Atmospheric flight model 2.0

Battlescape
42

It all depends on how much each one changes.

For subsonic it’s 1/2 * density*Velocity^2 * Cd * Area.

So for example, if you halve the Cd but double the area, you’d get no net gain.

For supersonic it’s alot more involved.

I was once told a simple answer for the question: “How do planes fly?”.
Answer: “The plane pushed down on the air, and the air pushes up on the plane” i.e Newton’s second law.

Picture that and I think you should get a basic understanding of how it should work without falling down the rabbit hole of complicated math that’s required to get precise numbers to make the plane perform as desired.

This mutual pushing comes in the form of pressure differences over an airfoil. Faster flowing air will have a lower pressure (not counting shockwaves).

If you picture the air moving across a symmetrical airfoil, particles will divide and go both above and below the airfoil. Since it’s symmetrical, the distance from leading to trailing edge is same on both the upper and lower surface. Therefore, the velocity of the particles should be roughly the same, and so, collectively, will the pressure they generate. This means that to generate lift with a symmetrical airfoil, you have to increase it’s angle of attack to the freestream. Symmetrical airfoils tend to be used for high speed aircraft due to reduced drag. In other words, the angle of attack affecting lift is because of the airfoil selection.

For a cambered airfoil, the distances are different. In order for the particles to meet at the trailing edge at the same time, one particle has to travel faster. So if the particle traveling along the upper surface travels a longer distance, its speed will increase and the collective pressure across the surface will decrease. Higher pressure on the bottom pushes harder than lower pressure on top and the net result is lift upwards.

Under certain conditions, the particles don’t meet up at the trailing edge at the same time. This likely results in flow separation (i.e. the air is no longer hugging the surface of your airfoil). Bad things happen if this occurs over your control surface because there is no longer any air for it to push against! You basically just created a vacuum.

These are all generally 2 dimensional considerations to simplify and break up the problem at hand.

TL;DR: The drag may or may not be more, it depends on the actual numbers. Blah blah, Lots of basic aerodynamics explanations that have been done to death already, blah blah. Lift at hypersonic speeds having more to do with angle of attack is actually because of the airfoils used.

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43

Isn’t it more apt to say the air pulls up on the wings?

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44

If you want to be that semantic(pedantic? I’m actually not sure what the correct word is here) you should go farther and just say that the pressure differential creates a net force in the direction from the higher pressure area to the lower pressure area. This direction happens to be upwards with respect to the wings due to their camber.

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45

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46

Yes well done magic.I thought so too when Harry Potter looked.

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47

No. There is nothing pulling in this case. It is, as Skyentist points out, the difference in forces pushing on the wing from all directions.

We are on a planet that has an atmosphere. At all times we are surrounded by it. It is a gas that, under normal conditions, obey’s the ideal gas law, Pressure*Volume = moles of substance *Universal gas constant * Temperature (PV=nRT ); can also be written as Pressure = density * specific gas constant * Temperature (P=rho Rsp T).

It is applying pressure to your body from all sides at approximately 1 atmosphere pressure (101325 Pascals aka N/m^2) (changes with altitude, e.g. density).

Since it effectively applies the same force to you from all sides, you don’t move because the forces cancel out. When they’re unbalanced, then you might move if the force is strong enough.

Dynamic pressure is the pressure of a fluid in motion and equals 1/2 * density * velocity^2 (1/2 rho V^2) for incompressible flow (Look familiar?). Gas behaves like a fluid if it is sufficiently dense, like our atmosphere.

Another definition for dynamic pressure is the kinetic energy per unit volume. Energy has to be conserved, and in cases where the velocity of the fluid increases, the fluid will cool down its temperature (energy was converted to motion), and by the ideal gas law, if the temperature goes down, so too does the pressure. Thus you will get a local pressure different from the rest of the atmosphere.

TL;DR The atmosphere squeezes things in it. But if you move really fast, it squeezes you less.

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Its mostly about plane design but its fairly relevant.


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50

I haven’t found any really good mechanical explanations of exactly why increased perpendicular velocity of the fluid produces less pressure on a surface.

So in spending time imagining how to produce lift in a simulation without explicitly coding for it, I emerged upon the following visualization. Note: this could be completely, utterly inaccurate.

A gas is comprised of individual particles moving at various velocities. In a convectively stationary clump of air with some thermal energy, the net average particle movement is zero, but zoom in close enough and you see that individual particles are moving in many different directions at varying speeds (the average speed being generally proportional to the thermal energy of the system).

Now introduce an object with surfaces. Particles randomly bounce off a surface, imparting some of their kinetic energy. En mass this collision force is experienced as a pressure, and when uniform along all surfaces of the object, zero net force occurs.

Now, move the volume of air perpendicular to the normal vector for a surface. Careful consideration will yield the insight that the average velocity vector of individual particles relative to the surface will become less normal to that surface as a whole. As a result, the amount of kinetic energy imparted to the surface during molecular collisions will be reduced, as the particles increasingly ‘glance’ off the surface vice striking it head-on.

Thus, felt ‘pressure’ decreases as the group velocity of air increases perpendicular to the surface. From this follows the standard differential pressure explanations for lift.

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Essentially yes:

If you increase the velocity of gas as a whole without adding or subtracting energy from the system, then you have to convert that energy from something else, in this case, temperature. By the universal gas law, if you decrease temperature, you also have to decrease pressure (or volume). A lot of that random motion of particles got converted to a uniform kinetic direction, and thus fewer particle collisions within the gas or its environment.

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This explanation is fine if you move the volume of gas as i suggested. But if the gas is static and the wing moves through it, then it does not sufficiently explain the reduction of felt pressure (other than by circularly referencing the gas laws it is trying to explain).

I admit my search was not rigorous, mostly limited to aerodynamics textbooks I used in college. But nowhere have I read the statement that the pressure is reduced on a surface because molecular collision vectors are made less perpendicular.

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53

The reference frame doesn’t matter.

The kinetic theory is on the molecular level and describes how pressure comes about. Most explanations of fluid dynamics stick to the macroscopic effect that is created, wherein pressure is a property of the gas.

Check out Bernoulli’s principle, it’s essentially what I’ve been saying (conservation of energy):

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