AS03: WINGTIP MORPHING TRAILING EDGE
AS leader: Andreas Wildschek - EADS D
AS Contributors: A-D, EADS UK, FACC, FHG, FIDAMC, NLR, ONERA, SAMTECH, VZLU
ABSTRACT
23,30
3,5
23,25
3
No Active Winglet
Winglet Trailing Edge Up (-15 deg.)
Winglet trailing edge down (+15 deg.)
2,5
Lift to drag ratio [-]
Factor of wing bending moment variation [-]
In application scenario AS03 a winglet with active trailing edge (WATE) is designed
based on EASA CS-25 certification requirements. Winglets are intended to improve
the aircraft’s aerodynamic efficiency but simultaneously they introduce significant
loads to the outer wing structure. These additional loads lead to heavier wing
structure and can thus diminish the initial benefit. Preliminary investigations show
that a WATE can significantly reduce these loads at critical flight points.
Additionally the WATE can adapt the winglet camber for improved lift to drag ratio
in off-design flight points. The main challenge with respect to WATE design is that
all parts such as actuator and kinematics must be contained within the winglet loft
line for undisturbed airflow. The objective of AS03 is to provide a full scale WATE
demonstrator for a low speed wind tunnel test in order to prove that the
realization of such a device is possible under consideration of certification
requirements.
4
2
23,20
23,15
23,10
1,5
23,05
1
0,5
23,00
0
2
4
6
8
10
12
14
16
18
20
Wing station [m]
0
1
2
3
4
5
6
7
8
9
10
11
12
Angle of WATE deflection [deg.]
Figure 1: Factor of wing bending moment variation over
wing span for different WATE deflections
Figure 2: Achievable lift to drag ratio over WATE deflection
Figure 3: Winglet 2D ultimate root loads (torsion
versus bending) for baseline (winglet without tab),
normal operation of load alleviation, failed load
alleviation, and jam of the tab
Figure 4: Drawing of winglet with WATE as
manufactured for the full scale wind tunnel
demonstrator
BODY TEXT
The key driver of today's aircraft development is without doubt the rise in fuel
prices. One common means for fuel efficiency improvement is installation of
winglets on the wingtips. Winglets are particularly useful for refit of existing aircraft
or if the wingspan is limited due to ground handling requirements. Main drawbacks
of winglets are that they introduce additional loads to the outer wing structure,
especially in gust and side wind, and that they make the aircraft more roll sensitive
to side wind. Application scenario AS03 provides a certifiable solution for a WATE
device which can counteract the drawbacks described above.
First investigations on a rigid aircraft wing show that significant effects on loads in
the outer wing can be achieved by moderate WATE deflections as illustrated in
figure 1. With a 15 deg. upwards deflection the bending moment is reduced by up
to almost 50% in the outer wing. However with a 15 deg. downwards deflection
the wing bending moment is increased by a factor of almost 4. The reason why
positive WATE deflections are considered at all is because of another potential use
of this trailing edge device. As shown in figure 2, a positive deflection of the WATE
can improve the lift to drag ratio by about 1% and thus allow for further fuel
saving. Assuming the lift distribution of an aircraft with winglets is optimized in
order to have the best lift to drag ratio at a certain flight point, the active winglet
then can help to achieve good lift to drag ratios also in off-design flight points, such
as climb. Conclusion from this preliminary investigation is that the range of WATE
deflections considered for the design is from -15 deg. (upwards) to +10 deg.
(downwards) in order to fully exploit the lift to drag ratio improvement capabilities,
but not to increase loads too much if the WATE is deflected in the wrong direction.
Since for refit, maintenance, and repair it is preferable not to have to introduce
hydraulic lines to the winglet, an electro-mechanic actuator (EMA) is implemented
for actuation of the WATE. One major drawback of EMAs however is that at least
according of today’s certification standards, actuator jam must be considered as a
failure case. That means that there is a non-negligible likeliness that the WATE can
get stuck in a position unfavorable for loads. Thus an ultimate loads envelope is
computed for torsion and bending moment in the winglet root, this time for the
flexible wing considering several flight points as well as positive and negative load
factors, see figure 3. These are the design loads introduced to the outer wing by
the winglet. One can see that the WATE leads to a reduction of the maximum
winglet root bending moment by about 15% compare red arrow in figure 3. Note
that jammed condition (orange triangles pointing to the right) is the limiting factor
for loads alleviation.
In order to provide a design which requires disassembly and maintenance only
every D-check (approximately every 5 years) the trailing edge tab is connected to
the winglet by 5 single hinges, so that the tab still remains safely connected even if
2 of the 5 hinges are ruptured (dormant failure). Morphing gap filler connects the
side of the tab to the winglet in order to avoid an aerodynamically disadvantageous
gap when the WATE is deflected. This morphing part is so stiff that it also
represents an additional damper which counteracts control surface flutter if the
screw drive shaft of the EMA is ruptured.
Based on the design loads for several flight points the structure for the active
winglet is designed. Access doors for maintenance are foreseen; see figure 4 for a
drawing of the active winglet as it will be manufactured for the full scale wind
tunnel demonstrator. The demonstrator will be equipped with sensors for pressure
measurements, accelerations, strains, and angular tab position, as well as with a
data acquisition unit, a control unit, and a servo controller (the latter two will be
installed in the outer wing) for the control of the WATE during wind tunnel test.
CONCLUSIONS
The design of a certifiable WATE is feasible. In order to ensure that actuation and
kinematics fit within the loft line it is recommended to consider a requirement for
available installation space in the winglet shape optimization.
Safety and reliability assessment needs to be performed already at an early design
phase and is a key driver for realization of the WATE device. Many design decisions
such as selection of kinematic concept, actuation concept, and hinge concept are
significantly influenced by safety considerations.
The winglet with WATE at a first glance is more than 10% heavier than a winglet
without WATE. In order to evaluate the overall benefit, structural mass saving must
be assessed for the whole aircraft wing considering load alleviation with and
without WATE.
Using an EMA for actuation of the WATE has the advantage that no hydraulic lines
are required in the winglet. One major drawback however is that according to
today’s certification standards EMAs cannot yet be regarded as jam-free, and thus
the jammed condition significantly limits the achievable load alleviation with
WATE.
REFERENCES
1. Wildschek A. and Maier R. European Patent EP2233395 (A1), "Winglet with
autonomously actuated tab", 09/2010.
2. Storm S. Wildschek A. Patent pending.
3. Heinen C. “Design of a Winglet Control Device for Active Load Alleviation”,
Master Thesis, Technische Universität München, 11/2012.
4. Heinen C. Wildschek A. and Herring M. “Design of a Winglet Control Device for
Active Load Alleviation”, Proceedings of the International Forum for
Aeroelasticity and Structural Dynamics, Bristol (UK), 06/2013.