Attention all ballisticians

I find it interesting that the discussion has turned to disagreement on the proper way to describe how asymmetrical airfoiils create lift.

Just to shift things a bit, don't you think that a bullet looks more like a symetrical airfoil?

Leading questions: How does a symmetrical airfoil generate lift? Also, typically, in an airplane, where is the center of pressure located relative to the center of mass, and how does that compare to their relative positions in a bullet as is moves between muzzle and target?
 
Boyd Allen said:
I find it interesting that the discussion has turned to disagreement on the proper way to describe how asymmetrical airfoiils create lift.

Just to shift things a bit, don't you think that a bullet looks more like a symetrical airfoil?

Leading questions: How does a symmetrical airfoil generate lift? Also, typically, in an airplane, where is the center of pressure located relative to the center of mass, and how does that compare to their relative positions in a bullet as is moves between muzzle and target?
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Good point thats why I'd mentioned sail boats. The Boat tail bullet got its name due to the resemblence and the ogive of a bullet can produce a similar effect to the combination of clipper bow and eliptical hull.
The eliptical or laminar flow wing also drew on the principle of the eliptical hull.

Symetrical airfoils require a higher angle of attack, asymetrical airfoils can generate lift at zero angle of attack.
Both generate the same low pressure zone.
The accuracy or inaccuracy of the long path theory is not relevant.
The Bernoule principle remains intact.


PS
The "language of appearances" which I demonstrated with the sunrize analogy holds with practically ever descrioption of an airfoil I've seen.
When you are in flight air doesn't move around you, instead you move through the air. Outside forces aren't whats in play, its the reaction of the air to the aircrafts application of force in the form of thrust.
 
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A little progress, maybe

Old Gunner said:
I said the atmosphere not the localized pressures on the aircraft's or airfoils skin. The following sentence should have made that clear enough, but perhaps my choice of words was not optimal.
The air pressure of the atmosphere remains the same everywhere around the aircraft other than in the low pressure zone above the wing. The increased impact of the aircarft to still air does not change the qualities of earth's atmosphere regardless of localized effects.
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Fine, then atmospheric pressure is constant around the entire plane. It is not different above the wing. That is a local effect, just like the positive pressure under the wing.


The air pressure on the under surface can not lift the aircraft against an equal amount of air pressure on the upper surface.
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Correct.

Without the low pressure zone there is no lift.
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Still wrong. If the pressure were atmospheric on the top, then the positive pressure on the bottom would still create lift equal to the integral of the vertical component of the pressure over the bottom surface area. Simple Newtonian physics.

Keith
 
Boyd Allen said:
I find it interesting that the discussion has turned to disagreement on the proper way to describe how asymmetrical airfoiils create lift.

Just to shift things a bit, don't you think that a bullet looks more like a symetrical airfoil?

Leading questions: How does a symmetrical airfoil generate lift? Also, typically, in an airplane, where is the center of pressure located relative to the center of mass, and how does that compare to their relative positions in a bullet as is moves between muzzle and target?
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Yes, a bullet is symmetrical, so it needs an angle of attack to generate lift. That's why I was bugging Bryan about calculating pitch. A right-hand spin stabilized body responds to an upward lift forward of its CG by turning right. (Wrap the fingers of your right hand around the bullet in the direction of spin. Your extended thumb will point in the downrange direction. Extend your index finger and move your hand so that your index finger points upward in the direction of lift. Now extend your second finger at 90 degrees to your index finger. It will point to the right, the direction that the yaw of repose. The same right-hand rule applies for wind forces from other directions. Just point your index finger in the direction of the force and your second finger will point in the direction of the deflection.) There has to be some pitch for there to be a pitching moment to cause the gyroscopic response. But as Bryan's 6DOF simulations show, it is really small and negligible for all but questions of why drift occurs.

For a flat plate (the simplest of symmetrical airfoils) in subsonic flow, the center of pressure can be found (with a potential flow solution) to be one quarter of the way back from the leading edge, independent of angle of attack. For real subsonic symmetric and asymmetric airfoils, CP is usually close to the same position. For a supersonic flat plate, CP is halfway back, again independent of angle of attack.

An important point to keep in mind is that while Bernoulli's equation helps explain why a subsonic airfoil works, it is not appropriate for supersonic airfoils. Bernoulli's equation is valid for constant density, but density changes significantly across the shock waves around a supersonic airfoil or bullet.

Cheers,
Keith
 
Mornin', guys and gals,

As a lifelong flier of one thing or another, I have often heard the question, "What makes airplanes fly; is it the lift on top of the wing or the pressure from below?" :eek:

After years of study and first hand experience, I finally figured it out. :cool: Yep, I can tell you exactly what makes airplanes fly; MONEY! and lots of it! :D

Gene Beggs
 
Can we say POWER

Gene Beggs said:
As a lifelong flier of one thing or another, I have often heard the question, "What makes airplanes fly; is it the lift on top of the wing or the pressure from below?" :eek:

After years of study and first hand experience, I finally figured it out. :cool: Yep, I can tell you exactly what makes airplanes fly; MONEY! and lots of it! :D

Gene Beggs
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Big POWER will cost you lots of money indeed
 
not just planes

I learned long ago that if it flies, floats or flirts it is cheaper to rent it! ;)
 
Good website

Boyd Allen said:
OK, look at this page http://www.nennstiel-ruprecht.de/bullfly/fig5.htm
and the two that it is linked to. Now, what is the author saying about the bullet nose into/with the wind issue? Slow down, and go over it carefully...a couple of times.
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Boyd,
By reading those three pages only, it would be easy to be confused about how a bullet responds to wind on its side. These pages explain that the nose of the bullet is blown downwind, which is pretty intuitive, but this is only the beginning of the story. If you follow the rest of the explanation on subsequent pages, you find that the overturning moment causes the bullet nose to deflect in the direction of the moment (see the right-hand rule that I tried to describe in post #144). That deflection changes the direction of the wind force on the bullet, which changes the direction of the moment, which changes the direction of deflection, and so on. The bullet chases its tail, so to speak, in the process we know as precession. There remains a very small average angle of deflection in the downwind direction, even as the bullet is precessing.

Cheers,
Keith
 
mks said:
Fine, then atmospheric pressure is constant around the entire plane. It is not different above the wing. That is a local effect, just like the positive pressure under the wing.
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Not entirely the same thing. any compression of the air is a positive action on the air locally, the lowered pressure area doesn't compress its an area where a part of the atmosphere is in a sense removed from the equation, though not physically removed, its just an area where the atmospheric pressure can not be maintained, A negative effect creating a positive action.








Still wrong. If the pressure were atmospheric on the top, then the positive pressure on the bottom would still create lift equal to the integral of the vertical component of the pressure over the bottom surface area. Simple Newtonian physics.

Keith
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How do you intend to create a sustainable increase in pressure on the bottom surface while isolating the upper surface at normal pressure?
It can work easily enough with enclosed structures such as a building in a storm, where drops in pressure can blow open windows and doors or great increases can cave in structures, but the aircraft is open to the atmospshere on all sides.
 
mks said:
An important point to keep in mind is that while Bernoulli's equation helps explain why a subsonic airfoil works, it is not appropriate for supersonic airfoils. Bernoulli's equation is valid for constant density, but density changes significantly across the shock waves around a supersonic airfoil or bullet.

Cheers,
Keith
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True, in fact I've read of supersonic airfoils which have more positive camber on the lower surface than on top, it has something to do with trans sonic flight, but I didn't look into it since light aircraft are all I was interested in at the time.
 
Keith,
The point that I was making was about which way the bullet points in response to a crosswind, nothing beyond that. There is more, but it is that one point that I was alluding to. Does he contradict himself a some later point, or is he progressing from that to the mechanism by which the bullet is moved by the wind? The reason for my post was the disagreement about that one point. It seems that Sierra has abandoned their previous position, but this fellow seems to believe that bullets point in a slightly downwind direction, based on the relative positions of a bullet's center of pressure and center of mass. I believe that many of the posters that have taken the opposite position, that bullets point slightly into the wind, have failed neglected this point entirely, and in some cases made comparisons in which the chosen object has these centers in the opposite order of a spin stabilized bullet.
 
Which way is the bullet nose pointing in a crosswind? I really don't care. What I care about is where it hits on the target paper.

So, lets think about the hole in the paper. Those of you that can see round holes (I can't) look at the individual holes like Tony Boyer and many other top shooters do. Those guys can evaluate some tune problems by looking to see if the hole is round and where the bullet nose struck. They are looking at yaw.

In an out of tune yaw situation, not all the individual holes will have the nose impacting in the same direction. But, if the gun was in tune, and was shot in a strong crosswind, would not all the bullets impact the paper with the nose pointing upwind if the bullet does, in fact, try to steer into the wind???
 
Boyd Allen said:
Does he contradict himself a some later point, or is he progressing from that to the mechanism by which the bullet is moved by the wind?
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Yes to the latter. He is telling the story one step at a time. Wind does INITIALLY push the nose downwind, which is the direction one would expect. But the gyroscopic effect, through stabilizing precession, quickly leads to the nose pointing toward the wind. Bryan's post #103 shows a nice example in which the nose is blown off by a full degree by a wind switch at 500 yds, but is stabilized back to a tenth in 100 yds or so.

But even that is not the complete story. There remains a very small deflection in the downwind direction that is a remnant of the original crosswind force, and/or due to the increasing influence of the crosswind relative to the downrange velocity as the bullet decelerates.

Cheers,
Keith
 
Found this
The Magnus effect can also be found in advanced external ballistics. Firstly, a spinning bullet in flight is often subject to a crosswind, which can be simplified as blowing either from the left or the right. In addition to this, even in completely calm air a bullet experiences a small sideways wind component due to its yawing motion. This yawing motion along the bullet's flight path means that the nose of the bullet is pointing in a slightly different direction from the direction in which the bullet is traveling. In other words, the bullet is "skidding" sideways at any given moment, and thus it experiences a small sideways wind component in addition to any crosswind component. (yaw of repose)

The combined sideways wind component of these two effects causes a Magnus force to act on the bullet, which is perpendicular both to the direction the bullet is pointing and the combined sideways wind. In a very simple case where we ignore various complicating factors, the Magnus force from the crosswind would cause an upward or downward force to act on the spinning bullet (depending on the left or right wind and rotation), causing an observable deflection in the bullet's flight path up or down, thus changing the point of impact.

Overall, the effect of the Magnus force on a bullet's flight path itself is usually insignificant compared to other forces such as aerodynamic drag. However, it greatly affects the bullet's stability, which in turn effects the amount of drag, how the bullet behaves upon impact, and many other factors. The stability of the bullet is impacted because the Magnus effect acts on the bullet's center of pressure instead of its center of gravity. This means that it affects the yaw angle of the bullet: it tends to twist the bullet along its flight path, either towards the axis of flight (decreasing the yaw thus stabilizing the bullet) or away from the axis of flight (increasing the yaw thus destabilizing the bullet). The critical factor is the location of the center of pressure, which depends on the flowfield structure, which in turn depends mainly on the bullet's speed (supersonic or subsonic), but also the shape, air density and surface features. If the center of pressure is ahead of the center of gravity, the effect is destabilizing, if the center of pressure is behind the center of gravity, the effect is stabilizing.
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[edit] History
German physicist Heinrich Magnus described the effect in 1852.[2] However, in 1672, Isaac Newton had described it and correctly inferred the cause after observing tennis players in his Cambridge college.[3][4] In 1742, Benjamin Robins (1707-1751), a British artillery engineer, explained deviations in the trajectories of musket balls in terms of the Magnus effect.[5][6]

[edit] Principle
When a body (such as a sphere or circular cylinder) is spinning in a fluid, it creates a boundary layer around itself, and the boundary layer induces a more widespread circular motion of the fluid. If the body is moving through the fluid with a velocity V, the velocity of the fluid close to the body is a little greater than V on one side and a little less than V on the other. This is because the induced velocity due to the boundary layer surrounding the spinning body is added to V on one side, and subtracted from V on the other. In accordance with Bernoulli's principle, where the velocity is greater the fluid pressure is less; and where the velocity is less, the fluid pressure is greater. This pressure gradient results in a net force on the body, and subsequent motion in a direction perpendicular to the relative velocity vector (i.e. the velocity of the body relative to the fluid flow).[7]
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http://en.wikipedia.org/wiki/Magnus_effect
 
alinwa said:
I haven't read the wiki but it needs to be corrected. This information is incorrect. The Magnus Effect is real but is not responsible for the "drifting" behaviour of spin-stabilized projectiles.

al
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I've dowloaded the Robbins "New Principles of Gunnery", but its something that will take time to wade through. My PDF copy is in a sort of printed long hand and in old English, first printed in 1740's.
Near as I can figure.
Its not that the Magnus Effect is responsible for bullet drill or throw from gyro scopic effects, but rather the Magnus Effect has a small but descernable effect on moderating or exagerating the bullet drill, at least at subsonic speeds towards the end or bullet travel. Its pretty much independent to the winds effects, but may affect the bullets attitude towards the wind.

I figure its tied to the buffeting of bullets as they go transonic at extreme long range.

Unless aerodynamicaly stable, such as an arrow or fletchette everything will either spin or tumble in flight for a portion of its travel. Bullets usually stablise in some direction other than point forwards if spin drops below the stabilization point, depending on shape.
A smoothbore musket or cannon ball will generally take on a rolling motion while the airsoft bb is given a backspin like a golf ball.
The cylindrical section of a bullet spun by rifling would be affected at a angle to flight and a slight angle away from direction of spin.

Its something I may have read up on forty or fifty years ago but never gave much thought to.



PS
Wiki is often wrong but also often correct, not the best source but it can lead to better sources.
 
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