The First Flight Pt.II

January 2, 2011

Work is really a mutual thing- I mean, the work and me. I’m glad I needed to work and the work needed me. So I’m also part of our modeling group. I know other teams mostly use CATIA. But we chose Inventor. With limited experience with this software, I was bold enough to deal with internal structure of wings.

Again, it was all blank in my database when it comes to internal structure. Neo briefed me that wing ribs, beams should be included; control surfaces should be split from the original wing surface, in order to move it in assembly mode. But how thick is a single wing rib? How distant is it between two ribs? What about the beams? And all those technical problems using the software… Ideally, a modeling group member should be spared asking such questions, as detailed parameters should be given before drawing begins. Unfortunately we’re a small team, we had to feed ourselves by looking for them on our own. It took me quite a while to get used to the instruction system of Inventor 2010. And the questions I raised to myself were all resolved by referring to the handbook. Thankfully, a lot of books out there concerning the wing structure are pretty convincing. Really have learnt a lot all the way down.

A story worth mentioning here is about the wing ribs. The front wing of our aircraft has a sweepback of 37 degrees. Initially Neo said the ribs were parallel to the airflow. While I thought it should be positioned perpendicular to the trailing edge, with James on my side. The great news was Aircraft Design Handbook has a whole chapter addressing this issue. I came to know that actually both layouts are seen in aircraft structure design. The former ensures a smoother wing surface in flight, but is heavier; the latter is more widely employed, the main advantage is that it’s lighter. With this in mind, the final layout was changed.

There do exist some tragic incidents, which I prefer not to put in detail 🙁 One of them is related to lack of experience with Inventor. The features I created on the aft wing were such a mess that it became impossible to modify in the end.’doh! A good planning before starting to draw is so important. This is the lesson I learnt. Then afterwards, I tackled the front wing, which was a little more complex than the aft wing, I finished it in two hours and got infinite praise from Neo, thanks to the conscious mentality this time. Yummy!

Other parts I did include landing gear and cargo container assembly. Genuinely a great sense of accomplishment when seeing the meat of our teamwork rotating on the screen before us. And as the final round is about to strike, I completed my part of report about seating/cabin door/cargo container arrangement, fuselage design and nacelle design.

It’s now time to address some non-technical problems here.

It keeps occurring that there are conflicting numbers regarding the same parameter. For example I was the one to set the seating capacity at 248, but to the group leader’s knowledge it was 247 and someone else has another version. No wonder in our presentation, the relative thickness of a NACA0012 airfoil could be stated as 8%. Poor data management creates confusion; and confusion creates frustration.

Another haunting problem is about the coursework itself. I’m not sure how precise we have to make it. There are certain limits of what we can do, while some, though not all questions raised by teachers implied mission impossible.

Moreover, sometimes it makes me blind when people say we’ve got to do a cool video, a cool presentation and some stuffs like that. I know these are essential to win the competition, but I never quite get a balancing point.

But again, what a road! It feels unbelievable how much I did and learnt. I’ve experienced the design process, full of paradoxes, conflicts while filled with joy, through playing multiple roles. It was great fun working with a bunch of guys generally named “awesome”. Peace :]

The First Flight Pt.I

The aircraft general design course has finally come close to an end, with each group working out the last bit and presenting their final solutions. Looking back, what a road! 😀

It’s not a least bit of exaggeration to say that everything is new to me. Like most of us, the driving force of being an aviation enthusiast mainly stems from exciting war planes. Before working on this project, commercial airliners looked like the same in my untrained eyes. Besides, it’s a great challenge to work with new people- in a mysterious new workplace. And above all, it seemed uncertain if we could work out a brand new aircraft from scratch.

The doubts were soon cast away. The guys I work with turned out to be just awesome and I earned myself some confidence by getting my ideas across. As soon as we stepped into phase I a.k.a conceptual design stage, the workflow got into the right track.

Through a lot of googling, everyone presented a sketch to meet the basic requirements for this civil aircraft, which is ~250 seats, 28 LD-3s and a range of a typical domestic flight. Nothing to be ashamed of, like any others would do, I compared the requirements with existing and confirmed forth-coming aircrafts, and found both Boeing and Airbus family have similar types to ours to offer, such as B-787 8 B-757-300 and A-310. Probably limited by the popular layout of these examples on the first sight, my initial sketch went nothing more special than adding a U-tail in substitution of the conventional horizontal + vertical tails.(Even the U-tail is inspired by Airbus’ new plane.) While around half of us thought in the way alike, the other half went so ‘far’ as to propose the employment of joint-wing design. Though I personally preferred to play safe, in order not to be trapped in fruitless bargain, I thought over and over again. James had his point, conventional layout uses horizontal tail for trim, this always feels like a waste of limited lift. While joint-wing generates lift from both front and aft wing, with the center of gravity located at somewhere in between. As equally charming is that more control surfaces lead to higher control efficiency.

But nodody had a good idea about it to take a final vote. So there came a second round of extensive reading and googling. Before my head exploded, the  five papers gave me a relatively clear idea of pros and cons. As I converted to the joint-wing, or more precisely, box-wing side, a solution to overcome its main defects was also formed in my mind. Since the box-wing design pretty much doubled the surface area- as it complicated the contour- a direct and simple method to counter this effect is to employ blended-wing-body design. OK, some guys are scared. There’s already a box-wing, and now this BWB thingy. Back at that time the only thing certain was that there was going to be much uncertainty unimaginable and many challenges unknown for both computing and modeling group. However, on a second thought, these guys had to agree with the simple logic. So, here we go, on to Phase II.

After the sketch selection, it was time to fill out the parameter form. the aircraft was divided into several sections, and I was assigned to work the fuselage out. James’ tips helped save a clueless mind. He hinted that the parameters of a fuselage should be determined by the basic requirements like seats, cargos etc. Following this roadmap, I collected all the available internal configuration of existing similar aircrafts. It’s so very kind of Boeing and Airbus to provide service brochures including all the information I need. But the blended-wing-body design was meant to create extra difficulty when it came to seating management. To get an idea of how variable cross-section varies with x-axis resulting from BWB, I also looked closely at B-1B, which possesses the closest blended surface to our desired one. It was like this, i) trying a seating configuration, ii) giving a corresponding variable cross-section, iii) creating a sketch to see how continuous the curves are. And it looped all over again in an iteration fashion until… well, until it looked “just fine” under the precondition that the basic requirements are met. Yes, “just fine” is such an obscure term. Yet after several classes we already learnt, flexibility in designing process can be find everywhere, especially in preliminary design stage.

At the meantime, I also worked on the design of the engine nacelle. Given an GE-CF-86A engine, with the help of Aircraft Design Handbook and design path suggested by an NUAA lecture note, plus my in-depth data analysis, the dimensions of the nacelle were determined.

You know behind every single digit in the table, there is blood and tears. And what you don’t know is that there is also blood and tears behind digits that are not even presented in the table. For example it’s crucial to allow for wall thickness of the fuselage, which is not presented. But a responsible person need to put a lot of work on it. I took two approaches to determine the exact thickness. The first one is to look up in the Handbook. The second is to make use of data collected. Subtracting fuselage width with the sum of all the seat width and passage width, I get the wall thickness by simple maths. The second method also include at least three typical aircrafts. Only when these figures coming from different approaches satisfy each other, can I safely say the it is reliable.

At the meantime, I’ll have to say Dudu’s weight/cg estimation report, James’ box-wing parameter and aerodynamic performance analysis, Alex’s landing gear report, they’re simply superfabulous!

Notably, we keep each other updated by publish everyone’s own portion of work on a public email. It makes other members aware of your progress, and makes it easier for revision and suggestion. Also it’s said to be encouraging when seeing buddies making effort. Well, maybe 😉 Although personally I’d recommend using Google Wave as it makes collaboration, comments even more smooth, due to extremely limited use of Gmail account, okey dockey, just forget about it!