Different VSI's

[Joseph II]

A while ago I flew with a test pilot. He was talking about two different types of VSI's in use.
One is usually found in the typical 172 trainer, but the other is found in test pilot aircraft.

All I remeber was he demonstrated in the 172 that if you very rapidly pitch down and level off (all
within 1/4 second), the VSI will show a climb instead of a descent. He said a test pilot's VSI
would show that rapid descent.

Does anyone have any information about this?

 

[Flymach2]

Hi...

The instrument which you refer to is probably the Instantaneous Vertical Speed Indicator, (IVSI).

The IVSI has has two little cylinders with pistons and springs inside of them. They're called "accelerometer dashpots". When the aircraft pitches up or down suddenly, the pistons will move and force air into or out of the diaphragm to get the pointer moving quickly.

However, in a steady rate climb or descent, the accelerometer dashpots will have no effect on the pointer.

Regards