# Gravity acting on aircraft

1. Apr 6, 2012

### mikedamike

what is a general formula for the gravity acting on an aircraft ?

regards
mike

2. Apr 6, 2012

### Whovian

Just the same as normal. Now if you're wondering why it doesn't accelerate downwards, that's because of Bernoulli's Principle and fluid mechanics in the case of a gas under nonconstant pressure everywhere, which I don't have much experience in.

3. Apr 6, 2012

### mikedamike

so if iv got this correct the gravity acting on an aircraft would be the same as gravity acting on any other possible object ? which is 9.80665 m/s2 (32.1737 ft/s2)

4. Apr 6, 2012

### Whovian

Absolutely. In this situation, the only difference between an aircraft and another object that needs to be considered is that some other force holds an aircraft aloft.

5. Apr 6, 2012

### mikedamike

The force that keeps the aircraft aloft does not how ever make a change to the gravity acting on it ? it just remains to be 9.80665 m/s2 (32.1737 ft/s2) ?

Thank you for you help

6. Apr 6, 2012

### Whovian

Correct. Gravity's approximately a constant MG near the surface of the earth and approximately GmM/r^2 otherwise. (Note that general relativity does come into play a bit here, but Newton's Universal law of Gravitation is a good enough approximation here.)

7. Apr 6, 2012

### DaveC426913

Yes. To be clear, the forces are in balance.

The 4 forces on a craft are gravity, lift, thrust and drag.

If the plane is in level flight, then the force of gravity is perfectly balanced by the force of lift.

And the force of gravity is the same everywhere near the surface of the Earth. (Depending on how high the craft can fly, and how accurate you want to be. )

8. Apr 6, 2012

9. Apr 6, 2012

### Neandethal00

hhhmmmm. But a pen dropped from my hand falls on the plane floor. Is the plane + passengers + everything on it an inertial frame?

This happens as if I climbed a 35000 ft tower and dropped a pen with acceleration=9.8 m/s2 roughly. What is really balanced here? Just curious.

10. Apr 6, 2012

### HallsofIvy

As long as the airplane is flying at a constant altitude, constant speed, yes, it an "inertial frame". The pen would fall from your hand to the floor accelerating at 9.8... m/s^2. (If the plane were accelerating upward [or downward] the pen would have acceleration equal to the sum of 9.8 and the vertical acceleration of the airplane.)

There is no "balancing" force on the pen- that's why it is accelerating downward. As far as you are concerned, the part of the tower you are standing on gives the balancing, upward, force that you feel as your "weight".

11. Apr 6, 2012

### Emilyjoint

there is a fifth force acting on a plane which usually can be ignored because it is insignificant but it does play a part in some flying machines....buoyancy.

12. Apr 6, 2012

### HallsofIvy

Particularly in lighter than air flying machines!

13. Apr 6, 2012

### DaveC426913

I do believe that the bouyant force is covered under "lift". i.e. there are many kinds of lift.

14. Apr 7, 2012

### haruspex

Mikedamike, you're quoting g to six places of decimals, so I assume you care about exactness here. A plane at ten kms up will experience 0.2% less gravity.
The earths's gravity actually varies quite a bit even at the surface. The poles are closer to the earth's centre than is the equator, making gravity a little stronger there. If you weigh objects stationary relative to the earth's surface then they will appear lighter at the equator also because of earth's spin. There are also local anomalies caused by variations in density of rocks/water.
Also, a plane flying W to E is orbiting the earth faster than the earth is spinning. This means it needs more centripetal force to stay at constant altitude. The lift it needs is lessened by that amount. Conversely, a plane flying E to W needs a bit more lift.

15. Apr 7, 2012

### rcgldr

The plane could be climbing or descending at constant speed (no acceleration), and still be in an inertial frame.