Even though the SVS is a backup system, my most common use is to get the gyros spooled up prior to departure. The run up will provide an inch or two of vacuum, but not near enough to really get things going. I select the SVS soon after engine start and get the gyros spinning, set the DG and make sure the AI is going to erect before I even taxi. All I have to do prior to takeoff is cross check the DG to the compass, and I'm not fiddling around on the runway getting it set.gahorn wrote:Dual venturies are a good, adequate system and shouldn't require any backup, in my opinion.
Best regards
George
Also, I'm not sure it make any real difference, but i feel better bumping along on the ground with the gyros spooled up rather than sitting dead in their gimbals. The same goes for keeping the gyros spooled up in the pattern prior to landing. Even though I've logged over 3000 landings, most can be classified as "arrivals" and I just feel better if the gyros are up to speed at touchdown(s) .
'98C may have some room for improvement as far as venturi performance at lower airspeeds. I've heard others say they get 4 inches or more from their venturis, even at pattern speeds. Some claim to be able to get the gyros to spool up on run up. When I'm slowed down to the 70-80mph range, the vacuum is down to 2 to 2.5 inches, and even a full power run up will give me less than 2 inches of vacuum. I have the modern style RC Allen AI and DG, and they get lazy anywhere under 3 inches.
I've changed to the 8-inch venturis with very little improvement. One thing that is different on '98C as opposed to many other 170's is that there is one venturi on each side of the airplane as opposed to both on the right as shown in the IPC. They have always been this way, as there is no evidence of a second venturi ever having been on the right side. Just as a test, I have plugged the venturis one at a time, and found that the left one will not pull adequate vacuum by itself, but the right one by itself is nearly as good as both together.
My questions: With the slipstram rotating around the airplane as it does, could the venturi on the left be at a disadvantageous angle of attack? The left venturi does "whistle" at idle on the ground, indicating that the airflow may may be somewaht across the face, rather than straight through(?) I might be amenable to moving the left venturi to the right if I had some assurance of an improvement. Is there an installation drawing that gives exact dimensions for locating the venturis?
Miles