Shaking of Su-25T at higher altitudes

[kontiuka]

When flying near sea level, the Su-25T starts shaking at around 650 kph IAS. I noticed, though, that at higher altitudes like 4,500 meters, the Su-25T starts shaking at around 520 kph IAS. Is this correct behaviour? I would have thought that the IAS at which the airframe starts shaking would be fairly constant.

[JABO2009]

difference of atmospheric pressure ?

[GGTharos]

No, the mach number is constant. The shaking is a function of the airflow type, not of the air speed (although components of your airframe may be limited to a certain IAS for thermal purposes). The airflow type changes with the mach number.

 

Have a look at this table, it will show you how mach changes with altitude. Also, the type of stores you carry on-board may change the mach number at which you start experiencing turbulent airflow.

 

Edited November 11, 2013 by GGTharos

[esb77]

The Su-25 (and Su-25T) are designed not just as sub-sonic, but really as sub-transsonic aircraft. The result is that they don't behave nicely when you're going at transsonic or supersonic speeds. There are assorted nasty aerodynamic side effects of being very close to the speed of sound, much of which involves drag from shockwaves as different parts of the plane break the sound barrier at different airspeeds.

 

As GGTharos showed with the charts, for the operational envelope of a Su-25 the speed of sound decreases with altitude. The effects are pretty well modeled.

 

External stores, especially high drag ones like Vikhrs, have a large effect on when the shake starts to become noticeable. I'd have to flight test to give exact figures, but the difference between no load and a high drag load can be on the order of 150 to 200 km/h difference in IAS for the start of noticeable vibration. The difference in onset speed decreases as altitude increases.

 

Aside from being really annoying it doesn't affect performance all that much. I've played SEAD missions using ingress at 30m or less altitude and IAS in excess of 950km/h. It's sort of like flying a paint mixer, but the plane does survive the shaking.

 

The only real concern with the Su-25s is that you not start a steep dive from high speed and high altitude. One of the well modeled things is that if you manage to get supersonic the tail looses almost all pitch control, meaning that it can take more than 7000m of altitude to pull out of a supersonic dive. This can happen to planes in real life and was probably first well documented on the North American F-86. It's why most modern fighters have a 'flying tail' where the entire tailplane moves rather than having a stationary horizontal stabilizer followed by a trailing edge elevator.

 

The Su-25T is happiest at altitudes below 1500m MSL and speeds in the 600-700 km/h range. The first flaps setting greatly improves handling if you are below 500 km/h. Still, if you don't care about fuel efficiency or having your virtual teeth rattled out of their sockets the Su-25s are happy to push right up next to the sound barrier if they aren't overloaded.

[kontiuka]

Thanks for your responses.