Turn and Bank Redux

[jmvolpp]

Ten months or so ago I wondered (on this forum) why the newly installed T&B on N44305 performed an indicated 2 minute turn in 1:15. At the time I had no gauge or regulator and was guessing that maybe the venturi was sucking in excess or that the T&B wasn't right. Since then, the T&B has been checked and recalibrated by Century (Aircraft Spruce kindly honored the slightly out of date warranty), and I now have both a regulator and surplus vacuum gauge (picture attached). With the gauge alone attached to the 2" venturi, it pulls 2.7" @2150 RPM, which is the high side of the T&B recommended limitation. Adding the regulator allows easy regulation to 2" or whatever you want. The gauge has 2 ports, P and V. The regulator has 3 ports, as you can see in the picture. The T&B has 3 ports, 2 for outlet and 1 for inlet. The inlet port is on top and normally has a screw-in disc type screen filter, which I have replaced with a hose fitting. The outlet port on the back of the instrument is too close to the fuel tank to be used and is plugged, so the venturi is attached to the outlet port on the bottom. Visualize the open regulator port in the picture attached via hose to the inlet port on the T&B, so altogether from left to right is the venturi, T&B, regulator with filter, suction gauge P port (V left open). The suction gauge works with this arrangement, but the T&B doesn't. Connected alone to the venturi, the T&B works, but still does the quickstep 2 minute turn, all the while in a bank that looks to be approximately standard rate. Whyizzat?

Mike V.

[Howard Wilson]

Re: Turn and Bank Redux

Mike, I may not be reading this right. Is the regulator between the venturi and the T&B? Or is the T&B between the venturi and the regulator? The systems I've seen usually have the regulator directly down line from the venturi then the instruments......?

[flyguy]

Re: Turn and Bank Redux

Venturi, regulator, gauge, T&B out, T&B, T&B in, filter.
DC

[Howard Wilson]

Re: Turn and Bank Redux

Venturi, regulator, gauge, T&B out, T&B, T&B in, filter.
DC

Yup..........

[jmvolpp]

Re: Turn and Bank Redux

Thanks for the suggestion, which I will try next flight. Do you agree that the V (vent) port on the gauge be left open to the atmosphere, which would necessitate a "T" fitting at the P port? Or that it (V) would be in the series, attached to T&B out?

Mike V.

[flyguy]

Re: Turn and Bank Redux

I am not familiar with this guage, but pressure gauges are not normally flow-thru devices and do require a T. Do you have instructions for the gauge that specify what the "P" and "V" connections are? Could that be vacuum and pressure as in a differential gauge?
DC

[jmvolpp]

Re: Turn and Bank Redux

No instructions for the gauge as it came from salvage, but I think P is for pressure and V for vent. The C150 service manual depicts the P port attached to one of the T&B out ports (pump and regulator to the other), and the V port attached to T&B in port along with the filter - pretty much what you guys have suggested. When the weather's a little more accomodating, I'll give it a whirl.

Mike V.

[flyguy]

Re: Turn and Bank Redux

Cessna hookup makes sense to me: Sensing vacuum at the out port and reference to atmospheric at the in/filter. That way you get a true differential across the T&B, clever.
DC

[taylorcraftbc65]

Re: Turn and Bank Redux

I'm lazy. I just strap my Garmin 196 to the yoke, and turn on the satalite driven instrument pack. Brie

[crispy critter]

Re: Turn and Bank Redux

No instructions for the gauge as it came from salvage, but I think P is for pressure and V for vent. The C150 service manual depicts the P port attached to one of the T&B out ports (pump and regulator to the other), and the V port attached to T&B in port along with the filter - pretty much what you guys have suggested. When the weather's a little more accomodating, I'll give it a whirl.

Mike V.

I'm not trying to be a stick in the mud but something your describing does not sound right. Let me study a little and I'll give you an answer tomorrow. I know I redone all the Venturi plumbing in my C-170 using a later model diagram from a vaccum pump system and it would NOT work because I read something wrong,it was a very simple mistake but took some rework to fix it. I will have an answer for you in the morning.

[crispy critter]

Re: Turn and Bank Redux

Here it is.....some gauges are marked differently so you wil need to figure out what stands for what, but I'm certain you will need to find a way to use the port in the center of your turn & bank(the one plugged because of tank clearence). You come off the Venturi to the regulator. Then from the regulator to the vaccum/inlet(should be the center port in the back of the T&B,the one you think you can't use because of clearence.....find a way because you will need it). From there you run off the bottom port of the T&B to the vaccum side of your suction gauge,the vent port from the suction guage is left open. The top port on your T&B should be hooked to a filter. You should only have one suction line coming off the regulator and all the others plugged unless you are running 2 vaccum insturments. Your suction gauge should not come directly off your regulator. This is how the system is suppose to work....your venturi creates a vaccum,it sucks air threw the regulator which sucks threw the inturment/insturments to be driven, with the suction guage getting it's suction threw the insturment or first insturment in line. Most insturments will have a mark for gauge(some older ones will not. Either way,you can not run the suction gauge directly from the regulator or from a T that is the primary vaccum sorce for the insturment to be driven. You will have to find a very close 90* fitting or relocate the insturment to make it work. Run from your regulator to the center port in the T&B,hook your suction gauge to the port on the bottom port from which you have been using to drive the insturment previously, hook the top port of the T&B to a filter,leave the vent side of the suction guage completely open. If this does not work then you have a bad gauge,insturment,venturi,or regulator. I hope I haven't made this too confusing.

[Forrest Barber]

Re: Turn and Bank Redux

The original set up was that way lots of times there was just the ( "2" ) venturi , no regulator, lots of times no filter either just a screen to keep out big chunks and of course Don't Smoke Will Flying !

[jmvolpp]

Re: Turn and Bank Redux

Thanks, boys! A report will be forthcoming. Then for some hood work.

MV

[flyguy]

Re: Turn and Bank Redux

" Either way,you can not run the suction gauge directly from the regulator or from a T that is the primary vaccum sorce for the insturment to be driven."

Why not? Case vacuum is case vacuum, whether you read it from the back port or the bottom port, right? Have I missed something here?
DC

[crispy critter]

Re: Turn and Bank Redux

The suction gauge is suppose to read the suction that is being pulled through the insturment not the regulator. If you run the suction gauge straight from the regulator you are loosing air volume going to the gauge. The system may work but it will not work as well. The suction gauge is suppose to come off of the nearest insturment to the regulator. The small 2 inch venturi's don't pull a lot of volume to begin with so you need what you have.