Hello Everyone,
I couldn’t sleep last night and started thinking about some really stupid things. Two things I was thinking about have really got me curious as I just don’t know the answers.
In a aircraft, anything in the plane is traveling as fast as the plane. This, for the sake of simplicity, is because you are “connected” to the plane by the fact that you are sitting or standing on it. So, what I was thinking is if you had a balloon and you were to weight it so that it would float in the cabin without touching anything, would the balloon continue to match the speed of the plane or would it eventually end up at the very back of the cabin. Meaning, would it stand still in a free float while the plane moved or would it continue to travel at the same speed as the plane? If it continues to travel at the same speed is this because of the pressure of the air in the cabin, which I assume is traveling at the same speed as the plane. I know this is probably a simple question, but given my lack of sleep I can’t seem to wrap my head around it.
Modern jet fighters for the most part have abandoned the traditional machine gun in favor of cannons. I have always thought that this was because the planes are now capable of traveling faster than a .50 caliber bullet. So, if you equipped a modern fighter with a .50 caliber machine gun and fired that gun while the plane was traveling faster than the bullet, what happens? Does the bullet even leave the barrel? Does it leave the barrel at its speed plus the speed the plane is traveling? Or do the bullets leave the barrel only to have the plane run into them? Once again, I am sure that this is simple physics, but I just can’t seem to get it straight in my head.
While the plane is accelerating (speeding up, slowing down, turning, banking, changing altitude etc), yes, the balloon will move. The same way a balloon moves around in your car. The same way everything slides across your dash or off your seat when you make a hard turn.
Any time you feel pushed against the back of your seat in the plane, the balloon feels the same forces, it just doesn’t stop until it hits the back of the plane. But when the plane is flying at a constant velocity the balloon will remain still…just like all the passengers.
ETA, the helium in the balloon might cause the balloon to move to the front of the plane on take off since the nose is higher then the tail, but I don’t think that’s what you were thinking of when you asked the question.
Sounds reasonable. I was thinking that because the balloon was floating and independent of the airplane (not touching anything) that the effect of acceleration and/or change of direction would not affect the balloon. That is why I was having trouble with it last night.
During acceleration, a helium balloon would actually move towards the front of the plane because it is lighter than air. The heavier air displaces it in a way that is counterintuitive unless you think about it a little. It moves the opposite way that you would. I’ve seen this in a car and can’t think of any reason that a plane would be different.
You’re forgetting the MASS OF AIR inside the plane. The balloon is touching THAT. The plane is carrying along this mass of air just as much as it’s carrying along the passengers and the “food”.
Same as when you’re in a car. Remove the windshield, and suddenly the mass of air isn’t moving along with you anymore. THEN, anything not attached to the car will “remain in place” (relative to the ground); i.e., “be pushed back quickly” relative to the still-moving car.
I haven’t noticed that effect so I won’t comment on it directly, but something that is very noticeable is that when the heater is on in your car and you go around (for example) a right turn, the heat rushes to the passenger side of the car and you get a blast of cold air.
No. A typical muzzel velocity of 3050 feet per second converts to 2079 mph. No dog fights are happening at that speed. Hell, the top speed of a F16 is something like mach 1.6.
I didn’t know that. I knew that the .50 traveled faster than sound , but I didn’t realize that it went that fast. Thanks for the information. I guess they abandoned machine guns due to not only missiles, but the superior killing power of the 20mm cannon.
The projectile leaves the barrel at muzzle velocity relative to the muzzle, so in actual terms, it is moving at the aircraft velocity plus the muzzle velocity. Of course, friction from air acts as the square of the velocity, so the projectiles lose speed rapidly.
There have been documented cases of self-shootdown, where a rare combination of factors allow an aircraft to fly into its own shells. This is not common, and the requirements of maneuvering in a dogfight slow aircraft to lower speeds so this is unlikely.
The wiki article on the M61 Vulcan cannon explains the thinking behind the move to 20mm cannons.
If the balloon has been configured to be neutrally buoyant in the air inside the plane (as the OP describes), it won’t do this - it won’t have any reason to behave differently from the air surrounding it.
Neutrally buoyant in the inertial plane. When the plane accelerates the air pressure falls at one end and rises at the other, and the balloon moves, yes? Of course it might move in either the direction of the acceleration or against it depending on which end of the airplane it started in.
Probably not. The balloon isn’t rigid, so the pressure probably just equalises. There may be some non-linearity in the fact that the internal pressure of the balloon is partly governed by the elasticity of the rubber, rather than just the ambient pressure.
Anyway, I thought (from other buoyancy-related threads) it was all the same - that buoyancy can just be explained by pressure gradients driven by gravity - and gravity is no different from acceleration…
Everyone already beat me, but the air in the plane it’s moving with the plane so a lighter than air balloon would move around in the direction of the plane’s acceleration. It does this because it’s lighter than air and if you turn left, the air shifts to the right just like a person in a car, but the balloon goes the opposite direction.
I may be splitting hairs but the balloon in the OP isn’t actually naturally buoyant, it’s just weighted so it doesn’t hit the roof so it will still move around in the opposite direction as an object normally would, just like a balloon that is tied inside a car with a string or held by a person’s fingers. It’s held from hitting the roof but the helium is still lighter than air.
You say that air fricton increases at the square of the velocity. Is there is distinction between air resistance and air friction or are they the same?
Thanks for all the replies. It all makes sense now. But to expand on question number 2 a bit. Someone up thread said that if a gun is fired from a moving plane that the speed of the bullet is the planes speed plus the speed of the bullet. So, it is safe to assume then that if a pilot pulled the trigger on a machine gun at Mach 2.5 (or faster than the bullet travels) that the bullets would travel faster than the plane?
It will pretty much instantaneously lose speed to friction, but when it leaves the barrel it will be traveling at the same speed it would if fired from a stationary barrel, plus Mach 2.5.
I think in this case, air resistance or drag would be the more correct term to use. I would describe friction as one component that makes up air resistance or drag.
A balloon that is weighted so that it floats in the cabin without touching anything is, by definition, neutrally buoyant; the average density of the balloon plus its cargo is exactly equal to the density of the air around it. You can accelerate the cabin in any direction you like, and the balloon will not exhibit any movement relative to the cabin walls.
Think about it this way: if you’re inside a passenger jet traveling at 600 MPH, and you throw a tennis ball toward the cockpit with your usual 20-MPH throw, does the plane overrun the ball? What is the ball’s speed relative to the plane, and what is the ball’s speed relative to the ground?
I’m not clear on your distinction between “cannon” and “traditional machine gun” in this context. The gun/cannon on modern military aircraft still uses self-contained rounds made of a projectile, propellant, and case. In principle, these are no different than what you find in handgun rounds, although they are quite a bit larger and more complex. The projectile may be armor-piercing (typically depleted uranium) or high-explosive/incendiary; typically the plane is loaded with a mix of the two, loaded onto the feed belt in a predetermined ratio. The HEI rounds include a fuze that triggers on some combination of the acceleration due to being fired, or the rotation of spinning (due to rifling in the barrel); they won’t explode if you just throw them on the ground. Muzzle velocity is on the order of 3000-4000 feet per second (2000-2700 MPH). Compare with the muzzle velocity for a .50-cal sniper rifle, around 2500 fps (1800 MPH).
As the tennis ball example suggests, the actual velocity of the projectiles relative to the ground will be the listed muzzle velocity plus the forward speed of the aircraft, so you could still shoot a small, slow 50-cal gun if you wanted to, no matter how fast your plane is going. however, when your target aircraft is moving across your field of view at 500 MPH, long times-of-flight for your projectles can make it difficult to aim with any accuracy. The predictive gunsights on military aircraft help by showing where your projectiles will be when they reach the range of your target aircraft, but it’s still difficult to score a hit if you and your target are maneuvering violenty and there’s a large distance involved. The faster your projectiles are, the easier it will be to aim/hit your target. And when you do score a hit, you want it to count - thus, the projectiles involved are large - 20-30mm - and either armor-piercing or HEI as described earlier; it’s hard to get useful HEI with 50-cal rounds.
There are two common sources of drag from the air. One is skin drag, related to the viscosity of the air. This is directly proportional to speed, and is actually relatively small, since air isn’t very viscous. It’s relevant when calculating flow losses in pipes and ducts, and also matters when calculating the terminal velocity of sub-micron particles, but it’s not a huge part of the drag on large, fast-moving vehicles.
The big factor is form drag, which is due to the fact that the vehicle has to redirect air around it in order to move forward. The vehicle exerts pressure on the air to make this happen, and in return the air exerts pressure on the vehicle. That pressure is proportional to the square of velocity. When air is pressurized, it gets hot due to the mechanical work being done on it. This is why the concorde used to get hot at cruise (1,400 MPH), and it’s why the SR-71 used to get really hot at cruise (2,100 MPH), and it’s why the space shuttle used to get mind-blowingly, god-damningly hot during reentry (17,000 MPH). In all those cases the highest temperatures are encountered on forward-facing surfaces, typically the nose and the leading edges of the wings, where the air goes SPLAT and then has to move around the vehicle.
For aircraft that are generating aerodynamic lift, there is also lift-induced drag due to the fact that the wings are redirecting air to create lift. This effect is actually greater at lower speeds and becomes a relatively small part of the total drag at cruising speeds.
That can’t be right. The air itself will fly to the back of the cabin, at least to some degree, so why wouldn’t the balloon do so as well? If it’s the same density as the air, then it should behave like the air. I assume it would “spring” back into place once the acceleration ceased, but it’d still move.