Air and Its Majestic Role in Producing Lift
By the late Paul Lipps, for Experimenter
Paul Lipps was an extraordinary thinker with an innocent sense of humor. Known most for his work in the advancement of propeller efficiency, Paul’s ability to see complicated things in simple terms had him question conventional wisdom in all aspects of his life, especially those used in aviation, a passion of his since his first flight in a Cub when he was just a teen. The following article was written in his usual tongue-in-cheek style that hopefully you’ll find as fun to read as he intended. The topic is a bit controversial, but no matter your opinion, it will get you thinking about what’s going on with your wing.
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Air Is Busy
The sea of air surrounding us consists of a mixture of molecules which have well-known proportions of oxygen, nitrogen, and various other elements, in free form and compounds. These molecules are in a highly agitated state of rushing around and banging into one another and whatever gets in their way. The average or mean-free velocity of these molecules is about Mach 1.46; yes, on average, they’re going faster than the speed of sound in all directions.
Often attributed to air are the concepts of pressure and temperature. Actually, these aren’t intrinsic attributes of air, but only to the effects of their collision with an object that happens to be in their way. Whenever a molecule of air strikes a surface, it does two things. It gives it a little push, and it gives it more speed or agitation. The push is what we call pressure, and the speed is what we call temperature. All molecules in a solid are vibrating with a certain speed, which is its temperature in what we term Brownian motion. If we heat up the solid, its molecules go faster and faster and eventually they get going so fast that they no longer stick together, either turning into a liquid or gas.
So if the outside air temperature (OAT) probe on your airplane is sticking out into the free stream of air flowing past your airplane in flight, it gets pushed back, which is drag, and its temperature goes up relative to the actual temperature of the air outside the plane. When you were putting around in your Cub or Cessna 152 at their slow speeds, it really didn’t matter much. But since the heating is a function of speed squared, as you go faster the error in your OAT goes up 2 percent for each 1 percent increase in true airspeed. You guys in your 200 mph to 300 mph planes are reporting erroneous temperatures when you give a pilot report to flight watch if you don’t compensate for this temperature rise from speed. Typically your probe will have about 70 to 80 percent recovery of the stagnation heating for your true airspeed, which is 7.2 degrees at 200 mph, 11.3 degrees at 250 mph, and 16.3 degrees at 300 mph!
But I digress. If I place a large, deflated balloon on the ground, place an object on it that’s 1 foot x 1 foot and weighs 1 pound, and then pump air into the balloon until it lifts the object above the ground, the air pressure inside the balloon will have to be at least 1 pound per square foot (psf). Are we on the same page with this?
In my bedroom I have a Select Comfort mattress resting on a platform on top of a structure with drawers in it. This mattress has a hose which connects to a pump below the bed and a solenoid-controlled relief valve. By toggling a switch in my headboard I can turn the pump on to inflate the mattress, or I can activate the valve to deflate it. When the air in it reaches a pressure that is my weight divided by the number of square inches of my body being supported on the mattress, the mattress will hold my body above the platform. In my garage I have a car which weighs about 2,400 pounds with approximately 600 pounds on each of the four tires.
What’s the Point?
In each of these cases it is the air in these devices that is supporting the load. Agree? With each of them the support contact patch has an area which is the weight divided by the air pressure. If the balloon in the first example is restrained from bulging out to the sides, as more air is pumped into it increasing the pressure, the amount of area that the balloon surface makes with the object will decrease.
It’s important to understand that any force acts in two places; this is usually referred to as action and reaction. You can’t have one without the other! When a weight lifter holds a weight above his head, its weight passes down through his body to the floor below him. We often forget that we’re constantly fighting the pull of gravity by our feet pressing down on the earth. Astronauts are well aware of this when they try to rotate a bolt with a wrench when in zero-gravity space. They must anchor themselves to take the reaction to their action. We on earth have gravity anchoring us.
On my mattress, when the pressure first starts to rise, lifting me up, the surface of the mattress will extend around my sides, increasing the contact area and making the mattress very soft. As I keep increasing the pressure, less and less of the surface will be in contact with my body, making the mattress stiffer and firmer. With the tires on the car, if they’re radial tires with flexible sidewalls rather than bias-ply tires with stiff sidewalls, the contact patch area the tire makes with the ground will, again, be the weight on the tire divided by the air pressure. So with 600 pounds per tire, if the tire pressure is 30 pounds per square inch (psi), the area under the tire is 20 square inches, and at 40 psi it will be 15 square inches. In each of these cases it is the air within these objects that provides the support by pushing up on one surface and pushing down on the other: action-reaction.
Where Am I Going With This?
Recently I wrote the following in a thread on the Van’s Air Force e-mail list:
Here I am clipping along at just a couple of feet above 1,000 feet where the outside air pressure is 2,040 psf. Some instrumentation tells me that the average pressure on the bottom of my 63-series airfoil is 2,020 psf, while the average pressure on the top of this wing is 2,000 psf.
The wing, except for some ribs and spars, is basically hollow and is filled with air at the ambient pressure of 2,040 psf. Looking at a diagram of the pressure acting on the upper and lower wing surfaces, I see that there is 2,020 psf pushing up on the bottom surface and 2,000 psf pushing down on the upper surface.
But the air in the wing is pushing down on the bottom surface with a pressure differential of 20 psf, and at the same time it’s pushing up on the top surface with a pressure differential of 40 psf.
So, could it be said that the wing’s lift is really a result of the air in the wing pushing up on the top surface 20 psf harder than it is on the bottom? What say you? And please don’t invoke the idea that there is a force called suction! Air doesn’t suck—it only blows!
And yes, airfoils with curved lower surfaces have lower pressure than ambient in flight.
Well, I have to tell you, this little piece stirred up a lot of controversy. I had people intimate in a very nice way that basically I didn’t know the first thing about aerodynamics and mass flow and downwash and the summation of forces acting all over the airplane and the Bernoulli effect and what if we put 1,000 psf inside the wing and what about a solid wing and so on. I, in turn, wasn’t as politic in my responses since I felt that a lot of responses were condescending; I felt that the respondents were skating the issue and stated as much. There were even messages directed at the moderator that he should shut down any further discussion as I wasn’t acting politely as a guest. It was intimated that I was like the people who said that a rocket wouldn’t work in outer space because there wasn’t any air for it to push against. Didn’t I know that the air inside a hollow wing was pushing down on the bottom surface just as it was pushing up on the top, and so the effect was neutralized? In a way I can understand their upset at a concept which is seldom, if ever, addressed in aerodynamics. I place this at the feet of the abysmal teaching that I’ve seen in aerodynamics and electrical courses conducted at our colleges and universities.
As I pointed out in the previous three examples of the balloon, the air mattress, and the car tires, yes, the air is pushing out equally on all of the surfaces, and yet it’s the air within each of them that is supporting the weight. So, why then is this different when in an airplane in flight? Probably because in each of the examples there was a hard surface below which supported them. But is an airplane in flight, then, different? What is it that is supporting the plane?
Occasionally the question is posed about a fly inside an airliner that is flying along. When the fly is sitting on the leg of a person sitting in a seat, obviously the weight of the fly is being carried by the wings of the plane; that goes without saying! But what happens when the fly takes off and starts buzzing around inside the plane? Has it now become disconnected from the airliner so that it is, as it were, a free and independent entity within the cabin? Did the gross weight of the plane change by the amount of the weight of the fly? The answer is no!
When anything is in flight, be it an insect, bird, or plane, its weight is still borne up by the earth below it. We fail to see that the air which is pushing up on the bottom of the wings of these flying objects is also pushing down on the earth below. Their flight through the air hasn’t somehow become isolated from the ground but still exerts a force on it the same as if it is at rest. The support force for the plane, though it may be 20 psf at the wing, gets spread out over the earth below it so that when a jet flies over we don’t feel a momentary impulse. It is similar to the effect that if you push down on a wooden plank of 10 square feet, equally supported on the ground, with a 1-pound force in the middle of the plank on 1 square inch, the force on the ground will be 1 pound but spread out over the 10 square feet, giving only 1/10 psf, assuming no flex in the plank.
The argument that the air in the wing is pushing down on the bottom surface as well as up on the top surface so that as a result the force is equalized and has no net effect disregards that the lower surface is basically resting on the ground, just like the balloon, tire, and mattress.
Let’s look at the example of a fabric-covered wing, where there’s little or no contact between the fabric and the wing spar except through the wing ribs. All of the flight loads are carried by the fabric. These loads are transferred from the fabric through the ribs and then into the wing spar and from it into the fuselage. In flight, the wing’s bottom surface basically has a rigid structure extending from it to the ground by virtue of the air below it pressing up on it while at the same time it’s pushing down on the ground, just as the air inside the wing, at outside static air pressure, is pushing up on the upper surface and holding it up by pushing down against the lower surface. So the load on the upper surface fabric is transferred by means of reinforcing tape and rib-stitching to the rib which in turn transfers it to the spar. The ribs are in tension, not compression!
Now as far as those who asked, “Well, what about a solid wing?” the answer is that the air takes the place of a solid supporting structure. The solid and the air do exactly the same job; they provide a means to convey the load from one side to the other, again, as with the balloon, tire, and mattress. So when looked at in its simplest terms, the load is conveyed from the top surface through the medium, air or solid, through the lower surface, through the air below the lower surface, and to the earth below, forming a rigid structure.
So, in the final analysis, what’s the answer to the question initially posed on the forum? It’s a resounding yes!