Venturi Suction

Robert Eilers · Post by **Robert Eilers** » Fri Mar 23, 2007 11:11 pm

I had one of those revelations today that happen every know and then. I have an IFR C170 with DG and AI driven by venturi suction (My first experince with a venturi system). Down wind abeam the numbers, I pulled the throttle back and happened to glance at the suction gauge (I have flown the airplane for over 200 hrs and must admit this is the first time I noticed). The suction had dropped off to zip. As I reapplied throttle the suction began to rise again. I had not previously realized how much the prop blast affected the suction being produced by the venturi. I assumed if I was moving forward throught the air at 60 MPH + the venturi would be producing sufficient suction.

cessna170bdriver · Post by **cessna170bdriver** » Sat Mar 24, 2007 1:45 am

Is it prop blast aiding suction, or is an idling propeller tending to block the venturis? Maybe some of both? (I really don't know, I'm asking the question.)

Regardless of the cause, it does happen. This is one of the reasons I installed an SVS backup manifold vacuum system. When on, the system uses a flapper valve arrangement to take vacuum from the strongest of two sources. When you close the throttle, the venturi suction backs off, but at the same time the manifold suction increases and takes over. I don't think it was specifically intended to be used this way, but I can find no prohibition against it.

Miles

varr · Post by **varr** » Sat Mar 24, 2007 2:42 am

I have an airborne pump that runs off the crank shaft. It works great.
with the ventura as back up with the same valve set up .
My deal is i am trying to put my 170 on a diet and dont fly it IFR,so the air borne pump is coming off . the ventura should be sufficant.