Originally Posted by
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)
?
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 :-)