K-14 Sight - Confused about "mil" unit in Air Forces Manual

[Lazerhawk]

Hello,

I'm reading "Air Forces Manual No.64 - Fighter Gunnery Firing Rockets Dive Bombing 1May1945.pdf" and I'm confused when it comes to the unit "mil" used in this passage on p. 88 when talking about the K-14 sight:

According to Wikipedia, "mil" has two meanings:
- abbreviation for "milliradians", a unit to measure angles (71.12 mil would be 4 deg).
- a "thousandth of an inch", a unit to measure length (71.12 mil would be 0.07112 inches or 1.8 mm)

As "thousandth of an inch" seems not to make sense here, I figure it's milliradians?

What does 71.12 mil mean here? And what does it mean if the circle is 71.12 mil or 4 deg as described in the manual?

Thanks!

[Zabuzard]

1 hour ago, Lazerhawk said:

As "thousandth of an inch" seems not to make sense here, I figure it's milliradians?

What does 71.12 mil mean here? And what does it mean if the circle is 71.12 mil or 4 deg as described in the manual?

Correct. Imagine there would be a full circle around you and you could say "I am aiming 20° more to the left and 5° up". But now fine tune that to a much smaller unit. And instead of using degrees, which go 0 to 360, you use radians, which go 0 to 1000.

Here a quick google images snapshot to get the idea:

Due to the involved geometry, you can also compute distances and similiar with it, as for example seen in the picture on the lower left.

It is also used in, for example, a classic scope you would see in a sniper rifle or similar (mil-dot):

Edited April 21, 2023 by Zabuzard

[Zabuzard]

Now, if you know that your mil-circle is 71.12mil big, you could do the math and use it as reference. For example in the german WW2 aircraft, with a 100-mil sight, if an enemy aircraft with a wingspan of 10m fit exactly into the circle, it was at a range of 10 * 10 = 100m. If it would fit twice into the circle, it was at a range of 200m and so on.

Edited April 21, 2023 by Zabuzard

[Zabuzard]

In the P-51D for example, you would use the 10-mil rocket-aid marker:

To place an enemy truck or vehicle within it. They have roughly a height of 1-3m, so when they fit exactly within the two markers, they are at a distance of about 250m (bc its 10-mil). Thats the distance at which you would roughly fire your rockets and likely hit the vehicles (if flown with the proper shallow dive angle and speed).

The math should be:

distance = size * 1000 / mils

So like, enemy wingspan is maybe 10m and fits exactly in the 70-mils circle. Then the enemy is at

10 * 1000 / 70 = 140m distance

It is essentially the kind of math that your gyro-sight does and the math that the pilot would need to be able to do (at least roughly) if they were using the fixed sight reticle.

Edited April 21, 2023 by Zabuzard

[Lazerhawk]

Hi @Zabuzard,

Thank you so much for taking your time to answer my question in such detail!

With your explanations I'm starting to get the hang of it now.

Thanks again!

[Lazerhawk]

Hi @Zabuzard, sorry to bother you again.

Judging from your examples above, am I correct in assuming that the diameter of the sight circle is 71 mils (blue in picture below) and not the radius (red in picture below)?

Best regards!

 

 

[Zabuzard]

2 hours ago, Lazerhawk said:

Judging from your examples above, am I correct in assuming that the diameter of the sight circle is 71 mils (blue in picture below) and not the radius (red in picture below)?

yes, the diameter is 71 mils.

With a wingspan of 30-35 ft for most of its enemy fighters, that means that when they fit fully into the circle (both their wingtips touch the circle), they are at a distance of 450-500 ft.

And when they fit half in the circle (one wingtip touches the circle, the other the center), they are at a distance of 1100ft... which happens to be the focus point of the 6 guns!

 

The main reason why the pilot would want to know the distance is because it gives them a reference of how much lead they have to give and similar. They have some rule of thumbs for that and practiced a lot on the ground with theory. There are some really funny illustrations in the old manuals and training lecture, like this:

And all of that depends on the distance.

Edited April 22, 2023 by Zabuzard

[Lazerhawk]

Thanks @Zabuzard again! It all starts to make sense now

[YoYo]

On 4/21/2023 at 1:15 PM, Zabuzard said:

In the P-51D for example, you would use the 10-mil rocket-aid marker:

To place an enemy truck or vehicle within it. They have roughly a height of 1-3m, so when they fit exactly within the two markers, they are at a distance of about 250m (bc its 10-mil). Thats the distance at which you would roughly fire your rockets and likely hit the vehicles (if flown with the proper shallow dive angle and speed).

The math should be:

distance = size * 1000 / mils

So like, enemy wingspan is maybe 10m and fits exactly in the 70-mils circle. Then the enemy is at

10 * 1000 / 70 = 140m distance

It is essentially the kind of math that your gyro-sight does and the math that the pilot would need to be able to do (at least roughly) if they were using the fixed sight reticle.

 

And it's interesting, from my trainings, without reading anything, I noticed that just like I usually fire rockets by eye, when I use this position I have the best hit results :). So nice.