View Full Version : IGV's & VSV's/VGV's Actuation Speeds.

Obi Offiah
20th Mar 2009, 20:57
I've been wondering, typically at what sort of rates can these vanes be re-positioned?


20th Mar 2009, 23:19
Dear Obi.

The speEd of controlling the bleedflow in a compressor is not measureable, this is controlled by the fadec/eec, depending on rotorspeed ,temperature and thrustsetting.
The manufacture develops a special program, and during development of an engine, several test will be done to figure out the optimum on rotorspool and blade position control.
To reduce load on an compressor to avoid a stall, the engine is every time controlled via temp and pressure sensors, when the VBV/VSV are not operating correct, bleedvalves open, to remove the air from the compressor. There is no abrupt change in movement, everything is smooth an in relation to avoid stall/surge


22nd Mar 2009, 02:05
During accel, all the variable components will track very closely the windup in N2. The lag is measured in milliseconds.

During decel/chop, the same is true (and even more critical) with the following exception: The VBV's (booster bleed doors) will "pop" wide open instantaneously to reduce core airflow. This means the LPT is developing less torque, and thus the fan spools down faster.

Obi Offiah
25th Mar 2009, 16:24

I've since managed to find a document which describes VGV's adjusting at almost 40 degrees per second, but it doesn't state this value as their maximum rate.


25th Mar 2009, 20:33
...VGV's adjusting at almost 40 degrees per second

I would not be surprised if the actuation system is capable of that slew rate, particularly on a throttle chop where the HP fuel pump is still turning at high speed but flow to the burner is cut way back. There's lots of excess fuel flow available.

But the VGV positioning demand is probably slaved to a core tachometer (either electronic or hydromechanical) - and thus if an acceleration from FI to TOGA takes (e.g.) 5.0 seconds, I would expect the VGV's to make their move in 5.005 seconds. If they moved faster than this, there would be an imbalance in pumping capability from the front of the HPC to the rear, and a stall/surge is risked.

If you ever get the chance to visit a test bench (test cell outside the UK) , you can see this in action. It's really impressive. :cool: