1 mach / in kmh

[Lenux]

Would someone please, explain, how is that possible, when i reach the speed of 1,5 mach, but I have only 1270 kmh?

I tried different weather conditions, weapons, altitude, unlimited fuel, etc

When I reached exactly 1 mach, which is 1225 kmh, i only could see 780 kmh ( F2 view) and a bit later, i reached the speed of 1.5 mach, and again, i checked F2 view, 1270 kmh

How is that possible, actually?

Thank you very much

Edited June 24, 2018 by Lenux

[ouPhrontis]

Mach numbers vary with air density.

[Lenux]

Okay, but which one was the real speed?

1270 kmh or the real 1.5 mach, which is 1852 kmh??

[Tricky11]

Would someone please, explain, how is that possible, when i reach the speed of 1,5 mach, but I have only 1270 kmh?

I tried different weather conditions, weapons, attitude, unlimited fuel, etc

When I reached exactly 1 mach, which is 1225 kmh, i only could see 780 kmh ( F2 view) and a bit later, i reached the speed of 1.5 mach, and again, i checked F2 view, 1270 kmh

How is that possible, actually?

Thank you very much

 

because its mph and not kmh?

[Lenux]

I have kmh, not mph

[Tricky11]

hmmm well those numbers you give are very close to mph?

[Lenux]

That's true, but as I said, i have kmh.

[ouPhrontis]

[snip]When I reached exactly 1 mach, which is 1225 kmh, i only could see 780 kmh[snip]

 

Mach is not a fixed speed, it varies with air density, temperature and some other variables.

Edited June 25, 2018 by ouPhrontis
reworded as I was off my trolly when I posted it.

[Bullitthead]

The speed of sound (mach) is slower at higher altitudes. https://www.fighter-planes.com/jetmach1.htm

Also measured airspeed (indicated airspeed IAS) goes down with altitude vs. ground and true airspeed, because of the way the speed measuring device is effected by the air density at high altitude. What you are seeing is probably related to these.

Edited June 24, 2018 by Bullitthead

[Lenux]

I was at, about 6 km altitude

Here, the weather conditions ( 1st of January, 11.00 PM )

[Lenux]

The speed of sound (mach) is slower at higher altitudes. https://www.fighter-planes.com/jetmach1.htm

 

Okay, thank you very much, I didn't know that

[antagonist]

Okay, but which one was the real speed?

1270 kmh or the real 1.5 mach, which is 1852 kmh??

 

M1.5 is 1852 kph only at sea level. As soon as you start climbing up into the atmosphere, the speed of sound decreases as it's dependent on ambient temperature and pressure.

 

As Mach factor is a dimension derived from the speed of sound, its value in absolute airspeed also decreases.

 

[ouPhrontis]

The speed of sound (mach) is slower at higher altitudes. https://www.fighter-planes.com/jetmach1.htm

 

Great chart, I recall seeing this years ago and made me smile seeing that it's still up; though it is tied to air density than altitude, altitude being a happy coincidence.

[Lenux]

Man, thanks a million, i didn't know that the speed of sound ( mach) could vary

Okay, one more question, off topic, guys : does a jet fighter ( F18, for example) behaves very different, at 6 km vs 2 km altitude?

I tried a dogfight at 6-7 km altitude, vs 1x MiG29, the F18 behaves very different, compared to lower altitudes

Could you please tell me why?

Thank you very much, again

Edited June 24, 2018 by Lenux

[Ironhand]

I was at, about 6 km altitude

Here, the weather conditions ( 1st of January, 11.00 PM )

Your figures for MAch 1.0 are just about on the money: Mach 1.075. Assuming you were still at the same altitude for M 1.5, you should have been around M 1.114.

[Aluminum Donkey]

Would someone please, explain, how is that possible, when i reach the speed of 1,5 mach, but I have only 1270 kmh?

I tried different weather conditions, weapons, attitude, unlimited fuel, etc

When I reached exactly 1 mach, which is 1225 kmh, i only could see 780 kmh ( F2 view) and a bit later, i reached the speed of 1.5 mach, and again, i checked F2 view, 1270 kmh

How is that possible, actually?

Thank you very much

 

Ok, Mach 1, the speed of sound, is about 1250 km/h if you're at sea level and 15 degrees C.

 

As you gain altitude, the air pressure decreases--and so does the speed of sound. So, Mach 1 at 10,000m (~35,000 ft.) is about 1100 km/h.

 

Typically, fast jets have to be at 10,000 m altitude or more to reach their top speeds--at sea level, you're lucky to do Mach 1.2 in a clean config, and that's it. It doesn't matter what kind of aircraft it is!

 

Finally, in DCS World when you're using external view speed readings or HUD speed readings, you're reading Indicated Airspeed (IAS) which shows a speed lower than True Airspeed (TAS) because airspeed indicators in aircraft don't compensate for air pressure drop as altitude increases. In other words, airspeed indicators don't actually measure airspeed--they measure ram air pressure in the pitot tube, and are only accurate at one altitude (usually sea level.)

 

If you were at 10,000m when you read Mach 1.5, your true airspeed was roughly 1,600 km/h, but showed less than that because of your altitude.

 

To get true airspeed in DCS World, use the F10 view and select your own unit.

 

Here you go!

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

 

AD

[Vitormouraa]

As people pointed out, Mach number varies with temperature and pressure.

 

Generally speaking, the Mach number tends to decrease with colder temps, and increase with hotter temps.

 

So make sure you are using Ground speed as a reference, which is the speed in relation to the ground (as the name says).

[ouPhrontis]

Man, thanks a million, i didn't know that the speed of sound ( mach) could vary

Okay, one more question, off topic, guys : does a jet fighter ( F18, for example) behaves very different, at 6 km vs 2 km altitude?

I tried a dogfight at 6-7 km altitude, vs 1x MiG29, the F18 behaves very different, compared to lower altitudes

Could you please tell me why?

Thank you very much, again

 

No problem, yes; it makes sense when you look at Mach being a measurement of the speed of sound through local mediums, sound propagates at different speeds depending on the thickness of the fluid it's travelling through.

 

On behaviour of jets at varying altitudes; read air pressures and temperatures; you will see variances in the power produced by the engine, so yes there will be differences in performance.

[ouPhrontis]

By way of example, the Harrier's Pegasus engine (this goes for any other conventional or similar turbine) suffers in performance from high ambient temperatures and low air pressure, enough that when in theatres such as the Gulf war; vertical takeoffs and landings in certain regions were rare, if not at all possible.

[Lenux]

I'm just speechless :)

Another off topic question : excepting the water, where the speed of light is about 225.000 kms vs air, 299.972 kms : is the speed of light, too, affected by cold / hot air, density, air pressure, etc??

[Vitormouraa]

All gas turbine engines will suffer from an increase in temperature.

 

One of the most important aspects of a gas turbine is the temperature between turbines. And this temperature, which is extremely hot, cannot exceed a specific limit. And this temperature is directly connected to the T1, this is the temperature at which the air enters the engine. In order to not overheat the engine, the fuel control unit must trim the fuel flow (decrease). With this decrease in fuel flow, the engine is going to produce less thrust or horsepower. It's very important to keep a constant air-fuel ratio.

 

Gas turbine engines run because of the air, whenever there is an increase or decrease of it, there must be an increase/decrease of fuel flow as well.

 

In the past airliner engines used to have a water injection system for thrust augmentation. When the air is colder, the density increases, so you'll have a higher amount of oxygen for the same volume.

 

Air would be dumped into the compressor inlet, thus decreasing the air temperature, making air denser. And in some cases, it would let the engine use a higher fuel flow for a specific amount of time.

 

The Pegasus engine for example (used as an example above) used water injection to increase its thrust. This was obtained by decreasing the combustion chamber temperature, therefore allowing a higher RPM.

[Lenux]

 

To get true airspeed in DCS World, use the F10 view and select your own unit.

 

 

AD

 

I'd like to see this small windows, from the cockpit view, without pressing F10, when i want to check the real speed

[Weta43]

I'm just speechless :)

Another off topic question : excepting the water, where the speed of light is about 225.000 kms vs air, 299.972 kms : is the speed of light, too, affected by cold / hot air, density, air pressure, etc??

 

yes, in water :

As an approximation, it is about 3/4 of its speed in a vacuum, or 225,000,000 meters per second. As Dipen said, you'd really need to know the index of refraction which depends on its temperature, etc. The index of refraction of water is about 1.33, so the speed of light in water is (c/1.33) = 2.25E8 m/s.

 

Refraction can be conceptualised as one side of the wave form slowing down - or speeding up - as the wave transitions between mediums that propagate the wave at different speeds.

[Bullitthead]

Great chart, I recall seeing this years ago and made me smile seeing that it's still up; though it is tied to air density than altitude, altitude being a happy coincidence.

 

Yeah, just trying to make it easier to understand for him.

[Bullitthead]

Man, thanks a million, i didn't know that the speed of sound ( mach) could vary

Okay, one more question, off topic, guys : does a jet fighter ( F18, for example) behaves very different, at 6 km vs 2 km altitude?

I tried a dogfight at 6-7 km altitude, vs 1x MiG29, the F18 behaves very different, compared to lower altitudes

Could you please tell me why?

Thank you very much, again

 

Yes, the plane will be less maneuverable at high altitude, again because of the reduction in air density. It's more difficult for the wings to provide lift in the less dense air.