Gun Recoil Testing

[funked]

I tested a P-51D at Sea Level in MIL Power, offline with 10 x ammo multiplier.  I reached 355 mph level speed without the guns firing.  I then commenced firing the guns and continued to do so until the speed stopped decreasing.  The minimum speed was 285 mph.

I then calculated the average recoil force required to reduce the speed by this much, assuming the airplane had 1590 hp (V-1650-7 engine) with 80% prop efficiency.  The average recoil force required to reduce speed from 355 mph to 285 mph is 807 lb.  I can discuss these calcs if you wish but I fear it will bore most of the audience.      

Looking at the 6 guns in this plane, firing a 48.5 gram projectile at 870 m/s and 750 rounds/minute, I get an expected average recoil force of 711 lb.  This is less than what we are getting in the game!  However the 711 lb calculation includes only projectile momentum and not the momentum of the gases being ejected down the barrel.  It seems quite reasonable that the extra 807-711 = 96 lb of recoil force in the game is due to these gases.

Summary:  We have recoil (at least on the Mustang) and it appears to be VERY accurately modeled.


[This message has been edited by funked (edited 04-19-2000).]

[funked]

Some equations to use if you are interested in testing other planes:

Aircraft Performance Equation:

1.  Thrust - Recoil = Drag

2.  Drag = K * Speed ^2  (K is a proportionality constant, you have to solve for it using the thrust in the non-firing case)

3.  Thrust = Efficiency * Power / Speed

Put these together and you get:

4.  Recoil Force = (Efficiency * Power / Speed) - (K * Speed ^2)

Looking at the guns directly:

5.  Recoil = Number of Guns * Projectile Mass * Muzzle Velocity * Rate of Fire  (Average recoil force, neglects momentum of barrel gases)




[This message has been edited by funked (edited 04-19-2000).]

[Pongo]

Thanks funked...My only concern about recoil is how it would effect long range gunnery..
I dont think that barrel lift etc would effect accuracy at 250-450 but certainly the effectivness at d1.1 will be hampered? just an impression.

------------------
Pongo
The Wrecking Crew

[funked]

In the planes I fly, I notice the nose moving around quite a bit due to recoil, both in Yaw and Pitch, during firing.  The Yaw is also quite visible on the slip indicator, and the Pitch is particularly obvious if you go to an external view while you are firing.

Furthermore there is significant dispersion of rounds even if the nose doesn't move, I assume due to motion of the gun and its mounts relative to the aircraft.

I'm not sure there's a problem.  Maybe the dispersion effects should be stronger, but I have no way of comparing the game of real life in that area.

Just for kicks, take a Typhoon and try strafing a tank from a dive.  I generally get an area of shell strikes about 5 times the size of the tank from 1000 yds!


[This message has been edited by funked (edited 04-19-2000).]

[hitech]

Physics are fun. Btw funked don't forget the plane mass in your calcs. The 10% difference you are getting can be just testing the plane at different weights.

HiTech

[funked]

Well I did it at steady state (zero acceleration), so plane mass shouldn't matter.    

I think the 10% difference is probably due to my assumptions on prop efficiency.  Also I have seen different figures for hp, muzzle velocity, and rate of fire.  Finally, the gun forces don't act precisely on the thrust line.

[This message has been edited by funked (edited 04-19-2000).]