posted on Jun, 18 2015 @ 01:44 PM
a reply to: TAGBOARD
I have a question that maybe should have been asked a year ago: why is the aircraft that is the subject of this discussion referred to as a
"spanloader"? I'm asking this as a professional aerospace engineer. The only thing we really know about it is that it seems to be a delta all-wing
planform with a slightly concave trailing edge and a wing sweep angle that appears to be slightly more than 45 degrees. This would imply that it is
intended for transonic cruise (around Mach = 0.9).
The term span "loading" refers to a load that is distributed in some manner along the span direction of the wing. I've seen this used to describe two
One idea is the aerodynamic loading; this is the lift (and drag) force exerted by the air flowing over the wing. It has long been known that the most
aerodynamically efficient aerodynamic span loading is elliptical. For a straight wing, the easiest way to achieve this is to keep the airfoil section
and angle of attack constant and make the wing planform elliptical. The classic example of this is the WWI Spitfire.
In today's world of computational fluid dynamics and manufacturing with composite materials, it is possible to design an airplane where the chord
size, the airfoil section, and the chord angle of attack are all varying in a complex manner as you move along the wing span in such a way as to get
an elliptical lift distribution even though the outline of the wing doesn't look anything like an ellipse. Bob Liebeck's Boeing X-48 is a good
Boeing X-48 Blended Wing Body
I assume that whoever built this West Texas bird would do as much of this span distribution of lift as they could to try to approach an elliptical
distribution. In fact, the concave trailing edge could be considered an attempt to control span-wise lift distribution.
The other way the term "span loading" was used when I was in Grad School was to refer to the span-wise distribution of weight along the wing. From a
structural viewpoint, an airplane has traditionally been seen as a fuselage (where most of the weight is concentrated) with a wing (where most of the
lift is concentrated) cantilevered out to one side. In this model, the lift on the wing is trying to bend the wing and break it off at the root. So
the wing spar (usually triply redundant) has to be built strong enough so that it won't reach the yield stress of the material it's made of, even
under the worst case lift load + margin (a minimum safety factor of about 2.5). Since the wing spar is usually the single heaviest piece of the
airplane other than the fueselage, it pays to try to save weight in the wing. One way to do that is to move as much of the wing weight as far out in
the direction of the wing tip as possible. That could include fuel, engines, and cargo, when appropriate. That gets to be real problematic for
swept, tapered wing designs since there's not much real estate out there where you can put heavy stuff, but can find application in specialized cases.
For example, the solar powered aircraft NASA Pathfinder could be considered a span-loader, because both the solar arrays and the drive motors are
distributed relatively uniformly along the span:
I'm not ranting here (at least I hope I'm not) and I'm not even suggesting that we change the terminology for this sighting. I'm just noting that we
seem to be using the term "spanloader" in a non-standard way--unless there's something I'm missing, here.