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  • How does softening natural frequencies of blades is affecting a helicopter rotor design?

For Smart Rotor Systems Inc. (SRS) and Carleton University, Rotorcraft Research Group, the main motivation behind developing the Active Pitch Link (APL) technology was to enable the practical implementation of the so-called “stiffness control” concept. The key idea of “stiffness control” is to change the effective stiffness of the blade for reducing rotor vibrations, instead of changing the aerodynamics of the blade as it is done with a “flow control” system, such as the actively controlled flap, active twist rotor or Higher Harmonics Control. The advantage of “stiffness control” over “flow control” would be the need for much lower stroke from an actuator, which is usually the main limitation in implementing any “flow control” technologies.

The original idea of “stiffness control” was first conceived by our colleague, Prof. Fred Nitzsche (co-founder and Vice-President of Smart Rotor Systems Inc.), in 1991 and was later independently verified by Prof. Gandhi at PennState University in 2001. While Prof. Nitzsche have envisioned controlling the torsional stiffness of the blade, Prof. Gandhi looked at controlling the flapping and lead-lag stiffness of the blade. Via simulations, both  researchers have showed that there is great potential in reducing vibrations via “stiffness control”. Prof. Nitzsche even conducted a simple wind tunnel test in 2001 (on a non-rotating blade equipped with a torsional stiffness control technology at the root), in which about 60% reduction of vibration was achieved. This is described in Nitzsche, F.,Zimcik, D.G.,Wickramasinghe, V.K.,Yong, C., et al. “Control laws for an active tunable vibration absorber designed for rotor blade damping augmentation“, (please click on the paper name to open). In Prof. Gandhi’s simulations, Anusonti-Inhra, P., Gandhi, F.“Optimal control of helicopter vibration through cyclic variations in blade root stiffness(please click on the paper name to open) as much as 90% reduction of vibratory loads was predicted with optimal 2/rev and 3/rev flap and lead-lag stiffness variations. In the latter paper, only 6-7% of the blade flapping and lead-lag stiffness change was required.

SRS Inc.’s team understands that monitoring the blade torsional stiffness as the Active Pitch Link is activated is crucial. For this reason, we have performed an experimental campaign on a blade, for which we have varied the pitch link stiffness. We have shown that no matter how soft the pitch link is, the torsional stiffness will  not change more than 6% at the nominal RPM. Please see Fig. 18 in the paper describing this (Feszty, D. et al. “Whirl Tower Demonstrations of the SHARCS Hybrid Control Concept (please click on the paper name to open))

Of course we have collected strain gauge data for the recent APL tests (showing the 80% reduction of the 2/rev vibration) but it takes time to generate the fan plots for these and we will have them soon. However, based on our experience, we are confident that the change in the blade torsional stiffness will not be larger than 5% even for these tests.

In conclusion: Yes, the Active Pitch Link does change the resultant torsional stiffness of the blade, but this change is less than 5% of the original value, when the vibration is reduced by about 80%. Please also note that the blade stiffness is only altered for a fraction of the cycle, keeping its original stiffness for the rest of the revolution.

  • Is a “stiff-to-soft” pitch link hard to design?

Designing and manufacturing a functional yet robust “stiff-to-soft” pitch link is indeed very challenging. This is why we are currently at the 3rd generation of our design, which finally seems to work as envisioned. For this, we had to converge to a design which is lightweight, compact and as simple as possible. On top of this, we also made it Fail Safe, i.e. in the case of power loss, the load path reverts to that corresponding to a solid pitch link. Over the course of February 2012 testing, the prototype of this 3rd generation Active Pitch Link (APL) has run for more than 5 hrs under heavy vibratory loads (with the blade AOA set to 2+7 degrees of periodic variation at 1/rev frequency) without any major issues. Note that the AOA change was enabled through a fan located asymmetrically under the rotor disk, generating about 14 m/s upwash for about 20 deg range of the azimuth. Our rotational frequency for these proof-of-concept tests was 700 RPM and we have a 1 m radius 1-bladed rotor (nominal rotational frequency is 1,550 RPM). After 3 hrs of operation we disassembled, inspected for wear and tear and reassembled the prototype APL before running it for another 2 hrs of tests, again with no problems and with repeatable results. Despite this success, we already have solid design ideas for the APL so its operating range (it appears to work best between 500 – 900 RPM) as well as its robustness can be improved. So, to answer your question: designing a “stiff-to-soft” pitch link is indeed challenging but based on our fresh experiments, we see that we already have a solid design, which can serve as a good basis for any practical application in the future.

  • How “soft” is the SRS Inc’s Active Pitch Link really is?

Firstly, please note that our technology should be viewed as a pitch link, which stiffness can be actively controlled between K1 and K2, where K1 corresponds to the (conventional) solid pitch link stiffness and K2 to the soft pitch link stiffness. The latter one (K2) is rotor-specific and can be selected to target any harmonics of vibration (i.e. 1/rev, 2/rev. 3/rev, etc.). The Active Pitch Link enables to switch between K1 and K2 at high frequency, multiple times within one revolution (i.e. anywhere between 1/rev and [N+1]/rev, depending on the number of blades and rotor characteristics). For our current tests, we aimed to keep the 1/rev vibrations unchanged – since this simulates the cyclic control in our whirl tower setup – and aimed to reduce the 2/rev (i.e. the [N+1]/rev) vibrations. The stroke of the APL is small and measured in microns, which means that the stiffness in the “soft mode” (K2) is not that far away from the stiffness in the “solid mode” (K1). In fact, if one looks at the change of the blade torsional stiffness when the APL is on/off, it shows no more than 5% reduction in the blade torsional stiffness. The key point is that small changes in the pitch link (or blade torsional) stiffness are enough to achieve significant reductions in the 2/rev vibrations. This proves two things: 1) that our unique “stiffness control” concept works not just in CFD but in experiment as well, and 2) that there is no need to disrupt the rotor hub design radically, since we only need to change the blade torsional stiffness by a very small amount. less than 5%.

In conclusion: designing a “stiff-to-soft link” is not an easy task, but it can certainly be accomplished – as we have shown- if the change in the pitch link stiffness does not need to be large.

  • Is the SRS Inc’s Active Pitch Link able to control higher harmonics?

Our goal is to control the higher harmonics of pitch link vibrations, specifically the 2/rev (or N+1/rev) vibrations in our 1-bladed system.

The test results (please see the FFT plot of the vibratory loads on the Active Pitch Link page) show that while the 1/rev vibratory load (corresponding to cyclic control in our system) remains largely unchanged, the 2/rev vibration is reduced by as much as 80%. We are confident that by tuning the spring stiffness and the control law, one can target any harmonics of the system. In other words, one could select such a combination of the spring stiffness and control law that any harmonics, i.e. the now undesired 1/rev, 2/rev, 3/rev, 4/rev, … [N+1]/rev etc. can be reduced.

In other words, the Active Pitch Link (APL) was deliberately designed to act as a filter for the higher-harmonics of the axial load vibration spectrum. From the very beginning of our work, the APL was envisioned to attenuate N-1/rev, N/rev and/or N+1/rev frequencies. To illustrate this idea, I attach one of our earlier papers from 2005, Nitzsche, F., Oxley, G., “Smart Spring Control of Vibration and Noise in Helicopter Blades (please click on the paper name to open) . This demonstrates via CFD simulations, that for a 4-bladed rotor with a properly “tuned” Active Pitch Link and control law, only the higher harmonics after 3/rev were reduced while leaving the 1/rev frequency content unchanged, thus guaranteeing the transfer of cyclic control through the pitch link. To achieve the reduction of the higher harmonics only, the control algorithm needs to be carefully designed, and the one used in the above simulations is described in detail in the 2nd paper attached: Nitzsche, F., Harold, T, et al. “Development of a maximum energy extraction control for the smart spring(please click on the paper name to open).

To summarize: The Active Pitch Link was deliberately designed to reduce higher than 1/rev harmonics and we have demonstrated both via CFD/CSD and experiment that this concept works.