Can gun recoil really slow a fighter?

[drgondog]

Simple answer - Yes. The negative acceleration due to recoil is expressed from F=d/dt(m*v)

The 'system' during cannon firing is characterized by a rapid change of MASS (i.e cannon projectiles) leaving the 'system') with respect to time

 

[Shortround6]

The formula for recoil velocity (not energy) is bullet momentum ( bullet weight x bullet velocity) + propellant momentum (propellant weight x propellant velocity) divided by gun/aircraft weight.

Propellant velocity is about 1200 meters/sec (+ or - 10%) for most rifles, machineguns and automatic cannon.

IF I am doing the math right it would take 100 rounds of .50 cal ammo to slow a 4000kg aircraft down by just under 1.5 meters a second.

Or look at it this way. A .50 cal round has about 5 times the recoil impulse of a .308 round, but the 4000kg airplane weighs 1000 times what the M-14 rifle does.

 

[R Pope]

I have a pic somewhere of the Missouri firing a broadside. You can see a wave at the bow, where the recoil is driving the ship sideways at a noticeable speed. And that's a 60K ton ship! Somewhat bigger guns, too, but still....

 

[pattle]

 

[parsifal]

The physics certainly says that gun recoil could theoretically have an effect. Its basically the aircrafts engines are applying a certain force in one direction, that is forward, whilst the projectile, according to newtons laws, are applying a force in exactly the opposite direction.

whether the discharge of the weapons affects aircraft velocity in applied terms, is another, and interesting question. heres my school boy bash at it be advised my maths is terrible.....

Say an aircraft is travelling at 500kmh and weighs 1000 kg. We have to convert the airspeed to m/s.
500kmh = 500x1000/60x60
= 140m/s


Assume the armament is 4 x 40mm cannon (AFAIK nobody ever carried that much). Assume a projectile weight of 1.6 lbs (0.8 kg). Assume a MV of 4000m/s and a rof of 250rpm for each gun. Effectively for the entire armament we have an rof of 1000 rpm

The kinetic energy of the aircraft is equivalent to its moments of force, which is expressed as a vector

P=MV where P= Momentum, m= mass and V=velocity
= 1000x 140 m/s (approx)
= 140000 units of momentum

This measuremnt of momentum (which is a moment of force) is in the same direction as the direction of the aircraft (assume straight line flight).

Force generated by the discharging armament

P=MV
=(rof of a/c x projectile weight)xv
=(1000x0.8) x 4000
= 3,200,000 units of momentum (ne force) in the reverse.

On that basis the armament discharging will very quickly cause the aircraft to lose speed and go into a stall. To maintain some speed the armament needs to be reduced, and the aircraft needs to be in a dive with the engines at full throttle.

But there should be little doubt, the armament, if heavy eanough, will affect airspeed

When I refer to "moment of force", i certainly stuffed my description of what was happening. This is what I think....What I was intending was that there is no force being applied (assuming no effects from friction or the like) when a body is not changing its weight or velocity. A moving body whose velocity is not changing has momentum, but it has no force being applied to it. Force is mass x accelaration, since there is no accelarartion there is no force, and no Work being done (work - Force x Distance). I also think the aircraft altitude is potential energy.

However if the mass (ie the aircraft) starts to alter its velocity, that momentum starts to translate to Force, even if decelarating. a car crashing has force being applied to it, by its own loss of velocity.

To describe it as a moment of force is technically wrong, I know, but it sometimes is used to describe (wronly) the amount of force being applied over a short period, say less than a second.


The second thing wrong with what I did above is I got my energy momentum calcs and gun estimates totally screwed up. The aircraft momentum calculations are per sec (ie the velocity is metres per second), but the gun component.....the thing applying force in the reverse direction......Ive calculated with rof measured in rounds per minute. They should be rounds per second. ive also gotten the the guns MV wrong

Correcting these errors, the estimated effect by the discharging guns should be as follows

P=MV
=(rof of a/c sec-1 x projectile weight)x projectile velocities (this falls down as well, it will not be 4000m/s, more like 1500, as the projectile is still accelarating when it leaves the barrel and in any case wil never get to 4000)

=(1000x0.8/60 kg/sec) x 1500 (m/sec)
= 13.33 x 1500
= 20000 units of force

The aircraft was producing 140K units of momentum, which will be reduced by 20000 units of force each and every second the weapons are firing. Of course the engines are producing a certain amount of power each and every second. So whilst the guns are applying a significant negative effect on the aircrafts momentum, the engines are applying a positive effect. If the aircraft is diving, almost certainly this will act to also counterbalance the loss of momentum caused by the guns. you would have to say there is probably very little effect I think.

 

[silence]

That's what I get for trying to think at night. Shame on me!

I have a pic somewhere of the Missouri firing a broadside. You can see a wave at the bow, where the recoil is driving the ship sideways at a noticeable speed. And that's a 60K ton ship! Somewhat bigger guns, too, but still....

Here's an interesting short paper on that very topic: Do Battleships move sideways when they fire?

 

[syscom3]

The only aircraft that I have heard of that actually slowed down to a considerable degree was the B25 gunship. Having a dozen .50's, or the firing of the 75mm cannon would slow it up. But not to any big degree.

 

[fastmongrel]

Mosquito Tsetse 6 pounder. It certainly bounces about on the ground when firing.
View: https://www.youtube.com/watch?v=M00f5RxhxLY

 

[GregP]

According to the A-10 pilots I knew back in Arizona (they flew them from Tucson), the aircraft slows significantly when the GAU-8 is fired. They said they were limited in how many shots could be fired in a row before flameout occurred. The self-sustaining combustors are not oxygen-supplying items, they simply keep the igniters on. If you fly though enough gun gas, the engines will flame out. They train so this doesn't happen … but it certainly CAN.

Talking with former pilots and crew members who flew the B-25 gunship, I heard them state the plane could lose 15 - 20 mph when firing the big gun. Now memory can play tricks on you, but these memories must have been pretty vivid because when one former pilot said it, the rest immediately nodded and agreed. The original field modification airframe to create the gunship B-25 only fired about 20 shots from the 75 mm cannon before the airframe was scrap due to the recoil loosening the rivets! If that isn't significant force, I don't know what would be.

The later North American Aviation gunships had a good recoil absorption mechanism to reduce the impact on the airframe and crew. It perhaps was a different story, but the group I heard at the former Champlin Fighter Museum in Mesa, Arizona were from the first squadron to fly these field mods. They flew with Pappy Gunn, who helped invent the plane. One was Paul Cherry, who was on Pappy Gunn's crew. I can't remeber the names of the others. The only reason I remember Paul is I worked with him at Motorola in Scottsdale, Ariziona for some years.

 

[GrauGeist]

As mentioned before, the A-10 can't "flame out" it's engines from cannonfire because it's equipped with a Self-Sustaining Combustion Section in each engine. This was developed during production and retro-fitted into early models based on performance of the very early models.

The GAU-8 is designed with recoil in mind as well as offest mounting to compensate for the recoil.

The Cannon can be held to long burst but speed equates to time over target and a long burst isn't nessecary for several reasons. One being the aforementioned ToT and the other is overkill/waste. A short burst from the Cannon is all it takes to re-arrrange devestate the landscape/buildings/armor/small nations and is a waste of ammo otherwise.

Pilots report deacceleration is negligible and a moderate burst will register about a 3 knot braking in a shallow angle of attack.

 

[wuzak]

The GAU-8 is designed with recoil in mind as well as offest mounting to compensate for the recoil.

How does that work?

I was always under th eimpression that the gun was off centre to allow room for th enose landing gear....

 

[GrauGeist]

How does that work?

I was always under th eimpression that the gun was off centre to allow room for th enose landing gear....

From "A-10: Developement Description", authored by Greg Goebel:

Each barrel fires when it reaches roughly the 9 o'clock position, when viewed from the front of the plane. Because the gun's recoil forces could push the entire plane off target during firing, the weapon itself is mounted off-center in the other direction, toward the 3 o'clock position, so that the firing barrel lies directly on the aircraft's center line. The firing barrel also lies just below the aircraft's center of gravity, being bore sighted along a line 2 degrees below the aircraft's line of flight. This arrangement accurately centers the recoil forces, preventing changes in pitch and/or yaw when fired. This configuration also provides space for the front landing gear, which is mounted slightly off-center on the right side of the nose.

While the cannon's recoil itself isn't enough to significantly slow the aircraft while firing, it is enough to skew the aim.

 

[Shortround6]

Some other aircraft may exhibit slewing or dipping when the guns fire, whether this is confused with the guns 'slowing' the aircraft I don't know but may be a possibility.

Figuring out the recoil impulse of a gun or guns is relatively easy, as above. Projectile mass in grams times projectile velocity in meters per second plus propellant weight times 1200meters a second ( good enough for most guns). Divide by the aircraft mass in KG to get the rearward speed (or speed reduction) then multiply by number of shots.

 

[Aozora]

About 19:27 on - the B-25Gs don't seem to slow down while firing their 75mm and they "get the hell out of there"...

View: https://www.youtube.com/watch?v=DGjGJHrQxy8

 

[GregP]

Hi Wuzak,

The gun is mounted off-center so the barrel that is firing is exactly on centerline, helping to not throw off the aim. The A-10 absolutely CAN flame out its engines. To help prevent same, they keep the igniters on while firing (self-sustaining item), but if you fire long enough, it'll STILL flame out due to lack of oxygen ... unless you are suggesting they carry oxygen to inject, too. I can tell you they don't do that.

The many melted APU's are mute testament to flameout all by themselves.

 

[GrauGeist]

Ah well, thought it was pretty well covered, I guess not...

Hi Wuzak,

The gun is mounted off-center so the barrel that is firing is exactly on centerline, helping to not throw off the aim. The A-10 absolutely CAN flame out its engines. To help prevent same, they keep the igniters on while firing (self-sustaining item), but if you fire long enough, it'll STILL flame out due to lack of oxygen ... unless you are suggesting they carry oxygen to inject, too. I can tell you they don't do that.

The many melted APU's are mute testament to flameout all by themselves.

On 8 June 1978, an A-10 (73-1669) was lost due to engine fouling by secondary gases. The pilot had to eject because the engines cooled and wouldn't relight. This led to an investigation as to why it happened and thus the secondary gas problem was addressed. This was the only known A-10 to ever experience total failure resulting in a crash.

The initial fix:

The Battelle device was developed by Battelle Laboratories as a gas diverter to be fitted to the barrel muzzles of the GAU-8/A. It was a relatively cheap fix but it was soon found to contribute to unacceptable stress fractures of the airframe along various locations of the forward fuselage. As such, the Battelle device was dropped from consideration.

Instead, the engines were fitted with a system that maintained continuous ignition for every moment that the 30mm gun was to be fired and for a short time after the trigger was depressed. Additionally, maintenance requirements now stipulated that engines were to be washed once for every 1,000 rounds fired.

Never heard of an APU "melting" in an A-10 due to engine fouling via secondary gas, but I'd be more than happy to read about it if you have the publication handy.

* Forgot to add a bit of info! *
Regarding the incident on 8 June, this was gunnery testing and the excersize was to make 5 passes, firing 100 rounds per pass. The engines fouled on the 4th pass.
Also, Maj. Gideon Jr. ejected safely, but he ejected at 2,000 feet approx. and suffered injuries.

 

[GregP]

I don't have an article on the melted APU's but did manage to see a couple of them at Honeywell in Phoenix, AZ in the 1980's.

Let's say I don't buy what you're selling but, that's OK. It likely won't shorten or extend the lifetime of the A-10 anyway. Cheers.

Speaking of the lifetime of the A-10, I wonder what we will get to replace it? I'd sure hate to hear the F-35 because the A-10's fly around at low altitude and are a bit vulnerable for an aircraft with the sticker price of an F-35.

 

[CobberKane]

As has been stated, firing the guns WILL slow the fighter, so the question is by how much, and whether it's significant. I guess under the right circumstances, for example just above stall speed, its something the pilot would have to take into account. That said, there seems to be a disconnect between the pilot's seat of the pants impression and the actual slowing of the plane. I've read numerous accounts where pilots have described the sensation of firing the guns with terms like 'hitting a wall', where some quick notepad calculations demonstrate that the effect was actually negligible. On a similar note, I have heard an experienced pilot claim that he rolled a railway carriage over with only the impact of his P-47s guns, which is obviously impossible, and whereas the Thunderbolt is generally touted as the best diver of the war, comparisons with the P-51 seem to generally have that fighter out-diving it. I guess the moral is, be careful of truisms, no matter how well established. Or as Scotty would say "Ye canna change the laws 'o physics, captain!"

 

[GregP]

Hi Cobber,

Like I've said before, the real-life recollections of a former combat pilot must be weighed against experience. I have spoken with many former P-51 pilots who stated it was the best fighter in the world. Later, when I asked them how many other fighters thay had flown, the answer was many times, "none," so their estimation of "the best" would have to be taken with a grain of salt ... at least in my book.

However, it stands to reason by simple Physics that a plane in steady flight with a steady engin epower ... that suddenly experiences firing of the guns would probably experience some small negative acceleration related to the sum total of the muzzle energy of the weapons. It is also quite possible that a 1 - 3 second burst, while it WOULD generate retarding energy, might not be sufficient to materiialy affect the speed of the aircraft unless the burst were sustained awhile.

I have seen a statement about the GAU-8, the cannon in the A-10, that the recoil force was on the order of 10,000 pounds ... but the article didn't cite any reference for it. So, the accuracy of the statement is a roll of the dice ... I make no claims for it or against it.

I have only the statements of pilots, who SHOULD know whether or not they lost speed. Can't say for sure myself; I wasn't there flying an armed fixed-wing aircraft and discharging the armament. When I was in Vietnam, I never noticed the Huey sliding sideways when someone fired a fixed M-60 out the side. But I also wasn't exactly trying to look for it, either. I was hoping the bad guys weren't good shots. Since I'm here, they weren't that good at that exact point in time, though we DID take some hits.

Maybe the combat recollections of pilots would be a bit suspect, but firing in practice runs during training would be a different story with little or no stress of combat to deal with. They SHOULD know, but that is only my opinion. Others may well feel differently.

I like that movie Aozora posted, but since there were nine B-25G's, they were very probably factory North AMerican planes with recoil absorption mechanisms, not the early hard-mounted 75's. Those didn't last very long and the recoil was murderous, if one can believe the first-hand accounts of the pilots and crewmen who flew them.

If you can't believe them, then what WOULD be believable? Hopefully not some opinion from someone who has never flown a B-25, much less an armed one. In any case, it'll be tough to prove one way or the other, so I won't try.

Cheers.

 

[GrauGeist]

Let's say I don't buy what you're selling but, that's OK. It likely won't shorten or extend the lifetime of the A-10 anyway.

Sorry if fact isn't as exciting as opinion or speculation

Speaking of the lifetime of the A-10, I wonder what we will get to replace it? I'd sure hate to hear the F-35 because the A-10's fly around at low altitude and are a bit vulnerable for an aircraft with the sticker price of an F-35.

They would be stupid to shelve the A-10 as it's a ground support force-multiplier of the highest order and has proven it's value on the battlefield countless times. They can keep the B-52 in service well over 60 years, they can certainly keep the A-10 around for a while longer as well.