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Variable CG?

Old 07-23-2015, 09:04 AM
  #1  
Jennifer Curtis
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Default Variable CG?

Has anyone seen some kind of variable
center of gravity setup? I'm thinking
of something like a sail winch servo
pulling a weight back to do wild aerobatics
and pulling it forward for normal flight.

Jenny
Old 07-23-2015, 10:11 AM
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ByLoudDesign
 
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Go to Servo City and use a coupling to make a jack screw the length by using a threaded rod vs a shaft. Threah the weight and drive it back and forth.
https://www.servocity.com/html/servo...l#.VbEtmrVRr2Q
Old 07-23-2015, 10:27 AM
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Jennifer Curtis
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For that it would probably take jackscrew
with about 0.5 to 1 thread per inch. Any
idea where to get one?

Jenny
Old 07-23-2015, 11:53 AM
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ByLoudDesign
 
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Use a 360 servo as a motor and a coupling from Servo City; selected by the servo mfg you select and the size of screw you wish to use. Get the screw long enough the there is some un-threaded shaft at the head, cut off the head and use that to fit into the coupling. You'll need some sort of guide/rail to keep the weight from spinning and guide its travel. You might consider using a lighter weight and moving it further, giving you a more finite adjustment.
Old 07-31-2015, 08:55 AM
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To have much effect on a regular plane the mass will either need to be quite a large portion of the model's weight or it will need to shift over good long distance within the model. You don't say what size of model you're considering but when you're talking about sail winches I'm thinking that it'll be up around a 40 to .60 size or electric model equivalent.

This suggests a 5 to 7 lb model weight. To shift the CG over about a 10 to 15% range of the chord to go from "mild to wild" you're looking at likely needing to shift a 7 or 8 oz mass over probably 5 or 6 inches of travel. Or a 4 to 5 oz lump over a longer distance such as around 10 to 12 inches.

I'm thinking that with something like this you might be better off with a weight on a guide of some sort and a closed loop "clothes line" setup with pulleys at each end and a geared motor turning a drum with a few wraps of a strong line around it. This can actually be made from a modified regular servo by removing the feedback pot and driver board and run it from a switch on a regular servo and limit switches on each end.

The jack screw idea is OK to but not the way you're considering it where you run the screw from a regular servo. You won't find any such thing as a 1/2 or 1 thread per inch "screw". The angles are such that it would take a massive torque to turn such a thing. Instead you're again looking at making up a custom "servo" or running a motor to a gear which is threaded internally to run on a normal length of small threaded rod such as 10-24. The power needed and the gearing down would come from the number of turns you need to travel the length of the rod with that fine a thread.

There's a few threads around that detail the amount of mass and distance needed to shift the CG of a model. Find one of those or look up the equations needed to calculate the mass torques and how it affects the weight of the model. That and running the model numbers through a CG calculator will give you a feel for how big a mass it needs to be and how far it has to move.

"Wild" actually implies being able to shift the CG to a slightly negative setting at a few % of the MAC behind the Neutral Point. The model will need to be constantly corrected but flying an "unstable" model up to a point is no worse then the tiny steering corrections we make while driving. It'll get worse as you approach a point where it's -3 to -5% for the Stability Margin but you won't know where the limit really is until you try it. There's a fair number of folks that fly with only 1 or 2% positive stability and don't find the models hard to control. And the flat foamy flippy models that do all the power hovering are commonly running negative stability margins of one amount or another since it makes the models hover in a more stable manner. Yet they don't have any problem flying around "normally" and being controllable.
Old 07-31-2015, 01:08 PM
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Jennifer Curtis
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I was thinking of something like a
HS-785HB sail winch servo. It runs
3.5 turns which with a 1.5 inch drum
would give about almost 16 inches of
cable travel.

In a 60-120 size plane there should be
plenty of room behind the wing to
run a weight back 16 inches to the
tail. The servo wouldn't have to be
installed in the rear, just where it
or the cables would not interfere with
any other workings. Elevator and
rudder servos and their pushrods would
be installed close to the turtledeck.
Sail winch servo, cables weight and
return pulley installed close to bottom
of the plane. The weight could slide
on a carbon rod if necessary.

I was hoping someone had already done
this so I could get some feedback about
what problems were encountered. Since
no one responded with anything remotely
like this, its likely this will be an
all original design.

It will take some time for me to work
out the details and construct everything,
so don't expect it anytime soon, maybe
next summer if everything goes well.
I also have other ideas and projects
I'm working on, so it may be even later.

Jenny
Old 07-31-2015, 04:06 PM
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The IDEA has been discussed a number of times. But the only time I've read about it being used was on a flying wing plank style design. And there were special performance reasons for that related to soaring rather than aerobatics.

Don't think of it as tail weight being shifted back. Likely as not you will find that you need a big lump of lead in the nose. Moving nose weight back to under the wing works equally well as moving a mid location weight to the tail at shifting the CG. And it has the advantage that you're not adding anything that isn't already there. It's better to shift what you have to have on board than to add more to the wing loading.

I know the fuel tank is sitting in the way but nothing says that it has to be a single ball or cylinder like lump. If a couple of flat plate pieces slide along BESIDE the tank in tracks or pocket or on carbon rods if there's room then it'll work just as well as a single lump like lump on a rod. So keep the thinking free form and work at locating it forward.

I'd suggest keeping it high though. Especially if it's a mid wing style design. If it's a low wing then you can mount the sliding weight under the gas tank. The key here being that on a competition model were you're judged on the flying you will find that the vertical location of the balance is still fairly important. Not AS important as the longitudinal balance but it still affects how the model flies vertical lines and does knife edge. So keep all that in mind.

While the concept isn't new at all I do believe that you'll be one of the first, if not THE first, to use it on a big jumbo aerobatic model.
Old 07-31-2015, 10:56 PM
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bogbeagle
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I think that Hanno Prettner was doing this back in the 80s.
Old 08-02-2015, 02:53 PM
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If that proves to be the case then I'm wrong yet again.... That's OK. I'm used to it.
Old 08-02-2015, 03:29 PM
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chuckk2
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Originally Posted by Jennifer Curtis View Post
Has anyone seen some kind of variable
center of gravity setup? I'm thinking
of something like a sail winch servo
pulling a weight back to do wild aerobatics
and pulling it forward for normal flight.
Jenny
Long ago with a glider. The battery pack was mounted on a flat plate, and could be moved forwards or backwards
with a small electric motor and gear box made from servo gears.A cord and pulley arrangement was used.
I never saw it fly, so I don't know how well it worked.
Old 11-20-2015, 12:33 AM
  #11  
Wizz-Racing
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This has been done with sliding trays and using the batteries as the balance source. the way it worded was the servo was mounted to the sliding tray itself. Then use a 360 degree servo with a spool mounted on the servo shaft. Then attach a cable that is anchored at both ends, The spool rides the cable up and down. The tray has to be mounted on tube type push rounds used for control surfaces. I would advise you put limit stop switches at each end to avoid any over run tension. As you have no clue when its reached its limit.
Old 11-20-2015, 05:34 AM
  #12  
Jennifer Curtis
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Now that's the solution I've been looking for.

Thank you.

Having the servo mounted on the sliding tray
is the way to eliminate the pulleys and such
associated problems.

I'm thinking to make the servo travel reach
its end before the tray reaches its limit to
avoid the need for stop switches.

With nyrods sliding over thick music
wire rods, there should be no flexing
or binding.

The whole setup should be very simple,
with very little to cause failure.

The only things I still need to consider
are how to handle unexpected events
like bad landings. The sliding weight
could cause a lot of damage. The
mountings and cables may need
considerable engineering to handle
the loads without themselves causing
further damage.

I'm running it through my mind, and
possibly the best solution is to make
a cushion to catch the weight, rather
than trying to restrain it with stronger
cables and mounts.

Jenny
Old 11-20-2015, 08:34 AM
  #13  
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A few considerations for a movable weight device:

1. Moving a weight affects the inertia of the airplane as well as CG. Since you said that you wanted to be able to move the CG aft for "wild aerobatics", you would probably want to have the least airplane inertia in the aerobatic mode. That means the movable mass would need to be closest to the airplane CG during aerobatics. Thus the weight would be up in the nose for stable flight and back near the CG for aerobatic flight. If you move weight back into the aft fuselage for aerobatic flight, you will be increasing your inertia. That slows down any rotary responses in pitch or yaw. Not what you want.

2. A variable CG system will change both pitch and yaw stability, and have no effect on roll. Is that what you want ? Your neutral point in yaw probably isn't at the same CG as the NP for pitch. But the CG moves the same distance when the weight moves. So having moved the CG say, one inch, you might be at the neutral point in pitch but still quite stable in yaw. Something to think about.

3. A lighter and perhaps less complex alternative might be to add movable canard surfaces to the forward fuselage. Deploying horizontal canards up front would destabilize the model in pitch. And of course deploying a vertical up front would destabilize in yaw. By tailoring the size of the canards you could have just the desired stability reduction in pitch and yaw, no more and no less.

Dick
Old 11-20-2015, 08:54 AM
  #14  
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After a moment's reflection, I should add that you really don't need deployable canards. You can just leave the canards deployed all the time if they are free to pivot in the slipstream when you want to be stable and then locked up in a neutral position when you want to destabilize. Very simple and lightweight system compared to moving a weight.

By the way, I'm not suggesting canard surfaces that are controlled by pilot input. Just rigidly fixed surfaces. The canard surfaces aren't really for aircraft control, but to move the neutral point forward. When they are fixed, they destabilize the airplane. When allowed to trail in the slipstream, they have little or no effect on the airplane.

Dick

Last edited by otrcman; 11-20-2015 at 03:08 PM.
Old 11-20-2015, 04:16 PM
  #15  
combatpigg
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I've fantasized about doing this with a vertical take-off and landing model. I'd winch the battery pack "to and fro" with a clothesline style mechanism.
Old 11-21-2015, 06:54 PM
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Actually you're just changing where the plane wants to natural balance. Which is why you need to setup Duel Rates for either forward CG or rear. If you have low, medium and high on the radio that's an easy setup.

As I remember it. The setup used little springs on each end of the high strength Nylon string. As metal cable cuts into the spool over time. The servo was mounted side ways so the spool was on one side and the string as far left or right as lose to the fuselage just so no stray servo cables could wrap around it. You wrap the string around the spool a few times then the springs cause tension. The tray had 2 sets of tubes. You use the inner tube cut to length then threaded rods on each end secured to thin sheets of ply on each end. Then use nuts on the threaded ends with washers to support it being sure to install the outside tube first. Glue the outside tube to the tray and test with batteries and notice where the CG moves.

With todays radios you can use the dial knob for forwards and back and set the servo travel. That way you know where center, forward and aft are all the time.

Now this was in the early 80's and the guy was a pattern flyer. He was so anal about his planes. I once seen him cut the whole rear stab and elevator section off just forward of them with a band saw. He said it was too nose heavy and off center. So he made it longer. As this guy said adding weights to an airframe to make it balance was for imbeciles that had no idea about aerodynamics and adding rocks to fix it means they didn't build it right. I didn't argue with him. He out weighed me by 60 lbs!
Old 12-04-2015, 11:08 AM
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BerndBrunner
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look here:
http://www.rc-network.de/forum/showt...werpunkt/page2
http://www.rcgroups.com/forums/showt...199539&page=11

/Bernd
Old 12-05-2015, 09:02 AM
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Would a fishing weight drilled to slide freely up a piece of piano wire do the trick.
fasten the wire each end rigidly. Then with a long arm on a servo it can be pushed back & forth along the wire
If the plane is correctly balanced it surely not take a lot to shift the C of G aft
The main thing is not to have too much weight. I had thought of a similar setup with a threaded rod so I could screw the weight back & forth by hand to make small , more permanent adjustments. On a nose light plane I considered poking it out under the engine & winding it back back inside the fuselage to move the weight aft
Old 04-09-2016, 05:35 AM
  #19  
ron ward
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in principle, that would work,....any weight moving fore and aft affects the cg. in reality, the system probably wouldn't move a weight enough to make a discernible change in stability. the servo arm, even a long arm would not move the weight enough. moving the weight on a track or plate by closed loop or jack-screw is about the best way, provided the right servo is used and you like to fiddle with apparatus.
possibly a small two-way switch, run by a servo, that controls the rotation direction of a small motor to drive a jack-screw either direction might simplify the idea in that simple motors and small switches are usually allot less expensive than drum servos or 360 servos. it would also facilitate being able to move the weight or what is essentially an unlimited distance, only defined by the length of jack-screw used. end limit switches would, of course be needed.
this system came to me one day while trying to figure out how to trim a sail boat with equipment I had on hand. the jack-screw ran a block on it, that the sail's sheet was tied to.
so in operation, a regular servo controls a directional switch the controls which way the jack-screw motor turns the jack-screw, moving the weight fore and aft .
with this system, the jack-screw could be 6 inches long or six feet long, it didn't matter.

small pulleys or micro chain can then be used to control how fast the weight moves and/or increase the torque of a small, low voltage motor to rotate the jack-screw.
Old 04-09-2016, 04:28 PM
  #20  
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Why bother with the mild setup? You could also just use retracts that fold rearward.

I have flown a quickie which is normally quite tail heavy ( an 1/8" of elevator travel from neutral is full up) with the CG 5% further aft. It was a very interesting flight, since the elevator trim was too course on the Futaba. Pitching up or down was one click of trim, with no position that would fly level. So I gave up trying to trim, and just pulsed the stick to keep it flying. Since this was during a heat race, I did 10 loose laps and landed. Moving the battery pack forward 10 inches corrected the balance. Roughly 2 1/2 oz. In a 3 1/2 lb. airplane.
Old 06-14-2016, 12:21 PM
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perdo
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why use weight and and this and that to move them with. Put two extra tanks and a smoke fluid pump and just pump the fluid back and forth? Just my .02 worth.
Old 06-14-2016, 04:48 PM
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Jennifer Curtis
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That would work, but there would be no way to
have a proportional control on the transmitter.

Jenny
Old 07-05-2016, 08:10 PM
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chuckk2
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Played with variable CG some time ago. Two models stuck in memory. One, an Alpha 25 trainer, required so much added weight that I relocated the wing to the rear by about one inch. After that minor changes in downthrust and other trim seemed to be all that was really needed without adding the extra weight.

The other, a low wing aerobatic trainer, basically ended up needing a larger flight power Lipo. (4S4000, instead of 4S3300)
Strong clues are usually the elevator position when trimmed for level cruise throttle, or fail safe (deadstick)

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