Re: [RCSE] Re: Gordy/Dive Test Quote..AND a question :-)

[Joe & Jan Wurts]

I guess I'll have to chime in on yet another thread started by Gordy that has become way overdone.  I'm not sure that I want to admit it, but there is almost a kernel of truth to his disparagement of the dive test.  A good pilot will measure his aircrafts static margin via one of a number of methods.  All of them typically use measuring the aircrafts response to a displacement from the trimmed condition in pitch.  I tend to do small excursions off of the trimmed flight speed to get a feel for the aircrafts stability.  As a validation, I'll sometimes roll the plane inverted and see just how much down elevator is needed.  Side note: I've had some aircraft that take up elevator when inverted to keep from stalling...  Think: dive test downsized.    If one dives the plane more than a second or so, one might also measure another property that could be important.  And that is, aeroelasticity.  There are quite a few lightly built TD designs that do not have enough stiffness between the wing and tail.  Any flexibility in either the tailboom or tail linkage can make the aircraft seem unstable at higher speeds.  Also, if the wing has poor torsional rigidity, along with a cambered airfoil, aeroelastic stuff can occur.  The concept of diving your plane for a multiple of seconds to determine your stability level is a bit too much, and will most likely show the effects of some flexibility.  One airplane that I flew a couple of years ago was fairly stable and would pull out easily at lower speeds, but would tuck fairly severely at higher speeds.  It was cured via a couple of carbon rods CA'ed to the top and bottom of the fuselage tailboom.   For finding the CG position that I want to fly the plane at, I just fly it and see if I am annoyed by having to hold the nose up for slow speed (result: remove nose weight).  Or, alternately, if I find that I am having trouble flying smoothly and am not able to consistently hold the desired airspeed, I add noseweight.  The airspeed excursion tests tend to help me dial the right CG in a little bit more quickly, in that I tend to want just a little bit of static margin, so I will start with some excursions to get me close.  After that, it is time to just fly and take note of what isn't working.   But, when all is said and done, what is important isn't what static margin value that the airplane is at.  It is, whether it is a good match for your preferences and skills.  I am happier with moderately aft CGs (low static margins), and find it annoying to fly high static margin aircraft.  Some other pilots would find my CG position annoying, and get more performance out of the plane with more nose weight.  For that matter, my preference is variable.  For F3J/TD, I use a more nose heavy balance than for F3B, or F3F.  Just is easier to fly an airplane a half mile away (or more) that is stable.  For flying close by, and especially if doing accelerated maneuvers, the aft CG works far better for me.  The bottom line, there is no single CG position that is right for all pilots, or even for a single pilot at all times and conditions.   Joe   PS  I really like the classic quotes thread.  I recognize a couple very well (Dan, I'm sure that Daryl and I were there!).  And to add to the collection, I'll provide my contest mantra: "I'd far rather be lucky than good", which goes with the Thermal Dorkation events...