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[Wing Walker]

I have no mods, just the current version of BAT.

You can observe everything that I've reported here by flying in the QMB.

What I am talking about is the later P-47D's, some have an ammo load out option, some like the P-47M have just the one. 

If you hit TAB key to view your load out while in the P-47M, or other later P-47D's, the guns will be "Browning M2".

This "Browning M2" is what has the problem.

[WxTech]

In my BAT 4.0 game all P-47s (including the M) behave fine except for these two, although I do use slightly modded gun classes:
P-47D
P-47D-27

Those two crates are the hateful garbage that immediately enter a harmonic oscillation of period roughly 1 second, swaying in yaw by up to a gun sight ring diameter (5.7 degrees) when firing the guns. I refuse to believe real planes went into service with such a horrid deficiency. To do this, the guns would just about have to fire alternately like this; a 1/2 second burst from the L guns, followed by a 1/2 burst from the R guns, and so on.

With real guns symmetrically distributed and all firing at a rate of something like 12/sec, the net effect will be an averaged, uniform net recoil having no chance to impart yaw of any import.

How come such 6x .50 birds like the P-40, the Hellcat, the Mustang have not reportedly suffered from such a recoil-induced yaw instability in their development? And they're lighter planes, thus having less inertia against recoil.

I maintain that the P-47's yaw instability was not down to the guns as fully causative; there might be a partial contribution, but some other factor was the principal instigator. And a 5 degree (!!!) sway would be instantly rejected for service. Our deplorable representation of these two planes is surely much exaggerated.

Interestingly, earlier P-47's having no dorsal fin areal enlargement don't suffer this, where we would expect they should be consistently treated. This only makes those two 'swing birds' all the more regrettable, and thus rejected utterly for service in my game. 

[SAS~Storebror]

With real guns symmetrically distributed and all firing at a rate of something like 12/sec, the net effect will be an averaged, uniform net recoil having no chance to impart yaw of any import.

In reality though, they did not act like you'd want them to.

How come such 6x .50 birds like the P-40, the Hellcat, the Mustang have not reportedly suffered from such a recoil-induced yaw instability in their development? And they're lighter planes, thus having less inertia against recoil.

If weight was what solves such issues, a B-29 would fly without a keel.

I maintain that the P-47's yaw instability was not down to the guns as fully causative; there might be a partial contribution, but some other factor was the principal instigator.

Sure the guns are just contributors, that's why the tail sways while cruising, too.

a 5 degree (!!!) sway would be instantly rejected for service.

That's why field crews applied dorsal fins on their own behalf.

Our deplorable representation of these two planes is surely much exaggerated.

Nice opinion. Nothing more, nothing less.

Interestingly, earlier P-47's having no dorsal fin areal enlargement don't suffer this, where we would expect they should be consistently treated.

Earlier P-47s were Razorbacks.

This only makes those two 'swing birds' all the more regrettable, and thus rejected utterly for service in my game.

Nobody forces you to fly them.
Nobody keeps you from modifying their behaviour.
It's just that if you take away the sway, you would have to deal with me complaining about the utterly unhistorical absence of the sway just like you do about their current behaviour


Mike

[Wing Walker]

My version of BAT is 4.2.2 Hotfix4... the current up to date version I'm sure.

I did not have this problem with the guns until this latest update, or the one before.

I have to apologize, I thought it was different earlier, but in actuality ALL of the P-47 models accept the few that have an alternative gun load out, have the "Browning M2" guns as a load out that do not function right and sound like crap.

Nice opinion. Nothing more, nothing less

Yes, but the current representation of yaw is also an opinion and nothing more.  Right?

We have to use reason and probability to estimate the extremity of that affect in BAT.  I mean I assume you didn't have a manual that said the degree of yaw, or have a former P-47D pilot test fly the mod?

[SAS~Storebror]

I assume you didn't have a manual that said the degree of yaw, or have a former P-47D pilot test fly the mod?

The one who claims that something is wrong and needs a change is the one who provides evidence for his claim, not vice versa.


Mike

[Wing Walker]

You are right.

Who ever developed the P-47D yaw effect made the claim with their opinion.

Would appreciate some historical reference then for the degree of yaw the effect has on the '47.

[SAS~Storebror]

Who ever developed the P-47D yaw effect made the claim with their opinion.

It doesn't work like that.
The mod is there, you're the one who complains so you have to prove your point.
I don't need to prove you're wrong in the same way like I don't need to prove that every rivet, bold and oil splat on the P-47 in it's current shape is right.

You want me (or someone else) to change it, so you have to provide evidence of what you say is right.
Otherwise, you may feel free to do it yourself at any time.


Mike

[WxTech]

Let's go back to fundamentals with a first order assessment, taking the .50 cal bullet mass as 48.5g, the muzzle velocity as 870 m/s, the rate of fire as 12/s, and the plane mass as 5,000kg.

From the Newtonian law of reciprocity where mass, velocity and force are under consideration, the recoil velocity imparted upon the plane by the firing of a single bullet is 0.0084m/s (8.4mm/s), assuming all energy is distributed between the exiting bullet and the plane, and that the force vector passes through the plane's center of mass. A close enough approximation for our purposes.

In reality, of course, an offset gun, such as when placed in a wing, will induce a rotational component upon the plane. Countering this, the bilaterally symmetrical placement of guns all firing together impart a mean force whose vector lies in at least the plane's lateral axis of symmetry. By itself, then, for a six-gun P-47, P-40, P-51, F6F etc., proper firing of all guns by itself does not induce a yaw moment to any material degree. (Else why not other planes similarly armed and also significantly lighter not having been beset with the issue?) This could be demonstrated by a ground firing with the plane standing on on a frictionless and freely moving sled. There's no way the plane will neatly oscillate in yaw.

The total recoil velocity on our 5,000kg plane after firing x6 such guns after 1 and 10 seconds is 0.6 and 6 m/s, respectively. That's the firing of 72 and 720 slugs.

Let's look at the rate of change in velocity. Suppose an initial plane speed of 100m/s, or nearly 200kt. While firing, the plane will slow by 0.6m/s. And so after 1 second the plane has slowed down to 99.4m/s, a change of 0.6%. After 10 seconds, the speed has slowed to 94m/s, a drop of 6%. The faster the plane is moving, the smaller the rate of velocity decrease.

Note that were are simplifying things a little bit here in this first order examination. For one, these calculations omit the effect of changing drag operating on the plane as its velocity changes. As the plane slows there will be reduced drag, it scaling as the velocity squared. And so the actual slowing will be a little smaller than naively calculated here because at given motor power output, with slowing there results a growing power:drag surplus which in turn results in a compensatory force retarding the rate of slowing. In the realm of velocities we're looking at, this can be ignored, although technically we can consider this simple analysis as being the 'worst' case.

The crucial question we are wrestling with is this. We know that the mean recoil and hence slowing that is imparted by symmetrically arranged guns all barking together act in concert as an essentially averaged and constant force pushing back against the plane more or less on its longitudinal axis. Whence, then, the yaw instability? It must derive from some aerodynamic force whose impact becomes apparent with the change in velocity, perhaps in concert with the engine power output. The kernel of the question is the magnitude of this force.

As noted earlier, the faster the plane is traveling the smaller the relative rate of change in velocity while firing the guns. And a faster velocity induces a stronger weathercocking effect, making departures in yaw harder to effect. These complementary considerations would suggest that any induced yaw should scale, to some degree, inversely as the indicated airspeed. Our game's silly treatment seems to apply the same huge moment at all airspeeds.



On a side note. When reading pilot reports of the marked slowing down while firing guns in combat, some figures are surely either exaggerated or are the result of contributions by other forces. Such as a Spit or Hurri being slowed by 30 mph after firing 8 (or perhaps 12) .303 guns for a second or two. Induced drag in a high-G turn will be the dominant force here. For instance, x12 .303 guns firing 10g bullets at a rate of 15/s and from a 2,000kg plane, after 10 (!) seconds will impart a slowing of 7.5m/s or about 15kt.

[SAS~Storebror]

the force vector passes through the plane's center of mass. A close enough approximation for our purposes.

Sorry but no. This assumption is simply wrong.

In reality, of course, an offset gun, such as when placed in a wing, will induce a rotational component upon the plane.

Correct.

Countering this, the bilaterally symmetrical placement of guns all firing together impart a mean force whose vector lies in at least the plane's lateral axis of symmetry.

If you cumulate the force for a long enough time: Yes.
Momentarily (and that's what induces yaw oscillation): No.

proper firing of all guns by itself does not induce a yaw moment to any material degree.

Wishful thinking, but wrong.

Else why not other planes similarly armed and also significantly lighter not having been beset with the issue?

They do.
They all do.
The difference lies in the inherent lateral instability of bubble top P-47s which makes them susceptible to yaw oscillation.
Other planes have sufficient lateral stability to counter the induced yaw oscillation.
Early bubbletop P-47s didn't, that's why the dorsal fin got attached which restored this stability.

It's similar to breaking up early Jumo 004s: All jet engines suffer from vibrations to a certain degreen. All 004s do. Yet only early ones disintegrated, and it's been a tiny yet crucial change that solved the issue.

This could be demonstrated by a ground firing with the plane standing on on a frictionless and freely moving sled. There's no way the plane will neatly oscillate in yaw.

Wrong. It will.
If you look close enough at the final second of this video, you will see how the tail moves up and down and left an right, despite the plane being fixed to the ground both at the wings (right where the guns are) and at the tail.
It's easy enough to imagine how the plane would have jumped off the rack if it wasn't latched properly:

We know that the mean recoil and hence slowing that is imparted by symmetrically arranged guns all barking together act in concert as an essentially averaged and constant force pushing back against the plane more or less on its longitudinal axis.

Wrong.
This is misinterpreting physics and drawing exactly the opposite conclusion of reality.
We know that momentarily the force is not constant and is not symmetrical to the plane axis.

some aerodynamic force whose impact becomes apparent with the change in velocity

Almost right.
It's an aerodynamic force whose impact only applies to moving wings.
If you take a look at the above video again, at the time where the guns are all firing, watch the massive amount of exhaust gases from the guns.
Without firing guns, air moves smoothly across the wing surface.
The moment you start shooting, the airflow become airbitrarily interrupted where the guns are, and this happens in a totally random, non-symmetrical, unpredictable, distortet manner.
This is what causes most of the vibration and oscillation when firing the guns.
The recoil in comparison is just a rather small contributor to the issue.

The kernel of the question is the magnitude of this force.

Buy yourself a P-47 and measure it.
There's no other way I'm afraid.
Or simply trust those who flew the kite before, reported back yaw oscillation issues while firing guns and eventually caused the dorsal fin modification to be invented.
I'm not sure why some of us seem to think that people back then did this out of pure boredom...

a faster velocity induces a stronger weathercocking effect, making departures in yaw harder to effect.

Wrong again.
Weathercocking on an already yaw-instable plane simply causes it to lose it's weak lateral stability.
Your logic would indicate that an aircraft could fly without keel in heavy crosswinds, because it can't fully counter it anyway?

any induced yaw should scale, to some degree, inversely as the indicated airspeed.

Wrong again, ignoring the aerodynamic impact of exhaust gases from gun barrels.

Our game's silly treatment

*Cough* eh... what?

seems to apply the same huge moment at all airspeeds.

Granted, the effect should be more pronounced in a certain speed range, but since I didn't find sources stating at which speed the effect was most noticeable, I skipped this part.
In theory, the effect would start off small at zero speed (only recoil effect contributing here, with zero lateral stability of the plane due to absence of airflow over control surfaces).
The it would start to rise (distorted airflow from gun barrel gases over the wings) up to a point where the airflow over the tail surfaces would cause a larger stabilisation effect than the distorted airflow over the wings causes a destabilisation.
From there on, the effect would become smaller.
What's the critical speed? God knows.

When reading pilot reports of the marked slowing down while firing guns in combat, some figures are surely either exaggerated or are the result of contributions by other forces. Such as a Spit or Hurri being slowed by 30 mph after firing 8 (or perhaps 12) .303 guns for a second or two. Induced drag in a high-G turn will be the dominant force here.

What you're saying here is that pilots - and not just a few of them - told blatant lies, while at the same time you consequently ignore the fact that distorted airflow has a significantly different effect to surfaces compared to laminar of at least constant, smooth airflow.


Mike

[WxTech]

Mike,
You raise a good point about the expanding, emergent gases from the barrels, which I didn't consider here. As it would be for the rapid recoil's net effect under the ~12/sec firing rate on each gun, these gas bubbles bursting forth at 12 times per second per gun, 3 sources per side, will have a net averaging effect.

As to additional retardation of aircraft velocity that might arise from these expanding gas bubbles... The first order calculation of induced recoil already effectively accounts for all aspects of the phenomenon by assuming all energy goes into the bullet and the plane. That's why I termed it the 'worst' case scenario in terms of aircraft slowing. The expanding gases actually carry away some part of the energy by virtue of expanding into the surrounding environs. And on top of that, it could be argued that these bubbles, by interfering with the oncoming airflow, might further work against slowing the plane due to some small reduction in the ram pressure.

Instantaneous effect for each shot has inertia to overcome. If there was just one gun firing at, say, 2 shots/sec, there would be ample time for each impulse to have a distinct, individual impact. But a rapid-fire hammering away results in these individual impulses tending toward an averaging. This is very much like driving a car over a heavily corrugated dirt road. At a slow speed, the car responds to the slower periodicity of the hump impulses, and so the passengers suffer a lot of annoying up and down bounce. But get up to a certain speed, the wheels don't have time to descend fully into the troughs, instead more skimming across the crests. Moreover, the more rapid bouncing of the wheels has less time to impart the effect of each oscillation to the chassis through the suspension. The ride becomes a lot smoother.

A jackhammer makes another good example. Let the hammer rate drop much below nominal, and the operator will suffer terribly.

About weathercocking and its impact on directional constraint at higher speed. This is evidenced by the trend to the requirement for greater force to be applied on the rudder. If the P-47 does require that a harder rudder pedal force be applied for given deflection with increasing airspeed, this would buttress the case for the expectation of decreasing oscillation in yaw resulting from the lateral instability. If this were reversed, where application of rudder force decreased with airspeed, in conjunction with yaw instability this could raise concern of a 'runaway' condition where the oscillation would get into a destructive feedback.

I'm admittedly just about as far from being an aeronautical engineer as one can get. 😉 But I do try to apply basic physical principles in an effort to understand phenomena. I struggle to understand how a plane would ever have got past the first prototype if it suffered an alarming 5 degree lateral oscillation just by firing its guns. Five degrees is huge! It renders the airframe as a gun platform as largely useless. Except perhaps for novice pilots to spray a large area on the ground. 

Has an actual set of figures been published on the angular range of sway? Or is this just a guess based on imprecise descriptions?

[SAS~Storebror]

a rapid-fire hammering away results in these individual impulses tending toward an averaging

No. They tend towards a resonance effect.
That's why I've mentioned the Jumo 004 example.

it could be argued that these bubbles, by interfering with the oncoming airflow, might further work against slowing the plane due to some small reduction in the ram pressure.

Fine with me, still it doesn't do that in a concerted, symmetrical way just to please the pilot

tending toward an averaging

You consistently employ the average as a basis for your arguments.
Get that out of your head.
On average, all planets sit inside their sun. Yet they don't.
Tail sway isn't an average issue, it's a momentary one.
 

If the P-47 does require that a harder rudder pedal force be applied for given deflection with increasing airspeed, this would buttress the case for the expectation of decreasing oscillation in yaw resulting from the lateral instability

Your mixing up the force required for controlled surface deflection with the force being applied by a fixed surface while increasing it's area.

Guys, this is getting tiresome.
You want to be right in your assumption and blank out all real-life encounters.
The yaw-swing was real, pilots didn't just imagine it, and ground crews didn't field-mod dorsal fins on these airplanes out of pure boredom.

Yet if you think that physics are just for lamers, and you are above historical reality, so be it.
Mod it the way you love it.
There will be people who like it, no matter what, and others who don't like it, that's life.
It's quite like the fancy flight model modifications popping up every now and then, where all of a sudden Corsairs and Helldivers outturn and outrun Zekes - even that has it's fanbase.
Just do it.
I'm not standing in your way.
It's just that I won't do it for you, since I believe in physics and real life encounters.


Mike

[Dimlee]

Interesting discussion.

I just remembered that the production of Il-2 3M (Il-2-37) was cancelled after several months because of the poor synchronisation of NS-37 cannons. A pilot could make just one shot in one attack. Rapid fire was impossible.
(Yes, yes, it's a 700+ g shell...)

Good video, by the way. A picture is worth a thousand words, a moving picture is worth ten thousand?