Do Super Decathlons have an alternate static valve?

Bartman

Bartman

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Saw a used parts site with what they said is an alternate air valve for an 8KCAB and I thought, nah, can't be. Can it? Where is it located if there is one?

I think I have the valve but figured it was from a Scout.

Thanks!
 
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Went looking through the parts manual for an answer.

Figures 11 and 12 have a part #21 which is the static source line and it's listed as going to an alternate static air valve but there's no valve shown in those diagrams or listed elsewhere. Figure #13 is for model years 1978 and up which shows the static source line but there's no reference to an alternate static valve so it must have been removed in model year 1978 as unnecessary.

So if they left it out as of 1978 but I'm technically rebuilding a 1977, do I have to include it or does deletion in later years mean I can delete it also?
 
Mine is 1977. It has an alternate air valve but it has nothing to do with the static system. On these airplanes you are lucky to have any static "system." The Cubs just leave the static port on each instrument open - lighter; less complicated.
 
My 1980 scout had at the alt static valve, hidden behind the panel on the left side. If I remember right, the alt static valve is an option, therefore, you can go without.
 
Bob Turner said:
Mine is 1977. It has an alternate air valve but it has nothing to do with the static system. On these airplanes you are lucky to have any static "system." The Cubs just leave the static port on each instrument open - lighter; less complicated.
Click to expand...

look under your panel on the left side like @Tangogawd said, is there a bare aluminum 1/4 turn valve with static lines attached to it?
 
Ok - tomorrow.
You'd think I would have spotted it in 2003 when I re-did all the instruments, the panel, and the vacuum system. Why would one need alternate static in a VFR airplane? Mine can be flown solely with reference to surrounding terrain. Oil pressure is my primary instrument.
 
I would have forgotten - climbed in for my weekly slow rolls and felt something hitting my foot. Long story short, I found no obvious static valve in there.

Long story - it was the brake reservoir. It is held to the firewall with two 10-32 screws and no lockwashers. The reservoir is behind the header tank and the screws are almost impossible to get to behind and below the left mag.

A buddy suggested studs. Yippee! Two 10-32 studs, jammed into the reservoir, and a sexy 90 degree socket driver and I was back in business in 30 minutes.

Remembered that I was going to look for the valve, but couldn't find my flashlight. ! I was getting ready to screw the upper cowl on with flashlight still aimed at the firewall.

So, long story longer - the search for the alternate static switch saved my new LED flashlight!
 
The static system draws pressure from both sides of the fuselage aft of the cabin. The tubing for each port comes together at a tee and the combined pressure is then routed to the instruments. It seems that prior to 1978 there was a valve under the panel that allowed ambient cabin pressure to be substituted for outside static pressure. Maybe there was only one port on the fuselage back then?

I'd bet someone still has the valve under their panel, who wants to look?
 
My 1976 dcat has a toggle switch on the panel labeled “alt static air”. I’d never tried it until today & when I did my altimeter rise by 300 ft and my airspeed indicator increased by 8 mph. My airspeed had always seemed to read low since I bought the plane.

Any theories on what the switch is actually doing?
 
The switch is simply opening a static port inside the cockpit. The inside of the plane has a slight negative pressure (less dense) due to the flow of air along the outside. Hence the slightly higher altitude.
 
Thanks... does that also account for the higher air speed reading? Which way will the switch provide the most accurate airspeed reading?
 
It would account for higher airspeed as well. Lower static pressure with the same pitot pressure would cause a larger differential of pressures, resulting in a higher airspeed reading.

Assuming correct calibration, it would read more accurate with the valve closed, reading outside static pressure.
 
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