June 19th, 2009

planet

flying backwards

this entry will have the exciting answer to yesterday's trivia question about airplanes flying backwards.

Is it possible for an airplane to fly backwards? By that I mean, let's say you're standing outside your house, look up at the sky, and there's an airplane that looks like it's passing over you tail first.
Yes, quite possible... mostly because the airplane is moving through the air, but you are standing on the ground. But let's continue...

If it's even possible, what does it mean?
It means the plane is going against one hell of a headwind. There's so much drag it's being pushed backwards, but wind over the wings also means you've still got plenty of lift. The magic word here is groundspeed... you have a gage that tells you what your airspeed is, and it's always going to be at least 60 knots or so in a little airplane. Your groundspeed is that, plus whatever the wind is doing. So if the wind is 60 knots against you (negative), you're just going to hover like a helicopter. 70 knots, and you're going backwards... relative to the ground. Relative to the wind you're pushing right through it like always.

What types of airplanes could do this?
In theory, any airplane could do this, but it's more likely in a small plane or glider with a low stall speed (meaning that they're capable of staying up even while flying slower). A little single engine can fly quite nicely at 55 knots or so... and it's not uncommon for winds at altitude to get to that. A Boeing that stalls at twice that speed is probably less likely to find winds fast enough. Winds do get faster at higher altitudes, but that almost cancels out my question because it's tough to notice what things are doing 39,000 feet above your house.

How long could an airplane sustain this configuration?
Cookie goes to metawidget for "4) as long as the wind, their fuel and their patience last". Since there's just as much air going over the wings, the airplane is just as stable as it always is. When you think about it, the plane really has no way of knowing the difference between lift from weather-related wind and lift from "we're moving through the air" wind.

What do you think's going through the pilot's mind?
crisco747 guessed that I was thinking "oh, shit..." but since I'm always thinking that it's not really a fair answer, I'm not giving him a cookie. Instead I'll give it to rynhollis who cheated and ran off experience, but yeah, we basically think it's a cool fun trick. Until we try to get somewhere, that is... then we're thinking, "I wish I had a more powerful airplane."

Technically an airplane can't fly backwards because you can't reverse the direction of the propeller. A propeller, just so you know, is like 2 or more little wings... all airfoils, and when the prop spins air moves across each blade and each blade gets lift, but since they're tilted forward instead of upward like a wing, you're pulled forward. If you could reverse the propeller I think the whole thing would just fall out of the sky because the airplane body just isn't designed to go that way... for one thing, the body itself would be blocking air to the propeller. But I'm open to other interpretations of this.

Anyway to make a short story long (and take up two lj entries) I got to do this the other day practicing slow flight in a 55 knot headwind. The "you can go this many miles on your remaining fuel" display sort of went nuts, because it wasy saying we couldn't go ANYWHERE which was true. Other than that, everything was normal, the plane flew just fine, my mental state wasn't any more heightened than it normally is. My instructor was more excited about it than I was because frankly everything right now is a big deal to me, so he has to be very specific about which things are also cool to non-new pilots.

If I were to give out cookies to everyone who answered some or all my questions correctly just about everyone would get one. There are of course other types of airplanes that are meant to fly backwards, this is just the only experience I've had. So... points.