I've recently gotten interested in the design of variable pitch propellers/rotors for the use in VTOL tilt-rotor UAVS. I have done a good amount of research with little fruition besides a few somewhat related pdfs. I was wondering if someone could direct me to a good source of information.I am specifically interested in designing the actual rotor airfoil itself and implementing it on a custom variable pitch motor unit. For now I would like the platform to just be a tiltrotor tricopter, with the focus mainly being propeller performance rather than the platform itself.I would prefer to use matlab if possible for any programming and simulations. I have access to a good machine shop and lab at the university I attend (Drexel). I am just looking for some sort of reliable documentation or consultation to direct me in the right direction.
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I guess I haven't really stated my intentions clearly, and may have been misleading. I am not concerned with controllability beyond stability, which could be either RPM or pitch control (after reading that VP thread, I'm leaning towards pitch control :). Also, I am really not all that concerned with efficiency for efficiency's sake. My main goal is to bring about completely new capabilities for UAVs not possible without variable pitch rotors. Namely, to allow hovering with a sizeable payload, as well as fast forward flight.
With a fixed pitch tilt rotor, a plane with X payload at hover could only reach X mph in forward flight, and the two are related to each other by pitch (at least partially). A fast, high pitch plane that performs well at high advance ratios would hover very inefficiently, meaning a lower payload. Furthermore, even a plane with large or wide, efficient wings cannot fly very fast due to drag.
A plane with large multi-blade variable pitch rotors could attain an efficient, high-payload hover, as well as support very low drag, swept wing, high velocity flight. Essentially, it could have not mediocre performance across all velocities, but excellent performance at both high and low velocities (haven't thought much about mid where the wing loading is too high).
Uriah George > Joshua GeatingMay 21, 2014 at 4:37am
Hi, I am working on the exact same problem. I am currently in the design phase of an airframe which I will use as a test-bed to develop several VTOL variations for experimentation. The first will employ a separate lift and thrust (SLT) design. It solves the problem you have stated, which is that propellers which are efficient during VTOL are inefficient during forward flight. SLT designs employ fixed lift rotors which are optimized for VTOL, usually in quad configuration because it has better longitudinal stability, and typically with a single thrust rotor optimized for horizontal forward flight.
Brad is right in many ways, and many engineers choose SLT because of the added complexity and points of failure of variable pitch and tilt rotors. However, nothing is impossible, and with proper design, integration and testing I think tilt-rotors with variable pitch props are the best solution for the application of VTOL aircraft with extended range and efficiency.
There are a number of reasons. First, the added weight of four fixed lift rotors that do not contribute to thrust during forward flight makes the aircraft less efficient. Not only because they are not generating thrust, but because when you tilt the rotors in-front and behind the wing during forward flight there is significant improvement in wing lift due to the higher airspeed generated by the props. I am going to set up an experiment to measure just how much lift can be generated on the wings with this configuration.
Second, if you are applying differential speed for yaw control during VTOL, the pitch and roll control are degraded somewhat. Especially if you are at full throttle already, the only way to get a speed differential is to reduce the speed, leading to negative control instead of positive control. With a tilt-rotor design it is possible to have excellent yaw control using differential tilt angle of the rotors. Not only is the yaw control more reactive and stable, but it does not degrade pitch and roll at all. Since the tilt servos are already present, differential tilt adds no physical complexity.
All that said, designing and integrating an assembly which takes care of rotor tilt and prop pitch is no easy task. If you don't do it correctly you end of with a lot of vibration in the best case.
I would be interested in organizing a group effort to develop such a system if people are interested. It would have significant benefits to the community and if done properly will add little weight and minimum complexity. I would go heavy on the durability of the design and the added weight will be made up for by the efficiency of the system.
I am apprehensively interested. It's a big project, and a lot to commit to. Can you either reply or PM with a few things like your background experience, location, facilities/capabilities, education/work, age, etc.?
You pose good questions and some of the data you ask about is measurable with 'amateur' techniques.
Regarding tip speed, this simple calculator might be of use.. Culver Propellors.
A text oriented toward prop dynamics.. Propellor Dynamics where the reviewer states..
Normally the shape and airfoil of propeller tips is not of much interest, as they don't much affect performance. However, in some racing classes the propeller tip speeds approach that of sound. This introduces a whole new realm of aerodynamics and the profile of the airfoil section becomes very critical indeed.
The first time I saw an Osprey 'in the wild' it was amazing. It is an aircraft that cannot be ignored.
I have always wondered how much the material in a prop distorts as the speed/load increases.
With the distortion, in your case a built in twist, at what point(s) does the 'bite' of the prop change?
There must be sections of the rotating disc that exhibit more thrust than others.
At least now we have some moderately decent model software.. if you can get a function that acurately describes the data.
I have a PhD friend, theoretical physics - surface modelling of particle scattering, that is always tweaking his software model trying to fit it to the collected data.
Measuring how your proposed blade variables affect performance will be the difficult part.
My PhD friend says I need to work in the experimental side of things.
I've seen that video several times, but haven't spent exorbitant amount of time looking it over. I was under the impression that the intent was more so for increased controllability than for efficiency. Their propellers appear to just be tapered flat rotors I've seen several times on hobbyking and similar hobby superstores. I am interested in propellers that actually have twist - could still possibly go in reverse (very inefficiently) if they needed to - but are primarily designed to operate over a wide velocity range. I would also like a much larger diameter, and possibly three blades instead of two. I can't help but notice how the osprey's rotors/propellers are much larger than a typical propeller, but smaller than a helicopter's rotors.
Does anyone know if R/C propellers come anywhere near supersonic at the tips? What is the primary factor limiting diameters? I would be interested in getting a motor like this one, http://www.hobbyking.com/hobbyking/store/__17924__NX_4008_620kv_Bru..., and increasing the diameter greatly for better efficiency.
Thanks for the reply. This is my first post on DIY Drones and I'm really glad to see there are professional, knowledgeable responses.
Although I have little experience in aeronautical engineering as a mechanical engineer (to my dismay), I can definitely believe you about the overhead associated with tilt-rotors, but I can't help but wonder if micro-UAVs somewhat inherently circumvent that problem because air frame loading is so low. I imagine even some servos could function entirely as the tilt mechanism when the entire aircraft may weigh 2-4 kg or less.
I am still confused about correct nomenclature. When I said variable pitch, I intended to imply changing the pitch rapidly and independently using servos for each propeller (although they would have independent motors as well).
Thanks for the resources! I'll be looking them over quite thoroughly.
I am resolved to hold an admittedly minority opinion when I say that tilt-rotor designs are a solution looking for a problem. It seems so many aeronautical engineers are fixated on the notion that the main problem with the current embodiment of the helicopter is that it doesn't fly fast enough. Unfortunately, all the added complexity and non-fault-tolerant mechanical points of failure added by the tilt-anything designs exacerbate what I see as the primary issue with VTOL flight - expensive operating overhead.
That said, I do agree it is patently true that the ultimate performance of any propeller requires variable pitch to accommodate different inflow velocities (advance ratios). To those that follow my random musings, note that this gentleman speaks of slower, symmetrical (both blades together) pitch changes on the order of 2 degrees per second or so, and therefore the propeller design would not be saddled with the pitch and inertial moment issues inherent in cyclic-pitch-control helicopters.
(hold on, I am going to answer the question. :-))
I know of no better resource on the web than Martin Hepperle's site, including his exemplary programs JavaFoil and JavaProp. All the tools are there, except there's not much focus on bridging the theoretical model gap between static and V/nD thrust. Heck, nobody seems to cover that much anyway, making me think there's an excellent PhD-candidate research paper project which awaits the intrepid academic experimenter. Anyway, here's the link:
If you're looking for an excellent technical foundation of the issues to get you started, this paper by Dr. Paul Pounds, now of Yale University, is a fairly complete treatise (see attached file).
Dr. Michael Selig of the University of Illinois and his team have done more actual investigation of low-Re propellers than most anyone. His papers and data can be had here:
Replies
I guess I haven't really stated my intentions clearly, and may have been misleading. I am not concerned with controllability beyond stability, which could be either RPM or pitch control (after reading that VP thread, I'm leaning towards pitch control :). Also, I am really not all that concerned with efficiency for efficiency's sake. My main goal is to bring about completely new capabilities for UAVs not possible without variable pitch rotors. Namely, to allow hovering with a sizeable payload, as well as fast forward flight.
With a fixed pitch tilt rotor, a plane with X payload at hover could only reach X mph in forward flight, and the two are related to each other by pitch (at least partially). A fast, high pitch plane that performs well at high advance ratios would hover very inefficiently, meaning a lower payload. Furthermore, even a plane with large or wide, efficient wings cannot fly very fast due to drag.
A plane with large multi-blade variable pitch rotors could attain an efficient, high-payload hover, as well as support very low drag, swept wing, high velocity flight. Essentially, it could have not mediocre performance across all velocities, but excellent performance at both high and low velocities (haven't thought much about mid where the wing loading is too high).
Hi, I am working on the exact same problem. I am currently in the design phase of an airframe which I will use as a test-bed to develop several VTOL variations for experimentation. The first will employ a separate lift and thrust (SLT) design. It solves the problem you have stated, which is that propellers which are efficient during VTOL are inefficient during forward flight. SLT designs employ fixed lift rotors which are optimized for VTOL, usually in quad configuration because it has better longitudinal stability, and typically with a single thrust rotor optimized for horizontal forward flight.
Brad is right in many ways, and many engineers choose SLT because of the added complexity and points of failure of variable pitch and tilt rotors. However, nothing is impossible, and with proper design, integration and testing I think tilt-rotors with variable pitch props are the best solution for the application of VTOL aircraft with extended range and efficiency.
There are a number of reasons. First, the added weight of four fixed lift rotors that do not contribute to thrust during forward flight makes the aircraft less efficient. Not only because they are not generating thrust, but because when you tilt the rotors in-front and behind the wing during forward flight there is significant improvement in wing lift due to the higher airspeed generated by the props. I am going to set up an experiment to measure just how much lift can be generated on the wings with this configuration.
Second, if you are applying differential speed for yaw control during VTOL, the pitch and roll control are degraded somewhat. Especially if you are at full throttle already, the only way to get a speed differential is to reduce the speed, leading to negative control instead of positive control. With a tilt-rotor design it is possible to have excellent yaw control using differential tilt angle of the rotors. Not only is the yaw control more reactive and stable, but it does not degrade pitch and roll at all. Since the tilt servos are already present, differential tilt adds no physical complexity.
All that said, designing and integrating an assembly which takes care of rotor tilt and prop pitch is no easy task. If you don't do it correctly you end of with a lot of vibration in the best case.
I would be interested in organizing a group effort to develop such a system if people are interested. It would have significant benefits to the community and if done properly will add little weight and minimum complexity. I would go heavy on the durability of the design and the added weight will be made up for by the efficiency of the system.
I would love to hear your thoughts!
Best regards,
Uriah George
Hey Uriah,
I am apprehensively interested. It's a big project, and a lot to commit to. Can you either reply or PM with a few things like your background experience, location, facilities/capabilities, education/work, age, etc.?
Thanks,
-Josh
Joshua,
Did you read William Premerlani's input on the discussion of VP from February?
LINK
It is not directly related to propellor voodoo but he has thought much about VP and quads.
You pose good questions and some of the data you ask about is measurable with 'amateur' techniques.
Regarding tip speed, this simple calculator might be of use.. Culver Propellors.
A text oriented toward prop dynamics.. Propellor Dynamics where the reviewer states..
The first time I saw an Osprey 'in the wild' it was amazing. It is an aircraft that cannot be ignored.
I have always wondered how much the material in a prop distorts as the speed/load increases.
With the distortion, in your case a built in twist, at what point(s) does the 'bite' of the prop change?
There must be sections of the rotating disc that exhibit more thrust than others.
At least now we have some moderately decent model software.. if you can get a function that acurately describes the data.
I have a PhD friend, theoretical physics - surface modelling of particle scattering, that is always tweaking his software model trying to fit it to the collected data.
Measuring how your proposed blade variables affect performance will be the difficult part.
My PhD friend says I need to work in the experimental side of things.
I've seen that video several times, but haven't spent exorbitant amount of time looking it over. I was under the impression that the intent was more so for increased controllability than for efficiency. Their propellers appear to just be tapered flat rotors I've seen several times on hobbyking and similar hobby superstores. I am interested in propellers that actually have twist - could still possibly go in reverse (very inefficiently) if they needed to - but are primarily designed to operate over a wide velocity range. I would also like a much larger diameter, and possibly three blades instead of two. I can't help but notice how the osprey's rotors/propellers are much larger than a typical propeller, but smaller than a helicopter's rotors.
Does anyone know if R/C propellers come anywhere near supersonic at the tips? What is the primary factor limiting diameters? I would be interested in getting a motor like this one, http://www.hobbyking.com/hobbyking/store/__17924__NX_4008_620kv_Bru..., and increasing the diameter greatly for better efficiency.
Are you familiar with the work shown in this video?
MIT ACL
I would hope your university connections allow some access to their information.
Although I have little experience in aeronautical engineering as a mechanical engineer (to my dismay), I can definitely believe you about the overhead associated with tilt-rotors, but I can't help but wonder if micro-UAVs somewhat inherently circumvent that problem because air frame loading is so low. I imagine even some servos could function entirely as the tilt mechanism when the entire aircraft may weigh 2-4 kg or less.
I am still confused about correct nomenclature. When I said variable pitch, I intended to imply changing the pitch rapidly and independently using servos for each propeller (although they would have independent motors as well).
Thanks for the resources! I'll be looking them over quite thoroughly.
I am resolved to hold an admittedly minority opinion when I say that tilt-rotor designs are a solution looking for a problem. It seems so many aeronautical engineers are fixated on the notion that the main problem with the current embodiment of the helicopter is that it doesn't fly fast enough. Unfortunately, all the added complexity and non-fault-tolerant mechanical points of failure added by the tilt-anything designs exacerbate what I see as the primary issue with VTOL flight - expensive operating overhead.
That said, I do agree it is patently true that the ultimate performance of any propeller requires variable pitch to accommodate different inflow velocities (advance ratios). To those that follow my random musings, note that this gentleman speaks of slower, symmetrical (both blades together) pitch changes on the order of 2 degrees per second or so, and therefore the propeller design would not be saddled with the pitch and inertial moment issues inherent in cyclic-pitch-control helicopters.
(hold on, I am going to answer the question. :-))
I know of no better resource on the web than Martin Hepperle's site, including his exemplary programs JavaFoil and JavaProp. All the tools are there, except there's not much focus on bridging the theoretical model gap between static and V/nD thrust. Heck, nobody seems to cover that much anyway, making me think there's an excellent PhD-candidate research paper project which awaits the intrepid academic experimenter. Anyway, here's the link:
http://www.mh-aerotools.de/airfoils/index.htm
If you're looking for an excellent technical foundation of the issues to get you started, this paper by Dr. Paul Pounds, now of Yale University, is a fairly complete treatise (see attached file).
Dr. Michael Selig of the University of Illinois and his team have done more actual investigation of low-Re propellers than most anyone. His papers and data can be had here:
http://www.ae.illinois.edu/m-selig/
Enjoy!
Pounds Modelling and Control of a Quad-Rotor Robot 33767.pdf