Reynolds Number Explained 

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

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.

Bro is INTENSE!!

RU-vid has made a great service for me today by showing me this channel lol

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.

I am very glad youtube recommended me this. You did a good job explaining the topic it in a short timespan.

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.

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).

I asked two aero-e’s to explain this. Neither could dumb it down to my level. Thanks!

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

Dude you are awesome! Excellent content and droll, over serious tone and demeanor! Excellent!

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.

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.

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!

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.

As someone who has applied for an aerospace course at university, I am subscribed

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!

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.

Come on man, keep those videos coming! I just discovered you and I am addicted...

Excellent analysis! Indeed, the Reynold's # is important to be considered.

Am i the only one who thinks that this guy is the real life version of sheldon cooper

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.

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.

Incredible vid. Please dont stop making these

Bro is on point. 10/10 on communication. Perfect!

That was a sweet explanation, man

This video is really interesting, thank you so much!

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.

Thank you. Nice explanation of very complicated subjects

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.

Excellent explanation. Thank you!

Excellent Explanation 👍

..your content very well explained with the two gliders; the same when ducks and geese stop swimming, the geese will go further!

Teach us how to use the Reynolds’s number to select an airfoil

Nice explanation - thanks!

This was great, thanks!

It’s a interesting field how to relate wind tunnel models to the real world by interpreting the RE number ( and wall effects)

I'm not into this thing, but I liked the content! Thank you.

thank you airplane sheldon

Thanks for that info.

Great vid. Thank you. Si, Christchurch, NZ.

Thank you. Well done

very interesting. thank you.

You are doing very well. Nicely explains the basics. Slowly you also add electronics and make remote control plane. Thanks

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.

Nice - thanks heaps

Awsome video

That was good. Thanks.

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

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.

Is there a series of reference books for aircraft design that you recommend? Like Jan Roskam

Thank you ❤

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!

Legend, thx

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.

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.

Bro does not BLINK!! Elite

Great!

Yes, thank you,,distance is scaled up along with size and velocity,,I better understand why my larger models travel farther...

How much does ground effect help the larger plane fly across the gym?

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)?

Very nice job, man! Greetings from Brasil

the "y" is silent in Reynolds.

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.

In low R nrs the air appears more viscous. Thus insects appear rather to swim in the air, than to fly.

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! ☀️✨🌟✈️

L=1/2 • density • airspeed ^2 • area • drag coefficient so why do you say the drag coefficient depends on airspeed and air density?

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.

Good very good video

Something that has concerned me since I read a book in high school… 🤔🤓

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.

Condors vs sparrows

So the ratio of wing length to cord length has nothhing to do with the Re #?

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 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?

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 think it caused me psychic damage reading Reynolds number out of nowhere on my feed

I learned that when the Reynolds number was below 2000 water would flow through a pipe in a laminar way and above 2000 in a turbulent way.

By the way you look to the camera I feel like this is some serious CIA instructions not a model plane video

Great explanation thank you very much. Can someone explain to me, why some Americans develop vocal fry ?

try to relax a bit in presentation. it looks like you're trying to blow up my head with your mind. 👍

u are one of the most educated American .

Wasn't the bigger aircraft heavier, thus starting with more energy?

Perfect

So a larger chord length= longer flight?

Hand launching the planes across the gym is a very poor test, since you can't apply the same force twice.

Parasitic drag not parasite drag

Both of my grandfathers were aerospace engineers, along with my dad, who, among other things, was part of the model maker for the wind tunnel testing. My name is Bicycle Bob and I approved this message and what is my last name?

Why are you pronouncing it Raaynolds number??

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! :)

Reynolds numbers are important. The standard atmosphere as lower Reynoldsnumbers at increased altitude from its own. Climbing to altitude with the same indicated airspeed, e.g. Vx, the Reynoldsnumber decreases slowly. This reduces the glide performance of the craft, the L/D reduces a little. The now obsolete Boeing Condor, a long endurance high altitude platform had an L/D of 44 at sea level. At 20.000 m the L/D was 39. This was a Reynoldsnumber effect. Flying high is a quest.

Or, as my Chinese fluids professor would say, Reeno nono.

Short explanation: Large plane need less speed than a small one

Nice, thanks a lot.

I don't understand.😢

lift to drag ratio must be worsened by speed, fast planes can't match the 50:1 glide ratios of slower gliders

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.