The Bootstrap Method of Performance Analysis

The first part of flight testing was relatively easy to plan.  For the first few hours, I was just interested in getting the airplane safely into the air and back on the ground.  I began to find the edges of the center part of the envelope in as incremental of a way as possible.  After those few hours, what was to come next?

I’ve read several articles, books, and Advisory Circulars that all have good guidance, but what I really wanted was a good way to start fleshing out the performance data that would come in the POH of a store-bought airplane.  I have read about sawtooth climbs and other “fly and record” approaches, but I didn’t see how that was going to give me as much detail as I knew I would want to have available.

Fortunately, some searching led me to a series of three articles by John T. Lowry in the Avweb archives.  Those articles led me to other articles of his, and to his book, Performance of Light Aircraft.  I’ve read enough of his writing to appreciate his approach and his style.  He is a physicist, but he is also a pilot.  He finds an excellent balance between the theoretical world and the actual world, and takes an approach to performance calculations that knows its limits so to speak.

He describes a method that he calls The Bootstrap Method of Performance Analysis.  The short version of the strategy is that we use the measured performance from one set of conditions, combined with his math-rich spreadsheets, to create predicted performance for many different conditions.  I’ll let you read the articles to learn how the strategy works.  If you’d like a little bit more detail, I’m planning to put an updated article in the first quarter 2014 Beartracks issue.  If you’d like to have much more detail, see if you can find a copy of his book, in which he goes step-by-step through his derivation of the various formulas.  Good luck finding a copy, since it has aparently been out of print for a while unfortunately.

The good news about his method is that it doesn’t take very much flight testing to get started.  The even better news about his method is that airplanes with constant-speed props have even fewer measurements to make.  The airborne flight tests amount to just a series of glides.  These glides do require a calm day with no convection, which means either an early morning, or an overcast, stable day.  

The first round of glide tests have shown that the minimum sink speed is somewhere between 50 and 55 knots indicated.  Lowry points out that the curves in most GA airplanes are very smoothly shaped, which is another way of saying that the glide performance difference between 50 and 55 is likely to vary little.  I’m planning to do more tests to see if I can pin down the specific speed, but for now my sample size is too small to say definitively whether it’s 52, 53 knots, etc.  

I’ll have much more to write about the method, especially if it works well.  The great thing about it gives me some insight about how various factors impact performance.  I’m less interested in knowing exactly what my climb rate will be at a particular circumstance.  I’m much more interested in knowing if, for example, the temperature were 20 degrees warmer, how much I should expect the rate to change.  Will it be 10% lower, or perhaps 80% lower?  Short of intuition or actual flight testing, I don’t have any other way to know.  

The Bootstrap Method also allows for comparing input factors like engine horsepower.  What if my engine produced 15 more horsepower at the same weight?  How would that impact my maximum level speed?  Similarly, what if I had installed a Hartzell 80″ prop instead of a 76″?  You can see how this method, even though it does make a few underlying assuptions, will be very useful in helping elaborate on how the Bearhawk can perform under certain circumstances.  Look for much more to follow, if not here, then perhaps elsewhere.