Question about Newton third law and NASA article

[late347]

I was confused by a NASA pdf document about flight physics. I provide quote from NASA document instructor's flight manual, flight testing Newton's laws. Document is categorized DFRC-X41-1.

"Thrust is just one of four primary forces which act upon aircraft in flight. The plane can't violate the third (Newton) law, therefore thrust must be opposed by equal and opposite force. This second force is called drag."

Did NASA use some fast-and-loose language right there? Imprecise definition of drag using Newton third law?

When thrust is more than drag, then acceleration occurs. Correct or not? When thrust and drag are equal, then aircraft remains at constant speed, unchancing speed.

Drag exists because aircraft pushes against airmass and airmass pushes against aircraft. Correct or not?

No drag exists in outer space. Rocket in vacuum has thrust, surely rocket can accelerate in outer space. Correct or not?

In outer space vacuum, rocket has thrust. Newton third law states that fuel gases push upon rocket body. Rocket body push upon fuel gases. Basically. No mention of drag and supposed relationship between thrust exists in this case of outer space vacuum rocket.

In practical terms outer space vacuum contains hydrogen atoms (?) so it's not a pure vacuum. But surely there must be some sensible reason behind this NASA logic? Can you guys help me understand Newton third law with respect to the normal aircraft in atmosphere, and also vacuum rocket with thrust.I was also wondering about another unrelated matter about flight physics, related to the compressibility phenomenon with certain WW2 aircraft. One such aircraft was P-38 Lightning. I was wondering what exactly prompted the flight surfaces to become so ineffective with this aircraft for example. The true cause was not understood during WW2, except that they installed a stopgap measure of an airbrake, to slow down a diving fast aircraft.

I remember watching a history channel show about the compressibility phenomenon of a P-38 aircraft. The documentary noted that the propeller blades became transsonic, which caused some kind of disturbant airflow to the flight surfaces. (such as elevator tabs in the tail)

Could the compressibility be explained in somehow more layman's terms? Why it happens at certain speed, as opposed to slower speeds that normally aircraft fly at ?
Compressibility tended to happen at high speed dives, during air combat. This made it difficult for a P-38 pilot to pull enough elevator to pull up from a dive. This tended to cause pilot deaths to crashes.

Soemtimes the thicker atmosphere at low altitudes allowed regaining of control for elevator tabs.

 

[rcgldr]

The Newton third law pairs aren't quite right. Assuming a propeller, then the propeller exerts thrust on the air, and the air exerts and equal and opposing force onto the propeller. The wings exert a downwards and somewhat forwards force on the air, and the air exerts and equal and opposing force onto the wings.

The "balance in forces" describes an aircraft in horizontal flight at a steady speed. Thrust doesn't have to equal drag, and lift doesn't have to equal weight. If thrust is greater than drag, the aircraft accelerates forwards. If lift is greater than weight, then the aircraft centripetally accelerates upwards relative to it's own frame of reference (such as a banked turn).

As for a rocket in space, assume a frame of reference where the rocket is initially at rest and that there are no external force (such as gravity). The center of mass of the rocket and it's fuel never moves with respect to that frame of reference. When the engine is operating, the pumps and engine exert a force onto the burning fuel mixture, and the fuel mixture exerts a force onto the engine and pumps. The fuel accelerates in one direction, and the rocket accelerates in the other direction. The fuel effectively has a terminal velocity relative to the rocket, and this velocity is used in equations for determining how the rocket will move. If the initial mass of rocket and fuel is nearly all fuel, then as the fuel is nearly depleted, the rockets velocity relative to the original frame of reference can exceed the fuel's exit velocity relative to the rocket.

P-38 - stability loss when approaching mach 1

Two issues. One is that the control surfaces cease to function normally, the other is mach tuck. Wiki article:

http://en.wikipedia.org/wiki/Mach_tuck

 

[late347]

The Newton third law pairs aren't quite right. Assuming a propeller, then the propeller exerts thrust on the air, and the air exerts and equal and opposing force onto the propeller. The wings exert a downwards and somewhat forwards force on the air, and the air exerts and equal and opposing force onto the wings.

The "balance in forces" describes an aircraft in horizontal flight at a steady speed. Thrust doesn't have to equal drag, and lift doesn't have to equal weight. If thrust is greate than drag, the aircraft accelerates forwards. If lift is greater than weight, then the aircraft centripetally accelerates upwards relative to it's own frame of reference (such as a banked turn).

So NASA was wrong and I was right according to the quotation?

Aircraft suffers drag upon its body. Drag increases as the speed increases.

Drag seems to be based on surface area of the body somehow.

Isnt that the reason behind the principle of an opened and effective parachute?

When the parachute is packed inside a bag, surface area seems small.

When the parachute surface extends open drag increases quite a bit...?

I suck at physics but it seems that sometimea even NASA makes mistakes too :D

 

[A.T.]

So NASA was wrong and I was right according to the quotation?

NASA has a well known problem with explaining Newtons 3rd Law correctly. See for example:
http://www.grc.nasa.gov/WWW/k-12/WindTunnel/Activities/third_law_motion.html

The book lying on the table is exerting a downward force on the table, while the table is exerting an upward reaction force on the book. Because the forces are equal and opposite, the book remains at rest.

 

[sardar]

I can provide some clarification on the topics you have brought up regarding Newton's third law and the compressibility phenomenon in flight physics.

Firstly, let's address the quote from the NASA document. The language used may seem imprecise, but it is important to understand that this document is likely intended for a general audience and not specifically for scientists or experts in the field. The main point being made is that thrust and drag are equal and opposite forces, and this is in accordance with Newton's third law. Drag is defined as the force acting in the opposite direction of motion and it is caused by the resistance of the air on the aircraft as it moves through it. So, while thrust is the force moving the aircraft forward, drag is the force opposing this motion.

In regards to your question about thrust and drag and their relationship to acceleration, your understanding is correct. When thrust is greater than drag, the aircraft will accelerate. When they are equal, the aircraft will maintain a constant speed. And when drag is greater than thrust, the aircraft will decelerate.

You are also correct in stating that there is no drag in outer space. This is because there is no air resistance in the vacuum of space. However, thrust can still accelerate a rocket in outer space because there is no opposing force acting on it.

Now, let's discuss the compressibility phenomenon. This occurs when an aircraft reaches a certain speed, known as the critical Mach number, where the airflow over the wings becomes supersonic. This can cause a shock wave to form, which can disrupt the airflow over the wings and decrease the effectiveness of the control surfaces, such as the elevator tabs. This can make it difficult for the pilot to control the aircraft, especially during high-speed dives.

The reason this happens at certain speeds is due to the properties of air at high speeds. As air flows over the wings, it can reach the speed of sound, causing the shock wave to form. This is why it tends to happen during high-speed dives, as the aircraft is accelerating and reaching these critical speeds.

I hope this helps to clarify some of your questions about Newton's third law and the compressibility phenomenon in flight physics. It is important to note that these are complex topics and can often be difficult to explain in layman's terms. However, I hope that my response has provided some understanding for you.