16 July, 2018 at 18:25 #58306
Managed to meet @david at Flite Fest Ohio ’18 and check out his latest flying contraption. He literally cut, threw it together, and got it flying in the week before heading over.
More to come…..
LitterBug16 July, 2018 at 18:48 #58309RCExplorer – DavidKeymaster
It was great meeting you, Mr bug sir!
I was very impressed with your builds 🙂16 July, 2018 at 19:50 #58310
Here’s some RAW flight video taken at Flite Fest Ohio ’18:
@David, Awesome meeting you too kind sir and keep up the great work! Any plans for a Rocket-Bi?17 July, 2018 at 04:03 #58312jihleinParticipant
@david – I watched the flight video of the bi-copter, and it caught my eye for a couple of reasons. First it’s darn cool looking.
Second – That shaking it exhibits, I’ve seen that before. I was working with a tilt rotor tricopter a while back, and in my case it exhibited the same behavior, except it shook in forward flight, and was good in reverse flight. I changed the front motors spin direction, and then it flew smooth in forward flight, and had the shakes in reverse. It would be an interesting experiment to see if by changing the bicopters motor spin directions, if the shaking behavior also reversed.
I think what is happening is that there is a desirable roll-yaw coupling in one direction, and an undesirable roll-yaw coupling in the other. Think of it this way, to roll, one motor throttles up, the other down, creating a roll moment. But in the case of the bicopter, the differential throttle also creates a yaw moment. In one direction, the yaw moment aides the turn, in the other, it fights it. When it fights it, the FC gets in the middle of things and makes it worse. Not sure if OpenAeroVtol would help here or not, got to think about this for a bit.17 July, 2018 at 18:01 #58314
Here’s some more pics from different angles. Right click on an image and open in another tab/browser to zoom in for more detail.
LitterBug17 July, 2018 at 18:59 #58316
From talking to David at flite fest, and looking at the pictures I took, here is the current build specifications:
Frame: Alpha design RCExplorer BiCopter
FC: RCExplorer F3FC Racing
PDB: RCExplorer Baby PDB
Servos: RCExplorer (bluebird) BMS-210DMH
Tilt: RCExplorer Tricopter Tilt Mechanism
Firmware: Stock BetaFlight
Motors: Emax Red Bottom 2205 2300kv
ESCs: Aikon AK32 35A
Props: 5152 Triblade
Feedback wire is not used on the servos.
Weight evenly balanced at the Front/rear and side/side centerlines.
LitterBug17 July, 2018 at 21:05 #58317
Would be interesting to see if Ducting could improve the bicopters stability.
In my testing the ducting tends to auto level the copter as the lift around the ring tries to even itself out. Besides, it looks as if the bicopter is reacting badly to being drawn through its own prop wash. A duct would help here as the prop tips wouldn’t be able to react with anything other than the incoming air.
Note: Best ducts I’ve made were 35 Grams, from a Nylon / Carbon mix. I’m sure this could be inproved upon considerably. Possibly use Nylon / Kelvar mix for Ducts that are extremely inpact resistant. Could also make the Tail Mechanism and Duct one single part to increase strength and reduce overall weight. (Think, impossible tail-mech + duct) My biggest issue with ducts wasn’t the added weight or change in flight characteristics but rather production issues. Making a reliable product that was free of defects or layer seperation is very difficult.17 July, 2018 at 22:29 #58318
I’ve thought about trying to print the ducts again, but my big printer is currently offline. Here’s some old video of another BiCopter David worked on during his Flite Test Days… (the EARLY FT days at that)
LitterBug17 July, 2018 at 22:41 #58319
Right off those Ducts don’t add anything to the bicopters performance. No added lift, no smoothing of the incoming flow or the outgoing for that matter. Rather thats just a simple cosmetic ring. IMO, it would most likely fly better without it than with it. In the Flite-Test video it looks like the propeller tips are sat just above the Duct. Any advandage gained from reducing prop-tip vortices is 100% lost with that setup. Idealy the propeller must be sat within the duct and the prop tips need to be within 1-2mm of the duct wall to have any positive effect. (Larger the prop the greater the gap can be)
I’ve the feeling that the Center of Gravity (CG) is lower than the Rotational axis of the Tilt Mechanism (TM). This could be exasperating the problem as the mass under the TM acts like a pendulum. Thus any oscillation could be getting amplified by this difference. Lowering the carbon arms may help with the oscillations severity as could mounting the battery above the frame. Furthermore, angling (Dihedral) the motor mounts (5-10 degrees) away from the body could help keep the Bicopter from running over its own propwash. (Thrust is directed away from under bicopter)22 July, 2018 at 19:14 #58389
Really stoked about @david’s bicopter but haven’t seen any info on a BiCopter kit yet. Decided to start working on a simple 3D printed version to work on testing and tuning flying such a contraption with dRonin. Designed an EZ plate in OpenSCAD that can be printed twice for the all the electronic gear and battery mount. Using the Impossible tilt designed by @terje (AKA @dkdarkness ) https://www.thingiverse.com/thing:1035497 in different colors to help with orientation. Printed the parts in PLA for now, and am just waiting on some TGY-211-DMH servos to arrive to complete the build in the next week.
Cut the tube to 28CM which should easily allow for 6″ props and also have room to throw some of @kevin_erik’s ducts on with the standard tilt and 5″ props.
LitterBug23 July, 2018 at 12:17 #58394
If the instability in reverse remains an issue you may find that a simple duct would help smooth the flow to keep it stable. Don’t have to be anything crazy, just something simple to prove the concept.
Question where are you going to mount the battery? (underslung or over)23 July, 2018 at 17:16 #58397
@Kevin_Eric, I made it boxy and generic so it would be easy to try all sorts of configurations and orientations. I can print a third plate for top mounting the battery. It will be easy to move the FC plate below if I want to put the battery right between the props.
I did a quick scan on Thingiverse for BiCopter, and was surprised at the handful that came up. Most of the ones that had videos didn’t fly well. There was one designed by YXC https://www.thingiverse.com/thing:1797031 that flew real well, but it uses vanes instead of motor tilt. His design has a top mounted battery. He has a SingleCopter https://www.thingiverse.com/thing:1555053 that I’ve played with, but I put the mass too low and it hasn’t flown too well for me. @jihlein has had success flying the singlecopter.23 July, 2018 at 18:05 #58398
Biggest issue I’ve seen with Bicopters is the overall stability is pretty bad. David’s flew really good until it went backwards. Thus I can imagine that a strong gust of wind would have a similar destabilizing effect if it came from behind.
One CG issue i did see right off was that the rotational axis of both the servos / motors is mounted high above the Flight Controller. (Your’s and David’s) This difference could cause the copter to oscillate as the FC is held within the pendulums lower arc rather than next to the point of rotation. Then add the battery mass under this and the whole thing will struggle to keep stable if it does anything but hover. Alternatively, mounting two smaller Lipo’s Fore and Aft of the FC and linking them to create a 4S configuration would also help wih the CG.23 July, 2018 at 18:37 #58399jihleinParticipant
Even assuming the aircraft cg and motor pivot points are in the same lateral plan, there still is a fundamental issue with the bi copter format. Say your boom length is 20 cm for arguments sake. For roll, you can use differential thrust, each motor acting over a 10 cm moment arm. For yaw, one motor tilts forward, one aft, again acting on a 10 cm moment arm. And then there’s pitch. Both motors tilt forward (or backward), but there is no moment arm for them to act on. Other multi rotor formats have the motors displaced from the cg both laterally and longitudinally, so there is a moment arm in both directions. There is no longitudinal displacement from the cg, so the pitch moment arm is effectively 0.
A low CG will help stabilize things in hovering flight, but as noted, forward flight gets creative. The full scale guys deal with this in various ways, none of which is easy to implement in our scale here.23 July, 2018 at 19:49 #58400
I get about the mass being offset to aid in motion but what i was refering to is having the FC better centered. Maybe i did a poor job of explaining this.
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