraft had supersonic prop tips even while stationary - XF-84H and the Harvard/ T-6 Texan also had tips that did more than Mach 1). And then you end up with a prop that's part supersonic and part subsonic: massively increased drag (and a f*cking awful noise!) with concomitant mechanical strength problems and possible problems from multiple shock waves hitting the aircraft structure i.e. could lead to failure of the fuselage due to "hammering" by the shock waves.

A Actually the tip speed is a function of rotational speed AND forward speed of the aircraft, but let's keep it simple.

Although your whole reply deserves a *Like*, this part I found particularly interesting.
• September 20th, 2014, 05:24 AM
One beer
Quote:

Originally Posted by Dywyddyr
........Mac didn't make any ridiculous assertions.......

Fair enough.

OB
• September 20th, 2014, 07:40 AM
MacGyver1968
The beer is in the mail.

Yeah...Dy is sorta my "go to guy" when it comes to questions about military technology and such. I knew when I posted the OP, he'd be one of the first to respond. Similarly, when he has questions on how to grease a midget....he calls me. :)