Hover Bother

We owe a debt to those YouTubers who painstakingly stitch together footage like this Ultimate Helicopter Crash Compilation:

https://www.youtube.com/watch?v=KRJSYRQnM7o

Given the fairly recent availability of inexpensive means of video capture as compared to the length of time the helicopter has been with us, it also represents the tip of the iceberg. The reason that flight statistics are so benign when compared say to driving the car is because they are vastly skewed by the safety of flying in a highly regulated environment in which say airliners operate, or indeed commercial helicopters. It is as if the safety of using an automobile were represented by, say, the number of fatalities suffered by passengers using a train or a bus.

Rather than the crash-bang-wallop ethos attaching any number of videos, its author does hazard too the generic cause behind each set of accidents or incidents. This is worth consideration, because much of this 'real world' operational experience will be carried over to the use of eVTOLs in future.

And the fact is, watching this it is hard not to think that in many scenarios eVTOLs will actually be relatively safer than a helicopter, and especially for the up close and personal spheres of operation at which rotorcraft have long excelled.

These are the tail-end of the operating environment in which the bulk of helicopter missions are flown, as elaborated above in relation to the apparent safety of vertical take-off types as drawn from the statistics. Nonetheless safety in the modern world is a double-edged sword that operates within disconnected spheres. We work in overly regulated environments and drive around in cars that are paragons of safety, whilst at the same time being much more likely to be knifed to death or killed by a drug-fuelled joy-riders than ever.

It is worth considering how eVTOLs will evolve, therefore, because they will do so like all technology in ways we could not envisage and beyond control of society in general and the law in particular, as is the case with AI or indeed the disintegration of society fuelled by seemingly harmless platforms like YouTube or TikTok.

For the centre will not hold, and the technology is likely to devolve between relatively safe and stupendously capital-expensive means of replacing helicopters in moving the affluent between down-town locations... and the rest of us. That rest of us is likely to use comparatively inexpensive electrical means of taking to the air much like those in these videos did, only moreso. The accidents involving eVTOLs will therefore provide a richer source altogether of footage that, should you wish to, you could watch all night.

Could we sleep afterward, though, having helped bring such things to fruition? I think we could, for the reason whatever we don't develop then someone else surely will. It is thus incumbent upon pioneers in the field ~ and I don't count myself among them ~ to make personal air vehicles as reasonably safe as we could expect them to be at the outset.

Post (-ponement)

These props - literal props ~ are growing cobwebs, whilst the tuna in the cans is most certainly off. Flight testing of the scale prototype is postponed until the second half of October whilst our engineer is re-allocated to work of a more military persuasion. In the UK when developing eVTOL concepts you're up against behemoths in the form of the grant-funded military-industrial complex on one hand and Hollywood's perennial need for aerial footage on the other. We in contrast are the rednecks in the back yard.

Pendulous Thoughts

Up there with questions like 'How does a wing produce lift?' an endless debate is the issue of whether or not pendulous stability actually exists in flight, at least beyond hanging from a balloon in a basket is concerned. Tom Stanton's excellent video would suggest that it's a mythical beast, whilst aircraft that are supposed to be extra stable in view of an overhead wing or rotor appear to be anything but to pilots.

The controversy is complicated by the interaction of forces attributable to drag and an interplay of all forces in a realm of aerodynamics. In an effort to grasp the essentials however I prefer to defer to real-world experience, and people who know a great deal about drones tell me that flying with a suspended load ~ whether tethered or fixed ~ is increasingly challenging when that load becomes further removed.

The issue is inherently inertial and could be considered in the absence of flight, as on the ground or suspended in space. The first diagram shows how all successful drone manufacturers start out in designing an airframe, not least because it is the easiest to produce in moulded plastic. Forces applied near the end of either cantilever have only to rotate the central mass to vector thrust, so there is clearly not a good deal of extra energy required. An example is moving boxes around a warehouse floor, best done by shouldering the load in line with its centre of gravity.

Further removing the centre of gravity, as in the second diagram, complicates matters considerably and matters not whether a drone is arranged with mass far above or far below. Take for instance balancing a pencil on a finger or telegraph pole in both arms. Whilst vertical, the force due gravity passes through the support, but falling produces a vector that also acts in the direction of lean.

Now that can be compensated for in the event whoever balances pencil or pole runs sufficiently fast as to counterbalance the additional vector. Clearly though a good deal of energy is required, and this is as true for the drone as for you or I. What it means in turn is the drone loses its ability to manoeuvre, because it requires a certain level of thrust to stay airborne, and a large margin of extra thrust in order to change direction.

Looking at the disposition of load and thrust in the second diagram it is clear that the drone has less purchase in altering the position of the mass; furthermore it is inclined to rotate around that mass like Earth orbiting the Sun. The only way of preventing this is again to add thrust and we don't know from Tom Stanton's experiment how much, because he doesn't measure it... take YouTube videos with a pinch of salt.

All of this suggests overhead or underslung rotors reduce baseline manoeuvrability, in one case because a larger moment arm is necessary (evidenced by the diameter of a helicopter main-rotor) or in the other because excess thrust is always going to be needed to correct departures from level flight. This suggests that helicopter look-alike such as Volocopter or Hexa will not set the world alight compared to air vehicles like Jetson or Airspeeder, which centre their mass.

The third diagram is why I concluded that if the TELEDRONE is to have eight rotors for redundancy (and to keep it compact), they are ideally distributed vertically as well as horizontally. All else being equal, thrust applied to the cantilevers seen in the diagram should act around a mid-point. This is about where the vehicle (and our own) centre of gravity is located, thus restoring the available purchase we see in the first diagram.

Whether this works in reality is something that we hope to see prior month's end.

Mantaaaaaargh!

It's always a treat when the Vertical Flight Society eVTOL round-robin drops into the inbox and I was especially taken by Manta Aircraft appointing a new board of directors, especially as mine includes me and the breadboard. Closer examination of the website however reveals Headquarters hovering here like a UFO over Google's Streetview of the company address.

I don't know about you, but if I ran a company out of somewhere like Maclaren's HQ in the UK, I'd invite people there unless they need loft insulation. Speaks volumes about an eVTOL industry that in past posts I've likened to an art exhibition.

I could be wrong, but as a precaution am currently hiding from a Mafia hit somewhere among the myriad rooms of TELEDRONE's registered address:

Buckingham Palace.

State of the (Dro-) Nation Address

Worth a pause in the proceedings to evaluate where we've come from, and where we are headed... what pilots call a progress log, or plog.

First and foremost, and as Robert Burns would say of the best-laid plans of mice or men, the airframe would not fit through the workshop door... although as I point out, better to be stuck outside the door than inside. And this also points to the potential benefit of an airframe that can be readily dismantled.

And so it may, ultimately. The upper quad has a central recess that allows it to rest securely on top of the phone-box (the dome you see here being cosmetic and merely a means of protecting the avionics). Meanwhile the lower quad has a central void that allows it to slide into position around the base of the phone-box, which really is the base as it appears here. In turn this transfers the static load of airframe and payload directly to the ground, allowing for a lighter build altogether.

Bolted afterward to the sides of each quad, those X-braces that you see secure the whole assembly. In theory therefore, once removed the airframe can be reduced to three modules able to negotiate the narrowest of passages. This counts in the long run, Hexa's single-seat eVTOL for example requiring not inconsiderable machinations in order to move it around (see below).

Failure of the previous prototype under flight-test was due to the undue flexibility of its airframe, and thus this one has been redesigned to be shorter, stiffer and lighter: It therefore forms the basis of whatever follows it going forward. At the same time its skids are easily modified so that the airframe could support octocopters instead of quads ~ that's sixteen motors in place of eight for those of a nervous disposition.

This counts for eVTOLs, incidentally. The (eight-motored) Vertical Aerospace prototype was recently destroyed following failure of two motors, or 25% of its installed power. To contrast that with conventional aviation, British Airways once flew a 747 to the UK from Los Angeles following 25% (i.e. one) of its engines failing shortly after take-off.

Accordingly for the purposes of maximum rigidity at this stage, all three modules of our 'DRONE are riveted together.

Finally, what happened to the four-pronged layout used so frequently in the past as against this more conventional arrangement with 'panniers' either side, not dissimilar from H-format racing drones albeit with the 'H' rotated through ninety degrees? Well the benefit of building and rebuilding at various scales is that strengths, weaknesses and practicalities of designs emerge that might not otherwise. You cannot beat lived experience, which is why YouTube is awash with 'ten worst' videos.

And this design from my own point of view ticks all of the right boxes, not least the ergonomics of flying an upright operator. What you see forms the front of the box will eventually hinged down to form a ramp, incidentally, so as to allow ingress and egress whilst still forming a shear-web to enhance the rigidity of the whole in flight. Aircraft are ever a delicate balance betwixt undue rigidity and undue flexibility and like all of us, are essentially somewhere on the spectrum.

A further reason too for flying this prototype as a 'whole' is that the flight computer fixed in the dome of the phone-box is wired to the lower quad in order to operate as an X-8 configuration. Ultimately I want that control duplexed so as to accommodate rapid assembly and disassembly, in order that we've an eVTOL that can be deployed by a single operator... and I don't see any others out there that fit that particular bill. Yes, Jetson's fabulous aircraft (like Blackfly's) can be wheeled out the garage, but I don't see the component part of either being racked on its walls any time soon.

Should this prove to fly ~ and I want it to fly better than any other at least in terms of outright manoeuvrability ~ then we'll scale up to full-size mannekin with motors that are currently surplus to requirements and somewhat larger, along with 27" propellers in place of 22". And then if that can be satisfactorily demonstrated ~ which should get us all excited ~ we'll up the power again along with the payload until such times as I'm up there myself.

Should it not fly to spec though, the towel passing your head is mine being thrown in.

Re: Move

The airframe begins its journey South for wiring, programming and test-flying. David Attenborough, superstar naturalist, narrates its passage:

"Here, in one of nature's most spectacular migrations, Octocopteris Rex mass upon the M6 in order to attach themselves to any member of the species Suzukus Jimnius that happens to be passing. After a long journey, the fledglings gather on the Somerset Levels in order to exercise their wings, taking to the air in a dazzling display."

Rebuild #25

Really the final touches prior to fitting out in the workshop down south with all of the electrical components, and here's a quick and dirty failsafe for securing battery packs (in whose absence I concocted this looky-likey).

Typically the VTOL community use Velcro to secure, though I prefer cable-ties in addition now that they make ones with ratchets that release. Otherwise should packs part company in flight (a) they may fall on your head with deadly consequence and (b) flying is discontinued, which is ideally something done on the ground. A cheeky trick here is to rivet the angle-alloy bracket over the cable-tie, ensuring it's the right way up. This provides sufficient clearance to insert fresh ties during repeated flight-tests.

It all exemplifies our current choice of airframe layout, which incidentally is based on a previous, albeit one truncated at waist-height (below). Though what we have now is effectively a 'stretch' ~ familiar from the evolution of every successful airliner ~ the practicalities of a fixing an upper quad above head-height are too numerous to list here. Actually they're not, I just cannot be bothered.

Pictured below is that prototype (listed in the VFS database) shortly before it crashed at Llanberis. This led to mutual acrimony and blame-shaming among the development team, which I am proud to say was principally driven by myself in a storm-the-capitol moment of madness.

Check it out though ~ in the DNA, isn't it?

Flight Club

Richard Browning demonstrates his jet-pack at the recent Oshkosh fly-in, increasingly the go-to event for demonstrating all variations of vertical flight. The company makes a living off the back of experience flights ~ not unlike those barnstorming events that showed how airplanes might prove to be more than a means of rapidly delivering mail.

Meantimes work commences on schedule September 11th on wiring our airframe prior flight-testing slated for later in the month. The GoFly Challenge itself winds up on the 26th, and so it's a make-or-break moment for us all.

Flight-Test Data

Returning to the data provided by T-motor for their U7 units, we can see the issues we face during forthcoming flight-tests this month. In truth we're underpowered, though that would have been the case with any number of aircraft flying today: like fine wines most airframes improve their performance as the years go by whilst the power-plants improve both their power output and efficiency along the way.

Sadly the above table is all that T-motor provide, and although the motor is slated as a 22.20V design the figures are for 24V and thus we lose 10% at the get-go. The vehicle will have a gross weight of around 16 to 17 kilos once fitted out, or well under the 20kg budget. Nonetheless the motors have to run at a middling RPM that leaves a degree of slack for variations in thrust necessary to stabilise it in the event of wind gusts, or yet simply to steer it in three dimensions.

We can see from the table that at 65% the motors will therefore produce around 2kg of thrust apiece, and altogether more at 75%. This should be sufficient even given the losses attributable to propellers on the lower quad operating in the efflux of those on the upper.

I've included the operating temperatures for our general amusement, which T-motor describe as a lawsuit avoiding HOT. Accordingly I have gone for higher amperage speed controllers, as they are the aspects most likely to fail under the considerable currents involved. Interestingly too, they are MORE likely to fail when reducing the RPM of each motor, presumably because of the heat that the necessary resistance generates.

You can thus see how my Plan B ~ that involves operating the lower quad as a form of 'collective pitch' providing only thrust ~ is a very cunning plan indeed. Running at close to 100% these motors provide practically twice the lift they would otherwise, while at the same time protecting the most failure-prone component. This is an overwhelming argument in favour of four foot-level rotors, even were it the case that the upper quad supported a failure-redundant conventional X-8 octocopter.

The cost of the rebuild (discounting electrical equipment retained from the previous prototype along with the mannekin) was around £300 besides the time involved and travelling expenses. A drop in the ocean compared to spend to date, and source of comfort for those who chose the round-the-world cruise instead.