Theory of the forward Swept Flying Wing

Consider the following:

A note from Bill Daniels:
Re: Marske Pioneer IId
Date: Tue, 13 Nov 2001
From: "Bill Daniels" <wdaniels@qwest.net>
Newsgroups: rec.aviation.soaring

During the flight tests of the Marske Pioneer 1 and 1A prototypes in the
late 1960's I flew the glider configured with a variety of CG positions to
determine the qualitative flight characteristics. Maybe the following will
help folks understand the Marske flying wings and explain why there are no
demons lurking in the stability and control characteristics of these unique
gliders.

At no time did these gliders ever exhibit any tendency toward stall/spin
departure. Regardless of the CG position, the up-elevator authority is
limited to less that required to stall the wing by the fact that the airflow
over the inboard wing separates ahead of the elevator as the wing approaches
the stalling angle of attack.

At one point, there was a concern that they might "tumble" end over end, but
this is impossible unless the CG is very near 50% MAC.

It is correct to say that any flying wing has a more limited CG range than
conventional gliders which is why gliders are probably the best application
of the flying wing concept in that the CG does not change during flight and
sailplanes are not required to carry cargo.

However, it is worth viewing the allowable CG range in two ways. One is the
CG range that optimizes performance and the CG range that provides
acceptable handling characteristics.

For safe handling, the allowable CG range is surprisingly wide.

For optimum performance, there is no range at all. It must be perfect for
the flight regime. This has led to a call for in-flight shiftable CG.
There are substantial performance benefits to running at high speeds with
the CG forward and thermaling with it aft. CG shifting serves the same
function as performance flaps.

If the CG is moved forward, the up elevator authority is limited so that the
minimum airspeed is increased. This is not unsafe unless the minimum
airspeed is so high that takeoffs and landings are difficult.

As the CG is moved back, the elevator authority is increased and the minimum
airspeed is decreased. The elevator forces also became lighter until, at
some aft CG location, the glider would not trim to any particular airspeed
and was neutrally stable in pitch. It is surprisingly easy to fly in this
condition but it does take more attention to pitch attitude. This is the
most aft CG location at which the glider should be flown. Moving the CG
slightly aft of the neutral point results in static instability where the
airspeed will tend to diverge toward a minimum or maximum depending on the
initial conditions. The glider is still quite flyable but the pilot
workload is high. The safe handling CG range, depending on the airfoil
chosen, is usually something like 17 - 24% MAC.

The above discussion addresses static pitch stability. One must also be
concerned about dynamic pitch stability defined as any tendency of the
glider to support oscillations in pitch. The Pioneer I and II series
gliders would, if the CG was forward of the neutral point, exhibit undamped
Phugoid oscillations typically at 17 a second period which is pretty typical
of all gliders. It is worth noting that phugoid damping is mainly due to
the overall drag of the glider and not to any "fly swatter effect" of a
fixed stabilizer on a long tail boom. (This is easily demonstrated with any
glider of medium to high performance by observing the phugoid behavior with
the spoilers open and again with them closed.)

They would also exhibit a highly damped resonance in pitch at about 2Hz.
This resonance was sufficiently damped that PIO's were not a problem. I
believe this high frequency resonance/damping is largely a function of the
moment of inertia around the pitch axis. The greater the mass distribution
around the pitch axis, the more likely this will become a problem.

It was an important observation that, as the CG was moved aft toward the
neutral point, the tendency toward dynamic instability decreased because the
restoring forces that support oscillations were also decreased.

It seems to me that most flying designers, with the notable exception of Jim
Marske, seemed to fear the pitch characteristics and compensated by
designing in far too much static pitch stability in the form of severe
airfoil reflex or, in the case of swept flying wings, excessive wing twist.
This resulted in dynamic instabilities that made these aircraft very
uncomfortable to fly. Put another way, for best handling, the static pitch
stability should be reduced to match the available damping. This also tends
to be nearer the best-performance CG.

I hope this helps.

Bill Daniels

----------

Forward sweep eliminates the need for washout

At low speeds most gliders have tremendous induced drag necessitating long wing spans with high aspect ratios but notice the very short elevator on the tails of most gliders, it is short, narrow and at right angles to the rudder, a prime suspect in the elimination of induce and interference drag in a sailplane. Why not put the elevator on the rear part of the main wing where it adds to the reynolds numbers, eliminates the interference drag where the rudder intersects the elevator and eliminates the weight of the long tail?

Weight is one of the determining factors when considering sink rate and airspeed. With a low sink rate of about 100 to 150 ft. per minute soaring can be done on what others consider marginal of no fly days. The ride is usually smoother and this occurs in the late afternoon and evening when most people have finished work. With a slower speed one can also turn in smaller circles.

Note from Bill Daniels:

The difficulty in comparing the tailless concept with the current state of
the art is that conventional tailed gliders benefit from nearly 100 years of
refinement and thousands of designs. On the other hand, even though the
tailless concept began in the first decade of the 20th century, only a
handful of tailless gliders have actually flown. No doubt if tailless
gliders had the benefit of the development history that conventional gliders
have, they would be far better than they are.

This is not criticism of tailless concept, just a statement of the facts.
Few designers want to step far beyond the current state of the art so they
copy most details of their previous designs, or those of perceived
competitors, adding a few design changes they hope will give their creation
an edge. This, in my view, has led us into a trap of diminishing returns.
In other words, the current design concept is very close to optimum. There
are no obvious opportunities to significantly improve the current design
paradigm.

Jim Marske's approach may offer a way out of this trap. Looking carefully
at the performance of the existing marske gliders one finds that they do
perform better than anyone would expect given their low aspect ratios and
wing loadings. To really answer the question, we need an all out,
composite, 15 meter tailless racer, built to the best standards, not wood
and fabric homebuilts. Contest scores will then tell the tale.

I think we all should tip our hats to the bold few who experiment with
flying wings. Put a brake on the criticism and enjoy the color and
diversity these designs bring. Jim Marske and Matt Redsell aren't heretics,
they are just experimenters having a lot of fun with their concept. I get a
lot pleasure watching their progress.

Bill Daniels

Keep in mind that swept forward and swept back flying wings are very
different craft. As different as canard and tail-aft aircraft. Swept back
wings are good if you are going to fly in the high Mach numbers. Swept
forward wings seem to offer some advantage for soaring flight. Each has a
different set of advantages and problems.

Swept forward wings with the elevator on the inboard trailing edge does
indeed increase the elevator moment arm. Forward sweep moves the center of
lift forward which requires the CG to be moved forward lengthening the
distance from the elevator to the CG.

The swept back wings of Horton, Northrop et. al. all had problems with
spanwise flow thickening the boundary layer near the wing tip elevons
leading to tricky handling at slow speeds unless the CG was well forward.
The forward CG, in turn, limited performance at low speeds.

It is true that a wing with substantial forward sweep will not permit the
use of winglets, but the swept forward wingtip itself accomplishes some of
the desired effect anyway. (At least one Pioneer II had winglets installed
but I have never heard how that affected handling.)

To summarize a previous post, flying wings of all sorts present a whole new
set of issues that need to be sorted out. To date, the limited knowledge
base related to these issues makes flying wing design a challenge. This
doesn't say that, when sorted out, the flying wing sailplane might not have
a significant advantage. Somebody just has to do the work.

I'm very glad that Akaflieg Braunschweig, Jim Marske and others are working
on the problem.

All else equal, swept forward wings may pitch up at
stall and swept back wings may pitch down. However, if the wing is tapered,
that can cancel the pitch up with forward sweep since the narrower chord is
likely to stall before the wider inboard section. This relates to a tailed
aircraft whose up elevator authority can bring the wing to a stalling angle
of attack.

However, (again) if the elevators are part of the inboard wing, you aren't
going to stall the main wing since, as soon as the flow separation starts on
the inboard upper surface, the up elevator authority is suddenly limited so
that a stalling angle of attack can't be reached anyway. Marske flying
wings will slow down and stabilize just short of a stall with the stick all
the way back. Trying to get one to fully stall or spin is a waste of time.

Bill Daniels