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Most failed aircraft idea fans know the Moller SkyCar saga, but for folk who aren’t or don’t, here’s a short telling of it:
Paul Moller dedicated his life and career to the simple dream of a making the flying equivalent of the family automobile, a fast (300+ MPH), long range (800+ mi) VTOL aircraft
that could obsolesce the ground vehicle. After decades of waiting for and inventing the enabling technologies, this culminated in 2003 in the M400, a sleek, sexy, 4 passenger machine sporting 4 rotatable ducted fans, each powered by a pair of purpose-built 180 HP Wankel engines (and maybe a pair of 120 HP electric boost motors – reports vary), all computer controlled.
Which just didn’t work. The best it managed were unmanned tethered hover test flights, which are heartbreaking to watch. Anyone with a good ear and a modicum of mechanical intuition can hear as much as see its problem: for all its computer control systems, the motor-fan system is irreparably lashy, unable to maintain a constant, steady speed and thrust, and engineering tragedy. An estimated $200,000,000 went into reaching this single prototype. In 2003 and 2006, Moller tried to auction it on eBay, but both time failed to get an offer over the $3,500,000 reserve price. A 2013 attempt at crowdfunding a mere $950,000 to resume test flight failed to raise $30,000 in pledges. A search of recent news paints Moller less as an engineer than as a con man, dodging shareholder lawsuits and hiding behind personal bankruptcy.
The conventional wisdom (mine, at least) is that if something like the M400 happens, it’ll be an all-electric motor, turbogenerator powered machine, essentially a giant kerosene fueled rotating fan drone.
But what if what the M400 needs to cure its lash ills is not more 21st Century computer control, but a 1910s-era heavier flywheel, in the form of a modernized rotary piston motor?
Take the basic design of the M400. Replace the low rotating mass Wankel rotor engines with 3-cylinder, fixed crankshaft, rotary piston motors, superficially resembling something ca 1900, but with modern fuel injection, timing, valves, etc. Ellipsoidal cylinders angled to fit inside airfoils replace the many thin, counter-rotating blades.
The resulting ducted fan has a lot of rotating mass. Lash won’t be a problem.
Moller M400 SkyCar
https://www.indiego...ycar-into-history#/
The ill-fated M400 test flight continuation Indigogo campain [CraigD, Mar 28 2016]
Tri-Dyne
https://books.googl...e%20tridyne&f=false
Maybe it just needed a better and smoother-running rotary engine. [Vernon, Mar 29 2016]
Le Rhone rotary
https://en.wikipedi.../wiki/Le_Rh%C3%B4ne
Bizarre, but they work .... [8th of 7, Mar 29 2016]
Attitude control using precession
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[FlyingToaster, Mar 30 2016]
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But wouldn't a high rotational inertia make the thing
less able to respond to changes in load or wind? |
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I do, though, like the idea of the cylinders being the
aerofoils. |
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The severe gyroscope effect can perhaps be counteracted by
making the engines alternately counterrotating; but it's going to
apply huge torsional forces to the airframe (altho composites
might be stiff enough, without a weight penalty). |
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The original rotaries used ignition-cut to control power output.
Since variable-flow carburettors were a known technology, this
begs the question of what peculiarity of rotaries precluded that
application. |
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Is it just the cylinders that have the external elipsoidal profile, or
are the pistons elipsoidal too ? |
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Most rotaries were 7 or 9 cylinder. A single failure on a 3-
cylinder would be much more serious than on a 7. |
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What about a 7 cylinder supercharged 2-stroke diesel ? |
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//torsional// Not as much as you'd think. Attitude adjustments (both fuselage and engine) would be precessionally accomplished by nudging the engines in a useful manner, so how much nudge is required to produce <x> amount of rotational force ? |
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//What about a 7 cylinder supercharged 2-stroke diesel ?// well, that would certainly have an interesting firing order. |
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//ellipsoidal cylinders// Which means the cylinders need some limited rotation abilities because if you lose an engine you really really want to be able to power-feather its diagonal counterpart. |
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// how much nudge is required to produce <x> amount of rotational force ? // |
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You'd have to do the math ... |
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// that would certainly have an interesting firing order. // |
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Worked OK for the Le Rhone ... <link> |
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// you really really want to be able to power-feather its diagonal counterpart. // |
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Yes, and then again somehow strangely not ... |
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It rather depends on the power-to-weight ratio. |
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No chance of an autorotation landing; the lightweight engines can't store enough kinetic energy. And it's reasonable to assume that the vehicle will require all four engines for normal operation - three would mean at best a rapid, semi-controlled plummet, being as 25% of your lift just wandered away. Reducing power on the diagonal unit does not sound like a good thing. Then again, attitude instability isn't a good thing either. So there will have to be a FADEC with a very quick response, and even then a siezed engine would be a Bad Thing - you'd probably flip before the opposite engine spun down.
It implies that the engines need a "buster" mode where they can deliver 200% of their normal output for brief periods (probably wrecking them) to allow a slightly more controlled contact with terrain. |
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Seats by Martin-Baker, please ... |
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Pretty sure they were 4-strokes: their relative lightweightedness stemmed from lacking a discrete flywheel. (//interesting firing order// was a brainfart) |
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//200%// from the nacelles adjacent, and 1.414x rotational velocity from the feathered one diagonal, to the bum engine, so the thing doesn't flip over when the pilot sneezes. Much FADEC when an engine goes, and even more when(if) transitioning with an engine out, but apart from that more or less normal control. |
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On the bright side, since attitude is taken care of by precession, and it ain't a glider or autorotator, fins become passe. |
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// Pretty sure they were 4-strokes // |
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There are 4-stroke rotaries, but the Le Rhone, the Gnome and
(IWRC) the Clerget are crankcase-scavenged 2-strokes, total-loss
lubricated with castor oil. 2-strokes have the dual advantages of
extreme simplicity, and high power-to-weight ratio. |
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Forget the airbags, and fit ballistic recovery parachutes. |
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Maybe a couple of big airbags underneath, for those final couple
of metres … |
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Hmmm. Assume that terminal (heh, heh) velocity is around the
same as for a human parachutist. That's equivalent to about a 3
metre drop, based on a non-steerable basic canopy with no
option to flare the landing. A properly-sized external airbag
would be light, compact, and would probably make the" landing"
walk-away survivable. |
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For the time being, we'll stay with fixed wing, if that's OK. |
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//the motor-fan system is irreparably lashy, unable to
maintain a constant, steady speed and thrust// |
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Try as I might, I don't understand this point. All my
attempts to self-educate ended up with me looking at
this idea in a google list. Am I right in thinking that this is
analogous to gear lash? In that the output and load aren't
stable? Are you suggesting that the pulsatile nature of the
Wankel engine creates lash/backlash against the
relatively stable aerodynamic load from each fan? From
what I gather* the engines are dual rotor, meaning 6
pulses per crank rotation. That's quite good, at say
10,000rpm you'd get a 1kHz vibration, disregarding any
horrible bending/twisting vibrations coming back from
the blades. I think the way to go would be a rotary
damper. It doesn't need the energy storage of a flywheel,
just some damping... this can be done with all kinds of
flexible couplings or some sort of twisty shaft. |
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I think the problems with the propulsion set up are
multiple. Sure, the contra-rotating ducted fan might have
high theoretical efficiency. You can probably get great
numbers on a test stand too. But it's already difficult to
get the pair of fans working together, the first fan
delivers air to the second, but with increased pressure,
linear and rotational kinetic components and all kinds of
swirls and eddys. So you need to modify the rear fan
design to account for the different angle of attack. You
can optimize this pretty well if you have a couple of main
conditions, like take off and cruise, but the SkyCar needs
constant adjustments in thrust at 4 points spread across
8 engines just to hover. Then throw in the transition to
horizontal flight which gives horrible asymmetry in air
pressure across the first fan face. A huge mess. No
coincidence that contra-rotating stuff is rare. |
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It would be better to go simple, single direction with
stators. |
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*gathering anything concrete on the whole Skycar project
is not for the faint hearted. |
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//Ellipsoidal cylinders angled to fit inside airfoils// |
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Honda did oval pistons in the VR750. Ellipses might
actually be easier, variable curvature rather than
curved-straight-curved. But don't prop blades twist down
their length? might be interesting. |
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I can't imagine that the piston rings would be easy to
manufacture. |
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// don't prop blades twist down their length? // |
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The angle of attack does tend to vary from root to tip, not counting the fact that anything more than a puddle-jumper is going to have a variable pitch constant speed prop. But that's less of an issue with a ducted fan, running at fairly constant revs and near maximum power. |
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Aircraft with rotary engines are far from viceless. Machines like the Sopwith Pup and the Fokker DR III can make flick turns in one yaw direction, but struggle in the other - thus an aircraft with an inline engine with a much less pronounced bias can easily elude them just by turning "against" their bias, and they can't follow, even with huge stick forces. This lead to the abandonment of the rotary as inline (and V) designs improved, altho their linear descendant the radial hung on for decades. |
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It follows that anything with large rotating masses is not going to be very agile, even with Die-By-Wire control, and the forces on the engine mounts will be high. |
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The piston ring thing .... yes. Round bores do have an advantage. |
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One of these (which I've been picturing as having 4 engines: one at each corner of a square) using precession to change orientation would be more agile than anything else. Engine mount forces would be equivalent to control surface forces necessary to accomplish a manoeuver, and it's spread out over all 4 engines. |
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Transitions are accomplished by uncoupling the engines from the frame (duh) and applying a nudge at the appropriate spot to get them to move themselves around to the new position. |
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The exceptions to having complete attitude control of course would be yaw while the engines are vertical and roll while horizontal, precession being inapplicable in the rotor disk plane. |
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Not to say you'd want to use centripetal force to (try to) maintain an unbalanced state: that _would_ put undue force on the mounts, but it should work just fine for the post's purpose: keeping the machine from flopping around while waiting (fractions of a second) for thrust adjustments to catch up. |
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