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Old Jul 11, 2012, 09:37 PM
B for Bruce
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The 'Wack, BC, Canada
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A concept for a weight trimmed plank wing with secondary pendulum stability

This idea came to me a couple of weeks ago but I just now got around to doodling a little concept model sketch to discuss. As shown below you can see that the wing sits up high on a pylon. The purpose is to keep the wing well above the radio controlled shifting balance trim slug of weight. The thinking is that we could get closer to neutral or perhaps even a small degree into negative stability as determined by regular means and by relying on the pendulum and high center of wing drag of the planform as an aid to increasing the pitch stability.

As shown the model does not show any full span camber altering flaps and elevons but adding such would certainly be worth the trouble so that the amount of reflex coud be dialed down a little to take advantage of the drag and pendulum factors.

This version is more a proof of concept. The final version, if this one proves workable, would involve gull wings instead of the pylon as well as some swoopy looking forward sweep in the inner gull panels and some mild sweep back of the outer panels to achieve more of a bird like look to the whole shebang.

For now though I'm wondering if anyone has any factors to offer that I may not have thought of for this conceptual exercise.

And Herk, it's some time away from being built since the renos on the new house shop areas have stalled a little due to summer things going on. I've still got about 3 to 4 months of a little framing to do then the drywall and finally making and installing all the cabinets. So it won't see the light of day as a radio controlled model any time really soon. Although some playing with a little toss about sheet balsa glider would be fun. Watch this space to see if I can get it to work with a near no reflex airfoil.
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Old Jul 12, 2012, 01:54 AM
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Hello Bruce,

Neat idea, I seem to recall (and I might be very wrong here) that gullwings are not really all that grand of an idea. I do not recall specifically why but somewhere in some article where the subject was the gull wing style wing that was prevalent in gliders many years ago it was discussed that this was not really effective. I think it had something to do with stability or lowering (???) the center of gravity to make the glider more stable but again I am not really sure.

It would certainly make a very attractive flyer with gull wings though. I have servos that would be ideal for a push/pull set up on such a plane as they are offer both a rotary output and a push/pull output. Might be very easy to build this with such a servo design.

I wonder how much motion is required for such an idea?

Jens
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Old Jul 12, 2012, 02:12 AM
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Hi Jens;

The big issue with gull wings is that for rudder and elevator control it puts the most dihedral angle in at the root where it does the least good for converting the yaw from the rudder into roll for the turn. But since I'd use ailerons anyway I would not need to worry about that aspect.

What gull wings would do for me with this multiple stability factor sort of scheme is raise most of the wing's lift and drag up much higher than the actualy true 3D Center of Gravity. And it would do it with what SHOULD be less drag than the pylon scheme shown above.

In order to get the power and throw I need for this trim weight shifting scheme I would likely use an RC sailboat winch servo that gives me a full 360 degrees of travel. With a 1 inch diameter output pulley that means I can shift a hunk of trim weight a full three inches worth. If that weight is the 4 oz of flight battery or similar then that SHOULD allow me to move the CG about 1/2 to 3/4 inch in total I'm hoping.
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Old Jul 12, 2012, 03:02 AM
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Hello Bruce,

Interesting and now that you explained it I recall that this was exactly what was discussed in the article. If that is all the motion that is needed then my Becker servos are perfect for the task. I have never built a "plank" but this idea is quite interesting. With a push pull servo of the type I have moving that amount of weight is a piece of cake for the servo. Most important is that it be as frictionless as possible. Perhaps teflon guide rails to locate the weight?

Jens
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Old Jul 12, 2012, 06:42 AM
Herk
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Quote:
Originally Posted by BMatthews View Post
For now though I'm wondering if anyone has any factors to offer that I may not have thought of for this conceptual exercise.

And Herk, it's some time away from being built since the renos on the new house shop areas have stalled a little due to summer things going on. I've still got about 3 to 4 months of a little framing to do then the drywall and finally making and installing all the cabinets. So it won't see the light of day as a radio controlled model any time really soon. Although some playing with a little toss about sheet balsa glider would be fun. Watch this space to see if I can get it to work with a near no reflex airfoil.
Hi Bruce - Would be interesting to see you pursue this. A plank with near zero SM could be quite efficient, though airfoil choice could have a big effect. Controlling the trailing edge along the span is a great way to experiment. Two sections on each side should be enough. I'd say build a small quickie and see how it works. Then elaborate from there.

Unless the model is to be powered the weight of your fuselage assembly will not be much - so the pendulum effect may not be all that helpful. With the weight of a power system in there you would get more effective mass.

After the flight of my finless plank I concluded that some downthrust would have reduced the P-effect. I believe that might have solved my left turn under power problem. Your concept would probably want even more downthrust just because of the low position of the fuselage.

My only other thought is that it might be a good idea to go for a more vertical hinge line on the rudder - avoid pitch effects when the rudder is deflected.
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Old Jul 12, 2012, 07:34 AM
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Another experiment!I love this aspect of the forum.As I've stated before one persons idea can be put out into the ether and refined into a practical project.
Jens,those Becker servos sound interesting,could you post a pic and specs.
Regards Stuart
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Old Jul 12, 2012, 07:59 AM
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Hi Stuart,

Here is the link to the Becker servos, http://www.becker-fm.de/shop/group/S...niversalservos all in German btw. I can translate if need be. Unless you have a Becker Rx you will have to change out the plugs. Becker uses 4 Pole plugs (only three pins connected). Also the servo square drive for the servo arms as opposed to the common round with ridges. I'll update this with a pic later. Cannot do it from work.

Jens
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Old Jul 12, 2012, 08:14 AM
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Grand idea! When you mentioned gull wing, I almost immediately thought of an old timer aircraft called Spook - plans attached, (NOTE: not a flying wing!) Might give some ideas for construction. The plans are updated and have ailerons. Anyway, if it helps at all, shiny... Interesting project, that's for sure.

Mark
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Old Jul 12, 2012, 11:38 AM
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Originally Posted by BMatthews View Post
The thinking is that we could get closer to neutral or perhaps even a small degree into negative stability as determined by regular means and by relying on the pendulum and high center of wing drag of the planform as an aid to increasing the pitch stability.
So the plan is to push the CG aft until pitch stability is a problem, then rely on the pendulum effect to provide pitch stability. Seems problematic to me. The pendulum effect may work predictably statically but when flying, the off center weight will be swinging around creating additional unwanted pitching forces. For example, as you enter a high speed loop, wouldn't that create a rotational force around the pitch axis that would be magnified by the inertial of the off center weight? I think that you'd be fighting the pendulum effect with any such maneuver, including turns. Of course all trim weight has this effect, but those weights are normally balanced fore and aft. The pendulum is different.

Based on my modest flying experience, weight shifting or CG adjustment is probably one of the most effective ways to improve performance on these flying wings. The price is loss of control. Having an instant method to recover control may best be done by weight shift. Having a very forward CG will solve most control problems in a flying wing. Personally, I'd like to have a system that had fine adjustment of the CG on a slider on the Tx then a secondary control with a switch that would throw the CG way way forward for maximum control. Kind of a panic switch.

I hope you get a chance to build a prototype Bruce. Should be interesting.


Kent
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Old Jul 12, 2012, 01:04 PM
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Perhaps the use of the term "pendulum" was bad wording. The intent isn't to have a free hanging weight in the fuselage, Instead it's simply about having a CG location that sits well below the wing to achieve stability of the same sort found on hang gliders. The movable trim weight would be fully controlled by a servo running off the throttle channel. The idea would be to work with it and find the limits of CG location which produces a still stable but oscillating sort of slow trim and on the other end a high speed almost dive at the fast end. These limits would be found and then the trim weight tuned up to where I can use the throttle stick as a... um... a throttle. I'd be able to stuff it forward for high speed trim or pull it back for slow almost oscillating flight. Mostly it would be used to vary between the minimum sink and best L/D speeds.

Yes, if the model is disturbed by some turbulence or at some trim setting the model could be upset by this and depending on the damping ability and resonance in the system there's a risk of some fugoid oscillations. Hopefully the system isn't so strongly resonant that it runs away at a speed of oscilation which is beyond my ability to stab in some correction to counter it.

Herk, that's a good point about the rudder hinge angle. It's not the sort of thing which is of any concern on any sort of normal planform but on something this closely coupled the elevator aspect could well be powerful enough to produce a noticable trim change.

As I mentioned above I still don't have a proper shop yet to be able to make up the traveler for the trim weight. Or even have a decent spot to build a wing. So I'll have to content myself with some little all sheet test gliders to try out the idea for now. But by doing so I'll get more of a handle on the pitch oscillation issue. There's likely some relationship between the chord to lengthwise weight distribution which will play out to my advantage.

For example I've seen descriptions of some plank gliders where it was found that the model tended to pitch oscillate rapidly. The conclusion being that a longer nose and tail so some weight could be placed out further to lengthen the period of resonance would have been worth trying. So I'll try this with the test gliders to see if it makes much difference.

If made as electric powered models there's no doubt that some extreme downthrust will be needed. And for the same sort of reason why gliders with power pylons require lots of thrust angle. With the high wing placement and resulting higher center of drag compared to the thrust line there's bound to be a strong pitch up force from a nose mounted motor.
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Old Jul 12, 2012, 03:52 PM
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Well it is kind of sort of pretty much like a hang glider with ailerons for even more control or?

Jens
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Old Jul 29, 2013, 01:27 AM
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OK, so some time back.... a LONG time back in fact.... I built up a test glider for this concept and puttered around with it. But other things caused me to set it aside. Well, tonight I didn't feel like doing anything energetic and hauled it out to play around.

My early testing was a bit of a failure. But at one point I realized that I had not installed enough weight down low to move the CG low enough. So first step was to install a little ball of lead at the base of the wing. Early results looked promising and you an see the results of this in this little video. Click on the picture and it'll take you to Photobucket to view the video;



But the last dive into the pathway busted the ball away from the model. So it was back into the shop and I came up with the ball being drilled and threaded so it is easier to adjust and it was set onto a length of 6-32 screw set into the belly. The wing was also glued into the slot so no more accidental landing adjustments. A picture of this adjustable weight is shown below.

Test glides with the screw adjustment are looking good. But by the time I got it out it was the last dregs of twighlight so I couldn't do any video. But in the next day or two if the wind is calm I'll do another video.
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Old Jul 29, 2013, 11:45 AM
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Nice glide !!!....and an attractive model too ! (note to self: It doesn't hurt to make a prototype look good)

Really amazing that you got such a good glide with no wing reflex. If you are anything like me, you'll post the best flight of a dozen tosses, but just the same, this must be a very aft CG in order to get any glide at all from no reflex. Color me baffled. You really threaded a needle with that set up.

So with the success of the free flight, a quick RC prototype can't be far off. Although it will look like , attached is quick test prototype that simply has a fishing weight hanging off of a servo arm. This would provide a very quick CG adjustment, just like a hang glider. In a hang glider you can use your body weight to jerk the glider into turn and other man-handling tricks. Lots of expo on that CG servo would probably be a good idea.

Also, for those of us who have not quite figured out free flight models, an AUW of your little glider would be instructive.

Thanks for the video.

Kent
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Old Jul 29, 2013, 11:45 AM
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That's a pretty neat setup. It brings to mind Lilienthal's weight shift control system, but you intend to use this for pitch only?
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Old Jul 29, 2013, 11:54 AM
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Yes the weight shift is for CG control in the pitch axis.
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