Back to the Future: A rough comparison of 70’s VTOL concepts to current eVTOL designs

Introduction

The combination of a sabbatical and the corona-induced lockdown makes up a lot of time to read. After finishing “Jesus lived in India”, I wanted to get back to something more technical and decided to dig a little bit in the old NASA archives. It is always fascinating to find out that our “super-disruptive-breakthrough-innovation” (have I missed a popular buzzword?), is not so new after all and can often be tracked back to some old NASA research (or even further to some German WW2-designs). Therefore, it might be not so surprising, that almost any of the current eVTOL designs can be traced back to ideas that were discussed starting from the 1930’s.

But first things first: My actual goal was to find some good literature that compares the noise produced by different kind of propulsors (like propellers, rotors, fans) depending on their geometry and operational parameters. Again, no surprise: The most comprehensive overview, I found to be a NASA report [1] from the 70s. This report was the actually the bait that brought me to the NASA archives: Once you’re are there, you can’t get away. It’s like Wikipedia for tech nerds ..

Not so new after all

I came across a very comprehensive overview on VTOLs and its history in the program report of the XV-15 [2]. It is the prototype predecessor of the better-known V-22 Boeing-Bell “Osprey” (s. Fig. 1).

Figure 1: The XV-15 (left) and the V-22 “Osprey” (right). The similarity is unmistakable.

Besides many interesting, technical details about the program, the authors devoted many pages to outline in detail the history of VTOLs and tilt-wing aero vehicles. Reading through this part, the image that can be seen in Fig. 2 caught my attention. It is an overview on all S/VTOL concepts by McDonnell Aircraft that were tested (or proposed) until 1967 (!). Not only, that it reminded me a lot of the eVTOL concept depiction wheels that you see nowadays, for instance by the Vertical Takeoff Society (Fig. 3), but the plurality of concepts is incredible. It seems like any concept that is developed today can be found in this compilation. The “Fan-in-Wing” concept (as proposed by Pegasus Air) can be found (20), just as the Pipistrel 801 idea (25).

In the bespoken program report another, very insightful publication [3] is cited, where more interesting concepts are discussed, such as

· X-wing Concept

· Folding tilt-rotor designs

· Trailing rotor designs

· Single stowed rotor concept

· Tilt-Wing with variable diameter rotor

Interestingly, while folding tilt-rotor designs and trailing rotor designs are pursued in current eVTOL projects, this seems not to be the case for an X-wing design or a tilt-wing concept with variable diameter rotor. However, the latter concepts seem to be the more favorable concerning noise and speed (s. Fig. 6).

Figure 2: Overview of STOL/VTOL concepts compiled in 1967 by McDonnell Aircraft. Taken from [2].

Figure 3: A glance at current eVTOL concepts. Image courtesy of Vertical Flight Society (vtol.org).

Looking at the numbers

Central quantities that are typically used to compare different type of VTOLs are disc loading, hover lift efficiency and cruise speed.

For all who don’t know what disc loading is and why it is used so frequently? The disc loading is basically the mass [kg] of the aircraft divided by the total area [m²] of the main rotor(s) used for hovering. It turns out that high disc loading has a bunch of unwanted consequences:

a) Low hover efficiency.

b) High downwash speed, i.e. the airspeed that is coming at you when you are close to the helipad. Typical for helicopters are downwash speeds are around 100 km/h (which is close to the wind speed of a hurricane). And this is the “lower end”. The downwash speed for a F 35 Lightning (that uses a lift-fan and vectorized thrust) is in the range 500 km/h. Surely, you don’t to be anywhere near when it takes-off.

c) High Noise [4].

What is hover lift efficiency? The hover lift efficiency is defined as the weight of the aircraft divided by the power of its power train (turbine, combustion engine or electric motor). Thus, in order to reduce energy consumption, a high hover efficiency is needed.

Therefore, the general rule of thumb for making a quiet and efficient VTOL is to keep the disc loading low during take-off, hovering and landing.

(I guess I don’t need to explain cruise speed here .. )

Today, due to their large main rotor helicopters have the lowest disc loading as can be seen in Fig. 4. Hence, they have lowest power requirements and noise footprint (I make here the simplified assumption that the impact of the disc loading is a major one.) Let’s see how modern eVTOL concepts compare to this regarding disc loading and hover lift efficiency. As a choice of designs, I took the ones proposed by the Vertical Flight Society (s. Fig. 3) and some others that I could find the necessarily data for. For the Uber eRCM designs I could find not sufficient data, just as for the Pipistrel 801, the EmbraerX and the ASX MOBi. For some designs I had to search for multiple sources and make reasonable estimations based on available material and good engineering practice.

If I could wish for something — it would be great if all companies working on eVTOLs would report the main specs (not only range, speed or ceiling) of their concepts in the way Airbus does it for Vahana or the CityAirbus. [5]

Figure 4: The famous image taken from [2] where typical disc loadings and hover lift efficiencies for non-electrified VTOLs are shown. Added are state-of-the-art eVTOL concepts that are under development. Abbreviations: VC2x: Volocopter 2x, AFS PAR: Aurora Flight Sciences PAR, PA VBJ: Pegasus Aerospace Vertical Business Jet, KW: Kitty Hawk, TAC: Transcendet Air Corporation. Not Good means bad.

As the general correlation between disc loading and hover lift efficiency is of the form 1/x^n, one cannot make a simple “the bigger the better” type of comparison. Taking as an example the Joby S4, it has a lower disc loading than a direct lift type of aircraft (and is therefore, we assume, quieter) it is also less efficient than comparable tiltrotor concepts. However, I would strongly argue that it is not meaningful to compare a tilt-rotor kind of aircraft with a jet and it is a good approach to compare similar kind of concepts.

Following this idea and comparing the CityAirbus, the eHang and the CarterCopter to helicopters, and Vahana to tilt wing or Joby S4 to tilt-rotor aircraft, one would conclude that almost all pursued eVTOL concepts would perform worse than certified state-of-the-art aero vehicles. This would be in strong contradiction to the high sums of corporate R&D and VC money that are currently invested to bring such concepts to certification.

I assume that this problem arises from the fact that mostly the maximum power of the propulsion systems is reported while the actual power required for hovering is sufficiently less than that. Indeed, almost all of the concepts are multi-rotor concepts. Hence, autorotation cannot be used to ensure a safe landing in the case of one engine inoperative failure (OEI) and the remaining engines have to provide more thrust than needed for regular hovering, in order to bring down the aircraft safely.

Knowing the propeller diameters, the number of propellers and the required thrust, an estimation of the required hovering power is possible. Indeed, it turns out that this gives power levels that are roughly ~ 50% below the installed power. Redoing the calculations based on these system power levels gives Fig. 5.

Figure 5: The same eVTOL concepts as in Fig. 4 are shown with recalculated power for hovering based on weight and propeller data. The green dashed line is a “best-guess-fit”.

Now, it gets interesting. The first observation is: The y-axis had to be extended to accommodate properly for the adjusted hover lift efficiency numbers. This is good news. Additional good news seems to be a shift in the trend line (dashed green line, which is an “out-of-the-guts fit”) in the beneficial directions (to lower disc loadings and higher hover lift efficiencies). So, let’s get into the details and start with Lilium.

While in Fig. 4 the Lilium Jet appeared at the upper end of Lift-Fan concepts, in Fig. 5 its hover lift efficiency is lower. Taking a closer look at the Lilium Jet, I would assume that the wing and placement of the lift-fans are arranged in such a way, that the airflow over the wing produces dynamic lift that contributes to the total lift similar to ideas of distributed propulsion for STOLs (X-57 Maxwell). An accurate estimation of this contribution is quite a difficult task for CFD even knowing the exact geometry. Therefore, I will help my-self with some “if-then” statement at this point: If this contribution is in the same range as the direct lift thrust of the fans (say 50 %), then the Lilium Jet would be at roughly 2.5 kg/kW and 500 kg/m² (in the same area as the Pegasus Aerospace VBJ) which would be superior to classical lift-fan concepts. If this contribution is even higher (say 70 % to 80 %, which seems to be very optimistic), then at 5.9 kg/kW and 200 kg/m², it would be an outstanding lift-fan concept with a hover lift efficiency comparable to helicopters. Nevertheless, the disc loading is probably higher than of other concepts. Also, small fans running at high speed would probably have an acoustic spectrum that is shifted towards higher frequencies where the atmospheric attenuation is higher as is also the subjective perception of noise. Although, the propulsion system power level is comparably small, these effects could cause a substantial acoustic footprint during take-off and landing.

The XTI TriFan600 and the TAC Vy400 appear to be quite in the middle of what is common for tilt-wing aircraft. As the TriFan600 is a mixture of a tilt-rotor and a lift-fan, it is reasonable to see it somewhere in-between these two types. The Vy400 appears to be slightly worse than what is expected for a tilt-rotor design. I must admit that this could be connected to the rotor diameter estimation. With a 30 % larger rotor, it could move to the tilt-rotor expectation space. Both concepts are intended to be hybrid-electric and their general configuration has similarities to non-electrified VTOL concepts, wherefore it is reasonable that they have similar efficiencies and disc loadings. Nevertheless, the fact, that the propeller is driven by an electric motor (and not by a combustion engine) could help to reduce the noise emissions of these concepts substantially: Characteristic acoustic frequencies of the turbine (or combustion engine) tend to increase the noise of propulsors [6]. Electric motors can be built to have a very low torque ripple, i.e. largely without excitation of such characteristic frequencies. Thus, reducing the overall noise level at least in the noise cone where the engine noise is dominant.

The next group in line are the two tilt-rotor configurations, namely the Joby S4 and the Bell Nexus. Again, as both have strong similarities with older concepts, it is not very surprising that they fall into the expected range. However, especially for these two concepts, it was hard to find good data, wherefore I would assume that the values could be better, i.e. around 4 kg/kW and 70 kg/m². Looking at this disc loading, it is hard to understand how Joby intends to achieve a 100x noise reduction compared to helicopers. Nonetheless, two effects might play in their favor: Firstly, the smaller propellers (compared to a helicopter) could have a lower tip speed and therefore be less noisy [4]. Secondly, a large noise contribution to the noise footprint of classic propellers is the blade flap [1] that might be avoided by the right combination of propeller size, position and cruise speed.

What might be somewhat surprising is that the Kitty Hawk Cora and the Aurora Flight Science PAR have a large spread in hover lift efficiency although both concepts share many similarities such as a large number small lift propellers, one large pusher propeller and a wing that allows dynamic lift during forward flight. Here again, the estimation of propeller sizes might be somewhat favorable in one direction than the other. However, structurally the PAR design has a closed frame structure which will save weight on the airframe as compared with the propulsor beams of Cora which do not integrate further structural functions. Hence, leading to a lower empty weight in the case of the PAR design. Comparing these two concepts (but also the tilt-wing and tilt-rotor designs) to the multi-copter concepts, it’s important to keep in mind that here only the hover lift efficiency is considered. In forward flight, the propulsion efficiency (i.e. what power is required to keep the aircraft in forward flight at constant speed) will be significantly higher due to the dynamic lift of the wing.

Further, there are both Airbus concepts that are very different in design but very close in terms of hover lift efficiency and disc loading. However, while in these coordinates the CityAirbus appears to be somewhere in the middle of state-of-the-art helicopters, Vahana is strongly superior to classic tilt-wing designs and can even compete to some extend in the helicopter design space. Low disc loading comparable to helicopters values and the smaller propellers could indicate indeed these concepts to be more silent than conventional helicopters. On the other hand, contra-rotation propellers proved to be rather noise enhancing than reducing as in the case of An-70. Also, speed control as is typical for commercial drones gives rise to strongly disturbing acoustic effects (comparable to a swarm of bees or to an orchestra that is out-of-tune). This effect would not only touch the Airbus concept’s but likewise any multirotor concept relying on speed control for maneuvering. Vahana tackles this problem by using thrust control that is implemented through blade pitch controls with hydraulic propeller governors.

At last there are the eHang, the CarperCopter and the VoloCopter concept. The Carper concept has a low disc loading and high hover lift efficiency as it is essentially an electrically driven compound helicopter. The eHang concept achieves very low disc loading and high hover lift efficiency by employing a very light-weight primary structure. In the case of VoloCopter, the very high hover lift efficiency is achieved by a very large number of small propellers, resulting a high total area. Although this design might appear strange, it makes a clever use of the scaling advantage of electric motors: In contrast to thermal engines, the power density and efficiency of electric motors stays nearly constant over a wide power range. Therefore, 18 small electric motors will have nearly the same weight and efficiency as one with the 18-fold power. However, the large number of small motors will allow a high level of redundancy and the total installed power can be greatly reduced.

The fact, that most concepts have substantially better values in Fig. 5 than in Fig. 4 gives room for further improvement. For instance, if the electric motors are built in such a way that a single point leads only to a partial loss of power, the total installed power can be further reduced and consequently the battery be optimized towards energy density rather than power density, allowing a higher range.

Before we turn to the speed plot, a last observation: It appears that concepts with one passenger have both lower disc loadings and higher hover lift efficiencies. Thus, it will be interesting to see, if these designs can scale to larger passenger numbers without sacrificing their good values.

In Fig. 6 the disc loading (normalized to helicopter values) versus the expected cruise speed are shown. The original figure is taken from [3]. For normalization I used 40 kg/m² — an average value for helicopters according to Fig. 4.

Figure 6: Disc loading vs. speed. Original figure taken from [3].

Compared to Fig. 4 or 5, it’s somewhat easier to judge in this case. It’s good if the design ends up at the lower right corner, i.e. at high speeds and a low disc loading (so hopefully at low noise levels).

With some exceptions, like especially Lilium and the Bell Nexus, most concepts seem to fall in the expected band. Interestingly, most concepts are going for rather lower than higher speeds than conventional helicopters. This is coherent with the targeted markets (short range intercity mobility, where the cruise phase is short compared to the take-off phase) and the fact that lower speeds reduce the energy consumption during forward flight, therefore extending the range that can be achieved with a battery.

Joby S4 exceeds this observation in speed, nevertheless not to the speeds that would be targeted by earlier tilt-rotor designs. I suspect that here again the battery is the limiting factor. Thus, a hybrid-electric concept could be meaningful to overcome the range and speed limitations, opening the possibility for a convenient intracity transportation. I would prefer to see hydrogen-based hybridization, i.e. using a fuel cell or hydrogen turbine, in order to maintain the zero-emission targets. Such a hybrid approach could also be useful for the Kitty-Hawk Cora or the Vahana concept (scaled to >2 PAX) to extend range and speed. Furthermore, I would advocate that older ideas such as the variable diameter rotor could be used to enhance the performance of the Joby S4 or the TAC Vy400.

Conclusions

Comparing new eVTOL concepts to the current and previous VTOL designs, a positive trend towards lower disc loadings and higher hover lift efficiencies seems to take place that is enabled by the scaling properties of electric motors. Indeed, if we think that an electric propulsion system is roughly 3 to 4 times more efficient and the hover efficiency is 2 times better (in the extreme case), an electric flying multicopter is feasible for missions below 50 km (i.e. intercity use-case).

Even if the market of urban air mobility is delayed due to regulation, certification, infrastructure issues or public acceptance, current eVTOL designs could replace conventional helicopters in cost-sensitive applications due to a lower energy consumption. If the range limitation is overcome, for instance, by hybridization, even a larger part of the helicopter market could be in sight.

From a noise perspective, lower disc loadings indicate that quieter VTOLs can be realized. However, it is not obvious if the advantage of electric motors (such as low torque ripple and quiet rotation) or problems such as speed control of propellers will overcompensate each other. Nevertheless, the flexibility that comes with electric motors enables a large design space for ideas to minimize the noise impact.

At last, I would be curious to see if some of the “old ideas” like the variable diameter tilt rotor or especially the X-wing idea could be particularly advantageous in combination with the design opportunities that electric propulsion offers.

P.S.

Dear reader, if you find that some of the data points are far off, don’t hesitate to get in touch and we try to see if there is better data available that allows to increase accuracy.

References

[1] Jack E. Made and Donald W. Kurtz: “A Review of Aerodynamic Noise From Propellers, Rofors, and Liff Fans”, Technical Report 32–7462 (1970)

[2] Martin D. Maisel, Demo J. Giulianetti and Daniel C. Dugan: “The History of the XV-15 Tilt Rotor Research Aircraft From Concept to Flight” in Monographs in Aerospace History #17 (2020), The NASA History Series

[3] Evan A. Fradenburgh: “The High Speed Challenge for Rotary Wing Aircraft” in SAE Transactions Vol. 100, Section 1: JOURNAL OF AEROSPACE, Part 2 (1991), pp. 1969–1987

[4] W. Z. Stepniewski and F. H. Schmitz: “Noise Implications for VTOL Development” in SAE Transactions Vol. 79, Section 2: Papers 700193–700395 (1970), pp. 941–956

[5] Airbus A³: Vahana Specifications

[6] J.P. Yin, S.R. Ahmed and S.R. Dobrzynski: “New Acoustic and Aerodynamics Phenomena due to non-uniform Rotation of Propellers” in Journal of Sound and Vibration (1999), 225(1), pp. 171–187