Choosing the right prop diameter for a drone of a certain weight: What prop for sub 250g?

[@fpvlive]

This is what we need in our hobby, just basic science facts instead of overhyped marketing quotes, thanks for your insight !

[@bkfpv]

You sir, are one of a handful of people who are shaping the development of this wonderful hobby.. Thank you for your contribution.

[@ChrisRosser]

Thank you sir, for your kind and positive comment!

[@JoshuaBardwell]

All I heard was, "250g five inch sucks JB was right the whole time."

[@ChrisRosser]

Hopefully knowing the reason behind it proves useful in your future conversations on the subject 👍

[@friendlynomad9840]

I think both JB and Rosser have science on their side. However, I still comfortably prefer a 5" quad, even at 250g. Keeping that weight equal, a 5" quad will never feel the same as a 3". I can tell the difference in split-s and power loop moves. I think the throttle and yaw feel of the 5" quad is noticeably different. Just my two cents.

[@Kabab]

~300g 5" is 👌. It just needs at the very least a 2203 or 2204 to be good.

[@friendlynomad9840]

@@Kabab Agreed. the 2203 is a big step up from 2004.

[@DeyRonUSA]

We have been so focused on 5 inch feel, and directly correlating prop size, I walked away with the weight and prop combination is maybe a focus worth exploring.

[@TheChickenLoop]

Loved this analysis. Finally someone who put the time in to debunk this arbitrary sub-250g 5" goal, which to me doesn't make sense if the weight is no longer 600g. I would love to see a video on aero drag vs AUW to try to optimize the "floatiness" of a quad. In other words, put quads on a spectrum between beach ball and golf ball.

[@ChrisRosser]

Parachute effect is linked to disc loading so a quad with a lower disc loading will feel more floaty with less fling.

[@trey4weaver]

I have several 2.5" quads that are just at 100gr. I think they are some of my best flyers. I am right on that green line. I am using 32gr 2S batteries too.

[@dflyfpv8765]

Finally, the part that was missing all the time on FPV, Science! 👨‍🎓👩‍🎓

[@aminordissonance]

Would have been skeptical of the smaller prop size a few months back, but ever since I received a set of T-Motor 4943 props I don't run anything else. The spool-up speed and responsiveness on them is fantastic. Which, if I've been paying enough attention to Chris, has to do with their moment of inertia.

[@ChrisRosser]

Oh you are so right!

[@DaxNealFPV]

Really enjoy your physics approach to finally answering our debates.

[@ChrisRosser]

Thanks for your support! Facts make everything better :)

[@jarosawrejer2117]

if JB is a racing drone engineer, you are all drone professor. thank you

[@eskay005]

Just changed frame to be able to mount 5.2 props and a bit steeper angled props to carry my gopro. I hit right that green sweet spot and that thing now flies smooth as butter without gopro - haven't tested it with a gopro yet, but already it feels a lot better to fly it based on your scientific insights :) thanks a lot!

[@JulianBauknecht]

perfectly in line with my own tests, but much better explained. The only thing I want to add is, the graph gets a bit distorted on the very low prop sizes side because it's only an approximation that the fixed electronic weight cancels out other scaling factors and that approximation fails on the very low weight side. That's why on the very light side if you only consider prop load I find the chart suggests a little bit too small props, but only a tiny bit (maybe 0.5")

[@ChrisRosser]

Absolutely right. Such a simplification can only be so good but it provides a starting point. Definitely small quads can benefit from slightly larger props. 👍

[@JulianBauknecht]

@@ChrisRosser do you know how the disc surface impacts wind resistance? I cant figure out if it only depends on the disc surface or also on the blade number because I feel like wind resistance per weight is the second most important part of the "feel" or "throw" in the air

[@ChrisRosser]

@@JulianBauknecht I think the disc loading will be the primary factor with blade count as a secondary consideration. The idle RPM of the prop also plays a part with a higher idle RPM providing more drag in reverse flow conditions.

[@JulianBauknecht]

@@ChrisRosser ah ok tyvm, that would explain why I can't get definitive results wheater or not blade count matters if it's in between in real life. That would partly explain why on 5" 250g kwads 2blades fly much nicer (besides the fact, that a super lightweight 2blade is stiffer than a similar heavy 3 blade)

[@BoilerFPV]

Great information! It would also be nice to see an analysis of the best prop diameter and motor rpm for maximum endurance for long range quads.

[@ScottTomlinson]

Another great video Chris! Will definitely be taking into consideration all this data designing future quads. Keep up the great work.

[@ChrisRosser]

Awesome, thank you!

[@blackpepperprepper2025]

I like your science based instruction/presentation. This I can get behind. This was very useful, even for my hexacopter.

[@markhutchens]

Than you Chris for applying the physics to debunk the hype. I love the way you are tactfully telling those who are driving trends that they are wrong.

[@markhutchens]

I wonder how many rtf models are designed using real data?

[@ChrisRosser]

I'm glad you enjoyed the video! Not trying to tell anyone they are wrong just trying to show how if you want a quad to behave a certain way there are concrete steps to get there 😁

[@2oldfpv987]

Here comes the 3.25-3.3" props!! Absolutely loving the Babyhawk 2HD and you just told me why!! Huge respect and appreciation for what you are doing!!!

[@Atlas_FPV]

I love engineering discussions, subscribed.

[@ChrisRosser]

Thank you for your support!

[@rctom1]

So when i saw the graph at 9.40 the Babyhawk 2 HD is a good BNF sub 250gr. Rig realy close to the sweetspot. Thank´s Chris Rosser for your amazing work due to the FPV Community.

[@ChrisRosser]

Thank you for your kind comment! And spot on with your interpretation of the data.

[@apair4002]

9:30 graph is really help me a lot. No more guessing and arguing lol. People build as light as possible 3 inch quad, for me it is too flooty. It turns out my 3 inch quad just at the point for flying outside, especially on windy days. My 3 and 7inch is on point with all of TPU protection and gopro 😅 but need 5.5inch prop for my 5inch quad 😅 Thank you sir.

[@smartfpv3992]

Awesome! I also figured out a while ago that 450-500g is the actual best performing weight for a 5 inch (at least in my opinion). I think that 4 inch is veery interessting for the sub250 class. I will definitly use your research as a base for future testing! Thanks a lot!

[@ChrisRosser]

I hope your future testing goes well. Good luck! 👍

[@LS-xb2fh]

Great video, although I think you skipped one important aspect: static thrust. Which is higher using a larger propeller (at fixed motor power). This is due to the lower exhaust speed, which increases specific thrust. It is more efficient to accelerate at lot of air by a little bit, than to accelerate a small amount of air by a lot. (That is why human powered helicopters are huge.) The flip side is a lower top speed. You don't get any thrust if the exhaust speed equals the speed of inflowing air. So by changing disk loading you can trade off top speed vs. acceleration (up to a point). For 250g: 2" too inefficient 3" low acceleration, crazy top speed 4" good balance imho 5" crazy acceleration, lower top speed 6+" unresponsive (small motors for props with large moment of inertia), frame flexing, low durability

[@ChrisRosser]

I don't think I neglected static thrust because static thrust is wrapped up in disc loading. I think the that static thrust/mass is among the most important criteria! If I understand you correctly your analysis of speed and acceleration vs prop size may neglect some limiting factors such as prop pitch, tip speed, and motor/battery mass. In real word testing 5 inch quads have a higher top speed than 3 inch quads because they are typically able to achieve higher prop tip speeds.

[@LS-xb2fh]

​@@ChrisRosser Static thrust is dependent on disk loading, but you did not explicitly talk about it. And the propulsive efficiency you used @2:40 is zero by definition when hovering (no work done at zero velocity). Given the importance of static thrust for drones I think it is quite important to cover it (and may be a topic for another video). Your anaylsis and presentation is great nonetheless and matches my experience when comparing my 3" and 4" 250g racers to my nazgul 5. Static thrust is propotional to the momentum change of air, whereas the power is proportional to the kinetic energy change of air. So you get T ~ m_dot * v_exhaust and P ~ m_dot * v²_exhaust **. Lower disk loading implies lower exhaust velocity (during hovering), which leads to more efficient hovering. The wikipedia article on disk loading has a nice graph of the relationship. As for top speed, at 250g and ~900W power (static thrust), a 5" is slower than a 3". The lower pitched 5" props just unload too much at higher speeds. If you want to play around with some numbers i can recommend the calculator on ecalc.ch. Of course when using the same power at top speed, a 5" should be faster. But then the 5" setup would not be able to handle full throttle punchouts. At lower power this changes as well. The extreme example would be a very low powered 3" setup that can just about hover. The increased efficiency of the 5" propellers means there is excess thrust, and forward flight is possible. ** The conrol volume needs to be chosen so that the exhaust is fully expanded, ie. at ambient pressure. See en.wikipedia.org/wiki/Momentum_theory

[@ChrisRosser]

These are great questions you are raising so I'll do my best to answer. The principle behind the video is a scaling analysis. The key is finding the things that stay the same as you change the size of the props. As you correctly point out static thrust changes with prop size so its not useful on its own for the scaling analysis. You run into a similar problem with power, a larger quad is more powerful than a smaller one so on its own power isn't useful for the scaling analysis. Disc loading and a constant tip speed capture the same variables but critically can stay constant as you change size. Power and thrust scale naturally as a result of disc loading and tip speed. I hope that explains why they don't appear separately in the video.

[@LS-xb2fh]

@@ChrisRosser You are right and I was not clear. Comparing absolute numbers is not that useful if you have scaling laws. So maybe my comments were just a long winded way of pointing out that the hover efficiency is also directly dependent on the disk loading. :) But creating a concise video is important, so I think you could have included that not you that you should have necessarily.

[@ChrisRosser]

@@LS-xb2fh I really enjoyed this conversation. Thank you for your suggestions and feedback. I hope we can have more chats like this about future videos! 😁

[@mehrana2722]

Many Thanks Chris, great work, really enjoy your presentations. Very nice, brief and effective analysis.

[@MrTempatel]

The most exciting video from motor volume / prop / disc loading series so far. Interestingly I was kinda intuitively leaning to that green line. Because I thought 5" 700g AUW having flight time 3.5min and landing with battery hot wasn't right. And according to the graph you need 6". And that's the size I stopped on. I had a 7" frame and I started to cruise around at about 60kph logging speed vs power. I replaced 7" with 6" and repeated. As a result I've got 2 elliptic clouds of dots (normal distribution) which were coinciding. Which meant 7" wasn't more efficient then 6". So why carry more? Now I'm building 250g free style / long range quad with 4" props and 1207 motors. And it also hits that sweet spot :)

[@ChrisRosser]

That's great to hear. I love it when theory aligns with people's real world experiences. Thanks for sharing!

[@alfpv2155]

Thanks!

[@ChrisRosser]

Very generous! Thank you 🙏

[@Brian-S]

Interesting. It's funny I've been flying a 4 inch apex a lot lately and I think it's my favorite flying quad. Wonder if I should give some 3.5 in props a try

[@DeyRonUSA]

Ha, I'm building one now. Can't wait to get done

[@woopweep9605]

Ha! Loooove this! I’ve always said “3” is the best size”. I’ve had 0 interest in sub250 5” and def think 3” is the future. Thanks for all of this great info Chris!

[@DeyRonUSA]

You my friend, are killing it. Allowing us to make Data driven decisions on our purchases. So tired of following the hype. This is good stuff. Working on a 4 inch build now, using this advice along the way. Keel it up.

[@ChrisRosser]

I like to think of my channel as a hype free zone. I'm glad you value this approach and thank you for your support!

[@DriftaholiC]

My A0S-5 with, Caddx Vista, DJI action 2, 1300mah battery and 5226 Gemfans came in at 665g. 654g if I drop to a 1200mah. Easiest quad to fly I've had yet.

[@Bruno-cb5gk]

I think what would be an interesting further development of this is to look more closely at the scaling of all the components and then look at the ideal prop size for the payload, rather than the AUW since that is impacted itself by size.

[@SufiShahHamidJalali]

This is one of the useful instructional videos I have seen. Very good.

[@stelic9515]

This is GREAT information THANK YOU

[@chrisyoung8062]

I love how you made this so easy to understand. Well done! I just sub'd.

[@ChrisRosser]

Thanks and welcome. I hope you enjoy my other videos 😁

[@happyvalley808]

Chris, thanks for sharing your knowledge!

[@ChrisRosser]

My pleasure!

[@grabejud]

no wonder of all the micro quads i have, the diatone 349 gives me the most smoothness of moves under 250 for freestyle. but I am eyeing for babyhawk HDII efficiency though.

[@MatteoAvalle]

Super informative! But I am having difficulties in following your reasoning about motors sizing. Why do they "need" to be bound by the prop diameter, up to the point they are not able to reach the maximum desirable speed anymore? If a motor is not able to bring the tip of the blades up to 450mph, couldn't you just use a bigger motor, by leaving all the other parameters the same? It's unclear to me why a sub-250g 5'' drone with oversized motors should still fly bad due to its uncapability to make props reach certain speeds. Maybe it will burn the battery sooner, due to the higher consumption with respect to its battery discharge rate, but according to your reasoning it SHOULD fly better, isn't it? I am sorry if I sound pedantic, but I just want to be sure I got everything right! I'd love to understand physics behind drones, and these videos are pure gold! Please keep up with the great work!

[@ChrisRosser]

If your motor cannot reach 450 mph tip speed then you need a bigger motor. You also need a bigger battery as well because a batteries ability to supply current depends on its size. Then you find you have a much heavier drone. That's the key takeaway from the chart.

[@aakashjana6225]

Chris I am really excited by the type of knowledge you have shared with regards to motors and propellers . It is ofcourse a little difficult to sum up the effects of motor stator volume to its torque and thereby the continuation of the same math with props . Would be actually helpful if you could show us a total guide based on these vids , how one of us could use these understandings to combine them up as a calculation and not make a mess out of them in our minds and make the best selection. Also how do motors draw power based on different props at particular voltages has always made me think if there happens to be some other fool way to determine exactly the power draw of a motor without doing a physical prop test but by simply using the size numbers.

[@ChrisRosser]

If you start with your target weight, that'll give you the right prop diameter using the graph in this video (green line). Then go to the motor video and the graph there will give you the motor volume you need. Pick your motor KV so that 0.7*KV*2*pi*Diameter ~ 450mph. Battery should be 1/3 of takeoff weight and that should get you in the ballpark :D

[@Switch180fpv]

It’s funny to see all The Who’s and who’s in the hobby gathering in your channel, proficiency in science will do that no more guessing 😊

[@ChrisRosser]

I'm very grateful that these very experienced pilots think the content is interesting and useful!

[@Gosuminer]

JFYI: 0.59 mach is roughly 200 m/s or 730 km/h, which is 30,400 rpm on a 5" prop or 50,700 rpm on 3". This feels a little high to me, maybe the 0.59 mach value does not fit 100% for tiny props (compared to man-carrying aircraft) but I could be wrong.

[@ChrisRosser]

It's not too bad when you consider that 1800KV on 6S will hit those numbers on a 5inch. 2800KV on 6S gets you right in the ballpark for 3inch as well. Smaller quads do struggle a little more because the electronics doesn't get much lighter so there is less weight for battery and motor.

[@kenb6565]

Thank you for well thought-out and presented data. I have always loved 3.5" props for micros and 5.5" for my GoPro rigs ( I also fly at 5000ft altitude). Hopefully we will see more 3.5" (thank you baby hawk hd) and 5.5" (gf 5551 only ones around these days) props come out. I usually have to cut down 6" props to 5.5" and it's a pain. It's hard to get a 5" freestyle rig with gopro in the 600 gram range. At 670 to 720g range 5.1" quad blades do help. Thanks again...you rock and keep up the great work.

[@marc_frank]

i like my 168g auw 4" 7min flights with a 750mAh 3s i wonder if a 3s 18650 pack would push it more into the optimal disk load area i'd like to see the tubular los quads in that graph btw amazing channel the vibration analysis results videos should be on every frames product page

[@phillipneyman9338]

BRILLIANT!!! thank you so much.... your channel is superb.....

[@ChrisRosser]

You're superb! Thank you for your support.

[@mihavuk]

Hey Chris, thanks for this confirmation - which is max. AUW between 600-700g for 5 inchers. One question I have is: how does (if it does) size of individual prop blade surface (for example in mm2) affect prop performance, it probably also affects ideal tip speed and everything else? For example if you look at Dalprop Cyclone 5 inch prop and compare it with for example Ethix S3 or S4, there is obvious difference in blade surface size. Or maybe this is too complex question. But at least recommendation from you like this example of blade shape and surface size is good/efficient and will work well with usual motor sizes and kVs and this is a bad example. Thanks and I appreciate your work. 👍

[@ChrisRosser]

Have you watched my video on choosing props? I think the answers you are searching for are in there.

[@tuhotomi]

Thanks for video! Now i know the science why my 300g 4” build feels good to fly 😀🤓

[@dezelvol]

Finally someone explained it. Thank you. But one question appeared. If goal is, for example, long range - is it beneficial to use lower KV motors and smaller props, to achieve more prop speed while cruising? Opposite situation - if you have crazy overpowered 8-12s build, isn't it better to use bigger props with same weight, just because motors can build more power/prop speed?

[@dezelvol]

Also, if we take into account power required to spin propeller will efficiency stay the same? Propeller speed to air movement speed is great relation, but propeller is easier to turn on lover RPM, so even with worse RPM to air speed efficiency, it could result in better battery consumption efficiency.

[@dezelvol]

After some research I'm now sure that I was right. 450mph(actually, it's 475) is best performance per rotation. So it's best thrust for the size. But if you can go bigger without adding much weight, you'll always get better efficiency. So, if you can get same weight with same motor size etc. in 10" it will be much more efficient than 3". Props, same as wings, are more efficient slower you go, and lower is motor kv, better is efficiency. If you do race drone of some size, you should stick to 475mph. But it's not a surprise that huge and light long range drones have efficiency even with low tip speed.

[@K001channel]

I do enjoy your engineering deep dives great job thank you

[@onemanmob6756]

Emax Babyhawk 2 HD - sub-250g, 3.5 inch props - now I know why all reviewers praise it so much....

[@ChrisRosser]

Absolutely, they've hit on an great size. 😁

[@onemanmob6756]

@@ChrisRosser also it seems like the frame design is interesting from the resonance mitigation pov... I've just ordered one, even though I have no DJI goggles yet 🙃

[@h2o-fpv623]

Great video like always full of information good inf. that we need i think i have to watch it again to get it right and dont get me wrong most of the time when the video is interesting and with good inf i watch it a couple of time thank u.

[@denismorgan9742]

If you have a triple toroidal with a built on cinewhoop with a rounded outside edge drones some of these graphs would change again. But good in depth work.

[@mouseFPV]

11:07 literally where my session 5 carrying freestyle quad sits with 5.1" p3 props lmao, you wizard.

[@ChrisRosser]

Engineer: Someone who does precision guesswork based on unreliable data from questionable sources, see also Wizard, Magician 😋

[@mouseFPV]

@@ChrisRosser maybe I missed it. At what rpm do common prop sizes hit 450mph at the tips? What is the equation for that? Ie: 31mm whoop 2" 3" 4" 5" 5.1" 6" What I am after is approximate optimal kv for a quad size. I know kv numbers on quads are not super accurate, but it would be nice to know. IE if 15000 RPM on a 3" gets you 480mph (pulled out of my butt) than anything over ~5000ish KV on 3s would basically be additional amp draw for no gain and less throttle resolution. I know it's not all about KV, and you need to the volume to manage the prop, but that would be useful to know.

[@RCRitterFPV]

@@mouseFPV I did the 4" and its 3222 on 4s... need to find a tip speed calculator to find RPM. I'm sure there is a better one than what I used... but

[@uavtech]

13:30 - "engineering effort". Make sure to quote that stuff. ;-)

[@aakashjana6225]

I knew I would find you here, sooner or later and here you are.

[@ChrisRosser]

Ooooh that's a nice burn... 😁

[@BogHopperFPV]

Chris - the physics of unloading please. You have touched on it in previous videos- AOA adjustment & Inflow Velocity. Can this be converted into rules of thumb to help when testing static thrust and knowing what will happen once the aircraft is in the air? Will it be different for fixed wing and multirotor?

[@ChrisRosser]

It will be very different for fixed wing and multirotor. It's very hard to give a rule of thumb for a quad as they fly over a huge range of speeds from negative inflow to fast forward flight. If you know how fast you want to go you can use an efficiency vs advance ratio plot to find the right prop for you.

[@MrTempatel]

If you have a 4" drone, does it make sense to run 3.5" 3 blade props in 250g acro config and 4" 2 blades in 300g long range config?

[@ChrisRosser]

I think it definitely makes sense if you want a bit more snappiness and performance for acro flying but still want a great LR cruiser. Great thinking! 👍

[@poporbit2432]

Thank you, the level of clarity you bring is fantastic. Im building model F35s derived from Nicholas Rehms design. His plane has 3 motors in a tri copter configuration. His plane weighs about 850gr uses 2205 motors and 5045 props on 4s batteries. They fly just fine. My design is scaled 50% larger, weighs 1090gr and uses 2306 1900kv emax motors with 5545 props on 5s battery. The motor overheats within 1min. of Hover time. Based on your data tip speed is to high so reduce prop dia to 5045 or change motor to 1400kv. Sound about right?

[@itsmeRizzG]

@Chris Rosser so, if I understand this correctly, using the tip speed calculator you reference, an XNOVA 1804 2400kv motor on 4S with a 5" prop gives a tip speed of ~570mph and with a 4" prop it gives ~450mph. All of this at only 14g a motor which makes an AUW under 250g with a Naked GP very doable (with eg. a GEPRC 660mah 90C HV that matches current requirements) for both scenarios with higher thrust and flight time efficiency than a smaller prop (granted with higher drag when flung). Is there a reason why this is not the better choice? I guess I don't understand what you mean @10:00 when you say that quads with a lower disk loading struggle to reach the maximum tip speeds? Could you potentially expand on this or provide sources in the literature? Thanks :) *Note: The same thing can be done with the new Ummagawd 2004 Aerolite 2400kv if you prefer a larger stator size.

[@ChrisRosser]

Are you making the assumption that max rpm = KV*voltage? It's much more complex than that unfortunately. The prop has a torque requirement for a given RPM depending on flow conditions and the motor has a current requirement to produce a given torque. The battery can only supply a certain amount of current at a given voltage. All of these factors depend to some extent on the weight of the components. The TLDR is an 1804 motor on 5" props won't get anywhere near 450mph no matter the KV. As for sources or literature on this I don't know of anyone who's done detailed research into this except myself and other enthusiastic hobbyists. There is no data on the torque curves for the props we use in moving air for example.

[@itsmeRizzG]

@@ChrisRosser I SEE! Upon more research I think I get it now. So as I understand it, higher KV produces less torque for a constant current. Larger propellers require higher torque supplied. So if we want to increase torque of the motor to overcome the torque consumed by the larger propeller, we have to lower KV. BUT by lowering KV too far you risk not achieving the desired tip speed for the maximum performance. Which leaves us no choice but to increase stator size for a given KV and maximum current limitations in order to overcome the torque consumed by the motor and reach that 450mph. That is to say that we are limited by battery technology and torque capabilities of the current motors available. And increasing stator size ultimately increases weight of the vehicle, making it harder to stay under 250g. Thus we need to look at less torque-intensive propeller sizes and then apply the design circle again until it is possible to reach the maximum tip speed for a given current supplied. Am I correct in this line of reasoning? And I guess that also means that thrust is lowered, but that wouldn't be such a bad thing in terms of flying characteristics when you want greater precision in flying. But the trade-off is then limitations in payload capacity when you want to add a naked GoPro under 250g, which decreases the size permitted for your battery, which again decreases current supplied and the design circle starts again. Is that also correct? Thanks again for the time spent educating us plebs XD

[@ChrisRosser]

@@itsmeRizzG Yes, your reasoning is broadly correct. The KV does not change the maximum torque of the motor. That is dependent on the motor size and construction. KV only changes the relationship between current and torque.

[@itsmeRizzG]

@@ChrisRosser Ah okay, so the correct interpretation is that a higher KV motor requires higher current to maintain the same torque in comparison with a lower KV motor?

[@Therberg2500]

Very interesting. Good stuff!

[@ChrisRosser]

Glad you enjoyed it

[@phiveone]

I knew there was something special about my Babyhawk HD II's flying characteristics...Oh it's just science.

[@honeybeedrones5510]

Excellent video series

[@noforyoutubepremium3108]

Kinda shows why the Emax Babyhawk 2 HD is so good! 😎🤘

[@tehllama42]

I feel like we should just set up a conversation channel in a discord you frequent to discuss some of this... although practically this is still a great venue (algorithm/engagement and all). I think the outright disc loading starts to grow wider error bars at the higher area/weight ranges, but that goes a fair way to explaining why 7" quads are pretty indifferent about all-up weight in my experience, and can broadly handle like race quads with a session on top if kept below 750g.

[@rafaelgonzalezmorales1824]

This is very useful. I switched to 3" from 5" sometime ago because I don't do use action cameras so I don't see the point... Anyway, the weight of my 3" racers is approximately 184 grams on 4S which is pretty good considering I'm using the Vista with original DJI camera. The bad news is that I need to loose approximately 15 grams to reach the green line. Deceasing the Vista and the Nebula Pro camera should get me there however.

[@ChrisRosser]

Yes I think decasing the vista is the clear next step 😁

[@camarochip]

My quad is over 800g on 4S and I'm trying to find the right pitch prop for efficiency,5146 does better then smaller pitch,but 5236 feels nice too.but idk bout which was more efficient because my flights time varies sometimes ,one minute I can get 5 minutes and then another flight only get four and a half minutes while doing same style flight.if that makes sense.

[@mouseFPV]

Wish the tp3 was on here! Disc loading king

[@divingfalconfpv4602]

It's so light. Makes less power than a lot of 3" so I feel it gets away with it. And kabab tried 4" arms couldn't handle it. He def tries everything to try and get the best

[@BogHopperFPV]

Thanks Chris

[@ChrisRosser]

No problem 👍

[@smartfpv3992]

Actually my own design and setup machtes perfect with your theory. 1950KV on 5s with 500g AUW with a Session style HD Camera. Perfect prop tip speed and perfect match between prop size and wight. And it just feels absolutly amazing to fly and is very easy to tune. I feel a little bit proud now lol :D

[@ChrisRosser]

Nice work! I love it when theory matches up with experience, seems we're on to something here. 😁

[@smartfpv3992]

@@ChrisRosser I would say so! I also ordered some pretty high and narrow motors to take a look at your motor volume theory. Your theory's really made me think again.

[@roystevenson9632]

Outstanding!

[@ChrisRosser]

Thank you kindly!

[@othermichigansaturn]

I love physics, and quads. Cheers!

[@peterboy209]

Excellent, subscribed!

[@ChrisRosser]

Welcome aboard!

[@samuelepaganoni9741]

Hi Chris, thank you so much for the info! Does the optimal tip speed of 450mph relate somehow to airfoil chord length of the prop? I found high chord length props likes to run at lower speed, for example the 2.5-inch Emax Avan Rush. It has a lot of thrust with low rpm but increasing the tip speed I noticed that the efficiency decreases a lot.

[@ChrisRosser]

I don't think so. Its to do with Mach number so more related to the air properties than anything about the prop.

[@Ugly_Baby_Gaming]

down to aspect ratio.. look at glider wings long and skinny, jet short and wide... the long skinny can handle higher aoa giving better low speed performance as lift is greatest just before the stall

[@divingfalconfpv4602]

I want the babyhawk ii hd with crossfire.. sold out before it came out 😳

[@ChrisRosser]

Me too 😢

[@denismorgan9742]

Another question is how many blades on the prop? Which amount gives the best results? Max/min props or prop? Should the prop get smaller towards the tip or wider?

[@alextravine9422]

Sir, do you have any good, scientifically sound info on motor stator height and effects on tip speed on props? You mentioned volume of stator being related to its torque and power.... can you elaborate?

[@surajbele4442]

Very good person you are,thank you.good day.

[@chasleichner5895]

Does the 6gin^-2 line imply that most x-class builds are too light and would be better served spending a bunch of weight on more powerful motors (e.g. 6521s for scaled volume up from a 2207) and ending up closer to 3.3kg?

[@ChrisRosser]

I'm not sure whether that line can be extrapolated out that far for X-Class. This analysis was really meant for mini quads. Your motors should be big enough that the battery can only just handle them.

[@chasleichner5895]

@@ChrisRosser Thanks for the response. I think it’s just barely feasible if you could take a bunch of weight off of e.g. Neumotor 6521 motors with 13” props and a single e.g. 6s 5100 mAh r-line battery, but even with over charging the battery, flight times might be very, very short if you didn’t want to completely cook it. I haven't seen anyone push much beyond 50xx in terms of motor volume for X-class, so that might be an interesting design line to explore, but that might also just be my lack of knowledge of standard x-class practices. Definitely a ton to figure out from a mechanical and electrical perspective to push on to the "feasible" side of "just barely feasible".

[@chasleichner5895]

The extra weight from the motors right at the ends of very long arms might me a major challenge in terms of resonance as well.

[@moneywithfox]

Thank you so much for this

[@nilsmensch]

im pretty confused with the tip speed calculation. i calculated in metric systhem, so 450mhp are 720kmh and as a formular i used: Pi * (7inch * 2,54 * 0,01)*(22,8V*3350kv/60) = 711km/h i mean why is it in this video, when even in this unrealistic configuration the tip speed is lower than the max droppoint of the efficiency? or did i something wrong?

[@damc7456]

So what's gonna happen when I hang enough 18650's from my X-Knight 5" to bring it up to 475g for intersecting the green 6g/sqin curve? Peak efficiency 😃, or fried silicon 😭?

[@SkoTTe666]

Do you think we will se a better selection of different pitched 3.5 " props than what we have now ? i think 2 and 2.8 is the only option right now!

[@jeet5579]

I love engineering and science aspect of drone .😍 Can someone please answer?🤗 I want to build little bit different then a freestyle drone which will weigh around 1.8kg-2kg . Can I use 2806/7/8 size 1300kv motor with 7inch prop? Especially for speed cruising.

[@ChrisRosser]

2807 should be fine for 7inch props. 1.8-2kg is way too heavy for a 7in though. You should be aiming for a disc loading of ~8g/sq in.

[@jeet5579]

@@ChrisRosser thanks for reply!😊 I m 16 yo so I don't understand too much . But from internet, theoretically on 6s 7inch it provides thrust about 1.9kg per motor gives about 3.5 : 1 TWR. So atleast on paper it will cruise.

[@savkamil8168]

ur very knowledgeableborderline genius lol. funny how all the new 3.5/3.6 inch/4.2 inch bind and flies are coming out

[@itsmeRizzG]

Hey Chris, thanks for the amazing info! My question is, if you want to design a Freestyle Quad that can also carry a GoPro whilst maintaining efficiency under 250g, my assumption is that you would have to trade off efficiency to thrust in order to carry the GoPro? Or is that fully compensated by the weight reduction?

[@itsmeRizzG]

And by this I am talking about battery life. Considering a smaller Quad, that would only carry a smaller battery, with a GoPro, I would think that the smaller props will give much worse flight times with less efficient thrust?

[@itsmeRizzG]

I guess you could also just increase the battery capacity with the weight saved by dropping a prop size in order to increase flight time? At some point you run out of frame space, but I guess you could always just bottom-mount a larger battery? Does this track?

[@ChrisRosser]

You have a huge sacrifice in efficiency as you try to carry the same weight with smaller props. Your disc loading increases. You need more power to lift the same wieght with smaller props.

[@itsmeRizzG]

@@ChrisRosser Thanks, so in that case, would something like a 4 inch prop build for a sub 250g drone carrying a GoPro be the better tradeoff for going sub-250g (thinking freestyle/cineflying) whilst still tapping into the Disc Loading optimisation and maintaining some of the payload capacity of a 5 inch? Or better off staying at 5 inch and taking the efficiency hit on "performance" if the goal is efficient payload carry?

[@ChrisRosser]

To go sub 250g your best bet is 3" with naked go pro or SMO 4k. No standard go pro is going to work.

[@rafaelgonzalezmorales1824]

Some are reporting 1,350 grams and 7" props on DRL's Racer 4; inneficient machine based on your data....

[@ChrisRosser]

Yes, DRL race quads are not optimum from a performance standpoint. DRL is spec racing, everyone races the same (relatively slow) quad. If you compare DRL to MultiGP you can see just how much faster MultiGP quads are able to go. I think DRL is designed for spectator enjoyment so they don't want the quads going too fast or the spectators won't be able to follow the racing.

[@isalihkilic]

For a local competition, I will make a 5 inch with 1200g all-up-weight. Is it wrong? It's at the line of cinewhoops

[@ChrisRosser]

That is a heavy rig. It's not suitable for freestyle but it'll probably fly.

[@isalihkilic]

@@ChrisRosser It won't do acrobatic flights, it will only go 40 meters, fly a circle, go 40 meters, turn around, go another 80 meters etc. The main factor is speed. We are gonna use 5045 prop 1900kv (f80 pro) 6s

[@narpat007]

hello Einstein, it's like theory of relativity, which most people still do not understand well, can you give PID's values for 3, 5, 7, 8, 9, 10, 12, 15 inch props on Arducopter with 3-4S lipo ?

[@sconerin1]

Got a quad with 1806 motors 3 inch 249 gams with a 850 bat lucky for me what prop do i put on that to get a longer flying thx.

[@ChrisRosser]

Have you watched my video on props? I think the information you need is in there. Link in the description.

[@jhayjay-2x]

I think there is no difference in sound and one way to really know is with a Decibal Meter. In regards to speed are both Propellers (Master Airscrew & DJI propellers) the same size in dimensions and diameter and weight or do they both have the same propeller specifications and design? Otherwise to claim which one is quieter or faster is kinda moot.

[@RCRitterFPV]

Your Graph... where does a 1.5kg quad land to get the in the green

[@ChrisRosser]

9inch props I think.

[@SkySeeker]

Thanks very informative as always! What about the effect of the arms length? Could it be that running longer arms provides increased stability? Or should the arm length always be proportional to the prop size?

[@ChrisRosser]

Run the shortest arms you can. Long arms can cause a lot of problems especially for larger quads.

[@andrewhnorris1]

Great vid

[@brezovprut4431]

For efficient cruiser (200-250g AUW) running on 2S 18650 idk if I should go for 4" bi-blade or 3.5" tri-blade props. PS. I am thinking between using 1303.5, 1404 or 1603 motors.

[@ChrisRosser]

For efficiency a 4" biblade on 1404 motors would be my choice. For freestyle 3.5" triblade on 1505s.

[@brezovprut4431]

@@ChrisRosser Would 1603 motor be even more efficient than 1404 for 2S 18650? 1603 stator has slightly less volume (603mm3) compared to 1404's (615mm3)? PS. I am bit confused why Dave_C used 1303.5 motor with 3.5 props instead of bi-blade 4" 1400-1600 setup for his 2S 18650 Rekon35.

[@grumpythumbs8703]

Hi Chris- any chance of an analysis of props vs noise - I high pitch long chord 3 blades seem to be quietest on my 2.5 inch toothpick- Gemfan D63s cinewhoop props - weird???

[@GregPless]

Omg! You are JB squared on tech info 😎

[@ChrisRosser]

I try, I like to think that I emulate UAVTech but for hardware components rather than filters and PIDs. 😁

[@DriftaholiC]

Oof sounds like all us pilots on DJI air units with GoPro's need 5.5" props. If I understood the prop video correctly adding blades to the prop won't reduce disc loading? I would enjoy a video showing some example builds with common payloads, taking all your research into consideration.

[@marc_frank]

well the disk stays the same with changing number of blades blade area loading might be better the projected area of the prop onto a parallel plane below

[@DriftaholiC]

Well.. now I find myself planning a 6" build and being a bit frivolous with the weight of components to hit the sweet spot.

[@marc_frank]

@@DriftaholiC 20x20 might be enough

[@gem-squared]

How about for, say, bicopters? Do I just match the overall prop area to get the same disk loading? If so, I assume for a 200-250 gram bicopter I use 4-5 inch prop. (planning on a tailsitter delta wing :D)

[@ChrisRosser]

That sounds like a good approach to me.

[@aakashjana6225]

At 5:49 I got interested to relate the propeller torque to the propeller power so , here is what I found by rearranging the terms, P = [ 2 x pi x T (prop torque) x Vtip ]/ D , where Vtip is 450miles an hour.

[@ChrisRosser]

That looks correct to me!

[@markhutchens]

Having been using dal folding 5” props for almost a year (tuned, including ramp up power with folding)I tried a fixed prop yesterday and the motors struggled; they sounded rough and there were lots of wobbles. I think my 2205 motors have a much easier time handling the response demands with a folding prop than fixed. I guess the difference wouldn’t be noticeable on a more powerful motor but it was interesting to experience. It’s made me realise how important it is to match the weight and pitch of the prop to the motor. Especially if your motors are a little underpowered. Do you think the lead/lag on the blades could ever have an effect on props so small with RPMs so high?

[@ChrisRosser]

That's very interesting. I have found fixed props to give better vibration performance myself but I run 2207s so I have a bit more torque to play with. I wonder how much lead/lag on the blades you get during flight. I might have to look into figuring that out.

[@Ugly_Baby_Gaming]

could be that there acting like a swept wing when ramped up helping with compressibility and will also reduce in diameter slightly maybe also changing the pitch slightly as they fold

[@markhutchens]

@@Ugly_Baby_Gaming I agree that the reduced prop diameter during spin up could be a factor. If this is happening then the blades would be spilling air, which would also reduce the loading on the motors.

[@Ugly_Baby_Gaming]

@@markhutchens would be interesting to get a high speed camera on them to see how much folding happens with throttle change. that's if they do??

[@markhutchens]

@@Ugly_Baby_Gaming indeed. If someone wants to send me a high speed camera, I’d be happy to carry out a study! 😉

[@aerialimagery3216]

What about voltage and voltage sag? If you punch the throttle and drop to 3.5 volts wouldn’t your dip speed drop a lot? So would you want to shoot for a higher tip speed knowing it will drop to 450mph due to voltage sag? Love the content!!

[@ChrisRosser]

Absolutely, I like to think that 450mph is a target. If you don't quite get there its not the end of the world. But you don't want to be too far away because then you are giving up performance.

[@tuckerfarce]

How does elevation influence the ~450mph tip speed? Is it altered by intial air density at sea level vs 8km vs 16km? I can't imagine it being a significant difference until reaching upperlayers of the atmosphere.

[@ChrisRosser]

The speed of sound is dependent on altitude. It decreases by ~2% from sea level to 5000ft. www.fighter-planes.com/jetmach1.htm

[@Ugly_Baby_Gaming]

@@ChrisRosser speed of sound is only dependent on temperature but temperature will decrease by 1.98C per 1000 foot.

[@杨洁溪]

Is it possible to achieve 450mph by reducing blade numbers and pitch in over light disc load conditions? Because the fewer blade and lighter pitch props need to spin faster to get the same thrust，it may achieve 450mph then. I'm building a 360g AUW freestyle quad with a 5' frame, it is hard to get optimum tip speed due to your chart. 4' props may be a good idea, but I think it wastes frame space of some kind. So how about using 5' 2 blades with a lighter pitch prop？ Thanks.

[@ChrisRosser]

It might help. Your AUW seems quite low for 5" props which is why you are struggling to achieve 450mph. Either use smaller props or bigger motors/battery.

[@杨洁溪]

@@ChrisRosser Thanks mate and great video by the way. Looking forward to your next one.

[@chaimaalebdaoui5759]

please how can I calculate the blade tip speed ?? i I found this formula (3,14 *D*rpm) with D: prop diameter but I'm not sure if it's the right formula or not?

[@Edga_Po]

Hi Chris, thanks again for such an interesting information. What about choosing higher kV motor and then limiting motor output to match 450mph prop tip speed according motor rpm log?

[@ChrisRosser]

You can, but be careful because having more KV than needed and limiting down increases risk of smoked motors and ESCs and also reduces efficiency.

[@Edga_Po]

@@ChrisRosserthanks. Just watched this video kzbin.info/www/bejne/h4KtZKx9pdeihck and as I understood, Max says otherwise about burning ESC’s.

[@ChrisRosser]

@@Edga_Po My concern is the stall current. If the ESC is trying to spin a stalled motor the current will be higher for a higher KV motor due to V = IR and R is lower for higher KV. If the current is too high the FETs in the ESC will overheat. Perhaps Max doesn't stall his motors that often or his ESCs are very durable. If the motor isn't stalled an output limit might help protect against situations where the motor is commanded to full throttle and then burns out. If you are using an output limit you are losing any benefit of the extra KV but still taking the efficiency hit. Only worth it if you need to run 4S and 6S on the same quad.

[@Edga_Po]

@@ChrisRosser Thank you for explanation! So the only solution for best performance and efficiency is proper mechanics, not cheating. 😃

[@ChrisRosser]

@@Edga_Po I wish it were otherwise! Happy flying!