THE FUTURE OF HELICOPTERS - PHYSICS AND AERODYNAMICS REMAIN UNBEATEN

There continue to be many press releases and announcements about new electric multi copter rotorcraft ?which, if you believe some of the hype, are about to erase the helicopter from the skies.?However, since nothing has changed with the standard atmosphere (apart from warming up of course) and the physics of flight our baseline for the understanding of aerodynamic principals remains as it was probably as far back as when Daniel Bernoulli first published his principal in 1738. In man’s search then for the optimum efficiency of flight, whether in a rotary or fixed wing aircraft, there remains a need to keep firmly within those physical norms in order to avoid the inevitable conceptual failures either within computer simulation or flight test ( in the past the former wasn’t an option and failure in flight test was a common occurrence of course).

What has changed, of course are the systems that seek to exploit those basic physical principals through the use of new materials, engines and fuel.?However, it still remains a fact that the one aerodynamic problem that has so far failed to come up with a single theoretical framework is that for the helicopter. Even with the best super computers there are still two main but slightly different physical theories, rotor disc and blade element that seek to answer the key power, performance and control principals.?

When I talk about helicopters and multicopters I am dividing horizontal rotor systems into two key groups the principals of which are as follows:

Helicopter??????????????????

·??????One or more horizontal or near horizontal rotor

·??????Noramlly fixed Rotor Speed between 225 and 500 RPM

·??????Piston Engine/Gas Turbine/Electric Power

·??????Variable hinge configurations to allow pitching, flapping and dragging

MultiCopter

·??????More than 3 horizontal rotors

·??????Normally variable Rotor Speed > 1000 RPM up to 20000 ?RPM

·??????Piston Engine/Gas Turbine/Electric Power

·??????Typically Hingeless

Anti-torque systems have not been included here because they vary so much and would be an article all on their own.

Hovering

Hovering any aircraft with has one clear physical requirement which is that vertical thrust must be ?equal to or greater than the weight of the aircraft.?This simple piece is of theory is complicated by some annoying momentum theory which basically shows that downwash in the far wake (>2 rotor diameters) must be twice the induced velocity at the rotor disk to allow for hovering.?The diagram below shows how efficiency varies according to the rotor or lift configuration and it baseline fact becomes readily apparent that efficiency rises as the rotor disc becomes larger and less heavily loaded – known as the figure of merit.

The figure of merit is equivalent to a static thrust efficiency and defined as the ratio of the ideal power required to hover to the actual power required,

This figure of merit number shows modern conventional ?helicopters with complex rotor blade profiles and with fully articulated rotor heads to be highly efficient, in fact higher than the diagram above at around 80-90%.?This is because the lower number of slower high aspect ratio blades (like any aircraft) are much more efficient in getting high lift for low drag. By comparison a high speed electric multi rotor system has the figure of merit sitting between 30% and 50% efficiency (ducted systems being slightly more efficient on average).

The problem with these high speed systems is that the tip vortices are wider and, therefore, stronger for higher angular velocities, increasing drag effects whilst the scale effects of on smaller platforms cause issues with Reynolds numbers ( too deep a principle for this article).?In addition the high solidity (lots of blades) ?of multirotor systems also leads to higher power consumption and lower FOM.?The following table shows the effects of this issue on power consumption as take-off weights increase.

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So when looking at profiles which favour helicopters we can pretty easily see that long duration heavy weight hovering is best suited to a conventional helicopter configuration irrespective of the power source chosen.?This includes things like load lifting, search and rescue, troop transport and those attack helicopter type roles which require lots of low speed operations are best delivered conventionally. That doesn’t mean other configurations, like tilt rotors, cannot be used (V22 and AW609 are clear examples) but only at the expense of very heavy and complex powerplants.?In forward flight it remains the case that a fixed wing system is always more efficient.

?If you can limit the need to hover for long periods, as in the urban air mobility sector, then Figure of Merit is less of an issue but only if you can keep the weight down.

Manoeuvrability

The second area where there are major differences in the performance of rotor systems concerns manoeuvrability.?Rotor systems have steadily developed over the last 70 years to provide more and more control power to the pilot.?There are both aerodynamic and physical reasons for this concerning the conservation of angular momentum and offset?of the rotor blade flapping hinge.?Modern rigid and articulated helicopter rotor systems push the hinge offset away from the rotor head increasing the moment arm of any particular offset control movement and the rates of pitching and rolling which can be induced.?This gives highly manoeuvrable helicopters which, in a military context are ideally suited for low level nap of the earth flying for a wide range of combat roles.

The same cannot be said of multi rotor systems. Whilst smaller platforms, with high speed rotor systems could easily move their small masses quickly the scale effects we saw related to rotor system efficiency also cross into this area.?As the platform mass increases the ability to induce differential rotor power across the platform is exponentially reduced.?Small multiple rotor systems, like that on the Volocopter, only provided limited control power and thus limited manoeuvrability.

Mulitcopters without wings

For those mutlicopters which avoid lift generating wing surfaces there are more problems to contend with.?In order to move through the air horizontally the disc needs to be tilted and when the inherent form of the design is high speed very rigid rotor systems this becomes even more important.?As a result unless the whole multi rotor system is tilted (increasing complexity and complicating certification) the platform is going to develop some really interesting body angles which will be susceptible to longitudinal stability problems – those hoping for high speed jinks in an unwinged mutlicopter maybe sadly disappointed.?It really depends how fast you want to go but I haven’t made that maths jump just yet.

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Additionally the plethora of small open multi-rotor systems currently being played as viable air mobility platforms really needs closer scrutiny. In the environment where these aircraft operate there are significant natural hazards to contend with (birds of course) who will not get out of the way and will do significant and likely catastrophic damage to at least one of the rotor systems.?Certification will be a real barrier here and is why most of the viable UAM projects are very much compounded fixed wing aircraft with tilt or other lift rotors.

Conclusion

So the helicopter still has a future built on some pretty clear physical issues which principally relate to the role. ?In UAM however, where however time can be very limited, and compound systems lend themselves to electrification, there is certainly much more opportunity but perhaps only up to a certain size.?That is why the main helicopter manufacturers are still developing new medium and large helicopters and why a great many unmanned systems are still using conventional rotor systems.

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