Why would it only be interesting at that point? At 500wh/kg, if the cost is low enough, you're already going to eliminate fossil fuels from basically everything except long range flights and shipping.

> Half the weight and double the range would be great

You don't need to quadruple energy density to achieve that. If you just half the weight (without increasing the total amount of energy in the battery), you're going to significantly increase range. The less weight you have, the less energy you need to move the vehicle.

> Plus, electrified ultralight "aircraft" start to have numerous advantages over the traditional 2 stoke engines.

I think you'll have plenty of benefits with ~300wh/kg (that's the target for many useful eVTOL aircraft).

The key challenge is you should redesign the whole aircraft around electric flight to get the full benefits. Look at NASAs Maxwell X-57 for an example of how that could look.

With 500wh/kg you can start taking over most regional flights. Yes, the range won't be as good as jet planes. But jet planes have FAR more range than they need because they don't design a special purpose aircraft for shorter range. They just put less fuel in.

But it'll probably take 10-20 years regardless of when we get good batteries because it'll take a long time to design and certify the aircrafts.

Ultralight have a very specific set of regulations. 500wh/kg is where you can really start to use it in that application. Currently there are models with electric, but it's about 1 hour flight time (advertised) and you don't have great margins. If you can reduce the weight and get a true 2 hours, then that would replicate the characteristics of today's 2 stroke.

Another point is that it won't take very long for ultralight since they aren't technically defined as an aircraft but as an air vehicle. You can home build them.

Yes, you might see a 10% increase in range with half the vehicle weight. If you tow or take long trips, you want double the range. At that density, you're choosing one or the other, or an "eh" compromise. I want 800 miles and less weight/size. This can especially be useful for retrofit kits for existing vehicles for people who hate all the tech in the EVs.

> But it'll probably take 10-20 years regardless of when we get good batteries because it'll take a long time to design and certify the aircrafts.

I'm wondering if large aircraft companies are actually already designing the next aircraft based on assumed battery densities? I know they put out press releases with nice looking renderings, but I am talking about serious development?

If you wait until you have the batteries on hand, and then spend 20 years to design a plane (and 20 years might be conservative, since arppovals will be harder to get for a brand new concept), you might be left behind. Instead, they could be already designing the plane and when 500Wh/Kg is available, boom, they are 15 years ahead.