Nose dipping (not dripping) when guns fired

[HagarTheHorrible]

It doesn't seem to happen in all aircraft, but I was curious as to why it might be ?

 

I can understand why a WW II fighter might be effected, lots of powerful guns, usually below the line of pitch stability ( not sure what the correct term is, maybe line of thrust ?), I read about this in the memoirs of a Hurricane pilot, the first time he test fired his guns. I can't get my head around the aerodynamics of why it would effect WW I biplanes as much.  Is the power of two guns, or even one in the case of the Brisfit, enough to significantly dip the nose of the aircraft ?  Does the fact that the guns are above, or below the propeller make any difference and does it matter if the guns are below the wing that provides most lift but is still above, usually more so, the set of wings below the guns.  If my admittedly goldfish memory span is correct, I think one of the aircraft less effected was the SE5a, which obviously has weapons both above and below the top wing, not that the aerodynamics change if only one weapon is working.

[AndyJWest]

If the guns are below the centre of gravity vertically, recoil should make the nose dip. Simple physics, nothing to do with aerodynamics.

Edited February 12, 2020 by AndyJWest

[HagarTheHorrible]

is it all about CoG ?

 

The guns are firing and creating a force opposite to the thrust, its not an up or down force, but a force opposite to the direction of thrust.  The guns, in WW 1 kites tend to be very close to that line of thrust and being on top of the fuselage I would  have thought they were above the CoG anyway and the resultant force would tip the nose back, if anything. If the force being created, by firing is large, or some distance from the opposite force, thrust, then I could understand why it might effect an aircraft in that way.  If the force is very closely placed next to the force it is opposing I would have thought the most obvious reaction, in this case, would be loss of speed.

[76SQN-FatherTed]

Sorry Hagar, but are you talking about real-life observations, or something that happens in game?  I'm not particularly sensitive to FMs, but I have to admit I've not seen this phenomenon in FC.

[HagarTheHorrible]

Observations in game, unfortunately I don’t have access to flying the real thing, or indeed firing weapons from them either, mores the shame. ?

[AndyJWest]

15 hours ago, HagarTheHorrible said:

is it all about CoG ?

 

The guns are firing and creating a force opposite to the thrust, its not an up or down force, but a force opposite to the direction of thrust.  The guns, in WW 1 kites tend to be very close to that line of thrust and being on top of the fuselage I would  have thought they were above the CoG anyway and the resultant force would tip the nose back, if anything. If the force being created, by firing is large, or some distance from the opposite force, thrust, then I could understand why it might effect an aircraft in that way.  If the force is very closely placed next to the force it is opposing I would have thought the most obvious reaction, in this case, would be loss of speed.

 

The centre of gravity of an object is also its centre of mass (Or at least, it is in a uniform gravitational field, which is near enough true to make no practical difference here). Apply a force along a vector that doesn't pass through the centre of mass, and you will get a resultant torque - as you say, upward for most WWI gun layouts. On the other hand, In theory at least, recoil should slow the aircraft down, which might(for an aircraft with positive longitudinal stability) cause the nose to dip slightly. So the nose could go either way.

 

As for what happens in-game, I can't say I've ever noticed any such effect. For a Hurricane (the example in your first post), the guns are mounted a little below the centre of mass I suspect, and the recoil effects are probably going to be stronger, so maybe one effect or another is sufficient to be detectable in real life, though I'd be wary of taking a single anecdotal observation as gospel. People often think they detect effects merely because they expect to.