Ultralight 7" vs. 4" | flight time comparison | 18650 Li-Ion 

Enjoyed this technical outlook on the two variations. Interesting to see which direction you head next. Subscribed

Love this episode. Finally a German man talks about metric curves like a real engineer.

I really love your channel dude thanks for sharing the results of your research

the effect of drag increases as disk loading is lowered

Very interesting. The heavier 7" quad needs more energy to stay airborne, so I think you just proved that disk loading is not the only factor involved here. A 7" quad is typically not flown with the same battery as a 4" quad. If you're aiming for the longest flight time you pick a battery that is optimal for the size of quad you're flying. I put a 6S2P VTC6 pack on my 7" and it flew for 22:40.

As an aero engineer for a rotorcraft company this was both amazing and frustrating to watch! I'm not going to go into all the details but suffice it to say that you're on the right track, keep exploring! Incredible work, and I love my flywoo explorer LR. It's very cool to be able to see how rotorcraft theory is playing out for you in reality with off the shelf parts. Rotorcraft are complex in their design, and playing with different parameters can produce very different results... often due to extremely nuanced reasons. Keep going!

Also food for thought... I learned years ago that the very ends of the blade is where the most drag not being converted to lift occurs. Its why some of the props now have the little spikes on the trailing edge out at the end. With bigger props this effect is exaggerated because the tips are moving faster all else being the same. Using ducted props is the most effective way to reduce the tip drag turbulence, but has the undesired effect of causing a drone to auto level due to the way ducts work when moving in a direction other then up (in the case of drones). Ducted propellers are the most efficient way to produce lift. Its why all modern jets use ducted engines. I'm thinking maybe a very thin duct just to break up the drag might work? Ducts are a whole rabbit hole. The guy on RCmodel Review explained them pretty well if I remember right. Also Also wind resistance/drag increases exponentially. I don't think the extra wind resistance on the body of the the 7" vs 5' would make much difference at 30mph. Like I said, ... Food for thought.

Interesting video, thanks Dave.

Exactly what i was looking for, thank you :)

Great video! I've found that the tune helps a bit with regard to efficiency.

Dave_C, great video and set of tests. Disc load and drag, are major factors. I’d like to see more testing on where the battery sweet spot falls. We can fly micro long range 2s, 3s, 4s,5s, even 6s. In each case, there are trade-offs between power delivered by each pack and weight. I typically fly 3s. For motors, we’re looking for the most thrust, able to exceed the total overall weight and provide power to fly in some wind conditions etc. so for motors, there is a sweet spot also of KV, weight & thrust. I have one build I fly with 6 inch, 3S 18650 pack, on Lumenier 1806 2300kv motors. I typically get 9 to over 20 minutes without carrying a GoPro.

You're becoming a mix of JOSHUA BARDWELL/KABOB with the style of your videos 👍🏻

Thanks for the info!

Loved the video it would be great to see props better suited to this ultralight 7 inch size factor

Those MB2204 are pretty neat for builds like this 7”. I’m surprised I had never seen them before. Apparently there was also a 2450kv variant that would be great for a 5” on 3-4S but those are next to impossible to find.

Thanks Dave for all your hard work to help us all get in the air. One metric it might be interesting to fix is mAh to disk loading? Probably set that from the 4" and scale the appropriate battery for the 7". Having 3000 mAh battery on the 4" then a 5250 mAh on the 7" would be about the mark for the same mAh per cm2. It is a more even point to make your testing from I believe. Thanks again for everything Dave I would be stuffed without you JB and all the others.

@Dave_C FPV , I was thinking if the bad tune on the 7" would use up the battery... seems logical that if it's wobbly it will have to correct more during each PID loop, thus taking up more unnecessary battery power. But since the results were massively different, other stuff like the props you mentioned is also a good idea. Love this channel, keep it up !. Happy new year !

Dave can you do a video on 2-blade props vrs 3-blade props?

I liked when the 7-10" quads were norm with naze32 and afroflight etc for the noise factor. Hopefully people will adopt endurance racing say the FPV version of motorcross and last guy flying wins with laps done. It's that in remote areas or indoor racing only in near future LOL!

Two things: So on my Sub 250g 7” with the 7038 props, I could get over 9m with a 4s 650mah battery, when I switch to the 7042 props the same battery only gave me around 8m. Sound, your prop area is greater, creating more wind resistance. Don’t match the speed of the two aircraft, match the attack angle and then compare distance and flight time of each aircraft.

That 7" looks epic

Removing the wobble out of the system is VERY important. As long as there is wobble the motors are constantly speeding up and slowing down to fix the wobble the flight controller is experiencing. This will DRAMATICALY reduce flight time. I would first try tuning, but if not stiffen up the frame. Until you do that you won't get meaningful results. It was fun to watch the attempt though. Good effort. Its cool you're doing this.

Hey Dave, interesting video. Do you think the tune could substantially decrease efficiency on the 7" quad? I have not been able to tune a 7" to perform decently.

Great test Dave!! Just an idea...make this frame little bigger to use DJI phantom motors with DJI low noise 9"props😊

great video, idea.... motor amp draw at same cruising speed would be higher on the 2204 than the 1404 and at both running 4s. What if you ran the 7" on 5s or 6s, bring disc load closer, and with a higher cell count you can pull fewer amps to get the same cruising speed?

Dave those 7inch bi-blade props look very high pitch? I know there are not many out there to choose in 7inch class, you should get Gemfan to make some 7035 bi-blade?

Can CNC made easy cut the 7 inch arms? Getting ready to order another 5inch mini LR . Awesome research by the way. Thank you for all the hard work you do.

Thanks, this was exactly the video I was looking for. Did you measure the resting voltage afterwards? Since the 7 probably draws a bit more current the LiIon would drop voltage harder. Anyhow - in my case I'd value the total trip distance before flight time, so I'm guessing the 7" would win.

Have a question. New to quads. Have a hubsan h501ss don't count that. I do fly long range fixed wing fpv however. Would like to get a long range quad and wondering what to get. All my batteries are 4s lipo and liions so would like to stick with those. Don't care about freestyle or racing or any of that, just long range cruising. What do you think would be a good place to start?

Dave, thanks for making this video. Really enjoyed it. On the graph of disk loading vs efficiency, you drew a curve that sort of looked like a smile (the mouth on a happy face). I could be wrong, but maybe the actual curve looks a bit more like a frown (the mouth on a sad face). If the curve did look like a tilted frown, then once you go below a certain disk loading, you get into a region of rapidly diminishing returns. I'm not sure about this, but it might be something to think about. Would explain the lack of significant endurance gains with 50% less disk loading on the 7 incher. Or maybe the 7 inch props just had too much pitch? Either way, super interesting comparison. Fyi, I bought myself a Flywoo Explorer LR4. Can't wait to get my hands on it. Thanks for making the LR4 a thing! Best regards from Brasilia, Brazil.

Great concepts, good test! Are you sure flying till 3.3v on 2 different packs (that might be in slightly different condition) gain a good comaprison? Have you checked how many MAHs you charged back into both packs after the flight? Could have been quite different MAH consumption / cell behaviour. I say this because I noticed in my last test (regarding LIONs) that temp playes a huge role. The one day (cold, around 0°c) I got horrible flighttimes (and only 2000mah out of a 3500mah pack until the volts dropped down and I had to land, the other day the same pack got me 2900mah and volts were still ~3v/cell only difference: 9°C this time and packs in a plastic bubble wrap... Just got into micro longrange by testing the iFlight Chimera4 and the "great" Shadow Fiend - sorry, both rip-offs of your Explorer - but the Flywoo LR is so hyped it wasnt available really ;) Today just for fun, I flew a few 650mah packs and got ~ 7mins of save flighttime ;) greets from Austria, Mario

Having a Flywoo explorer and built one of your mini long range frames (both excellent) I think you hit the sweet spot at 5 inch for li-ion and 4 inch for light weight li-po flying. The 7" might be a worthy experiment to test capability but less necessary (plus it's less of a looker!).

Omg can't wait to see tmount 7" motors and props. Now wondering if the new t-mount design from iflight 2005 might be a design testing for this.

Very interesting test and results. Maybe you could try another test where the battery mass fraction is the same for both quads....

I have 4 of your frames I am working with. My best flying, longest flying is the 4" frame but with 5" arms. I don't have the arm braces installed. The 4" with 4" arms flys great but it gets about 3 minutes less flight time. The 5" I have like the one you are showing with the bicycle connector flys great but does not get the flight time of the 4" with 5" arms even though the weights are within 10 grams of one another. IMHO I think the biggest difference are the props. GemFan Hurricane props fly better and longer with all of mine than similar HP or other props I have tried. I think the center of gravity is enough different on these to have an affect as well as the relationship of the motors in the design.

Glad you are putting yourself out there with your experiments. It is ok to be wrong. You can’t learn from always being right. Like the 7inch design. Maybe 5” props?

Have you thought about stacking the AIO and Vista and mounting the battery behind it? You could ditch the top plate and standoffs to save some weight, and basically get a top mounted unibody.

I am thinking if you could spin a 8" props on 7" frame and reduce its motor kv would give a longer flight time with the same battery capacity or may be a much longer flight time with a higher battery capacity.

Awesome content Dave!, I really love this deep dive into ultralight LR platforms! Anyway I don't think pure flight time is really a useful data for us. I mean what I care about is "how long (in term of distance, not time!) I can fly". So cruising speed is actually a thing! 18 minutes on the 4'' cruising at about 40 km/h means a12 km trip while 10 minutes on the 7'', cruising (I'm kind of guessing) at 70 km/h means... again roughly 12 km! While I loved my 4'' micro LR, I recently switched to 5'' mini LR because I can simply fly the same distance... but faster!

I have an dji pfantom 2 copy with 2212 920kv motors 9450 props. Do you think will work 3s sony li-ion battery.? Thank you for show the experiments.

Very educational! I tried to take in as much as possible 🙃 I am planning on ordering the 5" from cncmadness next week and wondered if they have the latest iteration?

It's obvious, the elephant in the room which you omitted from your calculations was...Drag. Those big 7 inch discs whilst giving bags of lift, also creat a lot of drag and need more power to drive them. Specifically, frontal area with the quad tilted at an angle to achieve 40kph. The bigger disc area = bigger drag...🤔🤔😀🇬🇧🇬🇧🇬🇧🇬🇧🇬🇧

You always put the GPS in the wrong place! Above the GoPro is by far the best place I've found. Thanks for the video!

I have the 5 and 6 inch rekon quads. It would be very nice if you would also provide a 7inch version of the quad, with a larger battery carrying capability for longer long-range flights.

I'm thinking you should be looking into the props as far as being properly matched to the motor - to produce 'enough' lift for the weight of the quad

Do you factor into overall frontal area drag of the discs into the equation?

Fantastic video! Thanks! Next, can you do a range test? For example, if there is a particular mountain I want surf down, how close must I drive my car to be close enough to fly my drone to the top of the mountain? If the 7" has a higher cruising speed than the 4", then perhaps less efficiency won't matter so much if I can fly to the mountain sooner. Much appreciation.

It's amazing bro. Where can i buy 7inch frame ?

What’s currently the pitch of the 7” prop? I guess you should have a very low pitch with a maybe a slightly lower stator but bigger diameter. Maybe you can divide the pitch to disk area and use that factor to divide that with the motor specs and get some factor out of there what you maybe can get some conclusions out of and optimize

I see Rekon7 coming soon.. real curious on motors. cause i was thinking when i was going to build myself 2303.5 with gemfan new lightweight LR 7035.. arm look so long and skinny.. surprised no braces connecting left and right motors along with front and back

Are you going to release the dxf for the 7inch frame?

Disc loading doesnt really account for the extra power needed to swing the larger props on the bigger motors, does it?

Pretty interesting results, Dave! Surprising! I'm curious to see what you do next! 😃 Happy new year! And stay safe there with your family! 🖖😊

Hi,I decided for your frame but I'm having a lot of yello. Have you some solution to suggest me? Thanks a lot

Hi~ Let me ask you a question. It's a 3 inch cine hoop. In the video, there are videos of making a pack with 4 lithium-ion 18650 batteries and flying it with an FPV long range. With excitement, I ordered batteries and made a pack. LGDBHG21865 3000mA. The problem is... When I hovered, the yaw tick symptom reappeared after it came up, and, after a few seconds, the low-voltage buzzer sounded and the aircraft sank. If I open it again, it opens again,,,,, repeat... So, I changed the low voltage warning in Beta Flight to 2.7V, and when I turned it on, there was no sound. If I take a break for a while, it will float again. But it sinks again... The battery is hot, and the voltage is 3.8V... TT I bought 40 tablets... but I can't use them all? How do people use? Batteries were also made by buying anything. Is my FC the problem? Or is there a special setting for Li-ion? FC is IFlight SucceX Mini F4 V3 Stack, and the manual says 2~6 Lipol. Do I have to use only Lipol? Can I change FC? Why is this happening? It's a waste of money/work time. I'm serious~~ Please advise.~~ TT

love the vids!

Here are my thoughts. You concentrated a lot on g/cm², but as you mentioned earlier on, we need to look at g/W too. So I think you're right at the end of the video that you need to further test motor and propeller combinations. A couple of ways you could test this against the 4" quad. 1) The in-situ method. Measure the wattage consumption while hovering until the battery runs down to your cut off voltage. BF black box should be useful here. This is subject to a lot more random errors, but would be more 'real world'. 2) The bench method. Get a table top thrust meter, measure the power to thrust relationship of the 4" prop and motor, then find the best motor and 7" prop combination that matches.

I like the happy compromise that a 6in prop brings. Your 5in frame adapted to 6in props, with t-mount 2204s and HQ T6x2.5x3 would be a nice combo.

Very interesting video as always! Well I think I'll have to try out Liion too... Where do you order your VTC6 cells? I'm from Germany too :)

A doubt: at 08:30 you said that a 7" have 993 cm2 prop area. Where is this number coming from? It's the prop specification or is there a calculation that you made? Thank you!

Dave, Can you compare them down to a specific voltage? Because no matter what the 7in is most likely drawing more amperage thus making the battery(especially li-ion) sag slightly more but I really don't know.(I know the cold hand thing my thumbs were gonna fall off the other day as I pitched the nose of the quad forward to get home asap) I've been doing these exact same comparisons between my Explorer LR 4in.(146g analog vtx weight) and my Tomoquads Katana V2 5in. w/fpvcycle 2204 2800kv t-mounts HQ 5030 Bi-Blades(206g analog vtx weight) 4s Li-ion(190g Pack)(18650 Samsung 25r-Green Wrap 2500mah 25amp Cell, best cell for the price at like 3$ each) and even though the katana fly's much much better especially if you add even more weight the amp draw is still slightly higher so flight times are lower on the same pack. I have a second Katana V2 5in. frame I'm going to build out with BrotherHobby 1504.5 2650kv motors I have and see how well it performs swinging a larger 5in prop and if its possible to beat the Explorer LR on the same pack using a 5in prop because the Explorer's efficiency really takes a dive if you add weight with a bigger battery or camera where as the bigger prop/bigger motor combination's efficiency is much less effected. So to get back to your test, the 7in will carry a larger battery and fly longer while of course burning up more mah/min BUT a really cool test would be to see how far each can go at there optimal cruising speed. I have a feeling that the 7in you will find has a much faster optimal cruising speed so where the flight time on the same battery might be less than the 4in, the 7in quad may cover twice the distance even with being in the air less time. Yea distance and finding optimal cruising for the 7in would def be a cool test.

You should look at the efficiency curve of brushless motors, even with the greater weight of a bigger motor, since you'd be at a much lower portion of it's torque potential, you should be able to get a very significant efficiency boost. I'd be curious to see you go in the complete opposite direction and get a 2806ish size motor then test the fight time. Should also significantly help with the feel of the quad.

...you would also need to fine-tune the drive-train. Check steeper and shallower pitch. Every motor has it's sweet spot of RPM vs torque - you just need to find the right prop to get it there... Keep going, man! :)

will the 7 inch be available to buy? ( or is it already?) thanks

It seemed to be pretty windy on that day. Maybe that caused the gap because of the much bigger surface area of the 7"? And how about going for 6S? Btw. Happy new year, Dave!

Maybe reducing the discload by using the same prop/motor combo like on the 4inch and choose a hexa/octacopter frame would be more comparable. The "right" motor/prop combo seems to be crucial in terms of amp draw.

Hi, Nice vid! The main benefit to go larger in propsize is the cruise speed for me. A couple of years ago I build a 3"ultralight with 2s 18650 for long range. I managed to get 25 minutes of flight but the biggest thing I found out that the cruise speed is quite low, and is you go faster then you lose efficiency. My 7" goes for 20+minutes on one 4s lion pack but at a cruise speed of about 70kmh. And got long range I find the speed also very important. What would be interesting to test them at there own cruise speed and look at the distance they flown. And what also would be interesting is to do the same test only then both at 70kmh. Ps, a bad tune also costs a lot of energy.

While the 7 inch has less disc loading, if you give it the same battery you still have more weight to carry. So what if you had the same ratio of battery weight to all up weight?

Dave you have 1450kV motors on the 7" and 2750kV motors on the 4", what would happen if the 7" motors were closer in kV to those on the 4"? In the tested setup the 7" has 3x the prop area but at the same current draw it is probably turning at less than half the speed of the 4", with the additional drag of a larger prop to overcome. Do the DJI goggles allow reading the amp draw during flight.

I think that you cannot start from the "disk load" as you do to judge the efficiency but that you must start from the effective efficiency of the propeller motor torque (grams per watt at X% Throttle where X is the gas level necessary to fly at 40km/h) before making the comparison. And again, even doing this using the engine manufacturers' tables, it is false, because these tests are done statically. There is no concept of "simple" wing loading for a rotary wing, and your test demonstrates this with talent. Since I'm thinking about the design of an ultra-light 7" I came across your remarkably interesting video 3 years later. Thank you for that.

Have you ever thought of the concept of having a small airfoil, like the tail of a DC-9, attached to the top of the quad which would generate lift at cruise speeds thereby allowing for much reduced power settings in cruise and longer flight times? The angle of attack of the wing would have to be optimized for a certain speed so as not to create more drag than lift. Anyway just a thought.

Hello Dave quick question have the flywoo exorer but I bought it without caddy Vista and I wanted to build a analog version. Witch I did I added my camera and a vtx but I need help with beta flight and making it fly any help?

Hi,what is the name of the 7 inch frame on the video?

Wouldn’t the 7” cover more ground than the 4”? If you were more concerned with a distance I think the 7” would still win if we are actually doing long range. Maybe instead of using a 7” with these motors, I wonder if changing the prop to 5.5”-6.5”? Looking for the efficiency/ distance covered sweet spot.

where we can collect the drawings to test the arms

There's a publication Modeling Flight - NASA that is a 6 MB PDF file on line. Flight efficiency does not scale at a linear rate. A rough estimate based on disc area would be energy consumed versus time. So if you scale up the disc area you would want to see how long it would take to consume the same amount of Watts. Because the discharge rate curve of the batteries are important you would want to use a battery that has proportionally higher weight and amp hours as disc area increases. You're dealing with the issue of cross-sectional surface area goes up by the square as the volume and possibly mass goes up by the cube. I like the smaller quads, they have lower inertia and are less likely to self-destruct if they hit something.

You should try the mavic motors and props. With the amount of money dji invests in there drones they are probably very efficient. Banggood has a frame that uses the mavic motors mounting holes.

you've just drawn the lift off logo 0:56 🤣

Hi Dave the TBS pizza box frame will be around 36g cut from Rohacell.com 1mm Carbon skins with polymethacrylimide plastic foam core, would love to see your experiment with this stuff.

Drag is higher on the bigger quad and there’s the prop+motor efficiency. Most 7” start to wobble because of the aerodynamics, the bigger prop starts to inhibit fixed wing properties and the quad starts to feel the wind like a flying wing does.

Where i can get the Arms for the 7 inch buidt?

You should try the 7" with a 21700 4S1P with 4100mah Li-Ion pack (Sony | Murata VTC6A 21700 4100mAh). Right now you are decreasing the discload but increasing the AUW with a unchanged capacity of battery, but I think with an increase of AUW you also need to increase the capacity of the battery, so 360gr on 3000mAh or with the 21700 batteries 580gr (I added 100gr for the heavier cells) on 4100mAh. your discload is still considerably less while the 21700 battery also gives you an increase in amp draw. I would also run a test with 18650 packs in 4S2P and 6S1P configuration.

by using the same battery the 4" had a much higher ratio of battery power to weight, there seems to be an efficiency curve here where not only is the right ratio of disk load to total mass important but the right ratio of battery weight to total mass.

The larger quad will have much more drag maybe more then double. also square/flat surfaces are not your friend, round/curved leg sections would be better. but why not just use the same 4" body/base with long 7" hollow round legs. Or/and try larger dia motors with lower volt/rpm

LOVE that someone is breaking out the graphs. On the flight testing, should you be using the same battery though? Id expect you'd want that scaled up proportionally to the weight no? Another variable: forward surface area -- the 7" is going to have a much larger silhouette. On a different note, Dave your frame layouts are elegant AF. Like they're aesthetic candy -- nice work dude.

I read an article about Vertically Offset Overlapped Propulsion System (VOOPS), improves efficiency could you give it a try?

The 7 inch had additional carbon fibre supports on the arms, while you flew the explorer without them. Air resistance is no joke. If the supports are necessary for the stability of the quad, then maybe the disc load theory simply needs materials science to catch up before it can be used in quads of this size. Perhaps folded aluminium would work

your 7 frame from rekon6? or what

Really very interesting, thank you! Definitely worth a subscription...;D. Here are some thoughts on why it didn't work out with flight time: A ballpark calculation (momentum theory) on the hover powers=f(AUW, DL) of the two configurations shows that the 7" drone should require about 12% less propulsive power than its smaller brother. Twelve percent is a decent potential, but it can be quickly eaten up by parasitic secondary effects. One of these is certainly the added drag of the larger configuration. However, I would take another look at the propeller selection in particular. A low disk loading also means that the propeller must deliver a comparatively low thrust for its size. So it may be that if the propeller is incorrectly selected, it will be operated almost exclusively at low part load. Drastic losses in efficiency are quite possible here. Maybe lowering the propeller pitch could be solution for you. BTW: Did you log the flight data?

load up the 7" with enough battery to get similar disc loading of the 4". I think that would be a more "apples to apples" comparison. People that are into long range don't use a larger quad to carry the same small battery - the point of a larger quad for long range is to carry the weight of more battery.

I think you should do the same thing again but switch the batterys because even though the batteries are the same the resistance might be different

Very good video 👍👍👀🇭🇷

I need to do a faster flight, but went 70 min with a lightweight rig based off of the Black Opal 6". Battery pack was a 3S2P 3500mAh Li-Ion. Arducopter was set for ~13kts running a circular do jump pattern. Non-HD rig, super light 215g before the Li-Ion(290g). I think if I increased my speed slightly, it might be more efficient.

Please make more videos about the Explorer LR 4 v2 maybe a ultimate build video without Dji components

I like the route TBS is going with their Pizza Box frame which is in production right now. The one piece 7” frame, which is SUPER light, as it’s very thin carbon fiber sandwiched around some type of foam. Then has a small canopy that will fit a Vista and and maybe a “Beast AIO”. I think they have their own AIO coming as well. The quad is not very crash worthy, but I think they are onto something.

a lower pitched 7 inch prop would be nice. implementing Prandtl lifting line theory in propellors could also give an enormous boost in flight time.

скажите, как называются пропеллеры на 7 дюймовом квадрокоптере

Can you buy THat seven inch frame ???

"1 year later..." lol Definitely the high pitch of 7"prop induces a lot of drag which ate the expected efficiency from the larger diameter prop. Another thing: ~120g lift per each 7in prop is very low and its rpm is very low as well. Not sure if this is the case but, at very low rpm, the efficiency is horrible (just imagine, e.g. 30 rpm producing 0 g thrust at 1A, efficiency = 0) and increases sharply as rpm increases reaching a peak followed by a gradual decrease at high rpms. It would be cool to see a bench test comparing W/g thrust per each motor in both setups.

A few years ago, I was going to build a medium/large camera platform (a lightweight quad with carbon/foam core, and 10-12" props)(never completed it), and at that time there was only big hexacopters being used. Some research led a link to Hobbyking, and did offer a motor that they developed to run on 8s. Their approach for longer flight time was higher voltage motors and lighter wiring gauge using 10c lipo batteries. That stuff never became popular and is no longer available. Here's a link for that concept if you are interested....ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-bNaiNNF7ZBs.html.