Discussion on how the speed of the aircraft affects velocity of munitions?

[naturboy99]

I'm sure on such an informed board this has been discussed before, but frankly I couldn't come up with the correct search terminology to find it I guess.




Is there a fairly simple way of explaining how the speed of the aircraft affects the velocity of the munitions fired? Surely it isn't as simple as adding the two together is it? Would the effective range of a weapon be increased by the additional momentum? I would think the energy of the round would increase drastically as compared to firing from a fixed platform? I can't imagine how devastating and terrifying a Jug coming in on a strafing run with 8 50 cals and rockets would be, to say nothing of what it's modern counterpart with a gajillion 30mm rounds coming at you would be like.

Again, I wager there is an existing thread, if someone could point me to it I would appreciate it, I truly did try to search for an existing discussion.


Thanks in advance!

[TinyTim]

What if a plane was fixed in a wind tunnel and the surrounding air had a velocity of 500kph while the plane fired its guns?

Yes, it's as simple as adding the two velocities together. If weapon's muzzle velocity is Vw and velocity of airplane is Va, then the relative velocity of bullets relative to the Earth is Vw+Va immediately after the bullet is fired. However - the greater air resistence will result in its velocity diminishing faster, that's why a weapon will have less range relative to the plane when triggered in flight compared to when triggered on the ground (given a plane continues the straight flight with constant velocity for the time of bullet flight - which is about 2 seconds max, so this assumption can clearly be made). In other words - if plane was fixed on the ground and fired at a target 300 m away, bullets will do more damage than a plane in the air, shooting at a target flying with approx. same speed from behind at 300m since bullets have to travel "up the stream" if you know what I mean.

Effective range of the weapon relative to the plane is thus decreased while in flight, absolute range however is increased. (Absolute range meaning the distance between the point in 3D space where the bullet was fired from, and the point where it looses its penetrating ability). In other words - it's the same as two bullets fired from the ground, one with Va, another with Vw+Va. Naturally, the latter will have better range.

[bruno_]

With the sole purpose to help getting a better insight about the qualitative roles played by aircraft speed in the air gunnery field, I would like to write down some short considerations about the influence of the aircraft speed on gunnery effectiveness, since the final result of a bullet burst is a matter of energy and trajectory as well. Shooting at the right moment (distance) is a crucial factor.
For the sake of simplicity, we will neglect the effect of altitude, humidity, air drag, etc. on the speed of the bullet once it goes out of the gun.
In this frame, beside its “absolute value effect” (bullet kinetic energy), the aircraft speed has an influence also on bullet path both in the horizontal and vertical planes.
When speaking about horizontal plane, the aircraft speed will affect the convergence of the bullet stream. In few words, the convergence will occur at an higher distance with respect to the “static” convergence distance (zero aircraft speed). The higher the aircraft speed, the higher the difference between the static convergence point and the dynamic will be.
Generally speaking, when shooting to another aircraft, the right distance to be used to get the maximum effect in terms of bullet concentration, the target speed too, will play an important role. More precisely, the key factor is the relative speed of your aircraft with respect to the target. This is the velocity to be combined with the bullet muzzle velocity for determining the order of magnitude of the right distance for opening fire. If you are chasing your prey from 6 o’clock and the enemy is flying at the same speed of you aircraft and you open fire when you are at the static convergence distance: no problem (relative speed equal to zero). Your bullet stream will converge where you suppose it should happen (static distance) and you will correctly open fire when the distance from your target will approach the static convergence distance. But, if you want to shoot at a still target (eg, an ammo depot on the ground) or at an “head on” flying target (the higher relative speed situation you can conceive), the dynamic convergence effect will taken place to its maximum extent and you need to take it into account, in order to get effective results. In this last case, the right target distance for an effective burst release may be significantly higher than the static convergence distance. Of course, this stuff is more important for wing mounted guns than for fuselage mounted guns.
When speaking about vertical plane, again, combining the aircraft speed to the bullet speed, means to consider that, with respect to the longitudinal axis: 1) guns have their own angle (depending on the gun offset with respect to the line of aim, the gun/bullet ballistic features and the nominal distances you want the bullet trajectory to cross the line of aim), 2) the angle of the aircraft longitudinal axis with respect to the aircraft speed, depends, beside other factors, on the speed itself (imagine you want to fly horizontally, the higher the aircraft speed, the lower the incidence angle you need to get the equilibrium between lift and weight). Once again, this must be taken into account when calibrating the gun elevation in order to get the bullet stream crossing the “line of aim”.
I hope these notes were someway helpful.