Message-ID: <19981215080145.28776.qmail@www02.netaddress.usa.net>
Date: Tue, 15 Dec 1998 08:01:44
From: Dan Schaefer <dfschaefer@usa.net>
To: lancair.list@olsusa.com
Subject: Water in the pitot/static lines
Mime-Version: 1.0

          <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>>
          <<  Lancair Builders' Mail List  >>
          <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> 
>>
Ran into something that got me to thinking (usually hurts) about the various 
V-speeds we observe in out Lancairs. As I've mentioned, my plane was 
weathered down in northern Calif. by heavy, blowing rain for almost 10 days. 
Didn't have my canopy cover with me which left the static ports exposed. 
Within minutes of take-off to come home, all the instruments that rely on 
pitot/static went bonkers. Having practiced flying like this before 
(inadvertently, to be sure) I returned to land and to get the water out. 
(No, I never installed an alternate static source - but I will now!) Anyway, 
I cut the static line in the cockpit to fly home planning to work on things 
in my hangar. Surprise! Surprise! My IAS, on average, was 20 - 30 MPH higher 
than I usually see. 

What this means, first, is that the static ports, placed where the manual 
calls out, could be in a bit of a high pressure field. Second, if my 
IAS has always been reading that low, I've exceeded Vno many times while 
expanding the plane's envelope. Several times at airshows, at approx. 100 
ft altitude, in front of a really big crowd! I have no idea if the effects 
I saw were linear over the entire IAS range - but if they were, I've 
inadvertantly pushed my envelope to at least 267 (and maybe 277) MPH. Maybe not, 
but it is quite sobering, to say the least.

What I'm getting at here is if a lot of people are using the location for the 
static ports called out in the manual, and are as questionable as 
mine seem to be, a lot of folks may be pushing into very dangerous areas.

On the other hand, there is some evidence that the ports are reporting the 
correct static pressure to the gauges and when opened to the cockpit, are being 
exposed to a low pressure (lower than actual ambient, or static) which will 
cause the airspeed to read high. The only evidence of this is inferential 
and right now I'm at the guessing stage.

I infer the possibility of low cockpit pressure from the fact that my header 
tank vent/overflow is near and slightly upstream of the little drain hole 
I put in the bottom of the fuselage. If, while filling the header, I allow a 
bit of fuel to overflow (which runs out the vent) there is an immediate smell 
of fuel in the cockpit. The only way this could happen is if the drain hole was 
sucking air, due to lower than ambient cockpit pressure.

I'm only mentioning this for the general good as I haven't figured out yet 
whether the standard (at least in the 235 plans) ports are resulting in 
accurate IAS indications or not.

If you have the tendency to do some hot-dog descents where you're pushing the 
pointer up near Vno (I know we all do it just to get the controller to 
wonder on air "what kind of an airplane IS that"?) you might take another 
look at your IAS calibration runs. You could possibly be the owner of some 
very interesting, 
but totally useless flight test data (like, for instance, "Yes indeedy, the 
tail will tear right off if you exceed Vno by that amount").

Seriously, I intend to redo my IAS calibration runs using the two static 
sources. Will pass any conclusions along as I get them. In the meantime, I'm 
going to stay below Vno minus about 20 - 30 MPH just so my wife won't have 
to figure out what to do with the insurance money, at least not just yet, anyway.

Cheers,

Dan Schaefer
N235SP


____________________________________________________________________
Get free e-mail and a permanent address at http://www.netaddress.com/?N=1