Your Questions to Boeing Engineer

[Bottlestein]

I'll be meeting K. Davis , one of the lead Aeroelastic Analysts of Boeing, later this week. (He'll be giving a brief talk to some of our research groups, and our group will talk to him for awhile.) Now, he was involved with the SST - II proposal, as well as the 787 Dreamliner.

Since SDN'ers have a basic interest in these things, but lack technical expertise, I will try to ask him questions that you guys put forth about composite construction methods, stress mechanics, aeroelastics, and general aerospace topics.

Ground rules: I will probably have time to ask him maybe five questions, so pick them well! Obviously, don't ask anything that a 2 second search into an appropriate book will turn up. His talk seems to be about the 787, so questions about the composites used on that would be to his liking.

Also, if you want to ask very detailed stuff in stress mechanics - that's fine, but try to phrase your question clearly, since I will not be able to clarify it very much - I only know a little bit of the terminology.

[Chardok]

Okay - how many flex-cycles is a modern wing assembly good for before succumbing to irreversible damage necessitating replacement of the entire assembly?

[Kyler]

What ever happened to the development & testing of aeroelstic main control sufraces for aircraft instead of standard control surfaces like flaps, rudders, elevators, and ailerons?

[aerius]

Conventional metal airplanes are built to try and prevent cracks from zipping all the down a seam and opening up giant holes in the fuselage, there's built-in "firebreaks" to limit the size of a crack. How would this be done with composite construction?

[Zixinus]

Is there news of the viability of the Venture Star prototypes? Were they cancelled by NASA prematurely?

[Bottlestein]

Thanks for the questions guys

Just got back- I got to ask all of your questions as well as a few of mine. I'll try to post all of his answers before my group meeting. (If I can't, I'll post the rest tonight. So - if you don't see your question answered immediately - don't be disheartened; I'll get all the stuff up tonight!)

I'll answer them in separate posts so you guys don't have to parse the wall of text needlessly.

Chardok asked

Okay - how many flex-cycles is a modern wing assembly good for before succumbing to irreversible damage necessitating replacement of the entire assembly?

There's 2 separate flex-modes : longitudinal (the wing bends, so the tip moves up or down), and torsional (the wing "twists" along its length, with different twist rates inboard and outboard of the engine).

There's different requirements for the spars (beams that support the wing and connect it to the fuselage), but generally all registering agencies (FAA, EASA, IOSA) agree on a minimum of 100,000 logitudinal flex-cycles before spar replacement. Note: Replacement is required well before structural failure - strain response that's 10% off is considered "not airworthy".

The Boeing 777, specifically, has 160,000 flex-cycles (longitudinal), and 533,000 outboard torsional cycles (it only has 1 engine per wing) allowable before spar re-assessment. This corresponds roughly to 4 years flying time. The design is expected to be able to undergo 3 to 4 spar refits, roughly an operational life of 12 to 16 years.

The 787 has roughly 250,000 cycles alotted, but due to other advances this is almost 6-7 years flying time (the wing flexes less). Plus, due to differing construction, there is no "spar refit", but component refitting. This extends the predicted operational life well into the 25 years + range. The number of torsional cycles allowed is roughly the same as the 777, but again, the wing twists a lot less, so first refit is again extended to 6-7 years.

EDIT: Cool fact, under a + 2.5g load (a 2.5g dive), the wing tip flexes up by 26.3 feet from its standard position.

[Bottlestein]

Kyler asked:

What ever happened to the development & testing of aeroelstic main control sufraces for aircraft instead of standard control surfaces like flaps, rudders, elevators, and ailerons?

There's some stuff that has this, particularly gliders. The main problem is the range of flows that the material has to handle. For jet planes, a lot of the flow becomes supersonic, especially at the wing trailing edge (even if the true airspeed is much lower than mach 1, such as an airliner cruising at 580 knots - since the airflow is accelerated to produce lift, the flow goes supersonic depending on the wing cross section). This means the aeroelastic component undergoes an extremely non-linear "bump" as the flow regime changes. This non-linear response makes writing control algorithms very difficult in this region, and similarly, for human applications the safety factor is compromised here.

However, for slower stuff that is very much in the subsonic region, aeroelastics are being tested now. Actually, there's a few road cars with aeroelastic diffusers (he thinks the Ferrari 458?) available now. For aircraft, some Airventure craft have aeroelastic control components - the main advantage is the weight and volume savings from ditching the servos.

[Bottlestein]

Aerius asked:

Conventional metal airplanes are built to try and prevent cracks from zipping all the down a seam and opening up giant holes in the fuselage, there's built-in "firebreaks" to limit the size of a crack. How would this be done with composite construction?

Cracks propagate differently in composites - mainly because composites are suspended polymers rather than a crystalline solid. Basically, carbon composites are some type of carbon chain bonded lengthwise to another chain, suspended in a "resin" or "matrix" of weaker bonded structures. It's extremely tough to break a carbon-carbon bond, so the composite has a natural "ply".

Essentially, damage prevention with composites follow two main guidelines: good understanding of which way the "natural ply" is pointing, so the direction of expected damage is always perpendicular to it, and building in small "patches" bonded together with some other polymer so a crack never becomes too big.

A 787 wing, for instance, has 168 different "types" of composite patches, from carbon laminate, to sandwich glassfibre, that cover it.

Another major problem with composites that isn't there with metals is "de-lamination". If you hit a piece of metal, you just dent it. If you hit kevlar, you can potentially take the top layer off. This may not penetrate immediately, but it reduces the structural strength of that piece, which may lead to failure.

Again, the 787 solves this by "patched" construction, and testing for "de-lamination" without critical failure - i.e. re-orienting each "sheet" to criss-cross the ply. The skin of the 787 is, in some parts of the fuselage, just 9 sheets of carbon laminate thick - so less than 8 mm thick.

[Bottlestein]

Zixinus asked:

Is there news of the viability of the Venture Star prototypes? Were they cancelled by NASA prematurely?

A reusable SSTO (Single Stage To Orbit) system with the type of planned payload of the Venture Star is not yet viable. He does think the NASA tests of the X-33 were cut off prematurely - but there is still a lot of research being done, and there is every indication that something with at least the capabilities of the X-33 can be built cheaply in a few months. (Mind you - he is from Boeing , a rival company to Lockheed, so maybe he has to say this )

As for the problems with the X-33:
1) The thermal protection system was a non ceramic one. In fact, this was a major selling point, as ceramic heat shields require massive between-flights maintenance. Now, at hypersonic flow, the rates of heat conduction change rapidly. Basically, a hot body no longer cools primarily by heating the air around it, but by radiating EM waves. Now, on the X-33, the thermal shields were metal based. The metals would not radiate as expected, cutting down cooling efficiency rapidly, and leading to failure.

2) Fuel tanks: the composites used in the fuel tanks began interacting with the hydrogen at the temperature and pressure experienced during flight. This lead to fuel feeding problems to the engine. These interactions have to be figured out, and this is still a bottleneck.

3) Generally, the aerospike engine was less efficient than predicted, and better fluid dynamics simulations are telling us where some of these inefficiencies lie. This is a setback, but aerospike engines are still possible, and efficient fuel cooling and feeding systems have already been tested, even with NASA's toroidal aerospikes.

However, X-33's viability depends on understanding the behavior of some of the new materials - which continue to bottleneck further development.

[Chardok]

Ha ha, bitches, I got the longest answer. That's awesome information, and actually helps allay my ridiculous fear of flying somewhat. Thanks for relaying that for me Bottle!

[aerius]

Essentially, damage prevention with composites follow two main guidelines: good understanding of which way the "natural ply" is pointing, so the direction of expected damage is always perpendicular to it, and building in small "patches" bonded together with some other polymer so a crack never becomes too big.

That is cool. I figured they must have a way of doing it since they're making giant single piece composite parts for the 787. I guessed the ply layup had something to do with it but had no idea that they also had "patches" and all that other stuff.

Ha ha, bitches, I got the longest answer. That's awesome information, and actually helps allay my ridiculous fear of flying somewhat. Thanks for relaying that for me Bottle!

Yeah, just don't fly on an Airbus and you'll be fine. Not only do the tails fall off, the engines blow up good. Let me repeat that: EXPLODING AIRPLANE ENGINES! Happy landings!

[Archaic`]

Yeah, just don't fly on an Airbus and you'll be fine. Not only do the tails fall off, the engines blow up good. Let me repeat that: EXPLODING AIRPLANE ENGINES! Happy landings!

It happens to Boeing aircraft as well.

[Chardok]

<snip>

...you're a bad person.

[Skgoa]

Yeah, just don't fly on an Airbus and you'll be fine. Not only do the tails fall off, the engines blow up good. Let me repeat that: EXPLODING AIRPLANE ENGINES! Happy landings!

Are engines not bought independent from the aircraft, anymore?

[UnderAGreySky]

Indeed, tensions at the Rolls Royce offices and factories are apparently quite high!

[Kyler]

Thanks for asking our questions, I appreciate the answers to the question.