Impossible Speed
 

G'day all,

Posted this up in the JB forum and was advised that this would probably be a better location for this topic.

Having just completed my pilot training I embarked on an exercise of greater understanding on the mechanics/physics behind flight, more specifically heavy aircraft and the design principles they are built on.

During my intellectual study I came across, unfortunately, an article and subsequent video's and forum posts from other sites from 9/11 "Truthers" claiming that UAL 175's estimated speed upon impact of the WTC, approximately 500kts, is impossible. They then back up their arguments with some pretty interesting points about aircraft structure and citing design speeds like Vmo and Vd. On a side note which I don't really want to discuss is that it seems alot of them then go on to peddle their theory that no aircraft struck the WTC which to me is quite stupid. :ugh:

I know that 9/11 conspiracy theories are a pain in the arse, but i'd like to open up a thread discussing the design characteristics and limits of aircraft and how "set in stone" they are.

I'll get the ball rolling based on my almost non-existant knowledge on aircraft engineering.

Design speeds and limits of aircraft, according to my understanding, seem to be purely numbers that are calculated and ultimately can never be tested on actual aircraft (ie testing the speed at which the aircraft will undergo structural failure is obviously stupid). I assume, ofcourse, that using known data on mechanical and structural strength of the materials, and through wind tunnel testing and ultimately controlled flight testing, they are calculated. How solid are these numbers? Obviously exceeding set safety limits such as Vmo can result in damage to an aircraft, which is why there are safety buffers on these speeds, but does exceeding guarentee immediate structural failure or do environmental factors and how the aircraft is handled play a major part?

Any thoughts from pilots, engineers and maintenance crew? Sorry Flight Attendants, don't think your knowledge base covers this topic http://images.ibsrv.net/ibsrv/res/sr...ies/tongue.gif

P.S. I'm gonna have to emphasis my VERY limited knowledge on how aircraft are designed, manufactured and tested. So if you can understand what i've stated above... I must be doing something right.

Cheers

Maximum speed
 

Wranga,

you have a bit of a point. Design speeds are indeed calculated and are invariably higher than certified maximum operating speeds. Then again, there exist a fair number of reports on airliners that exceeded their design speeds without immediate failure. Some spectacular ones (no guarantee for factual correctness) are here:

The DC-8 Supersonic Flight
and here:
China Airlines Flight 006 - Wikipedia, the free encyclopedia
and here:
FedEx Flight 705 - Wikipedia, the free encyclopedia
and here:
http://en.wikipedia.org/wiki/TWA_Flight_841_(1979)

I believe you could also find a number of less dramatic incidents when looking into the operations of certain flight haulers using Learjets. However, it is not possible to predict the exact speed at which a type of airliner will be damaged - due to uncertainties of weather, atmospheric density, fracture strenght of the metal, exact load case, previous micro fractures, and many more. That's why the designers determine the maximum speed at which they can guarantee the aircraft will hold together.

Hope this helps.

When designing an aircraft the stress experts go into huge amounts of detail to understand the forces acting on every part, using finite element techniques and the like. They therefore know where the weak points are and what effect various aerodynamic loads, including overspeed, will have on them. Sufficient margin is built in to provide an acceptable level of safety without an unacceptable level of extra weight. Flight testing with strain gauges fitted at critical points on the structure confirms the design calculations to be correct (hopefully!) and can be extrapolated up to the failure case without actually needing to fly to it.

Wranga,

Any structure, be it aircraft or bridges or cars will have a "limit load" which is described as "the highest load likely to occur in service" (or similar words to that effect).

In an aircraft all maximum permissible g loadings, speeds and manoeuvres are based on this limit load.

However, aircraft structures are designed up to "ultimate load" which is: limit load x safety factor. Aerospace sets this safety factor as 1.5 which is enough to give a good margin but will not cause the aircraft to be so heavy as to get off the ground!
Comparatively bridges can have safety factors up to 6 or higher as weight is not such an issue.

This means that while an aircraft may only be certified to fly at a maximum speed, or pull a certain amount of g-force, in fact it would almost certainly be able to withstand higher forces without structural failure.

It should be noted however that while it is technically possible to exceed the certified limits without immediate structural failure, the higher loads will increase fatigue and hence reduce the aircrafts safe operating life.

On a side note composites based aircraft (B787 A350) will typically be designed with safety factors upto 3 due to the way that failure of composites is not yet totally understood.

Regards
Alistair
3rd Year Aeronautical MEng

Consulted the boys on Airmech and got this definitive answer.


b, Kylie :O

Mr BNM must have given you that reply.

Shhhhh. Don't mention it. :}

Pls advise?!

Very interesting points and the websites provided give lots of insight.

However when calculating things like EAS, it seems like there isn't a great margin over Vmo.

Lets say an aircraft is flying at 500kts GS at 1000ft (using the UAL 175 example just for giggles). Would it be undergoing the same aerodynamic pressures as, lets say, 0.99M at 30,000ft with 354KEAS (assuming ISA ofcourse)?

Without going into too much of the physics behind it, can someone explain to me what kind of stress/pressure the aircraft is undergoing.

Thanks in advance

The larger point is the maneuverability of a commercial aircraft at high speeds, even if the aircraft could fly at that speed they would have been lucky to hit manhatten let alone the towers.

Also there is such a thing as a high speed stall, not 100 % but I am pretty sure this can happen at lower altitudes as well. It normall occurs at cruise. This happens basically when the center of lift moves aft of the wing.

Do UFOs exist?
 

Blista1989..wunnerful reply..the factor of 3 on the 787 is good..the way the wings flex kinda worries me.

Wranga..nice way of opening a tin of worms. John Lear has some very interesting ideas of events on that cathartic day.

Dodo..engineers are human therefore they are flawed.

Re-engineer..wahttya mean Wranga "has a bit of a point"?
Wranga's post is profound.

Wranga...Rolls Royce is a firm worth looking up on the internet..wunnerful videos on stress, strain and design.

Bit of a...
 

Derg,

I totally agree that Wranga's post was profound. I was just trying to formulate my view carefully in my first post here - maybe a bit over-carefully then.:ouch:

Don't get me wrong. I'm not a conspiracy theorist. I'm quite willing the accept the multitude of official and unofficial reports from a plethora of experts in their respective fields plus the hard evidence of witnesses etc etc. But it is a point that concerned me and having seen how this issue can infect the minds of people who have no idea about anything to do with aviation I decided that I would at least ask a few people who knew what they were talking about. Once again. I am NOT a conspiracy theorist. Just driven by pure curiosity.

Events of Oct 11
 

Rengineer I beg your pardon. Yes can see your caution.

Wranga..what dwells on my mind was the lump of a jet engine that was found a street away from the towers: it was foreign to both jets in the event.

Now I was born on the west side of the Iron Curtain. I had certain beliefs and values before 9/11..after 9/11 I lost my naivety.

Bugger, I think i've started a thread to talk about 911 conspiracies. :ugh:

DERG: All the wreckage that was found has been positively identified as being from their respective aircraft. The "conspiracy" theory claiming that the engine is too small for a large wide body airliner is false and those who bandy around this theory know nothing about Turbo fan engines.

Please people, can we all stick to the main question at hand.

Structural Integrity of Aircraft
 

Wranga YES indeed we can! blista1989 wrote an outstanding reply but I will add some stuff that may add to the subject.

Limit Loads: aircraft forces are dynamic and each load has to be asessed incrementally in time during flight. Materials change due to degradation...duty cycles...temperature effects....basically they age.

As blista notes in civil engineering the total mass of the structure is not an issue and he states a factor of 6 is used for design. In Civ Eng this is known as the "unknown load"...in civil aviation this category does not exist. There are no unknown loads in avaition design.

In aircraft the wing root and the under carriage is subject to severe dynamic loads. Blista noted a factor of 1.5. In fact many under carriages are designed to carry 30% greater than this for a short time. This is an impact load. Military aircraft such as those landing on an aircraft carrier are designed for much more.

In paragraph 5 blista tell us about how structures age. I think all of us have worked a thin piece of metal to one side and then the other until it breaks. See duty cycles above.

Environmental factors play a big role in this process. Temperature differentials....oxidation...chemical action..hard landings..."hot dogging"...over loading, or imbalanced pay loading all add to ageing.

Wranga if you stuff like this excites your brain I would go on to study engineering. You can still fly but no longer will you be a "machine minder" you will be an engineer.. a machine "designer". You will never be bored!!

Thanks Derg,

Might just do that. Have to first learn to fly my new aircraft type. I'm sure that will take up alot of my brainspace for a bit, and then there is the inevitable breaking in period where i'll still need to be studying the aircraft. After that i'm definately going to launch into an engineering degree of some descript. Who knows, might help me to become a test pilot in the future.

Cheers

P.S.

When you say unknown loads, You are referring to the fact that we know pretty accurately what the load of the aircraft will be?

In terms of weight? forces being imparted on the aircraft through all phases of flight etc etc?

definitions
 

No Wranga I mean that for any given mass of an aircraft we can predict what the LOAD will be on any given structural part at any point in the design envelope.

The only thing we can't predict is if the aircraft is subject to a massive environmental event like an EMP. Electro magnetic pulse..as in rare lightening.

Take the time to look up what
MASS
WEIGHT
LOAD mean to an engineer.

This is a very good example to work thru in your mind. See if you can work out what is going on in this pic.
FlightAware > Viewer

Overweight/high speed landing with high wing loading giving significant wing bending moment?