The Picture above is of my FlameWheel 450 with one added propeller motor Unit. It is for testing the feasibility and potential value of providing 4 opposed motor propeller units for providing position control instead of copter pitch and tilt. For testing purposes the horizontal motor / ESC is controlled manually from a separate receiver channel controlled by an analog "dial" on the transmitter.
Oliver Seeler and I are both pursuing the possibility of providing a more stable multicopter platform by separating the stability control of the copter from tilt and pitch based position control. Oliver is using a Hex Flamewheel with a setup very similar to the above at the moment.
This Blog is an outgrowth of a Forum item started by Oliver under "Aircraft platforms" entitled "The Witch Gets a Broom".
First test flights are complete on both of these copters and some significant data has been learned.
On Oliver's Hex considerable thrust was required to move the copter horizontally even at a slow pace. On my copter a very small horizontal thrust (< 1/3 throttle) moved the copter smoothly at a slow walking pace and my Quad's motor thrust is tiny in comparison with Oliver's Hex.
Oliver's Hexacopter's center of gravity is considerably lower than mine due to tall landing gear, a camera mount and GoPro camera but his horizontal motor and prop unit are mounted in the same location as mine. This means that the horizontal props center of thrust is considerably further above Olvier's Hexacopters CG than is true for my Quad.
From this result we have concluded that it is very important to have the horizontal thrust line aligned closely to the copters center of gravity. Failure to do so induces tilt which the stability mode of the APM counters by applying opposing vertical thrust, doing a surprisingly good job of cancelling out the horizontal thrust. My copter as seen above does not have this problem and requires very little power to smoothly produce horizontal flight.
Also, Oliver's Hex has the prop mounted in a pusher mode and on my Quad it is mounted in tractor (puller) mode. It appears either mode can be made to work satisfactorily. I was concerned that mine in tractor mode would have the horizontal prop wash interfering with the copters downward prop wash and might induce instability. At the power levels tried so far, no negative effect is observable and transition to and from horizontal flight is very smooth and without tilt.
My conclusion from the results achieved so far is that using a horizontal propeller for position control actually does make more stable flight possible with little or no pitch or roll of the airframe. I will continue to test in this configuration at higher power settings to insure stability is not significantly degraded.
But I am also beginning work on a setup that will place prop motor units between each of a Quads 4 vertical thrust rotors.
The intention of this configuration 4 x 4 is that the 4 horizontal motors will completely take over horizontal position control.
The existing APM programming and the Vertical thrust motors will be responsible for stabilization, altitude control and the yaw or horizontal orientation of the air frame. This way only balanced symmetrical vertical thrust on the air frame is required to adjust altitude and yaw and the only asymmetrical thrust is in direct response to outside forces trying to pitch or tilt the air frame. Basically it's entire vertical thrust resources are used keeping it level and there is no purposely induced tilt or pitch as is conventionally necessary for position control.
Control for the 4 x 4 unit will necessarily be more involved as it will be desirable at a minimum to produce differential control of all 4 props using a stick. This means that zero thrust of all 4 motors would be at "stick center" position and pushing in any direction would produce differential activation on one or 2 motors in that direction producing a horizontal displacement similar to what is done now with a quad copter using the APM.
This is not achievable from a transmitter directly and requires at least translation of stick center to = zero throttle on all four motors.
My initial solution to this will to be to either use an auxiliary (in addition to the APM) Arduino processor to take normal stick input servo data and translate it to the controls for the 4 motors or I may attempt to simply incorporate the control into the APM if my programming capabilities and available resources prove adequate to the task.
The hardware part is easy, the programming part, less so.
Oliver and I very much welcome and would like to solicit your comments, thoughts and contributions on this project, and if somebody with more APM programming experience than I would like to jump in there, please feel free.
Keep in mind, the silly thing in the picture actually works and there is no perceptible air frame tilt or pitch when you use the horizontal motor to move it.
This is a new kind of aircraft which so far appears to be feasible and also that it might have some really significant advantages and capabilities.
Current Design Above.
Comments
Hi Antonie, I looked at those little donkey motors a while ago and probably a good choice for this application especially since they would be lightly loaded and often off. I may end up using them myself. I have used the bigger one on fixed wing planes and aside from being a bit noisy and ugly you sure can't beat the price.
I'm working on a horizontal motor mount that allows easy vertical adjustment of motor position.
This is important because change in load - camera - battery etc has a major affect on CG.
I don't beleive the horizontal offset of the CG matters at all for this, but the better you are aligned with the horizontal CG the less pitch / roll the horizontal motors introduce.
My single horizontal motor is 1/2" to 3/4" above the CG and jamming it from zero to full throttle produces a barely noticeable forward pitch that can be eliminated completely just by slow smooth application of the horizontal motor throttle.
My friend Oliver's Hex horizontal motor is probably 2 to 3 inches above his CG and it pitches a lot and the APM's self stabilization in stabilize mode effectively fights against the horizontal motor requiring far more horizontal thrust than you would expect to produce horizontal movement.
Basically alignment with the vertical CG is really important so an adjustable motor mounting system is probably the best answer.
I think either tractor (pull) or pusher will work and if you have a little bit of clearance between the vertical motors propwash I think it is feasible to mount the motors close to the center of the copter (assuming prop clearance).
Using a smaller 5" prop makes this easier and there are 3 bladed ones commonly available and Eflite has a nice 4 bladed 100mm one with a steep pitch that might work really well on this motor.
Megapirates is doing some interesting stuff and adapting ArduPilot to other Arduino boards is an excellent pursuit like the new $35.00 Hobbyking MegaPirate and Multiwii board, but I am going to stick with the APM for now and just try and first put in a manual proportional control for the horizontal motors that I can switch the right stick too. Let the APM handle the throttle, stabilize and yaw and manually push it around with the horizontal motors. After that I'll try and blend it with the APMs inertial and GPS position maintenance.
They still haven't put in inertial horizontal position control, but it is next on their to do list.
Here is a 2560 mega board setup with a IMU(FreeIMU). I disconnected the receiver.
And the tug motors I intend to use:HK Donkey's ST2004 1500Kv. Brilliant little beasts for the price.
Hi Gary, appologies for the late reply. I've had a good long thought about this process. Agreed - using APM's stabalize system is great. It works well. Using the megapirates code(essentially the same principle) will allow the use of more experimental equipment like a 2560 mega arduino board with a sensor shield and external 10DOF IMU.
All this will cost you less than half a APM2.5 board. It is not as neat as the APM's and the IMU might not be as refined, but it works for experimental application. This will also open the door for borrowing code from the Multiwii stable. He has done compatibility with a lot of IMU's and uses standard arduino libraries - unlike the APM and clones using custom libraries.
I have not looked at the code to see how easy it would be to isolate this, will do so within the week. Setting up the inertial responce of the tug motors is, for me, the first step. I would look at this when time is available. Here is the snag - I code C#(Enterprise architecture) and used C++ yonks ago. The closest I've come to this was doing a bit of code for a robotic instrument in C#, but what the hell. Might just have to port it to Netduino.....
I'm planning some test flights with my Bixler and APM2 tomorrow. I'll get stuck into the quadroquad build after that. I'll also need a testing jig where I can mount the copter in testing....have to see what I do about that.
I'll post some pics when the deed is done. Finding the CG with such a small vertical displacement might be tricky. Any suggestions?
Inexpensive small Turnigy motors, ESCs and props on order for prototype QuadroQuad conversion. (Same as on Turnigy Microcopter (HobbyKing)).
Should be suitable for my FlameWheel F450, Gaui 500X and KK X525.
A small additional thought, currently the APM programmers are at work implementing inertial position control such as is found in the DJI Naza and WooKong and I believe ARParrot multicopters and I will need to take this into account in my programming to transfer all horizontal position control to the 4 horizontal motors when necessary and this is likely to be a bit trickier than described above.
Hi Antonie,
It might be possible to program some ESCs to operate that way, but that is not my plan.
I am going to wire the ESCs into the channels 5 through 8 outputs of the APM and then program the APM to operate the 4 horizontal motors from center stick zero translating each stick center to end motion into full Servo range. This will result in center zero and then each motor proportionally controlled from zero to full by stick displacement in the appropriate direction.
One additional transmit channel is needed to switch from normal flight mode on the pitch and roll stick to horizontal motor control.
This should be fairly reasonable to implement for manual flight mode.
Integrating it for GPS controlled flight will require adding a (switched) library for auto mode and will require some "interesting" but fairly straight forward programming.
For the most part if the 4 horizontal motor layout is handled properly (mostly lining up horizontal thrust lines with CG of Copter) the existing APM Stabilize programming will do a fine job of holding the copter level and throttle and yaw can still be controlled with the left stick and they are symmetrical on copter motor pairs so do not tend to induce pitch or roll.
Not having to mess with the existing Stabilize programming makes the other tasks much easier.
I will probably actually use a 3 position switch so I can switch from normal to QuadroQuad or to Dynamic QuadroQuad (which will actually run all 4 horizontal motors continuously at some preset but low rate). This should provide for noticeably faster response as it is in constant dynamic tension rather than shifting from static to dynamic.
Yes that would use a bit more electricity and even static mode will require a bit more electricity due to the slightly increased weight and electrical use, but I envision that normally you would fly it to the "filming" location using normal flight control mode the switch to stabilized QuadroQuad mode for the actual filming sequence.
Hi Gary, I like this idea. How would you program the 4 "tug" motors on the pitch/roll channels? Can you calibrate the ESC's to run from mid stick/side. I'd love to see how you solve this. I have a oldish quad and a 4 small donkey motors with 8A esc nagging me to do this.
The principle of keeping the tug motors at idle run for tighter positioning appeals to me. The mixing of the commands to the motors will require a bit of coding considering wind speed and direction and the intended speed and and and.
I'll follow your progress. Keep up the good work.
Thank you Daniel, I am now designing a "QuadroQuad" to use as a test bed and will begin construction shortly.
So far my test results with the single horizontal motion version have been very encouraging and it is possible to achieve horizontal motion with no change in level on the copter.
It is necessary that the horizontal motors / props line of thrust pass directly through the center of gravity of the copter or it causes a small tilt up / or down when it accelerates.
If you can get that line up right, the APM does a remarkably good job of holding the copter level even under gusty conditions. Tested by aligning my single motor version into a gusty wind and then manually maintaining position with the horizontal motor.
The copter should be able to be driven either as a regular copter or as a hybrid to and from a photo / video opportunity then operated in hybrid mode.
And it may be that for maximum stability you may even want to have all 4 horizontal motors operating at some low level of thrust, just varying them to maintain or change position.
That would provide a forced dynamic that should permit very tight position control.
Hi Flying Monkey, That was my original plan too, but there is a significant problem.
The problem is that the 4 motors go from zero to full with zero in the center and there is no way on a single transmitter stick to do that so it would still involve a separate control board to interpret normal pitch and roll stick input to translate to 4 motors throttles each zeroed from the stick center.
Basically two full range servo inputs to 4 throttle outputs each with a 2 to 1 multiplier.
That would give you full range proportional control and there are servo to servo controller boards that could probably handle this, but it is a considerable extra effort which would not carry over to the final configuration.
The APM effectively does that (and a lot more) for the vertical motors.
Effectively, what you suggest is what I am doing now with the single added motor and assigning an additional analog channel to the motor.
I have taken multiple test flights and so far behavior is very smooth and slight dip forward when given immediate full throttle on horizontal motor simply confirms necessity to have the horizontal motors thrust line in line with the CG of the copter.
Ideally I would like to be using a copter with the vertical motor prop units a bit further apart so that the prop wash from the horizontal thrust motors could not interact with the down thrust from the vertical motors although the tests I have performed so far do not seem to produce visible negative effects.
Thanks for the input, I may still go that route, partly because it would simplify the tuning process later but also because I would like to work with some of the simple servo to servo controllers as well.
Hi Patrick, very much appreciate the response.
As I understand it on aircraft, contra rotating props were originally introduced on high speed fighters as a way of dealing with the problems that were encountered when the blade tips exceeded the speed of sound. The main reason faster fighters like the Spitfire had short straight ended props was in and attempt to keep blade tip speed to below the speed of sound which induced vibration and resulted in vibration fatigue prop tip failure.
But short propellers are intrinsically less efficient than larger diameter ones.
Contra rotating props were introduced to assist with this problem and also to counteract severe torque problems on the vertical stabilizer especially during takeoff of heavy powerful single engine military planes.
Increased efficiency was a bonus result of being able to redirect and make use of the tangential air stream produced by a single propeller. Under the circumstances where the prop diameter is fixed an additional counter rotating prop can produce up to 20 percent increased efficiency, but if the prop diameter can be increased without running into turbulence problems (usually associated with the speed of sound) simply making the prop larger will increase efficiency as well and modern aerodynamically efficient propellers designed for a specific speed range are considerably more efficient than the old symmetrical designs in any case. Basically as I understand it the largest practical diameter well designed prop on our multicopters is probably at least if not more efficient than the best designed contra rotating or (coaxial) setup.
One additional thought, coaxial / contra rotating plane props generally have a rear propeller with a very different design and pitch from the front propeller to take better advantage of the high speed and rotating air from the front one.
However, my concern wasn't related to a simple contra rotating setup, it was related to yours in particular where the two 15 degree offset motors would produce a net 30 percent offset of the prop wash streams and my consideration was that under those circumstances there might be induced interference, higher losses and potential turbulence at various speeds. The fact that this does not seem to be the case is excellent.
In fact if the losses are not very large (< 20 percent) for your setup I can see it has enormous potential. I for one would very much like to know the comparative efficiency versus an identical symmetrically coaxial X8 copter.
If I, as originally intended, simply broke the problem out to an auxillary processor, I might have been able to get by without decoupling for flying around manually in a simple stabilize mode as the stabilize is intrinsic to the APM and simple proportional control of the 4 motors with zero at stick center should have moved it around OK at low speeds and acceleration rates anyway, with the APM still maintaining stability, but horizontal position control being entirely under separate manual control.
I have decided for several practical reasons to not attempt that method and am going to use the APM for control from the start and am going to proceed as Randy suggests above.
That will decouple from the beginning. My plan is that a transmitter switch/channel will shift from normal roll and tilt flight and reassign the roll and pitch stick to proportionally controlling the 4 horizontal motors only while the Normal Stabilize and Rate functions and PIDs will continue to control roll and pitch to keep the copter level.
My hope anyway, greatly appreciate any input on execution or alternate ideas though.
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