Saturday, January 24, 2015

UAS Excelsior Alpha - Vectored Thrust Tricopter

I was doing some fast, straight line FPV flying with one of my quadcopters and a few things struck me about how multicopters fly in a straight (horizontal) line;

  • The more you pitch forward (to go faster) the less aerodynamic you are
  • The angle of the pitch actually serves to reduce lift, effectively fighting the motors
  • Finding the maximum forward speed is a risky, manual process.

So what if you wanted to build a multicopter optimized for forward flight?  I've seen some 250mm FPV racers that actually had the front two motors angled forward to increase the forward speed at a lower pitch angle.  I found myself asking - what if you could dynamically actuate the front motors forward instead of pitching the whole aircraft?  (Similar to the motors on an Osprey, but sans wings.)

Vectored thrust on multirotors is uncommon, but not new.  Still, this idea has a singular purpose which makes working out the details and controls a bit easier.

So, I decided to build one.  I give you Excelsior Alpha - my proof of concept Vectored Thrust Tricopter:

And here it is in flight:

I settled on a tricopter design because I liked the idea of yaw (rotation around the Z axis) control being completely independent of the individual motor speeds.  Otherwise, vectoring the front motors forward is going to have some weird roll/yaw side effects.  Tricopters don't alter the motor speeds for yaw. Instead, they vector the tail motor on the Y axis to rotate the aircraft around the Z axis.

For the thrust vectoring, I decided the simplest design would be to mount the front motors on a single shaft running along the X axis at the front, and then couple that shaft to the airframe in a way that allowed a servo to rotate it - effectively pitching the front motors fore and aft.

Here's a video of the drive actuator mechanism: (Note the frame plates have changed a bit since this was taken.)

Build One Yourself!

DISCLAIMER I:  This is a largely unproven design.  It needs some improvement. I hacked it together in a couple of weeks as a proof-of-concept.  I'm only sharing it because in spite of these facts it worked better than I expected, and I've had lots of people ask me for the build details.

DISCLAIMER II:  This is not a beginner project.  I do not recommend undertaking this if you aren't already familiar with RC TX/RX hardware, flight controllers, ESCs, and the like.   I'll list the parts, but I unfortunately I don't have time to write a walkthrough or answer questions about the basics of multirotor construction / configuration / flight / &etc.

Purchased parts: 

(Links go to items I used from, feel free to substitute / experiment!)

Main components
498x12mm Carbon Fiber Tube (drive motor arm, cut to ~290mm length)
OrangeRX R100 DSM Satellite Receiver (or use a traditional R/C receiver)
ZIPPY Compact 1800mAh 3S 25C Lipo  (3S 1300mAh - 2200mAh seem to work well.)
2 x 7x4.5 3-blade counter rotating props

Misc assembly hardware  (May be incomplete!)
M3 screws (~14mm?) & nuts
heat shrink tubing (for solder-on bullet connectors)
4" zip ties
velcro strap (hold the battery)

3D Printed Parts 

(main frame plates and various mounts)
I've uploaded all the parts to Thingiverse:

I printed them from PLA at .2mm layer height on my Printrbot Simple Metal.  (~25% infill I think)

You'll need to print:
1 x excelsior_top_plate
1 x excelsior_bottom_plate
2 x 12mm_collar (used to hold the motor arm in position)
1 x 12mm_sleeve_servo_arm 
1 x servo_arm
1 x 12mm-to-14mm-tube-adapter (to mount the Talon Tricopter Tail Mount to the 12mm tube)
4 x 12mm_tube_clip (optional - I use 3 for the ESCs and one to hold the XT60 battery clip)
[2015/05 Update:]
1 x Tricopter Yaw Mount / Plate set :

Miscellaneous Construction Notes:

I drilled a 3mm hole through the CF tubes to mount:
 - The motor mounts
 - The collars left & right of the main plate
 - The sleeve servo arm
 - The tail servo assembly & adapter

In each case I positioned the part first, then drilled the hole (in each side separately if necessary), then threaded the screw through the part and tube together.

The main motor arm is 290mm long because that was the longest length of a broken 500mm tube leftover from a crash that I happened to have on my bench.  It seems to work well, but that size is fairly arbitrary as it was determined by chaos theory, not computation.

Flight Controller Setup

[2015/05 Update:]
See post on Flight Controller software here:

Former text for posterity:


Basic setup:
  • I reflashed the FC with Cleanflight and the modified it (provide patch?) to allow me to control an additional servo on S2 using the AUX3 channel input
  • I changed the channel map put AUX3 on the elevator (pitch) stick, and pitch on the AUX3 channel.
  • I put the FC in 'ANGLE' mode at all times so it will hold the craft level when I actuate the thrust vectoring.  This allows me to adjust the pitch with the AUX3 knob (as a result of the remapping above) and control the forward/reverse vector with the elevator stick.

You could accomplish something similar without modifying the FC software by using a traditional R/C receiver and connecting "ELEV" from the RX to the vector servo instead of the FC.  Just be sure to put the FC in "Angle" mode since you won't have any pitch control unless you connect an analog AUX channel to the FC Elev input.

If you undertake this, let me know what happens and GOOD LUCK!  ;)


  1. Update, I designed a better YAW mechanism:

    It's lighter, simpler, and has a larger range of motion than that mechanical design from the Turnigy Talon.

    1. Hi KC,

      I've got the electronics together for the EDF build, but I'm unsure how to proceed with flashing the KK board.

      Any tips?

    2. NB: It's the vectoring I'm not so sure about here. I feel lost!

    3. Every time I spent a few hours no it, I find the solutions to your work hehe!