# Thread: im proofing the lies in vertical landing airplanes

1. you can throw a pen with your finger in a vertiacal position(take off)
but if you want to catch a pen with your finger that comes down in vertical position( landing) thats nearly impossibble even for tthe most skilled machine in the world(the hand)
so whats the explanation for this
in both examples the net force(F) works in the same direction and so the acceleration is in the same direction but in take off the ,a' increase the speed and in landing it decreases it
so whats the difference then!

the difference is the momentum
in take off its in the same direction with net force and acceleration
but in landing the direction of momentum(m.v) is in the opposite of acceleration
so whats the importance of this!
in take off, any small deviation in the vertical position shall be balanced with the momentum(the vector of momentum and gravity are at opposite directions.the force applied from down(finger or engine) can not help the balance because it comes allways with 0 degrees.
in landing any deviation shall have no chance because the vectors(gravity and momentum) shall be to the same direction and to get the pen (or rocket) to its vertical position there is no other vector
after that point only GOD can help the rocket

2.

3. Please stop posting nonsensical bullshit over the physics forum.

Obviously VTOL jets are not a lie, because they exist...and they work....you don't have to be a physicist to calculate that.

4. The multiple engines of a rocket apply a force opposite to the gravitational force. When a rocket lands, the amount of upward thrust is maintained to be just a little less than the force applied by gravity, so the rocket slowly decends. The rocket has more than one engine. Each engine is balanced by a positive feedback system that keeps the thrust even between the engines, keeping the rocket upright. With what part of this do you have a problem?

5. PM: if you dont have knowledge of mechanics and dont have a open mind dont reply me
your a believer not a thinker
:-D

F=ma

If we have a constant mass, with an upwards force F<sub>1</sub> and a downwards force F<sub>2</sub>, then the overall force on the mass is given by F=F<sub>1</sub>+F<sub>2</sub> where F<sub>2</sub> is negative..representing a downwards force...this force is given by the weight of the plane...which is its mass multiplied by the acceleration due to gravity.

When the plane is stationary on the ground, |F<sub>1</sub>|=|F<sub>2</sub>|...so F=0 In other words, the upwards force (reaction force), and downwards force (weight) are equal...but opposite...meaning no nett force, and therefore no nett acceleraton. (Newtons second law)

When the plane takes off, its upwards force increases due to the rate of change of momentum of hot exhaust gasses from the jets. Now that |F<sub>1</sub>|>|F<sub>2</sub>|, the nett force now has a positive magnitude. Since the mass of the plane is the same, it also has some upwards acceleration given by a =F/m.

When the plane is hovering |F<sub>1</sub>|=|F<sub>2</sub>| again, no nett veritcal movent occurs...just as when the plane is on the ground.

When it comes down, the rate of change of momentum of exhaust gases from the jets decreases below the weight of the plane so |F<sub>1</sub>|<|F<sub>2</sub>|, and there is a slight nett force downwards...meaning the plane has some acceleration downwards. Obviously in landing the jets have to provide more thrust to balance out the forces again, and ensure that plane doesn't come down too fast. Understand?

6. Forget all physics, just go look at a video of harrier jet or i think maybe an F22, are saying they are a conspiracy? hahahahaha

7. Originally Posted by GrowlingDog
Forget all physics, just go look at a video of harrier jet or i think maybe an F22, are saying they are a conspiracy? hahahahaha
Lol Run! RUN for the hills!

The fact there are multiple engines means it is possible. Having one thrust engine in the middle would make it much more difficult due to the uneven distribution of mass.

8. Actually both the harrier jump jet and the F-35 VTOL Jets only have one engine...The F-35 engine is 'split' in the middle so you could say it has 2...but combustion only occurs in one, so really there is only one.

The Harrier for example has four nozzles but only one engine. The front two nozzles just expel compressed air after the first compression stage, and the rear nozzles expel combusted gasses from the rear nozzles, after passing through the rest of the engine. Obviously this creates more force, so the rear nozzles are actually aligned much closer to the centre of mass of the plane to compensate...and the front nozzles are really just to provide a more stable balance. They also have 4 extra outlets for stablizing the plane even more during vertical flight...one on the tip of each wing, one on the nose, and one on the tail...these expel 'bleed air' (pressurised from the first combustion stage) downwards, to keep the plane nicely level (Probably an automated system using tiltmeters). This air is also used to drive the mechanism that rotates the nozzles I think.

Another thing that creates complications is the gyroscopic forces that arise from the rapidly rotating turbine assembly within the engine. This is solved by having two or more spools that rotate in opposite directions to even it out.

9. Oops thanks for the correction! I guess I should have said multiple points of thrust!

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