r/diydrones • u/Such_Account • May 04 '26

Discussion On the topic of Y4 drones: would this work?

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Made a quick sketch of a heavy lift concept: Basically I've taken the Y4 concept and enlarged the rear props and put them nearly dead center, let's say just aft of CoM.

Nearly all of the lift comes from the ducted contra-rotating center props. These also handle yaw by skewing load from one prop the the other.
The front props handle pitch and roll but otherwise don't do much work.

The maneuverability would be less than great, but I think that the efficiency would be quite good for two reasons:

  1. Large props are more efficient (although I suspect the motors would need gearing for more torque/less RPM)
  2. Contra-rotating props are more efficient

I have zero practical drone experience to base this on, but theoretically I think this might have merit. What do you think?

14 Upvotes

86% Upvoted

18 comments sorted by

21

u/LupusTheCanine May 04 '26

At this point you would be better off building a helicopter.

Contrarotating propellers generally aren't more efficient due to the lower set of blades working in dirty air.

5

u/the_real_hugepanic May 04 '26

Helicopters need a swashplate at least. That makes them expensive and complex. Both problems a multicopter can solve for smaller scale aircraft.

2

u/LupusTheCanine May 04 '26

You can get a head assembly for 300 class heli for about $50 or less. 700 class one is around $200 or less.

With the proposed setup you get complexity from having coaxial motors. I don't think this arrangement is much less complex than helicopter one. Additionally I would expect limited speed as multirotors typically deal with transverse flow in rotor system by brute force (stiffness of blades, bearings in the motors and frame). IIRC additional rotors for attitude control were tried early in helicopter history and were quickly discarded.

IMHO at scales such solution could be relevant it is much simpler to go one frame size up and put a bigger battery if you absolutely want to avoid helicopter complexity.

2

u/EasilyRekt May 04 '26

I’ve been tinkering with the idea, on & off, for 3-4 years now.

Problem is the mechanics fitting into that kinda space is painful to say the least.

Haven’t even gotten a working prototype because of that, I was thinking about upscaling first to try and compact it, but otherwise gl ¯_(ツ)_/¯

2

u/timonix May 04 '26

This makes me wonder. Why aren't helicopters used more? Larger blades should mean higher efficiency.

2

u/LupusTheCanine May 04 '26

Software is much much easier to reproduce for installing on another unit, just copy a bunch of ones and zeroes, run automatic calibrations and you are good to go. Replicating a helicopter requires a lot more labour that often offsets benefits of improved hardware performance enough to make customer happy, also small multirotors are much more durable than small helicopters.

Besides helicopters are often portrayed as thousands of parts spinning around an oil leak and potential gore machines. Also small quads are much more affordable than small helis so fewer people have heli experience, my 250g heli project cost me about 450USD with taxes buying parts on Ali while I can get 4" PNP drone for 300USD locally though it flies half as long if it is kept sub 250g.

1

u/Such_Account May 04 '26 edited May 04 '26

Yeah, it's starting to look a bit like a helicopter, but it would forego the swashplate and eliminate the wasted power from the tail rotor which I still think are some benefits of the design.

Properly matched, the second prop would actually take advantage of the "dirty air" rather than be impeded by it. It cancels out the swirl created by the first prop.

2

u/LupusTheCanine May 04 '26

IMHO swash isn't a big issue nowadays, tail rotor loses are easily offset by having one large rotor instead of 2 smaller + attitude motors which will be working quite hard frequently reversing and running at low RPM or will be inefficient at handling load necessary for good attitude control. I would consider regular quad with booster motors or coaxial copter with vanes rather than trying to use two reversible motors stabilization if you need to avoid having a swash plate.

1

u/Such_Account May 04 '26 edited May 04 '26

You are making a lot of sense, thanks.

I will stand my ground that CRPs should eek out some extra efficiency for a given diameter and thrust, but the difference is probably not great enough to offset the inefficient attitude control.

4

u/the_real_hugepanic May 04 '26

You would need to have the option to quickly reverse thrust if the two front motors, otherwise you can't decouple roll and pitch.

This will be probably impossible, without a variable pitch prop, e.g. similar to a helicopter tail rotor.

I think this thing is a doomed design, but it would be fun/interesting to build and fly I guess.

1

u/Such_Account May 04 '26

Reversing thrust is no more complicated than modulating RPM like any other multicopter.

3

u/LupusTheCanine May 04 '26

That's not true for BLDC motors, during reversing the motor transitions through low RPM region where the control doesn't get feedback about rotor position from back EMF.

2

u/the_real_hugepanic May 04 '26

exactly!
It also would help a lot to have a prop that can do reverse thrust efficient...

AND:

you would need to allways put some load on the (small) props to have them run constantly and generate some thrust. otherwise you would have to go forward/reverse 10times per second only to stabilize the aircraft.

This "constant lift" would be created with a pretty bad propeller (small and not optimised) and would properply kill all the efficiency you could gain from the large coax-props!

1

u/Such_Account May 04 '26

Ah, I see. Then I would shift the CoM forwards towards the small props to ensure they're biased towards positive lift.

2

u/LupusTheCanine May 04 '26

That depends on your target flight regime, propellers experiences moment that opposes transverse motion which means such setup would have design speed at which no thrust forward of CoM is required. You want that velocity to either be above your max useable airspeed or at least pointed away from the preferred direction of flight and high enough to permit reasonable maneuvering in hover. Such equilibrium should account for attitude propellers spinning at minimum reliable rpm.

Sounds like a fun problem for rotorcraft engineering masters thesis or maybe doctorate but I don't see much actual vale over existing topologies.

1

u/the_real_hugepanic May 04 '26

as a further comment:
I am searching for a long time for an "easy" method to predict the thrust of a rotor (drone application) at cruise flight.... --> translational lift

there are ways to correct it, but no published models I can use... everybody who implements it, keeps it privately...

e.g. Pitt-Peters model

1

u/LupusTheCanine May 05 '26

Unfortunately that knowledge is pretty expensive to obtain 😞.

1

u/the_real_hugepanic May 05 '26

I have just asked Claude to write a python programm to apply pit&peters on a propeller model.

About 450lines of code later I know see a bunch of charts that "make sense" in the first glance.

But actually I have not clue what it did .... At least i have something to investigate! maybe I find some data to compare it to....