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Old 03-22-2014, 08:48 AM
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bonesbrigade
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Joined APC: Apr 2013
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Originally Posted by JamesNoBrakes View Post
I didn't think you were hanging in the Vmc thread, I thought you answered your own questions and a good understanding seemed to develop from it. I didn't think adding anything at that point would be helpful I may not have seen your last post or last bit either just due to a trip or something like that. Propellor controls to takeoff position is in 23.149, that's where the resulting "windmilling prop" comes from (after failure). Hopefully you passed that with flying colors!

It depends on how far down the rabbit hole you want to go for the static port. In my experience, those are the reasons (calibrated for the most likely scenario) for it. What i mean by this is you would have to probably call the Aircraft Certification Office and the Manufacturer's engineers to know for sure, but:



Also in my experience, just because you can't find any reference that says "it was designed for that scenario" in plain english doesn't mean that it isn't true. Often that kind of data is just not provided outside of the aircraft certification process.

Some food for thought, people always explained to me that the alt static pressure read inside of a cessna 172 was lower in flight because when you selected the alternate static pressure on the ground with the doors open it would read higher. Um, how would it read higher than the ambient on the ground with the doors open? That's physically impossible. What it boils down to is that it's just calibrated as specified in the POH, which is where you go, rather than trying to second guess all the dynamics involved with the instrument reading. In my personal opinion, the pressure is higher, because air vents and other sources "leak" air from in front of the cockpit into it, causing the air to "pile up" in the cockpit. The air in the cockpit is relatively still, while the air moving around the outside of the aircraft is moving fast. This generally means a pressure differential of high(inside) to low (outside), evidenced by just about every old convertible car you see on the road or someone who has used duct tape in place of a window. Maybe, just maybe, it reads higher on the ground with the doors open because in flight it IS actually higher pressure and it needs to read slightly higher to correct for the higher pressure (that will indicate lower altitude). That's just a guess though. We don't know for sure how they calibrate it and the error they publish and whatever it says in the POH is what should be followed.

I don't keep up on "systems" like I used to, as I'm required to be familiar with a much wider array of aircraft and systems, so unfortunately I can't go as deep as I used to. It's all still fun and interesting though!
In a 172 if you pull the ALT STATIC knob, the altimeter and VSI jump.

I would say that turning the heater on in the Seminole and closing the windows is the method that the manufacturer has come up with the counteract the installed error. No further explanation needed really if that is indeed the case. There is an error in the installation, here is how to fix it.
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