A large scale Variable Pitch Tilt Quadrotor implementation.
Here is a description of a Variable Pitch Blade Quadrotor build with some unique features. It is a large Quad, and can be quickly split into 2 halves, enabling easy packing into a carry case for simple transport. The ‘Upper’ half of the quad contains all the electronics, IMU, Autopilot, Payload control electronics, etc. The lower half carries the battery packs and the payload. The upper and lower halves slot together and are affixed by means of 4 captive spring release screws. The payload similarly mounts to the underside of the lower body half. The entire Quad can then be separated into an upper and lower body half, and payload.
Each motor has a standard ESC ( not any fancy hi-speed update type…) mounted beneath the motor. This keeps the 3phase motor leads very short, ensuring little electrical noise radiation from said leads.. Beneath each ESC is a small CPU module which receives a serial command from the Autopilot. This serial command contains the desired motor speed, the blade pitch angle, and the rotor tilt angle for Yaw control. This small CPU module generate the relevant PWM signal to the blade pitch servo, the rotor tilt servo, and to the ESC, while monitoring the motor RPM, ensuring constant motor speed, as commanded.
The upper half of the Quad has 2 Variable pitch, variable Tilt, same direction rotating rotors. The lower half has only two variable pitch same direction rotating rotors fitted. The motors are operated at commanded constant speed, and lift is varied by blade pitch, giving much faster control of individual blade lift for stabilization. This is a problem with conventional control by motor speed variation, as the rotor blades become larger in diameter. The inertial mass of the rotor increases, and it becomes increasingly more difficult to rapidly change rotor RPM, both to enable horizontal stabilization, and to control Yaw.
The Tilt rotors are both fitted to the upper half of the Quad, with the rotors spinning in the same direction. This ensures that when the rotors are tilted ( always in the same direction) to control Yaw, Gyroscopic Precession is equal and opposite across the two rotors, thereby canceling out. Gyroscopic Precession also begins to play a bigger role in larger diameter rotors, when the tilt rotor concept is employed.
The construction is almost complete. So far all simulations and math shows we should have around 2.5kg lift per rotor head, with an all up Quad weight of 5.5kg, inc 2 of 6cell, 6000mAH batteries, leaving around 2kg for payload. We should get around 15 to 18minutes flying time.
If it looks like it came out of the ALIGN Factory, it did not…I used parts from the Trex600 series chopper, the tail rotor pitch assy, etc, as well as tail boom, stays, and a few more bits and pieces…The composite parts were all modeled in 3D CAD, and molds made for them. All the custom Aluminium parts were likewise modeled and CNC machined.
Basic Specs:
Dimension from blade tip to blade tip – 2100mm
All up weight – 5.5KG, inc batteries, 3.4kg exc batteries.
Blades – Trex 450 style.
Variable pitch on all 4 blades.
Variable Rotor Tilt on 2 blades.
Max payload mass – 2Kg.
Estimated flight time with 1.5kg payload – 18minutes
Autopilot – NamPilot
I hope to fly it real soon now….!!
Just running out of space for all the aircraft here!, Guppy, Hornbill, Kiwit, all the 60 trainers, ……Need some sky-hooks!
Regards
Joe
‘The NamPilot’
Two unfinished body Halves:
Cutting out lightening Holes on the CNC:
2 halves with IMU:
Lightened Central Hub Assembly: (with unpainted autopilot cover)
Lightened Central Hub Assembly:
CNC Machining of Motor Arms:
The Pitch Tilt Arm:
The Variable Pitch Motor Assembly:
The Variable Pitch and Rotor Tilt Assembly:
Partial frames fitted Together: (floor squares are 300mm X 300mm)
Frames in Transport Mode:
Comments
Hello Elison,
This is the biggest problem with this concept! Brushless motor designers are not yet taking this mode of operation into account, and so most motor KV ratings are way to high. My motors are 355 KV, which is at least twice what I would like. They are Hacker A40-14 V2. We will see what happens when we start flying! I am doing all the RPM versus blade pitch versus Thrust measurements at the moment, to verify my simulations for all the control loops, and the results will show if the motors are going to work....The lower KV motors, 120 or so, are all very big and very heavy - good for a monster Quad..Adding gearing just adds weight and extra bits, the whole Catch 22 scenario.
Alex,
It is very difficult to keep the weight under control with a central motive power source. Drive belts just dont cut it - teeth slip, etc, and torque rods have there own problems - the arms of the quad need to be quite stiff to ensure the torque arms do not bend much under load, as they then start to suffer severe whiplash. The required gearing to drive the rotors all add weight, etc. The only advantage for this consideration is if one can use a gas engine to overcome all the losses and get longer flying times.
Interleaving the blades in rotation is not a good idea in this instance Alex. Unlike counter rotating props on the same axial (co-axial props), the turbulance seen by a blade half as it enters the other blades counter rotating airscrew causes very severe blade flapping, since the tip of that blade suddenly experiences accelerated airspeed, in either a forward or reverse direction, and suffers sudden increase or loss of lift, and flaps badly, with large vibrations and premature blade fatigue. Dont do it! You only gain effeciency if the one blade is fully and constantly in the other blades counter rotating airscrew.
With regard to the method of Yaw, pitch and roll control, please read my reply to Stuart - no differential blade pitch control needed.
This quad is also inended for Radio tag tracking of animals in the bush. The concept is that when the ground rangers wish to get to a tagged animal, they would send up the fixed wing UAV, flying sectors over the park, to find the general location of the animal, then proceed by 4X4 vehicle to that area. The quad will be fixed to a special roof rack on the vehicle, and when they are close to the area, the quad will lift off, climb to a few hundred meters and 'listen' for the tag again, get its current GPS position, and land back on the roof rack ( guided in by and infra red beam). The rangers can then get closer and repeat the process untill they can detect the animal by means of a handheld receiver and antenna and proceed futrher till they reach the animal.. The Quad will also carry a camera and video transmitter to give a view of the area to aid the possiblity of detection.
Watch This Space.....
I see you're driving the 450 blades without gear reduction. May I ask what kV motors you're using?
Stuart,
All four rotors have variable pitch control. The upper half of the quad has the North/South rotors which also have the rotor tilt mechanism. The East/West rotors are only variable pitch.
The North and South rotors rotate clockwise, the East/West, anti-clock. The North/South rotors control the airframe horizontal pitch, by applying different blade pitch angles to North compared to South Rotor. Likewise, the East/West rotors control the horizontal Roll. The North /South rotors when tilted control the airframe Yaw, as follows- To Yaw such that the airframe rotates clockwise around its vertical axis, the North rotor is tilted to the East, while the Southern rotor is tilted to the west, and vise-versa. When hovering, a 2degree rotor tilt on this Quad will give a yaw rate of around 80deg per second, so not much tilt is required. A huge advantage on this type of control is the fact that the control loops are well seperated and do not overlap much in cross coupled effects, unlike the high degere of coupling in a conventional control by varying only motor speeds. In such a system, motor RPM control yaw, and roll and pitch and lift, so all these have to be balanced against each other , each with PID control loops fighting for domination....
Cheers
joe
Thanks, but sorry I didn't explain myself. I understand the mechanism, but was struggling to understand the flight method. I presume you'll tilt the rotors, and the non-tilting rotors on the nose and tail will control pitch, while the tilted rotors will be yaw/roll (if not 90 degrees tilt). Is that right? Thanks
@ Robert - The 450 blades fit with about a 1.5mm gap - I made slotted Teflon Spacers to snug the blades up nicely.
@Steve - I hope so to! We have succeeded with many other planes ( see my other posts) but this is a first on a Quad! However, google MIT - Variable Pitch Quadrotor and watch the video....
@Stuart _ if you look at the 3rd and forth pictures from the end of my blog - the one 4th from the end shows a Variable pitch only motor mount - the Quad Tube ( that connects motor mount to frame) is shown connecting directly to the motor mount, ie a rigid connection. The rotor tilt arm has a bearing that fits inside the end of the Quad tube, so can pivot around the tubes axis. The larger servo you see then controls that pivot motion. Does that help?
I'm very interested to see this fly!
I've thought of building a variable pitch quad as a means to improving the actuation bandwidth of the rotor thrust, as you mentioned. I had assumed I'd drive the props via driveshafts from a motor or motors in the center of the airframe in order to reduce the rotational inertia of the airframe. If a single central motor and drive gear were used, it would allow interleaving rotor disks, with possible efficiency benefits. The cost versus this arrangement would be increased complexity and fragility.
Do you intend to use differential prop pitch for yaw control as well as the tilt control?
What is the intended mission for the quad? Is it an anti-poaching surveillance platform like the fixed wing aircraft you posted about a while back? What unique capabilities does it provide for that mission?
All in all, it looks very well engineered and executed!
Hope to see it flying. I have seen many attempt this and not get too far.
That looks beautiful, but I don't understand how the tilt works. Can you help? Thanks, stu
Simply said, two thumbs up!
-
1
-
2
-
3
-
4
-
5
of 5 Next