Top Ten Reasons For Difference Between Replies! ! !
Posted: Thu Apr 24, 2003 6:14 pm
Anybody know the top ten reasons for the difference in this recent George answer:
#9. Clear weather while answering with first response, strong thunderstorms in Austin area during second response.
#8. Keyboard had beer spilled on it before first answer, new keyboard before second.
Any ideas on 7 - 1?
And this George answer?gahorn wrote: No.
#10. Before the first answer Jamie was heard to sweetly say "Come in here George, I want you." And Jamie was out of town during the second answer.gahorn wrote:I'm always surprised that so many disparage venturis. I find them a very reliable and cheap source of vacuum. The few times I've discovered an airplane that had mal-functioning venturis had maintenance issues such as:
1. Improper fittings in the vacuum system. Using hydraulic or ordinary brass plumbing hardware creates restrictions due to internal design. (Most ordinary fittings are designed for pressure, not vacuum, and most make sharp turns internally adding restriction.) Fix: Use correct Aeroquip 816 (not the same as AN816) Aeroquip 8891, Statoflex, or Airborne (c) fittings, etc. in a vacuum system. These are identifiable by having their turns made with a long radius rather than sharp corners, and are usually mfd out of steel or spun aluminum, rather than cast, forged, or machined metal.
2. Old and/or incorrect hoses. Correct vacuum hoses designed for air/vacuum/pneumatic systems (such as Mil-H-5593, Aeroquip 306, or Stratoflex 111 or 193 hose) rather than pressure-type hoses. Pneumatic-type hoses have inner liners that are designed/vulcanized to their outer materials so they won't delaminate and/or collapse under vacuum and aren't dried-out by air. Other types are designed to carry fluids, oils, etc. under pressure and the inner liners may collapse, wrinkle, etc. diminishing the effective vacuum and may dry-out and send rubber particulates thru your gyros.
3. Venturis are covered in multiple layers of paint, dented, eroded, etc. and otherwise damaged. While they are pretty fool-proof devices, buying a used venturi is asking for trouble. If the inner throat is worn from exposure to sand, dust, accumulated debris, or has lost it's sharp edge at the exit,...or if the inner venturi is no longer concentrically located within the outer tube (the tolerance is only +or- .010") then it will never produce the design vacuum. It it's auxillary source plug is leaking, (or if in use, if the base gasket is leaking, dried out, missing, never-installed, etc.) then it cannot produce design vacuum.
The valid shortcomings of a venturi system include:
1. No vacuum produced before takeoff (unless a standby, or alternate source is used.)
2. Icing can reduce/prevent vacuum. (But it can do the same for flight in a 170!An iced up 170 with a vacuum pump is also in trouble, and even a vacuum pump equipped airplane is in short-term trouble without a heated pitot and anti-iced propeller.)
3. Installation location is important. (Some installations attempt to overcome icing paranoia's by locating the venturi near the lower cowling exit, reasoning that warm air there will prevent the ice from occuring. Unfortunately, this also locates the venturi where it will become contaminated with dirt blown-up/thrown by the prop and any oil misting/leaks from the engine compartment. Both will reduce the efficiency of a venturi.)
A venturi-driven vacuum system is subject to most of the same problems of a pump-driven system such as aged hoses, misrouted lines, incorrect fittings etc. But it has none of the sudden-failure habits of dry vacuum pumps and none of the oily-belly problems of a wet pump system. (And, by the way, all the problems mentioned above in #1 and #2 are also known to add to the short life of dry vacuum pumps. When a dry pump system doesn't provide enough vacuum due to the masking effects of those faults, the mechanic usually cranks up the vacuum regulator/relief. This seriously overworks the pump, and then the owner is surprised/disappointed when he goes through pumps regularly. (If I did have a pump-driven system, I'd opt for a wet-pump and live with the belly.)
In my own day-dreams, after I get a heated pitot, electric windshield, prop anti-ice and TKS system approved for the 170, I think I'd consider one of the engine-intake vacuum system backups for the all-weather interceptor 170. That way I could spin up the gyro's before takeoff and still have a backup vacuum source if the venturi ices up and the struts begin to look like popsickles. I just hope when it finally falls out of the sky, I hit the ground and wake up from the bad dream.
#9. Clear weather while answering with first response, strong thunderstorms in Austin area during second response.
#8. Keyboard had beer spilled on it before first answer, new keyboard before second.
Any ideas on 7 - 1?
(Just remember to run some bottled water thru when your done to prevent corrosion)