Homebuilt Helo #49

While we wait for those M3 bolts to arrive, let us consider another aspect of the proof of concept craft relating to battery stowage. Currently we've a pack addressing each of the motors, although we are likely to revert to past practise and use a single pack via a PDB or Power Distribution Board.

The reason for so doing is that I'd like to stow the substantial battery mass behind the head of the occupant on a perforated sheet upon which both this and a head-rest can be mounted.

A further reason is that there is a limited amount of room up top and from operational points of view a pack that can be swapped out conveniently from the rear works best.

The principal benefit of hanging battery packs here however is that it re-balances the vehicle given that the lower limbs of the occupant overhang the outline of its frame.

Building prototypes does take you down rabbit-holes and wondering what our lower limbs weigh is one such. There's a secure unit down the road here and one option was to have interviewed any number of chain-saw murderers to see if they could cast any light on the subject?

Alternatively you can scour the net at cost of being suspected of planning for body-part disposal yourself... though I've taken that bullet for you.

Using the table above we're looking at around 7.5% of body mass for the lower leg and foot (plural) and some 8.5% for a pair of thighs. I'm taking some of that seeing as how they overhang the edge of the seat, and so let's call it 12% all in. Give me a break here, because you're wearing a pair of boots unless you're levitating for enlightenment?

Apparently mens' perceived ideal weight in the UK is 77kg, which is me with clothes on... and no surprise that I'm perfect in that way too. That means we can hang 10kg of battery out back there on a full-scale prototype, which is a very respectable payload. In fact it is just about four of the largest six-cell LiPo battery-packs each providing for 22,000 milliamp hours.

A target application for a machine of this type is shuttling you upwards for a birds-eye view of the world ~ for which tourists will always part with money ~ or else launching parachutists or powder-skiers by a more cost-effective means. For most such uses a means of swapping out battery-packs quickly and efficiently would be required.

Homebuilt Helo #48

With the brackets cut out, apply the printouts from the motor specs ready for drilling the four holes necessary for connecting them.

Though these are for 3mm bolts I may well use a 4mm drill-bit firstly because it gives a little leeway for mounting the motors and secondly because I use the same bit for the pop-rivets and can't be bothered swapping it out.

We'll go for M3 Pan Pozi 8mm bolts at a cost of one penny apiece, I think.

Take a break now and make yourself a melted tuna-cheese toastie.

Homebuilt Helo #47

Here's how the motors will be mounted on the dog-bone mounting bracket. Practically this forms a demountable bogey that can be swapped out for rapid turnarounds in the event of breakage or failure.

In my experience drone-builders don't really get designing for personal air vehicles and not least because they've not flown their ass in a variety of machines as I've done over the course of some fifteen thousand hours.

One consideration for instance that rates this arrangement over co-axial propellers is that the brackets will be fitted with that left-most side nearest the airframe in order that the associated propeller is pitched clear of the occupant.

In a risk assessment scenario I've conducted, two people go flying in these suspended chairs in the Grand Canyon and one points to an interesting feature up above, loses a hand and bleeds out.

We need to eliminate each risk step-by-step while acknowledging flying is inherently risky. The reason people duck when boarding helicopters is that those not doing so do occasionally get their heads chopped off. Or die walking into tail-rotors or propellers on aeroplanes, or sucked into jet-engines.

That's why most of this type of work takes place beyond the shores of the UK, where what most needs saving from peril is the index-linked pensions of those overseeing its evolution. I should have gone to the US years ago, but it's too late now...

Because there are no rules for building electrified personal air vehicles and the reason we're working with parts off the shelf is that this is what happens at the birth of each technology.

I've seen the original steam locomotives with parts sealed with the leather and grease from cattle, and cars built from horse-drawn carriages. My old man was a telephone engineer for forty years and when I saw the first electrical computers being operated at Bletchley Park ~ where Enigma was cracked ~ I recognised the parts straightway.

From the analogue telephone exchanges I wandered as a child. 

Homebuilt Helo #46

I've gone ahead and continued with those tins of tuna in place of the motors as they are a close enough fit. Width-wise I've used a simple ruler as a masking template as again, it's good enough for government work.

If you think I'm kidding, a report into a nuclear submarine incident in the UK last week revealed that rather than using a new bolt in a component, the engineer used super glue to reattach a head that had sheared off.

If Putin's reading this, incidentally, I'd be heading for those shelters around about now.

Needless to say ~ okay, we need to say it ~ once you've cut one dog-bone out you can use that as a template for the remaining three.

Alternatively you may wish to use carbon-fibre sheet and a CNC or laser cutter... tho' that's not us, is it?

Homebuilt Helo #45

Why though are we using a pair of clockwise-rotating propellers, as here? Well from this photo it is clear that the overlapping areas of each disk will in fact rotate in the opposite direction.

This is important because overlapping propellers are not at all efficient, and altogether less so if the first swirls the efflux in the same direction as the next. An exception to this is the jet-engine, where the stages do all rotate the same way upon the same axis but are able to accommodate this with flow-straighteners in the interstitial stages.

Don't ask, basically.

Using the laser-guided set-squares I have here, however, it is clear to see that a ten-inch space (254mm) is a suitable separation distance for each pair of U7 motors.

Homebuilt Helo #44

The good news (and was there ever any bad about this?) is that relatively inexpensive carbon-fibre tubes at (!) 22mm are out there in the universe...

https://www.ecfibreglasssupplies.co.uk/carbon-fibre-tube-22mm-19mm

... and we'll likely need them before we fly as the sixteenth-inch alloy tubing that we saw yesterday will I fear not be as rigid as we need in order to preclude vibration.

Bear in mind too that I'll be leaving them in native black (though carbon-fibre comes in red now too) and thus I don't need anyone wanting their money back if the product doesn't match the picture... frankly nowadays, when does it ever?

Let's move on however to today's sermon in an effort to keep up the momentum. By way of recap we're looking at using our proprietary offset co-rotating propellers and not the conventional (X-8) co-axial arrangement because:

(a)    It's expected to be more efficient, and

(b)    We don't need complex motor-mounts.

Accordingly for today's exercise we shall be trialling what I shall call 'dog-bone' motor-mounts. This is one for the whole family to enjoy as it can be done indoors on a table-top like a giant game of Cluedo.

The ingredients include matching propellers, a pen, a sheet of plywood, U7 templates, tins of tuna and (if you can get hold of one) a Nigella Lawson.

The tins of tuna are subbing for motors at this stage because:

(a)    In the event of dropping one there are no more tears, and

(b)    They make a perfect melted tuna-cheese sub afterwards.

Homebuilt Helo #43

I'll call these cantilevers 'lances', because it's my party and I get to choose.

And you'll mail me of course, with"Master, how long shall we make our cantilevers?".

Foolish generation, can you not see fifteen inches is sufficient to clear the airframe, and twenty-two for the top-most propeller?

Go forth therefore, and make your cantilevers thirty-seven inches.