Winter ops -- reducing inlet area

Out for a nice little ride yesterday at 42F on the surface. I noticed that my CHT was running right at 265F in cruise. it's always been pretty cool to begin with but that seems like I need to bring it up a bit. Oil temp was also low at 165. It's not even winter yet !!

Has anybody got a good way to temporarily reduce the inlet openings for winter ? Ideally, I'ld like to make a "plug" to install in the inlets and then remove it in the Spring.

Looking forward to Winter......

John

Temps

John. you can also choke off the air on the exit.
Bullet

blocking exit air

I made up a quick and easy blocker for cowl exit air. Attached with one bolt and you can either run with one side or both sides on. Made a +30F improvment and now run CHT350's and Oil in the high 190'sF. That extra heat means more heat in the cabin as well.

A Plug Idea

I'm thinking of making a plug for each inlet that conforms to the inboad surface and uses the three platenuts on each side of the center part of the cowl and some long #8 screws to for mounting. This is purely for speed but increased temperatures are by-products. I'm still in the thinking stage. I have also been thinking over the outlet restriction but Wayne's photos show me some complications in what I was thinking of there.

Bob Axsom

Wayne two questions

Do you have any speed measurments taken before and after your outlet block plate was installed? If so, what was the impact?

Bob Axsom

Interesting...

I didn't think of outlet area. I was actually thinking along the lines of what Bob suggested. Make a couple of pieces to snug up to the inboard section of the cowl. Then just use longer screws to attach them. Might also try to add one screw top and one bottom to prevent the "plugs from being bent backward by the airflow. Along Wayne's idea, the plugs could be made from Aluminum and it might be even easier to make and secure.. hmmm......

The exit idea is interesting. One question for Wayne: when you attached the plates to the bottom of the cowl, you used what looks like a long strap and screwed them to it.

Did you consider adding a few platenuts to the plates and then just screwing the bottom cowl to them ? Seems like it would be simple and would leave a few small holes in the bottom rear of the cowl when you're done with it.

Looks like this might be a fun little project....

John

Speed change

Bob Axsom said:

Do you have any speed measurments taken before and after your outlet block plate was installed? If so, what was the impact?

Bob Axsom

Click to expand...

Thats a interesting question that I havent explored. I guess the fact is that it doesnt matter if it slows me down 1 or 2 kts but the important part is that I keep my engine warm & happy during the winter.

John_RV4 said:

The exit idea is interesting. One question for Wayne: when you attached the plates to the bottom of the cowl, you used what looks like a long strap and screwed them to it.

Did you consider adding a few platenuts to the plates and then just screwing the bottom cowl to them ? Seems like it would be simple and would leave a few small holes in the bottom rear of the cowl when you're done with it.
John

Click to expand...

The long strap that you see is part of the cowl attachment that is on the nose gear leg of my -7a. Not sure what the bottom of the -4 looks like around the outlet and exhaust.

I did two things to the plates to keep them in place besides the one bolt.

1) made the top "taller" than the opening so that the exit air pressure will push the palet against the firewall

2) bent the bottom at a 90 deg forward angle so that if the air pressure pushes the plate outwards, the hole will still be covered by the flange.

Living here in Alberta produces some wild temp swings in the winter from minus 40C to +15C in a matter of hours when a "Chinook" rolls through. Therefore the goal was to make the plates effective in the cold and easily removable on the tarmac in case the OAT increases.

Hope this helps and you can find a way to adapt it to your -4

Less flow means less drag

Wayne said:

Thats a interesting question that I havent explored. I guess the fact is that it doesnt matter if it slows me down 1 or 2 kts but the important part is that I keep my engine warm & happy during the winter.

Click to expand...

I would expect plugging the outlet some will increase speed.

Frank

I was curious about that too

Although the flat plate across the outlet would intuitively be considered a drag producer I was interested in whether the decrease in air flow mass through the system (clearly indicated by the CHT increase) resulted in an increase in speed. This would be a very important finding to me which ever way it came out.

Bob Axsom

How is this any different than cowl flaps?

It is similar but...

BillyBob said:

How is this any different than cowl flaps?

Click to expand...

It is similar but instead of going from maximum air mass flow and drag with pilot control and minimum flat plate obstructions to minimum air mass flow with with minimum flat plat obstructions you are simply blocking the air mass flow with a flat plate. Given the ability to redisign the cowl, the controllable flap configuration should be more efficient.

Bob Axsom

Cowl inlet mods

John_RV4 said:

Out for a nice little ride yesterday at 42F on the surface. I noticed that my CHT was running right at 265F in cruise. it's always been pretty cool to begin with but that seems like I need to bring it up a bit. Oil temp was also low at 165. It's not even winter yet !!

Has anybody got a good way to temporarily reduce the inlet openings for winter ? Ideally, I'ld like to make a "plug" to install in the inlets and then remove it in the Spring.

Looking forward to Winter......

John

Click to expand...

Hi John, My -6 has a Modified Barnard Cowl with sealed plenum and a 4" prop extension / round inlets. DO THIS AT YOUR OWN RISK. I have done this to my -6 only to see what happens. Get the duct tape out and block off the right side inlet completely, leaving the left inlet wide open and go fly the airplane on a COOL day close to the airport and observe the CHT's and Oil temp if you have 4 ea CHT probes. If temps are within reason put some heat in the engine by cranking it up to cruise speed. Observe your readings after temps have stabilized after 20 mins or so. If all ok, land then do the opposite, block off the left side inlet and open up the right inlet. Fly again and take notes. You should notice that in climb there will be variations but in level cruise the numbers will change again. You should see that the airflow between the left and right cowl inlets have different needs in climb and level cruise. I found this on my -6 but not all configurations are the same. The only reason I mentioned this is because you might want to make one cowl inlet plug / insert smaller than the other. My settup for winter / 35F air is a 2 round inlet on the left with a 2.125" round inlet on the right. My oil cooler is recieving air via a NACA duct with a controlable inlet door on the right side lower cowl so these small diffuser cowl inlets are not feeding air to the oil cooler. CHT's I'm seeing with this settup is averaging 350F in 35-40F ambient air. DO THIS AT YOUR OWN RISK!!! Just something to think about.

Bob Axsom said:

It is similar but instead of going from maximum air mass flow and drag with pilot control and minimum flat plate obstructions to minimum air mass flow with with minimum flat plat obstructions you are simply blocking the air mass flow with a flat plate. Given the ability to redisign the cowl, the controllable flap configuration should be more efficient.

Bob Axsom

Click to expand...

My thoughts were:
1. Best flexibility would be to have the exit "plates" controllable via a control cable, as an overall CHT/oil temperature control.
2. Maybe there is data already available for planes with cowl flaps that would indicate speed change with & without use.

more hmmm.....

Alan brings up another interesting point, which I hadn't considered (glad I asked....). I only have a single egt/cht on #3. So, if I modify the inlets, I won't really know what's happening. Good argument for a "real" engine monitor.

I have a cowl bumper down low in the back of the cowl. This might make a good place to attach an exit plate. That might be the place to start until I'm equipped to modify the inlets....

Anybody got an EI (or other) 2 1/4 sized monitor for sale ? Panel space is at a premium in the 4.

Thanks for all the info. This has been very helpful.

John

The heat is on...

When my uncle (Sam) moved me from my southern home to the North country, I flew my RV4 year round, even below zero in SD. Locals had a couple of hip-pocket tips I used. Always pre-heat (I used a hair dryer) the engine below 32F. Try to get your oil to 170 inflight to get rid of moisture. You can dump 100LL into your oil (poor man's oil dilution) if you don't run multi-vis and can't pre-heat on the road. Alaskan Charter operators don't fly piston airplanes below -30F, hopefully where you are in Yankee-land it doesn't get that cold.
The best fix for your oil temps is aluminum tape over 1/2 the inlet area on the cooler face, and check the temps. It's easy, cheap and no aluminum bending or 7-EH plates involved (Thanks Wayne). For the cyllinders you can try the same aluminum tape on the front sides over 1/2 the area but be careful, a little goes a long way. Good luck, EH...

Rob Ray

One of my older planes, a 1972 Citabria ECA, had a neat sliding panel on the bottom of the cowl which moved forward and back to open or restrict the cowl exit. Six screws (I think) were easily accessed to move that panel. The neat thing was the effect was fairly obvious and the amount of restriction could be adjusted from wide open to closed.

My -4 has an aluminum plate which attaches to the front of the oil cooler (port side cowl, front) with two bolts. This is minimally effective and I am going to consider the blocking of the exit air.

Good topic.

Cowling Insert Pics

Picture of inserts I made for my -6. Actually these are summer time inlets. 2.750" diameter each.

Fibreglass / carbon fibre inlet design

OK Alan - spill....,

HOW! How do you get / design the shape / profile for the inlet plugs. I'm thinking along the same sort of lines for the front inlets but have now ideas how to get a uniform cross section? I've even started thinking about CNC'ing a foam plug to lay onto!

Second related question: Can the air in the upper plenum be too slow? ie. if I have ~4" inlets and I make a airfoil plug, is there any disadvantage to slowing the air back to 4" - I'm assuming it is a good idea (the plenum is bigger still) and it is just starting the decelleration process.

Carl

Inlet plug shape

zkvii said:

OK Alan - spill....,

HOW! How do you get / design the shape / profile for the inlet plugs. I'm thinking along the same sort of lines for the front inlets but have now ideas how to get a uniform cross section? I've even started thinking about CNC'ing a foam plug to lay onto!

Second related question: Can the air in the upper plenum be too slow? ie. if I have ~4" inlets and I make a airfoil plug, is there any disadvantage to slowing the air back to 4" - I'm assuming it is a good idea (the plenum is bigger still) and it is just starting the decelleration process.

Carl

Click to expand...

Click to expand...

Good question. I've looked at NASA reports, airfoil shapes this that and have seen everything in the world. Answer: what is best, I don't think there is a best. One thing I did learn is the diffuser angle where it starts out small then increases in size shaped like a venturi worked best on my -6 with the round inlets. I kept the diffuser angle at around 7-10% to try and keep the air attached to the runner before it spills into the plenum. I started out with a plastic cup that you can get at a convience store. Just so happened the ones I found where the perfect angle and shape to lay the glass up on. I later had to change the shape a little bit to give clearance in the plenum since the engine moves around. The radius at the front edge is not the same around the entire inlet. The right inlet protrudes further forward than the top of it, idea ??? maybe to catch a little more air because of prop rotation on a climb out, don't know if it does any good but I did it. The radius shape is nothing out of a book, its just a nice gentle smooth radius. I did use an air mover with tuft string attached inside to see how things worked and the strings stayed attached but I don't know what the air is doing with the prop rotating. Whatever I did made the engine run cooler with the 2 3/4" inlets versus the stock 4 1/4" inlets. When I played around with different sizes I made 1/4" diameter changes. When I got to the 2 1/2" diam I started to see CHT's rise. So I settled with the 2 3/4" for now. I've got plans to make a bullet shaped devise that will be centered right in the middle of the inlet that I hope or plan to build a means to move it in and out to control CHT's. You must remember that my oil cooler is not picking up air off the plenum, so all of the air passing through the inlet rings is for cylinder and cyl heads only. I flew yesterday with 78F air, 23" x 2,450 rpm 4,000ft msl and here are my CHT's #1-314F, #2-314F, #3-316F, #4-312F. Yes I'm still to cold but I'm working on that. Hope this helps.

PS: Earlier in this thread I posted different CHT numbers with very small 2" inlets, I've made various other changes since then so the earlier post with CHT's in the 350F range are wrong now. When I got to playing around trying things my CHT's started running colder and that was not my intention, but I found out that the first few inches of the diffuser are critical and I'm guessing the velocity increased and that is why my CHT's started dropping more and more until I found a size that started bringing the CHTs up. AJ

More pictures of inlets

First picture, view of right inlet with lip protruding forward at the bottom.

Excellent Smither's

Nice creative work again Allan.

Ingenious how you down sized your existing inlets. The only "down-side" to the "down-size" (bad pun) I see, is you have a rather large lip/flange around the smaller hole insert, but this is unavoidable when down sizing an existing inlet. It probably has little effect on cooling, but may be a little more external drag than if your cowl was made to this size.

This would be too small for summer ops, and of course as you mention your engine is "oil cooled" and the huge oil cooler has its own air inlet source. However its a great idea and think I will steal it.

The asymmetric sizing is also interesting as well. All excellent application of theory, testing and out of the box thinking. Who Da Thunk that you should make inlets different sizes left/right, but we know the air going into the cowl is not balanced due to prop wash effects so why not balance the flow with with by sizing the inlet differently.

Alan,

I want to express my admiration for everything you've been doing and contributing to these forums lately.

What impresses me the most is that while you've made mods that have increased performance significantly and measurably, it doesn't appear that these mods have limited "the rest of the mission" in any way shape or form. Seems like all too often we see planes set up for top speed, racing, what have you, but that's all they do. That's all they can do. Your case seems distinct to me (and maybe it's just me) because it hasn't seemed to impact the overall capability of your airplane. It appears you have expanded a couple of envelopes without shrinking any others.

My hat is off to you. Please keep taking photos and sharing with us the things you've discovered. I'm sure there are now 100s of people benefitting directly from your hard work.

gmcjetpilot said:

Nice creative work again Allan.

Ingenious how you down sized your existing inlets. The only "down-side" to the "down-size" (bad pun) I see, is you have a rather large lip/flange around the smaller hole insert, but this is unavoidable when down sizing an existing inlet. It probably has little effect on cooling, but may be a little more external drag than if your cowl was made to this size.

This would be too small for summer ops, and of course as you mention your engine is "oil cooled" and the huge oil cooler has its own air inlet source. However its a great idea and think I will steal it.

The asymmetric sizing is also interesting as well. All excellent application of theory, testing and out of the box thinking. Who Da Thunk that you should make inlets different sizes left/right, but we know the air going into the cowl is not balanced due to prop wash effects so why not balance the flow with with by sizing the inlet differently.

Click to expand...

Click to expand...

To bad I've got finished paint on the cowl because if I didn't I'd cut the **** out of it and profile the nose to a nice taper like some of the Lancair guys have done. Another problem is prop clearance with the stock Barnard cowl, if I could I'd extend the inlets out further but I could only gain about 3/8" and now only have 1/8" clearance between the prop in low pitch. I'm still not done tinkering with these things yet. I agree that there is a lip / drag problem with the carbon fiber inlets not contouring the shape of the cowl towards the outside diameter and I hate the blunt shape because it's draggy. These are only prototypes for testing and I have more ideas to try. I did find that in climb, a bigger left inlet helps in a climb with #3 cylinder. The left inlet seemed to be the most effective in level cruise, my guess is that the #2 cylinder is spaced further back than cylinder #1 and the left diffuser inlet has a much longer run, more space to allow the air to flow where it needs to be, I'm guessing it's more effiecent, so I was able to downsize the left inlet more than the right one. I'm just guessing here OK. The pictures show the same size right now because I wanted them to look the same and were not talking about much difference. I'm dreaming up a way to install a bullet / torpedo shaped device in the middle of the inlet rings that I can move in and out with a servo or something to control the air. Anyone have any ideas?????

Alan: Kudos for doing real science and getting usable data, and most of all for sharing it with the rest of us. I asked my fluid dynamics literate son about what you have accomplished and here is what he told me: 1) The inlet lip should have a "generous" radius with a section known to have laminar flow properties, ie: an airfoil section. Probably doesn't much matter which airfoil since even turbulent sections are laminar over the nose portion. A sharp edge (small radius) at the inlet will encourage vortex shedding (ie: chaotic flow separation) which will sharply increase drag through the inlet and reduce flow, as you seem to have found out by getting more cooling with a smaller but more favorably shaped aperture. Too much drag at the aperture can cause the air to go around the hole rather than through it. 2) Your diffusers look good. He said it looks as though your outlet area is about double your inlet, which should roughly half the air velocity at the outlet compared to the inlet and lead to a quite large rise in outlet pressure, which is to be desired. I hope I have translated this properly and any errors in my explanation are my fault. I have only a layman's understanding of this. As for the bullet shaped obturator in the inlet, how about mounting it on a threaded rod that is rotated by a stepper motor, moving in and out of the throat? The telescope making guy use stepper motors to control the postion of their telescope mounts. The electronics are available off the shelf. The motor counts its steps and theoretically would provide position without further measurement. Downside: the thing goes ape and plugs up the hole. It also violates the rule of "simplicate and add lightness". There is little space for the mechanism in front of the cylinders, and it would need streamlining to keep drag low. A wind tunnel is suggested. Even better, sell the idea to NASA and let them do it. Unfortunately, little emphasis is given to the first A in NASA. All the money goes to the S.... In my opinion, having exchangable hot weather/cold weather inlet apertures and diffusers is the best (cheap, simple, reliable, light wt.) solution. How about measuring the pressure well back in the plenum using a piccolo tube, using another piccolo tube beneath the engine as a reference? An ASI could do the measuring. As long as the density of the air was the same between tests the pressure differentials should be directly comparable, providing another data point in addition to CHT.

Bang 4 Da $$$...

I think the scientific approach to cooling an air-cooled engine designed in the 1930's with fluid dynamics is noteworthy, but excessive. The RV4 design (I know, yours is a 6, but this is the RV4 section!) is a wonderful airplane, kept simple and light it provides amazing performance for a low budget.
I have a flying RV4, one that has been in 48 of the 50 states, AK, Mexico and Canada, eh. It has given me 10 amazing years of fun flying, dogfights, backcountry landings and untold adventures. I spent a grand total of $26K in 1995 dollars to finish it using Van's battle cry, "alot of bang for the buck", and haven't looked back.
I admire anyone who experiments and "good on ya" for all the hard work. However, I have some ideas for the RV4 masses: keep it simple, light, stock and fly the @#$% out of it! Take all the money you would spend on cowling mods and buy a bunch of 100LL, find a cute girl and take her to the bahamas...
You won't need any cowling air restrictions there...

RR

Well sorta of kinda

smokyray said:

I think the scientific approach to cooling an air-cooled engine designed in the 1930's with fluid dynamics is noteworthy, but excessive.

Take all the money you would spend on cowling mods and buy a bunch of 100LL, find a cute girl and take her to the bahamas...
You won't need any cowling air restrictions there... RR

Click to expand...

I hear you smokeyray, the KISS method and lets jut go fly attitude is not much unlike Van's bang for buck design philosophy, if it works well, good enough. It's like alternative engines, I don't get it but my hat is off to all of them for doing it.

However, let me ask you this, if you can make a cowl change that gives you 8 mph faster cruise speed with no down side or "excessive" effort or cost, than is that not awesome and worth it?

Lets assume ratio of new/old cruise speed is (198/190) = 1.04. Better miles per gal means more money for drinks on the beach w/ your new girlfriend. If you add EI, better wheel and gear fairings lets say you gain 10%. In a year say you by 1000 gal at $4.50/gal you may save $450 a year. That is probably over est by double, but $200 a year ain't bad.

Just look at old cowls and new cowls on a Lancair, Cirrus or Piper, they are different than antique classic planes. Clearly Van did not copy the cowl of a Luscombe. Van's base design is good enough, but there's room for improvement.

I'm sure Van approves of any mod that increases performance and is still safe. I know Van did and still does a lot of experimentation on different ideas. Many don't work, but for the ones that do, he included in the new kits.

What if Vas was satisfied with the *S-t-i-t-s Playboy he first modified and improved, there would be no RV-3 or RV-4 and so on........

What ever floats your boat smokyray, that is totally cool, you like to fly and not tinker. Sure an extra 10 mph will not make it more fun to acro or takeoff/land shorter, but Allan's my hero! I also have ideas but too lazy to do them. He does them and we all benifit, even if you don't follow them. Keep up the good work Allan.

*language filter does not like the slang for breast or teat, ha ha

One more picture

One more picture of a prototype inlet.

Old School...

You're absolutely right gmc, experimentation is what got the Wright Bros off the ground. It also bred one of my favorites, the GE F110 F16C "big inlet" mod which added 2500 lbs of thrust (27,500 total) simply by expanding the inlet. I have a bunch of time enjoying that one...woo hoo! Simple: more air in = more thrust out.
However comma, even as archaic as it is, the old Tcraft I owned with it's 65hp engine cruised at 98 mph on 4gph, mud on the struts and all. That's roughly 25 mpg. My original RV4 setup with stock cowl and 150HP cruised at 150 knots at 7.5 gph. Roughly 25 mpg. 40 years separates the two. My neighbor, after installing a James plenum and speed cowl, Van's recovery wheel pants, and new prop on his 160HP RV4 (and alot of hours and $$) still matched me RPM for RPM in speed afterward. Hmmm. The Wright Flyers props are rated 78% efficient, Hartzells best are 86%, over 100 years between them. Hmmm.

On the flip side, I think Alan's mods are incredible and will reap some benefits for all of us down the road, like I said "good on em". One of my Experi-mentors, Steve Wittman is legendary for thinking outside the box, just look at a Tailwind alongside anything else of it's genre'. It's still fast! I still marvel at Tracy Crooks Mazda powered RV4 every time I see it with the same hours mine has. (and a bunch more forced landings) I admire all of them for the time and effort spent tweaking an existing design and making it better.
I simply believe that for most folks who like to fly, the benefits gained are minor compared to the time, money and effort to get a couple of knots. I think much more is gained by getting air under your butt, in my humble opinion.

To quote my late great buddy Brian "Wolfman" Wolf:
Go Fly!

RR

Good Lord! Alan's work is freaking beautiful - but, really makes me feel kind of stupid.......

Here in good ole MN where we often fly in the single digits this time year - I use perfectly "adjustable" pieces of plain old duct tape or aluminum "speed tape". And to think....this might be costing me a mph or two!

Sorry for the sarcasm as usual....but I'm always a bit perplexed why people go through so much effort to do something that takes me literally 45 seconds....PER YEAR to accomplish. No bolts, not plates, no glass...and no speed that I can tell, but then again my plain isn't the fasted gun in the west anyway!

Lastly, the weather up here is crazy. We can go from 50 degrees one week to below zero the next. My adjustable tape keeps up!

I say to people like Alan - keep going because I truly respect that beautiful plane and all the mods (maybe perhaps a bit jealous To those experimenting with less "hacked" type solutions I say well done - just not for my lazy rear!

Anyway, just my 2 cents as usual. My Duct Tape or Aluminum Tape has worked well for many years and I maybe have 5 minutes total and $2 total over the past 7 years of doing it.

Cheers,
Stein.

These inserts are for Summer Use, OK, not Winter!!!

An update from todays flight. The prototype cowl inlet diffuser inserts that I'm flying with at present are 2.750" I.D .... They are installed for summer use but yes it is fall now and it's getting colder. I flew to Wichita, KS today, 1 hour flight at 7,500ft , 23.5 " MAP x 2,450 rpm with an outside air temp of 75F degrees, CHT's running #1 314, #2, 314, #3, 315, #4 305. EGT's at 125F ROP. Can anyone else match that with only 11.87 square inches of air inlet area to cool the engine????? Remember, the Cowl inlets do not feed air to the oil cooler OK!!!! This does not come easy BTW. Yes I did the duct tape thing in the past and boy did it look like cr%$^&p but yes it will work. Later, AJ

Roll the Tape...

Amen Stein, I used aluminum tape when I lived in SD, learned from the masters up there. Worked very well. Check out the KK-1 article on the home page feature today. Look closely at the inlet for some "cowling inlet modifying fairings" (duct tape).
Since I tallied up my RV4's total cost at under 30K, the tape fits in with the "bang for a buck" motto.
Gotta love it!

RR

SteinAir said:

Anyway, just my 2 cents as usual. My Duct Tape or Aluminum Tape has worked well for many years and I maybe have 5 minutes total and $2 total over the past 7 years of doing it.

Cheers,
Stein.

Click to expand...

Stein,

Being a transplant from Las Vegas to Minnesota I am trying to quickly learn about true winter flying. I like your duct tape idea, but wondered if you could actually use run-of-the-mill duct tape from Home Depot or if you have to use some special temp tollerant tape to block off part of the oil cooler. Also, are you having to block off part of the cylinders to get a reasonable CHT?

I have recently noticed that my CHTs can get under 300 when flying in the sub zero range, especially with LOP ops. Oil temps are consistantly 165, which I would much rather see around 185 degrees or a little more. I really want to see my CHTs closer to 330+ so I am thinking about a little experimenting with the duct tape blockage method on the cylinders if the oil cooler blockage is not enough! Any advice you or other have for types of tape to use and just how much you are blocking will give me a little headstart.

Travis
RV-7A @ 21D

Hi Travis,

On the oil cooler we use aluminum tape (same type found at Home Depot or wherever for HVAC stuff) and on the inlets I just use plane old grey duck tape! Worked well for years and easily adjustable. Something that is needed considering the wild fluctuations in temp. For those un-initiated to MN weather, less than a week ago we had HIGH temps of 3 degrees, today it was it was 38 but there will be many more single digit days ahead!

Cheers,
Stein