Electric VTOL aircraft are no longer a dream: they are flying today. "If" is no longer a question - only when and how they will reach the market. Electric propulsion and other technological advances make these aircraft much safer and much more efficient than conventionally powered aircraft. Our aEro 2 design is derived from 30 years of operating today's most common VTOL aircraft, helicopters, and from our experience building and flying electric aircraft.
For urban air mobility to become commonplace, aircraft and their operators will need to be able to address crucial concerns: performance, noise, downwash, and safety in both design and operation. In this essay I'll discuss our approach to these concerns at Dufour Aerospace.
In recent years, the gravimetric energy density of Li-ion and Li-po batteries and power density of electric motors have become high enough that pure electric flight has become feasible. We were the first to demonstrate this in an aerobatic plane with aEro 1. We experienced first hand the significant benefits of electric power systems, and learned how to manage battery performance and heat dissipation in an aircraft.
However, while their power output is high, the gravimetric energy density of batteries is still much lower than hydrocarbon fuels. So efficiency and mission type are crucial for aircraft using only batteries for energy storage. A light, efficient aerobatic aircraft such as aEro 1 can deliver extremely high torque and up to an hour flight time. With a delta-T (increase in battery temperature per flight) of 2-3°C, you can turn around quickly on the ground and easily fly the whole day long. We have flown aEro 1 up to 8 flights per day.
In contrast, batteries do not work so well for typical transport missions, where you need endurance and not high peak power output. Exchanging the powertrain in a typical piston aircraft for batteries and electric motors will often give you an aircraft with extremely poor performance.
Luckily electric power distribution brings us another advantage: we can make the system modular. So we can introduce conventional fuel and a small generator to boost endurance and total system energy density, while keeping a small reserve of batteries for phases of flight requiring higher power. And in future, batteries will continue to improve. But aircraft efficiency remains key to overall performance: an efficient solution will always beat an inefficient solution and give you a competitive advantage in the long run.
Efficiency is also key for VTOL aircraft, which must generate their own weight in thrust in order to hover. By the law of conservation of momentum, the smaller the rotor disc area, the faster it must accelerate the column of air to generate that thrust. To do this, small rotors and lift fans must turn faster, generating exponentially more drag (and also noise - see below). In contrast, large rotors can operate much more efficiently at lower RPM.
This is why a fan concept VTOL might use 720 kW to lift 600 kg weight while aEro 2 uses only 250 kW. Light helicopters with a single large rotor take this even further, using only about 100 kW to hover (Robinson 22). The required power increases further with altitude, so while fan concept VTOLs can hover only at very limited altitudes above sea level, aEro 2 can hover at 2500 m ASL with comfortable reserves. Even if the aircraft is intended only for use at low altitude, using small lift fans represents a significant reduction in safety margins.
Meanwhile for cruise flight, wings remain by far the most efficient way to generate lift, with props used only to generate forward thrust. So any aircraft concept that does not use wings will be limited in range, no matter the power source. Multicopters in particular do not gain any efficiency as they enter translation, and in fact require more power the faster they fly.
On top of range and endurance, aircraft efficiency offers other benefits. A fraction of the power used to hover will also be lost as heat due to internal resistance in the electrical system. While motors are usually easy to keep cool in the slipstream from their props, batteries require special attention. The complexity and mass of the battery management and cooling system scales with the power draw. The challenge to manage 720 kW is significantly greater than managing 250 kW.
In order to operate in urban areas, these aircraft will need to be quiet. According to a NASA study, the most important factor contributing to noise is the propeller tip speed:
Major design requirements for minimum noise can be summarized as follows:
- Low tip speed.
- Large number of blades.
- Low disc loading.
- Large blade chord.
- Minimum interference with rotor flow.
- Any features that will reduce the high-frequency airload fluctuations
[A review of aerodynamic noise from propellers, rotors, and lift fans, NASA 1970, p14]
The sound due to faster tip speeds is both louder (greater acoustic power) and higher frequency (more annoying). For example, most of the propeller noise from a helicopter comes from the fast spinning tail rotor, not the main rotor.
As discussed above, the smaller the aircraft's area of lifting rotor discs, the faster they will need to accelerate the air, and so the faster they will need to turn. Thus big rotors can operate at lower RPM and tip speeds, and so are quieter than small lift fans which have to spin very fast.
That's the theory. And then there's our real world experience: aEro 1 has a propeller tip speed of maximum 777 km/h. How does a human perceive that noise? The aircraft does make just a little bit of noise on take off at full power. But as soon as it's in the air, on the downwind circuit at our base airfield in Raron (above the highway) you cannot hear the aircraft anymore. The cars on the highway are louder!
aEro 2, our VTOL, actually operates at a lower prop RPM than aEro 1, but due to the larger diameter, propeller tip speed is approximately the same. Having twin props does increase the noise level, but the low frequencies will be far more comfortable to the ear than lift fans or a helicopter tail rotor. During cruise, the aircraft will have a very low noise profile as prop load and RPM are reduced.
Another critical point is downwash. After more than 50’000 helicopter landings in critical areas, I can confidently say this is one of the biggest risks of these new aircraft.
The smaller the propeller, the higher the air column velocity and the more downwash it produces for a given thrust. Already today you have huge issues in the helicopter world. An SA 315 Lama weighing 1950 kg with a rotor diameter of 11 m produces far less downwash than an EC135 at 2980 kg with a rotor diameter of 10.5 m. This fact makes it way more dangerous to operate a modern helicopter in an unknown area than an old SA 315.
Some concepts in the new generation of VTOL aircraft, especially those using small diameter lift fans, are at risk of having significant problems with downwash. They are designed to land in urban areas, but they may find it difficult to do that safely. They may cause disturbance and damage to the surrounding environment. At best, managing these risks by keeping passengers and other aircraft far away during take-off and landing will reduce operational efficiency.
To minimize these risks, we have designed aEro 2 with a propeller disc load that is almost as low as the SA 315 and therefore possible to land even in very densely populated areas.
A key advantage of electric propulsion is that it is easier to engineer for redundancy. This is true for the electric motors, cables, battery systems and even control systems.
(It is not true for structural components such as propellers. These elements need to be constructed to be "fail safe" or extremely reliable. The failure of a propeller in an aircraft will cause cascading damage to other propellers, structures and passengers with a catastrophic outcome.)
With aEro 2, safety in depth has been a core concern in our design, considering ground operations and all phases of flight, and ensuring redundancy in systems and components where it makes sense to do so.
We use only proven technology and aerodynamic concepts with a good safety record. During cruise, we have an inherently stable aircraft that flies, and if necessary glides, like a plane with a low stall speed.
In contrast, the safety of other concepts is not clear. What do you do if you run out of power reserves in a multicopter? What can you do if one of your thrust vectoring flap actuators fails in an electric jet?
We expect aEro 2 to spend about 5% of its time in helicopter configuration for take-off and landing, and the remaining 95% cruising in plane configuration. During the cruise phase, we can continue flying when one motor fails, when one of the electric systems fails (1 motor on each side can fail and it can still continue flying regularly), when one drive shaft fails (2 motors and the prop on one side fail), or when the tilt mechanism fails. Even if multiple power systems fail the aircraft can glide with a ratio of 1:12. Our aircraft can land vertically, STOL (short take-off and landing - the low approach speed increases the availability of emergency landing sites and dramatically lowers the risk of injury) and even like a regular plane with a stall speed below 61 kt (which places us in a lower risk certification band).
Each of aEro 2's props has two electric motors driving it. During hover flight phases, if either motor fails, the other will have the capacity to increase its power output by a factor 2 for up to 90 seconds (after which they begin to suffer heat damage). This gives the pilot plenty of time to land safely, or go into translation and convert to plane configuration. You should always have a plan B! That is how helicopter pilots think.
So aEro 2 can still fly when one electric system fails, one motor fails, one tail motor fails, or when the wing tilt mechanism fails.
Since the props are themselves critical components, why do we use just two, and not four or even more? Well, prop failure is not as easy to manage as motor failure. Do you have enough power to manage the increased load on the remaining props? Can you withstand the collateral damage caused by a failing prop?
The failure of a propeller is usually a dramatic event. It does not simply result in a loss of thrust, as in an engine failure. If even a small portion of the propeller blade is lost, the resulting imbalance may tear the engine from the aircraft, making the aircraft uncontrollable.
Increasing the number of propellers may appear to offer safety through redundancy, but in fact it also multiplies the probability of failure.
Conclusion: Propellers must be designed to be as safe as possible ("fail safe") and you should have no more of them than necessary. As it happens, this also offers the lowest disc loading and the greatest efficiency!
Electric propulsion technologies have the potential to dramatically reduce costs compared to today's VTOLs by simplifying systems and maintenance. However, to take advantage of this simplicity it's important to stick to proven technologies and aerodynamic concepts wherever possible. New concepts may potentially require longer and more expensive testing and certification phases of development.
With aEro 2, instead of reinventing everything and building a whole kind of new aircraft, we have based our design on existing and reliable concepts where a lot of data is available and that already proved their ability.
Tilt-wing aircrafts have been built before. The CL-84 and XC-142 for example were very highly rated by the many pilots that flew them. These aircraft are very stable and were flown without any computer assistance.
The wing is always under a positive airflow and therefore the standard control surfaces remain effective even in hover. This makes the aircraft even on very slow speeds aerodynamically stable and allows very short landings if necessary, where the risk of injury is massively lower.
Using a fan on the tail is as well a proven concept coming from NOTAR helicopters which have flown for 25 years in rugged and harsh environments, proving their ability as a control concept.
The separated flow (under and above the wing) makes the aircraft massively less susceptible to vortex ring state (helicopter stall). It is unlikely that both sided get into this air downdraft that causes the stall.
We also design for the minimum number of actuators, with just one wing tilt mechanism and two propellers. As previously discussed, these need to be failsafe. Adding more propellers may give the appearance of safety through redundancy, but it can in fact have unintended negative consequences.
We also want the control system to be as simple as possible. Developing the electronic control system will be a significant task for new aircraft with many props. Mission critical software must be developed and certified according to DO-178C, and its complexity increases exponentially with the number of interacting components. In particular, you have to demonstrate safe behavior in all failure scenarios: Does it work if fan 1 fails? Does it work if fans 3 and 5 fail? What may look at first like safety through redundancy becomes a combinatorial explosion of potential failure scenarios.
In contrast, with the minimum number of actuators, aEro 2's control system is as simple as possible. We will have significantly fewer development and certification risks with our concept.
No matter how safe the aircraft itself, the biggest risks lie in how it is operated. As a helicopter pilot with over 10'000 flight hours and 50’000 landings in all kinds of locations, I have learned a great deal about VTOL operations. Experience with high density operations (for example situations with up to 30 helicopters flying people and goods in and out of entire valleys because of interrupted ground transportation) has shown me what urban air mobility will look like and what the operational risks are.
We have designed aEro 2 with conservative safety margins and reserves so that it can handle not only the planned missions, but also the unexpected. For example, we don't quote its ability to hover at sea level but instead at 2'500 m ASL. Making the aircraft able to hover at higher altitudes gives you much higher power reserves. You will need them at some point - you have my word. Similarly, we ensure that all controls will be effective at altitude. This gives you additional freedom of operation, and the situation will arise that you will need it. We place the props well above head height for safety while loading and unloading passengers. Electric power allows us to spin them up and down quickly, but again, this is safety in depth. Large props maximize efficiency, and minimize noise and downwash which are big concerns in daily operation in congested areas.
In all, we are making our aircraft operations ready. Embarking people in these anticipated numbers requires rock solid and strong aircraft. How do you get in and out? What if they wear big jackets and carry bags with them? You want to be sure your aircraft is ready for that.
We designed aEro 2 according to lessons we have learned with our high performance aerobatic electric aircraft and from 30 years' experience in VTOL operations. It is not just a pretty design: we have focused on performance, noise, downwash, and safety in both design and operation. We at Dufour Aerospace believe this is the best way to build the aircraft of the future.
Thomas Pfammatter, Co-Founder