3D Robotics


I was a Kickstarter backer of the XPlusOne VTOL aircraft, and I was able to get it for $199 as an early backer.  Now it's sold at the regular price of $1,549 for the version I have, which has no autonomy (it's got a simple MultiWii controller board), or for $2,199 for an autonomous version with Pixhawk. 

I haven't tried the second, more expensive, one, but have now flown the first enough to give a quickie review. 

The promise of VTOL aircraft is that they can take off and land in small space like a copter, but have the duration and lifting power of a plane. Sadly, this is usually more true in theory than in practice. VTOL aircraft like these are often called "sporks" for a reason. As copters they are hard to control, unmaneuverable, and prone to being blown around by the wind due to the sail-like cross-section.  As planes, they tend to be draggy due to the extra props and as a result also struggle with the wind. 

The XPlusOne is no exception. As a copter it tends to be blown about by the wind, as you might expect for something with four (!) wings. But the idea is that you keep it in copter mode just long enough to take off and and land, and do most of your flying in plane mode. Unfortunately, the XPlusOne is even worse at that mode. Since it has no control surfaces, all steering is done with differential thrust of the props, which is sluggish. And since it has no tail surfaces, it has to fly with a very high angle of attack at all but top speed, which means that it's always close to a stall and thus also prone to being affected by wind. 

But the main problem is that, as you can see in the video below, it's almost impossible to see its orientation. With four wings in a symmetrical configuration you just can't tell which side is up, or left from right. Combine that with very poor control authority and you'll be lost within seconds of going into plane mode. 

I just got a few flights in, and only one in plane mode, before this caught up with me. On a windy day, i went to a safe altitude to transition, went into plane mode and I immediately lost orientation. So I switched back into copter mode and the winds at that altitude just blew it away to parts unknown in seconds. I was never able to regain enough control to bring it home. Bye-bye XPlusOne. Had I paid $1,549 I would have been pretty annoyed. 

In short, I think the XPlusOne is unflyable in its base MultiWii configuration. That said, a proper autopilot might have done a much better job than me and the Pixhawk-based version with GPS might have been able to bring it back against the wind in a way that I, from the ground, could not.  But I'm not going to pay $2,200 to find out. 

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  • Moderator

    We welcome all these VTOLs in the T3 http://diydrones.com/profiles/blogs/t3-the-vertical-one

  • Developer
    I'm hearing a lot of differential thrust talk. Sounds like we really need this issue implemented in APM.
  • Jared S

    AFAIK the TERN design is based on Convair Pogo one, which did not have any swash plates and for control it used only trailing edge control surfaces. I assume the prop pitch speed, control surface size and distance from CG define the control authority and ability to loiter in wind. 

  • Rob,

    I am designing a 2 twin engine plank flying wing; tail sitter VTOL; 600g take off weight; 40cm x 25cm wide to keep it small; 4x4 contra rotating props. Horizontal flight speed about 100km/h with prop pitch speed about  80km/h. In hover I will max out both props to get 300g static thrust each and the prop pitch speed will go to about 160km/h. Both props are blowing directly on larger ailerons to have enough control authority. No tail as I use differential engine thrust and Cm/4 pitch moment almost neutral symmetrical airfoils NACA4 00xx.

    So even with a stronger wind push I should be able to use GPS and compass to loiter/hover in about 8m circle  and point in desired direction. 

    I found these designs, which seem to me reasonably stable and should be able to survive wind with GPS loiter:




    The flying wing design seems to me relatively efficient for fixed wing flight, just with minor penalty for VTOL (twin engines and larger ailerons). Removing the tail and having blended fuselage should provide some efficiency as well. Obviously not like a glider ...  

  • Moderator
    Precision unmanned over here in Oregon has a flexrotor, and I know they love it.
    I've seen multiple hobby grade systems modeled after military grade drones-i'd love to see a hobby/small commercial option modeled after the flexrotor.
  • T3

    It's not directly scale, it's wing loading. Just look at the sensitivity to winds for huge manned sized solar powered aircraft. It just so happens that you can fit incredibly powerful turbines and huge fuel tanks in a large aircraft.

  • Damian, you also have to take scale factors into account.  40 km/h wind doesn't affect a 2 ton aircraft that can fly at 200 knots as much as a 2kg UAV with a top speed of 40 knots.

  • I had a couple questions. Which has better efficiency with autotransistion autopilot installed (like px4/pixhawk) , xplus1 or firefly?

    What is more important for sales, vtol efficiency or mass media exposure?
  • T3

    Aerovel's Flexrotor and NG's TERN both use large diameter props...rotors really because each prop has a swash plate like a helicopter. Big rotors with rigid rotor heads have huge control power relative to a quadwing. You can still see the Flexrotor getting tossed about but it should be able to fly in winds a similarly sized quadwing can't.

  • Seems like DARPA is not concerned too much with the wind resistance of tail sitters; Perhaps military GPS with cm resolution is good enough to maintain the static hover; this is TERN:


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