1 Field of the Invention
Lateral shake in helicopters is a typical aperiodic beat phenomenon that occurs in the lateral direction due to randomly oscillating loads of the tail boom. Lateral shake leads to increase in pilot work load and unpleasant flying qualities of helicopter. Lateral shake phenomenon is one amongst the most common issues that modern helicopters face [1-4]; which affect the overall handling qualities of the helicopter and also annoyance to the occupants including pilots. Inferred reasons for lateral shake phenomenon are; oscillatory loads of tail unit resulting from the strong wake formed around the main rotor hub, engine cowling and engine exhaust tail pipe interact with the tail boom, tail rotor and the empennage that consists of horizontal stabilizer, end plates and vertical stabilizer.
2 Description of the Invention
Having seen the lateral shake problem on a reference helicopter (Figure l); time history of the relevant
load sensors (strain gauges) were processed and filtered for the abnormal signatures. Loads pertaining
to tail boom lateral bending moment was found to have a peculiar beat phenomenon that was
repetitive and also random in nature. Figure 2 demonstrates the measured tail boom lateral bending
moment that comprises of the abrupt increase and decrease of lateral bending load at many
occurrences. This increase/decrease of oscillatory load was presumed as the reason for lateral shake
which was leading to annoyance to the pilots in the cockpit. Inferred reasons for oscillatory loads of tail
boom were; the strong wake formed around the main rotor hub, engine cowling and engine exhaust tail
pipe are interacting with the tail boom, tail rotor and the empennage that consists of horizontal
stabilizer, end plates and vertical stabilizer.
i
Extensive CFD analysis of the baseline helicopter has been carried out to unveil the presence of ;
undesired flow interactions that can lead to tail boom lateral bending load oscillations. CFD results
discovered that there were two pairs of strong vortex cores present. One pair originates around main
rotor hub and the other originates around the engine exhaust tail pipe. Vortex pairs originated around
the main rotor hub and engine exhaust tail pipe are shown in Figure 3 at moderate flight speed. These
vortex cores travel down the stream and interact dominantly with vertical fin and hence the oscillating
loads.
In order to eliminate the lateral shake; various proposals by means of passive flow control devices have been studied and found that a pair of strakes installed on the engine cowling were effective to mitigate the lateral shake. Retro fit-ability on to the existing helicopter structure is the main feature of the proposed strakes. Figure 4 illustrates the strake installation on the engine cowling along with the fairing of reference helicopter main rotor hub.
Main rotor hub fairing together with the strakes on cowling configuration is deflecting the vortex cores down and outboard substantially. Main rotor hub fairing upper surface is similar to that of airfoil.upper

surface which helps to direct the flow along the surface by which the vortex cores moves down. CFD predictions of the vortex cores position with respect to vertical fin obtained for main rotor hub fairing and strakes on cowling is depicted in Figure 5.
Flight tests of reference helicopter were carried out with the main rotor hub fairing and the strakes installed on the engine cowling. Flight test results show that the main rotor hub fairing and deflectors on cowling reduce lateral shake significantly to an imperceptible level. Time data of the tail boom lateral bending moment in level flight is presented in Figure 6 where no beat phenomenon can be observed. Devices were tested for individual effect also and found that main rotor hub reduces the lateral shake only in level flights but the shake remain present in descent flights. Addition of the strakes on the engine cowling along with main rotor hub fairing makes the helicopter shake free throughout the flight envelop. Figure 7 illustrates the tail boom bending moment measured during descent flight.

3 Use of the Invention
Strakes can be used on helicopter through which lateral shake in the descent can be eliminated while improving the overall helicopter handling qualities. However, present invention can be used for any other locations on the helicopters with similar requirements.
4 Aim of the Invention
Present invention of passive flow control devices on a helicopter is to achieve a high degree of
helicopter flying qualities while reducing the pilot control activity. On the other hand, the scheme is of
retro-fit-able type to adapt on to the existing platforms that need the lateral shake and pilot control
activity improvements. ,

5 Prior art Comparison
• In general many helicopters were undergone with large scale modifications of cowling and many other upstream systems. Proposed passive flow control devices does not require any major changes on the helicopter but minor additional provisions for installation.
6 Brief description of Drawings
Two strips on engine cowling; one on left hand (LH) side and one on right hand (RH) side installed as shown in Figure 8 to alter the flow physics. Altering the flow physics in such a way that the vortex cores to deflect away from the vertical fin. Main rotor hub fairing and the strakes deflect the flow (wake) vertically downwards from the rotor hub plane and laterally away from the helicopter.