Turn and Bank vacuum system

I'm using an overhauled vacuum turn and bank indicator, connected to a new venturi, and it does not work correctly. I think it isn't spinning up fully, because there is a little bit of needle deflection, but not even close to what it should be. I'm not sure where to start with trouble-shooting and am looking for suggestions.

The venturi is mounted behind the cowl cheek, so may not be getting adequate airflow. It is supposedly the correct size for running a turn and bank, and there is nothing else connected to it. Any experience with that location, successful or unsuccessful?

The venturi is connected to the indicator with 1/4" alum tubing, approximately a 2 ft run. Is that too small?

I don't have a vacuum gauge installed, so can't say whether the indicator is getting adequate airflow.

Maybe somebody experienced with this can tell me what's likely going on.

Thanks!

12 Geezer said:

The venturi is mounted behind the cowl cheek, so may not be getting adequate airflow.

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That's where mine is, just below the corner of the windshield. Mine (seems to) work fine.

NimpoCub said:

That's where mine is, just below the corner of the windshield. Mine (seems to) work fine.

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Thanks Logan - Mine's down lower, behind the wide part of the cowl. So I'm thinking that might have been less than bright of me. Could you maybe post or send a photo of yours?

Here it is, cropped from one of my initial teardown pix.
It's still a bit behind the cheek, but near the top so seems to get enough airflow hitting it.

Did you ever get your turn and bank indicator to work?
I am having a similar problem

Is this how you installed it? The venturi MUST be located where the air is flowing all of the time. Behind the cowl cheek, it is not. Also the airplane must go fast enough to move the air through the venturi. Nimpo's picture shows one location. Another would be low on the boot cowl directly in the prop blast. Some airplanes (Republic Seabee) tapped into the intake manifold of the engine for the suction. While a T&B will work on slow airplanes, usually a DG and Horizon which require more suction will have difficulty due to the low cruising speed. Any speeds much below 100 mph usually do not provide enough suction for "the whole enchilada".

TLubeck said:

Did you ever get your turn and bank indicator to work?
I am having a similar problem

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I installed a vacuum gage, and determined that the venturi is not producing vacuum behind the cowl cheek. But I haven't taken steps to move it, the Dynon D1 just works so well that it makes me lazy - -

A bigger venture will produce more vacuum.

I know of one C170 with a vacuum pump piggybacked onto the generator, and another with a belt-driven vacuum pump mounted just behind the prop. I know of others which were converted to the O-300D because the owner wanted an engine-driven vacuum system. I don't understand why people dislike venturi's. My old C170 cruised at about 115 mph, it had two venturi's to run a horizon & DG and they worked just fine. I've heard comments about venturi's icing up but unless you have a heated pitot (which most 170's don't) I'd think those would ice up before a venture would. The one bad aspect of a venture I can think of is that you have to get some airspeed up before the gyro's come to life-- not too good if you wanna take off into zero/zero.

I had the same problem with my PA-12 with the venturi on the left side of the boot cowl, forward and high. I've got a 160 with a Borer prop. Replaced the 2" venturi with a 9", and it still didn't work. Tested the gyros, a DG and TC, which worked on the bench. I wasn't trying all that hard because I didn't really want gyros anyway. I gave up; the DG was probably 20 years old, and the TC was older. I removed all the gyros, filters, regulator, gauge and venturi, updated the W&B, bought a Dynon D1. Got about 10 lbs of stuff off the ship.

I am certainly not planning any incursions into IMC, but this works just fine for me. I'd be happier with an air data computer for actual speed and altitude instead of the GPS speed and altitude that the D1 has, but my steam gauge altimeter and airspeed are right there if I need them.

No regrets, ain't 'going back.

I ran a 2" gyro on the J-3 for decades. Worked for thousands of hours, until the WWII gyro died. I installed it because I needed to demonstrate instrument stuff for my checkride.

I now run a large venturi on the Decathlon with a shutoff, to preserve my delicate instruments whilst upside down. I think it is 11", but it is the largest common size. It drives an attitude gyro, a DG, and a turn needle - all three - and pulls about 5" at 100 mph. It is adequate to keep me instrument current.

You need a vacuum gauge. Whether you want to permanently install it or not, is up to you. But without it you don't know if you are pulling a vacuum or not. Install a gauge and fly. If it pulls a good vacuum, you have a bad instrument. If it does not pull a vacuum, you can troubleshoot the system. disconnect the venturi, plug the filter fitting, and put a vacuum source on the line. If it holds vacuum, it does not leak. Reconnect the venturi and blow shop air through the venturi FROM A DISTANCE! If it pulls a vacuum, the venturi may be in a bad location.

Web

Dimensions for venturi location in lower right hand corner . . . . .

http://www.supercubproject.com/drawings/pdfs/A3310181.pdf