corrosion protection for venturi

[frainiea]

I just completed the conversion to a pair of eight inch venturi to drive my new DG. The final results where not all that I expected, I am getting about 5-7 degree's of counter clockwise procession on each 90 deg turn. the vacumn gage shows 5 inches. Not sure what could cause this so I am interested in feed back.
The next step is to test the vac. system at high altitudes, above 5000 ft, where the four inch venturi just couldn't make the grade.

The primary reason for the post is how to get input on how to protect the new venturi's from this gulf coast environment. It is easy to say paint'um, and I can paint about 60 percent of the surface. My worry is the inside surfaces. The old ones were painted inside and out but I am not sure how they accomplished it. Are most venturi's painted or bare metal??

[lowNslow]

You could soak it in Alodine, use clear or amber. After soaking a few minutes rinse with water and blow out any excess water with compressed air.

[Jeff Matthews]

You might try checking the DG filter. The suction gauge just tells you how hard the pump or venturi is trying to pull air through the instrument, not how much air is actually flowing through it. You'll get the highest reading on the gauge if the filter's totally blocked. Depending on how the system's plumbed, the other gyros (if any) might be hogging the airflow.

[scott]

Where you measure is as critical as critical as what you measure. The old large guages need a larger flow of air at a lower pressure. Modern artificial horizons and DGs (If I could call them that) need 4.5 to 5 inches of vaccuum but flow less air than the old 10 lb monsters originaly installed. (get your own exact #s from the book on all specs not from me please)
If you measure near the ventury you might get 5 inches and then after all the accumulated energy loss due to fitting induced friction you will see less energy transfered to the gyros in a dynamic flight & airflow condition.
This is due to flow restrictions on all the adapters you used to get it plumbed into place.
I suggest the use of clean system design from the start and the use of airborne fittings. (Warning they are very expensive)
With regard to painting your venturis you might consider powder coat or some other form of electrical discharge painting. That will get the paint in where you need it.
I normally do not recomend powder coat on anything. I do not like the way corrosion can travel unseen under the coating. It will only flake off after the corrosion is in an advanced state.

I use high chromate milspech DP 70 from PPG for priming aluminium surfaces that will see a severe environment, like a float plane. Concept DCC (allso PPG) paint is a good topcoat.

Hope this is helpfull to you

scott

[GAHorn]

Where you measure is as critical as critical as what you measure. The old large guages need a larger flow of air at a lower pressure. Modern artificial horizons and DGs (If I could call them that) need 4.5 to 5 inches of vaccuum but flow less air than the old 10 lb monsters originaly installed. (get your own exact #s from the book on all specs not from me please)
If you measure near the ventury you might get 5 inches and then after all the accumulated energy loss due to fitting induced friction you will see less energy transfered to the gyros in a dynamic flight & airflow condition.
This is due to flow restrictions on all the adapters you used to get it plumbed into place.
I suggest the use of clean system design from the start and the use of airborne fittings. (Warning they are very expensive)
With regard to painting your venturis you might consider powder coat or some other form of electrical discharge painting. That will get the paint in where you need it.
I normally do not recomend powder coat on anything. I do not like the way corrosion can travel unseen under the coating. It will only flake off after the corrosion is in an advanced state.

I use high chromate milspech DP 70 from PPG for priming aluminium surfaces that will see a severe environment, like a float plane. Concept DCC (allso PPG) paint is a good topcoat.

Hope this is helpfull to you

scott

The "older" AN style gyros do not require more "flow". They actually require less vacuum than newer 3 1/8" gyros. (Typically 3.5" or less.)
Do not powder coat venturis. Powder coating applies too much material to the throat and may reduce available vacuum. Either paint or anodize/alodine them.
Gyro precession problems, when correct vacuum is present, is a gyroscope problem....not a vacuum problem. Send the instrument for repair.