> >
>

# Why does aircraft fly at Mach?

Tech Log The very best in practical technical discussion on the web

# Why does aircraft fly at Mach?

25th Aug 2015, 16:53

Join Date: Apr 2008
Location: Test
Age: 31
Posts: 189
Why does aircraft fly at Mach?

Hi,

I'm just learning, so go easy.

Why does aircrafts fly Mach? Is it due to the speed of sound and density of air? How do i put forth an explanation?

Thanks
25th Aug 2015, 20:41

Join Date: Feb 2009
Location: Seattle
Posts: 379
Air flow - incompressible vs. compressible

This question should lead to taking a basic aerodynamics course or finding text books for that information on line. As a very simple, high-level over view the lift and drag characteristics of an airfoil change as Mach number approaches unity. At low speed, air flowing over an airplane is reasonably approximated as incompressible and scales with impact pressure that is proportional to the square of airspeed. As speed approaches Mach 1 the airflow experiences more and more compression yielding variation in the aerodynamic characteristics.

Airplanes are designed to optimize performance at a specific Mach number. To fly an airplane faster than its optimum cruise Mach number would require significantly more thrust as drag increases rapidly with higher Mach.

For most commercial transport airplanes the upper limit on operating velocity is airspeed at lower altitude and Mach number at high altitude. This gives rise to maximum operating velocity (Vmo) and maximum operating Mach number (Mmo). For any given airplane there will be an altitude at which Vmo and Mmo are reached simultaneously - this is called the Vmo/Mmo corner. Below that altitude Vmo is the operative airplane velocity limit. Above that altitude Mmo serves as the limit. The altitude of the Vmo/Mmo corner usually falls between 25K and 30K feet.

I hope this tiny intro encourages you to explore more understanding of these topics through course work and text books.
25th Aug 2015, 20:48

Join Date: Dec 2014
Location: Midlands
Posts: 114
I think the question might be more along the lines of, why is Mach used instead of conventional airspeed.

On the other hand I could be wrong.
25th Aug 2015, 21:16

Join Date: Aug 2006
Location: USA
Posts: 140
This may help

www.tscm.com/mach-as.pdf

When holding a constant Mach as altitude increases KIAS decreases. I don't have the charts at my finger tips, but Mach 0.84 at FL250 is about 340 KIAS. At FL350 M0.84 is about 280 KIAS.

Hope this helps.
26th Aug 2015, 03:38

Join Date: Apr 2008
Location: Denver,Co USA
Age: 72
Posts: 329
Referencing the previous post, once you climb above the Vmo/Mmo corner you can continue to climb at a constant Mach while if you are using airspeed you would have to reference charts to fly a constantly decreasing airspeed as you climb.
26th Aug 2015, 05:39

Join Date: Apr 2014
Location: Washstate
Age: 75
Posts: 0
RE MACH

One of the reasons mach is used at higher altitudes is that mach ( mach ) speed of sound varies as temperature. Standard temperature profiles ( absent storms, etc ) will show lower temperatures at higher altitudes.
Lower temperatures result in a lower speed of sound, so a given constant velocity ( airspeed ) results in a mach number which varies as altitude. The higher altitude = lower mach number and vice versa

OR IN SIMPLER TERMS The speed of sound is higher at sea level than at altitude 760 mph at sea level - 660 mph at 35,000 to 60,000 feet. temp is nearly stable 35 to 60 K feet.

would suggest looking at

http://www.fighter-planes.com/jetmach1.htm

Last edited by SAMPUBLIUS; 26th Aug 2015 at 16:10. Reason: posted wrong link - sorry
26th Aug 2015, 12:28

Join Date: Mar 2005
Location: Uh... Where was I?
Posts: 1,342
Airliners are limited in both IAS and Mach.

High and fast flight leads to relatively high Mach numbers.

For the same IAS, Mach increases with altitude, so in the climb there is a moment where the Mach number limit speed (IAS) goes below the IAS limit. From then on, Mach number becomes limiting.
27th Aug 2015, 06:58

Join Date: Aug 2001
Location: Above the Horizon
Posts: 114
Why does aircraft fly at Mach?

Nobody has provided the correct answer. Planet Basher has explained the question more clearly. "Why is Mach used instead of conventional airspeed?"
27th Aug 2015, 07:57

Join Date: Aug 2001
Location: Above the Horizon
Posts: 114
Why does aircraft fly at Mach?

Because of density error the ASI is not suitable fro speed indication above 25,000 feet. So the speed of an aircraft is given as a ratio of the TAS to the local speed of sound. Note the difference between IAS and TAS at high altitudes.
27th Aug 2015, 10:08

Join Date: Jan 2014
Location: N5109.2W10.5
Posts: 622
Hi extricate,
How do i put forth an explanation?
I suggest you read parts of:
http://www.skybrary.aero/bookshelf/books/2263.pdf
The graph on page 141 shows the "Flyable area" between two Mach Numbers and has the explanation maths.

Good luck.
27th Aug 2015, 10:34

Join Date: Jul 2013
Location: UK
Posts: 15
The nature of air flow changes at the speed of sound, hence the sonic boom. For subsonic aircraft you don't want any sonic boom anywhere so it is important to know how close to the speed of sound - Mach 1.0 - you are, and typically less than 0.9 to be sure no surfaces are in sonic boom territory.

The characteristics for supersonic flight change at faster than the speed of sound.

The speed of sound changes with air density and pressure so using air speed alone is not a good indicator.

Note 1: All this is as simple as I can make it
Note 2: Sorry to the OP if I've missed the point.
Note 3: Web search is your friend.
27th Aug 2015, 10:45

Join Date: Sep 1998
Location: wherever
Age: 50
Posts: 1,609
The only real answer these days is by convention. There were good reasons for the convention but with modern flight instrument systems those reasons become moot.
27th Aug 2015, 11:33

Join Date: Dec 2009
Location: usa
Posts: 10,461
Rick and Mustang are closest, because at higher altitudes and speeds, using an IMN gives you a constant number to reference in order to control for velocity/energy, vice chasing an ever changing indicated airspeed which varies greatly with changing temperature and altitude.

Propeller aircraft do not use IMN because they do not go very high or very fast.
27th Aug 2015, 13:13

Join Date: Jun 2014
Location: Village of Santo Poco
Posts: 794
I still like my answer the best. Too bad the mods deleted it.
27th Aug 2015, 14:08

Join Date: Jun 2001
Location: In the State of Perpetual Confusion
Posts: 185
The reason is much like why we fly the airplane using airspeed rather than ground speed. The things that we are concerned with as pilots, like when the wing stalls in level unaccelerated flight (I know it's angle of attack but work with me here), the best speed for approach, best cruise speed etc, are all functions of airspeed. At higher altitudes and speeds, things like the speed for high speed buffet, low speed buffet, minimum drag speed, best range speed etc, are most easily described as functions of Mach number. There are other variables as well (weight and temperature) but using the ratio of true airspeed to the speed of sound simplifies things somewhat. You could use indicated airspeed but the values that you would be concerned with would be all over the place as weight, altitude and temperature changed.

In a nutshell, the phenomena that you are most concerned with as a pilot (or performance engineer for that matter) are most easily described as a function of Mach number at higher altitudes.

Last edited by Gillegan; 27th Aug 2015 at 14:43.
27th Aug 2015, 16:32

Join Date: Feb 2012
Location: USA
Posts: 224
Airspeed in a nutshell

There are three airspeeds you care about....

1) Indicated airspeed (IAS) -- Think of this as how hard the wind would push back your arm if you stuck it out the window. Or, (oversimplified) how many air molecules per second are hitting the front of the airplane, times how hard they are hitting it. Higher altitudes and/warmer air = less air density (fewer air molecules per cubic foot of air) and so, at a given true airspeed, IAS is lower as you go higher or the air gets warmer. Most of the "normal" aerodynamic stuff -- drag, lift, response to the controls, etc. depends upon IAS. Stall speed, maneuvering speed, most "V" speeds are IAS.

2) True airspeed (TAS) -- actually how fast you are moving through the air. Think of it as how long it takes after the nose passes a given air molecule, for the tail to pass it. Some aerodynamic phenomena, most particularly flutter -- are dependent upon TAS rather than IAS, and so there will be limits -- Vne is TAS, for example.

3) Mach number -- how fast you are moving through the air relative to the speed of sound under those density / temperature conditions. Aerodynamic phenomena related to the compressibility of the air -- for example, how far back from the leading edge of the airfoil a shock wave forms -- are related to Mach number. Practically speaking, at lower altitudes a normal commercial aircraft is not going to be capable of getting anywhere close to the Mach limits, but at higher altitudes Mach limitations come into play.
29th Aug 2015, 15:18

Join Date: Mar 2005
Location: Uh... Where was I?
Posts: 1,342
Boyington, are you kidding me?

None of us gave correct answer and then you come up with that??

Because of density error the ASI is not suitable fro speed indication above 25,000 feet. So the speed of an aircraft is given as a ratio of the TAS to the local speed of sound. Note the difference between IAS and TAS at high altitudes.
29th Aug 2015, 15:20

Join Date: Mar 2005
Location: Uh... Where was I?
Posts: 1,342
Gillegan wins, so far.

Very good explanation!
30th Aug 2015, 08:13

Join Date: Apr 2008
Location: Test
Age: 31
Posts: 189
Thanks all.

Agreed with the above comment
30th Aug 2015, 14:38

Join Date: Aug 2001
Location: Above the Horizon
Posts: 114
Microburst
I was not kidding but merely quoting from 'Flying the Big Jets' by Stanley Stewart. The page number is 118.