But that is exactly my initial point, that calculation involves a perfect frictionless seal in a sealed cylinder does it not? If so, and I am betting it does, there are HUGE losses when you are talking a plate on a poorly "sealed" surface that instantly vents to atmosphere and does not remain perfectly horizontal in flight making such guesstimations several orders of magnitude off at best.
HeroHog, you're missing the point of that initial calculation/estimation. Surely, 0.28 psia isn't going to blow the cover sky high. It's a numerical guess at the minimum pressure to either just barely move it or just hold it up.
Certainly a real explosive overpressure of 5000 psia would, but as to how high? You could make some assumptions about the "barrel," meaning the rim of the manhole (call it, say, two inches), how far away from this "barrel" would the gas pulse still accelerate the manhole cover, etc., etc., etc., etc., etc. But this would only result in
estimated parameters for the empirical tests.
And these empirical tests would result in a whole raft of corrections to the original assumptions. (And probably uncover new, previously unthought-of variables.)
It's like when they first started to develop ballistic tables like the old "Ingall's Tables." They had to use a standardized projectile, do their measurements, and from this
empirical data, they developed parameters as to how to handle the variables of nose ogive, weights, sectional densities, "etc., etc., etc., etc., etc."
And from these tables* we got to the point where we could calculate, in advance, things like the nose force on a 150 grain flat-based cupro-jacketed lead-cored .30-06 bullet at 2743 feet per second.
As well as a 5.56 NATO bullet at 1534 feet per second --
which had not even been developed at the time the tables were generated.So your dysphoria about classroom theory versus engineering reality is unwarranted. After all, the "Ingall's Tables" for manhole covers have not yet been developed.
If the need for that ever arises, I'm sure I could get a juicy government grant to conduct the investigation for that.
Of course I'd have to hire birdman as a consultant, but I could at least competently
manage the research project.
Terry, 230RN
* and other derivative tables