I agree with Both.
Of course Higher pressure will cause lower readings on the baro and the opposite as well. Ram air pressure will add to this error so Gary is correct. However the outside airflow along the body of the plane will always be lower pressure than the inside thus creating a pressure differential. For the full size pilots on this forum it's easily demonstrated by opening a window durning flight. If you don't hold on to it will blow right open and you have to pull it shut. If you have a door that didn't shut correctly, depending on the airplane, good luck trying to shut it in flight since your flighting the pressure differential. So my point is that your both right but this error will never go away. It may be minimized but it will always exist. The only true way to correct for it is to use flying formulas.
Pressure differential between the inside and outside of a plane is dependant on its design and is not uniform over the entire surface. I have flown planes in which opening a widow or unlatching the canopy didn't cause either to be pulled outward by external low pressure. Conversely I'm sure there are planes that do have a significant differential. When there is a differential however it would be impossible to know if it were being cause by inside or outside change. That would require lab instruments and if you measured pressure at points around a plane body in-flight you would find areas of both high and low pressure.
What we are discussing is the pressurization affect on the RVOSD pressure sensor and on my plane the inside was being slightly pressurized by prop blast/ram air. Ideally you want it to measure ambient air pressure only and not to be influenced by pressure changes caused by flight. The air pressure change necessary to alter a barometer reading by 1 ft is tiny (about .0006 psi) so it doesn’t take much to throw it off.
Originally I was seeing the same barometer distortion from prop blast as from in-flight engine off ram air. With the reduced firewall vent size there is no measurable difference now on the bench with the motor on or off. The in-flight performance will be checked next time out but I expect the same result.
Really the only thing to fix this is to have a exit port slightly larger then the enter ports and use something porous to cover the rvosd or put it somewhere where it will not be "hit" with straight moving air. If you blow on the baro the values will change accordingly. Think of making it like a Microphone with the foam on it. You should try and get some of that material and it should dampen "direct air pressure" on the OSD barometer and make you values much more accurate. Or make a box and put tiny holes everywhere. that should make still air of the osd but also provide enough pressure change for it to work properly.
Last edited by man2000me; 07-14-2011 at 06:22 PM.
Do you have a picture of where the sensor is on the board?
On the smaller board, that little green chip with a metal ring on it. Labeled U11, but be very careful, the chip has a gel inside, if you puncture inside the GEL the baro will be destroyed.
Last edited by Alex Villa; 07-14-2011 at 08:00 PM.
Local air pressure WILL vary inside the fuselage due to the flow of air from inlet to outlet is not constant. As it passes by obstructions such as batteries,ESC, servos, bulkheads, wires, and the OSD... it has to speed up (increase dynamic P) Once past obstructions in the flow, it slows down (decrease dynamic P). Everytime the OSD, ESC, or batteries are moved, the local pressure at the point of measurement will have changed, i.e. every time you open the canopy and touch something or replace a battery.
One would be better off just measuring static pressure OUTSIDE the aircraft...no variables there... unless you bend or block the pitot/static tube.
A refresher course with high math but good graphics:
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Perfection is usually hard to achieve and in many cases unnecessary. IMO if by whatever means you can achieve altitude stability of plus or minus 10 ft that would be way good enough for displaying height and for use as a variometer.
If you were trying to do an instrument landing with your lunar module on the moon you might want more accuracy.
I wonder it we filled the space inbetween the 2 boards with soft foam then put the board back together and shrink wrap.
Heat! Temp already reading temperature around board and not really ambient. Plus components no likey heat.
Originally Posted by man2000me
I found wrapping the OSD in foam had the board temp up to 105F in minutes!