Proj Gets a Lift

The larger prototype I decide to fit with lift motors for a mock-up and a possible visit to the studio in Liverpool. The motors and propellers will be recessed in the slot seen here, which affords them more protection and because it's my party and I'll recess if I want to. In order to do so I patch in an extra foot of side panel so as to accommodate the width of a pair of 22" propellers.

My aim is to realise the concept of a self-launching USV and to inspire you all to sit on the sofa with a beer watching me having to do it on YouTube.

You bastards.

A Patchy Longbow

I wheel the original 8' build out of the hangar for a flotation test, and see that addition of motors compensates the unwanted buoyancy produced by extended hydroskis, so that it sits level unladen with a draft of 4". Adding a 5kg barbell to sub for 22,000mAh battery-packs increases draft by a further 4" yet provides for five times the endurance.

If (and it's a big one) we get this baby wired in time for Speed Week at Coniston, then it should do the 'flying mile' even at walking pace. Must be capable of that, or we may as well go home...

p.s. T-motor's perf figures are a bit of a moving target, tho' the final answer seems to be that these 280KV motors work best with 20" propellers instead of the 22" which I have in my possession, tho' I won't be dipping into my savings to swap them out.

One Small Step for a Pair of Waders...

...one giant leap for drone-kind. Bizarrely when I Googled 'swimming pools to hire' it turns out there was someone nearby with an outhouse who'd built an online business on that basis albeit for kid's parties (though I suggested to them that they tag it 'static flotation test-tanks' in order to cater to pressing needs like mine).

Here's a local pond near the heart of the old coal-mining industry hereabouts and I wondered whether stepping in with my waders I'd not be seen again having sunk to several fathoms. Frankly it wasn't something I was in a hurry to do, but as they say in Benidorm it's lovely once you're in.

Chopping the tail-end off the hydroskis ~ which provided for unwanted buoyancy at the rear end ~ did the trick, for with the equivalent of two 3,900 mAh battery-packs rear-loaded, it's back to looking the 'dog's bollocks' as we engineers say. Subbing for the packs incidentally is the quarter-brick featured earlier.

Merch Item #001: Static Buoyancy Test-Kit inc waders £175

Do you really want to be using 22,000 mAh battery-packs for that static buoyancy test in the local fish-pond? Suppose they slide off into the water at a cost of £350 apiece? Suppose they slide off in the water when you're in it, recreating the scene from the Bond film where he drops the electric fire into Blofeld's bath? You'd be toast, while your competitors watching online would be going 'Mwah-ha, mwah-ha, mwah-ha-ha-ha-ha-ha-ha...' wouldn't they?

Accordingly we are proud to offer this TELEDRONE static buoyancy test-kit comprising (instead of the battery-pack pictured) one plug for receipt of 1 x 2.5kg bar-bell in lieu of 22,000 mAh battery-pack; 1 x 5.0 kg bar-bell in lieu of two of same; quarter-brick in lieu of 2 x 3900 mAh packs; masking tape for use as plimsoll line; mooring-rope and tether to insure against loss at sea.

(Sadly in all this I had a colleague lashed for not coiling the rope to my satisfaction... though as my school principal did once, I told them it hurt them more than it did me.)

The Takeaways

I don't like maths any more than you do, so let's keep this simple and move on to the buffet. I've had to interpolate in two dimensions, first to guesstimate what the motors will produce at 80% power as a target setting and second to allow for the fact that we've 22.2V batteries whereas this table for reasons best known to T-motor represents 24.0V performance.

Reasons incidentally that I prefer powering boats to drones are principally (a) motors can be run flat out instead of around half throttle (b) in the event of a crash they are more likely to survive along with the props and (c) there are only two of them instead of four, or worse, eight.

Moving on, note the amperage at 80% is precisely that delivered by the larger battery-packs at 22.2A, which would mean that they would provide power literally by the hour.

Our chosen packs however will be usable at 80% power for five times less (3900 mAh as per 22000 mAh) or TEN minutes, and at 100% they will last for around SIX minutes.

There are two things though to bear in mind, firstly that LiPos can't be run down altogether so we'll consider duration as ten minutes at 80% and five at the maximum. Secondly on the plus side should the props pierce the water at slowest speeds then they can be used to recover prototypes at altogether lower power, for in this case they operate as water-screws.

The other takeaways from all of this is that the thrust on paper is around three kilos at 80% and four at 100%, which is around half and two-thirds the weight of the craft respectively. By way of comparison the most sizeable craft I've driven in the shape of the Airbus 321 weighed 205,000 pounds gross and produced a third of that in thrust... so we're in the right ball-park, albeit water is a lot stickier than air.

The last three columns are also worth examining before the bell goes for our break. These are the largest props available to the U7 motor at 22" instead of 18" or 20". In theory the most efficient rotors are the largest (and thus a helicopter is about twenty times more efficient in the hover than was the Harrier jump-jet).

The maximum thrust for this type of motor tends to come driving smaller props at a higher RPM, albeit at the cost of higher energy consumption. Larger props are more efficient at lower power settings, so ramping up the throttle does the same to torque (or work) whilst hammering efficiency. In terms of grams lifted per Watt expended, note that the efficiency halves between 50% and 100% throttle settings.

At the same time we've gone beyond what T-motor themselves have simply described as the motor getting HOT after ten minutes, to what I've called HOTTER and MELTING respectively.

Motors catching fire do though drive YouTube views and that's what most concerns us.

Power-Up

Time to decide upon which among the battery-packs amongst the inventory is usable for testing the prototype. The motor is rated at 24V but LiPo batteries (like the ones in your phone) for these purposes at least are produced like individual sheets of lasagne packed together: in each of these battery packs there are six (hence the 6S moniker).

Each of the cells in these packs is rated at 3.7 volts, so six together produce 22.2V as it says on the tin. The problem with powering a 24V motor is therefore that it falls between 6S (22.2V) and 7S (25.9V) battery-packs, although the latter would be usable albeit 'over-boosting' the motor at the higher throttle settings.

Note the different connectors: AS150 on the larger pack and XT60 on the smaller, and there are two key differences viz. (a) you can connect the former in series to double up on packs and extend your endurance at the risk of (b) joining the wrong connectors together to short-circuit the pack. I've done this with those larger packs ~ twice ~ and the effect is something like holding a small stick of gelignite whilst it goes off.

Perhaps the key difference between the packs though is the 'C' rating and in these terms at least the smaller pack wins hands down, which is why perhaps it is called a 'Supersport Pro' instead of a 'Supersport XL'. I guess the XL stands for XL, which it certainly is at 2.70 kilos in weight... which is why a 2.50 kilo bar-bell weight appears above, because during static testing in water it is a better means of ballasting the prototype altogether than using the real thing.

The 'C' rating represents the rate at which a pack can dump available energy if called upon to do so, and the smaller pack can do this at sixty-five times its regular rate in small bursts, and thirty-five times in short bursts i.e. 65C/35C as against 35C/20C in the case of the larger pack. This incidentally is why LiPos are used wherever higher instantaneous power is required ~ as with drones ~ while for endurance applications like motor vehicles Li-ion is the chemistry of choice.

At first glance then I am reassured that we can run with the smaller packs, not least because (a) they can deliver the extra amperage which we may need whilst (b) they are nearly eight times lighter at 366 grams instead of 2695.

Of course we could switch to higher voltage 7S packs to wring maximum power out of the motors, but as these retail at the best part of £200 per pack they can stay there.

Nor will the larger packs that we already have become redundant, as they'll be hooked up for the static test-runs that will call upon their greater endurance.

Next we'll get into performance data, which as ever we'll take with a pinch of salt.

Step by Step #16

Here's prototype #1 stowed safely out of the way whilst we focus our attention like a moderately powerful laser on wiring the current build for sound. The one here extends beyond the rear to allow for fitment of lift motors in all four corners, so it can hover itself over land or water. Rome however was not built in a day, so we'll be taking baby-steps first to get prototype #2 to simply travel upon that water.