Following Waynos’ proposal for the S-STOL (super-short take-off and landing) strike fighter, I have endeavored to research what are commonly called
“Bi-diamond”, “joined wing” and “box-wing” configurations.
The big advantages of the joined wing concept lie in it's ability to vary aspect ratio independent of fuselage length/stiffness at 'the roots' and
of course overall form/friction drag.
In /some/ designs, the wings cut the air at separate AOAs and thus allow you to design some pretty unique, 'paired', critical lift regions which do
not require the massive energy losses of vortice lift (the chord being so small that it's really more penalty than gain in terms of structural
Combined wings, in the most basic sense, are not unlike bi-planes; two wings couple together. We are particularly interested in configurations which
combine swept and forward-swept wings.
I have looked at these as an alternative to a high aspect ratio flying wing in a VTOL application for a small ship launched, high speed, UAV. They
offer some stability advantages IF you can make them 'variable geometry' to the extent that the wings either collapse like a scissor hinge or
decouple to slide up an external fuselage (chine) track or groove for deckspot factored as well as potentially cruise-Mach reasons (I have also
considered a rotary hinge for the fuselage itself).
The way I made mine, there was also a reasonable gain in propulsion efficiency by returning to an L+LC, dual-engine, configuration which effectively
used microejector technology along the wings to maintain a constant low pressure (think hovercraft) 'lift curtain' around the two principle lift
posts without an auxilliary roll duct system.
The forward engine essentially deactivates for cruise flight.
Given it was designed for a 'pure' ISR role, independent of landbased or big deck air, I still preferred to put the majority of the fuel in the
wings and foreward/aft primary fuselage tanks while retaining the centerbody for a modular EO/Radar based sensor suite that employed a stressed
composite (pressurizeable) conformal canoe radome along with endfire systems in the nose and tail.
Sizing an antenna suite to any particular bandwidth/dipole array functionality makes for difficulty in rewiring the system to other functional
roles/look angles at a later time and also adds weight outboard where it doesn't need to be.
The 'hollow' of the centerwings in this case makes it simpler to get to the dorsal hatch cover of the center mounted avionics systems on a vibration
dampened ready-raft set of fuselage beams that allows for the entire radar/EO head to be replaced as a single unit without further sealant
These types of configuration are increasingly popular although we have yet to see a production aircraft with them. They are however quite a recent
idea: The earliest joined wing concept I could find were two essentially similar ones from 1986 by a forward thinking aviation student. His design
centered on a novel twin fan VTOL arrangement and he makes scant mention of why he chose the joined wing:
Note that he places the leading wing lower and the aft wing higher –an arrangement which is consistent throughout my research. Waynos’ design
however reverses this, which we hope will improve high angle of attack capability because the leading wing will not mask the rear one.
You don't want to 'pod up' the wing because it's stiffness:weight advantage lies in the way it does not have to carry-thru on a fuselage lapjoint
or ringframe. I prefer slightly different wingsweeps for the forward and aft airfoils and a built in an/dihedral join differential that washes out to
a common tipcontrol plane.
With care there isn't too much worry about shock migration or blanking off the lead airfoil but you don't want them to have aeroelastic or vibration
harmonics in a common axis of motion or what bothers one will couple and harmonize to the other. The easiest way to assure that is to keep them from
biting the same air.
The best known combined-wing concept is probably Lockheed Martin’s future tanker/transport design featuring a box-wing:
Perceived advantages of this configuration are primarily that it can accommodate two refueling booms (on the wing tips) and marginal (?5%?) induced
drag advantages. Another advantages cited around box-wing designs (i.e. wings joined by endplates, typically without dihedral) is that they offer good
Generally speaking, good stability is not a desirable characteristic of a fighter though (opposite to agility, within reason).
Good fighters are never seen to be engaged and the gutted-skate design with planform and vertical silouhette reduction does admirably at both. Great
fighters have DIRCM which inhibits the effectiveness of 'dogfight' (as an exercise in agility dynamics) defined engagements by missiles. /Average/
fighters with good (sustained) EM performance can defeat 70% or more of all gunshots.
What else needs be said?
Joined wings with distinct anherdal/dihedral (producing a diamond shape when viewed from the front) offer greater potential for maneuverability
because the control surfaces could be used to push the aircraft sideways. Within certain maneuvers, this is considered super-agile. However they
certainly aren’t the only configuration which allows for super-maneuverability.
Stiffness at /weight/ defines the form by simplifying the amount of structural commitment to pylons and integral fuel cells and general twisting
moment structural paths needing stringer/spar 'connective tissue' reinforcement.
However; those same things also limit the absolute loads inherent to having a nice, thick, root:tip taper and _lift_ inherent to a more conventionally
Now consider what you do when, even assuming the monkey-as-hood-ornament is gone, you have to stuff ALL your weapons system (12ftX3ft weapons carriage
box), gas and sensor/propulsion weight into the primary fuselage. Even if it can be considered a pancake (Tomcat) equivalent lifting body, you lose
most of your structural benefits by having to go back to some kind of a very wide root, probably mounted well aft of the nose.
Direct Sideforce/Liftforce aside, I would never put the joined wing in a fighter classification.
Combined-wings in general also offer control surface redundancy which could be translated into better combat survivability, and potentially lighter
wing structures for equivalent lift/area (the joining providing rigidity allows lighter construction).
I think that survivability at least comes under the 'P-38s are harder to hit because they don't have a conventional fuselage centerline as
aimpoint' category of too-drunk-to-be-scared talk. The joined wing cannot sustain /any/ damage to ANY of it's airfoil group without essentially
destiffening the entire structural interlock. Since we are now dealing with missiles that throw out proximity fuzed fragment and blast kill
mechanization on the order of a dozen times more total lethality than a WWII gun system, the reality remains pretty high, IMO, that you just _would
not want_ to expose this aircraft to any kind of seriously strenuous as much as damage-intensive combat environment. Because you load a thin wing and
then cut it at any given point, and it will fold completely. While there just would not be any 'field goal' (AC-119 over the Trail) type misses by
something that came close enough to fuze.
What might be a little more interesting is the ability to couple and decouple descent rates and forward airspeed in the approach to a hover. Somewhat
like a combination of the F-15 SMTD nozzle effect (speed brakes on the forward fuselage) and the F-14 DLC spoilers (tip controls). Something which
should be a precondition of jetlift STOVL off of small decks without rotating the ejector cushion off of a level axis in achieving very 'repeatable'
approach behaviors. In this, the hollow wing will likely have limited gust response and there are no tails so about the most you might need would be
puffer nozzles at the joined tips.