barnowljenx

Is anyone using the Debowski Twin Motor Contra Rotating motor in F3A competition? I would like to compare notes on experiences.

barnowljenx

Thought it might be helpful to put a photograph up of the TMCR motor. This consists of 2 outrunners the rear one driving through a hollow shaft of the front motor to drive the front propeller. Each motor has its own ESC, a D3 in this case, and each can be controlled either though a Y lead or by separate channels if you want the flexibility of being able to adjust each ESC/motor separately. I tried the separate control but as you have to set up 4 idle points I found it simpler to use a Y lead and control both ESC together.



TMCR

TMCR installed with 2 D3 ESCs

Brenner

So how does it fly? What's the throttle response like? Does it have enough power? What props are you using?

big_G

I like the redundancy of 2 esc's and the very simple drive. No belts or gears. How is the weight?

barnowljenx

Hi Brenner,

it's a very easy set up as you just need a simple throttle curve and 4 idle conditions: Ground handling - 0 rpm; Spin ~150-200 rpm; land ~ 200-300 rpm; flight idle ~ 1,500 rpm although I have reduced this to 1,300 rpm to match my level speed. The ESCs are governing so that the amount you have to move the throttle stick is much less than you are used to. I know some folks have adjusted the top end of the throttle so that full stick movement gives the required upline speed. I've chosen to tighten up the throttle stick ratchet and use clicks of power. So, from datum, I will use 4-5 clicks up for a vertical, 3 clicks down for a 45 downline and fully closed throttle stick for a vertical downline. For snaps, I crank up 3 clicks before the snap and reduce by 3 clicks once the aircraft is stabilised after the snap. Seems to work for me. The useful thing about the governing ESC is that you can come back to idle for landing and really dive at the ground letting the props control the speed. On round out, you need to add about 3 clicks of power to avoid the drag from the props from dropping the speed to stall speed. Also, if you do land fast on tarmac, when you close the throttle, you can see the aircraft decelerate. One thing I've learned is not to close the throttle from approach speed till you have landed otherwise the instant response of the motor will cause the aircraft to stall as you round out.

As regards power, the highest I recorded was 3,600 W and 98 A. I'm using Falcon 23x20 F and R as recommended by Adam. He says you can also use 22x20 F and R for F3A use - as I have a pair of 22x20s I might try that but having that extra power on tap is useful on very windy days. I don't know how this is shared between the two ESCs as I only measured the total current flow. I'm not sure how you would measure both ESC power draws as this might need the telemetry data handled differently in the Tx. I'm sure someone will be able to do it using Open Tx etc but I'm using a JR XG11 and the SM Modelbau Unisens-E telemetry device. The ESC is claimed to be the fastest responding ESC out there and it certainly seems to be tied to the throttle stick movement exactly. In normal P25 flying, I rarely use more than 2,800 - 2,900 W and that's around 80 A so there is plenty of headroom.

I made a depron airflow guide to channel air to the motor and the D3s. Neither the motor, including the rear motor nor the D3s don't seem to get more than slightly warm as they are sharing the load so cooling is not as big an issue as it would be for a single D3 driving a single motor contra or any single motor for that matter.

This is a quote from the D3 website: "TMCR’21 uses standard Falcon propellers. Ultra light 3d printed spinner used with TMCR’21 may be printed with any diameter to match the user plane. Drive is controlled with two dedicated D3 F3A ESCs, which are smaller and lighter version of standard well known D3 controllers. Max current measured in flight 97A, max rpm in flight 4450 (limited with ESC to this value), max rpm in vertical flight with full power 4250 (with a pair of 23x20). Weight of the bare drive is 585g. With a pair of 23x20 current is distributed equally to both motors and this is the recommended setup. With light monoplane a pair of 22x20 is more than enough power."

Hi big_G - re the issue of weight, see the weight quoted in the text immediately above as 585 g. That is just the drive no props or spinners. The spinners are 3D printed and the early ones were pretty fragile but weighed in at 20 g total.

I think the combination of the 2 outrunners and two D3 ESCs have been quite amazing as regards speed control. You do have to get used to how quickly the ESCs respond and also the impact of having the rpm, and hence power, set by the throttle stick. You just have to remember not to close the throttle from the approach setting until after the round out and touchdown otherwise the aircraft just stops! Being outrunners, you can fly right after replacing the pack. No need to wait for things to cool down or use fans to dissipate heat.

In short, I'm very impressed with the setup of the TMCR and the D3 ESCs.