True Altitude?

[Soldier_Fortune]

Thank you, KG.
I did read it!

[FrankB]

Hi Soldier Fortune,

I did some tests using your formula to calculate TAS, but I still seem to have unsatisfactory results on winter maps.
In the past weeks I have learned how to use FMB to create custom mission for testing.

Test environment:
- stock 4.12.2
- SB-2, 100% fuel, 6x100kg bombs
- altitude 2000m
- IAS 330km/h
- bullseye placed at 0m elevation on the shore to simplify the calculations
- no wind on the maps
- coming in from the water

Calculations:
- got TAS from IAS and ground temperature as per your equation
- got the bombing angle using the calculated TAS and altitude

Results:
- good hits on the Crimea map (at least one bomb landed within the pattern)
- bombs fell short on Moscow winter map by at least 3 patterns

So some other variable comes to the play when calculating the bombing angle - maybe the air density/resistance...

[KG26_Alpha]

Work is in progress on the bombsites for 4.13

[Soldier_Fortune]

Hi FrankB!

So some other variable comes to the play when calculating the bombing angle - maybe the air density/resistance...

As far as i could test and see, no ballistic affects are modeled for the bombs within IL-2, and their motions my be described through the Laws of Newton.
I've made a set of charts (attached to post #30) for "fixed angle bombing" based on those laws, and I'm achieving good hits with them.

Test environment:
- stock 4.12.2
- SB-2, 100% fuel, 6x100kg bombs
- altitude 2000m
- IAS 330km/h
- bullseye placed at 0m elevation on the shore to simplify the calculations
- no wind on the maps
- coming in from the water

For these data, using the OPB-1 chart and an E-6B, I got the following settings:

Moscow winter: OAT@GL = -17 ºC
OAT @ Flight Altitude = -30 ºC
TAS = 340 km/h
BS Drop Angle= 44º [This value is affected by the interpolation while the chart is being read, and the player's judgement about the angle setting for the bombsight: the OPB-1 can be set with a precision of 0.25º/keystroke (it means: you need 4 keystrokes to increase or decrease the BS angle by 1º)].

I'll test them tomorrow, flying a SB-2, and then I'll tell you my results.

Results:
- good hits on the Crimea map (at least one bomb landed within the pattern)
- bombs fell short on Moscow winter map by at least 3 patterns

OK: your tests with the Crimea map are consistent with your data above.
But... How did you got "0 m ground altitude and shores" in the Moscow map?
In the Moscow map the ground is mostly flat... but the average ground altitudes are about 400 @MSL. If you've considered too low altitudes in your calculations, it may explain why your bombs fell short.

Let me give you some tips about level bombing:
- As lower is the chosen altitude for level bombing, as worse is the accuracy.
A good altitude is above 4000 m. Think like a bombardier: if you must carry on a pin point bomb mission flying at low altitude, it is better to use a bomber designed for such class of missions; e.g.: a Pe-2 or an IL-2.
Level bombing doesn't allow evasive maneuvers while you are flying the last leg of your bomb run. Therefore, if you are flying too low following a foreseeable path, you will make happy the flak crews.

A difference of 100 m @ MSL within an altitude of 2000 m, means an altitude error of 5% and 2.5% for the distance. The same difference of 100m, but within an altitude of 6000 m, gives an error of 0.85% for the distance.

It is easy to check the altitude for airfields: you only need to place a plane on the ranway of the targeted A/F, while you're designing your mission with the FMB, and read the altimeter. But it's more difficult to know the GL altitudes for other targets.
This other reason to bomb them from higher altitudes, to improve the chances to hit them.

- Don't use the Speed Bar data as inputs to set the bombisght.
The Speed Bar readings are less accurate than the gauges readings, with the sole excepcion of the Heading.
Speed Bar gives IAS and ALT in steps of 10. It means that you could read 230 km/h in the Speed Bar, but 239 km/h in your IAS-meter. What is true? The latter. For altitude, a difference of 10 m is negligible. But a difference of IAS = 9 km/h, it's not.

I hope these tips will help you to improve your enjoyment.

[FrankB]

Originally Posted by Soldier_Fortune

But... How did you got "0 m ground altitude and shores" in the Moscow map?

In the Moscow map the ground is mostly flat... but the average ground altitudes are about 400 @MSL. If you've considered too low altitudes in your calculations, it may explain why your bombs fell short.

It is a known thing that water in IL2 has to be at 0 altitude, hence my choice to put the targets adjacent to the water surfaces.
On the Moscow map it is not a seashore, more like a river bank, but the altitude is still 0 (landed on the target marker just to be sure.)

Originally Posted by Soldier_Fortune

- Don't use the Speed Bar data as inputs to set the bombisght.
The Speed Bar readings are less accurate than the gauges readings, with the sole excepcion of the Heading.
Speed Bar gives IAS and ALT in steps of 10. It means that you could read 230 km/h in the Speed Bar, but 239 km/h in your IAS-meter. What is true? The latter. For altitude, a difference of 10 m is negligible. But a difference of IAS = 9 km/h, it's not.

Yep, that is what I do since I fly mostly with speedbar off.
But one has to mention that the gauges are often unreadable - either blurry or the ticks are crammed together so sightly that it is hard to read exact value - that is why I chose SB-2 - as it was added recently, the gauges are more readable.

Originally Posted by Soldier_Fortune

I hope these tips will help you to improve your enjoyment.

Good tips there!

[Soldier_Fortune]

Hi again FrankB.

I was reviewing the equations, and it seems the Air density calculation is wrong.

It's rare, because they are the ISA (International Standard Atmosphere) ones. But it would be possible.

The formula I've posted has given you a TAS = 367 km/h, right?
I've calculated TAS = 340 km/h with an E6-B, and I confirmed this result with an app for Android which I have.

It would explain why your bombs fall short: calculated TAS is greater than the real, and therefore the calculated drop angle also is greater by about 2 or 3º. Finally this angle causes that you drop your bombs too soon, missing the target.

Please, use the following equations:

- Pressure at bombing altitude:

P = 101325*[(Tmap-0.0065*ALT) / Tmap]^5.25

- Air density at bombing altitude:

AD = 0.0036*[P/(Tmap-0.0065*ALT)]

The temperatures must be converted into Kelvin before they're used in the equations above (T+273).
The pressures are given in Pa (Pascals).

These equations are also from ISA and I've checked them right now.

For your data:

Tmap = -17 ºC = 256 K
T @ 2000 MSL = -30 ºC = 243 K
Po = 101325 Pa
P = 77071 Pa
AD = 1.142 kg/m3
AD_std = 1.225 kg/m3

Then:

TAS = IAS * SQRT(AD_std/AD) = 330 * SQRT(1.225/1.142) = 342 km/h

With this TAS your drop angle should be right.

Please: test these equations, and let me know how they work for you.

[FrankB]

Originally Posted by Soldier_Fortune

The formula I've posted has given you a TAS = 367 km/h, right?
I've calculated TAS = 340 km/h with an E6-B, and I confirmed this result with an app for Android which I have.

Right.

Originally Posted by Soldier_Fortune

- Air density at bombing altitude:

AD = 0.0036*[P/(Tmap-0.0065*ALT)]

While your formula works ingame, this does not seem to compute the correct density according to the ISA tables (at 11000m it should be 22632 Pa), so I used the source formula

AD = (P*M)/[R*(Tmap-0.0065*ALT)]

where R and M is universal gas constant and molar mass of dry air and that gives me the constant 0,00348, giving same results as ISA tables.

Originally Posted by Soldier_Fortune

Please: test these equations, and let me know how they work for you.

Your equations (and my slightly altered one) were spot on on both test scenarios as defined above.
I also tried to increase the altitude to 5000m as you suggested before, and at least one of the bombs was still hitting the target pattern. Great!

...until I switched from SB-2 to Ju-88 or B-25.
On both maps and both altitudes the bombs fell always short. Too short. Maybe 2-3 bombsight degrees.

[Jack_Aubrey]

Have you seen the new videos from 4.13 updates???? i think for us bombardiers it's a must seen .....

[Soldier_Fortune]

Originally Posted by FrankB

...until I switched from SB-2 to Ju-88 or B-25.
On both maps and both altitudes the bombs fell always short. Too short. Maybe 2-3 bombsight degrees.

This has nothing to do with the equations, but with the plane's attitude.

Both, B-25 and Ju-88, tend to sink more than other bombers. Therefore, when you engage the Level Stabilizer, automatically the plane will increase the Angle of Attack (AoA) trying too keep the altitude while damps the sinking.
This action avoids the bomber flies leveled, even if the level indicator and the variometer indicate that, and thus the bombisght is aiming to certain forward angle by default. You should substract this angle to that you've computed for a manual drop.

I've found that forward angle is about +3º when the Level Stabilizer is engaged and it is combined with fully trimmed elevator. For other elevator trimming, this angle would be a bit less... but I've not found an "universal" linear ratio for this matter. Too many variables are involved: TAS, altitude, payload, wind...
Therefore it's all about how well the player knows his plane's behavior at different speeds and altitudes under different external cirumstances, how well skilled he is to fly it using the proper trimming, and his jugdement about what is the actual AoA.