Welcome to Gaia! ::

So, here's a picture of the Blended Wing Body concept Boeing and NASA (with other smaller groups) are now developing. A lot of people think it is weird and ugly, but I happen to think it's the most beautiful airplane. What do you think?

A toast to $, the world's lubricant lol.


True. In aerospace, strictly speaking, looks is below functionality on the priority list. Nonetheless, planes are bought by airlines/air forces, which are supported by passengers/governments. Looks CAN help sell a plane. One of the reasons Boeing lost to Lockheed in the JSF bid for a VTOL fighter is because Boeing's plane look weirder/uglier, or so I heard.

It has a few less "wings" compared to conventional planes, but it's perfectly stable, and way more efficient than normal planes (it IS mostly a wing after all).

Nothing is perfectly stable ^_^; The only things that come close are a balloon or a big, flat rock, but even those aren't perfectly stable. Modern planes rely on being unstable - for example, to speed up they throw the balance of forces out of whack by increasing thrust and waiting until drag balances it out again.

i don't doubt that that thing is efficient - it's a big-a** wing and pretty much nothing else, which means it's generating lift over its whole body instead of just a small part of it. But efficient is not the same as stable.

Whenever you talk about stability, you gotta talk about the stability triangle. That's the imaginary triangle between the centre of gravity and the two farthest away supports on the same plane - for an aircraft, the engines are the supports. If you got a stability triangle that looks like this:then the bigger the value of b/h, the more stable your system.

You ever seen Jackass? Those guys, they take a shopping cart and strap a rocket booster on to it, and then launch it over a lake. What happens? Well, it takes off fine... then it flips over and spins out. Why? Because the stability triangle looks like this:so b/h is 0... totally unstable. Doesn't even get more than a couple metres before it spins out of control and the dude ends up in the lake.

Now, compare the stability triangle for that plane that has the engines up near the centerline versus a conventional plane that has the engines way out on the wings - the blended wing is the top one and the conventional plane is the bottom:See? The conventional plane's b/h is much bigger than that thing's. i'm not sayin that thing's not gonna fly, because it surely will, but it's gonna be wobbly and shaky compared to a conventional plane.

And then there's the problem of aerodynamic stability. There's a reason modern planes have tail sections. If you take a conventional plane, put it in the sky in any orientation, and then drop it, it will automatically - given enough time - level out. The tail section presents a bigger k-factor, a bigger drag cross-section. Even if you point it straight up, with the tail pointing down and the nose up, the plane will automatically right itself. If you're lucky, that will happen before you hit the ground.

But that wing? i don't see it. Obviously i can't be sure until i see the k-factor numbers for the different faces, but it doesn't look like it would be particularly stable.

Now i'm not sayin it won't fly, because it surely will. But i think it will shake and wobble like crazy. If you've travelled in a bunch of different planes, think about the difference in flyin in a single-prop plane versus a multi-prop plane. Very very shaky. No problem for a fighter plane (they're all really unstable), but for a passenger plane? Unless that thing's flyin really high, it's gonna be a bumpy flight.

i could tell you, but then i'd have to kill you. ^_^;

Honestly, bein able to say that and be somewhat serious is the best thing about my career. ^_^;


Ya, that's the stability triangle. The stability triangle tells you how much torque it'll take to tip it (m * g * a about A and m * g * b about B), and how far it has to tip before it topples (arctan(h/a) about A and arctan(h/b) about B).



We're not interested in how stable the plane is sitting on the ground (or cruising at constant velocity in impossibly completely calm air). ^_^; We're interested in how stable it is in (realistic) flight - that's dynamic stability.

In real flight, even if the plane is travelling at constant velocity, it is being buffeted by turbulence. If the plane is dynamically stable, every time the plane is nudged by a turbulent eddy it may jerk a little bit, but then it immediately starts correcting - unless it's a big jolt you don't even feel it. If the plane is dynamically unstable and it gets jerked by even a tiny eddy, it continues to move a lot (and if uncorrected the plane would spin out completely, but obviously that won't happen because either the pilot or the flight computer will correct) - and neither the pilot nor the flight computer will correct the displacement until they can "feel" it in some way or another... which means all the passengers feel it too, which means a bumpy ride.


An aircraft in flight is a 6 DOF system, which means 6 axes: 3 translational axes (up-down, left-right, forward-aft) and 3 rotational axes (yaw, pitch, roll).

i agree with you, i don't think the plane is particularly unstable around its yaw or roll axes. i think yaw will be taken care of just fine by the wing's span and the winglets. Roll... well geez, it's nothing but one big a** aileron ^_^ so it won't be a problem.

Pitch may be unstable. The CL of a wing moves around depending on a lot of factors - speed, wing configuration (aileron position, flaps up/down, etc.), angle of attack, etc. The CL moves around on a conventional plane, which is why you have elevator trim control (imagine tryin to fly a long trip without pitch trim ^_^; i'd die). But on this plane, you don't have elevators, but that's ok, because that's actually how this plane will fly - to take off they'll move the CL back behind the CG and the plane will pitch up, to pitch down they'll move the CL ahead of the CG. In steady flight, they will attempt to keep the CL and CG in line.

In a regular plane you have a regular stability triangle: the wing and elevator are the supports and the CG is the CG (you have to do some mathematical/geometric manipulation to turn the elevator's negative contribution into a positive contribution, but that's not really a big deal once you know how to do it). In this plane... there is no elevator, so there is no triangle. It is completely unstable. A computer will likely handle that in normal flight (and a human could take care of it for short flights, but it would be tiring), but it means the flight will be bumpy.

As for the translational axes, obviously the y axis (up/down) is no problem - it's a big damn wing. ^_^; The z axis (forward/back) is no problem either. But the x axis... there's a problem. What is to stop the plane from slipping sideways except those two tiny winglets? Not a whole hell of a lot, which means that if the plane is bumped on the side... you gonna feel it.


Of course it makes a difference. ^_^; Just do the math. Assume the plane is in level, constant-velocity flight. Looking down from the top, and assuming two engines for a 50 unit wingspan (each dash is one unit for scale), the two designs look like this:

Now, assuming the centre of mass in the centre of the wing span, calculate how big of a bump at X it would take to make the entire thing spin around. In this example, the conventional design is twice as stable as the blended wing design.

BIG difference. ^_^;


i don't see that? O_o How do you figure that?


Heh, you put way too much faith in computers man. ^_^; When i went to school for mechanical engineering, in the last year you get to choose your specialty. Mine was control systems theory.

News flash ^_^; no computer can minimize turbulence because it is a completely random phenomenon. A computer can only minimize what it can predict. All the computer can do is correct after the plane's already been knocked out of whack by the turbulence. Both the Boeing and the Airbus do not use their computer to eliminate turbulence, they use it to maximize efficiency (and give the pilot less to do in keeping the plane flat and level, but that's really not interesting - they've had "computers" that could do that since the 60s or 70s). In flight, the computer balances thrust and control surface configuration to maximize efficiency - because there's a lot of different ways you can maintain the same speed and altitude in level flight by balancing lift and thrust with the elevators, ailerons, trim surfaces and flaps... the computer just picks the most efficient balance. Neither the Airbus nor the Boeing is dynamically unstable, which is why you don't feel shaking (commercial planes without that level of computer control don't feel shaking either, hn?) - the computer does nothing about that.

The F117 is actually a good example of what really happens in an unstable plane. Think about what the pilot said realistically. ^_^; Cars are very, very stable, and very smooth rides. If a big chunk of your car suddenly got ripped off - a spoiler for example - don't you think you would feel it? ^_^; So why didn't the pilot feel it? Answer: because the plane was shaking so damn much! ^_^; Yes, the computer worked around not having one of its control surfaces, and good for the computer, but the fact that the pilot didn't even notice the thing fell off tells you how badly the plane was shaking around. All fighter planes are like that because they're so dynamically unstable - watch any real footage of pilots in flight and see how their heads are jerking around. Yes, the computer handles the flight control to make an unstable plane manageable... but it does not and cannot eliminate the shakes.

That's no problem for a fighter plane, but if a commercial plane was able to feel shakes and bumps and shimmies... don't order hot coffee. ^_^;

That's supposed to be a future version of a regular airplane? I'd totally fly in that!

Aren't those both the same thing?

For me, static stability is the plane's stability in static equilibrium - which is either at rest or moving at constant velocity with all forces balanced. Dynamic stability is the plane's stability in dynamic equilibrium, with forces not balanced (like a bump or buffet).


All correct... in theory.

But in real life, things are a little bit more complicated. Let's imagine you have a 300,000 pound plane that has had the air pressure under one wing drop suddenly (let's pretend a thin chunk of ice had formed on the wing face, and now broke off, so the lift characteristics of the wing changed dramatically). So, suddenly, this plane starts to roll. Now, this wonderful computer system can correct at - what would you like? 100 times a second? let's go with that. Let's say also that it is sensitive enough that it can immediately detect when the roll starts - so within 0.01 seconds of the roll starting. That's a pretty good computer - way better than anything you'll find out in industry.

So far all this sounds pretty good, right? Your plane is rolling, and you found this out within 0.01 seconds, before anyone can feel it. Now all you gotta do is stop the roll and correct it back to level flight. Do you really think you can stop a 300,000 pound plane from rolling:
* Fast enough that no one notices it rolling.
* Gentle enough so that no one feels the jerk.

Remember Newton! The faster you stop the roll, the bigger the jerk! ^_^ So there's no way you can win. Either you stop the roll quickly and make a big jerk, or you stop it slowly and people notice the rolling.

Just because you have a sweet computer with a sweet attitude detect and correct system, doesn't mean the laws of physics change. F still equals m*a, and planes are a shitload of m. Any fast correction means a lot of a, which means the passengers feel a lot of F.

And that's a simple case! A bump creates all kinds of new problems because you have correct the bump and then correct the correction! A faster computer will make that smoother, but you'll still feel a hell of a bump.


Those things on the wings are ailerons. That plane doesn't have elevators - it uses the difference between the CG and the CL to pitch (which it accomplishes by using computer control to translate pulling or pushing on the stick into combining the ailerons - and possibly pumping fuel, as you mention, but that's slow, dangerous and not really effective for fast changes (tho it's great for trim)).

That plane also doesn't have a stability triangle. Doesn't matter how wide the wing is, it's still only one wing, which means one lift vector. One lift vector, one gravity vector... that's only two. You need three for a triangle. (Conventional planes have two lift vectors - from the tail and from the wings - and you balance them with trim and smart loading for extra stability.)


Most turbulence *is* a bump at a single point (or over a very small area, less than a meter). Even if you happened to cross a pressure layer for a wide bump, the chance of you getting the bump all over the plane at the same time is microscopic - most likely one wing is going to hit before the other.

But i don't get what you mean with thrust differentiated to compensate for the motion?


Oh! i get it. But this is a four engine plane. If you lose an engine on a conventional plane, you don't *need* to use your rudder. In fact, it's better if you don't - less stress on the airframe. That's the same as with the blended wing design.

On a two engine plane, sure, that would be a problem. But if you lose an engine on a two engine plane, you best get your a** down on the ground (gently) as soon as possible, whether it's blended wing or not.


Nah, it's not just that they break down. You'd be just as ******** in an old fashioned plane if the wire connecting your stick to the ailerons broke.

It's that you're thinking *only* in terms of software and forgetting that there's a real mechanical, physical system to account for. Doesn't matter how fast the computer can detect and make changes if making changes that fast would rip the plane apart. Changing the motion a 100 ton mass doesn't happen easily. Software doesn't make inertia go away.


Yes, it requires fast detection, processing and correction... but that's not enough. You say "the comp kicks in and makes a correction" as if it happens like *snap* that. Maybe to the computer, but not to the plane. Even if a pilot could detect and think as fast as a computer, and realized the plane was starting to roll and yanked the stick to the side to prevent any motion - holy crap would you feel it. ^_^;

The only way to really prevent motion would be to react before it happens. That's how prototypes for ultra-smooth ride cars work - they scan the road ahead and look for bumps, and prepare hydraulics to correct the bump as it happens. In a plane, unless you can predict turbulence before it happens, you can't prevent feeling it. All you can do - and this is what good pilots and computer systems do - is try to make the ride smoother by riding out sharp bumps. Remember i said you could choose fast or gentle? They go for gentle - which means they roll and sway and rock a lot... but slowly.

Of course, this works better when there are fewer bumps to smooth out. In an unstable plane... that means it's not gonna work as good. i'll bet, and this is just a hunch from experience, that riding that plane is gonna feel a lot like riding an old-fashioned train - not one of them smooth riding bombardier cars - where you see people swaying as they stand.


No, i'm not kidding. ^_^; Just look at the videos of fighter pilots and tell me they got a smooth ride.

i never seen a x-29 fly, or spoke to anyone that flew one - but i trust the math, not what the pilots paid by the company testing the plane say. Most elevators have pretty sophisticated computer control systems, and you sure as hell feel the starts and stops in those, ya? And that's an elevator - it only goes up and down, and only in predictable ways. Computers don't make the motions go away. They either smooth them out (as in an elevator or civvy planes), or try to eliminate any effect they have on the flight path (as in a military plain). But you still feel them either way.

(And forward-swept wings aren't that unstable. In fact, they're even makin the next Cessna with forward swept wings. ^_^; )