Altimeter question

A while back I picked up a very nice precise altimeter from someone on another forum. They replaced it with a new one because it failed to meet the test criteria for instrument flight. I haven't done any testing at altitude, but it's accurate on the ground and the barometric pressure adjustment works as it should. We're roughly 1100-1300 ASL here, depending on where you are. From my limited testing, I'm guessing it may have a case leak... if I put enough suction on the port to run it up to 5-6 K it will leak down several hundred feet pretty quickly, then much more slowly after that. I am using a somewhat cheesy setup that may have its own leaks, it was just a quick check.

Now, having read the test procedures and remembering what I know about how altimeters work, it seems like a leaky case would really only matter if the static pressure and the cabin pressure were significantly different. As I'm building an open cockpit wooden airplane and the instrument enclosure will be pretty well ventilated, I wonder if I even care if the case leaks. I'll have a pitot tube for the airspeed indicator, of course, but I wonder if I'll even need an external static source. It seems that a simple filter on the static line might be enough, and a leaking case on the altimeter is probably a non-issue.

Thoughts?

Your cockpit may have a slightly higher pressure than static if it's open. It will capture air. My friend had a cub copy and the altimeter was sourced from the cockpit. It lost some altitude when he opened the window.

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Originally Posted by mmarien

Your cockpit may have a slightly higher pressure than static if it's open. It will capture air. My friend had a cub copy and the altimeter was sourced from the cockpit. It lost some altitude when he opened the window.

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ITOH, It also may have a slightly lower pressure than static, depending on the fuselage shape and how much air is escaping the cockpit. Extreme example; An ultralight type aircraft with a nose cone completely open to the rear will usually produce a lower pressure in the cockpit.

And, the pressure in the cockpit can change significantly (or not) depending on the aircraft's attitude and airspeed. The error may or may not be enough to matter.

Thanks for the replies, guys. Since posting my question, I did a bit more research. Up until now, the only P/S systems I've seen have been on metal airplanes. Those generally have at least two static ports on the sides of the fuselage, tied together with a T or Y. Seemed like overkill for this. And while I'm aware that the cabin pressure in an enclosed plane can change significantly, I don't know how much of a factor it would be in an open cockpit plane with the instruments essentially in an un-sealed box.

I have since found simple pitot/static probes like those available from Aircraft Spruce. I hadn't seen a simple perforated tube like that used before. I'll probably roll my own, but at least now I have an example of how it's done and can eliminate one potential source of error from the equation. There will be more than enough flight testing required, I don't really want to add a chapter on "How much are my instruments off and under what conditions". It might be negligible, but as I've got to run tubing out to the pitot anyway, an additional static line isn't going to be a big hassle.

When I bought my Fisher 404 the altimeter and ASI just vented behind the panel. The ASI was quite inaccurate at any airspeed other than 100 mph (which was over the aircraft's Vne!), and then the altimeter went bad, so I replaced them, but they were still off... I forget exactly, but I think the ASI was off by about 5-10 mph, so I added static ports to the fuselage sides, which brought it to dead on as accurately as I could determine.

In general, getting accurate static pressure around the aircraft can be challenging. Any place that the air is not going by the airplane at the same velocity as the air well ahead or behind the airplane will have a different static pressure. The open cockpit is probably the worst place possible both for accuracy and consistency.

There is an article by Tony Bingelis on the EAA site:

https://www.eaa.org/eaa/aircraft-bui...-static-system

I'm using it to build the static/pitot system for my Hatz