Go up and do some stalls with your typical loads. Document the stall speeds/weight and multiply them by 1.3 for approach speeds. This is how experimental aircraft arrive at their numbers but many, like me, also have AOA that we rely on since it is not affected by weight or CG.
Agreed on all points.
We pay very little attention to CAS. It’s a measure of how accurate our ASI reads which is a function of how well our pitot static systems work. So how well do our pitot static systems work?
I think the short answer is well enough. There are typically discrepancies and they are not linear up and down the airspeed scale. But they are generally on the order of less than 5 knots and linear enough so going out and doing some stalls in unaccelerated flight, then multiplying by 1.3 to come up with an approach speed is effective.
One place that they aren’t accurate enough is in comparing L/D polars of high performance gliders. The late Dick Johnson was a champion racer, an aeronautical engineer and meticulous assessor of sailplane polars. He demonstrated many times that CAS discrepancies caused many sailplane comparisons based on factory figures to be wrong.
To make an accurate comparison, the first thing he would do is determine calibration errors throughout the pertinent speed range. In pre-GPS times this usually done with a ‘static bomb’ which is a device that is trailed on a line behind the aircraft to measure static pressure. There were always errors, sometimes quite significant (thought <5knots) and they were almost always due to static system errors.
Turns out that pitot pressure measurement is pretty much accurate and non-critical. Static is another matter. Finding a place to tap static atmospheric pressure is difficult. There are few places on an aircraft where punching a hole to measure pressure will result in an accurate measure. And this is on a sailplane with no engine or prop wash and very smooth surfaces.
When experimentals were mentioned it made me think of my own static setup on the RV10. Many RV people use a pop rivet with the shaft removed. Some use a machined disk raised above the skin level a millimeter or two with a hole in the middle. Others simply punch a hole, clean it up and affixed a tube to it. Of course it’s done on both sides of the tailcone in a ‘reasonable looking spot’.
What occurs to me after years of watching Dick Johnson’s work is that none these solutions are particularly accurate, reproducible or necessarily agree with each other except by sheer chance. But its’ probably good enough for SEL experimental work.
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