One the subject of airplanes, a wacky idea I had about twenty years ago was designing aircraft cockpits to include periscopes.
A dumb idea? Not necessarily.
See, one of the problems with large aircraft is taxi-ing. Aircraft are designed to fly. They aren't really designed to be driven. So if you're in the cockpit of a 747, and you want to park it somewhere, you can't really see what you're doing. You're high up, your visibility sucks, and you have this nose-cone thing sticking out in front of you, guaranteeing that you have no chance of seeing what's just in front of your wheels. Consequently, expensive aircraft get trashed from time to time while they're still on the ground, while somebody is simply trying to find somewhere to park them.
What you need in this situation is a periscope – not to look up, but to see how things look from beneath the aircraft. You want to be able to pull a lever, and have a chunk of optics pop out of the bottom of the aircraft giving you a 180-degree or 270-degree view, relayed up to the cabin and perhaps projected onto a curved mirrored trim just below the windscreen. Worried about debris on the runway? Check the periscope. Trying to park? Check the periscope. Coming in to land, and not sure if your landing gear is deployed? Check the periscope. Unsure if one of your engines has just flamed out? ... you get the idea.
Consider the case of Concorde. One of the most difficult engineering tasks in designing Concorde was supposedly the design of the nose. Concorde has a looong nose, and it needs to be pointy and smoothly tapered for efficient supersonic flight. But when Concorde comes in to land, it glides in at an angle with its nose in the air, and the pilots can't see where they're landing. To get around this the engineers developed a "droop snoot" for the plane – an entire nose section that could swivel to point downwards when the plane landed, to give the pilots a fighting chance of seeing what they were doing. This was a difficult bit of engineering, with double windshields and so on.
Wouldn't a pop-down periscope system have been simpler?
Okay, so nowadays the idea's probably becoming a bit redundant. With recent aircraft, with their instrumentation displayed on LCD panels, it's probably easier to embed reliable cameras into key positions in the airframe and allow the copilot to switch one of their screens to camera view. The data networks are probably already in place, and the instrumentation panels are now flexible and modular. We're probably approaching the point where a pilot will trust a set of cameras more than a set of odd additional direct optics.
But for the last few decades, large planes probably really should have all had periscopes.
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