I took an aerodynamics class in college and still did not understand the Reynolds number. All I can remember is my professor essentially saying "always go with 1,000,000" since we were dealing with small real airplanes like the Cessna 172 and Piper Cherokee. You explained this significantly better. Thank you
Above this number, Reynolds number, turbulence happens, then it affects the fluid pressure at the measuring space, and also can cause vibrations that affects the surrounding material microstructures. ( Arthur Guyton’s Medical Physiology). Best Whishes from Rio.
These "small planes" are not small at all. Thanks to that heavy engine and the gas tanks, these monsters are two or three times the weight of a two seater glider, that does not need such a huge metal blob up front. And try parking such a "small plane" in a parking lot, you'll see it is far larger than a car. Even those ridiculous large American cars are dwarfed by what you call a "small plane". You are confused by the size of airliners. You think a 200 person plane is the normal size, well, it is close to the size limit. Any larger and it will break in two. Small planes are half the weight and size of the planes mentioned. Do check the Rutan designs, the smallest EZ's are twice as fast and burn way less fuel (mileage) than the giants you named. Because they are way smaller and way smarter designed. That tail in the back is pulling a plane to the ground! Did you ever think about that? It is safe, it is as stable as a badminton shuttle, but it is pulling you DOWN. And causing extra drag. Why do people keep using such fuel burners? I don't get it. Get out more. Do a conversion to an EZ. That does not imply you must buy one, but I bet you will want one. Because of the Reynolds number, maybe.
I kinda learned about it in numerical methods class without knowing what it really was. We were running simulations with particles that are sneezed out.
Rutans are not good concepts, at least the first 20 years worth@@voornaam3191. No one in his right mind would put the engine in back, and the tail up front. The Wrights were equally misguided!
Basically while the size of model aircraft and their control surfaces changes, the density, inertia (or force needed to move) and viscosity of air stays around the same, so model aircraft and the way that they fly doesn't scale linearly with respect to the properties of air and this has to be factored in when building model aircraft at different scale sizes.
Discovered Reynolds analogy in 1971 when I tried to design a paper airplane for the UCLA Engineering week paper airplane contest. It can be discussed without numbers. Basically as airfoil get smaller, the viscosity of the air becomes more important than inertial forces like lyft. That led me to use an auto gyro design that was very small with no camber. This about 3 inch tall auto gyro flew 3 times longer than 2nd place. Latter I made 1/2 inch tall auto gyros from vellum using surgical tools. These stayed aloft very long. The problem was they had to be handled with forceps and were easily lost.
You know what you are talking about. And when you learn something deeply, you will never forget. :) Thanks !.
4 місяці тому+9
It is indeed the case that parasitic or induced drag decreases with increasing Reynolds number (Re), but at the scales in question, it is not the Re that causes this decrease; rather, it is the design of the wing, especially the tips and the airfoil, that plays a role. Despite this minor detail, the video is of a high standard, the explanation is very intuitive, and the author explains it very well. I am a university professor and I teach in a Master of Drones programme. I would like to express my admiration for the author's ability to explain this topic so clearly and effectively.
Thank you, well explained. I asked many people who studied aeronautics at the university and so far no one was able to explain it to me so that I could understand it. Now you did that in 5 min. You did a great job. Now I got it.
Re number is also very important in measuring if a model will behave like the full size version (assuming you’re trying to compare a model with a real plane). To be able to compare, you need to have the same Reynolds number
Your pronunciation is Reynolds is something I've never heard, as far as I'm aware the y is silent, so it sounds like Renolds. But I think the best way to describe Reynolds number is that it is the ratio between inertial force effects and viscous force effects. Inertia = velocity, size, density. And viscous force effects = viscosity. So all the inertial terms are in the numerator, the viscous term is in the denominator. So at low Reynolds number, viscosity starts to become more dominant and viscosity tends to act to dissipate. In a high Reynolds number flow, the inertia of the air is more dominant and the flow tends towards more turbulent states because viscosity has a stabilising effect, so when Reynolds number is high, the flow is less stable and more prone to turbulence, but turbulence can have positive or negative effects depending on the situation. For very small aircraft (micro-aerial vehicle scale, or the scale of insects) conventional wings we use on large aircraft tend to not work very well, which is why insects get away with having non-aerofoil type wings, but on the flip side at low Reynolds number things like leading edge vortices can promote high lift (the mechanism used by many insects to achieve high performance).
What really helped me in school to understand what the Reynolds number is telling you is that it compares the pressure forces to that of the sheer forces, when the pressure forces far exceed the sheer you have a high Reynolds number!
Not quite right. Reynolds is defined as the ratio between inertial force effects and viscous force effects. Inertia = velocity, size, density. And viscous force effects = viscosity. That means a low Reynolds number is one where viscosity plays a large role at dissipating, a high Reynolds number tends to be one more prone to turbulence because viscosity acts to stabilise turbulence so when inertial forces are much much larger than viscous forces, the flow tends to break down into turbulence.
Correct. I think of it in a real life exaggeration. A 747 would go through a thick fluid like maple syrup much easier than a small bird. The inertial forces of size weight and all that moment moving fast overcomes the viscous forces much easier than a small bird with little weight and speed, being overcome by the viscous forces. The inertial forces of the fluid rival that of the bird and its motion… but not so much that of the 747!
Great video. It's worth noting the "knee" in the curve at about 4500. For best performance, you want your Reynolds number to be above about 5000. After that, the benefit drops off significantly. And we know drag increases greatly with velocity in general. So it's not so much "fly faster to go further", but "fly fast enough to get your R above 5000". This also means that small, slow indoor planes, with R below 4500, can have very different design characteristics from larger, faster outdoor planes. Slow park flyers can be stuck in the middle, halfway in between the viscous regime and the inertial regime. Anyway yeah fly faster or have a longer chord IF you're flying a tiny plane really slow, so your R is below 5000. Once you get above 5,000, increasing your speed gets costly because parasitic drag is proportional to airspeed SQUARED.
The point of Re is that different craft with the same Re behave the same way. This means that you can model a full size plane at a smaller scale by increasing the viscosity of the fluid.
Sixty years ago I struggled with this while studying for my commercial licence. I finally understand what I didn't really need to know. Thanks for the excellent presentation.
Neat! I like the format and the calm and technical narration. Reminds me of filming my paper airplanes and later models with my dad's VHS camera. This is, of course, taking it a couple of notches further and supporting it with engineering knowledge. Well done!
This is a bit misleading. Flying faster increases the Reynolds Number, and gives a lower coefficient of drag. BUT, for that same fuselage or whatever the drag force increases as the square of the speed. Because it is exerted over a larger distance, the power required goes up as the cube of the windspeed. This more than negates the advantage of the higher Reynolds Number. There are airfoils that work good at lower Reynolds Numbers, and airfoil lift coefficient definitely varies with RN. The BEST OVERALL BOOK on model design is Andy Lennon's R/C MODEL AIRCRAFT DESIGN.
You are correct that putting the glider into a steeper dive to fly faster makes it fly a shorter distance, not longer. Instead, you would need to add more weight onto the glider to make it fly faster, since a glider's weight does not (directly) affect its flight distance.
Um, you got that backwards. Efficiency is achieved with impossibly thin wings of high aspect ratio. For a given lift force, i.e. weight, a higher aspect ratio demands a lower induced drag for a given speed. Adding chord adds skin friction drag without providing the efficiency of aspect ratio. Adding weight increases speed because the weight must be balanced by lift, and lift is proportional to speed squared. Add weight to the smaller model to make it cross the distance faster, crossing the entire distance before hitting the ground, possibly.
It's not necessarily due to the reynolds number being better on the larger airplane, but that the smaller airplane is more dense than the larger one when they're compared to scale, if you used a hypothetical growth ray on the smaller one to make it exactly as large as the big one you'd find that it would be heavier than the formerly bigger one. This is the thing with making the same model at different sizes from the same material, the smaller ones are heavier to scale than their larger counterparts.
If I didn’t speak English I’d think this was a political rant with a major ultimatum at the end. Very intense and direct. I did like the video and I think you conveyed the information well.
3:25 Reynolds number *does* affect lift and drag coefficient. Especially at higher AOA. Most of airfoils with RN at least 2-3 millions will stall at about 15 degrees. At half million stall angle in most cases will decrease to around 10 degrees, sometimes around 7 degrees and that is less than half.
@@norbert.kiszka quote the title of one. I agree that the viscous effects are negligible at high speeds, and the divergence of the flow from the surface is due to the high ratio of the time integral of static pressure*area to momentum. But, in general, people don't talk about stalls outside of the critical angle of attack. Scaling models seems to be a dubious undertaking because the characteristic length is on the same side of the ratio as speed.
Reynolds number can actually be used in all sorts of fluid flow situations, not just for designing aeroplanes. The key point is that if the Reynolds number is kept constant for a fixed geometry, then the flow patterns will be the same. As a chemical engineer, I use it in pipe pressure drop calculations, for mixing calculations in agitated tanks, or when considering the flow of fluid around solid particles. It can be used to predict the fluid velocity at which the flow goes from laminar to turbulent (for a pipe, flow is laminar below 2100 and turbulent above 4000, where the characteristic dimension when calculating the Reynolds number is the inside diameter of the pipe). The boundary layer thickness depends on the Reynolds number: the higher the Reynolds number the thinner the boundary layer.
@@isaacmarkovitz7548 If you do the calculations for typical industrial plant using liquids like water, common solvents, gasoline, diesel or jet fuel etc. you will always be a long way into the turbulent regime. You need small diameters (3 mm or thereabouts) to make transitional flow a possibility.
Thanks. I have 14,000 hours flying and never really understood about Reynolds numbers- although I knew that they changed with size. I learned a few things today.
I seem to recall something about "dynamic similitude". Meaning, you cannot linearly scale (your small plane scaled 2x) to get twice the performance - or something like this.
What increases the Reynolds Number? Mostly size. The air is the air. It is a constant. So, the larger the aircraft, the proportionally thicker the air surrounding it is. This is why airliners appear to move laterally so slowly, compared to small model aircraft.
This is also the reason why a smaller plane has to be much more aerodynamically precise than a much larger plane. Thanks for sharing! Please have an excellent and awesome day! ☀️✨🌟✈️
This video is very informative, very on point, very useful, and you are looking at the camera like you are very mad. Subscribed! I need to learn how to select an airfoil for a 3d printed aircraft with swept wings. I know the spar dimensions, the expected speeds, and the size. I will continue watching your content.
I always wondered how Reynolds number affected wind tunnel modeling of airfoils. The Wright brothers were the first to empirically study airfoil lift and drag efficiencies through wing tunnel modeling, and as you have stated, lift coefficient is not significantly affected by Reynolds number. This is why their experiments to find the best lift characteristics using extremely small airfoil models in a low velocity wind tunnel were successful.
No doubt… especially the actual full size one on the 1903 flyer; blunt to say the least! Terrible stall characteristics. I’m convinced that their idea to control pitch through canard elevators (for safety reasons as the forward structure would absorb crash energies) saved their lives, but not for that reason. The canard configuration minimized/tempered main wing stalls… an added benefit they probably didn’t realize right away.
This case is only true when your Reynolds number is high enough that there is turbulent boundary layer over the wing. For low reynolds number the laminar boundary layer will separate quickly and the Cd will increase.
Great lecture, your knowledge is precious, and very few can explain it so comprehensively. Still, it can either be a Parasite OR a Drag, but not both at the same time... It can anyways be a Drag, with the qualities of a Parasite, thus, a "parasitic Drag" (grammar: an adjective for a noun)
El número de Reynolds expresa las relaciones entre fuerzas de viscosidad e inercia en un fluido en movimiento, que pueda ayudar a predecir el paso de flujo laminar a flujo turbulento es un uso práctico del número de Reynolds. Las aves vuelan en un entorno de números de Reynolds bajos
Ja, genau. Leider versteht der junge Mann das nicht. Ein wunderbares Buch, das er lesen sollte, trägt den Titel „Die einfache Wissenschaft des Fliegens“.
It does help a little bit, you can see the larger plane pull up slightly when it’s about half a wingspan off the ground. That’s another advantage of making the planes larger.
One potential flaw (?) in the glide comparison demo was not accounting for weight. The larger plane weighed more and thus possessed more potential energy launched from the same height as the smaller, lighter unit. For a more pure apples/apples comparison, I’d love to see a comparison between the two gliders with the smaller one weighted to the same value as the larger one.
The distance that a glider flies is equal to its lift-to-drag ratio multiplied by its starting height, so weight actually doesn't directly affect the distance it flies (although that's not the case for powered aircraft). Adding more weight to the small glider, however, would have made it fly faster, which would have increased its Reynolds number, decreased its parasite drag coefficient, and therefore made it fly further. In that way, adding more weight to the small glider could indirectly make it a match for the larger glider.
Reynolds number is SO much more than this. It's actually from the field of Chemical Engineering and is used to predict the transition from laminar to turbulent flow in fluids.
I actually think he already talks in a good speed. Not too fast, just like somebody who is really relaxed :). His "strong looking" face is probably a result of physical training :).
I am a glider pilot. I owned a glider with flaps and shifting to negative flap settings when gliding between thermals substantially increased performance. I had three negative settings, each one representing a different “drag bucket.” Any thoughts about that phenomenon?
May I guess? I'm sure the Soaring Society of America has a magazine article with the answer. Anyway, the thinner the wing, the lower the drag. Ideally, a wing would have zero thickness. That is impossible for structural reasons. Reflexing the flap effectively flattens the profile of the airfoil, thinning it slightly, and reducing drag.
@@DumbledoreMcCracken I agree with your statement about effectively thinning the wing. It’s fascinating. And when you click in negative flaps, the nose rises slightly and you sort of automatically achieve an optimal speed. Excellent for cross-country flying. I’ve never seen a “Soaring” (the SSA magazine) article about it, but it’s likely there. I did find a dissertation by a NC State aeronautical engineering student about slow speed aerodynamics but drag buckets and negative flaps were not mentioned.
@@DumbledoreMcCracken that's true (not exactly - I will explain it little later in this comment), but only for very low angle of attack. Slower planes has more thick wings (mostly 12-18% of chord) and faster planes has thin wings (4-12% of chord). That's because of mentioned R. number, air viscosity and in some cases mach speed (thicker airfoil will generate more increase of speed at top layer). If You decrease thickness, then stall AOA in most cases will be lower. Drag will be lower for AOA=0. But at some point (like 1-2 deg.) drag will be higher.
@norbert.kiszka there is no reason for a slow plane to have a thick wing. Modern sailplanes have a very very thin wing. While the usable range of Angle of Attack is narrower for a thin wing, that is not an issue for real airplanes. Aircraft that require a large range of AoA have a bendable chord via leading edge flaps, or fixed or retractable slats. My buddy has let me stall his de Havilland Chipmunk a lot. It has a beautifully gentle stall. I attribute that characteristic to its thick, round, gentle airfoil. The Chipmunk is an aerobatic training airplane, so de Havilland may have wished to give the students a more relaxed plane to fly. However, and in general, most planes are designed for speed and low fuel consumption, and require a relatively thin wing (plus flaps, at a minimum) to achieve those goals. Training prevents stall from being a serious issue. Nobody flies with a high drag AoA unless he wants to lose altitude without gaining speed.
@@DumbledoreMcCracken then try to look at many different airfoils, especially designed for low Reynolds numbers (in most channels links are not allowed). In previous comment I missed a small fact, that most thick airfoils (like a 25% chord) are used in wind power plants. Making airfoil thin at around 4% it will be very good for fast fighter jets, but it will be terrible for slow planes. Changing thickness from (for example) 12 % to 11 or 13 will generate significant change in characteristics at slowest and highest speeds. Try any aerodynamic simulation software and You will see that what Im saying is the true.
I think that statement is incorrect. Common misunderstanding. While the non-dimensional Reynolds number ratio is lower for lower velocities, for a constant characteristic length, the viscous force does not contribute to lift in a meaningful way because the viscous force points in the wrong direction. Notice the orthogonality of the statement. If we invert the Re number, it is clear that the viscous force is miniscule to the inertial force. Flight must be the result of the inertial forces. Q.E.D.
Reynolds number formula indicates precisely when where and which factors are causing turbulent flow, no matter the surface, wing, rocket combustion chamber, or even heart valves.
I just finished my second year in aeronautical engineering and you just taught me more about Re number in 5 min than in my 2 years, such a shit education system
i mean it's not a difficult concept (it's quite literally just a ratio between dynamic and viscous/friction forces) but the application of this figure is much more technical which you will see when you study incompressible aerodynamics. it's hard to introduce such a figure without being able to go in depth on what it actually does so i can see why you might be confused. i'm sure you will learn a lot more over the next year so don't be discouraged. btw, reynolds nuymber is much less important past a certain point as your coefficient of lift is invariant all the way down to your level flight stall speed. this guy just happens to be running a model plane (small and slow aircraft) channel and not a general aviation (much much larger and faster) channel so in this case, it is much more relevant. ig that's why a drones prof has shown up in the comments lol.
Thank you for your nice and informative videos. What about the Cubic Wing Loading WCL? planes with low WCL float in the air, are slow, and have low stall speed. On the other hand, planes with high WCL are fast and have high stall speed. For both airplanes used in your demonstration, what are the WCL? is there any direct relation between the Reynold's number and WCL? I know there is not, but what about an indirect relation?
airspeed influence drag coefficient that is biggest misconception. Geometry of wing under different airspeeds changes drag coefficient, that is complete nonsense. Many solutions disregard engine thrust (airspeed), which sustain energy conservation condition.
This is a spectacular video! You are incredibly informative. Being an airplane builder myself, Reynolds’s number always confused me. Thank you very much, and have a nice day!
So, would it be true to say... All else being equal, if reducing parasitic drag will increase the Re number? In other words... The smaller the airframe (scale), the lower the Re, the more critical that parasitic drag becomes in design, (if wanting to improve L/D and/or minimum sink)?
Yes, I’ve found that below a Reynolds number of about 50,000, the parasite drag coefficient of airfoils increases significantly, which would make streamlining more important. The drag coefficients of other shapes may be affected by the Reynolds number in different ways though.
You keep saying further, but mean farther. (i.e. The gas station is farther from my home is an imperial measurement.) (i.e. He is spiritually further ahead in life than I am, cant not be measured with imperial measurements) I hope this helps.
What about the scaled travel ratios ? If we throw 1x and 2x scaled planes, measure their distance and divide the distance of larger plane by 2, would it give the same result ?
The distance a glider flies is determined by the ratio of the lift coefficient to the drag coefficient, multiplied by the height it was thrown from. The small plane was at a disadvantage because it had a higher drag coefficient.
If all the structural geometries are identical in scale, the smaller plane is at a disadvantage. However structural strength doesn't scale linearly either, and favors smaller, lighter, stronger structural solutions as you scale down to some degree. Replacing the fuselage surface area with a long soda straw, removes parasitic drag while retaining wing area. Likewise the smaller slower model has smaller control forces, so the tail section can be significantly smaller if positioned farther from the CG or CM, with a larger lever length. Scaling planes up has similar results, often requiring significantly heavier and stronger structural elements.
info synthesized nicely! please consider taking more care while editing the video chops next time, specially when it comes to volume variations, it was hard to understand sometimes as the trafic noises filtered through my window. with that siad, please keep the great work i cant wait to see your next video! :)
You can use any length you want to calculate the Reynolds number, as long as you stay consistent when comparing different Reynolds numbers. The aspect ratio does not directly affect the Reynolds number, but it is possible that changing it could also change the characteristic length and therefore the Reynolds number too.
@@Toomanydaysthe chord length is the determining factor, and speed, when discussing Reynolds number. The wing span is irrelevant. Unfortunately, Reynolds number is described in an algebraically simplified form. The factors are more understandable when the form of the equation is not algebracially simplified.
5 minutes in one sentence: Small things moving through air (e.g. bees) see it as sirup. Large things (e.g. B747) see it as a fluid like water). Hard to explain, I also needed time to understand. Running through my fathers garden with a Airfix model in my hand I wondered: he is small, the real aircraft big, the air is the same. How does the model aircraft feel the air? Doing my batchelor, and later master in aviation, it was eventually explained.
Great video. I dont have foam board available in my area so can you please make the next plane with some other material please.And can you also teach us how to make a rubber powered plane. Great videos thank you very much.
In a later video I plan on talking about building materials. You can also use pressed or corrugated cardboard, balsa wood, corrugated plastic, thick paper, or insulation foam. I'll consider making a video about rubber powered planes.
I am curious as to your background. The Reynolds Number is the ratio of inertia stresses to viscous stresses. It does affect lift coefficient as well as drag. This requires understanding boundary layer theory.
I majored in aerospace engineering. The boundary layer and stresses are beyond the scope of these videos so I didn’t mention these subjects to keep things simple. The main point of this video is that if you make your glider too small and/or too slow, it could reduce your flight distance.
My undergrad degree was in aero/astro engineering from Purdue in ‘82. This was followed by graduate degrees in aerospace engineering from a joint program from the University of Dayton and AFIT. My research interests were aero elastic effects at low Reynolds Number. I retired as Aerospace Fellow from Honeywell in 2021. You were wise in selecting Aerospace Engineering as a career. It does not get better than that. Aerospace Engineers rock! Cheers!
I took an aerodynamics class in college and still did not understand the Reynolds number. All I can remember is my professor essentially saying "always go with 1,000,000" since we were dealing with small real airplanes like the Cessna 172 and Piper Cherokee. You explained this significantly better. Thank you
Above this number, Reynolds number, turbulence happens, then it affects the fluid pressure at the measuring space, and also can cause vibrations that affects the surrounding material microstructures. ( Arthur Guyton’s Medical Physiology). Best Whishes from Rio.
These "small planes" are not small at all. Thanks to that heavy engine and the gas tanks, these monsters are two or three times the weight of a two seater glider, that does not need such a huge metal blob up front. And try parking such a "small plane" in a parking lot, you'll see it is far larger than a car. Even those ridiculous large American cars are dwarfed by what you call a "small plane". You are confused by the size of airliners. You think a 200 person plane is the normal size, well, it is close to the size limit. Any larger and it will break in two. Small planes are half the weight and size of the planes mentioned. Do check the Rutan designs, the smallest EZ's are twice as fast and burn way less fuel (mileage) than the giants you named. Because they are way smaller and way smarter designed. That tail in the back is pulling a plane to the ground! Did you ever think about that? It is safe, it is as stable as a badminton shuttle, but it is pulling you DOWN. And causing extra drag. Why do people keep using such fuel burners? I don't get it. Get out more. Do a conversion to an EZ. That does not imply you must buy one, but I bet you will want one. Because of the Reynolds number, maybe.
I kinda learned about it in numerical methods class without knowing what it really was. We were running simulations with particles that are sneezed out.
Reynolds number is an adimensional number, has no units, that indicates relationships between inertia and viscosity forces inside a moving fluid
Rutans are not good concepts, at least the first 20 years worth@@voornaam3191. No one in his right mind would put the engine in back, and the tail up front. The Wrights were equally misguided!
Basically while the size of model aircraft and their control surfaces changes, the density, inertia (or force needed to move) and viscosity of air stays around the same, so model aircraft and the way that they fly doesn't scale linearly with respect to the properties of air and this has to be factored in when building model aircraft at different scale sizes.
I asked two aero-e’s to explain this. Neither could dumb it down to my level. Thanks!
Discovered Reynolds analogy in 1971 when I tried to design a paper airplane for the UCLA Engineering week paper airplane contest. It can be discussed without numbers. Basically as airfoil get smaller, the viscosity of the air becomes more important than inertial forces like lyft. That led me to use an auto gyro design that was very small with no camber. This about 3 inch tall auto gyro flew 3 times longer than 2nd place.
Latter I made 1/2 inch tall auto gyros from vellum using surgical tools. These stayed aloft very long. The problem was they had to be handled with forceps and were easily lost.
You know what you are talking about. And when you learn something deeply, you will never forget. :) Thanks !.
It is indeed the case that parasitic or induced drag decreases with increasing Reynolds number (Re), but at the scales in question, it is not the Re that causes this decrease; rather, it is the design of the wing, especially the tips and the airfoil, that plays a role. Despite this minor detail, the video is of a high standard, the explanation is very intuitive, and the author explains it very well. I am a university professor and I teach in a Master of Drones programme. I would like to express my admiration for the author's ability to explain this topic so clearly and effectively.
UA-cam has made a great service for me today by showing me this channel lol
I am very glad youtube recommended me this. You did a good job explaining the topic it in a short timespan.
Thank you, well explained. I asked many people who studied aeronautics at the university and so far no one was able to explain it to me so that I could understand it. Now you did that in 5 min. You did a great job. Now I got it.
As someone who has applied for an aerospace course at university, I am subscribed
Dude you are awesome! Excellent content and droll, over serious tone and demeanor! Excellent!
Re number is also very important in measuring if a model will behave like the full size version (assuming you’re trying to compare a model with a real plane). To be able to compare, you need to have the same Reynolds number
But how would you do that when the number is in the millions for large planes?
Your pronunciation is Reynolds is something I've never heard, as far as I'm aware the y is silent, so it sounds like Renolds.
But I think the best way to describe Reynolds number is that it is the ratio between inertial force effects and viscous force effects. Inertia = velocity, size, density. And viscous force effects = viscosity. So all the inertial terms are in the numerator, the viscous term is in the denominator.
So at low Reynolds number, viscosity starts to become more dominant and viscosity tends to act to dissipate. In a high Reynolds number flow, the inertia of the air is more dominant and the flow tends towards more turbulent states because viscosity has a stabilising effect, so when Reynolds number is high, the flow is less stable and more prone to turbulence, but turbulence can have positive or negative effects depending on the situation.
For very small aircraft (micro-aerial vehicle scale, or the scale of insects) conventional wings we use on large aircraft tend to not work very well, which is why insects get away with having non-aerofoil type wings, but on the flip side at low Reynolds number things like leading edge vortices can promote high lift (the mechanism used by many insects to achieve high performance).
Wow you made it so clear. Thx
What really helped me in school to understand what the Reynolds number is telling you is that it compares the pressure forces to that of the sheer forces, when the pressure forces far exceed the sheer you have a high Reynolds number!
Not quite right. Reynolds is defined as the ratio between inertial force effects and viscous force effects. Inertia = velocity, size, density. And viscous force effects = viscosity.
That means a low Reynolds number is one where viscosity plays a large role at dissipating, a high Reynolds number tends to be one more prone to turbulence because viscosity acts to stabilise turbulence so when inertial forces are much much larger than viscous forces, the flow tends to break down into turbulence.
Correct. I think of it in a real life exaggeration. A 747 would go through a thick fluid like maple syrup much easier than a small bird. The inertial forces of size weight and all that moment moving fast overcomes the viscous forces much easier than a small bird with little weight and speed, being overcome by the viscous forces. The inertial forces of the fluid rival that of the bird and its motion… but not so much that of the 747!
Short and to the point - a great video!
Great video. It's worth noting the "knee" in the curve at about 4500. For best performance, you want your Reynolds number to be above about 5000. After that, the benefit drops off significantly.
And we know drag increases greatly with velocity in general. So it's not so much "fly faster to go further", but "fly fast enough to get your R above 5000".
This also means that small, slow indoor planes, with R below 4500, can have very different design characteristics from larger, faster outdoor planes.
Slow park flyers can be stuck in the middle, halfway in between the viscous regime and the inertial regime.
Anyway yeah fly faster or have a longer chord IF you're flying a tiny plane really slow, so your R is below 5000. Once you get above 5,000, increasing your speed gets costly because parasitic drag is proportional to airspeed SQUARED.
Bro is INTENSE!!
Aspergers
The point of Re is that different craft with the same Re behave the same way. This means that you can model a full size plane at a smaller scale by increasing the viscosity of the fluid.
Sixty years ago I struggled with this while studying for my commercial licence. I finally understand what I didn't really need to know. Thanks for the excellent presentation.
Neat! I like the format and the calm and technical narration.
Reminds me of filming my paper airplanes and later models with my dad's VHS camera.
This is, of course, taking it a couple of notches further and supporting it with engineering knowledge.
Well done!
Come on man, keep those videos coming!
I just discovered you and I am addicted...
Bro is on point. 10/10 on communication. Perfect!
This is a bit misleading. Flying faster increases the Reynolds Number, and gives a lower coefficient of drag. BUT, for that same fuselage or whatever the drag force increases as the square of the speed. Because it is exerted over a larger distance, the power required goes up as the cube of the windspeed. This more than negates the advantage of the higher Reynolds Number. There are airfoils that work good at lower Reynolds Numbers, and airfoil lift coefficient definitely varies with RN. The BEST OVERALL BOOK on model design is Andy Lennon's R/C MODEL AIRCRAFT DESIGN.
Interesting!
You are correct that putting the glider into a steeper dive to fly faster makes it fly a shorter distance, not longer. Instead, you would need to add more weight onto the glider to make it fly faster, since a glider's weight does not (directly) affect its flight distance.
Um, you got that backwards.
Efficiency is achieved with impossibly thin wings of high aspect ratio.
For a given lift force, i.e. weight, a higher aspect ratio demands a lower induced drag for a given speed. Adding chord adds skin friction drag without providing the efficiency of aspect ratio.
Adding weight increases speed because the weight must be balanced by lift, and lift is proportional to speed squared.
Add weight to the smaller model to make it cross the distance faster, crossing the entire distance before hitting the ground, possibly.
It's not necessarily due to the reynolds number being better on the larger airplane, but that the smaller airplane is more dense than the larger one when they're compared to scale, if you used a hypothetical growth ray on the smaller one to make it exactly as large as the big one you'd find that it would be heavier than the formerly bigger one.
This is the thing with making the same model at different sizes from the same material, the smaller ones are heavier to scale than their larger counterparts.
If I didn’t speak English I’d think this was a political rant with a major ultimatum at the end. Very intense and direct. I did like the video and I think you conveyed the information well.
thank you airplane sheldon
Incredible vid. Please dont stop making these
3:25 Reynolds number *does* affect lift and drag coefficient. Especially at higher AOA. Most of airfoils with RN at least 2-3 millions will stall at about 15 degrees. At half million stall angle in most cases will decrease to around 10 degrees, sometimes around 7 degrees and that is less than half.
What is your justification for the statement?
@@DumbledoreMcCracken NACA papers, papers about fixed-wing UAVs (especially slow ones) and my own experience.
@@norbert.kiszka quote the title of one.
I agree that the viscous effects are negligible at high speeds, and the divergence of the flow from the surface is due to the high ratio of the time integral of static pressure*area to momentum.
But, in general, people don't talk about stalls outside of the critical angle of attack.
Scaling models seems to be a dubious undertaking because the characteristic length is on the same side of the ratio as speed.
That was a sweet explanation, man
Reynolds number can actually be used in all sorts of fluid flow situations, not just for designing aeroplanes.
The key point is that if the Reynolds number is kept constant for a fixed geometry, then the flow patterns will be the same.
As a chemical engineer, I use it in pipe pressure drop calculations, for mixing calculations in agitated tanks, or when considering the flow of fluid around solid particles.
It can be used to predict the fluid velocity at which the flow goes from laminar to turbulent (for a pipe, flow is laminar below 2100 and turbulent above 4000, where the characteristic dimension when calculating the Reynolds number is the inside diameter of the pipe). The boundary layer thickness depends on the Reynolds number: the higher the Reynolds number the thinner the boundary layer.
How do you handle the transition region between 2100 and 4000? Do you just design away from it or go with whichever f value is more pessimistic?
@@isaacmarkovitz7548
If you do the calculations for typical industrial plant using liquids like water, common solvents, gasoline, diesel or jet fuel etc. you will always be a long way into the turbulent regime. You need small diameters (3 mm or thereabouts) to make transitional flow a possibility.
Thanks. I have 14,000 hours flying and never really understood about Reynolds numbers- although I knew that they changed with size. I learned a few things today.
Am i the only one who thinks that this guy is the real life version of sheldon cooper
Excellent analysis! Indeed, the Reynold's # is important to be considered.
I seem to recall something about "dynamic similitude". Meaning, you cannot linearly scale (your small plane scaled 2x) to get twice the performance - or something like this.
What increases the Reynolds Number? Mostly size. The air is the air. It is a constant. So, the larger the aircraft, the proportionally thicker the air surrounding it is. This is why airliners appear to move laterally so slowly, compared to small model aircraft.
This is also the reason why a smaller plane has to be much more aerodynamically precise than a much larger plane.
Thanks for sharing!
Please have an excellent and awesome day!
☀️✨🌟✈️
This video is very informative, very on point, very useful, and you are looking at the camera like you are very mad. Subscribed!
I need to learn how to select an airfoil for a 3d printed aircraft with swept wings. I know the spar dimensions, the expected speeds, and the size. I will continue watching your content.
Love your approach to the aviation
Very nicely presented! Clear and understandable.
bro where are the next videos??
I am really invested in them.
Excellent Explanation 👍
Teach us how to use the Reynolds’s number to select an airfoil
I always wondered how Reynolds number affected wind tunnel modeling of airfoils. The Wright brothers were the first to empirically study airfoil lift and drag efficiencies through wing tunnel modeling, and as you have stated, lift coefficient is not significantly affected by Reynolds number. This is why their experiments to find the best lift characteristics using extremely small airfoil models in a low velocity wind tunnel were successful.
Surprisingly, the type of airfoil the Wright Brothers were using is very uncritical about the Reynolds number because of its sharp leading edge.
No doubt… especially the actual full size one on the 1903 flyer; blunt to say the least! Terrible stall characteristics. I’m convinced that their idea to control pitch through canard elevators (for safety reasons as the forward structure would absorb crash energies) saved their lives, but not for that reason. The canard configuration minimized/tempered main wing stalls… an added benefit they probably didn’t realize right away.
wow great stuff. I shall now watch every single one of your videos
This video is really interesting, thank you so much!
? Increasing reynolds number does increase the sectional lift coefficient and hence the overall lift coefficient is also enhanced.
Bro does not BLINK!! Elite
I'm not into this thing, but I liked the content! Thank you.
Is there a series of reference books for aircraft design that you recommend? Like Jan Roskam
This case is only true when your Reynolds number is high enough that there is turbulent boundary layer over the wing.
For low reynolds number the laminar boundary layer will separate quickly and the Cd will increase.
..your content very well explained with the two gliders; the same when ducks and geese stop swimming, the geese will go further!
Great lecture, your knowledge is precious, and very few can explain it so comprehensively. Still, it can either be a Parasite OR a Drag, but not both at the same time... It can anyways be a Drag, with the qualities of a Parasite, thus, a "parasitic Drag" (grammar: an adjective for a noun)
Great vid. Thank you. Si, Christchurch, NZ.
Interesting and thanks. Once you set up a spreadsheet, you might be able to figure our and draw useful graphs to predict airplane behaviors?
The lower the Reynolds number, the more it’s like swimming through thick soup.
El número de Reynolds expresa las relaciones entre fuerzas de viscosidad e inercia en un fluido en movimiento, que pueda ayudar a predecir el paso de flujo laminar a flujo turbulento es un uso práctico del número de Reynolds.
Las aves vuelan en un entorno de números de Reynolds bajos
Ja, genau.
Leider versteht der junge Mann das nicht.
Ein wunderbares Buch, das er lesen sollte, trägt den Titel „Die einfache Wissenschaft des Fliegens“.
How much does ground effect help the larger plane fly across the gym?
It does help a little bit, you can see the larger plane pull up slightly when it’s about half a wingspan off the ground. That’s another advantage of making the planes larger.
One potential flaw (?) in the glide comparison demo was not accounting for weight. The larger plane weighed more and thus possessed more potential energy launched from the same height as the smaller, lighter unit. For a more pure apples/apples comparison, I’d love to see a comparison between the two gliders with the smaller one weighted to the same value as the larger one.
Interseting thought - Theory suggests the big one WILL still go further
but
*Speaking as an empiricist*
SHOW ME *THEN* I'll be convinced
The distance that a glider flies is equal to its lift-to-drag ratio multiplied by its starting height, so weight actually doesn't directly affect the distance it flies (although that's not the case for powered aircraft). Adding more weight to the small glider, however, would have made it fly faster, which would have increased its Reynolds number, decreased its parasite drag coefficient, and therefore made it fly further. In that way, adding more weight to the small glider could indirectly make it a match for the larger glider.
You are doing very well. Nicely explains the basics. Slowly you also add electronics and make remote control plane.
Thanks
Add electronics to stabilise planes and get reproduce able results. Avoid human error and remote control.
By the way you look to the camera I feel like this is some serious CIA instructions not a model plane video
We need more videos, good stuff
What a wonderful channel. Subscribed. Have you calculated the reynolds number of your tache? (I don't know why I felt compelled to ask that)
Nicely done.
I've been looking for exactly what you're doing.
Thanks.
Reynolds number is SO much more than this. It's actually from the field of Chemical Engineering and is used to predict the transition from laminar to turbulent flow in fluids.
And mechanical engineering
Yes, our guide doesn't understand what he proselytizes.
…. Waiting for your UA-cam video and description of RN… 🙄
@@rjhinnj and waiting for yours
Dude, I did not disagree with this video instruction. YOU did.
Immediately subbed thanks
I understand with practical examples mostly. Did CSE by mistake bt this is what I really wanted do.
It’s a interesting field how to relate wind tunnel models to the real world by interpreting the RE number ( and wall effects)
Thanks for that info.
Thank you. Nice explanation of very complicated subjects
try to relax a bit in presentation. it looks like you're trying to blow up my head with your mind. 👍
I actually think he already talks in a good speed. Not too fast, just like somebody who is really relaxed :). His "strong looking" face is probably a result of physical training :).
@@quaditz 50% of the problem is the lighting. A strong almost directly overhead light is not kind.
Are you picking on aspies?
@@brucebaxter6923 no
@@KX36
You sure.
Sure looks like it.
Condors vs sparrows
Low wing loading vs. high wing loading
I am a glider pilot. I owned a glider with flaps and shifting to negative flap settings when gliding between thermals substantially increased performance. I had three negative settings, each one representing a different “drag bucket.” Any thoughts about that phenomenon?
May I guess? I'm sure the Soaring Society of America has a magazine article with the answer.
Anyway, the thinner the wing, the lower the drag. Ideally, a wing would have zero thickness. That is impossible for structural reasons. Reflexing the flap effectively flattens the profile of the airfoil, thinning it slightly, and reducing drag.
@@DumbledoreMcCracken I agree with your statement about effectively thinning the wing. It’s fascinating. And when you click in negative flaps, the nose rises slightly and you sort of automatically achieve an optimal speed. Excellent for cross-country flying.
I’ve never seen a “Soaring” (the SSA magazine) article about it, but it’s likely there. I did find a dissertation by a NC State aeronautical engineering student about slow speed aerodynamics but drag buckets and negative flaps were not mentioned.
@@DumbledoreMcCracken that's true (not exactly - I will explain it little later in this comment), but only for very low angle of attack. Slower planes has more thick wings (mostly 12-18% of chord) and faster planes has thin wings (4-12% of chord). That's because of mentioned R. number, air viscosity and in some cases mach speed (thicker airfoil will generate more increase of speed at top layer).
If You decrease thickness, then stall AOA in most cases will be lower. Drag will be lower for AOA=0. But at some point (like 1-2 deg.) drag will be higher.
@norbert.kiszka there is no reason for a slow plane to have a thick wing.
Modern sailplanes have a very very thin wing.
While the usable range of Angle of Attack is narrower for a thin wing, that is not an issue for real airplanes. Aircraft that require a large range of AoA have a bendable chord via leading edge flaps, or fixed or retractable slats.
My buddy has let me stall his de Havilland Chipmunk a lot. It has a beautifully gentle stall. I attribute that characteristic to its thick, round, gentle airfoil. The Chipmunk is an aerobatic training airplane, so de Havilland may have wished to give the students a more relaxed plane to fly. However, and in general, most planes are designed for speed and low fuel consumption, and require a relatively thin wing (plus flaps, at a minimum) to achieve those goals. Training prevents stall from being a serious issue.
Nobody flies with a high drag AoA unless he wants to lose altitude without gaining speed.
@@DumbledoreMcCracken then try to look at many different airfoils, especially designed for low Reynolds numbers (in most channels links are not allowed). In previous comment I missed a small fact, that most thick airfoils (like a 25% chord) are used in wind power plants. Making airfoil thin at around 4% it will be very good for fast fighter jets, but it will be terrible for slow planes. Changing thickness from (for example) 12 % to 11 or 13 will generate significant change in characteristics at slowest and highest speeds. Try any aerodynamic simulation software and You will see that what Im saying is the true.
the "y" is silent in Reynolds.
I like it. Kinda fun when ya think of how much of his generation is shortening everything.
Are we sure we know how Osborne Reynolds said it himself, in 1880s Britain with Irish parents? I like the way he says it here.
@@redbaron07 Could be, likely verifiable. Let us know if you find out.
@@redbaron07 Modern day usage is with a silent 'y". But you can be strange and different and say it anyway you like :)
There are a lot more of self taught people nowadays. A lot of people learned a word from Wikipedia, but heard it pronounced in real life
In low R nrs the air appears more viscous. Thus insects appear rather to swim in the air, than to fly.
I think that statement is incorrect. Common misunderstanding.
While the non-dimensional Reynolds number ratio is lower for lower velocities, for a constant characteristic length, the viscous force does not contribute to lift in a meaningful way because the viscous force points in the wrong direction. Notice the orthogonality of the statement.
If we invert the Re number, it is clear that the viscous force is miniscule to the inertial force. Flight must be the result of the inertial forces. Q.E.D.
Reynolds number formula indicates precisely when where and which factors are causing turbulent flow, no matter the surface, wing, rocket combustion chamber, or even heart valves.
Utter bullpoop!
Yes, thank you,,distance is scaled up along with size and velocity,,I better understand why my larger models travel farther...
Did both models of aircraft have the same wing loading?
No, the larger one had less and therefore was a little slower than the smaller one.
Are the weights of those 2 different size planes scaled equal to their size difference?
Thank you. Well done
I just finished my second year in aeronautical engineering and you just taught me more about Re number in 5 min than in my 2 years, such a shit education system
i mean it's not a difficult concept (it's quite literally just a ratio between dynamic and viscous/friction forces) but the application of this figure is much more technical which you will see when you study incompressible aerodynamics. it's hard to introduce such a figure without being able to go in depth on what it actually does so i can see why you might be confused. i'm sure you will learn a lot more over the next year so don't be discouraged.
btw, reynolds nuymber is much less important past a certain point as your coefficient of lift is invariant all the way down to your level flight stall speed. this guy just happens to be running a model plane (small and slow aircraft) channel and not a general aviation (much much larger and faster) channel so in this case, it is much more relevant. ig that's why a drones prof has shown up in the comments lol.
All the good stuff is free. Stop paying 10'000s to a stupid liberal university.
Thank you for your nice and informative videos.
What about the Cubic Wing Loading WCL? planes with low WCL float in the air, are slow, and have low stall speed. On the other hand, planes with high WCL are fast and have high stall speed. For both airplanes used in your demonstration, what are the WCL? is there any direct relation between the Reynold's number and WCL? I know there is not, but what about an indirect relation?
A plane with a higher WCL would fly faster and have a higher Reynolds number.
very interesting. thank you.
Excellent explanation. Thank you!
very helpfull video thanks :)
This was great, thanks!
Nice explanation - thanks!
L=1/2 • density • airspeed ^2 • area • drag coefficient so why do you say the drag coefficient depends on airspeed and air density?
The air will flow differently around the object and create a different drag force.
airspeed influence drag coefficient that is biggest misconception. Geometry of wing under different airspeeds changes drag coefficient, that is complete nonsense. Many solutions disregard engine thrust (airspeed), which sustain energy conservation condition.
This is a spectacular video! You are incredibly informative. Being an airplane builder myself, Reynolds’s number always confused me. Thank you very much, and have a nice day!
So, would it be true to say... All else being equal, if reducing parasitic drag will increase the Re number? In other words... The smaller the airframe (scale), the lower the Re, the more critical that parasitic drag becomes in design, (if wanting to improve L/D and/or minimum sink)?
Yes, I’ve found that below a Reynolds number of about 50,000, the parasite drag coefficient of airfoils increases significantly, which would make streamlining more important. The drag coefficients of other shapes may be affected by the Reynolds number in different ways though.
Great video 👍🏻 Relax and smile 😎
"Relax and smile"
Hell no.
It's perfect composure. Remember, You are building Your own airplane, unironically.
You keep saying further, but mean farther. (i.e. The gas station is farther from my home is an imperial measurement.) (i.e. He is spiritually further ahead in life than I am, cant not be measured with imperial measurements) I hope this helps.
Brownean motion.
That explains Reynolds number.
What about the scaled travel ratios ? If we throw 1x and 2x scaled planes, measure their distance and divide the distance of larger plane by 2, would it give the same result ?
The distance a glider flies is determined by the ratio of the lift coefficient to the drag coefficient, multiplied by the height it was thrown from. The small plane was at a disadvantage because it had a higher drag coefficient.
You can see in the diagrams, that it is a sort of exponential function. So the answer should clearly be "No".
If all the structural geometries are identical in scale, the smaller plane is at a disadvantage. However structural strength doesn't scale linearly either, and favors smaller, lighter, stronger structural solutions as you scale down to some degree. Replacing the fuselage surface area with a long soda straw, removes parasitic drag while retaining wing area. Likewise the smaller slower model has smaller control forces, so the tail section can be significantly smaller if positioned farther from the CG or CM, with a larger lever length. Scaling planes up has similar results, often requiring significantly heavier and stronger structural elements.
Nice - thanks heaps
info synthesized nicely! please consider taking more care while editing the video chops next time, specially when it comes to volume variations, it was hard to understand sometimes as the trafic noises filtered through my window. with that siad, please keep the great work i cant wait to see your next video! :)
So the ratio of wing length to cord length has nothhing to do with the Re #?
You can use any length you want to calculate the Reynolds number, as long as you stay consistent when comparing different Reynolds numbers. The aspect ratio does not directly affect the Reynolds number, but it is possible that changing it could also change the characteristic length and therefore the Reynolds number too.
@@DesignYourOwnAirplanes-xd6lz Does an albatross wing have a higher Re number than a crows wing?
@@Toomanydaysthe chord length is the determining factor, and speed, when discussing Reynolds number.
The wing span is irrelevant.
Unfortunately, Reynolds number is described in an algebraically simplified form. The factors are more understandable when the form of the equation is not algebracially simplified.
5 minutes in one sentence:
Small things moving through air (e.g. bees) see it as sirup. Large things (e.g. B747) see it as a fluid like water). Hard to explain, I also needed time to understand.
Running through my fathers garden with a Airfix model in my hand I wondered: he is small, the real aircraft big, the air is the same. How does the model aircraft feel the air?
Doing my batchelor, and later master in aviation, it was eventually explained.
Legend, thx
Great video. I dont have foam board available in my area so can you please make the next plane with some other material please.And can you also teach us how to make a rubber powered plane. Great videos thank you very much.
In a later video I plan on talking about building materials. You can also use pressed or corrugated cardboard, balsa wood, corrugated plastic, thick paper, or insulation foam. I'll consider making a video about rubber powered planes.
u are one of the most educated American .
I am curious as to your background. The Reynolds Number is the ratio of inertia stresses to viscous stresses. It does affect lift coefficient as well as drag. This requires understanding boundary layer theory.
I majored in aerospace engineering. The boundary layer and stresses are beyond the scope of these videos so I didn’t mention these subjects to keep things simple. The main point of this video is that if you make your glider too small and/or too slow, it could reduce your flight distance.
My undergrad degree was in aero/astro engineering from Purdue in ‘82. This was followed by graduate degrees in aerospace engineering from a joint program from the University of Dayton and AFIT. My research interests were aero elastic effects at low Reynolds Number. I retired as Aerospace Fellow from Honeywell in 2021. You were wise in selecting Aerospace Engineering as a career. It does not get better than that. Aerospace Engineers rock! Cheers!