With Idle time on my hands I found myself grinding up a bunch of “at some point in the past” important papers that had been laying on my desk collecting mold. While doing so I was surprised to discover, buried in the midst of them, was an 8.5" X 11" computer print out of Bob Kruger’s plan of his modestly modified F’ized Twister that almost certainly came directly from him while we were corresponding about it back in 2002 or so (I told you the stack of papers had a history!).
I reviewed it and, noting the changes obvious from the original 1987 MA articles, had a few memories refreshed and thought I’d add some comments that may be of interest to current and/or future builder/fliers. Although Bob made some very well thought out construction mods (false ribs, thicker sheet for the stab/elevator, etc) the primary changes to the fundamental airworthiness of the alteration of the kit was, once again, at the tail end of things…reduced fuselage stretching and stab/elevator area increases to about 23% of the 490 square inch wing area shown on the plans, a significant increase over the stock tail feathers.
I’m sure Bob and I discussed the changes and the reasons for doing so. We corresponded for a couple of weeks while he developed the drawing he later forwarded for my approval…the one I found in my stack of stuff!
The kitbash aerodynamic changes (both the ’87 and ’02 versions} were intended to update the kit to include the longer tail moment movement initiated by myself and others in the late ‘70s and early ‘80s and later, with Bob, to include the larger stabilizer/elevator…the combination of which provides improved stability accompanied by more aggressive cornering capability and much improved bad (windy) weather stunt patterns; i.e. the now ubiquitous larger tail volume/aft CG standard that gets rehashed regularly on-line and stunt guy face to stunt guy face.
The reduction to a two inch stretch from the original 1987 three inch was the result of the larger tail area limiting any possible value of the longer stretch and the need to minimize the potential for the design ending up tail heavy if both the bigger (heavier) tail and three inch extension were employed.
After spending a leisurely hour or so reviewing the plan I could find only a couple of things I thought might further improve the concept as shown on the Kruger plans. One a physical change and one enhanced flight trim capability not readily apparent on the plans. Both, however, directly affecting the improved performance level of the points covered above.
The physical change I would advise would be to move the hingeline on the stab elevator aft a half inch or so making the stabilizer chord longer and the elevator chord shorter. The Twister will generally build quite light (a good thing) but one result of lighter weight is reduced line tension for a given lap time. Reduced line tension simultaneously reduces the amount of force the pilot has available to deflect the controls surfaces against the airflow forces resisting their deflection(we refer to that resistance as Hinge Loading). The most significant factor in excessive hinge loading is the chord of the moveable surfaces…flaps and elevators. The elevators on Bob’s drawing are noticeably greater in chord than the stock surfaces (as well as in span). Reducing the elevator chord will reduce the hinge loading but will not, surprisingly to some, significantly effect the power of the stab/elevator combination to do our tricks. The result will be better responsiveness in pitch which translates to less large muscle input to fly those tricks…a “very” good thing IMHO.
Bob’s plan in the side view appears upon first glance to indicate a very forward CG location (the classic circle, sectored into two black and two white slices of pie) appears “very” far forward for a design now utilizing the larger tail volume produced by the configuration mods. The larger tail volume’s biggest asset is its impact on where the CG needs to be to provide stability while still allowing aggressive maneuvering.
Bob’s tail area is roughly 23% of the wing area and, when combined with the lengthened arm, is quite capable of allowing the CG to be further aft and still be perfectly stable and require less control input to achieve the desired radius of pitch axis maneuvering. (Remember, that doesn’t mean “twitchy controls” in the modern adjustable handle era…trim the airplane to be its most aerodynamically sound for great patterns and then make the pilot happy by adjusting the line spacing at the handle to make the response rate and the pilot’s input sympatico!)
Based on some WAG measuring and a familiarity with the design I’m confident initial flights of a Kruger based F’ized Twister could have the dry (empty tank) CG located ¼ to 3/8 inch forward of the main spars and likely end up fractionally further aft when optimized for both the plane and its pilot.
Now, there appear to be many happy F'ized Twister pilots out there and I don't for a minute suggest they cut up their current happy ships because they're now dinosaurs. If, however, you're about to build a new one and are using Bob's fine plans you might want to consider just a bit of additonal F'izing.
Ted Fancher
Topic: Fancherized Twister Redux (Read 5279 times)
