Saturday, June 22, 2024

What Airbus should build next

Boeing should obviously build a 737 replacement next.  As I understand it, this project has been underway for years.

Airbus, however, could do something interesting.  They could build an airplane optimized for much greater passenger comfort without sacrificing fuel burn. 

Here's the idea: make the cabin bigger.  WAY bigger.  This is the number one complaint of passengers.

To be clear, I’m suggesting scaling up the cabin dimensions while keeping the cargo weight capacity the same. Obviously if you increase the weight the plane can carry, it’s going to burn proportionally more fuel and not much has changed.

As the cabin gets bigger, then parasitic drag from the fuselage gets larger. This can be fixed by flying the plane at lower dynamic pressure (indicated airspeed). If the surface area is doubled, then flying at half the dynamic pressure will keep drag about the same.

But we don’t want to fly slower, so instead we fly higher. For instance, assuming surface area doubled and dynamic pressure halved, we’d have to fly at 52,000 feet (like the Concorde) instead of 38,000 feet. The speed of sound does not change between these two altitudes, so there shouldn’t be any problem with Mach buffet.

The lower dynamic pressure will require the wing to fly at either higher coefficient of lift or with more wing area, and it will require more wing span. This does not imply more drag from the wing! To a first approximation the L/D will stay the same so the wing drag stays the same.

A bigger cabin would have huge benefits for passenger comfort, so why not do it?

The answer is safety in the case of a blowout. At 43,000 feet, someone breathing pure oxygen (from a simple mask) is getting the same partial pressure of oxygen as someone breathing normal air at 7400 feet. That’s about the same as the pressure altitude of the cabin at cruise, and anyone safe to fly a plane normally should be okay.

But at 52,000 feet, someone breathing pure oxygen would be getting the same PPO2 as someone breathing normal air at 18,000 feet. I’ve done that, and I started hallucinating at 17,000 feet and was pretty woozy at 19,340 feet. It wasn’t terrible for me, but I was in my 20s at the time and running 20+ miles a week. For someone with compromised breathing ability, this exposure could kill them.

All commercial planes are able to maintain internal pressure above outside pressure, even when a couple of windows are blown out, by pumping huge amounts of air into the cabin. To ensure it could hold an even bigger pressure differential (so that it could cruise at 55,000 to 60,000 feet), the Concorde had tiny windows.

So the safety case has been closed in the past. They could make a plane that flew higher, with a bigger cabin. They might even be able to do it with large windows by fitting larger fresh air pumps.

The plane could have a simpler wing, by landing at speeds similar to todays planes. The smaller ratio of cruise to landing speed would require fewer enhanced lift devices on the wing. In fact, for the ratio suggested above (cruising at half normal dynamic pressure), it should be possible to have a wing with no flaps at all. That will save a significant amount of weight and cost, and make it possible to load more fuel.

I think it would be a radical and welcome innovation for long distance flying.

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