Blast from the past: Venturi-driven vacuum system on a 172

On the grass parking area in front of the Redcliffe Aero Club is an old 172 with a fairly unusual design feature: a dual Venturi-driven vacuum system.

The VH register says that VH-DCO was first registered in Australia in 1962. It is however quite likely that the airplane was first registered in the US and later imported into Australia. The registration data shows the model as a plain “172” and not “172A” or “172B”, which seems to indicate that it was built between 1956 and 1960, according to the Cessna 172 page on Wikipedia.

Another cue is the square tail of the original 172 model, which contrasts with the more familiar swept tail introduced in 1960 with the 172A model. Lots of other details on the original 172 can be found in this pilot report. I learned for example that the rear window was first introduced in 1963 with the 172D model. I personally love this feature of the 172, if only because it makes it easier to check for drift on climbout after take-off by having a quick glance at the runway behind.

Externally, the Venturi vacuum system looks like two brass horns bolted to the right-hand side of the engine cowling. However, instead of being one long cone-shaped tube as with the horns of vintage automobiles, each of the tubes is narrower about one-third of the way down, which is exactly what you want for exploiting the Venturi effect in order to create a vacuum.

As can be seen on the picture, the diameter at the choke point is about half the diameter of the intake end of the tube. Half the diameter means that the cross-section is reduced down by three quarters, and therefore the speed of the air at the narrowest part of the tube is four times the speed at the intake. I am no aerodynamics expert, but I guess we can assume the speed of the air at the intake to be fairly close to the airspeed of the plane.

I’ll spare you the calculation, but using the Venturi formulas given by the irreplaceable Wikipedia and for ISA conditions at sea level (air density of 1.225 kg/m3), such a Venturi device would ideally generate a pressure drop of 243 hPa for a cruise speed of 100 knots. Of course this assumes that the air is a non-compressible fluid, that it flows nicely in a laminar manner down the tube and that there is no friction on the walls of the tube. In addition, one can see on the picture that there is an inside ring near the intake end of the tube, which probably slows down the air a little on entry.

But you get the idea: the suction produced is strong enough to drive the gyros of the Attitude Indicator (AI) and the Directional Gyro (DG). I assume that the gyro of the Turn Indicator is electrically-driven, but I could not confirm that.

Just like with more modern airplanes which use an engine-driven vacuum pump, a suction gauge on the instrument panel tells the pilot that the vacuum system is working correctly (or not, depending). This can be seen on a picture of the instrument panel of G-BSEP, a 1959 Cessna 172 flying in the UK.

Obviously, the main problem with a Venturi-driven vacuum system is that it requires the aircraft to have reached cruise speed for the gyros to spin fast enough to give a reliable indication of either attitude or heading. This means no AI or DG during taxi and climb-out. I can't think this is too much of a problem for VFR pilots, since attitude and heading information are readily available from just looking outside or at the compass.

There is still one thing I am curious about however. How does the vacuum system behave in slow flight with a high angle of attack, such as when coming in to land? Can the pilot see a drop on the suction gauge? And if so, are the gyros still spinning fast enough to be relied on? Again, not having any AI or DG indication at this stage of flight would be nothing dramatic, since VFR pilots have their eyes looking out at the runway by then. I would love to go out and try it out for myself though :-)