Unclematt
Well-Known Member
http://aeroguy.snu.ac.kr/cyclocopter/paper/AHS_forum60.pdf
http://serve.me.nus.edu.sg/cyclocopter/
http://www7.miami.edu/ftp/acfdlab/projects/AIAA_2005_1260_Slide.pdf
http://www7.miami.edu/ftp/acfdlab/publications/AIAA-2011-941-884.pdf
http://www.d-dalus.com/
http://www.popsci.com/science/article/2012-01/how-d-dalus-flies-nothing-else
CycloidalPropellers for Aviation and a Concept to Optimize Them
(forgive the long post)
Above are links to papers discussing cycloidal propellers for aviation use, co-flow jet designs, and info on D-Dalus. For those of you unfamiliar with these cycloidal rotors, they incorporate blades that rotate axially around a center of rotation, not radially as propellers and helicopter rotors do, with the blades oriented horizontally to the ground. The angle of attack of the rotor blades is changed on opposite sides of the rotor by a cycloidal actuation mechanism to create lift/thrust,in any desired direction 360 degrees around the axis of the rotor. The whole lengths of these blades are exposed to airstream velocities that are ordinarily exposed only to the outer parts of the blades of most rotor systems. They are also much quieter than typical rotors, and have demonstrated the potential to create greater thrust per horsepower than typical rotors or propellers. The idea is not new, and has been around for 80 years or more in the form of cyclogyrosand cyclocopters, but now new attention is being paid to the concept.
However, they do have some challenges to overcome or designaround. Depending on how you orient and place them on an airframe, they made need additional rotors to offset gyroscopic effects. Each blade must move through an arc of 30 to 45 degrees or more of cyclic with each rotation of therotor. The G-forces created at the periphery of the rotors is quite high, and the whole blade length is subject to that force. Bearing loads at blade pivot points are subsequently quite high as well. D-Dalus is currently working on those challenges, and claims to have solved the bearing issue, though they haveyet to present a stable flying craft to the public. I like their idea of usingcarbon graphite discs to construct the rotor and mount the blades on, becauseit produces less drag than designs that use tubular spars to connect blades to the main hub, and would provide an area to place air ducting in the discs.
On that segway, one of the reoccurring themes I noticed in my research of this concept was the need to reduce drag (of course) of the rotor to increase efficiency even further. The unique circular flight path ofthe horizontal blades in this rotor configuration allow the blades to avoid stalling at angles of attack where airfoils in normal linear airstreams would stall. And though this helps itcreate lift efficiently, it also increases drag dramatically. The tubular spars most researchers used to mount the blades to the hub also caused a great deal of drag, which I would also like to avoid buy using carbon graphite discs as D-Dalus has done.
After reading threads on this site, and many papers online,about Goldschmieds theories and ways to reduce drag, I had an idea to optimize the cycloidal propeller. Leading edge blowing and trailing edge sucking on anairfoil, or otherwise known as a Co-FlowJet airfoil, offered a way to not only reduce drag, but to actually create negativedrag (thrust) to an airfoil. I also learned that further gains to lift could be realized from installing tabs inthe blowing slots to create discrete jets for the Co-Flow airfoil. The papers above show it adds even more gain in lift to those already made by the Co-FlowJet system, indeed as high as 150% increase to identical non-circulationairfoils. But none of my research shows that anyone has applied these methodsto Cycloidal Rotor airfoils.
Of course, to make this type of discrete co-flow jet cycloidal propeller work will require some ducting of high pressure air through the rotor hub and discs, and perhaps valving the air to the blowing/sucking slots on alternate sides of the rotor blades ( And all the negative effects on efficiency that goalong with that kind of ducting.) I am still formulating and analyzing this concept, so no drawings to show for now. But if I can pull it off, I am hoping to create a quiet rotor for VTOL/forward flight use that can lift more poundsper horsepower than current rotors or cycloidal propellers.
However, I have some questions I need some input on:
The airfoils in thecycloidal propeller must produce lift while in an “upright” and an “inverted”position, as they rotate. Subsequently the airfoils have zero camber and must be symmetrical in cross section. NACA 0015 or NACA 0012 are the favorite airfoilsused by experimenters posted above. To incorporate the required air passages for both sides of the airfoil, I was planning on using a thicker blade, perhap sNACA 0020 to NACA 0025. These would have 2 blowing slots at the leading edge, one on each side, and 2 sucking slots ahead of the trailing edge, again one oneach side. Should air valving not be required to alternate circulation from oneside of the blade to the other, the blades could be thinner in cross sectiondue to reduced ducting requirements.
Now then, the question : is it necessary or advantageous toblow the LE slots and suck the TE slots atall times, or only when the LE & TE slots are oriented "on top"?
In other words, do I need to control air flow so that it only feeds the “upper” LE & TE slots, or would it be better to flow through all slots at all times? If this were a normal airfoil application, of course the answer would be obvious: that only the “upper” slots would need to beactive as the blades rotate into proper position to create lift/thrust.
But recall this will be occurring at very high angles of attack. Even without my idea to incorporate this co-flow jet circulation system, normal cycloidal propeller blades can achieve angles of attack as high as 30 degrees or more without stalling! With this concept I hope to take that angle of attack to the highest practicable point, with zero drag from the airfoils, and hopefully thrust. Indeed it would be ideal if I could feed enough air mass through my Discrete Co-Flow-Jet Cycloidal Propeller to provide for powered rotation without any additional mechanical means. (Though powering the rotor iscurrently planned).
And if the slots are active on the “bottom” of the airfoil in lift areas of rotation, that “lower” airfoil surface will be facing into theairstream at a high angle of attack. Would the circulation flow from the “lower”LE slot simply be blown away/dissipated by the force of the airstream under those conditions, or would the blown air merely accelerate and add flow energyto the incoming airstream? Would having the circulation active on both sides ofthe blade effect lift in general at high AoA? Would it help, hurt, or have no effect? Also, would the lower TE sucking slot be overwhelmed by the high velocity airstream and serve to “catch” the airflow like a ledge, reducingefficiency in the process?
Considering the answers to all this, the questionis: full time air flow through all circulation slots, or only when slots are“on top?
In a separate scenario centered on the alternate circulationscheme, I also need to understand the dynamics played by the slots on the“lower” surface of the airfoil when no circulation is occurring under high AoA. Do the inactive lower slots cause unexpected effects to the airstream such as lift reduction/drag being produced when not actively blowing/sucking? Especially the TE sucking slot, and the possibilityit could cause extra drag under those circumstances? Is full time circulationneeded from all slots to avoid any of those problems, if they do exist?
I am in planning stages now to build a crude air tunnel anddo some testing to find some answers to these questions, but wanted to get asmuch input from knowledgeable sources as possible.
And please, I know this is a kooky idea, and has a lot of challenges, but only looking forconstructive input related to the questions posed. And of course, reading the posted referencematerials prior to offering an opinion would be appreciated too. Thanks.
http://serve.me.nus.edu.sg/cyclocopter/
http://www7.miami.edu/ftp/acfdlab/projects/AIAA_2005_1260_Slide.pdf
http://www7.miami.edu/ftp/acfdlab/publications/AIAA-2011-941-884.pdf
http://www.d-dalus.com/
http://www.popsci.com/science/article/2012-01/how-d-dalus-flies-nothing-else
CycloidalPropellers for Aviation and a Concept to Optimize Them
(forgive the long post)
Above are links to papers discussing cycloidal propellers for aviation use, co-flow jet designs, and info on D-Dalus. For those of you unfamiliar with these cycloidal rotors, they incorporate blades that rotate axially around a center of rotation, not radially as propellers and helicopter rotors do, with the blades oriented horizontally to the ground. The angle of attack of the rotor blades is changed on opposite sides of the rotor by a cycloidal actuation mechanism to create lift/thrust,in any desired direction 360 degrees around the axis of the rotor. The whole lengths of these blades are exposed to airstream velocities that are ordinarily exposed only to the outer parts of the blades of most rotor systems. They are also much quieter than typical rotors, and have demonstrated the potential to create greater thrust per horsepower than typical rotors or propellers. The idea is not new, and has been around for 80 years or more in the form of cyclogyrosand cyclocopters, but now new attention is being paid to the concept.
However, they do have some challenges to overcome or designaround. Depending on how you orient and place them on an airframe, they made need additional rotors to offset gyroscopic effects. Each blade must move through an arc of 30 to 45 degrees or more of cyclic with each rotation of therotor. The G-forces created at the periphery of the rotors is quite high, and the whole blade length is subject to that force. Bearing loads at blade pivot points are subsequently quite high as well. D-Dalus is currently working on those challenges, and claims to have solved the bearing issue, though they haveyet to present a stable flying craft to the public. I like their idea of usingcarbon graphite discs to construct the rotor and mount the blades on, becauseit produces less drag than designs that use tubular spars to connect blades to the main hub, and would provide an area to place air ducting in the discs.
On that segway, one of the reoccurring themes I noticed in my research of this concept was the need to reduce drag (of course) of the rotor to increase efficiency even further. The unique circular flight path ofthe horizontal blades in this rotor configuration allow the blades to avoid stalling at angles of attack where airfoils in normal linear airstreams would stall. And though this helps itcreate lift efficiently, it also increases drag dramatically. The tubular spars most researchers used to mount the blades to the hub also caused a great deal of drag, which I would also like to avoid buy using carbon graphite discs as D-Dalus has done.
After reading threads on this site, and many papers online,about Goldschmieds theories and ways to reduce drag, I had an idea to optimize the cycloidal propeller. Leading edge blowing and trailing edge sucking on anairfoil, or otherwise known as a Co-FlowJet airfoil, offered a way to not only reduce drag, but to actually create negativedrag (thrust) to an airfoil. I also learned that further gains to lift could be realized from installing tabs inthe blowing slots to create discrete jets for the Co-Flow airfoil. The papers above show it adds even more gain in lift to those already made by the Co-FlowJet system, indeed as high as 150% increase to identical non-circulationairfoils. But none of my research shows that anyone has applied these methodsto Cycloidal Rotor airfoils.
Of course, to make this type of discrete co-flow jet cycloidal propeller work will require some ducting of high pressure air through the rotor hub and discs, and perhaps valving the air to the blowing/sucking slots on alternate sides of the rotor blades ( And all the negative effects on efficiency that goalong with that kind of ducting.) I am still formulating and analyzing this concept, so no drawings to show for now. But if I can pull it off, I am hoping to create a quiet rotor for VTOL/forward flight use that can lift more poundsper horsepower than current rotors or cycloidal propellers.
However, I have some questions I need some input on:
The airfoils in thecycloidal propeller must produce lift while in an “upright” and an “inverted”position, as they rotate. Subsequently the airfoils have zero camber and must be symmetrical in cross section. NACA 0015 or NACA 0012 are the favorite airfoilsused by experimenters posted above. To incorporate the required air passages for both sides of the airfoil, I was planning on using a thicker blade, perhap sNACA 0020 to NACA 0025. These would have 2 blowing slots at the leading edge, one on each side, and 2 sucking slots ahead of the trailing edge, again one oneach side. Should air valving not be required to alternate circulation from oneside of the blade to the other, the blades could be thinner in cross sectiondue to reduced ducting requirements.
Now then, the question : is it necessary or advantageous toblow the LE slots and suck the TE slots atall times, or only when the LE & TE slots are oriented "on top"?
In other words, do I need to control air flow so that it only feeds the “upper” LE & TE slots, or would it be better to flow through all slots at all times? If this were a normal airfoil application, of course the answer would be obvious: that only the “upper” slots would need to beactive as the blades rotate into proper position to create lift/thrust.
But recall this will be occurring at very high angles of attack. Even without my idea to incorporate this co-flow jet circulation system, normal cycloidal propeller blades can achieve angles of attack as high as 30 degrees or more without stalling! With this concept I hope to take that angle of attack to the highest practicable point, with zero drag from the airfoils, and hopefully thrust. Indeed it would be ideal if I could feed enough air mass through my Discrete Co-Flow-Jet Cycloidal Propeller to provide for powered rotation without any additional mechanical means. (Though powering the rotor iscurrently planned).
And if the slots are active on the “bottom” of the airfoil in lift areas of rotation, that “lower” airfoil surface will be facing into theairstream at a high angle of attack. Would the circulation flow from the “lower”LE slot simply be blown away/dissipated by the force of the airstream under those conditions, or would the blown air merely accelerate and add flow energyto the incoming airstream? Would having the circulation active on both sides ofthe blade effect lift in general at high AoA? Would it help, hurt, or have no effect? Also, would the lower TE sucking slot be overwhelmed by the high velocity airstream and serve to “catch” the airflow like a ledge, reducingefficiency in the process?
Considering the answers to all this, the questionis: full time air flow through all circulation slots, or only when slots are“on top?
In a separate scenario centered on the alternate circulationscheme, I also need to understand the dynamics played by the slots on the“lower” surface of the airfoil when no circulation is occurring under high AoA. Do the inactive lower slots cause unexpected effects to the airstream such as lift reduction/drag being produced when not actively blowing/sucking? Especially the TE sucking slot, and the possibilityit could cause extra drag under those circumstances? Is full time circulationneeded from all slots to avoid any of those problems, if they do exist?
I am in planning stages now to build a crude air tunnel anddo some testing to find some answers to these questions, but wanted to get asmuch input from knowledgeable sources as possible.
And please, I know this is a kooky idea, and has a lot of challenges, but only looking forconstructive input related to the questions posed. And of course, reading the posted referencematerials prior to offering an opinion would be appreciated too. Thanks.
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