C172 pitot has a large position error at low speeds.
DO.
Good point about accuracy of measurements
. Not only are the static and dynamic pressure instruments at risk of increased error in unusual attitudes and at the extremes of their performance range but the observation by the pilot is constrained by the need to safely fly the aircraft while making the observations. I had a yoke mounted Garmin GPS 96 running during the flight. I have downloaded the information it recorded to provide a check of the observations while flying the stall and mushing down and have put some graphs of the data on Facebook at: http://www.facebook.com/album.php?aid=2025023&id=1118091118&l=b2b72910cc
I used Garmin MapSource to display the track and profile of the route I flew. From this I was able to identify one of the stalls in the data set. I copied just the data for the stall and clime out into a Microsoft Excel spreadsheet to graph out the height against time. This was to allow me identify a section of the stalled descent that is clearly where the aircraft is mushing down so I could get a measurement of the rate it was descending.
The measurement section I chose starts at 16:05:17 with the aircraft at 3467 feet and finishes at 16:06:23 with the aircraft at 2705 feet. The rate of decent was therefore 693 feet per minute. A simple linear regression line through the data shows it to be a stable descent along a straight line since the R-squared is 0.94. Which I interpret to mean that only about 6% of the variation in this data was due to a deviation from the linear trend line.
Before 16:05:17 my interpretation of the graph is that the aircraft is losing lift and entering the stall mushing. The section in the descent after 16:06:23 is likely to be where stall recovery has been initiated and the aircraft is being reconfigured to clime out.
The ground speed variation during the descent was not stable as can be seen from the very low R-squared value. It is likely that this is simply reflecting the prevailing wind conditions. I carried out the stall into wind over a reservoir in Essex just south of Colchester on the 25th Oct. The weather at the time was clear but windy. A nearby local private automated weather station in Colchester reports reading 270 degrees 5 to 10 Knots at the time of the flight http://www.jbest.net/weather/weatherSearch.aspx?fd=25%2f10%2f2009
I have included graphs of the landing and roll out at the end of the flight to provide a basis for a rough comparison for this aircraft between the descent rate and ground speed for the stalled configuration and a conventional landing. Since the aircraft was flown down final with 30 degrees of flap at 65 Knots indicated air speed the ground speed shows a head wind was present of around 10 Knots.