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general_kalle
01-21-2010, 02:32 AM
what confuses me is that i always thought that the free fall a bombs as used in world war 2, upon being released would fall in a bend Trajektory going flat imidiatly after release and then turning downwards going falling(vertical speed) faster and faster till (if) it reaches terminal Velocity.

However that Conflicts with the theory that an objekt with a forwards velocity falls at same vertical speed, as an objekt without forward velocity taking the bomb as an example.

does the bomb accelerate during the fall or does it have terminal velocity instantly after release?

is the law true?
does it have something to do with the aorodynamics of the bomb? (Lift?)

so i was sitting in Physics class and we were told that the Gravitational acceleration is independent of the velocity of an objekt.
as i understood it, it basically means that an objekt moving at speed is still affected by the same Gravitational acceleration as an objekt that falls vertically downwards (if there's no lift)

Edit: ok i have understood this now, however ill let the question remain for others to reflect upon.

Erkki_M
01-21-2010, 02:57 AM
After the bomb is released, there are three two forces affecting it: gravitation, giving it an acceleration towards the center of earth, and air resistance, which slows it down(might even give it some lift in the very beginning, but air resistance quickly rolls the bomb to point where it goes).

If the bomb was dropped from an aircraft in level flight, it would hit the ground somewhere below the plane's flight path. However, thanks to the air resistance, they are nearly almost a little off to the sides of the calculated impact point, and drop slightly short. I dont know if the WW2 era bomb sights knew to calculate the total work the air resistance will at least affect the bomb with(simplish calculation) or not.

Bomb have very small cross section compared to their mass. As you know, even a WW2 era plane can break the sound barrier(with fatal effects, though) in freefall. A bomb will, too, if dropped high enough. If there was no air resistance, a free falling object dropped just below 6000m would reach the speed of sound...

Just some quick thoughts, someone surely knows better(or actually knows).

AndyJWest
01-21-2010, 08:22 AM
Other than gravitational forces and air resistance, another factor is wind strength and direction. This will vary with altitude, making precision bombing more difficult with height.

na85
01-21-2010, 08:30 AM
Originally posted by general_kalle:
what confuses me is that i always thought that the free fall a bombs as used in world war 2, upon being released would fall in a bend Trajektory going flat imidiatly after release and then turning downwards going falling(vertical speed) faster and faster till (if) it reaches terminal Velocity.

However that Conflicts with the theory that an objekt with a forwards velocity falls at same vertical speed, as an objekt without forward velocity taking the bomb as an example.

does the bomb accelerate during the fall or does it have terminal velocity instantly after release?

is the law true?
does it have something to do with the aorodynamics of the bomb? (Lift?)

so i was sitting in Physics class and we were told that the Gravitational acceleration is independent of the velocity of an objekt.
as i understood it, it basically means that an objekt moving at speed is still affected by the same Gravitational acceleration as an objekt that falls vertically downwards (if there's no lift)

Edit: ok i have understood this now, however ill let the question remain for others to reflect upon.

Imagine you have two bombs. One released from a helicopter and one released from a bomber in level flight. Imagine they are identical bombs, and are dropped from identical positions (alt, weather conditions, etc)

Both bombs will hit the ground at the same time, even though the 2nd bomb travels a greater distance.

Both bombs will accelerate downward at the same rate: 9.8 m/s^2, and their vertical speeds will be the same. However the first bomb has a horizontal speed of zero, while the 2nd bomb has a horizontal speed of non-zero.

So in terms of total speed, the 2nd bomb is actually moving faster, even though their downward movements are the same.

JtD
01-21-2010, 08:33 AM
Some interesting info on bomb trajectories in this (http://cgsc.cdmhost.com/cgi-bin/showfile.exe?CISOROOT=/p4013coll8&CISOPTR=2342&filename=2331.PDF) interesting WW2 US army document.

Choctaw111
01-21-2010, 12:48 PM
This is an interesting thread.
It is true that two objects of the same size, shape and weight will fall at the same rate.
It is also true that an object with a horizontal speed will fall at the same rate as the same object at rest despite the fact that it will cover a greater distance.
There are several factors that can come into play that may have the two objects hitting the ground at different times but if all the conditions are identical for both, then they will hit the ground simultaneously.

na85
01-21-2010, 01:45 PM
More interesting is if you were to drop from sufficient height, the bomb with a horizontal velocity component will reach terminal velocity before the other one, and as a result the bomb dropped straight down impacts first.

I love physics

M_Gunz
01-21-2010, 02:49 PM
I don't think that's how it works. It doesn't work that way for bullets and they start out higher than
terminal velocity, for example.

na85
01-21-2010, 03:40 PM
Pretty sure... but willing to be corrected.

Makes sense to me; terminal velocity = sqrt( 2*m*g/rho*A*Cd )

so if your object is traveling on a 45 degree down angle, and is at terminal velocity which is 100 km/h, but 30 km/h of that is horizontal velocity you're looking at only ~95 km/h downward. The other object falling straight down at 100 km/h ought to hit the ground first..

unless I'm greatly mistaken and skipped over some simple concept http://forums.ubi.com/images/smilies/bigtears.gif

thefruitbat
01-21-2010, 04:00 PM
Originally posted by JtD:
Some interesting info on bomb trajectories in this (http://cgsc.cdmhost.com/cgi-bin/showfile.exe?CISOROOT=/p4013coll8&CISOPTR=2342&filename=2331.PDF) interesting WW2 US army document.

Thanks for posting JTD, interesting read http://forums.ubi.com/images/smilies/25.gif

M_Gunz
01-21-2010, 04:46 PM
Originally posted by na85:
Pretty sure... but willing to be corrected.

Makes sense to me; terminal velocity = sqrt( 2*m*g/rho*A*Cd )

so if your object is traveling on a 45 degree down angle, and is at terminal velocity which is 100 km/h, but 30 km/h of that is horizontal velocity you're looking at only ~95 km/h downward. The other object falling straight down at 100 km/h ought to hit the ground first..

unless I'm greatly mistaken and skipped over some simple concept http://forums.ubi.com/images/smilies/bigtears.gif

Like the horizontal component had it's own drag component the whole time and the angle of drop is not static?
Also it would be a very low-density bomb that falls half as fast as a human would?

I started to think it through with angles, speed and drag in components and that's when I remembered rifle fire.
The bullet starts out say horizontal well above terminal velocity but still manages to drop nicely anyway long
before slowing to terminal velocity.
I don't think that terminal velocity pertains to any direction but down, you see.

Viper2005_
01-21-2010, 05:14 PM
Terminal velocity is just thrust=drag.

In freefall the thrust comes from gravity.

If you want to tackle the problem numerically you need to split the drag into horizontal and vertical components, and then calculate the nett force on the object.

F=ma; integrate once to get velocity and twice to get displacements. Numerically, over a small timestep, F may be assumed constant. Iterate to your heart's content. That's how flightsims work.

Viper2005_
01-21-2010, 07:25 PM
Meanwhile, you can check the physics in IL2 as follows.

Fly in WW view with wind & turbulence off and note your TAS & altitude. Make sure that your bombs have zero delay. Set the bomb sight appropriately and note the release angle.

You can then compare this with a zero bomb drag approximation.

Altitude is already in m.

TAS in km/h divided by 3.6 gives TAS in m/s.

We can calculate the time it takes the bomb to fall:

s=ut+(at^2)/2

u = 0 because we're in level flight. Therefore

s=(at^2)/2

rearrange

2s=at^2
2s/a=t^2
(2s/a)^0.5=t

s = your altitude
a= 9.81 m/s^2

so:

(2*altitude/9.81)^0.5=t

You can plug this into google substituting your own numbers as required to get the time of flight.

Then the horizontal distance travelled is trivially just v*t, where v is your velocity in m/s.

You can then draw a right-angle triangle, with the height and distance travelled, and calculate the bomb release angle.

The bomb notionally travels along the hypotenuse. We're interested in the angle at the top, so the height is the adjacent side, and the horizontal distance travelled is the opposite.

SOHCAHTOA

Tan = opposite/adjacent

Note that google will give you an answer in radians which must be converted to degrees. It'll do that for you if you ask nicely.

Note also that the angle given will be the internal angle of the imaginary triangle. IIRC this is what is required, because you increase the bomb sight elevation to get closer to the horizontal.

Enjoy.

AndyJWest
01-21-2010, 07:33 PM
Then the horizontal distance travelled is trivially just v*t, where v is your velocity in m/s.

This would be true in a vacuum. But does IL-2 simulate air resistance? This will also affect the vertical component of velocity.

I don't know for sure if IL-2 does factor in aerodynamic drag with bombs, but it seems to with guns - the rounds loose horizontal velocity over time, giving less damage at long range.

na85
01-21-2010, 07:35 PM
Originally posted by M_Gunz:

Also it would be a very low-density bomb that falls half as fast as a human would?

I think it's pretty obvious I was just making those numbers up for the sake of illustration http://forums.ubi.com/groupee_common/emoticons/icon_rolleyes.gif

Viper2005_
01-21-2010, 07:42 PM
Originally posted by AndyJWest:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Then the horizontal distance travelled is trivially just v*t, where v is your velocity in m/s.

This would be true in a vacuum. But does IL-2 simulate air resistance? This will also affect the vertical component of velocity.

I don't know for sure if IL-2 does factor in aerodynamic drag with bombs, but it seems to with guns - the rounds loose horizontal velocity over time, giving less damage at long range. </div></BLOCKQUOTE>

The equations allow you to test whether or not IL2 accounts for air resistance. That's essentially the reason that I posted them.

If you bomb from high altitude over the sea on Crimea with something fast like the Ar-234 it should be pretty easy to detect even quite a small drag coefficient...

AndyJWest
01-21-2010, 08:19 PM
I've done a bit of testing in IL-2, and there isn't much that is 'pretty easy' beyond the raw figures you can get from DeviceLink etc. Still, the particular case you cite should be easier than some: without air resistance, a bomb dropped from an aircraft moving at constant velocity shoul hit the ground directly below the aircraft. Does it?

M_Gunz
01-21-2010, 10:19 PM
Originally posted by Viper2005_:
Terminal velocity is just thrust=drag.

In freefall the thrust comes from gravity.

Damn, you went and spoiled it. http://forums.ubi.com/images/smilies/16x16_smiley-wink.gif

M_Gunz
01-21-2010, 10:43 PM
Just time the drop to hit from high alt and you'll know if the bombs have significant drag.

Nate, I know it's an example for illustration but compared to the velocity of a falling iron
bomb at terminal velocity the horizontal component would have to be very high TAS which would
be scrubbing off even as the bomb picked up vertical speed.

But since Viper put it in such good terms, the TV only matters in the direction of gravity.

Waldo.Pepper
01-23-2010, 04:41 AM
This is a superb book if you can track it down.

The science of bombing : operational research in RAF Bomber Command / Randall T. Wakelam. Published Toronto : University of Toronto Press, 2009.

Choctaw111
01-23-2010, 06:44 AM
Originally posted by na85:
More interesting is if you were to drop from sufficient height, the bomb with a horizontal velocity component will reach terminal velocity before the other one, and as a result the bomb dropped straight down impacts first.

I love physics

You are looking at it the wrong way.
You are thinking terminal velocity in the horizontal rather than the vertical.
It does not matter how fast the bomb is traveling horizontally. It will start to slow down in the horizontal path as soon as it is released anyway.
You need to focus on the vertical path as the bomb falls from gravity.
In this regard, BOTH of the bombs will fall at the same rate regardless of any horizontal speed...
PROVIDED that the bomb with the horizontal speed is traveling PERFECTLY PERPENDICULAR to the gravitational force.

na85
01-23-2010, 11:59 AM
No, my thinking was that the horizontal component of velocity contributed to the total drag, which it does, but my mistake was thinking that the total drag had to be opposed by gravity...

or something like that.

Either way I was mistaken

Phoenix-21
01-24-2010, 02:23 PM
You guys have gotten my interest up so I pulled out my Students manual of Bombing and the Bombardiers information file and based on the finding there let me present this. Using an altitude of 16,000 feet and a AN M43 500 lb general purpose bomb and if the only variable is airspeed the following is presented from the bombing table. For time of fall, which will mean actual shackel release to impact is as follows. 200 mph-32.78 seconds, 300mph-32.95 seconds and, 400mph-33.19 seconds. I hope this dosen't muddy the water to much. Good luck Rick

M_Gunz
01-24-2010, 05:29 PM
Faster you go, longer the bomb is falling pointing sideways to downwards (= bit more drag)?

JtD
01-26-2010, 09:47 AM
There's a difference between bombs gliding and bombs falling. Bombs had fins, even in WW2. It is fair game to approximate their behaviour as falling, though there certainly was a minimal gliding component. Check Phoenix-21 numbers, it's a 1% effect. Thanks for these numbers, btw.