Engineering Challenges Facing New VTOL Aircraft

[60FltMech]

I decided to start a new thread to maybe contain discussions around engineering challenges that some of the new VTOL technologies currently being developed are facing, and possibly how systems are developed, by some of the people that know how “the sausage is made”.

There’s naturally a lot of discussion around V-280, X2 technology etc what with the various US Army programs that have been awarded or will be awarded in the future, but I’d love to see discussion on other VTOL tech as well.

Even the Bell Invictus, while appearing to be a more “conventional” helicopter, will undoubtedly seek to leverage advancements in flight controls and drivetrain technology that have happened in the past decades since our current frontline rotary wing platforms entered service.

I know several members here have the education and real-world experience working through these sort of issues and can explain the physics of it to laymen like myself, I greatly appreciate reading and learning and I’m sure I’m not the only one.

Anyway, here’s the thread. Whoever has questions about all this new technology, throw them up here!

FltMech

[SASless]

Excellent offering and hopefully it shall turn out to be a learning source for a lot of us not so involved in the technical side of things.

[helispotter]

60FltMech: I'll enjoy reading posts prompted by your thread.

I had been reading posts in the threads "Hill Helicopters HX50", "Marenco Swiss Helicopter" and "new gyroplane". These are start-up ventures, or began that way. That seems brave nowadays. The competition in the market and required finances are perhaps greater challenges than any engineering challenges they will face.

[SplineDrive]

60FltMech: I'll enjoy reading posts prompted by your thread.

I had been reading posts in the threads "Hill Helicopters HX50", "Marenco Swiss Helicopter" and "new gyroplane". These are start-up ventures, or began that way. That seems brave nowadays. The competition in the market and required finances are perhaps greater challenges than any engineering challenges they will face.

Aside from the technology involved, which is hard enough, you're right that the finances required to get a certified product in customer hands and have the stability to support a growing fleet is often an even more daunting task. Most, if not all, these start ups and eVTOL firms really don't know the paperwork ****storm they're running into when they try and get a new aircraft certified by a brand new company without any institutional history, data, processes, process controls, etc. I honestly don't think a new company will be held to the same standards as an established OEM... what the cert authorities put the existing OEMs through seems like an insurmountable bar if you're starting from square zero.

[wrench1]

Most, if not all, these start ups and eVTOL firms really don't know the paperwork ****storm they're running into when they try and get a new aircraft certified by a brand new company
I think you'll find the VTOL and the eVTOL industries are two distinct and separate markets especially on the civilian side. I watched the civil VTOL market evaporate from the anticipated 1000 projected units to basically none. On the other hand, the eVTOL market has exploded and is projected to be a $30B market by 2030. Plus it has the high end money behind it.

As to the eVTOL certification side, I believe the EASA has elected to create a new regulatory part for eVTOLs that will be much leaner than the conventional aircraft rules and use more consensus standards. I don't follow the EASA side much but the FAA has decided the same for a new part after trying to fit eVTOLs into the Part 23 structure. Haven't seen any details as yet, but several of the leading eVTOL manufacturers are progressing through the certification requirements on both sides of the pond as they are written. From what I've seen and read in a limited capability, I think we'll see certified Part 135 eVTOLs pax ops before you know it provided they don't start falling out the air and killing people.

While the designs and mechanics of conventional VTOLs are very interesting and which I've followed since the XV-15 days, I think outside of the military and a few AW609 customers the eVTOL industry will be the common player. How the 4-6 pax eVTOL will fit into the big regulatory picture with conventional VTOLs, rotorcraft, and other hybrid aircraft will be the most interesting development.

[JohnDixson]

One area of technology where there exists a vacuum is the absence of design standards for modern ( i.e., Fly-By-Wire ) control systems. Some might argue that is good, because the technology leaders in this area are all working for private companies, and asking for either the FAA or the various military organizations to generate standards is in fact asking for trouble because neither has experienced staff in this area. I am of that bent. So, how does this get done? Or, does it need to be done?

[60FltMech]

JohnDix,

When I wrote the post opening this thread I had two questions in mind that I wanted to ask: first was about fly by wire and the second question, which is regarding flight control hydraulic systems.

It seems that if you could develop a rugged fly by wire system you would be well on your way to simplifying your flight control hydraulic systems.

Regarding V-280, I assume Bell will use what they have learned on their 525 FBW and hydraulic systems. But as you pointed out the standards for such systems for VTOL aircraft aren’t fully established, so how does it all come together in the end?

I just can’t see an aircraft with a mass of push pull tubes, mixers, bellcranks and Hydraulic lines being the future of Army Aviation(or VTOL in general), someone has to crack the code on simplifying these systems, while also keeping redundancy for battle damage or other emergencies.

FltMech

[Chock Puller]

I too look forward to seeing this kind of discussion becoming more prevalent as the forum needs some fresh topics for consideration.

Thank you 60FltMech for creating this thread and I shall look forward to more in the future.



A 2016 article, but some interesting points being made re DARPA and its ability to be innovative.

Is there a genuine inability of the US Military (US DOD) to find solutions for the new generation rotorcraft/tilt rotor/or whatever you want to call the new concepts?

https://www.darpa.mil/attachments/DARPA_Innovation_2016.pdf


A later article reporting DARPA and Sikorsky UH-60 FBW research.

https://idstch.com/military/air/darpa-alias-fly-by-wire-kit-turns-commercial-helicopters-and-aircrafts-into-unmanned-drones-with-autonomous-landing/

[wrench1]

So, how does this get done? Or, does it need to be done?
From my understanding it depends on the integration of the FBW. For the Bell 525 FBW the FAA issued Special Conditions to its certification basis under Part 29. See below. I also believe for the AW609, its flight control certifications will be dealt with under the new Powered-Lift category's certification requirements which has been described as a collection of existing Part 23, 25, and 29 requirements. However, my info is a bit dated on the 609.
Special Conditions: Bell Textron Inc. Model 525 Helicopter; Fly-By-Wire Flight Control System (https://www.regulations.gov/document/FAA-2021-0065-0004)

[Petit-Lion]

I for one think that the most obvious improvement to a conventional helicopter would be a swiveling tail rotor. I am surprised that only Karem ventured into that technology... to no avail so far. Why is it so difficult to implement, compared to seemingly much more complex designs?

[SplineDrive]

I for one think that the most obvious improvement to a conventional helicopter would be a swiveling tail rotor. I am surprised that only Karem ventured into that technology... to no avail so far. Why is it so difficult to implement, compared to seemingly much more complex designs?

Swiveling tail rotor as in, rotating the tail rotor from a horizontal thruster to a largely axial thruster as the aircraft accelerates so it becomes like a propeller on the back of the aircraft (like X-2 or AH-56)? Sikorsky actually built and flew this mechanism (they called it "Roto-Prop") on a modified S-61 used as a technology demonstrator (S-61F). Basic issue why you don't see more of this is for the prop is only more effective than the main rotor at providing forward thrust at fairly high speeds. To really utilize the prop to push the aircraft to speeds a good conventional helicopter cannot achieve requires a lot of power installed, just for the prop, and thus the tail of the aircraft becomes home to the high power transmission system as well as the swiveling mechanism. An edgewise flow main rotor will always have pretty terrible drag compared to a fixed wing aircraft with axial flow propellers. So the power required to hit, say, 250 knots, looks pretty ridiculous for the size of the aircraft.

In the end, you can achieve near 200 knots in a more conventional helicopter without a prop or swiveling prop. If you desire going much faster than that, a tilt rotor ends up being the better answer and really the only answer at around 250 knots and above. So there is a possible range of 200 to 250 knot target cruise speed where some sort of thrust compounded helicopter MIGHT be a better balanced design than the alternatives, but that's an awfully narrow range to spend a lot of development time/effort into. It also requires a lower drag main rotor/hub and aircraft than is typically achievable.

The Airbus Racer or X3 concept is interesting and can achieve high speeds, but the main rotor is significantly offloaded via wings (like other high speed conventional helicopters) and there is no swivel function as yaw control comes from differential blade pitch across two smaller props. This configuration might practically fill that 200 - 250 knot gap, maybe.

Short answer: wings are simpler and cheaper than props to get a helicopter to 200 knots... if you want more speed than that, the tilt rotor appears to be the ideal answer. Airbus is working a hybrid concept that might fill the gap between the two.

[SplineDrive]

I think you'll find the VTOL and the eVTOL industries are two distinct and separate markets especially on the civilian side. I watched the civil VTOL market evaporate from the anticipated 1000 projected units to basically none. On the other hand, the eVTOL market has exploded and is projected to be a $30B market by 2030. Plus it has the high end money behind it.

As to the eVTOL certification side, I believe the EASA has elected to create a new regulatory part for eVTOLs that will be much leaner than the conventional aircraft rules and use more consensus standards. I don't follow the EASA side much but the FAA has decided the same for a new part after trying to fit eVTOLs into the Part 23 structure. Haven't seen any details as yet, but several of the leading eVTOL manufacturers are progressing through the certification requirements on both sides of the pond as they are written. From what I've seen and read in a limited capability, I think we'll see certified Part 135 eVTOLs pax ops before you know it provided they don't start falling out the air and killing people.

While the designs and mechanics of conventional VTOLs are very interesting and which I've followed since the XV-15 days, I think outside of the military and a few AW609 customers the eVTOL industry will be the common player. How the 4-6 pax eVTOL will fit into the big regulatory picture with conventional VTOLs, rotorcraft, and other hybrid aircraft will be the most interesting development.

eVTOL got "high end money" back when interest rates were near zero. A cash crunch is coming soon and most of the players won't survive their cash burn rates as certification is delayed. The FAA cert basis documents for Joby and Archer are out and they both reference a requirement to be able to perform an "controlled emergency power off landing equivalent to a glide or autorotation". Joby's aircraft MIGHT be able to autorotate as its props are fairly large for the multi rotor industry. Jaunt Journey can autorotate. None of the others can, they simply lack enough inertia, even if they had the control authority/range to perform the required maneuvers to enter a stabilized auto. So that rule alone, which IS a good rule, complicates certification for most of these aircraft. The industry is also settling on a configuration with high part count, lots of blades and bearings to inspect, low yaw authority in VTOL ops, and one that doesn't scale in size, limiting the utility of the aircraft concept.

I think it's largely a capital and brainpower bonfire.

They may claim to be different markets but we all fight the same laws of physics and economics. Few of these companies will survive to 2030 much less be in commercial operation by 2030 in a 30 billion dollar market.

[Petit-Lion]

Many thanks SplineDrive for your detailed explanation. I feel a little less dumb now :)

[helispotter]

...The Airbus Racer or X3 concept is interesting and can achieve high speeds...

I hadn't seen anything about the X3 for a while, so looked it up on Wikipedia (https://en.wikipedia.org/wiki/Eurocopter_X%C2%B3). It apparently achieved 255 knots in level flight in 2013, indicated to be an unofficial helicopter speed record. Yet it has already been in a museum since 2014!

I hadn't come across Racer (Rapid and Cost-Effective Rotorcraft), so looked it up too (https://en.wikipedia.org/wiki/Airbus_RACER ) and now understand it is an evolution from the X3. Wiki reports "first flight anticipated at the beginning of Q2 in 2022" but that has clearly passed so progress presumably still slowed due to COVID?

The Racer article in turn reminded me of the Piasecki X-49 and Sikorsky X2 as compound helicopters of 'comparable' configuration. The X-49 (and its predecessors) seemed like a mechanically straightforward alternative to any swivelling tail rotor concepts. Is there any future in such an arrangement as opposed to the X3 / Racer configuration?

Finally, is there any future in revisiting the DTNSRDC X-wing concept that had been built and fitted to the S-72 (https://en.wikipedia.org/wiki/Sikorsky_S-72), but was never flight tested? It seemed like a radical idea to achieve high speed flight of a rotorcraft at the time. Rotor is only used for take-off then becomes stationary for high-speed forward flight. Not sure how they managed to solve the problem of the aerodynamic loading on the forward swept pair of stationary blades (wings)!

[OldLurker]

Maybe this doesn’t make sense, but here goes: one thing that puzzles me a little, perhaps only because of my own ignorance, is that for pushing air downwards (ie VTOL) development efforts have focused almost entirely on what are effectively propellers – one or two big unshielded rotating wings, or sometimes lots of small ones – whereas for pushing air backwards (runway takeoff), jet propulsion is increasingly the norm, on smaller and smaller aircraft, now down to the VLJ category.

Of course the military have played with jet propulsion to push air downwards, with vectored thrust as in the Harrier, or with lift jets or fans, but AFAIK they’re few, and vectored-thrust has mostly gone away from VTOL towards improving manoeuvrability of fighters.

[SplineDrive]

I hadn't seen anything about the X3 for a while, so looked it up on Wikipedia (https://en.wikipedia.org/wiki/Eurocopter_X%C2%B3). It apparently achieved 255 knots in level flight in 2013, indicated to be an unofficial helicopter speed record. Yet it has already been in a museum since 2014!

I hadn't come across Racer (Rapid and Cost-Effective Rotorcraft), so looked it up too (https://en.wikipedia.org/wiki/Airbus_RACER ) and now understand it is an evolution from the X3. Wiki reports "first flight anticipated at the beginning of Q2 in 2022" but that has clearly passed so progress presumably still slowed due to COVID?

The Racer article in turn reminded me of the Piasecki X-49 and Sikorsky X2 as compound helicopters of 'comparable' configuration. The X-49 (and its predecessors) seemed like a mechanically straightforward alternative to any swivelling tail rotor concepts. Is there any future in such an arrangement as opposed to the X3 / Racer configuration?

Finally, is there any future in revisiting the DTNSRDC X-wing concept that had been built and fitted to the S-72 (https://en.wikipedia.org/wiki/Sikorsky_S-72), but was never flight tested? It seemed like a radical idea to achieve high speed flight of a rotorcraft at the time. Rotor is only used for take-off then becomes stationary for high-speed forward flight. Not sure how they managed to solve the problem of the aerodynamic loading on the forward swept pair of stationary blades (wings)!

X-49 as a research platform has been going on for a while, but not a ton of public data. The configuration adds a wing (which is key to high speed performance with a flapping rotor) and the ducted prop. Yaw control is via rotating segments of a turning duct structure into the prop thrust. The entire tail assy looks really heavy and I suspect yaw control is still somewhat marginal. A production configuration was slated to add a third engine to handle the required prop power (similar to the discussion above) and a nearly 4 foot fuselage extension which I imagine goes forward of the wing to help move the center of gravity close to the main rotor mast axis. So... can the configuration work? Yes. In fact, with the new ITE engine and a new drive train, a third engine probably isn't needed. Is it better than X3? Not sure. The Airbus X3 concept splits the anti torque and forward thrust into two, though smaller, diameter propellers. Two props is more drive system and rotor components, but the turning duct isn't needed, shaft torques are lower, and the hardware is much closer to the aircraft center which reduces yaw inertia. The ducted prop has protection for people on the ground which is something the X3 notably lacks.

The Sikorsky X-2 is a different beast. Remember my comment about X-49, X3, Invictus, and other high speed helo having wings to achieve high speed? The wing is a solution for overcoming the drop in lift/thrust performance of a flapping main rotor as the aircraft speed increases. In textbooks, this is the "Ct/sigma vs mu" plot, a way of plotting lots of rotor data in a non-dimensionalized form. Adding a wing offloads the rotor so that as aircraft speed increases the lift demand from the rotor drops to stay within its capabilities. On an aircraft without a propeller, this also means the lift available to pull the aircraft through the air via rotor flapping also drops, but if you can flap the rotor a lot and/or have a low drag fuselage in a nose down attitude, you can still achieve higher speed than usual for a helo. The propellers on X3, X-49, and AH-56, etc. replace the missing forward thrust from the main rotor with pure horizontal thrust... at the cost of adding more rotors. In the extreme, a large enough wing and prop can completely unload the main rotor and then you just need to slow the main rotor down to keep the advancing tip below Mach 1 and add tip masses to keep the rotor stable. This is the Carter Copter (and Jaunt Journey) solution. Larger and larger wings do add download, though and harm hover performance, but going fast does require more installed power on the aircraft, so that can be overcome with a larger main rotor (and more mass). There's no free lunch in any of this.

Back to X-2... it does not have a flapping main rotor, so it's Ct/sigma vs mu plot doesn't show rotor lift decreasing rapidly with increasing airspeed. The rotor itself really can move to a different level of performance at speed than conventional flapping rotors. Eliminating the flapping compliance largely eliminates the natural flap response of a rotor that keeps the center of lift over the rotor mast and rotor controls can trim the retreating blade to produce less lift and shift the rotor lift heavily onto the advancing blades. The rotor now behaves a bit more like an airplane wing which is why X-2 aircraft don't have the wing. You also eliminate the download from the wing, so the aircraft is more efficient at hover (coaxial rotors also have some efficiencies here). All this sounds great... on paper... but the Iron Law of No Free Lunch applies. The X-2 eliminates the most important invention in the history of rotary winged flight, the flapping hinge, and the consequences are severe. Rotor loads are roughly an order of magnitude higher than on a flapping rotor. Vibrations in the aircraft are severe. Hub drag is high. Part count is high. Yaw agility is low and sometimes nearly non-existent. The configuration has gone head to head with tilt rotors twice now (XH-59A vs XV-15 and SB>1 vs V-280) and lost both times. It might even lose to a simple winged helicopter in a few years. The prop isn't the key technology, its the rigid rotor that eliminates the wing, but the rigid rotor is also the source of all the aircraft problems.

X-Wing IS even more radical. It is a rigid rotor (bad) that uses a hugely complex pneumatic "swashplate" to direct variable amount of air to leading edge and trailing edge vents along the elliptical cross section blade airfoils to both control lift and simulate cyclic blade pitch (bad). Had it flown, the vibrations in horizontal flight would have been severe. The rotor design was an innovative stiff in/out of plane "bearingless" hub design that used composites to achieve the stiffnesses required to make forward swept wings work. Integrating the mechanical, pneumatic, software, and other systems together for such an aircraft led to the development of the "System Integration Lab" approach that is now common for advanced helicopter programs. The volume and power inefficiencies of the pneumatic systems and rigid rotor probably doom this concept from further development. Remember... No Free Lunch.

[SplineDrive]

Maybe this doesn’t make sense, but here goes: one thing that puzzles me a little, perhaps only because of my own ignorance, is that for pushing air downwards (ie VTOL) development efforts have focused almost entirely on what are effectively propellers – one or two big unshielded rotating wings, or sometimes lots of small ones – whereas for pushing air backwards (runway takeoff), jet propulsion is increasingly the norm, on smaller and smaller aircraft, now down to the VLJ category.

Of course the military have played with jet propulsion to push air downwards, with vectored thrust as in the Harrier, or with lift jets or fans, but AFAIK they’re few, and vectored-thrust has mostly gone away from VTOL towards improving manoeuvrability of fighters.

There's a couple of key differences between lifting an aircraft vertically with "blowing air" and pushing it forward via "blowing air". Lifting an aircraft requires generating enough thrust to completely lift the aircraft and then add control margin on top of that. Pushing an aircraft through the air just requires overcoming the drag of the airframe and wings. So the lift demands are an order of magnitude higher for VTOL flight. There is a relationship between thrust/disc area and the ability to turn horsepower into thrust. Low disc loadings (large area for a given thrust) is far more efficient than high disc loadings at turning horsepower into thrust. Classic example diagram below:


https://cimg5.ibsrv.net/gimg/pprune.org-vbulletin/623x519/0_ugccoe2vshqog70q_29fc7728fd9ac9da3777717cee0ef2c0dd6472f8. png

So using jets, etc. to lift the aircraft requires FAR more power than using a large rotor does.

Another key difference is the speed of the air entering the rotor/fan/jet... In a helicopter, the velocity of the air flowing in "inflow" is relatively low. In an axial flow fan or jet, it's quite high. This affects the design each system and it's hard to optimize the same thruster for both near zero inflow and very fast inflow. There's plenty more to it, but these are some big ones.

This discussion is basically the genesis of the tilt rotor: large enough rotors to be near the helicopter end of the hover efficiency plot and rotors that are too large to be optimal propellers, but good enough. The somewhat undersized rotors mean hovering requires more power installed than a normal helicopter, but that power then enables much higher cruise speeds as an airplane.

The intro sections of the NASA XV-15 monograph discuss a lot of attempts at working around the physics of the above chart. Worth a read to those interested.

https://history.nasa.gov/monograph17.pdf

[OldLurker]

SplineDrive, thanks very much for that admirably clear explanation of lifting vs pushing! The graph is very helpful. It all makes complete sense now. Thanks too for the pointer to the NASA monograph – I've downloaded it for future reading.

[wrench1]

eVTOL got "high end money" back when interest rates were near zero.
The high end money I am referring to is still in place and has been. But agree the huge amounts given several years ago that were covered by a media fanfare were simply “big promises”. However, there still is solid funding from both private and public entities for mature eVTOL programs and UAM/AAM support in general.
The FAA cert basis documents for Joby and Archer are out and they both reference a requirement to be able to perform an "controlled emergency power off landing equivalent to a glide or autorotation".
If referring to the proposed airworthiness criteria published in the federal register a while back that was a procedural requirement and is not the final certification basis document. 10 to 1 those conventional requirements like autorotation ability, or its equivalent, will not survive in the final eVTOL certification basis in lieu of other methods or revised criteria.
Jaunt Journey can autorotate.
I thought Jaunt is pursuing a Part 27/29 certification?

[SplineDrive]

If referring to the proposed airworthiness criteria published in the federal register a while back that was a procedural requirement and is not the final certification basis document. 10 to 1 those conventional requirements like autorotation ability, or its equivalent, will not survive in the final eVTOL certification basis in lieu of other methods or revised criteria.

I thought Jaunt is pursuing a Part 27/29 certification?

Yes, Jaunt is pursuing a more conventional certification. As for eVTOL platforms and the common lack of autorotation and/or glide capabilities, why should the flying public tolerate aircraft that are missing such a fundamental safety feature or it's equivalent? And will insurers tolerate that?

https://www.federalregister.gov/documents/2022/11/08/2022-23962/airworthiness-criteria-special-class-airworthiness-criteria-for-the-joby-aero-inc-model-jas4-1

It's my understanding that this is the agreed to certification basis for the Joby S4 and it agrees to modify existing language and create a new definition of "emergency controlled landing".

JS4.2105
(f) Continued safe flight and landing must be possible from any point within the flight envelope following a critical loss of thrust not shown to be extremely improbable.
(g) The aircraft must be capable of a controlled emergency landing, after loss of power or thrust, by gliding or autorotation, or an equivalent means, to mitigate the risk of loss of power or thrust.

My read is that (g) means the aircraft can protect the passengers in a power off landing, though means other than gliding or autorotation can be acceptable (like a ballistic chute). Might be a market opening for chute solutions that work at low altitudes and airspeeds.

We'll see. All these ships have a long road to certification.

[Mechta]

Imposing a requirement (autorotation) from one type of aircraft (helicopter) onto another type (multirotor) seems a negative step. What is required is the same or better level of reliability and survivability. If a multirotor can achieve this with two or more distinctly separate power and control systems, why should it be saddled with the weight, complexity and additional possible failure modes of an autorotation system?

[SplineDrive]

Imposing a requirement (autorotation) from one type of aircraft (helicopter) onto another type (multirotor) seems a negative step. What is required is the same or better level of reliability and survivability. If a multirotor can achieve this with two or more distinctly separate power and control systems, why should it be saddled with the weight, complexity and additional possible failure modes of an autorotation system?

Because people fly aircraft to the point of energy exhaustion and errors/mistakes are made in the energy loading/consumption process. Should we have passenger fixed wing aircraft that can't glide? Call me old fashioned, but running out of stored electrical or chemical energy in the air shouldn't result in everyone plummeting to their deaths. A survivable controlled emergency landing from a stored energy starved flight state isn't a bad requirement.

[helispotter]

SplineDrive: Thanks for your comprehensive review / assessment in post #16. A fair bit to absorb!

I had overlooked you already mentioned X-2 in your post #11 (as 60FltMech did in the original post!) and have only now realised it formed the principles of their SB-1 Defiant bid. You made the comment "It [rigid contra-rotating rotors] might even lose to a simple winged helicopter in a few years". Do you mean something like the Sikorsky S-67 (https://en.wikipedia.org/wiki/Sikorsky_S-67_Blackhawk)!!

As you have obviously thought carefully about overall rotorcraft design, you might also like to comment on whether there could be a future in a modern-day version of the Fairey Rotodyne gyroplane (https://en.wikipedia.org/wiki/Fairey_Rotodyne)? I am sure design techniques are now available to substantially reduce the noise generated by the rotor tip jets. Perhaps this isn't so much a technical challenge anymore, rather one of overcoming protest over use of such rotorcraft in built up areas (where they might otherwise still attract a commuter market).

[SplineDrive]

SplineDrive: Thanks for your comprehensive review / assessment in post #16. A fair bit to absorb!

I had overlooked you already mentioned X-2 in your post #11 and have only now realised it formed the principles of their SB-1 Defiant bid. You made the comment "It [rigid contra-rotating rotors] might even lose to a simple winged helicopter in a few years". Do you mean something like the Sikorsky S-67 (https://en.wikipedia.org/wiki/Sikorsky_S-67_Blackhawk)!!

As you have obviously thought carefully about overall rotorcraft design, you might also like to comment on whether there could be a future in a modern-day version of the Fairey Rotodyne gyroplane (https://en.wikipedia.org/wiki/Fairey_Rotodyne)? I am sure design techniques are now available to substantially reduce the noise generated by the rotor tip jets. Perhaps this isn't so much a technical challenge anymore, rather one of overcoming protest over use of such rotorcraft in built up areas (where they might otherwise still attract a commuter market).

My comment about an X-2 style aircraft losing to a simple winged helicopter was a reference to the US Army FARA competition where the Sikorsky Raider-X is pitched against the Bell Invictus.

I have a hard time imagining that tip jet powered aircraft will make a comeback. Noise, fuel and aerodynamic efficiency, etc. are pretty serious problems. I could be wrong, though. Historically abandoned ideas do make the occasional revival.

[MLH]

Imposing a requirement (autorotation) from one type of aircraft (helicopter) onto another type (multirotor) seems a negative step. What is required is the same or better level of reliability and survivability. If a multirotor can achieve this with two or more distinctly separate power and control systems, why should it be saddled with the weight, complexity and additional possible failure modes of an autorotation system?


A distinctly separate battery is a non-starter. In the event of a main power bus (single battery) failure, the aircraft becomes a brick.

[bellblade2014]

JohnDix,

I just can’t see an aircraft with a mass of push pull tubes, mixers, bellcranks and Hydraulic lines being the future of Army Aviation(or VTOL in general), someone has to crack the code on simplifying these systems, while also keeping redundancy for battle damage or other emergencies.

FltMech

the illusion of FBW is simplicity. It is imminently more complex than push pull or cable systems. Software is a vast ocean of possibilities and decisions with many hidden (and deadly) traps as evidenced by numerous complex failures from software based systems of the past.

no one has certified a civil FBW rotorcraft or tiltrotor yet and it is not because of the workload. It’s very challenging to prove safety to dozens of strongly opinionated regulatory specialists from all major authorities. EVTOL’s can naively project confidence but they are massively unprepared for the gauntlet of “what if-isms” that will come on the way to type cert.

it sure is a ton simpler to just use SAS/SCAS to reduce pilot workload while still preserving basic mechanical function in nearly all conditions. Mechanical failure is extremely rare in reality.

[wrench1]

It's my understanding that this is the agreed to certification basis for the Joby S4 and it agrees to modify existing language and create a new definition of "emergency controlled landing"..
Proposed criteria. Its not been finalized yet which is needed to make it a rule. Its undergoing review/revision after an extended comment period. If this were a normal certification process these criteria would have been simply listed in an FAA Issue Letter.

Since the Joby and others are being certified under Part 21, they need to post those criteria in the federal register for the legal framework. Same with Archer and the others needing their separate posted criteria. This is one place the EASA is ahead of the FAA as they already have official guidance in place: SC-VTOL-1 (https://www.easa.europa.eu/sites/default/files/dfu/SC-VTOL-01.pdf).
My read is that (g) means the aircraft can protect the passengers in a power off landing, though means other than gliding or autorotation can be acceptable (like a ballistic chute).
Given the FAA and EASA have publicly stated not all powered-lift aircraft have the ability to autorotate or glide that should have been the end of it. Instead, they proposed new rules like (f) and (g) in JS4.2105 which include: “critical loss of thrust not shown to be extremely improbable” and “or an equivalent means, to mitigate the risk of loss of power or thrust.”

I think you’ll find the plan forward has always been around failure probability vs chutes or autos. Hence the reason to modify the existing regulatory language. In general terms, if one can prove any “critical loss of thrust” is “extremely improbable” then the "controlled emergency landing" required in paragraph (g) becomes moot or limited.

And to note, “extremely improbable” is defined as one failure in 1 billion flight hours. The EASA SC-VTOL-1 has a similar requirement.
We'll see. All these ships have a long road to certification
I have 3 standing bets there will be a certified eVTOL by end of 2025. My opponents are betting end of 2024*.
*not including China.

[Lonewolf_50]

10 to 1 those conventional requirements like autorotation ability, or its equivalent, will not survive in the final eVTOL certification basis in lieu of other methods or revised criteria. I'll be interested to see how that plays out. John Dixson has mentioned any number of times the mismatch between the early FBW development for rotary wing versus the capacity to figure out requirements for that among regulators (at least on the FAA side).
Call me old fashioned, but running out of stored electrical or chemical energy in the air shouldn't result in everyone plummeting to their deaths. A survivable controlled emergency landing from a stored energy starved flight state isn't a bad requirement. The future passengers of these new aircraft agree with you. :ok:
the illusion of FBW is simplicity. It is imminently more complex than push pull or cable systems. Software is a vast ocean of possibilities and decisions with many hidden (and deadly) traps as evidenced by numerous complex failures from software based systems of the past. Two that come to mind are the 609 (civilian tilt rotor) and the 525 (Bell). I am sure that there are others, to include the odd event with S-97 - WoW and fly by wire mode conflict - a few years ago at West Palm.
no one has certified a civil FBW rotorcraft or tiltrotor yet and it is not because of the workload. Isn't Relentless (Bell 525) almost there?
it sure is a ton simpler to just use SAS/SCAS to reduce pilot workload while still preserving basic mechanical function in nearly all conditions. Mechanical failure is extremely rare in reality. I agree. Sometimes I wonder if the "can" and "should" thinking fits the needs of the aircraft.

[Otterotor]

I'll be interested to see how that plays out. John Dixson has mentioned any number of times the mismatch between the early FBW development for rotary wing versus the capacity to figure out requirements for that among regulators (at least on the FAA side).
The future passengers of these new aircraft agree with you. :ok:
Two that come to mind are the 609 (civilian tilt rotor) and the 525 (Bell). I am sure that there are others, to include the odd event with S-97 - WoW and fly by wire mode conflict - a few years ago at West Palm.
Isn't Relentless (Bell 525) almost there?
I agree. Sometimes I wonder if the "can" and "should" thinking fits the needs of the aircraft.

I believe the 525 Relentless and the 609 tiltrotor are one to two years away from certification (my guestemation). The V-22 provides a significant amount of reliability data for FBW. (The first crash in Delaware as I understand it, was attributed to human error of incorrect cable attachment to two of the three onboard computers, while the North Carolina hydraulic fluid incident was truly a software glitch in the warning light system.)

[megan]

North Carolina hydraulic fluid incident was truly a software glitch in the warning light systemWard Carroll who was involved in the program at the time gave a safety lecture reinforced the message to know your aircraft systems, apparently the mishap was caused by the pilot cancelling the warning (eight times), had he left the caution on a safe approach and landing could have been made.The test pilot kept resetting his master reset & eventually bled all his hydraulics away. The computer analyzes & contains hydraulic leaks through a protocol, but when the master reset is initiated, it assumes all is well & must run through the protocol again.

Anything like the V-22 I'd imagine would have to be certified to continue to an airport for a safe landing, autorotation is not possible with ROD's in the 15 to 20,000 FPM range (quote from the lecture), nor was the ability to autorotate part of the design brief..

[Otterotor]

Megan,
As I understand it, all that you wrote is true. You didn't mention that the Push-To-Reset function was incorrectly connected to an incremental increase in the helicopter flight mode collective position of one of the rotors, The rabbit punching of the reset button incrementally increased one of the rotor's collective position (which was obviously not understood by the pilot) until the aircraft rolled over and went in. This is the software glitch I was referring to. (Correct me if I'm wrong.) Ott.

[megan]

Correct me if I'm wrongNo personal knowledge Otto, just repeat of what was said. I think the protocol referred to is probably as you said, but just guessing as I have no personal idea of the systems involved.Push-To-Reset function was incorrectly connected to an incremental increase in the helicopter flight mode collective position of one of the rotors

[helispotter]

Would anyone like to comment on the viability of the 'Wisk' aircraft with its series of small rotors to provide vertical lift (https://wisk.aero (https://wisk.aero/?fbclid=IwAR2LyKcpk6NEa5_hhxkh8WY6gTkjslzMr4em60cnN6x5XPWjn9-AX2p2ows))?

I guess the aircraft has to contend with the weight and drag penalty of the vertical lift system when in forward flight, and the graph SplineDrive shared in post #17 also hasn't gone un-noticed, but this arrangement is probably less mechanically complicated than a tilt-rotor?

[CTR]

JohnDix,

When I wrote the post opening this thread I had two questions in mind that I wanted to ask: first was about fly by wire and the second question, which is regarding flight control hydraulic systems.

It seems that if you could develop a rugged fly by wire system you would be well on your way to simplifying your flight control hydraulic systems.

Regarding V-280, I assume Bell will use what they have learned on their 525 FBW and hydraulic systems. But as you pointed out the standards for such systems for VTOL aircraft aren’t fully established, so how does it all come together in the end?

I just can’t see an aircraft with a mass of push pull tubes, mixers, bellcranks and Hydraulic lines being the future of Army Aviation(or VTOL in general), someone has to crack the code on simplifying these systems, while also keeping redundancy for battle damage or other emergencies.

FltMech

60FltMech,

First, thanks for starting this thread, this site has become dry of new topics of late.

Many people don’t realize that the FAA FARs are only the equivalent of a cover page. Interpretation of FARs correctly is impossible without referencing the advisory circulars (ACs). ACs provide guidance and describe acceptable methods for demonstrating compliance to the FARs.

Where the rotorcraft ACs fall short, are for new and novel technologies, like FBW. But FBW is only new on commercial rotorcraft (Part 27 and 29). For Part 25, Transport aircraft, FBW is well established. So using Part 25 ACs to provide guidance for Part 27 and 29 FBW has been the path for the AW609 and Bell 525.

This path drives FBW rotorcraft to be designed to many of the same requirements as a large Transport aircraft like a Boeing 777 or Airbus 350 airliners. So cost and weight become a major factors. This is why eVTOL developers are trying to circumvent this path to certification. Like you, I fear this will result in the loss of many lives if it is allowed to happen.

The only three production FBW rotorcraft, the V-22, NH90, and CH-148 were built to military or company standards, and don’t meet FAA certification requirements. Sikorsky attempted to get FAA certification for the CH-148 FBW system, but design’s architecture was inadequate.

The AW609 and Bell 525 will be the first civil certified FBW rotorcraft, and both have the same FBW system design. Their FBW systems build on the lessons learned from the V-22, but are very different in design. Bell realized 25 years ago that a V-22 FBW architecture could not be FAA certified. Additionally, cost, reliability, and maintainability aspects of the V-22 architecture were incompatible with a commercial customer. So Bell started from scratch, designing a FBW design based on the KISS principle (as simple as a FBW system can be). Because of the long delays in AW609 and 525 certification, Bell (and Leonardo) engineers have had decades to develop these FBW systems.

When the FAA eventually publishes ACs providing guidance on FBW for rotorcraft, it will be based on the standards set by AW609 and 525.

The Bell V-280 FLRAA and 360 FARA FBW systems are based on the 609 and 525 architectures. Since the US Army is now requiring FAA levels of critical failure reliability for these platforms, Bell was well prepared with an FBW system solution.

[CTR]

I have a hard time imagining that tip jet powered aircraft will make a comeback. Noise, fuel and aerodynamic efficiency, etc. are pretty serious problems. I could be wrong, though. Historically abandoned ideas do make the occasional revival.

SplineDrive,

Perhaps the revival time for a tip jet may have come ;-)

https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/679x910/img_6502_6cf28830641164c52a78dbdf244cecf854584ddb.jpeg
Bell Patent for Electric TipJet Rotorcraft

[SplineDrive]

SplineDrive,

Perhaps the revival time for a tip jet may have come ;-)

https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/679x910/img_6502_6cf28830641164c52a78dbdf244cecf854584ddb.jpeg
Bell Patent for Electric TipJet Rotorcraft

Now you've added some of the problems of the X-Wing pneumatic rotor system! :)

[SplineDrive]

Would anyone like to comment on the viability of the 'Wisk' aircraft with its series of small rotors to provide vertical lift (https://wisk.aero (https://wisk.aero/?fbclid=IwAR2LyKcpk6NEa5_hhxkh8WY6gTkjslzMr4em60cnN6x5XPWjn9-AX2p2ows))?

I guess the aircraft has to contend with the weight and drag penalty of the vertical lift system when in forward flight, and the graph SplineDrive shared in post #17 also hasn't gone un-noticed, but this arrangement is probably less mechanically complicated than a tilt-rotor?

I'll bite, but I'll preface this that my opinion is counter to the eVTOL industry trend.

Wisk has moved to the increasingly common "lift + tilt cruise" approach for distributed lift. Their front 6 props are five blade each and appear to have collective pitch control. The aft 6 lift props are four blades each and I can't tell if they have pitch control or will use variable RPM to control thrust. Most of the "lift + tilt cruise" aircraft are focusing on 2 bladed lift props but these will have significant 2P load and vibration problems during transition , particularly as the prop diameter grows to accommodate realistic payloads and vehicle weights. Since these aircraft tend to have "rigid" lift props with collective control (and sometimes not even that), the maximum practical rotor diameter is limited. I'm not sure where that limit is for the rear lift props, and it depends on a number of factors, but it could be somewhere in the range of 6-8 feet unless you either have low blade loading, significant flapping compliance in the blade, or some form of active vibration control. So vehicle growth has to happen by adding more rotors instead of larger diameter rotors.

The front tilt props have higher blade count which will reduce vibrations but there will still be significant loads/vibrations that a propeller designer is entirely not used to.

I question the argument of reduced part count and complexity on the multi rotor configurations and the claim that there are no single point failure elements. At a bare minimum, each blade is is a wear item that has to be inspected periodically and each blade (that has pitch control) has bearings and more bearings than a conventional variable pitch prop. Wisk Gen 6 has 54 blades and potentially 108 blade feathering/retention bearings and yet more bearings in the pitch link / control system for each blade. Add in actuators for each prop and actuators for any moving airfoil surfaces and you've got a lot of parts that I just do not believe will not require some scheduled inspection in service.

All of the lift + tilt cruise aircraft look (to me) weak in yaw authority in hover operations. Landing on a targeted pad in variable 20+ knot winds might not be easy. Cruise speed, range, and payload all compare poorly to a light turbine helicopter or even some piston helicopters. There is zero point zero chance of autorotation (as discussed earlier in the thread), so any controlled emergency landing capability from a VTOL flight condition will involve some low altitude ballistic chute? As an autonomous aircraft, I guess Wisk will argue the computer will never fly the aircraft to energy starvation and there is enough electrical redundancy to avoid a complete loss of energy onboard. The difficulty in certifying a fly by wire system and the autonomy required, in an FAA environment, I think has been grossly underestimated.

To me, these lift + cruise aircraft don't compete favorably with a conventional helicopter and the configuration doesn't scale well and will remain limited to the price sensitive small end of the VTOL pool (or museums). High development and certification costs and lower profit margins make for a questionable financial plan, though the vertical integration of vehicle OEM and operator will attempt to bypass some of the economics. I also wonder if firms like Wisk and Joby are going to self-insure the aircraft and operations or if they've been engaging the insurance industry to make sure they're onboard with the configuration and concept of operations as well.

Obviously, a bunch of startup eVTOL firms and billions of (previously) low interest rate dollars think I'm wrong. My opinion is worth exactly what you paid for it :-)

[CTR]

Now you've added some of the problems of the X-Wing pneumatic rotor system! :)

I believe Bell is basing their design on the more successful SO 1221 Djinn cold tip jet propulsion applied to a gyrocopter.


https://en.wikipedia.org/wiki/SNCASO_SO.1221_Djinn

[FH1100 Pilot]

The difficulty in certifying a fly by wire system and the autonomy required, in an FAA environment, I think has been grossly underestimated.

And right there we have the crux of the issue. GROSSLY underestimated. You got that right! These LSD-induced eVTOL fantasies of pilot-less drones flitting about urban areas *below* the tops of the skyscrapers and landing on rooftops or ground-level drone-pads are just silly. First of all, they neglect to factor in the perhaps insurmountable hurdles that will be put in place by the FAA (here in the U.S., anyway). The inability to autorotate may be able to be overcome, but as SplineDrive notes, the FAA *will* require a level of redundancy that may not be possible. Batteries can malfunction; I can see the FAA requiring an extra battery for redundancy. The thought of an emergency parachute is simply laughable. And let's not even talk about crashworthiness! This is 2023, not 1953.

Proponents of these little proof-of-concept vehicles never talk about other things as well. To accommodate four (or so) passengers who don't know much about aircraft and don't care, the hardware (door handles and such) will have to be really heavy-duty, not flimsy like an Enstrom or R-22 door. Passengers are going to want creature comforts - you know, comfortable seats, a nicely-appointed interior...heat in the winter and a/c in the summer. How much weight does that add up to? Can these little eggshells be scaled-up that far? I'm sure that battery technology will improve...but...really? Oh yeah, what happens if a passenger gets airsick and pukes? (Think it won't happen?) Will the drone be able to detect it? Either way, that machine goes out of service.

The idea of autonomous eVTOL drones zooming around crowded cities is enticing, I'll grant you that. And I don't want to be so arrogant and pessimistic as to say it will *never* happen... But it won't, let's be honest. Not as long as the FAA governs things that fly in the air, and not as long as the public are as risk-averse as they are today. But perhaps I'm wrong! Perhaps people will become *less* risk-averse in the future. Yeah, that could happen...not.

[JohnDixson]

60 Flt Mech. Sorry for not responding to the following from Post #7:

“It seems that if you could develop a rugged fly by wire system you would be well on your way to simplifying your flight control hydraulic systems.
Regarding V-280, I assume Bell will use what they have learned on their 525 FBW and hydraulic systems. But as you pointed out the standards for such systems for VTOL aircraft aren’t fully established, so how does it all come together in the end?
I just can’t see an aircraft with a mass of push pull tubes, mixers, bellcranks and Hydraulic lines being the future of Army Aviation(or VTOL in general), someone has to crack the code on simplifying these systems, while also keeping redundancy for battle damage or other emergencies.”

As to simplifying hydraulic system, the Canadian MHP S-92 offers a glimpse of the future. All of the hydraulic clap-trap that is required on the top deck in front of the main rotor on S-70 and S-92 models, boost actuators, SAS actuators, control mixer devices, and in the tail cone, the tail rotor control quadrant . There is weight to be saved, maintenance man hours/inspection hours to be eliminated, areas where mistakes in maintenance can cause fatal accidents to be eliminated. There are other capabilities that beckon in the future for FBW, present in some military machines.
This is the one area of helicopter design where general standards do not exist. One sympathizes with the Bell/Augusta 609 and Bell 525 teams in their quest for certification.
Given that the expertise is resident in the OEM’s engineering departments, is the solution a VFS Committee of FBW specialists writing general standards?

[CTR]

The eVTOL startups are dreading the prospect of being held to the same stringent standard for certification as the 609. “It appears the FAA intends to certify the AW609 as a special class aircraft under FAR 21.17 (b) and before issuing formal powered-lift airworthiness standards, the latter widely anticipated in 2024 and that will largely impact eVTOL aircraft. Ahead of that, the FAA is announcing applicable regulations and other airworthiness criteria developed specifically for the AW609 that include applicable parts of certification standards for both helicopters and fixed-wing aircraft under FAR Parts 23, 25, 27, 29, 31, 33, and 35.

According to the FAA, “The proposed certification basis incorporates by reference existing transport category airplane and rotorcraft standards, one normal category airplane standard, Category A rotorcraft standards, optional Category B rotorcraft standards, and criteria for operation under instrument flight rules.”

https://www.ainonline.com/aviation-news/business-aviation/2023-06-09/faa-floats-certification-path-leonardo-aw609-tiltrotor

[Agile]

Enjoyed this recent video on the technical back room at Joby aviation,
Motors, batteries, software validation .. a lot of technology rich motivation.
Very much the Hill Helicopter spirit and philosophy on steroid.
https://www.youtube.com/watch?v=cQFH1Z9DMuY

[Lonewolf_50]

But will it scale up? That is a single pilot toy for a rich person at present. (And yes, I want one! :})

[wrench1]

The eVTOL startups are dreading the prospect of being held to the same stringent standard for certification as the 609. “It appears the FAA intends to certify the AW609 as a special class aircraft under FAR 21.17 (b) and before issuing formal powered-lift airworthiness standards, the latter widely anticipated in 2024 and that will largely impact eVTOL aircraft.
FYI: There will be various powered-lift aircraft certification catagories based on various requirements. So an eVTOL will not be certified under the same rules as a tilt-rotor.

The issue was the FAA planned to certify powered-lift under the existing Part 23 rules whereas EASA planned to create a new certification rule. The FAA finally realized their "error" and 2 years ago elected to create a new route similar to EASA.

The FAA special class process as been around for years and is a catch all for new aircraft certification that does not completely fit into an existing FAR. For example, the Bell 525 has a special class for its FBW system.

Regardless, with this new commonality between FAA/EASA eVTOL cert rules there are several recently release guidance documents that show eVTOLs will have their own specific requirements to meet. But from what I've seen I dont expect any formal FAA/EASA eVTOL full certifications until 2028. Interesting times ahead.

[DavidSmithHeli]

The FAA special class process as been around for years and is a catch all for new aircraft certification that does not completely fit into an existing FAR. For example, the Bell 525 has a special class for its FBW system.
.

Can you describe this in more detail, are you speaking about the use of “Special Conditions”? That is different in FAA-speak from “Classes”. so I’m not following your reference to the 525 which has special conditions but no special class that I’m aware of.

I think you may be mixing words or topics, but want to understand your intent.

[wrench1]

so I’m not following your reference to the 525 which has special conditions but no special class that I’m aware of..
Nothing to follow. The 525 falls under Part 21.17(a) special conditons and the eVTOL ceritification falls under Part 21.17(b) special classes. My bad on the use of "classes" for the 525.