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On 27 Feb 2002 23:04:33 GMT, Richard Carlisle wrote: >> While I have no factual knowledge of this airframe and thus do NOT >> suggest that it is less than safe, I do question the structural >> efficiency of its design. There is essentially no triangulation in the >> space frame; this violates the most basic of structural design >> principles. > >I agree...And the reason his design works is because of the gauge of tubing >he uses. It is strong, but also very heavy. It could be made much lighter >if it were built using a triangulated space frame design in thin wall 4130. >It would also probably be stronger. Applying the principles of mechanics will show that a properly designed space frame would yield the highest reasonably-attainable structural efficiency. When loads are diffused rather than "pointy", a monocoque structure is better, but that makes it very hard to carry engines, pilots, landing gear, and the beer cooler. In a simple structure such as a light aircraft fuselage, the tubular steel space frame is an excellent compromise among the conflicting factors involved. When one vigorously pursues efficiency over of ease of construction, one may occasionally find that high efficiency carries its own price tags. I would not wish to be inside a weight-optimized carbon-epoxy composite fuselage in a CFIT. People inside ancient light aircraft fuselages have survived impacts that would have been fatal in lighter, more brittle aerostructures. >>> (b) He has a means of adjusting the hang point for and aft. >> >> Obviously. I had enquired what this means of adjustment actually is. > >The point where the nylon web straps are supported on top of the airframe >can be moved back and forth...I think. In that event, the only real difference is in how his design carries the flight loads from the canopy suspension down to where they are reacted by the pilots weight. It is a trivial difference in that these loads are pure tension (the engineers favourite), and anything thats strong enough will work. Because tension members are generally weight efficient (100% utilization of the materials tensile strength), only massive overdesign would make the alternatives markedly worse in efficiency, and massive underdesign would very quickly manifest itself. Probably just the once, though... > >Once again...I agree...If you knew what the all up weight on one of these >things was in flight ready configuration, youd fall over. One single place >model that I know of weighs 100lbs more than my Phantom. The reasons you >state above are exactly the reasons why I have not yet built one. I do not >agree with the use of heavy pig iron in any sort of airframe. The design is >popular among folks who have limited building experience. Using that heavy >wall steel tube, you can screw it up pretty bad and still have a machine >strong enough to fly safely. Given that some of these will be built by unsophisticated constructors, a broad structural margin may be a very good feature. If one cannot analyze all the flight loads and calculate strengths required, then its safest simply to oversize everything and use fault-tolerant materials and techniques. The price paid in flight performance probably shows up least in this type of airplane. > >If you are interested in a more high tech PPC...Id look elsewhere. > >Personally...I prefer flying the old fashioned way. You know...Wing, tail, >rudder, stick, etc. So do I, but having flown stuff that taxis faster than the average powered parachutes red line (for those who didnt notice, that was a joke), I am fascinated by something so slow that you can count blades of grass on the ground as you putter past. - Curious |
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