Note: While
this article appears under my byline I must emphasise the text was co-authored
with Jean-Luc Foucher and the beautiful pictures were a collaboration with
Frédéric Hours. My deepest thanks to them for their participation in the
creation of this article. — PR
Introduction
Iam taking
my pen today to tell you about a nice project led by two longtime friends,
Frédéric Hours and Joël Carlin who are both F3F competition pilots. This
project was born three years ago and the first flight — I was lucky enough to
attend — was made at the end of May 2020, at the end of the first lockdown. But
let’s not rush the steps and let’s start by talking a little about the genesis
of this glider.
A Simple Yet Ambitious Idea
The initial
idea was to design and build a large, electrified F3F glider with a 5m wingspan
and a semi-scale look. Intended for slope and mountain flying and eventually a
little GPS triangle racing (GPSTR), it will be equipped with a motor in the
nose — a front electric self-launch (FES) system. While being discrete it will
enable going up in case of weak air. This is a key point in my opinion if you
want to fully enjoy this kind of glider without risking it. Elevation in the
mountains is sometimes important and it is not rare to go down low and fast
without necessarily having the possibility of landing safely. The motor is
beneficial as it allows to push the search for the lift a little further or a
little longer.
The wings
will be built from a vacuum laminated core while the fuselage would be
constructed using an existing homemade mold, to be determined later in the
process.
It remained
to define which model specifically. The type of construction of the wings
imposed a multi-panel wing requirement. Joel and Frédéric reviewed the
different models on the RC glider market such as the Diana 2, or
the AN66 GPSTR type or the large gliders such as the ASG-29 and ASW-27 for
which it is quite easy to find a 1/3-scale mold. The choice was quickly made
for the ASW-28 in its 15m version. It offers the possibility
of a wing with a reasonable aspect ratio of 19.5 while also allowing the use of
a thin airfoil. It also employed the geometry of trapezoids to approach an
elliptical lift distribution while remaining close to the wing shape of the
full-size glider. Note however that the model has six wide chord control
surfaces and not just ailerons and airbrakes as found on the full-size
aircraft.
Some wing
geometries of well-know RC gliders of the same wingspan and type.
Once the
geometry of the wing was defined, our two friends called upon Jean-Luc Foucher
— the father of the Alliaj HM, but also of the Prodij HM by
Aeromod, the Pingouin F3F and the Cosmos F3F — to
design the airfoils of this new 5m glider. Jean-Luc accepted the challenge and
what better way than to tell us about it himself, so I’ll pass him the pen.
Aerodynamics (Jean-Luc Foucher)
Aerodynamics
is just an exciting hobby for me and is treated as any other physics topic. I
employ my experience in fluid dynamics related to various topics such as: F3F,
dynamic soaring, 3D helicopter blades, racers and windfoils. By experience we
address three main phases: objectives/constraints, flight mechanics/specs and
airfoil computation.
Lift and Cz
repartition along the wing.
Objectives and Constraints
The first
objective is to achieve an ‘F3F-like’ behavior. This means
maneuverability, acceleration, turn efficiency and a wide range of speed. This
also means a speed range from 15m/s to 50m/s, and the highest turn Cl/Cd ratio.
The second objective is to increase the overall soaring capability compared to
F3F. One constraint is to have very ‘friendly’ behavior with no hard stall so
that it can be used by non-expert pilots. The second constraint is to keep
the ASW-28 shape leading to an airfoil chord (from 77mm to
308mm), mass range from 9kg to 13kg and wing area of 117dm². And finally, the
third constraint is to have at least 11% of airfoil thickness at the wing root
for mechanical strength.
Flight Mechanics and Technical Specifications
For this
purpose general aerodynamic equations are used: one output is that airfoils
along the wingspan will have to be analyzed over a wide Re range —
from 1e5 to 1e6. Another output is a Cl range between 0 and around
0.7 all over Re range. For soaring we have to focus on Cl up
to 0.8 for lower Re, optimizing Cl¹.5 / Cd for
various speeds and flap configurations.
Airfoil and Wing Computations
Software
used was Profili for 2D design, then XFLR5 for
the 2.5D design mainly to check targets are achieved. Work has been performed
starting from F3F airfoils, as ranges of Cz and Re are
comparable. Then thickness at root has been increased to 11% and enhanced for
soaring purposes including the use of adequate flaps. Then airfoil adequacy
along the wingspan is computed taking into account: Re distribution,
elliptical shape of Cl at high wing lift with flap and efficiency
of ailerons for roll axis. To achieve the desired friendly behavior, one
important point is to get a smooth Cl / Cd curve, even
with flaps, to be more forgiving to wind and pitch changes.
Airfoil Comparison with A7026 Series
Finally, we
compare these airfoils to existing airfoils such as the very popular A7026
series; both airfoils being set at 11% for comparison purposes. A few
simulations show that these A7026 series are very efficient over a wide speed
range. Optimization looks to be performed around Cl = 0.4 and can
be extended to lower Cl using negative flaps. This means that A7026
series seems optimized for rather big and loaded scaled airplanes. A7026–1 has
a 2.55% camber as our camber is 2%. The two airfoil polar curves are comparable
when putting -3° flap at 25% to A7026, with a small advantage to our airfoil as
it has been optimized in lower Cl range. Putting a few degrees flap
to our airfoil, we stick to A7026 polar curves. Nevertheless, with some
positive flap to A7026 series, it becomes more efficient above Cl =
0.8 which is falling outside our objectives.
In
conclusion, despite the differences between these two airfoils concerning
camber and Cl area of optimization, I would guess there is not so
much difference in flight behavior: of course, using adequate flap (and
wing/fuselage angle setting) for the desired flight domain and taking into
account fuselage and tail drag, as well as induced drag for high Cz. So
again, the actual need and technical spec is of great importance in the design
process!
The
winglets geometry used, and 3D-printed on the ASW-28.
Few Words about the Construction (Pierre Rondel)
I take the
pen back to continue on the construction chapter, starting with the wings.
This is a
laminated core and vacuum construction with a spar. The first step was to do
some tests to determine the best skin layup. Initially, Frédéric did some tests
with a single layer of 200g/m² triaxial carbon fabric under the 50g/m²
fiberglass finishing fabric, but the weave always stood out, despite several
tests at different vacuum pressures.
This
approach was therefore finally abandoned, and the following solution was
chosen: one layer finishing fabric of 50g/m² fiberglass, then two layers of
160g/m² from HP-Textiles (see Resources, below) positioned at 45°
for better torsional rigidity. In practice, in order to optimize the use of the
fabric which was 1.50m wide, the fiber is in fact at 38° on the 2.50m of the
wing, to avoid the joins which are always seen under vacuum, what the tests
confirmed.
To avoid
deformations on the top surface of the wing, the lamination is done in several
steps, the first one being to laminate the top surface on the raw core, without
any insert, and using a mylar sheet (also bought from HP-Textiles) for a nice
surface finish. The core is simply a Knauf ‘small ball’ polystyrene.
Frédéric
then removed the material on the spar area to put six layers of 220g/m²
unidirectional carbon fabric on the top surface and four layers on the bottom
surface decreasing in thickness and length. Carbon works less well in
compression, so you have to use a little more.
The spar is
prepared separately and calculated according to an Excel spreadsheet created in
2003 by Jean-Luc Delort and available on a French website Les Grands
Planeurs RC (see Resources). The spar is calculated to
support acceleration up to 10G to 15G for a glider of about 10kg.
In green,
the spar.
The spar,
composed of two carbon soles joined by a composite made of two 50/10 balsa
planks, vertically oriented fiber, then reinforced with vertical fiberglass,
with a foam core in the middle.
The
structure of the wing root, with the joiner box terminating the spar.
The wing
joiner is a square section of 25mm x 25mm in carbon, with dihedral. The joiner
box is made of fiberglass plus Kevlar strands, with a 1.5mm plywood cap on top
and bottom. The complete spar and joiner box is then inserted in a carbon sock,
dry before installation and lamination ‘in situ’ in the wing. Note that the
gluing of the spar is not done under vacuum pressure, just a little weight to
press on it during the curing to avoid any deformation of the top surface.
The wing
and control surface spars are also made of 50/10 vertical fiber balsa and
fiberglassed on both sides. There is no spar on the last tip panel, because its
dimensions are too small to fabricate something correctly.
Once cured
we fabricated the bottom surface reinforcement, composed of four layers 220g/m²
unidirectional carbon fabric, and we finish with the bottom surface skin. The
leading edge is reinforced with a little micro balloons filler and silica.
The
fuselage comes from a homemade mold. However, the fuselage wing root had to be
modified to fit the wing section. The canopy was bought from Ulmer (link
in Resources). The winglets are simply 3D-printed, modelled on
the ASW-28's Maughmer tip.
Work on the
tailplane and on the fuselage, mainly the wing root to adapt to the wing
airfoil.
The tail,
which uses a TP29 profile at 10% to accommodate a 10mm servo, uses the same
manufacturing process, but this time using a TeXtreme 110g/m² carbon fabric
(see Resources) laid at 45° and a 50g/m² fiberglass surface layer.
A spar with balsa board 50/10 vertical fiber and 50g/m² fiberglass is employed
to make the connection between the underside and the upperside.
The locking of the wings in flight is made with a threaded rod of 6mm in the wing to which we mate in the fuselage with 3D-printed nuts.
The wings
finally came out around 1900g the half wing, the tail is 182g, so the weight
estimate is within the limits for a target of 9kg to 11kg empty with the
possibility to go up to 13kg to 15kg ballasted.
Motor Setup
It is the
most standard, using a direct drive motor for simplicity and avoid maintenance
:
A last
motor test, just in case of poor lift — but that didn’t happen!
This
configuration certainly does not allow climbing vertically but it is not the
goal! The objective is to have a climbing rate around 7m/s and to aim for three
or four climbs of 200m, which allows you to get out of a bad situation in the
mountains and save the glider.
For the
spinner, Frédéric and Joël contacted Georgi Mirov of GM Propellers (see Resources),
who kindly did some 3D simulations to select the best available spinner for the
shape of the ASW-28 nose. The result is quite impressive as
you can see in the sketches and on the pictures.
Propeller
simulation and in real life to compare the accuracy of the simulation.
Radio Installation
Not much to
report except for the dual reception with each half wing connected to a
receiver, each receiver having its dedicated 7.4v Li-ion battery (2S format
18650), while the elevator and rudder are in redundant mode, i.e. connected to
both receivers through the use of JETI ENlink 2RS plus modules
(see Resources). This module then provides the highest voltage and
best signal to the servo for optimum safety. The elevator servo is 10mm with
direct drive. Same for the rudder servo, both mounted in the fin. Three servos
per half-wing to avoid having a rudder longer than 1m for obvious torsional
rigidity problems and also to relieve the servos. Moreover, this enables more
complex mixes, for example in ‘butterfly’ mode for landing.
Radio installation in the fin and fuselage (lot of place), and radio
settings in the workshop before driving to the slope.
In Flight
I was able
to attend the first flight which took place on the beautiful slope of Corps,
south of Grenoble. The flight went perfectly, the CG was immediately correct.
The geometry of the glider was entered on PredimRC (see Resources)
beforehand to calculate the CG using a static margin of 2%.
Joël Carlin
ready to launch Fred’s ASW-28.
Despite its
5m wingspan and 8.5kg, the glider seems light and agile in flight, responding
like an F3F glider to the pilot’s commands, with the inertia and responsiveness
of a larger and heavier model. The stability is excellent; the glider is like
it’s on a rail in flight and is very precise.
Few
pictures in flight to appreciate the result of this nice project!
Acceleration
is excellent, and the energy retention in turns and other manoeuvres is
incredible!
Elementary
aerobatics are performed with ease and elegance. The six control surfaces allow
the glider to slow down on landing, making you forget the size. The rigidity of
the wings is satisfactory both in bending and in torsion.
Finally,
the presence in the air of the glider with its large fuselage and the curve of
the forward part is superb.
During
another flight session, I was able to take the radio transmitter and fly this
fantastic glider. The feeling on the sticks is excellent, a mix of lightness,
precision and agility which is unusual on a glider of this size. A beautiful
machine!
To finish,
here are three videos you should be sure to watch. The first one is of the first
flight in 2020 :
And a third one with me at the sticks:
Conclusion
At the end
of the day, it is safe to say this project has been a success, with all
objectives met or exceeded. The satisfaction brought by this model makes the
builders forget the hours spent in the workshop during the construction or in
front of the computer for the design and the calculations. I can easily imagine
that Joël Carlin is now impatiently waiting for his model, so that we can film
or photograph the patrol flights of two 5m ASW-28s!
I hope that
sharing these few lines on this project will have brought you some information
and will give you the desire to launch yourself in this kind of construction!
Finally, a
new French craftsman should emerge in the next few months with the ambition of
marketing a glider of similar size using the series of profiles developed for
this ASW-28 and working in close collaboration with Jean-Luc
Foucher.
Big smile
on Fred’s face after the sucessful maiden flight!
Specifications
©2023 Pierre Rondel, Jean-Luc Foucher and Frédéric
Hours
Resources
- GM
Propellers on Facebook. — “High performance carbon propellers for hobby
and competition…”
- HP-Textiles
GmbH —
“Since the company was founded in 2004, HP-Textiles GmbH has stood for the
development and distribution of fibre composites…”
- JETI
ENlink 2RS plus — “a device for increasing safety and reliability of radio
controled models to the highest level … parallel connection of systems
that are detached maximally leads to the significant increase in
reliability…”
- Knauf
Insulation — “Our mission is to challenge conventional thinking, create
innovative insulation solutions that shape the way we live and build in
the future…”
- Les
Grands Planeurs RC (Large Scale RC Gliders) — The home of the spar calculation
Excel spreadsheet created in 2003 by Jean-Luc Delort.
- PredimRC — “an aerodynamic
analysis and simulation tool for aircraft models … for designers but also
for pilots anxious to adjust correctly their device or to improve its
behavior or performance…”
- Profili — “This new version can
assist you in: · Searching for the right airfoil for your application ·
Creating new airfoils · Analyzing the airfoil aerodynamic…”
- TeXstreme — “spread tow
reinforcements are a uniquely adaptable, safe and ultra light supportive
solution for your carbon fiber composites…”
- ULMER
Kunststoffteile GmbH — “We see ourselves as a competent solution partner in the
development and production of high-quality and technically high-quality
plastic products in vacuum thermoforming technology and hot forming…”
- XFLR5 — “an analysis tool for
airfoils, wings and planes operating at low Reynolds Numbers…”
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