109 nose cannon

[trap78]

It's a chart that shows the BF109g6 with 20mm gondolas line of fire as it relates to the line of sight.

Okay cool thanks for the explanation. Then this graph tells us that a MG 151 round would drop about 100cm (39 inches) at 200 meters and about 180 cm (70 inches) at 300 meters. So my question is could the hub cannon be angled up enough to compensate for this trajectory. I'm guessing no but will have to verify this to be sure. The machine guns definitely had x-y adjustments for harmonization. I don't know a whole lot about the Revi sight but in the pictures I've seen there didn't appear to be an elevation adjustment.

[Motherland]

Okay cool thanks for the explanation. Then this graph tells us that a MG 151 round would drop about 100cm (39 inches) at 200 meters and about 180 cm (70 inches) at 300 meters. So my question is could the hub cannon be angled up enough to compensate for this trajectory. I'm guessing no but will have to verify this to be sure. The machine guns definitely had x-y adjustments for harmonization. I don't know a whole lot about the Revi sight but in the pictures I've seen there didn't appear to be an elevation adjustment.

Here's a quick-and-dirty translation of the important parts of the chart based on my spotty German abilities. It's not completely translated but I think the rest of it is fairly obvious or not really relevant to the discussion.



You can see that the MK108 30mm motorcannon not only is angled up high enough to intersect the sight line, it barrels over it and then drops back down. The MG151's are in wing gondolas and thus don't really matter for this discussion.

[Blooz]

Okay cool thanks for the explanation. Then this graph tells us that a MG 151 round would drop about 100cm (39 inches) at 200 meters and about 180 cm (70 inches) at 300 meters. So my question is could the hub cannon be angled up enough to compensate for this trajectory. I'm guessing no but will have to verify this to be sure. The machine guns definitely had x-y adjustments for harmonization. I don't know a whole lot about the Revi sight but in the pictures I've seen there didn't appear to be an elevation adjustment.

lol...you had it right two pages ago when you realized that it's the same as a hunting rifle with a telescopic sight. When sighting a rifle, you don't move the barrel to make the bullet intersect your line of sight, you move the sight to intersect your line of fire at a certain known range. That's what his chart is for. It gives the pilot the ability to know where the bullets are (high or low) for the different ammo types at different ranges. The amount of drop doesn't matter. What matters is how far the bullets are from the line of sight. In this case, at 300 meters, the bullets of all three calibers are about 60 centimeters (30 inches?) above the line of sight. So if a P51 was at 300 meters range the only need to put the pipper at the base of the Mustangs tail to get good hits with all weapons. If the P51 were at 400 meters, the 30mm and 13mm guns would hit with the pipper dead center of the target.

[Blooz]

If the 30mm was angled up the line of fire would curve up. No. The 30mm starts at 0 on the chart and the line of sight starts at about 70 cm above it (at the gunsight in the cockpit). The pilot is actually looking down through the lines of fire (shown by the line of sight line) and the bullets are arcing down (none go up) through the line of sight.

[Stoney]

(Image removed from quote.)

But we're still looking at a fixed convergence for the Mk108 at 400m, right?

[bustr]

80cm = 31.496 inches

Average person's eye is aprox 80cm up from the seat of his chair.

Revi is fixed at 80cm up level with your eye so you can look to infinity.

The Revi3, 12 and 16 series had a virtual reflected light ring the was equal to a 450mm dia iron ring at about 18 inches from your eye. Or on the reflector glass itself a ring about 45mm - 50mm in diameter 18 inches from your eye.

The piper is fixed on the Revi. Maybe its housing mounts could be shimed a bit but, the reflector gunsight is just a dot at your eyeline to infinity. In the included chart it looks like the pilots line of sight is at an angle down over the top edge of the engine to about 400 meters. Or if he aims using the lower stadia ticks on the vertical line of the Revi16b graticule the first tick down would be about 200 meters for the Mk108.

I've seen one of these charts for the FW and the sight line is level out while the MG and cannon are set to arch up to the sight line. With a Mk108 in the nose of a 109 it would seem all of the MG and 20mm are set to drop their rounds rapidly out of the barrel to try and harmonise with the very rapid drop of the 30mm. The sight line's down angle to 450 meters would indicate the pilot expects to start aiming with the lower stadia marks on the verticle graticule line.

The game seems to auto level the convergence sight line for us with the MK 108 in a hub mounted configuration rather than force us to remember to aim with the lower stadia marks. Unless they elevated the Revi in the 109 and fitted a Berlin phone book under the piolts kester to raise his eyeline over the nose of the plane while flattening the top of his head to fit up against the top of the canopy. Or the late war luftweenie's were all circus midgets.....

[bustr]

Does anyone have this chart but with a 20mm HUB cannon?

My thought being that the Revi and PBP russian graticules have a primary cross with stadia marks in both. Both are used with hub mounted cannon which look to not have much ability to be elevated from the engines datum line. This would mean shots past 250 meters would require aiming asistance in the form of bullet drop compensation marks. If so, HTC should have the HUB cannons fixed on the 109, 152 and Yak at about 250 meters and we learn to use the stadia marks.

Unless they were put in the graticules like some of us hang fuzzy dice from our rear view mirrors.

On one of the WW2 Warbirds forums I read a comment that 109 20mm hub cannons were fixed at 400 meters from the factory. But, thats in effect anecdotal information. The chart provided in this post shows the tragectory of the MG 151/20 dropping away rapidly at about 250 meters even if you were to superimpose its line up 40cm to the 0-datum line. You would still be aiming for the top of the rudder with the center of the graticule or placing the first stadia mark down dead center at 200-250 meters.

It would be an interesting addition in the hanger to have charts like this one generated when we adjust convergence that could be pulled up. Even have it show in real time above the aircraft and as you change the convergence the chart changes with it.

[morfiend]

The only point that of interest is the point at which the shell exits the prop spinner. The 50mm blast tube could be move in the x-y axis as could the MK108 as long as it was a straight line. The thrust line of the engine only sets the general line of the trajectory.

  Milo, I've read that the blast tube was 70 mm, either way having a larger blast tube might have allowed some positive/negitive adjustment at the mount. I've been looking for reference on this but it seems difficult to obtain.


  I hope to check with Mr.Baumer to see if he has any reference material on the subject,he has some excelent balistics and convernce settings for the FW.

     

[bustr]

From: Schliephake

The problem with fitting large guns in engine mountings was that there had to be enough space between the back of the engine and the pilot's seat to fit the gun breech and ammunition magazine. The gun barrel wasn't a problem - there was lots of length running from the back of the engine to the prop - as long as it was less than 70mm wide (the diameter of the tube running between the cylinder banks of the 109). The problem with the MK 103 was that there was a gas duct running alongside the barrel for part of its length, and this meant that the barrel couldn't fit in the tube. The MK 103M was redesigned to avoid the need for the gas duct, so the entire barrel could fit within the 70mm tube through the engine.

http://www.ww2aircraft.net/forum/weapons-systems-tech/luftwaffe-cannons-machineguns-topic-6368-13.html

If you account for barrel wall thickness to handel preassure and not rupture, I'm betting the 70mm allowence throught the DB605 cylanders does not allow alot of play if you want to keep a straight line to get the rounds out of the extension tube. A few pages back on the forum I linked is a MK108 with adaptor for mounting in a 109. Question would then be based on the adaptor.

Is the tube running between the cylanders fixed in place with no adjustment thus requiring the adapter to mate the MK108 barrel to the tube inline to the engine 0-datum line?

[nrshida]

Doesn't that information pertain to the MK103? I think that was only fitted to the Do 335 and was too big for the Bf109. The Mk108 was an APIB system and hence really stumpy. I think only a tiny piece of the barrel protruded into the cannon tunnel, unlike the Mg151/20.

Yup the tube / tunnel has to be fixed, look again at the gears fore and aft.

[nrshida]

[MiloMorai]

  Milo, I've read that the blast tube was 70 mm, either way having a larger blast tube might have allowed some positive/negitive adjustment at the mount. I've been looking for reference on this but it seems difficult to obtain.


  I hope to check with Mr.Baumer to see if he has any reference material on the subject,he has some excelent balistics and convernce settings for the FW.

     

It is quite possible it is 70mm as it was a looong time ago when told the size of the blast tube.

[morfiend]

 I checked with Mr Baumer and he said he doesnt have anything definitive on the subject,his TA152 manual doesnt even cover this!


   Milo,NP. I suffer from the same problem,I call it sometimers,you remember sometimes......



    

 PS: I never did find that reference to the JG that requested the gondies be removed from their K4's,  So we're even

[bustr]

I have a copy of Janes 1945/46 edition with a section on engines in the back. Dailmer Benz engines had hollow airscrew shafts with a square mounting flang on the back end of the shaft. If the engine was used in a multi engine mount or a none hub cannon mount a cover was bolted over it.

The flang and it's four bolt points make me think the cannon is solidly fixed along the 0-datum line of the engine so that there is no chance of the barrel rubbing on the inside of the rotating hollow shaft introducing unncessary vibrations to the engine or prop.

I'm not sure if the square flang rotates with the hollow shaft or the hollow shaft rotates inside of it via a race bearing of some type.

Half way down the page here is a picture of the MK108 with a DB605 shaft adaptor.

http://www.ww2aircraft.net/forum/weapons-systems-tech/luftwaffe-cannons-machineguns-topic-6368-7.html

[bustr]

Check the picture of this DB605 with motorkanon attached. I beleive the cannon is fixed to the 0-datum line of the DB605 via the hollow prop shaft so the convergence is fixed at the factory. Probably around 275-350 meters for a 20mm depending on ammo. You can calculate the drop for a 30mm based on the graph for the G6 in this thread. Probably 250 meters for the MK108.

http://historicaviationjournalandmarketrepo.blogspot.com/2010/12/for-sale-messerschmitt-bf-109-lake-find.html