Matt, a student pilot in Florida, writes: I’m studying lift as part of my pilot training. I can get my head around Bernoulli and Newton, and how that works with the airfoil, but none of my instructors can explain how a simple paper airplane — having no airfoil — generates lift to fly.“
Well, in their defense, your instructors probably don’t have the rare, difficult to obtain paper airplane rating on their pilot certificates. Luckily for you, I actually do.
But seriously, this one confuses a lot of folks, and I’m happy to help. But before we dig into it, let me back-taxi a bit and make sure we didn’t leave anyone at the FBO.
Aerodynamic lift, as currently taught to pilots, comes in two flavors: Bernoullian lift and Newtonian lift. Those labels don’t come from the FAA, by the way. Instead, they come straight from your Uncle Will’s flight bag of teaching tricks, but it’s still a good way of keeping them straight in your head.
Bernoullian lift is created by the airfoil-shape of the wing. Due to that shape — and other complex reasons that we are not getting into today — air moves faster over the top of the wing than it does along the bottom of the wing. When air, or any fluid for that matter, speeds up, its pressure drops. This gives us a comparatively low(er) pressure zone atop the wing, and a comparatively high(er) pressure zone below the wing.
As Mother Nature finds pressure differences distasteful, and tries her level best to equalize them, the high pressure air flows toward the low pressure. Or, if you prefer, the low pressure air sucks in the higher pressure air. Your choice. It’s just opposing semantics for the same phenomena.
Of course, the wing is in the way. It’s blocking the path of the high-to-low flow. But while the presence of the wing prevents the two zones from equalizing, it doesn’t stop them from trying to merge, and this tension between the two zones is lift and one of the primary forms of lift that support the wing in flight.
Unscientifically stated: The high pressure below is pushing the wing upward while the lower pressure air above is simultaneously sucking the wing upward so that it “floats.”
Bernoullian lift is named for Daniel Bernoulli. The Bernoullis were a family of 17th and 18th Century Swiss mathematicians and if you think you have a bad relationship with your father, you should read up on the relationship Daniel had with his father Johann. Anyway, the younger Bernoulli is famous for mathematically describing that whole fluid speed and pressure relationship, which is now called Bernoulli’s Principle.
Moving on to Newtonian lift, which is named for Sir Issac Newton, it’s created by angling an airplane’s wing to leverage his Third Law, which is often boiled down to “for every action, there is an equal and opposite reaction.”
The wing is installed so that its leading edge is angled slightly upward so that moving air smacks into the bottom of the wing, deflecting it downward, in turn creating a Newton’s Third opposing upward force that lifts the wing. Pretty clever. By the way, this wing angle is called the angle of incidence.
But with no airfoil for Bernoullian lift and no angle of incidence for Newtonian lift, how does a paper airplane generate lift?
It may surprise you to learn that our foldable flyer still uses the same two principles.
Here’s how that works: A paper airplane is a form of glider. It gets its forward motion by cashing in altitude for “thrust.” So while some designs boast impressive range, they are still falling out of the sky in flight.
As a paper airplane falls, that flat wing compresses the air it’s falling on. This makes the air below the wing more dense than the air above the wing. Dense air is higher pressure than less dense air, so we still have Bernoullian lift at play. And, as the paper plane falls onto this column of self-compressed air, it pushes down on it, triggering a Newtownian opposite upward reaction.
Unlike with powered flight, the paper airplane isn’t able to generate enough Bernoullian and Newtonian lift to remain aloft, but it does generate enough lift to dramatically reduce the speed of its “fall” to earth.
Don’t believe me? Simultaneously launch a paper airplane and drop a tennis ball. The tennis ball also generates lift, but it’s not as good at it as a paper airplane, so with substantially less lift to off-set weight, it will drop much more quickly.
So there you have it — it’s the same friendly forces that you had your head wrapped around with the airfoil, just deployed slightly differently.
