Mk108 & MG151/20, two questions for Pyro and gun experts

[MANDOBLE]

Firing wing ROOT mounted MG151/20 produce a noticeable yaw side to side effect in the plane. Is that correct?

Firing Mk108 from a landed 262 I noticed a very pronounced shake in the plane, that means very poor aiming chances in flight.
Mk108 was famous for having no-recoil effect. Is that wrong?

[Karnak]

Mandoble,

The "cockpit shake" is preset.  All guns have exactly the same recoil effects in AH.  I agree that it should vary based on the gun(s) being fired.

Take 4 20mm cannon on the Fw190A-5 and fire just the cannon, note the shake.  Now fire just the 7.92mm guns.  Its the same shake.  This is true on all aircraft.  The 4 .303s on the Mossie shake it just as much as the 4 20mms. The single 7.7mm on the A6M5b shakes it just as much as the 2 20mms.

The plane itself is not bouncing, just the head position.

[MANDOBLE]

My point is that if wing root mounted guns do not produce side to side movements nor shakes, and if Mk108 had no recoil effect, then those head shaking movements should not be present disturbing the aiming. In the other hand, IMO, these are real yaw movements instead just head bounces. If not, In any case, where is the cause of my head bouncing like that?

In the first case, outer 151/20 should have much more dispersion/shake than wing root ones. In the second case (Mk108) dispersion should be mainly due ballistics ahead 300 yards than just due gun shake.

[Raubvogel]

My favorite is firing the 7.92mm on the Ju88 and having the plane shake.

[Vermillion]

Where did you get the idea that the Mk108 had no recoil effect?

Its basic physics guys.  

For every action there is a reaction.

Unless your using some sort of "recoiless" cannon, which isn't really recoiless but you get the idea, your gonna have recoil.  Muzzle brakes and such designs can minimize "recoil effects", but thats just redirecting some of the forces.

In general the more powerful the cannon, the more recoil your going to get.

[MANDOBLE]

Verm, read, for example that:


http://visi.net/~djohnson/armament/mk108.html

An interesting feature was that neither the barrel or receiver moved in recoil, the entire force of firing was absorbed by the rearward movement of the bolt against the driving springs, which buffered against the recoil.

It that is not enough for you, Mk108 was anything but a powerful cannon, their mine shells where powerful, the cannon not.

Here, in AH, the shake effect firing these recoiless guns is very very noticeable.

[pbirmingham]

(Stupid American flubs up his SI units.  Original conversion of Newtons to pounds was low by a factor of 2.2)

Originally posted by MANDOBLE

It that is not enough for you, Mk108 was anything but a powerful cannon, their mine shells where powerful, the cannon not.

Here, in AH, the shake effect firing these recoiless guns is very very noticeable.

You're reading too much into Tiff's description of the freely-moving bolt.  The gun wasn't recoilless; what happened was that instead of the bolt being locked in the breech as with most guns, the bolt flew backwards against a spring.  This turned the sharp shock of a normal recoil into more of a push, but it was still quite a push -- the 312 gram shell flew at 550 m/s, giving it a momentum of 157.6 kg-m/s.  To stop the bolt in the 1/10 second of a firing cycle takes 1576N of force on the average -- about 354 pounds.  The spring exerts this force on the gun, which will push back against the airframe.  Consider that you didn't have the whole firing cycle available, and the force becomes (1) greater and (2) less continuous.

Also, while the MK108's shell was quite slow, it was also very heavy.  It had as a result quite high momentum and, therefore, kick -- more than the Hispano Mk V, whose 840 m/s, 130 gram shell had 109 kg-m/s of momentum.  In the areas relevant to this discussion, the MK108 was powerful indeed.

(gun data courtesy of Emmanuel Gustin at The Fighter Gun Debate )

[GRUNHERZ]

The Mk108 in AH is great, nothing to complain about here.

[Kweassa]

So can we assume that this thread is a request for different level of gun recoil according to planes?

[MANDOBLE]

pbirmingham, what I've read is just that the ENTIRE recoil force was absorbed by the springs. That is, that force affects inmediately the initial part of the spring, and the spring compression will absorb all the impact, supposedly, in the interval between shots. If the final effect is, like you say, a small push, it may produce a minimal "brake" effect, but not a tremendous shake.

Pick up a 262, and, from the runaway, fire the four guns and check the effect ...

[hitech]

MANDOBLE, What absorbed the force on the other side of the spring?

HiTech

[pbirmingham]

Originally posted by MANDOBLE
pbirmingham, what I've read is just that the ENTIRE recoil force was absorbed by the springs. That is, that force affects inmediately the initial part of the spring, and the spring compression will absorb all the impact, supposedly, in the interval between shots. If the final effect is, like you say, a small push, it may produce a minimal "brake" effect, but not a tremendous shake.

Pick up a 262, and, from the runaway, fire the four guns and check the effect ...

It's not a [I[small{/I] push.  For four MK108s, the total force from recoil (unrealistically assuming that the force is constant over the entire firing cycle, which it won't be,) is 1415 pounds.  Compare it to the thrust of the Junkers Jumo 004 at 1986 pounds each  and you see that this "small" push is nearly half the available thrust on the airplane.

Actually, the force on the spring is not even -- it increases as the spring is compressed by the bolt, and as HiTech and I are telling you, ultimately that force is transmitted to the airframe.

Consider that according to this account of the He 162  the Salamander was equipped with MG 151/20 cannons because the recoil of the originally-planned MK108 was too great (look under "Prototypes.")  Obviously this gun taxed the airframe greatly.

This was not a recoilless gun.  It exerted forces on the Me262 that were significant relative to the other forces on the plane.  While I don't know what a realistic amount of  shake is (I've only seen one 262, at the Deutsches Museum.  They didn't let me fly it, or even shoot the guns on the ground.  Bastards.) I don't think any of us have enough information to say that it's too much now.

[MANDOBLE]

Hitech and Pbirmingham, the working principle is just like a modern pneumatic "ground flatter" or pneumatic hammer. The device is applying tremendous forces able to break big rocks while being manned confortably by a single human worker. These modern devices have a ratio of hit similar to the Mk108 or even higher. Basically, the spring system is converting the recoil shock into more or less smooth push. The "sinusoidal push" graph would not cause the shake effect of the "digital violent hit" graph. In the other hand, these pushes go directly towards the GC of the plane from the nose, paralel to the flying course, so there is no explanations for lateral neither vertical bouncing shakes. IMO, the effect of a Mk108 burst should be as a constant and soft brake, not just like hitting an asteroids field.

[pbirmingham]

Originally posted by MANDOBLE
Hitech and Pbirmingham, the working principle is just like a modern pneumatic "ground flatter" or pneumatic hammer. The device is applying tremendous forces able to break big rocks while being manned confortably by a single human worker. These modern devices have a ratio of hit similar to the Mk108 or even higher. Basically, the spring system is converting the recoil shock into more or less smooth push. The "sinusoidal push" graph would not cause the shake effect of the "digital violent hit" graph. In the other hand, these pushes go directly towards the GC of the plane from the nose, paralel to the flying course, so there is no explanations for lateral neither vertical bouncing shakes. IMO, the effect of a Mk108 burst should be as a constant and soft brake, not just like hitting an asteroids field.

If your opinion were correct, why did the Heinkel engineers decide that the He 162 airframe could not handle the recoil of two MK108s?  From the above-cited account:

The V2 was the prototype for the A-1 variant and was fitted with the two 30mm MK108 cannons, and it became clear that even with these low velocity guns the recoil was too much for the plywood nose area to handle.

  In other words, they didn;t use the MK108 because it would tear up the plane.

Your pneumatic hammer (jackhammer) analogy is an interesting one, but flawed.  The problem with it is that the forces involved are MUCH greater when you impart 157 kg-m/s momentum to ten shells a second.  This takes an average force, for four guns, of three quarters of a TON.

Breaking rocks, by comparison, is much easier.  A person can do it with a twelve-pound hammer -- the jackhammer is used because it can strike more frequently, doesn't use the worker's muscle power, and often is designed so that the part that hits the rocks is pointed, increasing the pressure at the point of contact so less force is needed.   So, I dispute any implication that the forces are anywhere near equal.

Finally, do not forget that the jackhammer's own weight assists the operator in holding it down.  In watching the construction on Wacker Drive here in Chicago, I've never seen a jackhammer being operated in a position far from the vertical, with the exception of the big crane-mounted ones they use for the serious rock-breaking.

[Vermillion]

It just doesn't work like that Mandoble.

True, the spring would work as a dampener, but it would not have the effect your describing.

For it to work that way,  each gun would have to be precisely located off the aircraft centerline in relation to one another to counteract the other gun, and then each gun would have to fire precisely at the same moment as the counteracting weapon, and the aircraft has to be perfectly homogenous and perfectly balanced.  

I'm talking nuclear weapons grade machine work, and weapon timing firing mechanism.  And THEN each gun would have to have the exact same amount of powder, exact same shell weight, and be chemically identical to the other shells powder to something on the order of 99.999999 %

Take a basic Mechanical Engineering Statics course (no not statistics, statics ) and you will rapidly see that what your describing is literally impossible.