Can gun recoil really slow a fighter?

[CobberKane]

This one comes up time and again in pilot combat reports; that recoil from the guns significantly slows a fighter aircraft. I'm not going to spoil the fun by doing the maths straight off, but to me this seems highly unlikely. I suspect the idea comes from seat of the pants impressions but doesn't hold up to the laws of physics, but if anyone can prove otherwise, go for it.

 

[vikingBerserker]

I certainly think there is the potential from the recoil. IMHO it depends on the number and type of guns firing versus the weight of the aircraft.

 

[CobberKane]

I certainly think there is the potential from the recoil. IMHO it depends on the number and type of guns firing versus the weight of the aircraft.

Absolutely. The weight of the projectile and it's muzzle velocity will give a figure for the recoil of each weapon upon each discharge. Rate of fire should then give us an idea of how much reactionary force is created per second of firing, and with reference to the mass of the aircraft we could then estimate how much the recoil of the guns would slow it over a given period.. Shouldn't be too difficult. Personally though, I'm about to head off fishing, so I'll check back later.

 

[turbo831]

Stars and Stripes article addressing this on the A-10 Does the A-10's gun slow the plane when fired? - The Rumor Doctor - Stripes

 

[syscom3]

I certainly think there is the potential from the recoil. IMHO it depends on the number and type of guns firing versus the weight of the aircraft.

You mean the energy state of the aircraft. If its going fast, the recoil has less of an effect.

 

[vikingBerserker]

If you are referring to speed in relation to mass then I would agree somewhat. I would think speed would certainly come into play but I would assume mass/weight would have a greater impact. The recoil of a 30mm would probably stop a paper airplane in flight going 100mph, but would have no effect on a 747 sitting still.

 

[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

 

[wuzak]

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

I don't follow that reasoning at all.

Firstly, kinetic energy is proportional to mass and the square of the velocity. Momentum is proportional to mass and velocity.

Momentum is also not a "moment of force".

 

[GregP]

According to a copuple of friends of mine who flew the A-10, the recoil of THAT gun slowed the aircraft considerably. Fortunately, you could not fire many rounds or the accumulation of gun gas in front of the plane would flame out the engines and the pilot would be in a panic to restart, usually melting the APU in the tail.

Whether or not the recoil of six .50 caliber MG for a short hurst would slow a Mustang is another story, but four 20 mm cannon would. The trick is to aim, get in a shooting solution, and fire a few rounds for effect ... and do it again. The trick is NOT to hose the ammo all over the sky, slow down and be happy with your ONE victory.

 

[Greyman]

The physics clearly points to this being the case.

I've also read many anecdotes where pilots felt the aircraft slow down when they fired, being pressed forward into their straps, etc.

I've even read of a few anecdotes of pilots in dogfights with enemy fighters and not being able to fire at their opponent due to being at stall speed, even though the enemy was right in their sights. This was always at extreme low level where they weren't going to risk it. Must have been insanely frustrating.

 

[Cave Tonitrum]

"significantly slows a fighter aircraft"

No.

 

[Greyman]

Depends what you mean by 'significant', I suppose.

Flying right on the edge of a stall in a turn just above the waves ... that'd be significant in my books.

 

[turbo831]

From Stars and Stripes article :

But while the A-10's endurance and firepower are legendary, the myth that its cannon drastically decelerates the aircraft is pure "Hawg-wash," said retired Air Force Col. Steve Ruehl.

"I have fired as many as 500 rounds in one trigger burst, that takes just about seven, eight seconds, and [it had] no impact on the air speed of the aircraft," said Ruhel, who has logged 3,500 hours flying A-10s.

It boils down to simple physics: Force = Mass x Acceleration.

"The bullets are only about three-quarters of a pound," Ruehl said. "The airplane, in a typical combat mission, is 40,000 pounds. When you got 40,000 pounds moving at 350 knots, that's a lot more [force] than a three-quarter pound bullet moving at 2,000 feet per second."

Does the A-10's gun slow the plane when fired? - The Rumor Doctor - Stripes

 

[GrauGeist]

The GAU-8/A in a full 5 second burst may slow the A-10 about 3 knots and the self-sustaining combustion modules in the engines engage when the A-10 fires it's weapon to prevent oxygen starvation.

 

[CobberKane]

The physics clearly points to this being the case.

I've also read many anecdotes where pilots felt the aircraft slow down when they fired, being pressed forward into their straps, etc.

I've even read of a few anecdotes of pilots in dogfights with enemy fighters and not being able to fire at their opponent due to being at stall speed, even though the enemy was right in their sights. This was always at extreme low level where they weren't going to risk it. Must have been insanely frustrating.

Absoultely no doubt that a fighter WILL slow when it fires it's guns - Newtons third law demands it. The question is, how much?
Here's my take on this with, say, a P-51D.
A .50 bullet weighs about 45g. I don't know how much the propellant in the cartridge weighs, but we have to take that into account as well because it's also being ejected from the muzzle, adding to recoil. I'll double the weight of the projectile to 90g to cover it.
At 750rpm the Mustang will be ejecting about 13 rounds plus propellant per gun per second, for a total ejected weight of about 7kg per second. To my thinking it would make no difference if the mustang fired seventy eight .50 cal rounds per second weighing 7kg total or one 7kg round; the equal and opposite force accelerating the fighter in the opposite direction would be the same over the course of one second.
The muzzle velocity of 7kg/sec of stuff coming out of the guns is about 750 mps. Therefore, if the Mustang could weigh exactly seven kilograms, and (for the sake of simplification) it fired a single 7kg projectile with a muzzle velocity of 750mps, the Mustang would go backwards at 325mps and the projectile would for forward at 325mps. But of course the mustang is much heavier than one second's worth of its firepower: at nine metric tons its about 1300 times heavier, in fact. So divide the 325 mps of force on Mustang by 1300 and we get .25 mps or 9kph – about five and a half mph. Ergo, each second of fire will take about five and a half miles per hour off the mustangs speed. That would be irrespective of whatever speed the Mustang was travelling at when it fired.
Is that a significant amount? Probably not if the Mustang is travelling at a fair clip to start with, but given that a five second burst would slow the plane by nearly 30mph, I can see how a fighter would be pushed over the edge into a stall during a low speed dogfight, or if the wings were at a high angle of attack.

 

[bob44]

I remember a watching documentary years ago, a P47 pilot said firing the 8 50cals would slow the plane down considerably (cannot remember the exact words).
I remember I was very impressed by how much it did slow down.

 

[silence]

Absoultely no doubt that a fighter WILL slow when it fires it's guns - Newtons third law demands it. The question is, how much?
Here's my take on this with, say, a P-51D.
A .50 bullet weighs about 45g. I don't know how much the propellant in the cartridge weighs, but we have to take that into account as well because it's also being ejected from the muzzle, adding to recoil. I'll double the weight of the projectile to 90g to cover it.
At 750rpm the Mustang will be ejecting about 13 rounds plus propellant per gun per second, for a total ejected weight of about 7kg per second. To my thinking it would make no difference if the mustang fired seventy eight .50 cal rounds per second weighing 7kg total or one 7kg round; the equal and opposite force accelerating the fighter in the opposite direction would be the same over the course of one second.
The muzzle velocity of 7kg/sec of stuff coming out of the guns is about 750 mps. Therefore, if the Mustang could weigh exactly seven kilograms, and (for the sake of simplification) it fired a single 7kg projectile with a muzzle velocity of 750mps, the Mustang would go backwards at 325mps and the projectile would for forward at 325mps. But of course the mustang is much heavier than one second's worth of its firepower: at nine metric tons its about 1300 times heavier, in fact. So divide the 325 mps of force on Mustang by 1300 and we get .25 mps or 9kph – about five and a half mph. Ergo, each second of fire will take about five and a half miles per hour off the mustangs speed. That would be irrespective of whatever speed the Mustang was travelling at when it fired.
Is that a significant amount? Probably not if the Mustang is travelling at a fair clip to start with, but given that a five second burst would slow the plane by nearly 30mph, I can see how a fighter would be pushed over the edge into a stall during a low speed dogfight, or if the wings were at a high angle of attack.

Not too sure about the propellant being ejected: its being burned and converted into energy to drive both the projectile and the recoil. These may balance out, or one may mitigate the other to a certain degree. Hmmm... been too long since I studied dynamic mechanics. I'm not even sure I'm making sense here. Wouldn't be the first time.

I shouldn't have read this thread: now my brain will be mulling this over whether I want it to or not. Hope someone can clear this up for me so I don't lose too much sleep!

 

[CobberKane]

Not too sure about the propellant being ejected: its being burned and converted into energy to drive both the projectile and the recoil. !

A chemical reaction such as the burning of the propellant in a cartridge does not destroy matter, i.e. the propellant is not 'converted' into energy. The mass of the gases produced by combustion in the cartridge would be exactly the same as the mass of the propellant before ignition, and as these gases are ejected they would contribute to recoil accordingly.
Incidentally, I don't know how much energy would be released to the propellant in a .50 cal round WERE fully converted to energy, but you are talking about the equivalent of a number of decent hydrogen bombs. We might need to make the breach and barrel a bit heavier.

 

[pattle]

 

[stona]

its being burned and converted into energy to drive both the projectile and the recoil.

It is being largely converted into gas (albeit by an exothermic process) not energy, otherwise each cartridge would go off like a nuclear explosion All the laws of conservation still apply. The problem becomes somewhat tricky as not all the gas and other combustion products are ejected forwards, the action of the gun is driven by a component for example.

The answer to the original question must be yes, as already noted, as not even the USAAF cannot defy the laws of physics.

Cheers

Steve