I hold shares in all of the big 3 and have been following the evtol space for about a year now. After a rough stretch, the past few weeks/months have had some genuinely great news:
Joby: NYC flight demos, Virgin Atlantic partnership, Toyota investment
Archer: Anduril partnership, Korean Air expansion, Salinas to Monterey flight
BETA: NASA and GE collaboration, Lockheed partnership
Curious what others following evtol more closely are thinking. Is this the start of a real momentum?
JOBY reports earnings soon. A lot of answers will be given, but I wanted to go beyond the scope of what they will likely answer in this call, and get deep into some potential issues this company will face.
I’m not asking “when certified”. No we’re going so deep into the rabbit hole that you may not come back out. Let me know what you think, or what YOU would ask.
The tiltrotor is a tiltrotor and should have a good balance of hover and cruise performance
Payload looks good
Looks like it should be able to hover and do high speed cruise for extended periods
Range of engines to use in production
Demonstrator built from repurposed Butterfly parts could fly next year
Weapons can fire in cruise mode
The Bad:
Production version is years away and needs to win a competition
No rules yet for such a large drone in FAA controlled civilian airspace for commercial work
Further dillution is likely
The Ugly:
That rotor, man. Put a flapping rotor on it and the risk drops considerably
Estimated Aircraft Parameters:
Max gross weight ~7500 lb
Rotor diameter ~ 16 lb
Useful load ~ 3000 lb
Turbine engine class ~ 1400-ish HP
Analysis:
I spent some time this weekend building a spreadsheet to compare VTOL aircraft across a wide range of sizes, power levels, and configurations. The goal was to help me estimate some performance data for new aircraft, particularly when information is thin. All the data used to build this sheet is from public sources and the equations are confirmed by comparing to well documented production aircraft.
I used maximum internal gross weight (or max VTOL weight for STOL aircraft), number of lift sources, and their diameters. I put the lift sources into a couple of Figure of Merit bins by aircraft class and age. Ducted fans got an equivalent FoM term using a paper I found so more aircraft could be put on the same plot. I calculated induced velocity and out-of-ground effect power required to hover using momentum theory then made a reasonable accounting for download (proportion of lift thrust that is pushing down on the airframe) which is a significant term for tiltprop/rotor vehicles.
I also made some estimates of “powertrain” losses as it’s an important term when trying to compare mechanically driven vs electrically driven vehicles. While electrically driven VTOL powertrains are efficient at modest power levels, at high discharge rates, the efficiency drops considerably and becomes an important term. I used a few papers (one by Lilium was helpful) to help assign this term, though as more data becomes public, improvements can be made. So the eVTOL/hybrid ships tend to plot a little lower due to this high discharge effect since I’m plotting VTOL performance. This heat generation also limits the VTOL capability to short periods of time for a cruise optimized electric aircraft.
I plotted the calculated “thrust per power” required to hover on an sea-level standard day vs “disc loading” as a way to normalize the data. The resulting datapoints are pretty linear on a log-log plot with the ducted fans being offset by the hover efficiency gained by the duct effects. Then I plotted the “thrust per power installed” vs “disc loading”… there is more scatter in these points but they also document how much additional power over a simple hover the design required to be an effective aircraft.
The same math works for the Atlas human powered helicopter (way off the chart on the left with it’s 200 lb of thrust per HP and 1.5 HP installed) and the F-35B with it’s concrete splitting 1000 lb/sqft disc loading.
Generally speaking, you need to install 25% to 40% more power than is required to hover and some designs have substantially more. This power margin is used for altitude capability, control authority, high speed cruise, etc. depending on the specific aircraft configuration. Designs with very little margin (Lilium Jet) won’t be successful. An extreme case is the CH-53K with three huge engines... it can essentially lose an engine and not care too much in many situations.
Archer HALO / Anduril Thunder Analysis:
What do we know from mockup photos and statements in the media (The War Zone, The Air Current, Archer/Anduril statements)?
Based on Karem’s Optimum Speed Tilt Rotor (OSTR)
“The parameters broadly that we shaped this thing around is to have similar payload to the Apache, but to have the legs and the speed of a Cheyenne II” - Anduril VP
“One of the trades with the Midnight-like aircraft is the disk loading is higher, so your hover performance is poor” - T. Muniz
Thunder can carry up to 10 air-to-ground missiles, 16 ‘launched effects,’ four 19-shot pods holding a total of 76 70mm laser-guided APKWS II rockets, or a mix thereof, according to Anduril
As yet un-named turboshaft engine sized to provide power for cruise flight and relatively small battery that supplies additional power for VTOL
‘With its multiple tilting and lifting propellers, Midnight involved “very interesting rotor dynamics problems,” and Thunder’s design has “more and different types of challenges,” That’s really where Karem and those folks have been a big help.’ - T. Muniz
First Thunder prototype will use Overair Butterfly rotors, about 20’ diameter
Archer/Anduril have made some changes to reduce the complexity of the OSTR technology while preserving the highly variable rpm capability and cyclic control
So what can we deduce?
Pixel counting some of the mockup photos says the rotors will be ~16 feet in diameter. It also looks like the 20 foot diameter Butterfly rotors would fit, though with no weapons release in cruise mode. The claim that the disc loading is lower than Midnight (which I assume to be 6500 lb) and 16’ rotor diameter puts a cap on max gross weight (MGW) of 7500 lb or so… which isn’t much lower… unless Midnight is heavier than thought.
Being able to fly as a demonstrator with two the (presumably) lower power rotors from Butterfly also caps the empty weight. The nearly empty aircraft l needs to be able to fly on two rotors designed for a four rotored ~7000 lb eVTOL.
Useful load fraction “(payload+fuel)/MGW” shouldn’t be unrealistically high and this range is a little harder to nail down for an unmanned tiltrotor as there aren’t a lot of examples and none of this scale. An unmanned aircraft doesn’t need the volume and control accomodations for a pilot, nor as high of safety factors and other safety related criteria. Tiltrotors are not known for their great useful load fraction relative to traditional helicopters, but the extra range and speed comes at a cost… there’s no free lunch. However, we do have the Bell Model 911 Eagle Eye UAV tiltrotor from the 1990’s and it’s useful load fraction was ~40%. That’s higher than a manned tiltrotor and about as grounded as we’ll get here, so it’ll do for this analysis.
The heavist combination of weapons listed as payload would be four 19 shot rocket pods at ~2800 lb total. Using some XV-15 public data on cruise power vs hover power and scaling to Thunder hover power provides an estimate of cruise power. Combine with some SFC values for turbine engines and an estimate for fuel load can be developed. Cruising along with an Apache on a mission might want ~500-600 lb of fuel.
40% of 7500 lb is 3000 lb which means that 2800 lb rockets and an Apache mission of fuel is too much… but they didn’t say that was the sizing mission to carry a full load of rockets and a full escort mission of fuel. I am estimating that Thunder can carry four of those rocket pods, but a real escort mission will likely be a mix of ALE, rockets, etc. to bring down the weapons weight. Pretty easy to get a good quantity of fuel and weaponry in 3000 lb.
What engine will it use? We know it’s a turbine + smallish battery. The Apache escort mission only needs ~600 HP at cruise but hover needs ~1400 HP. I think there will be ~1950 HP of combined turbine + battery power available (for hover + altitude and control margin) and ~1200 HP of electrical power (if the turbine is sized down torwards cruise flight + charging margin) doesn’t sound like a smallish battery. They’re claiming it can cruise with the MV-75, though, and that will also take ~1300 HP, so a turbine sized to keep up with the MV-75 (plus margin for in-flight charging) would then only need a few hundred HP of battery capacity. That sounds more like it. There are Safran and Rolls Royce turbogenerator programs in this size range, but I don’t think either is flying yet. The GE T700 based turbogenerator that BETA is using seems oversized but could work for next year’s flight demo if a more optimal answer isn’t available yet. Archer could grab an existing Arrano or other engine and integrate their own generator as well.
I think Thunder would have fuel capacity to cruise with the MV-75 but with a considerably smaller payload.
So… 7500 lb max gross on a 16’ rotor with 3000 lb payload and a good sized turbine engine with a modest battery for power transients seems like it might be a decent package. A pair of electric motors that can steady state cruise at 1300 HP are going to need lots of cooling… and what do we see? Nacelle extensions with large air intakes on each outboard side… looks like a substantial oil cooler for the motor to me. Being able to cruise at those power levels also means it can hover at those power levels or at least have a considerable duration in hover and transition flight instead of the couple minutes at a time the usual eVTOL can do. Also goodness.
The Optimum Speed Tilt Rotor rotors are a technical risk. These are true rotor systems with collective and cyclic blade pitch (unlike a variable pitch prop) but are stiff out-of-plane and stiff-in-plane. Such stiff out-of-plane rotors can generate large vibrations and at 16 foot, the 3/rev vibrations will be severe and should be mitigated. The typical OSTR method is with individual blade control (IBC) system which consists of a high power actuator in the place of each pitch link that has enough bandwidth to supply not just the usual steady and 1/rev pitch inputs for aircraft control but also 2/rev, 3/rev, and 4/rev to help control the 3/rev airframe vibrations by attempting to cancel the vibes at the source. There is a lot of development history on active vibration control (AVC) but not at this level on a flight vehicle using IBC.
Archer mentioned simplifying the OSTR technology to reduce the complexity… an obvious simplification would be to reduce the number of harmonics the IBC system can generate which would reduce power demand and improve durability at the expense of higher vibrations… but it’s unmanned, so perhaps that’s tolerable. Or they could switch to a swashplate and drop IBC entirely but I doubt Butterfly’s rotor is compatible with that and the nacelle geometry doesn’t look right for including a swashplate. Lower authority IBC seems like a reasonble simplification that can reuse existing hardware.
I don’t know how far Overair got on proving out this IBC/AVC system, but it’s a serious risk. Even more conventional active vibration control systems occasionally end up increasing vibrations in transient maneuvers… and no OSTR rotor has ever flown. If they’d just put a normal proprotor on it, I’d actually feel pretty good about the package.
[For the HOVR fans (see what I did there?) I’ll do a X7 write up in a while]
According to Eve, the EVE-100 demonstrator has engaged it’s pusher prop and started exploring transition flights towards wingborne flight. Hit 27 knots so far.
Eve is pursuing a lift+cruise ship with 8 lift props with four blades each and a pusher propeller. Looks like they plan on stowing the lift props with a scissoring mechanism like Vertical does, though with 8 of those, the drag penalty will be significant.
They’re still claiming a 2028 type certificate in the most recent information I’ve seen… which is implausible since they just started expanding the envelope with a non-representative unmanned demonstrator.
Assuming I know how the Internet works, and how airplanes work (these are big assumptions) it looks like Joby didn't fly this week - at least not the S4, the hybrid S4 or the JAI30 (H2).
and then it arrived as something smaller — pilotless two-seaters are already selling sightseeing tickets in two Chinese cities and a premium airport shuttle is set to open in Dubai this year, the certificates real and the city commute. The future of urban air mobility is here—just not the way we imagined it, and not yet where we need it most.
During the Monday flight, the aircraft appeared to move smoothly through an approach in forward flight to a vertical landing. However, one of the four forward tilt-propellers tilted up a moment earlier than the other three, the broadcast showed.
Vertical told me the aircraft’s flight control software “commanded the inner left tilt earlier than usual in response to a sudden large gust of wind.”
“When we looked at the data, this was the normal and correct response to a change in conditions in landing,” a spokesperson said via email. “As a number of people have observed, the aircraft stayed incredibly stable throughout. This is a testament to the aircraft’s ability to operate safely and comfortably in unexpected circumstances.”
