Optimization of an Ultralight Glider Towing Aircraft

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Optimization of an Ultralight Glider Towing Aircraft

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Optimization of an Ultralight Glider Towing Aircraft “Nynja”
R. Lisazo
Institut Supérieur de l’Aéronautique et de l’Espace
31500 Toulouse, France

Abstract

2. Methods

In recent years, the use of Ultralight aircraft in the
field of glider towing has gone through a formidable
growth. The Nynja is the pioneer model and
companies seek to maximize the number of rotations
in order to increase their benefits. The inclusion of a
spoiler on the wing permits to controllably enhance
the drag, which enables the aircraft to descend much
faster in a safe way. Possible configurations were
examined and tested, and the optimum was identified
after data treatment. Finally the viability of this
solution was qualitatively evaluated.

A huge number of investigations carried out on high
lift devices show that spoilers, sometimes called lift
dumpers, are an effective way of creating carefully
controlled stall over the portion of the wing behind it
[2], [3]. By varying the position and deflection of the
spoiler, different values of drag and lift coefficients
are obtained.

Index Term: ultralight, spoiler, lift, drag, deflection
1. Introduction
Ultralight aircraft constitute a range of aircraft with
certain characteristics in terms of weight and size
which are currently widely used in the context of
private aviation. However, its reliability, along with
its low price and low maintenance costs have also
raised great interest for commercial use. Indeed, the
market of Ultralights has experienced tremendous
growth through the business of glider tugs. In
particular, the Nynja is the sector-leading model and
has been adopted by the vast majority of the
companies.
The major concern in the design of these aircraft is
to maximize the number of rotations per hour in
order to obtain the highest profit.
The aim of this paper is to examine an aerodynamic
modification of the Nynja’s wing, which will allow
an increase of the permitted rate of descent. In order
to achieve this, the drag coefficient (Cx) must be as
high as possible, while maintaining a required
minimum lift coefficient (Cz) in order to comply
with regulations [1]. Finally, a value of drag
coefficient corresponding to an optimum
configuration is to be established.

According to the Nynja’s technical datasheet [5], its
wing is composed of NACA 23012 airfoils [4],
equally spaced along the wingspan. The maximum
vertical speed admissible for the structure is 110
km/h. Through the equations of Flight Mechanics,
this value imposes a minimum value of Cz = 0.85, as
specified below:
=

2

In this equation, W represents the weight of the
aircraft, ρ the density of the air, S the wing surface.
V is the speed, which was considered to be the
maximum in this calculus.
A series of tests were conducted on a model of this
airfoil in a wind tunnel for various positions and
deflections of the spoiler at this speed. In total, four
positions (0.5c, 0.6c, 0.7c and 0.8c, expressed in
portions of the aerodynamic chord ‘c’) and three
deflections (30º, 60º and 90º) were examined. For
each configuration, the software returned the values
of Cz and Cx.

3. Results
The data obtained were plotted to enable visual
comparison. A curve of CX as a function of CZ was
generated to illustrate their relative performance.

Figures 1, 2 and 3 illustrate the effect of the position
of the spoiler for three different deflections. On the
other hand, Figures 4 and 5 indicate the impact of a
change in deflections in two different positions.
Visual inspection shows a clear increase in drag as
the position is closer to the leading edge, although
this evolution is much more noticeable when
accompanied by high deflection values.

Figure 1. Curve Cz vs Cx for a deflection of 30º and
four different positions.

Figure 4. Curve Cz vs Cx for a position of 0.6c and
four different deflections.

Figure 2. Curve Cz vs Cx for a deflection of 60º and
four different positions.

Figure 5. Curve Cz vs Cx for a position of 0.8c and
four different deflections.

Figure 3. Curve Cz vs Cx for a deflection of 90º and
four different positions.

A remarkable growth of drag occurs along with the
deflexion of the spoiler. However, there is a
significant stabilization for values greater than 60º.
To determine the most appropriate solution, the
different values of Cx for the examined
configurations are provided in Table 1.

Position

δ (º)

Cx

0.5c

30

0.22

0.6c

30

0.21

0.7c

30

0.21

0.8c

30

0.21

0.5c

60

0.26

0.6c

60

0.26

0.7c

60

0.26

0.8c

60

0.24

0.5c

90

0.28

0.6c

90

0.27

0.7c

90

0.27

0.8c

90

0.27

The study identified the aimed maximum value for
the drag coefficient (Cx=0.28), which corresponds to
a spoiler located at 0.5 and deflected by 90º. This
highlights the caution that should be borne in mind
in order not to exceed structural limitations.

6. Recommendations
Though it may seem attractive to use the highest
values of Cx, the more refined question is to what
degree these configurations jeopardize the structure.
Further analyse on the effects caused by the
aerodynamic surfaces on the structure should be
carried out before certifying the chosen
configuration.

7. References
1.

Direction Générale de l’Aviation Civile,
“Conditions
techniques
complémentaires
spécifiques à l’aptitude au remorquage de
planeur par un ULM”, December 2011.

2.

Mashud, M., Ferdous, M. and Omee, S.H.,
“Effects of Spoiler Position on Aerdynamic
Characteristics of an Airfoil”, International
Journal of Mechanical & Mechatronics
Engineering, vol. 12, no. 6, December 2012.

3.

Harley, C.D., “Aerodynamic Performance of
Low Form Factor Spoilers”, School of
Mechanical, Aerospace and Civil Engineering,
2010.

4.

UIUC Applied Aerodynamics Group, Available
at: http://aerospace.illinois.edu/ (Accessed:
March 2014)

5.

BestOffAircraft Company. Available at:
http://www.bestoffaircraft.com/index.php/en
(Accessed: March 2014)

Table 1. Drag coefficient values for all configurations

It can be seen that there is no significant
improvement between the values obtained for spoiler
locations from 0.6 to 0.8.

4. Discussion
The results show that a remarkably strong drag
coefficient is obtained for deflections over 60º. In
fact, it reaches a maximum of 0.28 for a 90º
deflection of a spoiler located at 0.5. Nevertheless,
this phenomenon decreases for higher values and
stabilizes as previously stated. Indeed, the difference
between two deflections of 60º and 90º, in terms of
drag coefficient for the same spoiler location, is less
than 2%.
While the values of deflection cover the range
between 60º and 90º, the stall created by this device
tends to exceed the acceptable, thereby leading to
structural instabilities on the model. This results in
an undesired variability of the data, as shown in
figures 1 to 5.

5. Conclusion
This paper has examined, separately as well as
regrouped, the effects of both the position and
deflection of a spoiler situated on the upper surface
(suction side) of an Ultralight aircraft called Nynja.

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