.338 Whisper?

[mks]

Greg,
It is nice to run into someone who is actually interested in the minutia of fluid mechanics. The point I am trying to make is that the drag on shapes other than spheres can be reduced by boundary layer trips.

My quote: A sphere is not a special shape that is the only one that can benefit from dimples or other boundary layer trips.
Yours:

Sure it is.

I think you have backed yourself into a corner on this one. Airplane wings are not spheres, and their drag is reduced by vortex generators. http://www.nasa.gov/centers/langley/news/factsheets/Micro-VG.html

If you don't like that example, let's stretch the sphere a tiny bit into a prolate ellipsoid. It is no longer a sphere, even though it looks like one. We may have to use calipers to tell the difference. Are you saying that this tiny difference means that the boundary layer no longer responds to trips? Now let's squash a golf ball into a slightly oblate ellipsoid. It not a sphere any more. Do the dimples quit reducing drag? Careful now, you will run into a wall in either direction!

The yacht is a bad example. It's riblets are not vortex generators, like dolphins, sharks and Olympic swim suits.

The yellow tie story was funny. Just trying add what is known about low Mach number drag on bullets, even if it doesn't prove anything.

Cheers,
Keith

 

[Greg Culpepper]

Greg,
It is nice to run into someone who is actually interested in the minutia of fluid mechanics. The point I am trying to make is that the drag on shapes other than spheres can be reduced by boundary layer trips.

My quote: A sphere is not a special shape that is the only one that can benefit from dimples or other boundary layer trips.
Yours:

I think you have backed yourself into a corner on this one. Airplane wings are not spheres, and their drag is reduced by vortex generators. http://www.nasa.gov/centers/langley/news/factsheets/Micro-VG.html

Dimples on a sphere lower drag by keeping the boundary layer stuck on the object for a greater "wrap" around it in order to reduce the area of the low pressure wake on the back or downstream side of the moving object (sphere). An airplane wing (that is not stalled) by its shape doesn't have much wake compared to its area because it tapers, ideally, to a knife edge.
What a wing does have is lots of surface compared to its frontal and trailing area. On a subsonic wing much, maybe most, drag comes from skin drag. Air is "sticky" on surfaces and itself. It acts like high speed molasses or taffy. Vortex generators reduce drag by separating air from the surface to reduce drag, exactly the opposite of what the dimples do to keep the air attached longer.

If you don't like that example, let's stretch the sphere a tiny bit into a prolate ellipsoid. It is no longer a sphere, even though it looks like one. We may have to use calipers to tell the difference. Are you saying that this tiny difference means that the boundary layer no longer responds to trips?

Nope, I'm not. What I'll agree to is that as you increasingly prolate a sphere there is a point where drag along its pole becomes indifferent to dimples, and beyond that amount of prolating, dimples increase drag and no longer reduce it. Now, put a cylindrical skirt on that lozenge (or sphere) at the equator and have a shape for which drag, with or without dimples is equal, doesn't exist. Dimples will always add more drag on that shape than they reduce. And that shape looks a lot like a 380 ACP bullet.[/QUOTE]

Now let's squash a golf ball into a slightly oblate ellipsoid. It not a sphere any more. Do the dimples quit reducing drag? Careful now, you will run into a wall in either direction!

Same as above.

The yacht is a bad example. It's riblets are not vortex generators, like dolphins, sharks and Olympic swim suits.

The yellow tie story was funny. Just trying add what is known about low Mach number drag on bullets, even if it doesn't prove anything.

Cheers,
Keith

The point of this is that by the time that you streamline a round ball into something that looks even a little bit like a bullet, the application of surface features that create turbulence adds drag in (now I'm going to be absolute here and give you an opportunity to parse obscure contrarian examples) all cases. Grooves on the sides of bullets (or dimples on the front) won't reduce drag.

I noticed that SSK has introduced a 95 caliber rifle that drives a 3600 gr bullet 2200 fps. If we reduce the velocity enough do you think we can eliminate the eighteen pound muzzle brake? Should we hurdle right past SSK and start development of ultralight trebuchet's for punkin' chunkin'? BTW Dimples will work on a pumpkin to reduce drag.

All the best,

Greg

 

[Greg Culpepper]

Hey, Melvin,

I remember those guys! Except for that Shehane guy. I'm a little vague about that name. You motivated me to give Jim a call. He's gone the other way now and has a new Rocket, a 20 Rocket that gets over 4200 fps out of a 13". Yikes! He'll be 91 on Christmas eve. Good visit. Thanks for the nudge. I miss being able to carry everything I needed for a match in a gym bag. Miss my young eyes too.

Greg

 

[mks]

On a subsonic wing much, maybe most, drag comes from skin drag. ...Vortex generators reduce drag by separating air from the surface to reduce drag, exactly the opposite of what the dimples do to keep the air attached longer.

This may be the root of our disagreement. Skin drag does increase with decreasing Reynolds number, but on normal size wings traveling at normal speeds in air, the skin drag coefficient is less than 0.01.

http://adg.stanford.edu/aa241/drag/skinfriction.html

compared to combined drag coefficient (skin + pressure) of 0.045 or so.

http://www.grc.nasa.gov/WWW/k-12/airplane/shaped.html

Pressure drag is typically the larger portion of the drag.

Vortex generators work the same way as dimples - by mixing high speed air down toward the surface to keep the boundary layer attached ("re-energizing the boundary layer"). This decreases pressure drag by delaying separation. It increases skin drag, but since pressure drag is larger than skin drag, the combined effect is reduced overall drag.

http://en.wikipedia.org/wiki/Vortex_generator

The point of this is that by the time that you streamline a round ball into something that looks even a little bit like a bullet, the application of surface features that create turbulence adds drag in (now I'm going to be absolute here and give you an opportunity to parse obscure contrarian examples) all cases. Grooves on the sides of bullets (or dimples on the front) won't reduce drag.

Again, surface features increase skin drag, but if put in the right place, reduce pressure drag enough that overall drag is decreased. This can work wherever there are adverse pressure gradients, which occur on all flying bodies. Even on laminar flow airfoils, an adverse pressure gradient exists on 25-70% of the wing.

http://www.aviation-history.com/theory/lam-flow.htm

You are probably right that grooves don't work on bullets. They are really bad airfoils, too short and their tails too abrupt for separation to occur anywhere except at the tail. They have lots of pressure drag (subsonic bullets), but no effective way to reduce it. But if we ever start shooting subsonic bullets with tails streamlined more like wings, grooves could work.

Cheers,
Keith

 

[alinwa]

This may be the root of our disagreement. Skin drag does increase with decreasing Reynolds number, but on normal size wings traveling at normal speeds in air, the skin drag coefficient is less than 0.01.

http://adg.stanford.edu/aa241/drag/skinfriction.html

compared to combined drag coefficient (skin + pressure) of 0.045 or so.

http://www.grc.nasa.gov/WWW/k-12/airplane/shaped.html

Pressure drag is typically the larger portion of the drag.

Vortex generators work the same way as dimples - by mixing high speed air down toward the surface to keep the boundary layer attached ("re-energizing the boundary layer"). This decreases pressure drag by delaying separation. It increases skin drag, but since pressure drag is larger than skin drag, the combined effect is reduced overall drag.

http://en.wikipedia.org/wiki/Vortex_generator





Again, surface features increase skin drag, but if put in the right place, reduce pressure drag enough that overall drag is decreased. This can work wherever there are adverse pressure gradients, which occur on all flying bodies. Even on laminar flow airfoils, an adverse pressure gradient exists on 25-70% of the wing.

http://www.aviation-history.com/theory/lam-flow.htm

You are probably right that grooves don't work on bullets. They are really bad airfoils, too short and their tails too abrupt for separation to occur anywhere except at the tail. They have lots of pressure drag (subsonic bullets), but no effective way to reduce it. But if we ever start shooting subsonic bullets with tails streamlined more like wings, grooves could work.

Cheers,
Keith

hmmm, a guy who actually understands that airplanes are pushed upwards by the ambient........ you must not be a pilot

LOL

al

 

[mks]

hmmm, a guy who actually understands that airplanes are pushed upwards by the ambient........ you must not be a pilot

LOL

al

Uh oh, let's not pick on pilots. (You're trying to get me in trouble!)

So wings are pushed, but bullets are sucked?

Cheers,
Keith

 

[alinwa]

Uh oh, let's not pick on pilots. (You're trying to get me in trouble!)

So wings are pushed, but bullets are sucked?

Cheers,
Keith

ohh no, planes are also sucked upwards

al

 

[Charles E]

ohh no, planes are also sucked upwards

al

Until they aren't. You guys gotta remember Al is a contrarian, as in "Nature abhors a vacuum, the world (in the nontechnical sense) sucks."

BTW, I've given up on the idea. Got an email from Dave Tooley, saying, essentially, that subsonic rifles are notoriously inaccurate. Of the 14 or so unfinished projects I've got going on, this one isn't going to have an easy answer. But if anyone wants to give it a start, I'd say forget golf balls. Think "double cantilevered beam."

Charles

 

[mks]

... subsonic rifles are notoriously inaccurate. Charles

How are they inaccurate? Vertical, horizontal... or both?

 

[Charles E]

Dave didn't say.He did say "hard to tune," which makes one think vertical -- though contrary to popular opinion, tuning effects both.

If you want to build one & work out the bugs, I'd be happy to copy it ;-)

 

[Greg Culpepper]

This may be the root of our disagreement. Skin drag does increase with decreasing Reynolds number, but on normal size wings traveling at normal speeds in air, the skin drag coefficient is less than 0.01.

http://adg.stanford.edu/aa241/drag/skinfriction.html

compared to combined drag coefficient (skin + pressure) of 0.045 or so.

http://www.grc.nasa.gov/WWW/k-12/airplane/shaped.html

Pressure drag is typically the larger portion of the drag.

Yes, pressure drag is typically the larger portion of total drag. But your final citation in this your post states that surface drag can amount to 30% at normal cruise. Normal cruise is at small angle of attack. High angle of attack is when vortex generators are most useful on airplanes to postpone stall and allow slower flight and higher rate of climb. Bullets fly at cruise, low angle of attack because they orient themselves to minimize drag just like a weather vane. Pressure drag on an airplane wing at cruise is mostly on leading surfaces as trailing edges on wings are very tapered and create small wakes (low pressure drag). At higher angles of attack than cruise, pressure drag increases on the bottom surface of the wing and adverse drag on the top surface moves toward the leading edge. This is when a wing can most benefit from vortex generators. But a bullet isn't shaped like a wing and it never flies at a high angle of attack, it just points into the wind since it has no control surfaces to control it. it is in ballistic flight, not controlled flight.

So, a bullet isn't shaped like a wing and it doesn't fly like a wing. What difference does that make? Well the difference is that it can't benefit from increased turbulence.

Vortex generators work the same way as dimples - by mixing high speed air down toward the surface to keep the boundary layer attached ("re-energizing the boundary layer"). This decreases pressure drag by delaying separation. It increases skin drag, but since pressure drag is larger than skin drag, the combined effect is reduced overall drag.

http://en.wikipedia.org/wiki/Vortex_generator

Yes, this is right and it can work for wings and does work for a golf balls, but not bullets. Turbulence can wrap air farther around a sphere on the downstream side to provide a superior cd and bore a smaller hole in the air or keep air attached farther back on a wing at high angles of attack to soften/postpone stall response or allow slower landings or provide higher rate of climb or other desirable performance objectives involving controlled flight as contrasted to ballistic flight.

Again, surface features increase skin drag, but if put in the right place, reduce pressure drag enough that overall drag is decreased. This can work wherever there are adverse pressure gradients, which occur on all flying bodies. Even on laminar flow airfoils, an adverse pressure gradient exists on 25-70% of the wing.

http://www.aviation-history.com/theory/lam-flow.htm

You are probably right that grooves don't work on bullets. They are really bad airfoils, too short and their tails too abrupt for separation to occur anywhere except at the tail. They have lots of pressure drag (subsonic bullets), but no effective way to reduce it. But if we ever start shooting subsonic bullets with tails streamlined more like wings, grooves could work.

Cheers,
Keith

So, where would the right place be? Not very far ahead of the adverse pressure on the bullet, which occurs at the end of the bearing surface. I think the rebated boat tails on some Lapua Scenar bullets may be an attempt to address this opportunity.

Anyway, I'm glad to learn that we don't disagree that surface features on leading or parallel surfaces of bullets won't reduce drag. I'm left imagining what a double ended bullet designed to minimize drift at subsonic velocities might require of the multitude of competing compromises of internal ballistics, manufacturabilty and accuracy.

Greg

 

[Greg Culpepper]

Dave didn't say.He did say "hard to tune," which makes one think vertical -- though contrary to popular opinion, tuning effects both.

If you want to build one & work out the bugs, I'd be happy to copy it ;-)

Here you go Keith. Flexistock time.

 

[Charles E]

Flexistock time.

You proally thought I wouldn't ask. What's a flexistock? Different than our "don't flexastock" (the 3+ inch o.d., 0.50+ inch wall, tube)? Actually, the "don't flexastock" is also the double-cantilevered-beam barrel. Scaled down to meet point-blank HV rules, there are some issues. Some work, some don't. If I wanted to prove out the theory, I'd try the big guy first.

Keith, Greg's seen these rifles -- mine, Joel's, Charles Bailey's.

 

[Greg Culpepper]

Charles,

Some time back Keith was ruminating on varying ballast weight position on the vertical plane in a stock to adjust its response in recoil for the purpose of tuning vertical. I think I remember Keith pointing out that weight position would affect the amount of deflection in the stock in recoil and as Enfield rifles with two piece (hinge in the middle) stocks have shown reduced vertical at long range with crap ammo compared to intrinsically more accurate rifles shooting the same ammo, I speculated that deliberately building single plane flexibility into a stock could show a benefit for ammo that had an extreme velocity spread of greater than zero (all ammo). I suggested further that the stock be constructed to allow tune-ability by installing an elastic material in the top of the wrist of the stock, providing or establishing a hinge point at the bottom of the wrist, and loading that "damped spring" with a through bolt to provide spring rate adjustment for tuning.

The presumption that Keith and I share is that in a subsonic system with comparatively long barrel time, that establishing a first order tune to affect launch angle for reduced vertical at the target across varying muzzle velocities might be easier with a stock tune than with a barrel tune. What frequency and compensating amplitude do you need and do you chase it with a stiff stock and a flexible barrel or the converse. That is the flexi-stock concept in a nutshell.

Keith has the curiosity, insight, interest and talent to make it work if it is workable, just as you Joel, Charles Bailey, Dan Wesson and a few others have established a body of knowledge (and shared it, thank you) with tensioned barrels. I hope he chases this line of inquiry.

Greg

 

[Greg Culpepper]

Charles,

I've had another thought that relates to tensioned barrel guns. As you have explained to me (unless I've "misremembered") more than tuning, a tensioned barrel gun largely takes tuning out of the equation and will generally produce round groups that are small round groups with good (meaning low extreme velocity spread) ammo. This might be particularly true for guns like Pendergraft's and Bailey's heavy guns who's center of gravity is nearly concentric with the bore (and front support) and recoil forces by comparison to a presumptive short range light gun with stock rules.

What advantage might arise if a tensioned barrel gun was designed to be tunable by non-concentric (on the vertical plane) cg in combination with a barrel contoured for differential flex. Flats milled on the top and bottom of a clocked barrel could focus direction of vibrational movement in the vertical plane. A cg below (or above from the scope mass) would predictably excite barrel vibration in the vertical plane. Barrel tension might be employed to alter frequency and amplitude for tune to converge POI at a distance across varying velocities by launch angle. It might require that the front barrel attachment point be accomplished by spherical or horizontally oriented cylindrical bearing.

Your thoughts?

Greg

 

[mks]

Greg,
I think we are largely in agreement now on aerodynamics (Whew). Here is an interesting student project on a human-powered plane that has a wing predicted to have a friction drag coefficient of 0.006 and a pressure drag coefficient of 0.0014, so friction drag greater than pressure drag is possible.

http://www.dept.aoe.vt.edu/~mason/Mason_f/HPAFinalRptS06.pdf

Bryan Litz mentions a bullet smith saying that rebated boat tails don't help much:

http://www.appliedballisticsllc.com/index_files/BulletDesign.htm

 

[Charles E]

My thinking is that the center-of-mass = center-of-bore rifle is not what matters. My big tube guns, if you remember, have a piece of 0.75 inch thick, 13-inch long piece of aluminum functioning as a butt. Don't remember what it weighs; at the back, it is 5 inches deep, then to the bottom of that is bolted the 3-inch wide plate plate with steel guide rails to fit the rear rest (a short-range front sandbag). The weight is not insignificant.

This handles the torque, letting me use "only" a seven-inch wide front plate. Both Joel and CB had to go to a dual-post front rest.

My first tensioned-barrel rifle use a small 2-inch tube, and the stock was one of those 20-pound McMillan 50-caliber stocks, all below the bore line. Both Jeff Rogers and Tony Z's (Australia) rifles are the same (homemade stock), as was Dave Tooley's *compression* fitted barrel -- that was a Light Gun, as I remember.

So now we have compression and tension; to complete the picture remember Phil Jusilus' HG, which used a big tube, but neither tension nor compression, he simply used a spider-type bolt system to hold the muzzle in place.

All of these shoot the characteristic round groups. The load can be tuned, but the shape of the group stays the same. It just gets a bit bigger or smaller

That's why I think the double-cantilevered beam is the correct model.

You could argue that Joel's rifle shot better, and that was certainly true of the first barrel. But his second barrel wasn't so good, my tube gun stayed right with it. And Jeff Rogers "not-COM" guns hold a lot of Australian records.

Ideally, both the butt & muzzle would be cemented into a brick wall. But getting that to the line would be hard, never mind bench rotation...

 

[Greg Culpepper]

Charles,

Thanks for the quick reply. Do you think there might be benefit to tuning between fixed supports for launch angle to compensate for mv variation and resultant trajectory differences?

Greg

 

[Charles E]

I'm not sure -- maybe.

I have started to shoot a little rimfire, indoors. After I set the tuner, I added an old Henrick device I had laying around. While figuring out it's best position, I could sit there and shoot 5-shot groups on the rather generous IBS target, and if the groups were centered up, no shot would touch the 10-ring -- all would be inside it. But go and shoot a card! -- with the down, left & right, down movement, and I'd get close to a scratch 10 sometimes. Not so bad as to need to be plugged, but a lot worse than with shooting groups. Particularly worse when I'd drop a row. And I did take the time to push the rifle back & forth on each bull, & even pound it (well, tap it) a bit when dropping a row.

My thinking was that was due to the sandbags; still using the rather flat front bag I use for CF. But maybe not, maybe it is the differing attitude of the muzzle.

What do you have in mind?

 

[Greg Culpepper]

What I had in mind had to do with a tensioned barrel with more flexibility (by contour) in the vertical plane deflecting to produce differences in launch angle by bending behind a fixed point instead of the muzzle itself moving on a vertical line.

But you rimfire comments open another discussion.

I haven't often and not for quite now awhile competed in RFBR, but I'm not devoid of opinion (I know this surprises you).

I've got a rimfire that shoots uncommonly small groups. Like you, when I shoot a card and move from target to target I find that there are new issues to deal with. Like you, I use the same three inch rest I use in LRBR and I think that changing the angle of the forend in the front bag between shots has a negative influence on poa vs poi. That difference seems to be more pronounced if the change includes both vertical and horizontal rifle displacement as when changing row and column.

Observation suggests that most competitors move from target to target in the same manner that we read, top left to bottom right, returning to the left margin after completing a row. When I mimicked this I was all over the place. Magnitude of angular displacement between shots seems to contribute to magnitude of unpleasant surprise in bullet hole location. For example, I've shot 1 moa grids in the same pattern without the accuracy problems that a RF target with 5 moa between bulls produces. It may be that front rests with delrin stock riding buttons that were in evidence at the time overcame this apparent problem with bag/forend interaction.

Accordingly, I adopted a different protocol and it seemed to work for me. I shoot columns. I think I can chase trends in vertical influences on poi from bag/rifle angular changes and horizontal windage issues from a condition change more easily if I separate them and don't have to integrate them additively. Another difference is that I shoot fast and try to maintain a cadence. I shoot a sighter or two to verify and provided the two shots agree and I decide the condition is holding, I hold off accordingly and run the column, adjusting poa from bull to bull, not by adjusting the rest but rather by pulling the gun back in the bags and using the draft angle between forend and butt to select the next lower target. When I complete a column, I go all the way up and right one and shoot a two sighters to settle the gun and evaluate condition changes during the rest adjustment (windage only) interval. Two sighter shots also gives me an opportunity re-establish shot cadence.

This shot cadence consideration may have two benefits. I think rimfire accuracy can benefit more from constant barrel temp than centerfire. A centerfire barrel can drift poi with increasing temp if the bore doesn't run up the middle. A rimfire barrel doesn't get hot enough to change poi much but bore temp probably has an influence on lubricity of waxy bullet lube on rimfire target ammo and that seems to me to affect group size. (That's my story and I'm stickin' to it, at least for now.)

Over time, accuracy arts develop and I don't know what's going on in the RFBR world right now. But at the time, what I've described (and really good ammo) allowed limited experience, and absence of a tuner or wind flags to keep up and even win some.

What do you think about these rest/bag/forend considerations?

Greg