Why cannot the airplane accelerate for ever in the sky?

[albertlee]

These 2 possibilities is what bi can come up:

 

1) According to Newton's first law, if the force of the airplance is equaled to the force of the air, it goes in a straight line.... which means the air force (I mean not the millitary stuff ) also increases as the the force of the airplance increases.... but how can the airplane accelerate in the bigginning...?

 

2) According to Newton's third law, when the force exirting on the air, the air exerts back, but therefore the plane cannot even move, since the plane also exerting backwards and the air exerting back.......

 

Anyway, ithink the reason for this is all about pressure of both (air + plane) and its transformation of force resulting the rate of air resistance........

 

Any way, back to my topic, please explain why..And tell me some specific ways of calculation to it

 

Thx

 

Albert

[blike]

I'm not sure what you're asking?

[Pinch Paxton]

Push your hand down on bath water slowly, and your hand easily moves through it. The faster you slam your hand through the water the more resistance you feel. I think it is like a concertina effect, the water needs time to push the next layer out of the way. Same with air I guess.

 

Pincho.

[Sayonara]

I think something along these lines is what you need to be thinking about.

 

The weight is the resultant of gravity on the plane's mass. Or something.

[ski_power]

Firstly, at low speeds the plane is able to accelerate because there is very less air resistance(How are you able to sprint?, Same type). But as Pincho said, the faster you go, more the resistance(Drag). Hence, you are able to accelerate to normal speeds of around 0.7-1.0 Mach. Also, the aerodynamics play an important factor over here. More streamlined is the design, lesser is the air-resistance, reater is the possibility to accelerate to higher speeds.

Also, the Newton laws thing, it's crazy. The air doesn't offer any reaction(atleast that's what I think, correct me if wrong), because you aren't applying any action on the air. You're pushing the plane forwards, not the air backwards.

[albertlee]

Also, the Newton laws thing, it's crazy. The air doesn't offer any reaction(atleast that's what I think, correct me if wrong), because you aren't applying any action on the air. You're pushing the plane forwards, not the air backwards.

From skipower

 

When u push the plane forwards, is'nt it also pushing air forwards? Then if so, does Newton's law still apply,here? if not, how and why?

 

On the other hand, back to my first message, Can anybody give some mathematical calculations towards this topic? because all i got is just concept....but to me, i think we always measure in Physics........

 

Will Apreciate for the responds of all my questions

 

Thx

[Skye]

Two seperate forces, each with their own 'equal and opposite reaction'.

 

1) Thrust. Force of the propeller on the air, pushing BACKWARDS, with an equal and opposite force FORWARDS.

 

2) Drag. Force of the whole plane travelling through the air, pushing FORWARDS, with an equal and opposite force BACKWARDS.

 

Thrust and drag aren't equal and opposite forces of each other, they are only related in that thrust accelerates the plane which increases it's velocity, which then means that it shoves more air out of the way leading to more drag.

[albertlee]

Thx for the reply froM skye, It is very useful.

 

On the other hand, can any one explain to Me More clearly about the difference of Thrust and Drag about their forces and relationship?

 

Say it More clearly, how are thrust and drag not equaled?

 

very apreciate for the responds

[Dave]

Like skye said, thrust is the force pushing the plane through the air, which comes from the propeller or jet engine on the plane. Drag is effectively the air resistance from the plane travelling through the air.

 

(this is quite a basic description)

[ski_power]

Yeah, but still we don't know how thrust and drag are equalled(if they actually are, or not)?

[albertlee]

I really wonder why drag force is less than the thrust? just as simple as that......

but it really takes a big time on thinking.........

 

Any help?

[Skye]

The thrust depends on the power of the engine, more or less under control of the pilot by controlling the throttle and thus the revs per minute. The drag depends on the speed of the plane. So if you have a plane that is stationary, then switch the engine on to a certain rpm, it will have a certain amount of thrust, and no drag. The plane will then accelerate, and it's drag will increase gradually, untill it reaches a speed at which drag equals thrust. It will then continue at a constant speed. If you increase the rpm some more, the thrust will again be higher than the drag, and the plane will accelerate till drag equals thrust again. If you reduce the rpm to it's original value, drag will now be greater than thrust and the plane will deaccelerate back to it's original speed.

[albertlee]

Sorry, still confusing.......

It is almost telling the laws in more useful in Physics.....

Can anybody tell me why not the thrust equalled to the drag? since according to the Newton's third law..

 

Any help?

[swansont]

Sorry' date=' still confusing.......

It is almost telling the laws in more useful in Physics.....

Can anybody tell me [b']why not the thrust equalled to the drag? [/b] since according to the Newton's third law..

 

Any help?

 

Drag isn't the reaction force to the thrust. Thrust action/reaction is the force the propeller exerts on the air passing through it, and the force the air exerts on the propeller. Drag action/reaction is the force the plane exerts on the air by moving through it, and the force the air exerts on the whole plane.

 

You can have thrust with no drag (e.g. plane bolted down with the prop running) and drag with no thrust (e.g. a glider).

[Guest kpcoder]

swansont is correct. Drag is not equal to thrust because these 2 forces are independant. You were mistaking Drag and Thrust as action/reaction which they are not. The correction action reaction pairs are as follows.

 

1) action: Engine Thrusts gas in a south direction

equal and opposite reaction: Jet is accelerated North.

 

2) Action: air pushed out of the way by the jet (drag)

equal and opposite reaction: Jet is accelerated South (meaning it slows down)

 

This is why Drag does not equal thrust, they are not an action/reaction pair, they are independant.

[albertlee]

Ok....So, can anybody tell me the equation of drag? with also some explanations?

 

Apreciate

[Guest kpcoder]

Drag = .5 * Coefficientdrag * densityair * Velocity^2 * Area

 

Coefficientdrag = coefficient of drag. This value accounts for the shape of the body, its orientation/inclination, air viscosity, and compressibility.

Click here for examples of drag coefficients for various shapes

[albertlee]

As a conclusion..........Why drag and thrust are independable? since the stronger the thrust, the stronger the drag........therefore i think they both apply Newton's third law in some ways..........

 

Any helps? or suggestions?

 

Apreciate

[YT2095]

when you think about it logicaly,it has to be more or less entirely about Air friction/resistance.

 

imagine that plane could fly in space, it`s a special type of plane with it`s own oxygen supply onboard (in otherwords a rocket).

this rocket can burn for months! (this is only hypothetical for the point), it has a thrust of say 10 newtons (only a small single thruster), the rocket will continue to accelerate until the fuel burns out, still using that 10 newton thrust, it doesn`t matter how much the mass of the craft it, it will still keep acceletating, faster and faster

 

a "Normal" planes engine in Air, set to a constant thrust, will reach a maximum speed then stay there?

 

so break the problem down. what is the difference between the 2 scenarios (factoring out the engine types as that`s only to present thrust) ?

 

maybe that will help elucidate the problem a little more clearly?

[Kedas]

I didn't read all of above but:

air resistance is determent by speed and shape of the airplain and surrounding air.

So completely independent of the trust.

and action = reaction meaning the trust will be equal to the sum of F1=m.a and the resistence of the air F2

meaning if speed=constant then trust=resistance/drag

(m=mass of the airplain, a=acceleration of the )

[albertlee]

Ok.....Let me sort it out..........So....., is it possible to have drag without thrust? and secondly, if so, where does the force come from to have the drag? since drag is a mechanical force......

 

Any help?

[YT2095]

no, it`s not possible to have ANY resistance, be it from air or in an electric circuit without force or energy being applied to "recognise" it

[albertlee]

Yes, that's it, YT, if the drag needs thrust, which means this apply to Newton's third law....... Simply, what i wonder is why not Thrust is the action, and Drag is the reaction in one way or another? Sorry for being confused so long, but what I really confuse is always about the Newton's third law as role in "force and flight"......

 

Any help?

[Kedas]

Ok.....Let me sort it out..........So.....' date=' is it possible to have drag without thrust? and secondly, if so, where does the force come from to have the drag? since drag is a mechanical force......

Any help?[/quote']

Yes, that is completely possible.

The force comes from the air that doesn't want to move easily.

If trust = 0 then resistance = m.a

a bullet that is horizontally flying through the air has no trust and still a lot of drag until it stops. (gravity force ignored)

(keep in mind a falling object has a vertical 'trust' of F=m.g)

 

maybe your problem is that you don't understand F=m.a very well?

 

no' date=' it`s not possible to have ANY resistance, be it from air or in an electric circuit without force or energy being applied to "recognise" it

[/quote']

Correct but you don't need trust to be able to talk about energy being transfered.

so it's "yes,....."

[swansont]

Yes' date=' that's it, YT, if the drag needs thrust, which means this apply to Newton's third law....... Simply, [b']what i wonder is why not Thrust is the action, and Drag is the reaction in one way or another?[/b] Sorry for being confused so long, but what I really confuse is always about the Newton's third law as role in "force and flight"......

 

Any help?

 

Newton's third law states that every force has a reaction. That is, F_A-B = F_B-A. A exerts the same force on B that B exerts on A.

 

But thrust and drag are both exerted on the plane - the same object. The reaction force to either has to be a force that the plane exerts on the air.