Recoil

[HagarTheHorrible]

When you fired the guns, on the ground, in the original IL2 game the aircraft would roll backwards.  As of the present build, in this game, firing the guns has no similar effect.

 

I have no idea which is closer to real, but I was struck by a passage I've just read in a book about the effects of recoil on a Hurricane.  The pilot was Eric Carter and he joined 615 Sqn just after the BoB, fresh from training.  He describes his first flight, wonderfully, in a Hurricane, his first front line fighter and trying out the guns for the first time flying over the sea off Wales (Not at).

 

"The recoil from those eight machine guns the first time I fired hem was amazing and, although the gun itself absorbed some of it, it still had a marked effect on the Hurricane, slowing it down noticeably and making the nose sink.  That really focuses the mind when you're flying at just 100ft!"

[Matt]

There is a noticable recoil when firing the guns while flying. The nose goes up quite a bit, especially on the La-5 (not sure if it goes down when you have gunpods on the 109?).

Edited June 6, 2014 by Matt

[Finkeren]

Recoil seems nicely implemented in the air, not sure why it doesn't have any effect at all on the ground.

 

However, the effect in the old IL2 was ridiculous. There was no friction modelled for the tires, so the plane would glide around on the tarmac like an ice skater holding a running cooling fan. Definately better to have nothing at all than that bit of nonsense

 

And yes, I understand, that IL2 is 13 years old and was brilliant for its time, heck is still mighty entertaining today.

[HagarTheHorrible]

I expect the nose dipped when firing the guns because the convergence point was above the wing line.   I tried firing the guns in the G2, but apart from the shaking I couldn't discern any noticable slowing down of the aircraft.   Would a couple of cannon and machine guns have more or less of a recoil effect than eight, rifle calibre, machine guns, presumably firing at a greater rate ?

[Finkeren]

I expect the nose dipped when firing the guns because the convergence point was above the wing line.   I tried firing the guns in the G2, but apart from the shaking I couldn't discern any noticable slowing down of the aircraft.   Would a couple of cannon and machine guns have more or less of a recoil effect than eight, rifle calibre, machine guns, presumably firing at a greater rate ?

I don't have the time to do all of the calculations now, but with a firing rate of 1200 rpm of the Browning M2 and a muzzle velocity slightly better than that of the MG 151/20, I'll say the 8 light MGs of a Hurrri produces about the same recoil force as one slow firing (600 rpm) MG 151, maybe a bit more.

 

Remember that, all other things being equal, a doubling of the diameter of the projectile leads to an 800% increase in mass of the projectile and in turn recoil force. This means, that if you simply doubled the size of the Browning gun and its ammunition (which would still make it smaller than the MG 151) you'd have a gun that produced eight times as muc recoil force.

Edited June 6, 2014 by Finkeren

[69th_chuter]

The massive recoil of the unbraked 75mm gun in the B-25, according to my figurings (25000lb aircraft), would back that plane up 0.75 mph.  Just in case anyone is looking for those specific numbers.   

[CreepiJim]

Firing twin 3,7 BK rocks the plane on the ground. Tried to calcuate recoil forces yesterday, but lack some data (charge and charge velocity) to make a well-educated guess. The recoil force was huge, firing a 380gram AP projectile at over 1170m/s. The weight of the weapon pods helped, the mounting not so much.

[Holtzauge]

Interesting question  

I think you could work out a ballpark figure like this:

British 303 bullet data: Bullet 150 grain, power 46 grain and muzzle velicity 2700 fps.

Going Metric: Bullet 9.72 gram, powder 2.99 gram muzzle velocity 821 m/s

Assume Hurricane weight 3400 Kg and eight guns:

Law of conversation of momemtum: m1*v1=m2*v2

3400*v1=12.71/1000*821*8 (Where the 8 comes from the number of guns)

Gives v1=0.025 m/s or 0.09 Km/h per firing

If we assume a 3 s burst and rate of fire and 1400 round per minute gives 70 round per gun.

So this gives a speed reduction of 0.09*70=6.3 Km/h which would be noticable.....

Actually it is a bit more (probably around 10-20%) because the power gases leave the barrel at a higher velocity than the bullet but this gives a ballpark figure anyway...
 

[69th_chuter]

as the aircraft slows down propeller efficiency goes up so speed loss is not linear.

[Holtzauge]

You are right. The reduction will not be linear so what I posted is a bit on the high side.

 

However the effect comes not from a reduction in the prop efficiency but from the power being constant right? i.e. there will an increase in thrust due to a reduction in velocity in the equation Thrust=prop_eff*Power/velocity.

Edited June 7, 2014 by Holtzauge

[CreepiJim]

Even the A-10 would, theoretically, move backwards with full thrust. However, the time the guns fire is too short to stall the plane (it just slows it down).

 

This thread was about rolling backwards when firing. And as you can imagine, it can happen, because the aircraft has no speed, the roll friction is what restricts the aircraft from rolling with small to medium size calibers. However with 37mm and up it could happen, especially when firing multiple shots at some point the momentum would suffice to beat the roll friction.

 

Firing multiple times mid-air won't make you stall, but it will push you off-target when the gun is not in the center of gravity (nose mounted might not push you off, wing mounted will)