Do supersonic inatakes contravene Newton's laws?N

[Andybart]

There are a number of threads in the Tech Log dealing with the subject of supersonic air intakes. Some of the post in these threads provide excellent explanations of how they produce thrust. But none of these threads deal with the apparent conflict between the creation of this thrust and Newton's laws.

In supersonic flight the intakes decelerate the air from supersonic speed to subsonic speed, and in doing this they convert kinetic energy into static pressure energy. This static pressure then acts on the internal surfaces of the intake to produce thrust.

But according to Newton the force that the intake exerts on the incoming air to decelerate it, should result in an equal and opposite force exerted on the intake by the air. This reaction force will produce drag.

Please understand that the purpose of my post is not to deny the fact that these intakes produce thrust. I am quite convinced that they do. But I would like to get to the bottom of this apparent contradiction between intake thrust and Newtons laws.

[ECAM_Actions]

It's rather simple - it's another exception to Newtons Laws (of which the list grows daily).

Basically, we need a new theory. Newton is good for teaching kids the basics, but is no good for serious research anymore (though it has its uses).

I suggest Einstein has a better idea: conservation of energy. You need to analyze HOW the airflow is slowed in the intake. To simply state that it should create drag is wrong. Drag should be considered a total force, and not a force acting in its own right. Drag is what you get as a result of other forces. Take the other forces away, and drag vanishes.

[Mr Optimistic]

not to mention the law of monotonic increase of nonsense.

[roulishollandais]

Drag and Lift are the TWO projections of the same ONE Force on TWO orthogonal axis from a TWO dimension space (plane).

What is important is not the n-Vector mathematical representation or artefacts -projections on less than 3 or 4 dimensions that human people are able to imagine easily - but the n degrees of freedom of the Vector in the physical world.:p.

[Wizofoz]

I suggest Einstein has a better idea: conservation of energy.

Einstein came up with conservation of energy?

That must have come as a surprise to the physicists who were using it as a principal law of physics in the mid 1800s.....

There is no contradiction to Newtons laws. Yes the decellerated air produces drag- but the re-accelerated air from the eflux produces a greater gain than the loss from the drag, so the net effect is thrust.

[Torque Tonight]

Andybart, to get half an understanding of this sort of thing you need to be taking into account mass flow rates, temperature changes, density, entropy, enthalpy and all sorts of horrible stuff. Thermodynamics is a b1tch of a subject, and I dumped most of it from my brain immediately after my university finals. Without a solid grip on this subject you have litte chance of understanding what!s going on.

[rudderrudderrat]

Hi Andybart,

Wizofoz is correct.
There is a really good explanation somewhere in the Concorde thread (by M2Dude).
Basically if the engine was not "vacuum cleaning" the excess pressure from the rear of the intake duct, then there would be no net thrust from the intake - only drag.
Edit: see
http://www.pprune.org/tech-log/426900-concorde-engine-intake-thrust.html
Post 5

[MathFox]

A very simple explanation is that part of the force needed to compress the intake air is provided by the frontal surface of the next engine stage and deducted from the thrust that part of the engine provides. From an aerodynamic point of view it would make sense to assign that drag component to the nozzle, I'm sure that the calculation then shows a net drag caused by the nozzle.

[MG23]

Newtonian mechanics is perfectly adequate for almost any practical scenario. For example, it's perfectly good enough for calculating the paths of spacecraft through the solar system, and indeed that's how JPL do it.

That's not entirely true. Pure Newtonian physics doesn't work for Mercury's orbit, which is one of the observational oddities which lead to the theory of relativity.

[rudderrudderrat]

Hi MG23,
Pure Newtonian physics doesn't work for Mercury's orbit
http://www.donaldgburkhard.com/pdfs/dgbch2.pdf
Newtonian physics is accurate to within 43 seconds of Arc per Century.
Not bad for a very simple 1687 theory.

[awblain]

That's a great question, and it's not a relativity-vs-classical mechanics issue, it's a what-happens-to-the-air issue.

The problem is stated as:

The momentum of the intake air is reduced as it proceeds through the engine - thus the engine pushes the air forward (Newton II), so the air pushes the engine back (Newton III).

Right?

The momentum of the air does go down in the frame of the engine, as the mass flux is fixed, and the speed is falling.

However, the escape route is considering the total force acting on the air in a thin slice through the engine via the increasing pressure as it travels towards the compressor. It gets pushed backwards by the air in front, pushed forwards by the air behind, and pushed normal to the walls by the walls of the intake. The net force on the air is indeed forwards, as it's slowing (Newton II). However, the force on the air due to the walls of the intake is rearwards, and so the force on the intake due to the air is forwards. Newton is satisfied from the mix of pressure forces in the airflow, and the intake still produces thrust due to the action of the increasing pressure on the walls.

[MG23]

Newtonian physics is accurate to within 43 seconds of Arc per Century.
Not bad for a very simple 1687 theory.

Sure, but that was a measurable difference that was impossible to explain with Newtonian physics. Everyone knew something odd had to be going on.

[Andybart]

Awblain

Thank you for your reply. It makes perfect sense to me now. I was making the mistake of thinking that the intake structure was decelerating the air, so I expected an equal and opposite force to act on the intake.

While attempting to research this subject I found some interesting material in the Rolls Royce book "The Jet Engine" The book explains a method whereby the thrust of an engine can be calculated by calculating the conditions in each section using the formula:

Thrust = (Pressure x Exit Area) + (Air Mass Flow x Exit Velocity)

The nett thrust generated by each section is then calculated by taking the result of the above calculation and subtracting from it the result for the preceding section. The overall thrust of the engine is the sum of the individual thrusts for each section.

This process reveals the fact that with the exception of the compressor section, positive thrust is exerted only in the divergent sections where pressure increases. But these are also the sections in which the flow velocity decreases. So although thrust is created by accelerating the air rearwards, it acts upon the engine only in the sections where flow velocity decreases.

In the convergent sections, other than the compressor, the flow velocity increases, but the net force exerted on the engine is drag. So the convergent propelling nozzle for example, experiences a drag force.

The compressor is the only convergent section in which thrust is exerted on the engine structure. This may be due to the fact that both pressure and velocity increase in the compressor.











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[awblain]

The force acting on a sectional slice of air should indeed be the change in pressure multiplied by area d(p A), plus a change in momentum term d(rho A v-squared). Integrate through the engine and you should get the appropriate answer.

Each compressor and turbine stage and the combustor modify the pressure and density, and overall Newton should be satisfied by a thrust or drag on the shaft of each of the rotating parts reacting to the airflow. The thermodynamics comes in from changing pressure, density and speed of the flow.

The clearest case of this is a large fan, from which about 100,000lb of thrust appears at its bearings due to pumping the bypass air around the engine core in a single stage.