Metric... or Imperial?

© TELEDRONE LTD. 3rd November 2022

Mix-and-match really because propellers follow US practise and come in inches, while stock material here and elsewhere in the world is retailed in metric. A good example is that centre-section, which at 20" will suit a half-metre square shear-web.

I've come over a bit Jackson Pollock today and transferred sketches from an A5 pad to lining paper taped to the floor. I was interested to see that Tesla still model in clay at full-scale, a method dating back a century which auto-manufacturers still adhere to today for the final 'look and feel' of each model.

Similarly when ships like the Titanic were designed, each cross-section or 'station' would be chalked out at full-scale on the floor like this prior to their engineering.

The overall dimension here stands at 42" if you include the overhang of each motor, but discount the arc of its propeller. Folding two-blade propellers are slowly making their way up to the larger drones, so it is conceivable that an airframe like this might yet be stored in a four-foot box.

Alloy... or Carbon?

There are options in the UK to purchase 20mm pultruded carbon-fibre sections with a wall-thickness of 1.50mm to suit tube-connectors, but there are reasons not to do so:

Despite recent rises in commodities, aluminium remains almost ten times cheaper.

It can be cut to length from stock unlike preset fibre lengths brought in from Holland.

It can be re-purposed in future prototypes or else recycled.

I can drive up the road to collect and doughnuts usually do for cutting by the staff.

It comes in a range of dimensions that are easy to scale, whereas carbon does not.

You can re-work metal whereas composites are effectively single-use use plastics.

Enough said?

Re: Bar

Conventionally with the sort of hand-built commercial drones that sell for five-figure sums, the means of mounting the motor is to design a plate or outline that can be cut from carbon-fibre sheet using a laser-cutter... which is expensive.

I bolt them direct to inch-square tubular alloy as I have done generally for each of the prototypes that we have flown (including the last in December of 2021).

At the scale considered for our current effort, 20mm square alloy is a better choice, but then at the thickness it comes from the supplier (2mm) the tube-connectors that I use do not fit inside. Our world rotates round stock dimensions of one inch or 25mm.

Accordingly looking at that section superimposed on T-motor's technical drawings, we can see that the four bolt-holes for fixing U-7 motors range around a 30mm diameter circle... which means that the distance between each is rather less. In fact, at around an inch (as seen in the figure on the left) it does not at all suit direct attachment.

What I've done in the past is to rotate it through forty-five degrees and then fix the motor with two bolts directly through the spar, with the outliers pinned to angle-alloy fixed along its sides. In this case as you can see, it is a close call... whereas with U-11 motors which I used previously, the bolt-holes were arrayed around a 40mm circle.

Nonetheless it should be do-able. In fact we discovered that airframes flew perfectly safely with just two bolts ~ those within the confines of the spar ~ in use. 

Experts in the field had endless worries over this with torsion and resonance issues, none of which materialised. Whilst people with giant drones are generally patronised, on one hand I flew jet airliners that had been brought to grief by such issues, and on the other I'm aware that the Wright Brothers were also dismissed as cranks.

So as Andy Warhol once said, If people hate what you are doing then keep on doing it.

Power Play

The first choice to make when it comes to fitting battery-packs to eVTOLs is between lithium-ion and lithium-polymer. The former carry more weight, but deliver power over a longer period and are therefore the cells of choice for electric cars, aeroplanes and upcoming air taxis. The latter pack more punch and are preferred at the lower end of the scale when budgets dictate shorter flight times at lower weights altogether. They also come as flat-packs, instead of bumper packs of cylindrical cells the size of AAs.

The second choice is whether to 'bus' the batteries together, and supply the motors by a sort of spider-web array of wires from the centre-section, or to have packs address motors directly. Smaller drones are conventionally wired from a single battery in this way, but many personal air vehicles fit the batteries directly adjacent to motors, like for instance LIFT's 'Hexa' eVTOL.

An advantage of a single energy source is that it might feature one large battery-pack or a pair thereof wired in series to attain a higher voltage, instead of say eight smaller. 

Thirdly there is a choice between higher and lower voltages, with the number of cells in each pack determining the voltage in multiples of 3.7V. T-motor recommend running our motor of choice on six-cell packs (6S) like the one above, which supply around 22 of the rated 24V. Voltage determines the RPM of the motor and is thus tailored for the fixed pitch propeller to optimise thrust at expected operational values.

On balance I prefer to go down the distributed energy route, for even though a single battery failure would affect one of the eight motors, this will not be an issue because there are eight more independent motors and power sources available for recovery.

Finally we can calculate the flight endurance from the battery rating, by examining the T-motor data below. This says that the motors will be drawing around 22A at a typical setting, and a 22000mAh battery-pack like those I have used in the past would run the motor for one hour therefore. The smaller pack (2200 mAh) seen here would therefore be expected to run for a tenth of that time, or six minutes. LiPo batteries however are not run flat, however, so that the ones in your phone and laptop for example are wired to a battery-management circuit (BMC) that prevents this from happening.

Accordingly eight of the above packs (else the slightly larger at the same price) would suit the prototype, at a cost of £520. This brings the budget nearer to £3000 all in and shows why few people build large drones for fun, let alone people-carrying types. My plan this time however is to let you power up my prototype, the way Liberace used to tell people that he hoped they liked his jewellery... as they had paid for it.

(Note that the figures below are the results of tests at 25V instead of 22, although we can address this by upping the number of cells in the battery-pack with a 7S type, so that we've nearer 26V available to us.)

 

Pound for Pound?

In many ways the development of eVTOLs re-visits that of the PC (which itself evolved into the smartphone), in that both the data and equipment are becoming integrated. So whereas you once had to build computers out of components and find out as you went along how to put them together ~ whereas now it is all on a plate ~ slowly the technology behind multicopters is beginning to be rationalised in the same way.

Which makes it easy for dummies like me and you. Accordingly here at the outset I've  approached things in a scientific and disciplined manner, which is a personal first.

The above selection of motor, propeller and speed controller provide around 26 kilos of installed thrust at a throttle setting of 60% according to the data. Thrust settings of that sort are what are required to keep something 'in hand' with which to address the contingencies of both steering a trajectory as well as engine failure.

This amount of lift happily accords with what is broadly becoming an internationally recognised limit to the gross weight of drones before things get more complicated to register i.e. 25 kilos. This in turn means we can build a sizeable scale prototype, and subtracting whatever its airframe weighs from that figure yields a theoretical payload.

Budgeting for development ~ again a novelty for me ~ is key to progress, however, and so it pays to build the airframe prior to shelling out on the motors etc. Happily we can now do this, because having the diameter of the propellers we can refer to the post of 28th October so as to draft an outline of the drone itself.

Above us only sky?

Let's explain the rationale behind this.

Prior to the introduction of the jet-turbine, helicopters did not have the option to mount the motor on the roof, where with the exception of the Robison R66 it has remained ever since. The object lesson of helicopter design throughout its evolution is therefore that (a) the ideal place for rotors is the overhead and (b) the ideal place for motors is the overhead.

People-carrying drones, however, don't come awfully light. This is principally because what you have gained in terms of weight-savings from the power of electric motors, you have generally lost in replacing high-density fuel i.e. kerosene, with battery-packs.

To support a sizeable drone at two metres you would therefore need a substantial structure (phone-box as originally conceived), besides an extensive undercarriage to stop it from toppling.

The 'method and means' above solves each problem at a stroke.

At the same time it preserves a unique selling point ~ ours a flat-pack flying machine and yours not ~ because the columns can be inserted individually into the base from above, through the four channels in the drone included in its centre-section with that in view.

Expert UAV builders caution that the drone will be insufficient manoeuvrable in view of its low-pitched centre of gravity ~ but by and large we have progressed more often with expert advice having been ignored, than with it having been acknowledged.

Let's start building.

Re-Boot

As personal air vehicles go as distinct from flying taxis, there's currently only one game in town and that's Jetson, who've done for Sweden what Spotify did earlier and proven that everything need not necessarily emerge from California... just most of it.

After a four-month break whilst I needed to step away from the vehicle, I've had to ask myself what counts? What's in the DNA?

And the conclusion I've come to is that it has to be about moving people a shortish distance in three dimensions in the simplest superstructure possible, by using wholly unique home-grown means. I don't believe that you succeed by copying everyone else in the longer term, and that goes for products as well as it does for personalities.

So we'll take it from the top ~ literally ~ and draw upon the rich inventory that is the WIPO patent specification and in particular, this configuration for eight or more rotors.

Electrical air vehicles have been something of a fool's errand, as they looked like they could replace the main-rotor of helicopters at a stroke. They did so on the shoulders of drones, which got away with as few as four maintenance-free means of lift at each corner.

What distinguished the develop of drones and subsequently eVTOLs was the fact that the majority of people involved had never been (nor intended to be) pilots. And when it comes to life-or-death scenarios like engine failures, it's the gladiator in the arena that counts and not the spectators.

As a consequence, the number of motors on practical multicopters has proliferated, and as things stand eight is the minimum for entry to the ball.

Why though abandon the X-8 configuration of over-and-under power units used by most commercial drones and current eVTOLs? Well the estimates for the losses of aerodynamic efficiency involved I have heard range from an eerily precise 14% to T-motor's own estimate (or warning) of as much as 25%.

And that's far too much power to trade at the outset.

The advantage of the outline above however is that it continues as a simple four-point proposition, with pairs of motors addressed as modules that I'd describe as cassettes as they relate to two rotors, or bogeys where they are attached (over-and-under) in a set of four.

The only addition to the configuration as originally conceived are those torsion bars around the perimeter, pictured in black.

The dots featured at each corner of the centre-section we'll investigate anon.

I had intended to discontinue this blog ~ Jetson thriving without one ~ but relented as the stats disclosed a consistent readership. If you're reading this, then rest assured you make a difference.