**3. Challenges for smart-systems inside helicopter blades**

Smart-systems need to answer challenges specific to the integration in helicopter blades. The combination of these challenges make smart-blade concepts very difficult to design.

#### **3.1. Weight and space constraints**

8 Will-be-set-by-IN-TECH

active blade have shown an increase in the lift without significant increase in the lift-to-drag

Active flow control devices take another approach to improve the lift on a profile. Instead of modifying the airfoil geometry to act on the flow, they directly modify the air flow by re-energising the boundary layer on the top of the profile with a high speed jet. Such a flow is called a synthetic jet. The objective is to bring the separation point closer to the trailing-edge and therefore improve laminar flow over a larger portion of the airfoil [23]. Actuators for this application are placed inside a cavity which has a tiny opening [26] or a full slot perpendicular to the flow direction on the top part of the profile [67]. Figures 7 and 8 show these two types

Pulsating flow

Vibrating membrane

Pulsating flow Cavity

Vibrating membrane

Side view Front view

Wind tunnel experimentations have shown that synthetic jets improve the aerodynamic performance when driven at a specific frequency [23]. Much better performance is obtained when the actuation mechanism is combined with sensors arrays before and after the position of the synthetic jet system [57]. The sensors monitor the instabilities that will trigger the flow transition and actuate the synthetic jet system so that it damped the instabilities delaying further the transition. The actuation frequencies are in the kHz range and are related to the

Narrow slot

Cavity

ratio [36].

**2.3. Active flow control**

of synthetic jet actuator.

**Figure 7.** Sketch of a slot synthetic jet system.

**Figure 8.** Sketch of a synthetic jet system with a circular orifice.

The weight and space are the main constrains in helicopter blades. Helicopter blades are designed to handle large centrifugal loads. The structural material takes most of the section of a rotorblade. Carbon fibre composites provide strength in the direction of the blade and reinforced layers give the blade impact resistance. The only space available is in the tail of the profile. Therefore, it is very difficult to integrate a system directly in the rotorblade skin for structural reasons. Concerning the weight, not only it cannot increase much, but its distribution in the profile should not affect the chordwise balance of the blade. Therefore, any weight added behind the aerodynamic centre needs to be compensated by an extra mass in the leading edge. For the whirl tower test of the SMART active flap rotor, weight was added in the leading edge to maintain the blade balance [55]. This constraint makes distributed and light systems like the active twist very relevant to maintain the distribution of mass along the profile chord. In comparison, the variable droop leading edge requires a very heavy mechanism to deform the leading edge of the profile that would change completely the weight distribution around the aerodynamic centre [29].

#### **3.2. Mechanical constraints**

Mechanical constraints are significant in a helicopter blade. The centrifugal loads are by far the main issue. The centrifugal loads come from the large rotational speeds of the blade. The centrifugal acceleration *a* is calculated with the following formula

$$a = \omega^2 r \tag{7}$$

where *ω* is the rotational speed of the blade in rad/sec and *r* is the position along the length of the blade. An 8m rotorblade rotating at 250rpm will generate close to 560g of acceleration at the tip. Because of the aerodynamics of an helicopter blade, the active system needs to be integrated near the tip of the blade where the centrifugal acceleration is the largest. The centrifugal loads resulting from the acceleration depend on the mass of the actuation system. Thus, a very light system does not lead to large loads. Some small actuation systems are very small and robust. The "Squiggle" linear drive motor, developed by NewScale technology, features a shock resistance of 2500g [40, 64]. For larger mechanisms most of the designs limit the load transfer along the blade [42, 55, 60]. Hence the design can be approximated to a bi-dimensional structure that is extended along the blade axis. For distributed systems that use patch actuators bonded onto the structure, like the active-twist technology developed by DLR, the actuators are being supported by the blade structure [46]. The main concern with these actuators is related to the deformation of the blade during its rotation. The peak strain


**Table 1.** Comparison of the life time of an 10<sup>9</sup> cycles actuation mechanism in a 250rpm rotor blade system for various active control concepts. For the Active flow control system, the system is in operation only on the retreating side of the helicopter.

at the surface of the blade must not exceed the maximum strain of the actuator that will lead to breaking or debonding.

In addition to centrifugal loads, helicopter blades are subjected to large vibrations. In the most common configuration, helicopter blades are attached to the rotor by a joint that allows rotations with three degrees of freedom. The motions of the blade relative to the joint are defined as flapping, leading-lagging and feathering. Each of these motions are associated with one degree of freedom. While the blade is rotating the cyclic loads excite each degree of freedom causing vibrations at frequencies that are multiples of the blade rotational frequency [6, 48]. As a consequence, the design of a mechanism must address the resulting loads. Moreover, these loads constrain the use of mechanical elements like hinges or friction surfaces that will tend to jam and fail.

## **3.3. Reliability and environmental constraints**

Any mechanism built in a commercial aircraft must comply to a set of rules to ensure the reliability of the system after a large number of actuation cycles, as well as the safety and the integrity of the aircraft in the event of a failure. Helicopter blades in a general purpose helicopter are designed for 10,000 flight hours [30]. Although composite blade design can handle even more loading cycles, manufacturers specify helicopter blades to be maintained and replaced on a much shorter basis [25, 30]. Actuation mechanisms for the active blade need to have a design life superior to the blade design life and have to maintain performances through their operational life. In aircrafts, hydraulic and pneumatic mechanisms are widely used due to these concerns. They are especially utilized for moving control surfaces that must satisfy a reliability requirement of 10−<sup>9</sup> failure per hour and their performance is hardly affected even after a large number of cycles. It is only recently that electrical mechanisms have reached equivalent levels of safety and have been used to control control-surfaces in aircraft [4].

The reliability of the actuation system is also depending a lot on the application type. For alleviating the lift asymmetry on the retreating side, the mechanism performs a cycle once per revolution. This figure is to be compared with an actuation system for actively cancelling vibrations that need to operate at 2/rev and 4/rev or even at 5/rev in the case of torsional frequencies [18, 66]. Table 1 shows the various expected life time for a mechanism that has been design for 109 cycles in the case of various active blade concepts for a 250 rpm rotor system.

Although all the cases satisfy the 10,000 hours of operating life, the vibration damping case shows that a high quality actuation system certified for 10<sup>9</sup> has an operating life close to that of the blade. Therefore, it would be very difficult to qualify an actuation system for damping vibration at 6/rev. For flow control devices, very much higher quality actuators are required to be certified for active flow control.

Finally, helicopters need to operate under a large range of environmental conditions. Blades are certified to perform over a large temperature range: from high altitude freezing conditions to high temperatures and with very high moisture content. It is therefore very difficult to design a fail-safe mechanism in these conditions, especially on small helicopters which do not have a de-icing system. Furthermore, some specific environments subject helicopter components to very difficult conditions such as sea and desert environments where corrosion and erosion are important matters.
