How would you design a cyclocopter?

[OCPatch]

Hey Nick, Jude, or anyone else:
What do the Cl vs. alpha and Cd vs. alpha curves look like during the dynamic stall phenomenon? I get that it is a transient condition at best, but it must predictably happen with every revolution of a cyclorotor blade if you guys are getting best thrust at 35 or even 45 degrees incidence wrt the blade path.

 

[Gabik09]

Hey Nick, Jude, or anyone else:
What do the Cl vs. alpha and Cd vs. alpha curves look like during the dynamic stall phenomenon? I get that it is a transient condition at best, but it must predictably happen with every revolution of a cyclorotor blade if you guys are getting best thrust at 35 or even 45 degrees incidence wrt the blade path.

Hey! During dynamic stall, the Cl vs. alpha curve typically shows a sharp drop after a peak lift coefficient as the flow separates and the stall begins. It often leads to a sharp decrease in lift and a significant increase in drag. The Cd vs. alpha curve will show a sharp rise in drag at high angles of attack, reflecting the increased flow separation and turbulence.

For cyclorotors, if the blade is reaching such high incidences (35-45 degrees), you'd expect these dynamics to occur every revolution, with periodic stall and recovery. This transient behavior is what makes achieving high thrust at these angles a bit tricky, but also why dynamic stall is crucial to the performance at certain RPMs.

 

[Caeden]

Thanks a lot for your feedback
At the Momment I am Testing the influence of Ground Effect on Cyclocopters at high pitching Angles of about 45°. As a Reference I also tested a 5inch Propeller does anyone know ho

w to interpret these Numbers. With the Propeller there is a very clear distinction that is almost not noticeable with the Cyclocopter

 

[2jujube7]

@Caeden That's really cool, I don't think I've ever seen anyone look at the ground effect for cyclorotors.

Here's some CFD simulation of a large rotor: https://core.ac.uk/reader/286449981 I would guess that the decreased ground effect might be caused by a rotors airflow spreading out more? Or from taking in air from the sides vs the top?

 

[Caeden]

Yeah that does seam quite interesting, might be worth investigating. For the time being I've been testing out different LW PLA 3D printed wings with fibre glass, which habe been working quite well.

 

[OCPatch]

Hi All,
I'm glad I checked back in here - more interesting developments!
It is a impossible to be certain without the particular parameters of the tests, but I'll lay out my assumptions that I'm making that influence my comments.

With the relative distances and (I assume comparable size of cyclorotor to propeller) the two cases each for propeller and cyclorotor are essentially IN ground effect and OUTSIDE of ground effect.

I think the sharp drop in efficiency for the propellers in likely transonic tip effects. If your 125 mm propeller is really producing ~58 Newtons of thrust (5900 gr-force) and the in-ground-effect case is producing ~78 newtons of thrust (7900 gr-force) then it is almost certainly at very high RPMs. Propellers don't work well when the tips approach supersonic speeds.

The cyclorotor is likely turning at considerably slower RPMs and rotor airspeed to generate comparable thrust per watt so transonic effects are likely not a factor.

As far as the marked difference, particularly for the propeller in- versus out-of-ground-effect, here is one possible explanation:
Thrust is mass flow rate multiplied by the velocity of outflow. Power is mass flow rate multiplied by the square of the velocity of outflow. Higher outflow velocity implies less power-efficient thrust generation.

The fluid outflow behind an unducted propeller running in compressible fluid tends to condense or contract in to a smaller, faster jet or plume. When in ground effect, the result is the opposite because the static pressure of the stopped fluid in the center of the outlet jet causes the flow to spread instead of contract. If we assume the same mass flow into the propeller disc in both cases then the flow through the smaller area at the exit must be faster to have the same mass outflow. Higher velocity of the outflow means less power-efficiency.

And if we look at the data and conclude that the cyclorotor is less sensitive to ground effects than the propeller, then two points might explain this:
The outflow side of a cyclorotor is producing vortex pairs from the trailing edge of each passing blade. The lower surface vortex that contacts the ground surface tends to 'roll' itself along that ground surface with lower velocity relative to the surface (lower relative velocity equals lower drag and losses) and the vortex shed from the upper surface effectively 'rolls' itself like a meshing gear on the lower surface vortex. It is possible that this behavior has the effect of causing relatively efficient flow out to the 'spinward', 'windward' side of the outflow instead of allowing for the stagnation point that occurs below the large, planar area below the in-ground-effect propeller. Note also that the entire rotor disc area below the propeller is in close proximity to the ground while the cyclorotor only has a small area of its 'drum' shape that is close to the ground.

Thanks for sharing your data. Best of luck on your research.

 

[Caeden]

Hi All,
I'm glad I checked back in here - more interesting developments!
It is a impossible to be certain without the particular parameters of the tests, but I'll lay out my assumptions that I'm making that influence my comments.

With the relative distances and (I assume comparable size of cyclorotor to propeller) the two cases each for propeller and cyclorotor are essentially IN ground effect and OUTSIDE of ground effect.

I think the sharp drop in efficiency for the propellers in likely transonic tip effects. If your 125 mm propeller is really producing ~58 Newtons of thrust (5900 gr-force) and the in-ground-effect case is producing ~78 newtons of thrust (7900 gr-force) then it is almost certainly at very high RPMs. Propellers don't work well when the tips approach supersonic speeds.

The cyclorotor is likely turning at considerably slower RPMs and rotor airspeed to generate comparable thrust per watt so transonic effects are likely not a factor.

As far as the marked difference, particularly for the propeller in- versus out-of-ground-effect, here is one possible explanation:
Thrust is mass flow rate multiplied by the velocity of outflow. Power is mass flow rate multiplied by the square of the velocity of outflow. Higher outflow velocity implies less power-efficient thrust generation. View attachment 249057
The fluid outflow behind an unducted propeller running in compressible fluid tends to condense or contract in to a smaller, faster jet or plume. When in ground effect, the result is the opposite because the static pressure of the stopped fluid in the center of the outlet jet causes the flow to spread instead of contract. If we assume the same mass flow into the propeller disc in both cases then the flow through the smaller area at the exit must be faster to have the same mass outflow. Higher velocity of the outflow means less power-efficiency.

And if we look at the data and conclude that the cyclorotor is less sensitive to ground effects than the propeller, then two points might explain this:
The outflow side of a cyclorotor is producing vortex pairs from the trailing edge of each passing blade. The lower surface vortex that contacts the ground surface tends to 'roll' itself along that ground surface with lower velocity relative to the surface (lower relative velocity equals lower drag and losses) and the vortex shed from the upper surface effectively 'rolls' itself like a meshing gear on the lower surface vortex. It is possible that this behavior has the effect of causing relatively efficient flow out to the 'spinward', 'windward' side of the outflow instead of allowing for the stagnation point that occurs below the large, planar area below the in-ground-effect propeller. Note also that the entire rotor disc area below the propeller is in close proximity to the ground while the cyclorotor only has a small area of its 'drum' shape that is close to the ground.

View attachment 249059

Thanks for sharing your data. Best of luck on your research.

Wow, thank you so much for your time and insight! It's clear that your knowledge of aerodynamics is far greater than mine.

Just to clarify, I made a significant conversion error with my load cell. The Newton values should be a factor of 10 smaller, meaning the maximum thrust is about 7.8N.

For the propeller, I used a 5-inch Ethix S3 Watermelon prop with three blades and a pitch of 3.1. The motor I used was a T-Motor F90 1300KV (2806.5) running at approximately 16V. Based on rough calculations, the propeller tips should be well below the speed of sound.

For the cyclorotor, I used a wingspan of 30 cm, a wing chord of 10 cm, and a rotor diameter of 20 cm. The motor and power source remained the same, with a 1:7 ratio between the motor and rotor. The wing shape is based on the NACA 0012 airfoil. The maximum blade pitch was close to 50°, so they may have been in a stalled condition.

Regarding the propeller’s behavior, one widely accepted explanation is that a propeller operating in ground effect (IGE) experiences higher pressure beneath the wing compared to a propeller out of ground effect (OGE). As a result, at the same rotational speed, it generates more thrust.

Unfortunately, I didn't record the RPM during the propeller test. However, if we use PWM as an approximation for RPM, we can see that the propeller produces more thrust at the same PWM but also consumes slightly more power. This suggests that, in ground effect, the propeller encounters more parasitic drag. However, the additional power consumption is nearly within the margin of error, so I’m unsure if we can confidently draw that conclusion—although it does align with what I've seen online.

As for the cyclorotor, I currently don’t have the means to visualize the vortices to confirm or disprove any hypotheses. If you have any suggestions for cost-effective ways to analyze airflow over the wing, I’d love to hear them.

Once again, thank you all for your insights!

Also, if you don’t mind me asking, what do you do for work, and what did you study?

 

[OCPatch]

Hi Caeden,
Last questions first: I have worked at corporate and private aerospace jobs - mostly scientific programming - and in medical device development where the bulk of the work was designing electronics and firmware. I attended university in the 1980's and I received degrees in aerospace engineering and computer systems engineering.

The thrust that your propeller produces seems much more reasonable now that you have explained the scaling error on the graph of efficiency versus Newtons of thrust. And it also gives me one more idea about why the propeller efficiency appears to hit a brick wall at a particular point. I agree with your assessment that the issue is not likely transonic tip speed. I think the issue is one of electrons, not air molecules.

As I look at your PWM graph I think that maybe those sharp drops in prop efficiency occur at approximately the same PWM setting. There is a phenomenon that occurs with DC motors called "Back ElectroMotive Force" or Back EMF. Basically a DC motor will produce a voltage that acts against the supply (battery) voltage that is nearly proportional to the motor RPMs. A DC motor will produce its greatest torque at nearly zero RPM and then successively less power as RPMs increase. There is less and less net voltage left over to actually drive a propeller as the Back EMF voltage increases. Maybe this causes the sharp loss of efficiency, but Back EMF is is usually seen as a gradual flattening of the thrust versus power applied curve. (Or the down-sloping efficiency curve above)
The more likely reason may just be your Electronic Speed Controller (ESC), or if you are using batteries to power the tests, then a Battery Management System (BMS). These electronic devices may be limiting the current drawn from the batteries at the highest PWM settings. You already know that the ESC does not turn on until it sees nearly 1200 microseconds of pulse width on its signal input. If the motor is turning excessively fast then the ESC may not be able to switch phases efficiently, correctly sense the voltage rise on the other phases, etc.
I have an annoying "smart" ESC on a solar-powered plane project which tries to determine how many lithium cells are in its (assumed) battery pack when it first powers up and then won't drive the motor if the input voltage is below 2.x volts per cell . Well, I don't have a battery pack - I have solar cells that don't have to be protected from over-current or under-voltage. But the ESC faithfully denies my PWM requests for more throttle if the panel voltage drops to some lower voltage. After that, even if the panel voltage increases again to what it thinks would be 4.0 volts per cell, it won't allow motor power above the equivalent of about half throttle because it thinks it has batteries that it needs to protect. So I have to 'trick' the ESC by never letting my controller give a throttle command high enough to cause the ESC input voltage to go too low. The ESCs that I have can be programmed for many different things, but if the ESC is designed to handle different battery voltages (2S, 3S, 4S, ...) then this battery protection 'feature' seems like it cannot be disabled. And I don't have enough free time on my hands that I want to go off and design my own ESC.

I contend that if you used a high enough gear ratio on your cyclorotor (let the motor spin faster) you would eventually see similar behavior to the thrust "brick wall".
The problem at the low end of the RPM range is that the motor can draw too much current from each phase as it is driven. Rotor locked and full pulse width, your 16 volts, F90 1300KV TMotor could (until it melted!) pull over 200 Amps of current. If you run the motor too slowly then too much power is lost as heat (current squared multiplied by resistance) and you just have a slow spinning, very hot motor. :-/
I also contend that if you use a larger propeller (6x4 or 7x4 two bladed prop) on the motor then you would both see higher baseline efficiency numbers (see my post above on thrust versus power efficiency where flow velocity is considered), and those sharp drops in power efficiency would occur at higher values of thrust (or maybe not at all).
And as one of my old professors used to say "Go figure out the optimum solution, or at least a sufficient one. That is why engineers get paid the big bucks!" ;-)

Visualizing flow and vortices produced by the cyclorotor would be useful if it could be done. I'm not sure what the state of the art currently is, but here is an older YouTube video on Particle Image Velocimetry (

). Presumably for the cyclorotor you would have the laser sheet running parallel with the flow direction and you would be interested in a series of images just as a rotor passes the bottom position. Here a two-bladed cyclorotor would allow best visualization. The stationary camera would look from the side, perpendicular to the laser sheet, and you would take a series of images as the blade passes through the field of view and then animate them into a short video. You might be able to get really clever about releasing successive vertical lines of smoke puffs or particles upwind of the test area to form a grid as the blade passes. Schlieren imaging probably wouldn't illustrate the vortices very well.

I just now realize that I assumed, but didn't say it explicitly except in the diagram, that the direction of rotation of the cyclorotor and the airflow are such that the airflow and the rotor blades at the bottom are going the same direction for the ground effect test. I think that is the more interesting case. The cyclorotor blades are only in proximity to the ground for a short time and then they are up in almost unaffected air the rest of the time.

 

[OCPatch]

This is a follow-on to the message above:
If my calculations are correct then the 'brick wall' drop in efficiency seen in both the IGE and OGE propeller cases occur at approximately 156 watts. It seems like there is a power limit or current limit in your power supply.
Oddly enough, if my calculations are correct, the cyclorotor cases seem to be able to pull over 250 watts at their maximums.