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In reply to the discussion: Skygate 911 [View all]William Seger
(11,731 posts)8. Cowboy Bob rides again
The design dive velocity is one of the design inputs used to calculate the loads the plane needs to withstand. As I said, this is definitely NOT the point at which the plane will begin to fall apart:
http://www.flyingmag.com/technique/proficiency/technicalities-are-you-feeling-lucky#YilDUX7dgskkUFgk.99
All of these loadings are due to air pressure, which grows in proportion to the square of the speed double the speed produces four times the force; they are functions of the indicated airspeed, not the true. Now, VD, which is an indicated speed, is by definition a safe speed; the forces at VNE are just 81 percent of those at VD (0.9 x 0.9 = 0.81), and so there is a comfortable margin of safety, so far as structural strength is concerned, at VNE.
But there is a complication that muddies the water considerably. It is flutter. Flutter is a vibration that may be augmented by aerodynamic forces. It is the one challenge to aircraft structures that does not increase gradually with speed. It is possible for a structure to perform normally right up to a certain speed and then, with a gain of two or three more knots, to explode into fragments in a split second. That is what most likely happened to the South African VL-3. The accident has not yet been investigated, but it has the earmarks of wing flutter induced by a vibrating aileron.
Flutter is affected by a number of factors, one of which is the true, not the indicated, airspeed. As you will have immediately perceived, this fact raises a logical difficulty. VNE, the redline on the airspeed indicator, is an indicated airspeed, but the critical flutter speed may be a true airspeed. So the margin separating VNE from the critical flutter speed gets smaller as you gain altitude. Furthermore, if you get really high up, the difference can be larger than the margin that separates VD from VNE, simply because the difference between indicated and true airspeed is greater than 10 percent.
That doesn't mean the airplane will flutter, because VD is not the critical flutter speed. Manufacturers are not required to determine the critical flutter speed for each design, but only to demonstrate that it is free of flutter up to VD and that there is good reason to believe, based on various kinds of ground tests and mathematical analyses, that it will remain so up to 1.2 times VD. It is noteworthy that the section of Part 23 regarding flutter, 23.629, makes no mention of altitude. The cumulative margin between VNE and 1.2 times VD is 33 percent, and this probably provides a good cushion in all normal operations, but if I were to ride a wave to 35,000 feet in a 172, I would not be in a hurry to peg the airspeed at redline on the way back down.
But there is a complication that muddies the water considerably. It is flutter. Flutter is a vibration that may be augmented by aerodynamic forces. It is the one challenge to aircraft structures that does not increase gradually with speed. It is possible for a structure to perform normally right up to a certain speed and then, with a gain of two or three more knots, to explode into fragments in a split second. That is what most likely happened to the South African VL-3. The accident has not yet been investigated, but it has the earmarks of wing flutter induced by a vibrating aileron.
Flutter is affected by a number of factors, one of which is the true, not the indicated, airspeed. As you will have immediately perceived, this fact raises a logical difficulty. VNE, the redline on the airspeed indicator, is an indicated airspeed, but the critical flutter speed may be a true airspeed. So the margin separating VNE from the critical flutter speed gets smaller as you gain altitude. Furthermore, if you get really high up, the difference can be larger than the margin that separates VD from VNE, simply because the difference between indicated and true airspeed is greater than 10 percent.
That doesn't mean the airplane will flutter, because VD is not the critical flutter speed. Manufacturers are not required to determine the critical flutter speed for each design, but only to demonstrate that it is free of flutter up to VD and that there is good reason to believe, based on various kinds of ground tests and mathematical analyses, that it will remain so up to 1.2 times VD. It is noteworthy that the section of Part 23 regarding flutter, 23.629, makes no mention of altitude. The cumulative margin between VNE and 1.2 times VD is 33 percent, and this probably provides a good cushion in all normal operations, but if I were to ride a wave to 35,000 feet in a 172, I would not be in a hurry to peg the airspeed at redline on the way back down.
And as I said, the way that the engineers will insure that Vd "is by definition a safe speed" is to multiply the calculated loads by a Factor of Safety, which is typically 1.5:
http://en.wikipedia.org/wiki/Factor_of_safety
Choosing design factors
Appropriate design factors are based on several considerations, such as the accuracy of predictions on the imposed loads, strength, wear estimates, and the environmental effects to which the product will be exposed in service; the consequences of engineering failure; and the cost of over-engineering the component to achieve that factor of safety. For example, components whose failure could result in substantial financial loss, serious injury, or death may use a safety factor of four or higher (often ten). Non-critical components generally might have a design factor of two. Risk analysis, failure mode and effects analysis, and other tools are commonly used. Design factors for specific applications are often mandated by law, policy, or industry standards.
Buildings commonly use a factor of safety of 2.0 for each structural member. The value for buildings is relatively low because the loads are well understood and most structures are redundant. Pressure vessels use 3.5 to 4.0, automobiles use 3.0, and aircraft and spacecraft use 1.2 to 3.0 depending on the application and materials. Ductile, metallic materials tend to use the lower value while brittle materials use the higher values. The field of aerospace engineering uses generally lower design factors because the costs associated with structural weight are high (i.e. an aircraft with an overall safety factor of 5 would probably be too heavy to get off the ground). This low design factor is why aerospace parts and materials are subject to very stringent quality control and strict preventative maintenance schedules to help ensure reliability. A usually applied Safety Factor is 1.5, but for pressurized fuselage it is 2.0, and for main landing gear structures it is often 1.25.[11]
Appropriate design factors are based on several considerations, such as the accuracy of predictions on the imposed loads, strength, wear estimates, and the environmental effects to which the product will be exposed in service; the consequences of engineering failure; and the cost of over-engineering the component to achieve that factor of safety. For example, components whose failure could result in substantial financial loss, serious injury, or death may use a safety factor of four or higher (often ten). Non-critical components generally might have a design factor of two. Risk analysis, failure mode and effects analysis, and other tools are commonly used. Design factors for specific applications are often mandated by law, policy, or industry standards.
Buildings commonly use a factor of safety of 2.0 for each structural member. The value for buildings is relatively low because the loads are well understood and most structures are redundant. Pressure vessels use 3.5 to 4.0, automobiles use 3.0, and aircraft and spacecraft use 1.2 to 3.0 depending on the application and materials. Ductile, metallic materials tend to use the lower value while brittle materials use the higher values. The field of aerospace engineering uses generally lower design factors because the costs associated with structural weight are high (i.e. an aircraft with an overall safety factor of 5 would probably be too heavy to get off the ground). This low design factor is why aerospace parts and materials are subject to very stringent quality control and strict preventative maintenance schedules to help ensure reliability. A usually applied Safety Factor is 1.5, but for pressurized fuselage it is 2.0, and for main landing gear structures it is often 1.25.[11]
Seems to me that you are the one who needs to do some reading and learning if you want to pretend to be an expert. Without justification and contrary to the facts, you are claiming that 757s and 767s cannot withstand 1.2 times Vd, despite Boeing's reputation for producing rugged planes. Anyone who cares to search for it will see how well that claim is holding up around the internets, but I won't waste time beyond substantiating what I said.
Speaking of which, are you saying that you still don't know the difference between critical Mach and drag divergence Mach -- even though the first sentence in this Wikipedia definition warns against that confusion? And are you trying to deny that planes typically cruise just above Mcrit to save fuel? This is the "expertise" you're selling in your videos?
Anyone who still takes you seriously hasn't been paying attention.
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Jetblue Captain and Aeronautical Engineer falls for Skygate 911 lies and fake Vg diagram
superbeachnut
Jun 2014
#71
math expert debunks pilots for truth math, pilot for truth forum thread confirms it
superbeachnut
Jun 2014
#44
pilots for truth fail to decode what a mathematician can, so much for experts
superbeachnut
Jun 2014
#48
Fake Vg diagram, inability to post the structural failure speed - pilots for truth
superbeachnut
Jun 2014
#53
Why can't the super pilots for truth source what they say they can source
superbeachnut
Jun 2014
#77
pilots for truth talk Technobabble with aerodynamics and can't explain their dumb-speak
superbeachnut
Jun 2014
#81
Where is the core, why do they not help spread lies of impossible speeds, and fake Vg diagrams
superbeachnut
Jun 2014
#111
pilots for truth unable to state the structural failure speed on their fake Vg diagram
superbeachnut
Jun 2014
#43
Fake Vg diagram supported with... nothing, pilots for truth fake Vg diagram supports lies about 911
superbeachnut
Jun 2014
#60
sign of no evidence, for lies of "structual failure at 425 KEAS", and fake Vg diagram, flying a desk
superbeachnut
Jun 2014
#88
How does this save the fake Vg diagram or the structural failure speed lie
superbeachnut
Jun 2014
#55
Paranoid conspiracy theorist fall for pilot for truth fake Vg diagrams and other lies
superbeachnut
Jun 2014
#65
paranoid conspiracy theorist post more lies and paranoia instead of evidence
superbeachnut
Jun 2014
#67
A fake Vg diagram appears in the Skygate video with the lie of structual failure at 425 KEAS
superbeachnut
Jun 2014
#78
pilots for truth make fake Vg diagram and explain how to fake the Vg diagram, without engineering
superbeachnut
Jun 2014
#86
Pilots for truth can't find the spec the 767 was built to, a reflection of their fake 767 Vg diagram
superbeachnut
Jun 2014
#129
Seger, are you familiar with real world exercise, practical application, and precedent?
johndoeX
Jun 2014
#134
Fake speeds, fake Vg diagram, failed physics, what is the next fake claim from pilots for truth
superbeachnut
Jun 2014
#132
To any person with a WORKING brain SOMETHING IS TERRIBLY WRONG WITH WHAT WE'VE BEEN TOLD ABOUT 9/11.
dballance
Jun 2014
#158
More nonsense sponsored by pilots for truth, more hearsay and exageration
superbeachnut
Jun 2014
#171
Working brain? You fell for lies in the "The Big Bamboozle", you were Bamboozled
superbeachnut
Jun 2014
#168
pilots for truth can't defend impossible speed lie, no support from rational Aero Engineers
superbeachnut
Jun 2014
#176