You will, with optimization of all the variables. be lucky to get 40%
of the lift/drag ratio of an equivalent conventional planform.

Can you elaborate? I don't see why this should be true.

Well, let's see. The back wing(s) operate in the downwash of the
forward wings, there's a hit there. The upper wings operate in a flow
field affected by the lower wings, there's a hit there. Twice as many
wingtip vortices, take a hit there, and at some angles of attack, the
aft wing(s) will be operating in the vortice of the front wing(s).


But the bigger problem will be control. Pitch stability, in and out of
ground effect, will be a formidable problem, as will stall
characteristics. Compromises needed to make the handling acceptable
may make the efficiency even worse.

Well, most tandem wing aircraft are designed to make normal stall
impossible. (the rutan designs for instance)

There is a price paid in efficiency, and in landing speed in making
this NECESSARY trait possible. It's necessary because a canard or
tandem wing design is very vulnerable to an un-recoverable deep stall.
The consequence is that you cannot optimize the angle of attack for
both wings simultaneously, and that the C ell Max of the combined
system is degraded, making the landing speed higher, or the wings
bigger (which will hurt efficiency AGAIN).

Pitch stability is a problem
that I thought had been pretty well handled by airfoil design in canard
aircraft years ago. My thoughts (I wouldn't call it a design) are simply
two sets of biplane wings mounted fore and aft. Biplane wings don't
normally present much of an efficiency problem except for the bracing which
isn't stricly necessary (The hyperbipe was a pretty efficent design)

Pretty efficient for a biplane, but nowhere near as efficient as a
conventional design.

I certainly agree that handling especially in the pitch axis is the major
challenge, but I don't see why it should present a much bigger problem than
the flying flea family of aircraft where it was eventually solved
satisfactorily.

Again, by limitations that hurt efficiency. And a good half-dozen
people died before the pitch stability issue was solved. That was
actually a problem at cruise/top speed.

The transition between operating in ground effect and out of it is
pretty tricky for a equal area tandem wing airplane. This was seen in
some of the first experimental Wing In Ground effect surface skimmers.
They had tremendous pitch stability (a problem if you're trying to
rotate) until they suddenly didn't, and they would pitch up quite
violently. That's one reason the Quickies have ANHEDRAL on the forward
wing, and Dihedral on the aft wing, as well as mounting the forward
wing lower than the aft wing. In this way, with a pitch up to rotate,
both wings come out of ground effect at much closer to the same
instant, without a sharp pitch divergence.

I agree with Ernst - a low aspect ratio delta/lifting body makes more
sense. Perhaps a 2 seat Facetmobile with the outer portions folding
inward like a Dyke Delta.

Perhaps, but it's been tried many times and with very limited success.

Various low aspect ratio designs have been flow since the twenties,
it's true. The Burnellis, the Spratt, the Fike designs. The Dyke Delta
is a low aspect double delta, with the main cabin airfoil shaped. But
true lifting bodies were basically unknown until the 1960's. John
McPhee wrote about one of them in "The Deltoid Pumpkin Seed". The the
Facetmobile is, IMHO, the most successful general aviaition true
lifting body design.