Turn and Bank Redux

[flyguy]

Re: Turn and Bank Redux

"... don't pull a lot of volume..."
Are you saying that there is air flow thru the gauge? I'm just trying to find out if there is some type of gauge out there that works quite differently than the ones I have calibrated and worked with in aerospace applications and the standards laboratory.
Darryl

[crispy critter]

Re: Turn and Bank Redux

Try it,read your books,look at the manuals. The suction gauge works threw the first insturment in line....or at least that's the way the C-170,172,and 182 manuals show it(all early straight tail models with venturi's). I have also hooked up several 2 inch venturi's on t-crafts,a Cub,and a couple other birds. I have had the same problem as described above and have hooked it up every way known to man and this is the only way it will work properly. I'm not trying to act as if I know more then anyone else but I know what problems I have faced threw trail and error and this is what it took to fix the problem. This is also how the manuals I have says to do it with a venturi system. I don't have any maunals or do know of any that exist that show how to hook up a 2 inch venturi to an old T&B and suction gauge on a T-craft or a Cub. Show me an aircraft manual with a section on vaccum system and venturi's that has the suction gauge coming off the regulator and I will say no more. I know volume is not a big deal but pressure is and if you hook the vaccum gauge threw the regulator before the insturment it does take away from the insturment.Also if you hook the suction gauge up threw a seperate line off the regulator the 2 inch venturi now has to lines to supply for. This might not make a difference if your using a 4,8,or 9 inch venturi but with the little 2 inch venturi that is hooked up to drive a 2-2.5 inch insturment it does make a difference because they don't pull the volume pressure of the bigger venturi's.

[flyguy]

Re: Turn and Bank Redux

First: you are correct that sensing the vacuum off one of the case ports is the "accurate" way to do it.

The reason for that is not because the gauge "uses" or depletes any of the vacuum. Contrary to the little arrows in the manuals, nothing flows THROUGH the gauge. It is just atmospheric on one side and vacuum on the other.

For any given setting of the regulator there will be a increased vacuum level right at the regulator compared to anywhere down the line toward the gyro and especially inside the gyro case as atmospheric air is being blown into the case. That is because there IS flow through the manifold (or line) and therefore some presssure drop along it's length.

Because of that drop, when you attach the gauge to the supply manifold anywhere not close to the case it will indicate more vacuum than is actually present in the case. Since it is likely that the manufacturer rates the required vacuum for a given rotor rpm at the case, then the best way to measure that would be off the "extra" case port that has no flow coming out of it. You are absolutely correct about that.

I was careless in stating to just measure the vacuum between the regulator and the gyro out. My expericence has been more with calibrating gauges and such in test systems where, once obtained, the vacuum is static and there is no flow.

I do suspect that sensing it off a T, on the vacuum out line, close to either case port, would indicate very close to the actual case vacuum level.

The reason for all my questions and this diatribe on my part is because of the the way it was expressed that the gauge took away vacuum from the supply as if it was leaking atmospheric pressure into the system, or something along that line. I wondered if I hadn't missed something over the years working with gauges. Don't like to be, or stay, ignorant.

Second reason is that someone who doesn't know about this stuff is going to read this thread and get the wrong ideas if it isn't accurate in both results/applications, AND explanations of why things work that way.

DC

[crispy critter]

Re: Turn and Bank Redux

Now I think we are in complete agreement. I just hope we haven't confussed the poop out of everyone else who is trying to learn things in simple terms they can understand,lol. I have tried using a "T" before on a 2 inch venturi on a t-craft and it did work but not as well as running it threw the seperate inlet port on the T&B,plus I was with a 4" venturi,the 2" wouldn't pull it threw a T . I also had clearance problems with the fuel tank and tried to run it from the bottom inlet port. It would work but not very well best I remember and when the suction gauge was hooked up threw a T on either side of the insturment,the suction gauge would work a little but the insturment would not. I first assumed a bad insturment so I tried hooking up a DG the same way and had the same results. I then suspected the venturi so I hooked a 4 inch to it. With my dad flying I held it out the window and it would work either insturment with the suction gauge on a T but we were having to regulate about 3 inches to make it work. Then my father suggested we tool up,gas up and try switching things around in the air between both venturi's and try several different combo's until we got the best readings.I had the T&B hooked to the 4" venturi and swapped it over to the 2" with the regulator set at 3 inches. I simply unhooked everything and just plugged the T&B in directly to the back port and it worked great,spun up instantly.I could feel the torque in my hand the second I connected the line. So then I hooked up the suction gauge and the easiest way from where I was at was to simply plug it directly into the bottom inlet port.....guess what,we had instant 2.5+ inches of vaccum from a 2 inch venturi and everthing worked great. After landing I reinstalled the T&B in a differnt location so I could use the center port and I used a T in the back going to the suction gauge to simplify hook up. When we flew it the next time the vaccum was barely pulling 1.5 inches and the T&B would not work. So I removed the T and hooked the suction gauge to the bottom inlet and everything worked great!

[jmvolpp]

Re: Turn and Bank Redux

I've finally had a chance to hook things up as suggested and test fly it. The T&B works (timing still not what's indicated), but the gauge shows only 1.8", so evidently everything together is asking more than the 2" venturi can deliver. I'll drop the regulator and its length of hose, but if that doesn't give me at least 2" indicated, I'll drop the gauge also, and be satisfied without either. For my intended purpose, 4 degrees/second will be just as useful as 3.
Thanks for all your comments.

Mike V.

[Howard Wilson]

Re: Turn and Bank Redux

Mike,
I just went to the 'super' venturis on the 172. The old ones are more than 2" I think (will have to look). If your interested in one let me know. I don't need 2 extra venturis, just one for the Parrakeet project. Howard

[Larry Lyons]

Re: Turn and Bank Redux

Just for my edification; what do I have? My venturi is an all brass BENDIX about 6.5" long, 1.75" O.D. with a 1.5" I.D. at the exhaust and 1.125" O.D. with a .75" I.D. at the intake. This is off a 41BC-65. Thanks, Larry

[jmvolpp]

Re: Turn and Bank Redux

Larry, my 2" venturi is 6 13/16" long with 11/16 intake I.D. and 1 3/8 exhaust I.D.

Mike V.