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Old Jan 24, 2013, 06:17 PM
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Some Design Concepts

Some Design Concepts
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Old Jan 24, 2013, 07:06 PM
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Very cool! A much more rational approach than most other multi-rotor conversion planes.

Steve
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Old Jan 24, 2013, 07:18 PM
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Originally Posted by Ran D. St. Clair View Post
Some Design Concepts
I think on the tri setup, you will not be able to control yaw in a hover, unless the tail tilts. You are correct for forward flight, but in hover, no dice.
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Old Jan 24, 2013, 08:18 PM
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Originally Posted by ilektron View Post
I think on the tri setup, you will not be able to control yaw in a hover, unless the tail tilts. You are correct for forward flight, but in hover, no dice.
I assume you are referring to the tri-copter design depicted at the end of the PDF file. Imagine that the forward motors can tilt as far forward as 90 degrees for forward flight, and as far as 10 degrees back in a hover. The nominal hover angle would be straight up, or zero degrees. Now imagine that the left front motor tilts back 10 degrees, and the right front motor tilts forward 10 degrees. The result will be a strong yaw to the left.

It's essentially the same concept as tilting the tail rotor as is done with a conventional tri-copter except that the tilt axis is not perfectly aligned with the CG of the aircraft. It is close enough, however, and the forward translation vector of the right motor is canceled out by the rear translation vector of the left motor (in this example for a left yaw), so it all works out as almost pure yaw.

If you don't think it will work then go ahead and have the tail rotor tilt fore and aft for forward flight, and side to side for yaw control. You will need a 2 axis motor mount gimbal though. Such a thing is entirely possible but a bit harder to build.

If you were to tilt the motors up and down as part of a control scheme while in forward flight it would result in roll, not yaw. It wouldn't work particularly well because the wing would mostly straighten the flow from the propeller, regardless of which way the motor was tilted. Ailerons are a much better solution.
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Old Jan 24, 2013, 08:45 PM
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The tilting front rotors with stationary rear rotor has already been done successfully. It was one of the configurations I built and tested on my F-35.

F-35, Dual tilting front fans flight testing (2 min 50 sec)
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Old Jan 24, 2013, 08:46 PM
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Quote:
Originally Posted by Ran D. St. Clair View Post
I assume you are referring to the tri-copter design depicted at the end of the PDF file. Imagine that the forward motors can tilt as far forward as 90 degrees for forward flight, and as far as 10 degrees back in a hover. The nominal hover angle would be straight up, or zero degrees. Now imagine that the left front motor tilts back 10 degrees, and the right front motor tilts forward 10 degrees. The result will be a strong yaw to the left.

It's essentially the same concept as tilting the tail rotor as is done with a conventional tri-copter except that the tilt axis is not perfectly aligned with the CG of the aircraft. It is close enough, however, and the forward translation vector of the right motor is canceled out by the rear translation vector of the left motor (in this example for a left yaw), so it all works out as almost pure yaw.

If you don't think it will work then go ahead and have the tail rotor tilt fore and aft for forward flight, and side to side for yaw control. You will need a 2 axis motor mount gimbal though. Such a thing is entirely possible but a bit harder to build.

If you were to tilt the motors up and down as part of a control scheme while in forward flight it would result in roll, not yaw. It wouldn't work particularly well because the wing would mostly straighten the flow from the propeller, regardless of which way the motor was tilted. Ailerons are a much better solution.
Der... You are right. about the yaw. I'll let you know about the ailerons in forward flight. My current tilt rotor build in progress won't have them until I deem them necessary.
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Old Jan 24, 2013, 10:13 PM
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That F-35 is very cool. Much more charismatic than my plain and functional design. If this were a marketing competition I would say you have me beat.
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Old Jan 24, 2013, 10:46 PM
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Please, may I have feedback on the gear selection for the Bixler to quad-copter conversion project. As I said earlier, I am not claiming to be a quad-copter expert, as a matter of fact, other than the little Blade MQX the only quad I have flown is the VTOL in the video.

I am thinking to use 4 each of the CarbonBird Motor NMB-1175kV 175w 50g on 3S. The multiwiicopter.com website has them listed as out of stock at the moment.

I figure the battery will be 3S, something like 3,500mAh.

Propellers will be the GWS HD-9050X3, 9in diameter, 5in pitch, 3 blade.

Motor controller will be the ESC CarbonBird 18A MultiCopter ESC 4S double heatsink JST

In addition to putting together a package that is good quality and reasonably priced, I want to make sure I pick stuff that is readily available so I can give the recipe to anyone and they will be off to a good start.

Of course it all has to play well together. I dont have any good data on this combination of motors, propeller, and ESC. The CarbonBird site suggest that they will work well together, but offers no actual data.

So what do you think? I am not looking to start a brand war here. Just good middle of the road stuff that is widely available.
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Old Jan 25, 2013, 01:37 AM
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Ran, those are good stuff. It should work. Check with Jason. I bet he can help you out.
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Old Jan 25, 2013, 03:43 AM
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Quote:
Originally Posted by Ran D. St. Clair View Post
VTOLicious, Your design places the motor at the end of a lever arm of an inch or two. In the event of any kind of shock, like a bad landing, it will pound back into the servo output. Depending on the strength of the servo output gears, weight of the motor, etc. The servo may not be able to take it.Your design does have the advantage of raising the prop farther away from the boom in hover mode so a prop strike on the boom is pretty much impossible.
You are right, its a compromise between "distance to boom" and "strength requirements". However, for my design it was necessary to raise the prop. Im not entirely happy with this solution and plan to upgrade it with a mechanical stop that absorbs shocks when the motor is in vertical or horizontal position. Inbetween the full load is anyways on the servo.
...regardless of the motor mount design the whole assembly incl. servo will always be a weak point of a tilt rotor. Same for the tilt mech of a common tricopter

Quote:
Unfortunately, in the back, where the motor has to rotate down your design will drop the propeller down into the grass. I am already cutting the grass with my rear prop in hover mode as it is. Dropping it down another inch or 3 would be a problem. You could just make the landing gear longer, but then it gets wobbly, and causes other design problems.
You could mount the boom above the wing...no more grass cutting

As said before, Im happy to help and can design/build a mechanism out of laser cut plywood parts and share the CAD-files with the rcgroups-community, just let me know if my contribution is desired...

Regards Michael
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Old Jan 25, 2013, 07:21 AM
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Ran D,

For your Bixler project I would like to offer you a suggestion. Mount the motors on separate spars. This gives several advantages:

The motors can be mounted tractor or pusher as long as the wing and ground clearnces are there.
The motor spars become a very simple tilt mechanism.
The wings will be easily removable for storage.

Also, the simplest programing will be to run it as a quad full time and simply tilt the fans. When tilted forward, pitch modulations of the fans have no effect on flight, yaw modulation of the fans have no effect on flight, roll inputs will modulate the fans to give a yaw force. The yaw will be in the correct direction for the turn and with the Bixler's upswept tips any yaw force will induce some roll (yaw roll coupling). When going from hover to forward flight; as the fans lose their control effectiveness (past about 45 degree tile); the fixed wing control surfaces take over in a smooth transition.

Check out this experiment I did. It shows the pivoting spar arrangement. It also shows a little jet bing controlled in hover and forward flight by the fans only, no control surfaces.

Hover Jet Experiment, Maiden (2 min 27 sec)


Jump Jet, no control surfaces, R&D test #2 (2 min 23 sec)
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Old Jan 25, 2013, 12:04 PM
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leadfeather,

I will have to give some serious thought to the relative advantages and disadvantages of a rotating spar concept. It clearly works well for your little jump jets.

You raise a valid point about the ability to remove the wings. I had planned to glue them in and then brace them on the underside for torsional rigidity. Rotating spars would separate the motors from the wings and they would remain standard wings which are removable for easy transport.

I am not crazy about having the spars sticking out at 90 degrees to the forward flight airflow. It will never be a fast plane due to the low propeller pitch but I wanted to keep the drag to a reasonable level. I could easily add a little balsa trailing edge to make them teardrop shaped though.

This is a somewhat bigger plane, so the spars would need to be of a reasonable diameter, at least 6mm carbon, and even that would be a bit wimpy. It might not be trivial to integrate the tilt mechanism into the existing fuselage in a clean way. I would have to figure out all the various issues with bearings, pushrods, etc. That and it all needs to be both strong and stiff enough. There are a lot of torsion loads induced by yaw control input. Stronger and stiffer means larger diameter, more drag, more bearing surface, more power from the tilt servo, etc.

There is a minor aesthetic issue with spars. They would clearly look like an afterthought. Motor pod configurations look normal for a twin, and the pusher puller motor pod is just an extension of that concept. I admit, my tilting motor mounts are not exactly pretty and motor pods on the end of a spar could be made more elegant.

I think my motor pods will generally be stronger and more durable though. Motors on the end of a spar will always be subject to getting snapped off unless you go with a really large diameter carbon tube spar, and at that point the drag and other issues become more of a concern. Even then you are transmitting crash forces back into the fuselage via hard points at the bearings and the fuselage itself probably won't fare well. At least transmitting those forces back through the entire wing root spreads the forces along a big chunk of the fuselage.

I think all of these issues are less of a concern the smaller the aircraft. Smaller things are inherently stronger and fly slower so drag is less of a concern.

Ultimately, I think both methods can work, and I am sure will be made to work in various designs.

You have given me something to think about, especially the issue of being able to remove the wings for easier transport. A fully assembled Bixler isn't that large, but it might not fit nicely in some vehicles. Perhaps a one piece removable wing with motor pods attached??? Ideally such a wing would have a big torsion tube embedded in the foam, but this is a kit bashing project.

I am thinking that these things could eventually become the FPV vehicle of choice. Properly configured they should have twice the flight time of an equivelent quad copter, but still be able to hover when you want to.

If on the other hand, it's more about the cool factor of a nice little jump jet that's just a blast to fly, then I think you already have the right formula.
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Old Jan 26, 2013, 07:31 AM
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Ran D,

There are lots of paths to VTOL that can work. I think your design philosophy is very good.

I also think the choice of the Bixler is good... tough airframe, good performance, widely available at a reasonable price.

I also think you are right on with the idea of keeping it simple...simple to make, simple to set up, simple to fly.

I'll continue to offer suggestion that I think are worth considering, but I think you make decision that will follow your original (very good) vision.

Here is another idea for your interest. Imagine a Bixler that has a quad adapted to it rather than fully integrated into the airframe. The quad could be flyable on it's own and a simple mount would connect it to the Bixler in a couple of minutes. I did something similar with a Tricopter and a Stryker....it worked very well.

Flying Monkey mod (1 min 20 sec)



Flying Monkey Mod, Success! (2 min 26 sec)
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Old Jan 26, 2013, 01:38 PM
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Leadfeather,

Really good stuff. I will invite a minor hijack and ask you to share as much as you are willing on the specific construction. It looks like you are using 6MM carbon tube for the structure. Possibly some sort of nylon landing gear blocks for bearings. possibly drilled out servo arms for the tilt linkage. So how does it really work?

Likewise the control systems. You don't seem to be having much problem with control interactions when in forward flight. Are you doing something special to avoid them or are they just not that big of a problem.

I do like the idea of only tilting the front rotors for some applications. It may be a nice design simplification. Can you speak to the pro's and con's?
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Old Jan 26, 2013, 10:39 PM
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Quote:
Originally Posted by Ran D. St. Clair View Post
Leadfeather,

Really good stuff. I will invite a minor hijack and ask you to share as much as you are willing on the specific construction. It looks like you are using 6MM carbon tube for the structure. Possibly some sort of nylon landing gear blocks for bearings. possibly drilled out servo arms for the tilt linkage. So how does it really work?

Likewise the control systems. You don't seem to be having much problem with control interactions when in forward flight. Are you doing something special to avoid them or are they just not that big of a problem.

I do like the idea of only tilting the front rotors for some applications. It may be a nice design simplification. Can you speak to the pro's and con's?
There are details in the jump jet thread. The purpose of the jump jet build/experiment was to see if a standard quad FCB and programming would work for VTOL no mods to the quad code. It worked very well...much better than I expected. The tilt servo was hooked directly to the Rx with a servo slower device.

I think the spars I used for the fans in the jump jet were a stout 6mm. The motors are pretty protected by their location being nested in the airframe...less vulnerable to impact than plain quad. A single flush mounted servo in the fuselage tilted the fans.

I think tilting all 4 fans is probably better for a quad VTOL. As all 4 motors approach full forward tilt they will have little effect on forward flight control.

For the Bixler project I would suggest:
  • Beefing up main spar in wing...the spar that came with my Bixler was pretty wimpy. Also fans mounted on wings may cause wing twist with yaw inputs in hover.
  • Quad with all 4 fans tilting together
  • Tilt input direct from Rx with adjustable rate servo slower
  • Standard quad programing at first
  • After all else is working good make minor tweaks to programming such as turning off gyro to fans at full forward tilt.
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Last edited by leadfeather; Jan 26, 2013 at 10:47 PM.
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