Su 25 shaking error at high altitude

[plane00]

At about below 1000m , at 1000+km/h shaking begins

At about 9000m, at about 900 km/h shaking begins (true speed ,not IAS)

Is this Reasonable??

[Abburo]

SU25s max speed is around 950 km/h according with the constructor. So yes, the values indicated by you are very reasonable.

 

Also, max speed varies according with your current loadout.

[esb77]

At about below 1000m , at 1000+km/h shaking begins

At about 9000m, at about 900 km/h shaking begins (true speed ,not IAS)

Is this Reasonable??

 

Yes, when an airplane not designed for supersonic flight gets to the transonic speed range (usually about .85 to 1.0 Mach) it will suffer from a variety of aerodynamic problems.

 

Buffeting is one of them.

 

In the game however, buffeting does not have any bad affects other than making aiming weapons very difficult.

 

Entering a dive at high speeds however, is not a good idea. It can take over 7000m of altitude to pull out of a dive if the plane is too fast.

[Erdem]

Speed of sound decreases with altitude, so it's possible that you're reaching it and experiencing its effects sooner at altitude.

[BitMaster]

It heavily depends on your load out when this will happen.

 

For example, when doing SEAD missions I usually take 2 x Kh-58u and 4 x 25MPU and 2 pods BUT no Air-2-Air on the outside pylons. As soon as those are mounted top speed is really limited, when I leave them off instead I have another approx. 100km/h that I can go faster without shaking.

 

Bit

[plane00]

I can't accept these explanation.

Edited October 29, 2014 by plane00

[MiG21bisFishbedL]

I can't accept these explanation.

 

Tough, then.

[FoReIgNeR]

At about below 1000m , at 1000+km/h shaking begins

At about 9000m, at about 900 km/h shaking begins (true speed ,not IAS)

Is this Reasonable??

 

High speed buffeting occurs due to compressibility effects (proportional to speed of sound)...

 

Mach number of 1000kph @ 1000m:

---------------------------

Temperature = 273.15 - 0.0065 * 1000 = 266.7K

Speed of sound = sqrt(266.7 * 1.4 * 287) = 327.4 m/s

1000kph in ms = 1000 / 3.6 = 277.8 m/s

Mach number = 277.8/327.4 = M.849

 

 

Mach number of 900kph @ 9000m:

---------------------------

Temperature = 273.15 - 0.0065 * 9000= 214.7K

Speed of sound = sqrt(214.7 * 1.4 * 287) = 293.7 m/s

900kph in ms = 900/ 3.6 = 250.0 m/s

Mach number = 250.0 /293.7 = M.851

 

 

Conclusion: yes it is reasonable, shaking begins at approximately the same Mach number

[FoReIgNeR]

And some additional info:

 

Shaking is the result of part of the flow becoming transonic. In a nutshell: lift is created by a pressure differential between the top and bottom of the wing. The pressure differential is created by a speed differential of the air on the top and bottom of the wing. The shape of an airfoil -in essence- `accelerates' the speed of the airflow at the top of the wing.

 

At a certain critical Mach number the airflow over the top of the wing reaches transonic speeds (i.e., ~M1.0). As a result shockwaves start to form which, for sub-sonic wing profiles, cause induced drag, reduced controllability and a lot of turbulence. Buffeting is mainly caused by this turbulence hitting the tailplane of the aircraft.

 

Hence, high speed buffeting occurs at a given Mach number, which in the case of the SU-25 seems to be M.85.

 

Cheers

[Ironhand]

I can't accept these explanation.

What is it about the explanation that you find hard to accept?

 

 

Rich

[plane00]

Isn't sound barrier force lower at high altitude than at lowe altitude ?

 

Should su 25 , su 25t has lower speed limit at high altitude?

[mvsgas]

This information was already provided to you.

Speed of sound decreases with altitude, so it's possible that you're reaching it and experiencing its effects sooner at altitude.

 

High speed buffeting occurs due to compressibility effects (proportional to speed of sound)...

 

Mach number of 1000kph @ 1000m:

---------------------------

Temperature = 273.15 - 0.0065 * 1000 = 266.7K

Speed of sound = sqrt(266.7 * 1.4 * 287) = 327.4 m/s

1000kph in ms = 1000 / 3.6 = 277.8 m/s

Mach number = 277.8/327.4 = M.849

 

 

Mach number of 900kph @ 9000m:

---------------------------

Temperature = 273.15 - 0.0065 * 9000= 214.7K

Speed of sound = sqrt(214.7 * 1.4 * 287) = 293.7 m/s

900kph in ms = 900/ 3.6 = 250.0 m/s

Mach number = 250.0 /293.7 = M.851

 

 

Conclusion: yes it is reasonable, shaking begins at approximately the same Mach number

 

And some additional info:

 

Shaking is the result of part of the flow becoming transonic. In a nutshell: lift is created by a pressure differential between the top and bottom of the wing. The pressure differential is created by a speed differential of the air on the top and bottom of the wing. The shape of an airfoil -in essence- `accelerates' the speed of the airflow at the top of the wing.

 

At a certain critical Mach number the airflow over the top of the wing reaches transonic speeds (i.e., ~M1.0). As a result shockwaves start to form which, for sub-sonic wing profiles, cause induced drag, reduced controllability and a lot of turbulence. Buffeting is mainly caused by this turbulence hitting the tailplane of the aircraft.

 

Hence, high speed buffeting occurs at a given Mach number, which in the case of the SU-25 seems to be M.85.

 

Cheers

[Ironhand]

At about below 1000m , at 1000+km/h shaking begins

At about 9000m, at about 900 km/h shaking begins (true speed ,not IAS)

Is this Reasonable??

In the part of the atmosphere in which we fly, the thing that has the greatest impact on the speed of sound is temperature. Up to roughly 11km, the temperature of the air decreases as you get higher. So, the speed of sound is reduced, the higher you fly. You hit that transonic range and shaking sooner at higher altitudes.

 

 

Rich

[FoReIgNeR]

Isn't sound barrier force lower at high altitude than at lowe altitude ?

 

Universal gas law:

pressure = density * specific gas constant * temperature

 

Density and temperature in the troposphere (up to 11km) decrease with altitude. So yes, the pressure in the shockwaves will be lower than at sea level (albeit only slighty). But the fact is, high speed buffeting will occur at the same Mach number, and the speed of sound decreases with altitude. Thus buffeting occurs at a lower TAS the higher up you go.

[FoReIgNeR]

And further, as for heavier payloads:

 

1) The payload itself causes induced turbulence

2) The added weight of the aircraft requirs a higher AoA to remain in stationary flight. Higher AoA in general means higher vorticity over the wing = higher speed over the top of the wing. That's why the critical Mach number is lower for a heavier loadout

[Ironhand]

Isn't sound barrier force lower at high altitude than at lowe altitude ?

 

Should su 25 , su 25t has lower speed limit at high altitude?

Ah, I think I understand what you're saying. Are you thinking that, because air pressure is lower at higher altitude, you should be able to go faster, not slower? The problem is that air pressure itself isn't the issue.

 

The faster you fly, the more closely packed the sound waves ahead of you become. At some point you catch up with those densely packed sound waves you've been propagating and that's the so-called sound barrier. But the thing that determines that speed more than anything else is air temperature--about 0.6 m/s for every degree Centigrade you lose.

 

 

Rich