Wind Tunnel Testing: During
1987 -1991, Seair conducted exploratory wind tunnel tests on 1/20
scale models of several wing, body, tail, and endplate combinations.
This testing was accomplished in a small research tunnel at Aeronautical
Testing Services of Arlington, Washington. Data from these tests
were used to develop practical aerodynamic prediction methods, identify
viable configuration options, and to help understand various ground
effect phenomena. Wind tunnel tests helped determine several viable
tail and endplate shapes from more than a dozen options. Wing pressure
measurements were used to assess the influence of several airfoil
section shapes and flaps on takeoff lift and cruise height stability.
The wind tunnel program employed several measurement methods to
gather a range of data:
- Longitudinal force and moment data
- Off-surface and downstream wake survey
- Flow visualization (tufts)
- Wing and ground plane pressures
- Free-to-trim pitch-heave dynamics
Some the data obtained over eight tunnel entries
was the first of its kind collected on WIG's anywhere (and some
may still be unique in 2001). Pressures measured on the ground-plane
under a WIG wing with endplates graphically showed why the ride
of some WIG's has been described as feeling like that of a large
luxury car with soft suspension. A hovercraft must support itself
on a short cushion of air bounded by the skirt around its hull,
and a hydrofoil is supported only at its ends on stiff struts. In
contrast, the experimental data showed that a WIG's supporting cushion
extends well forward and aft of the wing. Its total pressure "footprint"
on the water (or in this case on the tunnel ground-plane) is three
to four times the length of the wing chord. The length of the WIG's
air cushion acts as the aerodynamic equivalent of a long wheelbase
that smoothes out the bumps when the craft is "platforming"
over a choppy surface. Just as the craft exerts a force on the water
surface well out in front of the wing, oncoming waves feed a pressure
force back to the wing of the craft allowing it's motion a slight
"lead" on the shape of the surface when "contouring"
at an angle to long swells.
From 1987-1996 Seair Craft Inc. conducted
extensive outdoor testing with 1/5 and 1/4 scale powered models,
and two small hand-launched gliders. Hull planing characteristics
were studied with towed and radio controlled models.
Just as the wind tunnel models provided insight
into the aerodynamic lift and pitch stability, the first generations
of towed-hull and R/C model tests provided an understanding of the
hydrodynamics. We found that traditional seaplane float and boat
hull design guidelines could be used to build WIG endplates and
hulls that had good "boating" characteristics, but these
were not necessarily compatible with the aerodynamic requirements.
Conversely, the lowest drag and highest lift aerodynamic shapes
had very poor hydrodynamics. These were difficult to get over the
"hump" and onto plane, to unstuck from the water, and
could have bad "slamming" characteristics in choppy water.
When properly executed, the reverse-delta planform
of Lippisch type WIG's has been shown to offer good height and pitch
stability, but the arrangement is inherently larger in width x length
for a given square footage of lifting surface. Besides being difficult
to build compactly, the wing geometry is potentially more difficult
and costly to build than a rectangular planform. The simple tandem
rectangular wing arrangement favored by Jorg is perhaps the most
straight-forward and results in very fast cruise speeds over smooth
waters. We were concerned that the tandem wing craft did not seem
to be very robust in stability or takeoff performance when in choppy
waters or gusty winds.
We initially investigated single-piece rectangular
wings with simple trailing edge flaps and flat-bottomed or S-shaped
airfoils. First results were disappointing. After performing computational
fluid dynamics (CFD) on nearly 50 airfoil modifications, we arrived
at a family of custom hybrid airfoils and wing planforms that provided
reasonable pitch stability, lift, unstick from the water, when combined
with a specially designed center hull and sponson/endplates. We
a preferred general arrangement formula consisting of a mid-mount
low aspect ratio main wing, coupled with a high-mounted, moderately
tapered horizontal tail surface of high aspect ratio. The horizontal
tail is supported and stiffened by twin vertical fins (in the case
of single engined craft) or a single centerline tail behind side-mounted
twin engines. Although most testing was conducted with open props,
the preferred design employs shrouded props or ducted fans. The
resulting combination enables takeoffs without the use of auxiliary
engines or PAR devices at static thrust-to-weight ratios or less
the 1:4. The horsepower to weight ratios are comparable to efficient
PAR WIGS. This makes the complexity of powered lift or hovercraft
type air cushions unnecessary (unless amphibious capability is desired).
Material on this website
is copyrighted ©2000 by C.P.Nelson, Seair Craft Inc.