Physics, engineers and wives
RVbySDI said:
Engineers are like wives. Neither wants to accept generalities.........If 1800 lbs is the limit will the airplane crash and burn at 1801 lbs? Why the numbers are set the way they are is not always based on metalurgic qualities, physics, aerodynamics or any other "scientific" calculations. Sometimes they may be based on best estimation generalizations. RVBYSDI Steve
Humm engineers are like wives..... I think the analogy is funny.
What are the generalities? My take on this is wives and engineers are alike because they are ridged and follow a set or rules or principles (assumptions). Some of those rules or principles are the mystery (but no doubt they are always right
). Let me try and explain since many years ago I worked as a structural engineer for a Big airplane maker.
The big differnce between the wife and engineer is that if you don't follow the her rules, you make your wife mad. With the engineer, not following the rules, physics can come up and bite you, resulting in a bent/broken airplane. May be the rules are too conservative or restrictive but are intended to keep you in good stead (not divorced or balling your airplane up).
Engineers HAVE to follow criteria, at least in certified airplanes. These criteria
ARE arbitrary but have stood the test of time, meaning airframes that takeoff in one piece safely land with all the same pieces attached, provided the pilot flys within the limits.
Strength of materials and aerodynamic loads are precise. There is no generalized issues. It's hard science.
However there are assumptions made which may the generalization's you alluded to. Like your wife, engineers will not waiver or give on these rules. It's not a matter of accepting generalization's, its a matter of compromising on their assumptions and rules. If you ask why, it's because that's the way we do it.
These assumptions involve what the critical load cases are, like how strong vertical gust are (turbulence). The other assumption or arbitrary rule is a catch all called margin of safety; All are arbitrary but based on sound common sense (or FAR's). Example: margin of safety in an airliner is arbitrarily 1.50, in a fighter it's 1.25 and non-manned rockets even less. Why the arbitrary difference? Well one is carrying many people, the another has an ejection seat, and in the last example, a rocket, it has no people.
What is "Margin of Safety"? Its and arbitrary allowance for variation in construction, materials, extream conditions, pilot screw-up and possible loss of structure strength over time (corrosion). People consider that the 1.5 factor as the ultimate load factor.
SO UNDER ULTIMATE load you will not break or fail structure HOWEVER YOU MAY BEND IT. That is where the LIMIT load comes in. ANYTHING OVER LIMIT LOAD CAN INVOLVE permanent deformation. So limit load has no factor of safety if you will. For example our ultimate aerobatic G factor is 9, but limit 9/1.5= 6 g's. Up to 6 g's you are good and this is the LIMIT LOAD. Over 6 to 9 g's you can and very well perminatly bend the plane. Over 9 g's all bets off, parts can bend and can come off.
I don't want to get into fatigue, since our little planes fly so little.
However higher stress reduces fatigue life. We all recall caeses in the news of old T-34's or pipe patrol planes flown several 10's of thousands of hours experiencing catastrophic fatigue failure.
We of course do not need to follow anyone assumptions with experimental aircraft, but as was pointed out before, Van's Aircraft sees the wisdom in having a safety margin. I do also. Just remember exceeding any limit does have impact. It's not a matter of IF it impacts you, it's a matter of how much. Any change in weight will affect everything to a degree.
As far as speeds, the pilot controls the max G's limit at slower speeds like at Va, where the maneuver is intentional, under your control. So if you are flying at HIGHER gross you can just take it easy. Vne may need to be reduced slightly. The theoretical (FAR) vertical gust I recall is 50 feet/sec; I never meet or felt a "certified gust", but we know there are gusts or wake turbulence; some gust may be worse than 50ft/sec?
Bottom line, if you raise the gross know what you are getting into, there are limits. The higher the weight the closer you're to the maximum (ultimate failure). With the robust structure and excess performance RV's have we can expand the gross with a fair degree of safety (with in reasonable limits), but you need to reduce the "envelope" both in intentional maneuvering and unintentional (lower Vne). Also high (over) gross landings, especially soft field was mentioned a few times as an issue. Landing a extra heavy RV or taxing over soft dirt might strain the gear. My concern is the small 5x5 tires in soft grass with high weights.
Cheers George