In-flight CG adjustment

[ghoti]

Wanting to explore the flight behaviour of my slowstick at "two-mistakes high" altitude I devised an easy way to achieve moving the battery pack fore and aft about an inch, while in flight. It will be a while before I fly it yet. The key is to use a flat file to open the square holes in the black plastic battery tray holders so they slide freely on the square fusalage tube. A rubber band pulls the battery sled forward and the servo arm pulls against it, controlling the sled position. A kevlar thread links the servo arm to the sled. The sled has the usual velcro for battery security. My ESC will ride in the top side of the sled, moving with the battery.

[Tom Harper]

Neat and interesting, but why would you adjust the CG in flight?

[ghoti]

I want to explore dynamics of fore and aft CG. Slow flight wants one position, aerobatic flight wants the CG more to the rear. Where is the best place for maximizing drag to bleed off altitude, for example? I want to learn the symptoms of too much and too little up high, then I can better recognise what is happening during low altitude maiden flights. Like that.

[Ollie]

Uses of CG by the B2 to change stability.

http://www.charlesriverrc.org/articles/asfwpp/helmutlelke_asfwpp.htm

http://www.glide.dyndns.org/on-the-wing1/index.html
http://www.glide.dyndns.org/on-the-wing3/index.html
http://www.glide.dyndns.org/on-the-wing3/index.html

[Sparky Paul]

Fish, good luck! :)

[davidfee]

Very neat! I can think of some "interesting" things you could use this for. :) You might find that the rubber band puts too much load on the servo, causing a greatly increased servo current. I think it might be better to use a rigid pushrod instead of the rubber band and Kevlar thread.

[mtnmnstr]

I have been thinking this one over with a 3D pattern pilot friend. I came up with moving a 1 ounce weight back and forth on a threaded rod. Get a servo, remove the limiters and place the out put shaft to the end of the allthread and mount it. Move the weight FWD for some manuvers then run it AFT for others that requier a aft CG the run it FWD again. Also came up with the idea of the weight on a guide rod and have a servo with a 3" or so arm moving the weight back and forth, like a old wiper speed control.

Gene

[ghoti]

Gene: Putting a weight on a multi-turn lead screw requires 1) limit stops and/or some position indicator so you know where the weight is. A turns counter or something. My servo arm moves less than a full circle and the knob on the transmitter tracks its travel. I know where it is by touch any time I need to know.

[ghoti]

Ollie: Many thanks for the pointer to dynamic balancing. It is an awesome concept and right up my alley. I will spend a lot of time on that one. Bill

[John O'Sullivan]

The Swiss F3B (R/C international glider class) team had adjustable CGs in their very state of the art 12' Spartacus designs at the World Championships in the mid 80's.
It consisted of a weight on a threaded rod extending back inside the tailboom.
The idea was to launch and set up the flight using a conventional stable position for the CG. Then for the speed run task the CG was moved back to a point where pitch stability was neutral, allowing the model to track straighter. The idea did not catch on and the Swiss did not use it again. Obviously the complexity of the system was more trouble than the benefits (if any) that it gave.
As in the case of most things, simple is better.
John O'Sullivan

[Ollie]

I forgot.

A UK modeler used CG adjusting by means of a swing wing.

Ten years ago a Flying Wing (100" span) kit used battery position with servo to do CG adjusting.

Of course, hang gliders shift the pilot's CG for control in both pitch and yaw.

Most models with tanks shift the CG as fuel used up.

[Tom Harper]

Interesting, keep us posted on results!

[Andy W]

I've thought about this for tuning the CG on an existing model - a removable (and probably externally attached) system that could be used to push the CG back from the current (and hopefully stable) position, to see how much further you could go but still be able to restore it if it's too unstable. Mostly for F5B and sailplane use, but could be useful for pattern/3d as well.

My plan was to use a weight on a kevlar or similar string, running in a tube of some kind, and pull it back and forth by adjusting a slider or knob on the Tx. When you find the position you feel works best, land and balance the model. Now remove the gadget, and relocate equipment so it balances at that point again.
..a

[iiifly]

moving the cg forward as the aircraft gains speed enhances stability. this is where this will come in handy, but only in a really fast airplane. You can probably gain quite a few extra knots by doing this.

[Ollie]

Moving the CG forward loads the tail and increases the drag. The is how the tail load limits speed.

[Tom Harper]

Fish,

Are you flying this thing yet?

[ghoti]

Fish,

Are you flying this thing yet?

Not yet. I must wait for a replacement ESC thaI fried after my plane fell into water. (offtopic) If you ever wet your BL ESC, pull the battery off immediately and dry it FIRST before reconnecting a battery. It might save it.

Another couple of weeks, at most, and I will have a flight report for you.

Thanks all, for the rich stream of comments and suggestions! The wing-door dive brakes sounds very interesting, I may go with it in future.
fish

[ghoti]

Woops! That comment on wing doors was meant for another thread. Sorry about that. Fish

[ghoti]

Very neat! I can think of some "interesting" things you could use this for. :) You might find that the rubber band puts too much load on the servo, causing a greatly increased servo current. I think it might be better to use a rigid pushrod instead of the rubber band and Kevlar thread.

Davidfee: Good point. Today I replaced the rubber and kevlar with a ball joint at the servo arm driving a short length of bicycle spoke. Surely that small web of plywood will break open on the first hard landing? Pictures of this will appear soon. Fish

[ghoti]

Today I assembled another SS dedicated to this project. Had to cannabalize parts from my old Gentle Lady, but that means I will fly soon with moving CG. I don't want to cycle another mail order so am using an old 350 brushed motor.

New installation moved the servo to rear of front wing mount, for balance. Bicycle spoke passes through one of the two "windows" molded in the front wing support. For wing clearance I had to flip everything upside down putting the battery on top. As it is I get about 3/8 inch travel from center, in either direction. With a longer servo arm I could get more.

I chose the rudder stick control for this.It gives me a center position I can always return to but keeping it from moving while I work throttle will be a challange. This way my hands never have to leave the sticks and I like that. Fumbling around for a knob and leaving the stick is ackward.

The end of the bicycle spoke is not very visible. It is the standard mushroomed end bent 90 degrees to fit the bike rim. Here, it fits the hole in the plywood sled. That web of plywood aft of the sled it going to break easily. Next one I make will have a full 1/4 inch of wood all around the hole.

I hope the pictures come through. In the preview I am confronted with an invalid attachment message. Fish
Fish

[John O'Sullivan]

Seems a lot of moving weight to hang on a servo. A hard landing will likely strip your servo gears. Also moving your battery over this travel distance would probably only result in less than a 1/8" overall change in CG position, hardly enough to notice any trim changes.
John O'Sullivan

[Andy W]

Nice idea, Fish!
..a

[davidfee]

A longer output arm on a "servo saver" from an RC car will resolve the potential issues John mentioned.

[ghoti]

My test flight area is only 100 by 300 feet surrounded by tall trees so I can only do short flights. Yesterday I made three 20 second flights shifting the Lipo pack fore and aft. John, the short travel distance is sufficient. I have observed dozens, hundreds of times certain airplane behaviours but these remained isolated data points, one flight to the next. Balance point has always been a struggle for me. If off, which way? Now, with the ability to sweep through the whole range and watch the characteristics change, to meld one into the other, from too nose heavy to too tail heavy, is a truly reinforcing experience. "I thought that meant nose heavy-- now I know it is", I say to myself. When I can get this to the high school, where I have room to play, I will spend as long as I can with this. I love it as a teaching tool. John, I hope that small web of plywood will break before the servo gear does. It really needs a mechanical fuse, a breakaway link to save the servo. Now I just try to land gently. I hope to run it through this weekend. Thanks for your thoughts all of you! Happy landings. Fish

[HELModels]

ghoti,
I think this is very innovative. I apologize for the misinformation I briefly insisted on in another thread on reverse thrust. I realize that a prop still puts out thrust if all you change is direction of rotation. I admit it, I was wrong and it really is not a big deal because for the moment I thought I was correct. If it was so obviously wrong then it wasnt affecting your project.

I'll tell you when you should jump on someone for being wrong is when they believe they sense ignorance and intentionally tell you the wrong thing. It happens on here and it happens at the field, if you fly at a sanctioned field. I had some old fart tell me there is no boundary layer on the top of the leading edge of a wing. that is ignorant as he presents himself as an authority on anything airborn.

Beware of authority when it comes to this stuff and believe in your own ideas.

Keep on innovating!

[Tom Harper]

Well?.....anymore flights......need input!

[Bill Glover]

Mostly for F5B and sailplane use, but could be useful for pattern/3d as well.

... but on a Slow Stick ?!

[Sparky Paul]

"Mostly for F5B and sailplane use, but could be useful for pattern/3d as well."
... but on a Slow Stick ?!
.
As some of us folk get along in years, our horizons shrink to the feasible.. :)

[ghoti]

Well?.....anymore flights......need input!

I expected to fly this a lot on last weekend but I left my foam wings in a hot car and they warped horribly. Foam expands and tape does not when hot. Now I am again ready to fly and will give it another try today. I rearranged the SS and now use 3 cell LiPO so am shifting more weight than before with a full inch of travel.

I can tell you that the rudder stick is not the preferred channel to use. It is better to use a knob that will hold its setting (not spring loaded). This way, you can find the sweet spot, bring the plane in, and locate the balance point at that optimum place, move the battery until "normal" operation is optimized and leave it there.

I'll be back soon with a report. fish

[ghoti]

This makes leveling repeatable. fish

[ghoti]

... but on a Slow Stick ?!
Bill,
I have three functioning slow sticks, two carry cameras and one is for experiments like this one. The SS is the most versatile kid on the block. I am only an intermediate pilot and who wants speed? At 12 MPH drag is not a big thing and so you get to hang stuff aboard with great convenience. And it is big enough to see.
Finally, the wind is going away and flying weather has come. Hot dog!
Fish

[Bill Glover]

Not knocking the SS at all, just querying the benefit of something as sophisticated as in-flight CG adjustment on a rudder/elevator slow flyer! But hey it's something fun to try, so good luck! :)

[Andy W]

It's a mule, great platform for developing ideas.
..a

[ghoti]

Today I flew this variable CG bird and learned what it offered. First, moving the LiPO pack made less difference than I thought. Second, and most important to me, I can fly now and tell whether it is off balance, which way it is off, and have a good feel for how much to adjust when I change.

This technique would be interesting to play with as replacement for elevator control. Perhaps in a flying wing and for pitch control and that might require a rate gyro. (I use a rate gyro on the pitch axis of my camera planes). The old Airtronics servo I used worked fine here but seemed a bit slow to travel for quick pitch changes from CG shift. Who was it said some sailplanes change CG with a lead screw for enhanced performance? With modern electronics it should now be possible to remove the drag and weight of the stabilizer/elevator.

The sled and servo linkage worked flawlessly and smoothly. If you try this, the battery and motor wires flexing as the pack slides need to be anchored, tied off, for strain relief at critical places.

Now I use the servo to my next application: paper HLG drop from 300 feet.
Happy Landings. fish

[Tom Harper]

Good show fish! Are you going to build the weight stabilized flying wing?

[ghoti]

Good show fish! Are you going to build the weight stabilized flying wing?


I thought about that and it sure would be fascinating to pursue weight stabilizing. But my passion is AP and mostly I am interested in making a better camera platform. My next project will be to fly my SS using an extended length Lazy Bee aileron wing so I can have aileron/flaps-down or flaps up as spoilers, on my camera carrier. It will add about 4 ounces and increase wing area by 10-15%. I want to find a way to drop out of the sky, lose altitude and come through a tunnel of trees down to my feet. A "down elevator"? Dive brakes would do it, or crow mode. Perhaps I'll start another thread when I start that one. Thanks smart people for sharing your thoughts.

[Bill Glover]

I want to find a way to drop out of the sky, lose altitude and come through a tunnel of trees down to my feet. A "down elevator"? Dive brakes would do it, or crow mode.

I flew 14 lb IC AP planes in the 80s and 90s and used trailing edge airbrakes for exactly that. See pictures, my blue & white pattern ship had similar brakes ... you can see them a bit more clearly there.

Most people make the mistake of using flaps, I did on my first dedicated AP plane. They certainly allowed it to fly slower, but not to descend much steeper ... and it floated for ages in ground effect when you started to flare on landing. The orange plane had flapperons and separate air brakes and was extremely successful (the 8mm cine camera was strictly for fun).