View Full Version : VSI gauge lags?

11-29-2004, 03:39 PM
Currently, I hold a private pilots license in general aviation aircraft. By no means am I an expert on combat manuvers, especially in WW2 aircraft. That out of the way, my question is this...

Why does the vertical speed indicator gauge lag so bad? When I adjust the pitch of the aircraft I could see the altimeter increasing or decreasing, but the VSI will show the opposite for a few seconds. All the gauges should work together to show actual attitude of the aircraft, right?

That is my question. Thank you for taking the time to answer it. It is not a complaint, just an observation of the gauges. I have tried to ignor the VSI when trimming the aircraft. http://forums.ubi.com/images/smilies/16x16_smiley-very-happy.gif

11-29-2004, 04:40 PM
That€s what they do. http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif Use the altimeter to maintain altitude. The VSI will tell you your RoC in a prolonged climb or descent, but for the short term it is (and was!) largely useless.

The lag is due to the VSI measuring the rate at which the pressure changes, by comparing the pressures between ambient and a vessel which only slowly equalizes with the ambient pressure. A high rate of change of ambient pressure means there will be a large pressure difference.

This, however, means that the climb will have to go on for a bit before the pressure difference has reached the magnitude dictated by the rate of climb. You also have a built-in accuracies due to volumes in the system.

On modern VSIs, much has been done to improve this. You can e g have small weights in tubes, suspended on springs. These move due to inertia as the aircraft accelerates vertically. This means that they will push air around in the tubes, which is then fed to the chambers of the VSI to compensate for the lag. Such devices are called IVSIs, or Instant Vertical Speed Indicators. They probably weren€t in WWII crates.

Fred, typing away before bedtime

11-29-2004, 05:54 PM
Effte correct. VSIs back in the day would only indicate trends, not instantaneous changes. In your IFR instrument scan, you gave it a back seat. Really only useful back then to set up constant rates of climb and glide for instrument work, like the classic 500 fpm ILS descent.


11-29-2004, 06:26 PM
Bac in the day... AND today! The VSI on my P-3C/EP-3 lags like that. Tells you you are going up well after you are going up!

You can't fly the VSI. Thats way to reactive. You have to be proactive and anticipate what you are doing.