Quad tilt; can you control FF without traditional flight surfaces? - RC Groups
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Feb 18, 2013, 06:03 PM
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Quad tilt; can you control FF without traditional flight surfaces?

Working on a design for a Quad tilt-fan and had a question...Is it possible to control fast forward flight motions without traditional (plane) flight surfaces, but by using the fans themselves instead?

I've been trying to work out how to induce flight characteristics with a 30-45* tilt, and having difficulty sorting out pitch and yaw. Once the fans are in this configuration, not certain their inertial effect would work similarly as when on a horizontal plane.

The idea is a "fan-only" design without typical airplane surfaces, allowing for a more compact, less complicated design.

Just to make things more difficult, looking into ducted fans, not traditional rotors. Again, I'm not sure if they will even have the same inertial effect as larger rotors, even when on a horizontal plane.



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Feb 18, 2013, 06:42 PM
It's a really big number.
Hey M,

I did a pile of research / experimentation into this on a now defunct project - and It turned out much simpler to stick a horizontal thruster on the thing and use it to scoot the quad around flat at high speed. No need for flight surfaces that way either, but I found a CG similar to a flying disk was best (approx 10 - 15% forward of the center of the quad), else like a flying disk, the aircraft became pitch sensitive in fast horizontal flight. A little thrust vectoring on the pusher made the machine hillariously manouverable...

I spend ages building a microprocesseor board that would redistribute control to compensate for the cross control effects induced by the angled fans, but ended back with the thruster for simplicity as you could use off the shelf parts.

People have made EDF based quads and they fly just fine, but EDF is power hungry compared to a large shallow pitch props.
Feb 18, 2013, 06:42 PM
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In short: yes, but you can't glide, though the ducts might act a bit as control surfaces. And the mix for the hover will be a completely different one than that for forward flight, which will make transition more complex as well. Also, I suspect that the traditional method to control yaw in a quadcopter will not work as well for ducted fans, probably differential tilt of a pair of duct will work better, and allow you to use fans that spin all in the same direction.
Feb 18, 2013, 07:04 PM
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leadfeather's Avatar
Sure you can, my experimental quad jet even looped and rolled without control surfaces.

But better to have control surfaces too. Better control and maintains control in dead stick situation.


Hover Jet Experiment, Maiden (2 min 27 sec)
Feb 18, 2013, 07:10 PM
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Wow, thanks for all the quick, great replies! Lead, saw your videos right after making this post. Watching your build carefully.

So if I were to build a tilt-quad (EDF) with no flight surfaces, best to just utilize articulation of the fans for "quad" mode. Using the same design, I imagine having the programming done for controlling the articulation for forward flight control would be a nightmare?

Anyone up to working on something like this with me?
Feb 18, 2013, 07:12 PM
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I think that a quad tilt wing might manage even a decent glide in a dead stick situation, though without yaw control if using tilt only. And it would need dual rates when not using vectored thrust.
Feb 18, 2013, 07:20 PM
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Looking around, I'm seeing just a few EDF VTOL projects that seem to fly like a quad.
Considering this "phase 1", I wonder how they're achieving this?
Feb 18, 2013, 08:26 PM
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leadfeather's Avatar
Originally Posted by Michael_Blue
Looking around, I'm seeing just a few EDF VTOL projects that seem to fly like a quad.
Considering this "phase 1", I wonder how they're achieving this?

There is a trick you might be able to use to avoid complicated programming with your quad. With a little appropriate tilt to the fans you can make it yaw stable and have yaw control.

The little Alien Jump Jet used this technique. All the props spin the same direction
Feb 18, 2013, 10:43 PM
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Ran D. St. Clair's Avatar
I am not sure if we are all talking about the same thing. Leadfeather, correct me if I am wrong, but you were in slow forward flight mode, with the fans tilted about 45 degrees, when you did the loop and the roll. At that point the quadcopter is still mostly a quadcopter carying the airplane around to look cool. Yes, the wing is certainly providing some lift but the control mode is still pretty much a quadcopter.

If we are talking about full forward flight (FFF) mode, then things get more complicated. By full forward flight mode, I mean that the rotors or fans are facing forward, not 90 degrees up or even 45 degrees up. In that mode 3 axis control is possible but more complicated.

All four fans would need to have high speed tilt control from straight up to about 10 degrees past straight forward (tilting down).

For pitch in FFF you tilt the front fans one way, and the rear fans the other. For example, up elevator would tilt the forward fans up and the rear fans down. When I say up and down, I mean up to 10 degrees from horizontal.

For left roll you tilt the right fans up and the left fans down.

For left yaw you add power to the right fans and reduce power to the left fans.

If the fans were ducted there would almost certainly be enough surface area in the ducts to give pitch and roll control in glide, though yaw control would still require just a little power.

I doubt the glide issue would matter much though. Such a design would have a lot of drag and would probably not glide very well anyway.

As was mentioned earlier, ducted fans and VTOL don't go together well.The laws of physics dictate that it will take a lot more power to generate the necessary thrust that way.

You have set yourself a difficult task. What is the reason for going about it in this way?
Feb 18, 2013, 11:24 PM
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leadfeather's Avatar
"At that point the quadcopter is still mostly a quadcopter carrying the airplane around to look cool. Yes, the wing is certainly providing some lift but the control mode is still pretty much a quadcopter." Yep, the wing is providing significant lift, but the control is all quad; that's why I call this mode fast hover; the fans dominate the control even if the plane has control surfaces.

The nice thing about combining a quad and a well matched airframe is that the roll yaw coupling works in a good way. Delta wings, swept wings, dihedral craft, and even dihedral tipped craft such as the Bixler have a good yaw/roll coupling....yaw input results in roll response in normal forward flight. With the quad fans tilted past 45 the roll(stick) input gives a yaw(fan) result which results in a roll (craft) response...nice synergy!

Pitch control from the fans goes to zero as the fans tilt full forward...unless the rear fans are mounted high and the front fans are mounted low, then some pitch control remains from the fans. For the jump jet experiment with no control surfaces, I couldn't go past about 85 degree fan tilt and still have enough pitch control...my fans were not offset vertically.

For M-Blue; a Quad with no flight surfaces that you mentioned in post #5 you will probably not be able to tilt past about 45 degrees...just not enough lift. 45 degree fan tilt or less it should behave (control wise) pretty much as a quad as already mentioned above.

Randy's point about ducted fans is good. Not efficient and heavy. Looks cool, but expect very short flights.
Last edited by leadfeather; Feb 19, 2013 at 08:29 AM.
Feb 19, 2013, 08:50 AM
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Here are a couple of other examples of quads with tilted fans. The tilt helps improve the yaw rate and give more yaw authority. Looks cool too.
Feb 19, 2013, 12:06 PM
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Awesome input guys, I really appreciate it.
Regarding my intent to use a ducted fan or this, my hope is eventually to significantly scale-up the project into a large-scale working prototype, and need to final design to be efficient and compact, with a minor regard also to aesthetics.
In larger scale, my understanding is, ducted fans can be smaller than traditional rotors because they are more efficient, they're quieter, safer (to bystanders or those entering and exiting the craft), etc. Maybe EDFs are too far removed from normal rotors and I need to build something of a hybrid?

The Bell X-22A is something similar to what I'm trying to achieve, though on a smaller scale, with a non-traditional airframe and smaller fans.

In the sketches I'm working on (I'll post some eventually) my design is a central craft with the passenger compartment in the front, power source and ancillaries in the rear, with 4 fans in an X configuration which are level in hover and tilt into a staggered arrangement (rear fans tilt up from the leading edge, front fans tilt down from the rear edge) in an attempt to provide better airflow to each fan. This design should operate as a quad in hover mode (or by nacelle tilting if inertial effect doesn't work with this design) and apparently may still work in that fashion so long as the tilt doesn't pass 45* (fast hover), only needing to vary the function of the fans for flight control when in fast forward flight. (Thanks Ran D for the description, this was one of my questions).

If speed and range can be attained in "fast hover", "fast forward" flight would not be necessary, significantly simplifying the project. This would have to be similar to other small private aircraft.
Feb 19, 2013, 12:23 PM
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The X-22 is a ducted prop design rather than a ducted fan. With a properly shaped inlet lip a ducted prop can achieve a greater static thrust than the same diameter un-ducted prop. But that same bellmouth shaped intake lip will cause trouble in conventional flight. There's also another problem common with all quad-tilt platform: in forward flight the rear rotors fly in the wake of the front rotors, which causes all sorts of issues with vibration and loss of efficiency. that's the reason why the rear rotors on the X-22a are more widely spaced than the front ones.
Feb 19, 2013, 12:31 PM
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Thanks for the correction. As I'm sure you've noticed, I am pretty new to this and don't have all the terminology. "Ducted prop" is definitely what I'm looking for in this design.

"Wake" issues are the same reason is why I'm tilting the props in a staggered configuration and why I hope "fast hover" will achieve the airspeed and range I'm looking for. If that mode can't be competitive with other private aircraft, I'll need to know the best route to move ahead into "fast forward" flight. Keeping this design as simple as possible is still the goal, so I would still very likely look to thrust vectoring and/or nacelle tilt for directional control in that mode.

To answer another question, no, this design is really not designed to "glide" at all. If there is a failure, there would be a "whole-craft" safety system in the form of a large parachute system that would allow the craft to descend at a non-lethal rate. Yes, gliding would be a more glamorous solution, but adding large, controllable flight surfaces would over complicate this design, imho.
Feb 19, 2013, 12:48 PM
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leadfeather's Avatar
Ducted props will be much better than edf's.

You'll be able to find counter rotating props pretty easily so you can just use an ordinary quad controller and no weird fan tilts to control yaw.

The prop system is much lighter and more efficient for the same amount of static thrust when compared to an edf.

If you are going for a scale looks, the larger ducted props might work better for your X-22 in this way also.

Not going to full tilt for forward flight is another big simplification and a good starting point. Once you get it flying well, you can always attempt more/full tilt later.

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