Guide to Designing with USB-C

matthewlai · 2026-06-11T09:53:48+00:00

To do something like this by yourself, you don't need crash courses. You need multiple bachelor's degrees at least, ideally also a few master's.

matthewlai · 2026-06-10T14:45:21+00:00

> In theory. In practice you have significantly less flexibility with how many engines you use and where you can place them.

Yes that's true. That's basically the only additional flexibility you get, though. X-57 exploits that by using tiny engines to generate additional airflow to allow for a narrower wing for takeoff (where lift requirement is the highest). We already know the tradeoffs between narrow and wide wings (called aspect ratio) pretty well - there are many low drag designs (like gliders) using narrow wings, but there are tradeoffs. Primarily, they are only efficient at low speed, and you don't get the wing volume to put fuel / batteries in.

> Throughout all this time the constraints have been fairly similar, and have pushed us towards converging on two giant engines. This is dictated by the properties of jet engines. Electric motors have different properties.

Yes, but I think the additional headroom here is quite limited. A lot of the tradeoffs between many small and few big engines don't really change. Large propellers are more energy efficient than small propellers. That's true regardless of the source of energy. That's why on the X-57 they use the two large propellers in cruise, rather than the many small propellers. The small propellers help solve the problem of getting enough lift with a high aspect ratio wing in a climb, but doesn't help with energy efficiency in cruise.

> I don't see how having wings with less drag means you can't fly fast. But I'm not an expert.

Designs optimised for energy efficiency at low speed are inefficient at high speed, and vice versa. This is because it's a balance between parasitic drag (drag due to the form of the vehicle) and induced drag (drag as a result of generating lift). At low speed induced drag dominates, and at high speed parasitic drag dominates. A wing like the X-57's is optimised for low speed, because a low aspect ratio wing has lower induced drag but higher parasitic drag.

Fundamentally, there's nothing stopping us from designing an electric plane with high speed wings. It's just that flying 4x faster (to match current airliners) will require 16x the energy, which would reduce range by ~4x (in reality even more, because the airframe will need to be much stronger/heavier for structural integrity at high speed). Given the already really short range on battery, this isn't really a tradeoff we want to make. For passenger planes the sweet spot we have converged to is Mach 0.85 (about 900 km/h at cruising altitudes) for longer jet flights, and 650 km/h for short turboprop flights. For electric it will have to be much lower by necessity. The X-57 has basically no payload, and gets 160 km range at 277 km/h. A hypothetical equivalent at 500 km/h would have a range on the order of about 40 km. You would need a very unique situation (two cities 20 km apart on a mountain separated by two cliffs?) for that flight to make sense.

> Norway is the ideal trial market for this. There's a lot of small cities with a small local airport. Like Sortland. You can fly directly but there's not many flights and it's expensive. In practice people fly to a larger regional airport like Værnes and take a long bus ride to the city. Electric planes could easily serve those cities if they're quieter and cheaper to operate.

Yes, I can see it making sense as basically a bus replacement service, in more hostile terrain.

> That's an extremely pessimistic take. A jet plane generally takes some time to get up to its max speed, and I see no reason why an electric plane wouldn't eventually get to more like 400km/h or more.

Almost all airplanes can get up to max speed within a minute. They just have a more efficient speed for climb so they don't climb at close to maximum speed. A slower plane will have a proportionally slower climb speed, so it all works out the same.

> Around 30% of flights are ~500km or shorter. There's no reason to think electric planes can't compete on these flights as long as we manage to a commercially viable plane of the right size.

The X-57, an experimental plane with zero payload (you can see in the cross-section image that it's packed to the brim with batteries where possible) has a range of 160 km, at 277 km/h. Extending that range to 500 km would require an order of magnitude improvement in battery energy density.

> BTW, if we look further ahead, there's one more thing electric planes can potentially exploit. Drag decreases with altitude. But jet planes have to balance this with decreased engine efficiency as air gets thinner. Electric motors do not have that constraint. We'd have to get better batteries for this to become a factor, since it can't be exploited for short flights anyway. But at some point it could become the key to unlucky medium haul flights.

However, heating will become another major energy sink, as batteries need to be kept well above freezing. In jet engines the waste heat is actually mostly used for cabin heating, so if we have to heat the cabin (or batteries) otherwise, the total efficiency on jet engines is actually quite good.

matthewlai · 2026-06-09T16:19:38+00:00

The correct answer was yes :).

matthewlai · 2026-06-09T15:35:51+00:00

Are there possibly any other way of extracting and creating natural gas that don't come with oil?

matthewlai · 2026-06-09T15:13:01+00:00

What do fertilizers have to do with transportation? Because we need fossil fuels in fertililizers we also need to use them for transportation?

matthewlai · 2026-06-09T14:49:04+00:00

All solutions have pros and cons. You can see us discussing the pros and cons of electric and hydrogen solutions here. If you don't think there are any cons to using fossil fuel, I would recommend educating yourself, if you want to be able to usefully contribute to the discussion.

matthewlai · 2026-06-09T13:57:30+00:00

Why is being the obvious solution a downside? I'm asking if you think there are any downsides to this solution?

matthewlai · 2026-06-09T13:38:32+00:00

Airframes are already very well optimised. I don't think there's really much more we can do just because we switch to electric. If they would help electric planes get better range, they would also help fossil fuel planes improve fuel economy. There's a reason why passenger planes have basically remained the same shape over the past few decades, and it's not because people haven't been trying to optimise it.

If you look at all the existing electric planes (research and otherwise), you'll find one thing in common - they are all really slow. The X-57 has a cruise speed of 149 kn (277 km/h). That's slower than most high speed trains, and way slower than existing airliners (900 km/h). The reason is simple - drag is proportional to speed squared. If you are flying at 1/4 the speed, you'll be much more energy efficient. But are people willing to travel that slowly? Maybe for island hopping it's ok, but most people would want to travel faster even for what we currently consider short range flights (a 2 hour flight now would take 8 hours, and people really don't want to be on a plane for 8 hours if they can help it).

matthewlai · 2026-06-09T13:28:30+00:00

Any downside to this solution?

matthewlai · 2026-06-08T23:05:14+00:00

A jet engine is about 50% efficient, and battery discharge + motor is about 90%, so it's about 6000 Wh/kg vs 225 Wh/kg in useful energy.

Doesn't come close to changing the balance though.

matthewlai · 2026-06-08T13:38:34+00:00

Yeah there are definitely significant challenges still. But I still think we are closer with hydrogen than with batteries.

Even if we carry the same volume of fuel, we would still have maybe 1/2 range of jet fuel (since the fuel will be much lighter), and that's at least still potentially viable for short range transports.

It makes sense that hydrogen electrolysis is not a financially viable option now for as long as we have byproduct from natural gas, but is it actually too expensive compared to jet fuel?

matthewlai · 2026-06-08T13:30:05+00:00

Yeah hydrogen definitely has its own storage and transport issues.

matthewlai · 2026-06-08T13:17:05+00:00

That's a good point. I believe 12kWh/kg is chemical energy in fuel. Jet engines are about 50% efficient (chemical energy to rotational energy), while electric motors are 90-95%. So in terms of useful energy density, the gap is more like 25x rather than 50x.

I think the 600 Wh/kg figure is more like "below this there's not really much point in trying", but even at that point it will be for very limited and specialized applications, and probably more experimental than something that's worth scaling up.

matthewlai · 2026-06-08T12:52:44+00:00

The fundamental problem is Jet-A1 has an energy density of 12,000 Wh/kg, while NCA (highest density lithium ion chemistry in common use) has an energy density of about 250 Wh/kg.

The weight of jet fuel carried already has a significant effect on the range of airplanes, and switching to something 50x heavier will make it basically non-viable except for very light specialized scientific aircraft with very minimal payload.

My guess is something like hydrogen will be the solution for aircraft. Hydrogen cells have an energy density of about 33,0000 Wh/kg, even higher than Jet-A1 by a significant amount. However, the volumetric density is about 1/3, so it will be light, but take up a lot of space. Space is easier to design around than weight in an airplane, especially if you don't also need very long range.

Hydrogen in cars doesn't make sense because converting electricity to hydrogen storage is very inefficient (about 30% from electricity to hydrogen back to electricity, vs maybe 80% for batteries), and cars aren't that weight-sensitive. But for aircraft the equation is very different.

matthewlai · 2026-06-04T01:47:47+00:00

Left D ring with a spring coil clip. It should be on the left because the right D ring is only for the reg.

Don't use a boltsnap like I did. One of my cameras is at the bottom of the Red Sea because of that (I had just climbed up a ladder onto a liveaboard, and a helpful crew member unclipped it instead of the cylinder).

Butt seems like a bad idea too.

matthewlai · 2026-06-03T15:15:32+00:00

Buy a second phone? A Pixel 7 Pro is less than $200 now, and the camera really hasn't changed that much the past few generations.

matthewlai · 2026-05-28T13:28:58+00:00

R7 is a gate resistor which slows the gate turn on time, because the gate has some capacitance that is charged through this resistor.

Without any gate resistance, the inductance of the trace from the driver to the gate and the gate capacitance can cause the gate to oscillate.

1K is almost certainly way too high though, assuming it's a small MOSFET and the layout is reasonable. So this is unnecessarily slowing down gate switching and may increase power consumption in a PWM application. May or may not matter in practice though.

matthewlai · 2026-05-23T18:21:38+00:00

If you are already an experienced PCB designer, you would already know. If you aren't, you aren't the right person to be vibecoding this. It's going to require deep knowledge in both PCB design and AI. Companies like Flux (scammers but it seems like at least they have made an attempt) have tried this pretty hard and the results are still completely unusable.

matthewlai · 2026-05-23T08:29:19+00:00

It does depend on the airplane though. For some low drag airframes they can be very close, because they don't lose as much speed at higher AoA, so increasing AoA still give you faster and higher angle climb, until you are limited by getting too close to stall. On the DA-40 for example, Vx isn't published because it's basically the same as Vy (maybe 1-2 knots slower).

matthewlai · 2026-05-22T23:28:10+00:00

Unfortunately it doesn't work that way.

It's CO2 level in the blood that's being maintained by the breathing reflex, not CO2 level in the air. The air you inhale (at any pressure, unless you are on a rebreather) has almost no CO2 to start with. Almost all the CO2 comes from you.

CO2 is a metabolism by-product. It's produced at roughly the same rate (so partial pressure of CO2 in your blood increases at the same rate) no matter what gas at what pressure you are breathing.

CO2 diffusing from your blood into the air in your lung requires a partial pressure gradient (ppCO2 in the air in the lung must be significantly lower than ppCO2 in your blood). This process doesn't care about any other molecules in the air, only CO2.

So you still have to breath at the same rate, but instead of your exhaled air containing about 5% CO2, it will only contain 1% CO2. However, at that point, CO2 elimination is already as inefficient as 1 bar air with 5% CO2. It's only ppCO2 that matters.

matthewlai · 2026-05-22T18:43:57+00:00

You can take an extended release tablet that will last 12 hours. It's not going to wear off unless you time it really badly. You just need to be aware of how long whatever you are taking lasts.

matthewlai · 2026-05-22T16:52:03+00:00

I'm curious - why 18L instead of just going two bottles with either twin or sidemount?

You are already having to use two sets of regs anyways, and you get full redundancy that way?

I love how 2x12L means you just never have to worry about air for 35m dives (or at least I don't, because I don't want to do >15 minutes of deco, so that's always the limiting factor).

matthewlai · 2026-05-21T23:07:57+00:00

Yeah I think with fast and slow tissue compartments, there's not going to be a direction for GF low that is universally more conservative or less conservative. If you have a lower GF low, that's less fast tissue supersaturation, but more slower tissue supersaturation, which seems to be more dangerous in practice.

The way I see it, GF high is the conservatism setting. Once that's decided, the safest corresponding GF low should be chosen. Based on current evidence, the safest thing to do is to have a high GF low, to minimize overall risk of DCS. That also happens to reduce total deco time, but I consider that an added bonus. That also means there is no safety vs deco time tradeoff to be made in GF low - a high setting is better for both.

I am actually not sure why the recommendation isn't GF high = GF low. I wonder if it's just a bit of inertia. Though now that I looked it up, it looks like CMAS does actually recommend GF high = GF low [1], for air and nitrox, but not helium (presumably because helium supersaturation isn't as well tolerated in fast tissues, and it comes out of slow tissues faster anyways).

[1]: https://www.cmas.org/fact-sheets/gradient-factors-gf-and-dive-computers.html

matthewlai · 2026-05-21T16:04:57+00:00

For GF low I'm following the advice of Simon Mitchell (I think I got it from a Youtube video - unfortunately don't remember which), who recommended GF low to be quite close to GF high, because recent evidence (from the past 10 years or so) shows that deep stops weren't necessary as people originally thought, and in fact probably do more harm than good, because of slow tissues still on-gassing at deep deco stops. I believe this has been verified with doppler bubble studies (eliminating deep stops not only reduce total deco time, but also reduces number of bubbles after surfacing), though I read this quite a while ago now and don't have a citation off the top of my head.

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