How do planes fly upside-down?

Question

I understand that the wing of the plane creates downforce, lifting the plane into the air, so how do planes fly upside-down? Wouldn’t the wings start pushing the plane down into the ground once flipped?

In: 373

Answer 1

There are three guys keeping an aircraft in the air (and two of them rides on the back on the first one):

* Newton. The wing has an angle of attack. Basically, it’s tilted, so it pushes the air downwards. Newton has the law of action and reaction. Air is pushed down, so the wing is pushed up. You can look at simple model aircraft, which has a flat wing profile, for a clean example of this.

* Bernoulli. The wing has a profile which is longer on the top side, which means the same amount of air needs to be spread over a larger area, so lower pressure. Air is sucked down. In pops Newton and says “Something is going down, something must go up!” and that’s the wing going up.

* Coanda. The wing has a curved profile. A flow tends to follow a curved surface (try the back side of a spoon under a water faucet). This means that the air will follow the top side of the wing downwards. Once again, Newton stomps in and proclaims “Air is going down, wing must go up!”.

All these effects work together, and depending on what you are doing (speed, attitude, height, wingload and so on), there will be more or less of each of them.

So, enough basics, back to flying up side down:

Bernoulli isn’t much use here, but if the plane lifts its nose a bit more (relative the horizon), the wing will have a higher angle of attack. In this situation, Newton and Coanda steps in and saves the day, because their principles will still push air downwards, and thus moving the wing upwards.

Bernoulli isn’t completely out, though. When you change the angle of attack like this, the leading edge point of the wing moves a little bit (hard to put in words, but if you look at a wing profile and tilt it, you’ll see it), so that it cuts the air stream a bit more evently. It doesn’t really make much lift, but it helps reduce the downforce the Bernoulli effect would otherwise create.

Some aerobatic planes have a symmetric wing profile, and, of course, that works equally well in either orientation. It isn’t as effective, but they aren’t built for efficiency, they are build for agility in all orientations.