13

With commercial aircraft flying slightly above Mach 0.8, is there any surface exposed to speed of air significantly larger, e.g. near Mach 1? ignoring:

  • The engine components,
  • The boundary layer as clarified by Trebia Project.

Edit: Reading useful information provided in comments and answers, I realize the key notion is "critical Mach number", and the question boils down to:

Are commercial jets flying at critical Mach number speed or higher?

since the critical Mach number speed is -- by definition -- the one at which some airfoil portion experiments a supersonic airflow. See also:

How does exceeding the critical Mach number of an aircraft affect its operation?

Community
  • 1
mins
  • 72,728
  • 27
  • 308
  • 441
  • 5
    The outside. – fooot Jan 22 '15 at 19:04
  • 1
    Fair enough. +1. I updated the speed to near Mach 1. – mins Jan 22 '15 at 19:28
  • Flow speed through engines will be much slower than mach 1 (excepting scram/hyperjets). For highest velocity flow look for lifting surfaces (e.g. the wing) and propeller tips. – casey Jan 22 '15 at 20:05
  • 3
    @casey Yes, flow through the engines is slower than Mach 1, but the fan blade tips themselves exceed Mach 1 on many turbofan designs, resulting in shockwaves, which I'm assuming is why mins excluded the engines from consideration. – reirab Jan 22 '15 at 22:28
  • http://aviation.stackexchange.com/questions/11884/do-nacelles-around-aircraft-engines-helps-in-reducing-the-engine-sound This answer states the blades of engines runs at mach 1.5 – Antzi Jan 23 '15 at 03:16
  • 3
    Since the A310 new jet aircraft use supercritical airfoils, and they have their name from flow speeds above the critical speed at their design point. Speeds of up to Mach 1.2 are possible and are normal in everyday operations. – Peter Kämpf Jan 23 '15 at 09:37
  • @fooot Here is another picture of one of the optical effects of shockwaves generated by commercial craft: http://www.pprune.org/tech-log/443649-0-98-mach-747-8-a.html#post6264909. I have seen this several times while traveling as a passenger, and in one case, the wave was visible as a distortion of the view through it (this was on a Tu-154, which I don't think has a supercritical wing.) – sdenham Nov 09 '15 at 20:52

2 Answers2

12

Of course! If you check the wikipedia page the Critical Mach Number of an aircraft is the lowest Mach number at which the airflow over some point of the aircraft reaches the speed of sound, but does not exceed it. The Drag Divergence Mach Number is usually bigger than the Critical Mach Number, it means that at that speed there are big surfaces exposes to speed of air larger than 1.

It can happen because the figure of an airplane is shaped in order to avoid shock waves with flow and to postpone the maximum as possible the Drag divergence Mach number (in order to have a greater cruise speed).

Where? On the upper part of the wings, where the flow is accelerated in order to produce lift.

Clarification about engines (thanks to the help of one of the comments):

The flow from a jet engine has a speed about double of the cruise speed of the airplane. Anyway the outflow from an airliner jet engine is much hotter than the surrounding air, so the speed of sound in this gas stream is much higher; its Mach number is still subsonic.

Community
  • 1
Gianni Alessandro
  • 3,154
  • 7
  • 30
  • 51
  • it doesn't hit any surface -- unless you consider the engine wall a surface ;) – yo' Jan 22 '15 at 23:26
  • 10
    The outflow from an airliner jet engine is much hotter than the surrounding air, so the speed of sound in this gas stream is much higher. While you are right that the gas speed is about twice the flight speed, its Mach number is still subsonic. – Peter Kämpf Jan 23 '15 at 09:03
3

If we take the question as it is, I will say that never ever any point of the surface of any airplane will be at Mach > 1. Close to the surface, at typicial airliner, exist an thin layer of air that adapts the speed from outside to the speed of the airplane. So, the air at the surface of the airplane is at velocity relative to the airplane of 0. So the Mach = 0.

This layer is called boundary layer and, in typical airliners, some centimeters over the the edge of the surface of the airplane the air will recover the speed and everything that has been mentioned will apply (and so there will be points close to the surface where the Mach will be > 1).

Question seems as well focused in the cruise condition, however, when high lift devices are deployed (flaps and slats) the acceleration on the upper surface is much bigger, you can have one or several points "close to the surface" where the Mach is 1 having the airplane flying at relatively low Mach number (0.3-0.5). This depends strongly of the high lift devices used by the airplane.

Trebia Project.
  • 3,913
  • 1
  • 21
  • 40