Yes, it’s really just that a stable airborne object must have the CG in front of the CP.
A taildragger taking off isn’t airborne yet, though. There’s still weight on the wheels. While that’s the case, you have to lift the tail to lower the wing’s angle of attack and get it on the forward side of the power curve.
That part I pretty much grasped already. Is there really no way to meet that requirement without producing downforce from the horizontal stabilizer?
Since stable flying wings exist, it must be possible to fly with no stabilizer.
You can produce up-force with a canard instead of down-force with a stabilizer.
Flying wings are swept, so that the net CP is farther aft than if they were straight (that helps reduce transonic drag, too, which is a different topic). A typical subsonic wing airfoil will have the CP at about 25% of chord, or roughly at the point of max thickness, so with a straight wing the CG must be well forward of that in the absence of any other surfaces’ effects.
Yeah, I meant the H stab in a conventional layout must supply downward force (nose up moment was where my train of thought was)
In a flying wing, some part of the wing which is aft of the CG must serve this function. If the wing is swept rearward, the tips can be twisted LE downward (washout) to do this. This also improves spin resistance, and is commonly used on conventional aircraft. Alternatively, reflexed airfoils can create large upward lift toward the front of the wing, and a small downward force at the back of the wing.
http://www.mh-aerotools.de/airfoils/nf_3.htm
Oh, and I am embarrassed to have neglected to mention the ME-163 Komet as a fine example of a fast, efficient, completely manually controlled flying wing. Prior to the X-1 it was the fastest manned aircraft. They reportedly handled quite well when you could keep them from spontaneously exploding or the fuel from dissolving the ground crew.
I guess I don’t understand why we’re talking about the characteristics of the wing in isolation. The airplane is a solid structure. If the CP of the entire airplane is aft of the CG, what difference does it make if the wing CP is ahead of the wing CG?
I can understand that that might require a stronger, stiffer, heavier wing, and that the engineering tradeoffs favor lightness, but that doesn’t make it impossible.
If the wing’s CL is ahead of the CG, then it tends to pitch the nose up increasing the angle of attack (AOA)which causes more lift, which exacerbates the situation.
In a stable, conventional airplane, any excess lift from the wing will tend to pitch the nose down, decreasing the AOA, relieving the excess lift, and will also tend to increase the negative AOA of the H stab which will tend to restore the AOA that existed prior to the disturbance.
Extending your train of thought: Because the wing and H stab normally operate with opposing incidence angles, the effect of AOA is that the CL of the airplane as a whole moves in response to AOA changes, and that movement tends to produce stable flight in pitch.
Suppose I built a plane with the wing CL behind the CG, and the stabilizer at a positive AOA. Excess lift (if I’m understanding that correctly) would cause the wing to want to pitch down, but that pitch change would decrease the lift of the tailplane and also tend to restore the original AOA.
I can see that the horizontal stabilizer would need to be more responsive to changes in angle-of-attack; that for a given AOA change, the tail would need to have a proportionally greater increase or decrease in lift for it to act as a restoring force to return to level flight.
I can also see that a tricycle-geared airplane would need the tail to produce downforce, at least momentarily. At rest, the CG has to be ahead of the main gear to keep from tipping over backwards. At rotation, the tail would need to push downward to pitch the plane upward for takeoff.
So what is it I’m not seeing?
Look at a pic of a B-2 and you’ll see the bulges for the crew/bombbay area and each set of engines. The wing is relatively thin otherwise. If you plan on carrying passengers or cargo inside a larger area of the wing, the wing will also have to be thick in those areas to accommodate that.
A thick wing cannot go as fast as thin wings (like a normal airliner) so there will either be speed limits, or it will need a more thrust than a normal airliner to overcome the drag. This would hurt fuel economy, requiring more stops, or larger tanks, which would make for a thicker wing, etc (compounding the problem).
Silly question but doesn’t it come down to $s - at least for cargo?
If a wing design could somehow be shown to be considerably* more cost effective given some measure like tons of cargo X miles hauled it wold probably be used.
*“considerably” meaning more than enough to offset production and logisitc (and other thing I’m not thinking of) costs.
Just posted on YouTube: The Story of the Flying Wing. Neato 15-min film from Northrop on YB-49.
** Zombie reply **
Yes, it definitely comes down to $$.
If FedEx, UPS, Atlas, Kalitta, and all the other cargo operators worldwide were able to make more money from a dedicated cargo-only flying wing/blended-wing-body, even after paying for all the infrastructure changes necessary for such a change, they’d do it. But there are so many roadblocks to such a path that unless passenger airlines were to make the change, it’s not likely that cargo airlines would make the change either.
The vast majority of the commercial aviation world, from engineering & materials science to airport layout, and almost everything in-between, has been optimized to support the “tubes with wings” aircraft that we’ve been designing & building for ~70 years. A relevant example is the IATA-standardized Unit Load Device (ULD).
There are some very practical reasons involving loading and unloading, which have to be quick for a cargo operator. A tube you can get at from the sides or even ends is straightforward, but how do you easily get at the cargo holds on a flying wing? You could design something that loads from the bottom like a bomb bay, maybe, but that means losing interchangeability with any other kind of container and aircraft. It’s bad enough making *passenger *doors accessible that way, in the quantities you need to meet emergency-exit requirements.
The aircraft industry regularly looks at how to make a specialized freighter economically, but the quantities and economies of scale just aren’t there. There’s only a small market for new airplanes that get made with wide doors and cargo decks, and a much larger one for converting cheap, depreciated, used-up airliners - the low acquisition cost makes up for higher fuel burn and maintenance. To make a new high-efficiency airplane economical requires fuel costs *much *higher than can be plausibly predicted, and if on-ground operating inefficiencies are built in, the break-even point is even higher.
Another reason I have not seen mentioned, for passenger flights, terminals are not designed to accommodate the larger wing spans and how it would be much closer to the jetways.
Modern military jets are inherently unstable. With the notable exception of the MD-11 (and possibly the TriStar), civil passenger and freight aircraft *are *inherently stable (“positive stability”). Maneuverability is far less significant for civil aircraft than safety.
To turn the question on its head then, given all of the limitations, why does the B-2 bomber use a flying wing design?
I don’t understand what makes a flying wing harder to load and unload than a conventional shape. A conventional fuselage is a long, narrow shape. A flying wing is also a long, narrow shape, just oriented a different way. Why can’t you put a ramp on the left or right end of the wing, just like you put a ramp at the back of a conventional plane? Or for passengers, doors and windows along the front surface, instead of along the sides?
Another point is that if you put your engines on swivels, you can easily change the sweep angle of a flying wing, to get the optimum sweep for a variety of different speeds.
Trouble is, neither putting engines on swivels nor active alteration of wing sweep angle are in any way easy to accomplish. If built to acceptable reliability standards, they would add considerable weight and cost.
AIUI, to reduce its radar profile.
At the time it was designed, and with the materials available at that time, a flying wing with no vertical tail surfaces had the lowest RCS.
Existing airport infrastructure is designed for the passenger cabin to be on the aircraft’s longitudinal axis. For flying wing or blended-wing-body aircraft to be used in passenger service, every gate, at every terminal, of every airport, would have to be redesigned and rebuilt to accommodate them. Calling this a “significant expense” would be an understatement.
To put passengers and/or cargo spanwise in the entire wing, the wing would have to be several feet thick across it’s entire span. This is highly inefficient, and severely limits the aircraft’s speed. The weight of the structure required to support passenger/cargo doors or ramps all the way out at the wing tips would also be prohibitive.
If pax & cargo are in the wings, where is the fuel going to be stored?
Variable-sweep wings were attempted (B-1, F-111, F-14, and some others), and had a reasonable degree of success. But they all suffered weight penalties and maintenance headaches dealing with the structure & mechanisms required for sweeping the wings. The F-111 had two weapons pylons on the wings that swiveled as the wings swept, to keep the weapons aligned with the airflow under the wing. I can’t find a cite, but I’ve heard from people who used to maintain F-111s that those swiveling pylons were a major maintenance headache. No current designs I’m aware of incorporate variable-sweep wings - the current state of the art in aerospace engineering, aerodynamics, and materials science has negated the need for such a system in today’s aircraft designs.
There were these guys in Ohio who were experimenting with flight. They were bicycle guys, who had a unique insight that seemed to be eluding just about everyone else at the time. You see, when riding a bicycle above about 10 mph, making a turn means you have to lean into it: other engineers were thinking, oh, to turn, we will just turn that way, like in a carriage.
So these guys designed a biplane glider that used “wing warping” to force the glider into a turning roll. They took it out and tested it, only to discover an effect called “adverse yaw”. When banking for a turn, the upper wingtip encounters higher drag, in part because it has to move faster than the lower wingtip. This means that the wing wants to turn the wrong way. They corrected for this by adding a rudder.
An aircraft wants the rudder to be offset from the wing. More is usually better, unless you are stunting. So you will still want a rudder for turn control. You could do this with engine differential, but jet engines in particular do not like to wind up and down, they like to run steady all the time.
Once you get to adding a nice rudder, the flying wing idea starts to look inelegant. I mean, why not just put a full fuselage in there. For most applications, it just makes no sense. Instead, start exploring lifting bodies, if you want to find more practical theory. There might be something of value there.