abufletcher

Sure, I suppose I could build some sort of sturdy support structure from which to hang a custom balancing jig of the sort you described. But as I said, then the idea of your method being "easier" is out the window. The "weigh and measure" method just seems simpler to me.

Propworn

My Feedback: (3)

One structure build it to support the largest you will build then it will work for everything else. One balancing jig fits all, set the adjustable stop so the wing rests in line with the required balance point and suspend. Simpler than running around with scales and a chart where its possible to make a mistake. No calculations just hang it and observe. Everyone to their own of course. That's the great thing about this hobby find what works for you and use it. I make mine from 1X3 clear pine and use hinges with the pins removed and Hairpin clips for quick disassembly. A few minutes to set up on the table balance then pull the pins and everything goes neatly away.

Dennis

abufletcher

It would be great to see a photo of your rig. Maybe I'm just imagining something more complicated.

Propworn

My Feedback: (3)

The last rig was just barely big enough for a 1/4 scale Aeronca. I was intending to build one that would handle my new project a 1/2 scale cub. The nice thing is the design I have used has been scaled up several times from when I first started each time getting larger as the size of my models increased. Yet each time it will still work with the smaller ones. Perhaps it might be easier for you to get an idea if I were to show you what I intend to build for this new project. Yes it would still fit your table and work in your work area on your model in the pictures. There would be no problem scaling it to the size model you build.

Dennis

abufletcher

A photo would be very welcome. At the moment I'm drawing a big mental blank.

abufletcher

Looking around on google I found this fine example of KISS thinking.

Propworn

My Feedback: (3)

Your picture is in essence the same idea with mine working right on your bench top. Like you the room I have is limited and taking the bench apart is not an option. With my fixture I assemble the model on the bench and roll it forward. Slip the cradle on the wing and lift it a few inches until it sits at flying attitude with the wheels only a few inches above the table.

This is the basic design for the hanger. Fours sides are cut and in this case with a 32 inch cord the sides will be 1/4 ply and 3/4 pine block spacers between two sides making two cradles. The two holes are the balance points the top for suspended CofG points and the bottom holes for supported CoGs positions like on biplanes. The hangers should be balanced before use for best results. In Canada our models can weigh up to 75 lbs without having to get special permission to fly and I have confidence these two cradles constructed this way will support that weight. It is important to position these as close to the fuselage sides as possible.

abufletcher

That looks good. BUT from what are the cradles suspended? I assume you're connecting them to the ceiling with a cord. I'm not sure what sort of support beams there are in my ceiling.

The KISS principle inspired me to head to the local hardware store and think creatively. For $15 I got two sturdy traffic cones (not the flexible type) and two fixtures for hanging pipes with a rounded top (which screw onto the flat tops of the cones). For a further $5 I got a couple of wood slats to spread the load. When I got home I tested out the idea and while it's not perfect it looks like it will do the job (of visually double-checking the calculated balance). The tail-wheel is over a discarded (i.e. free) tripod with a adjustable height center column. I need to change the position of the slats a bit so that the weight is equally distributed along the pre-established CG location. But this looks promising...all for about $20.

abufletcher

Oh, and how would you use your cradle with the hole on the bottom (for biplanes)?

abufletcher

I added some cut-out to the slats so that they fit around the wing struts. This allows the centerline of the slat to be right on the desired CG (that way the slat doesn't affect the balance). The top of the fixture is then placed dead center on the slat. The CG position given in the manual is 124mm back from the wing leading edge. Generally I trust this because the recommended CG on the smaller 80" Paulistinha seemed perfect. A more conservative (for a first flight) position is at about 90mm back. I can't recall at this moment, but I think I worked this out as being the 25% MAC position. Obviously, I'll check again.

At the recommended 124mm mark the model is definitely nose-heavy...heavier than can be corrected just by moving the batteries around in the cockpit. It would be necessary to add weight to the tail. At the 90mm position, the model looks (to my eye) very close to having the right attitude. In this position the stab is level. But it's really hard to judge the overall attitude and I've been wrong about this before and have had some very scary maidens as a result. The photo below shows the model at the 90mm mark.

My balancing equipment seems to do the job nicely and it will be interesting to compare with the model after it has been balanced using the weight-and-calculate method.

Propworn

My Feedback: (3)

You have to pay attention where on the plans they print the balance point and whether it’s an arrow pointing to a surface or a circle with center point. Arrows usually point to the upper or lower surface of the wing. It means the model is meant to be balanced suspended from this direction. In the picture with the ladder the arrow most likely pointed to the top of the wing so it was easiest to balance upside down or you could use a set up like my drawing. If the arrow is below the wing then balancing from the bottom as you have done is correct. Sometimes with biplanes you will find a circle drawn somewhere between the upper and lower wing usually near the top of the fuselage. Very accurate balancing is done at these circles or designated points as center of mass. At these points as the aircraft rotates through dive to climb the calculated mass and lift dynamics will rotate about this point with the least amount of change.

If you extend a vertical line from any point of balance observe as you rotate your model through nose down to nose up that the line will move slightly along the top of the wing/wings. The center of pressure or lift of the wing/wings remain the same in relation to the airfoil but will move forward or back in relationship to this point.

Unlike full scale our models due to the light wing loading do not render this relationship as critical as in the full scale applications. That is why most plans/prints use an arrow to show a convenient place to support the model airframe to give an acceptable surface to balance from. If the arrow points to the bottom of the wing it should be balanced as you have it in your picture. If the arrow points to the top of the wing it should be balanced suspended as in my diagram or upside down as in your picture. In biplane applications statically the lower wing adds to the pendulum effect which changes as that lower wing begins to add lift. That is the reason with biplanes balancing at center of mass is more critical.

In regard to your question about balancing biplanes and the bottom hole a pin can be inserted and by resting on the lower pivot point with the wing raised up to a proper height on the cradle you may be able to match the center of mass point on the plans. If not then a flat plate with the proper distance would need to be made.

Dennis

Propworn

My Feedback: (3)

Well 90 mm works out to about 3.5 inches and 120 mm is about 4.75. Better to be nose heavy than tail heavy. I heard someone say when I first started out that a nose heavy model flies poorly a tail heavy may only fly once. I am looking at your tail wheel area and there is plenty of room to wrap lead solder around the wire struts. Thin solder works best cut 12 inch lengths bend a hook in each and start hanging them from the wire struts for the horizontal stab. Find out how many needed to move the balance point to 100 mm then 105 and 110 all the way to the 120 recomended. Fly it and add the lengths of solder by wrapping it around the wire struts moving the balance point rearward until you are pleased with how it fly's. Now you know where you need to set your balance point. Looking at your model it looks like the tail section comes off just before the servos. You could make up a battery box and fix it in place back there running heavier wire to the switch so there would be no loss of voltage. I had to do this with my Piper Pawnee. The battery was put in the fuselage right in front of the tail wheel.

Dennis

Rafael23cc

My Feedback: (6)

A lot of knowledge being spilled here. A very good thing.

In my mind, as a future full scale pilot. If it is good enough to balance an airplane that will carry my behind, it should be plenty good to balance a model. It also helps that I'm an engineer and I am very confident that math works.

Rafael

Propworn

My Feedback: (3)

I must admit I have never tried the balance scale method as I have always been able to balance mine suspended over the table. It’s only a reference point anyways as I have always moved weight around until I found the balance point that suits me. Sometimes a little more forward sometimes back a little. Very few have ended up where the plans called for.

Remember these are models not man carrying so if I balance mine to the point of instability the only thing I loose is a model.

I remember helping as a kid when I flew full size gliders there was only a central wheel and skid for landing gear and the home built gliders were suspended like Abuflecher has done with plates close to the fuselage on the underside of the wing. Seemed to work pretty good for them.

Personally I am a little bit leery using full size methods on models as nothing else scales down. Rarely do full size airfoils work well for models, if a 1/4 scale model weighed 1/4 the weight of the full size it wouldn’t fly the wing loading would be out of this world. Engine size, weight and power do not scale. Air speeds least of all do not scale. A 1/4 scale cub flying at scale speed would mean a cruise of 65/4=16.3 knots and a stall 33/4= less than 9 knots. Smaller scales even slower.

Sorry from many years experience engineering for engineering sake doesn’t mean much to me. Lots of so called engineer over complicate things when there is no need to. It’s a good exercise for them, it keeps them busy and while they are home surrounded by their calculations and instrumentation I am out flying. Thank goodness there is room for everyone in this hobby.

Dennis

abufletcher

My view is that I want different methods to confirm each other. I want to learn the weigh-and-measure method because it seems like that might be more practical for complicated (i.e. heavily rigged) WWI biplane models. But after I do the calculation I want to have some method for visually confirming that the calculated weight looks right on the balance. And if a model "looks right" on the visual balance setup, then it would be nice to run the numbers and find that they are as expected. If either method were out of sync with the other, I'd say there's a problem that needs to be looked into.

Oh, and it might not be my behind in the model, but I've got $500 in the ARF, with a $1,200 engine up front, with another $300 (or more) in radio gear. I'm not willing to "eyeball" a model with that kind of investment. Ditto on a precision scale model that I've spent three years (or more) building. Seriously, it takes less than an hour to make the measurements and run the numbers.

rtstestpilot

My Feedback: (2)

Smart and thumbs up!

Propworn

My Feedback: (3)

There is nothing wrong with double and triple checking. What you’re doing works for you. There is nothing wrong with your method.

I wasn’t being caviler about loosing a model just making a point that mistakes (they do happen) are not life threatening.

I took a brief look on the internet and there seems to be no known issues with the balance point stated in the manual. I would think if there were problems they would be all over the forums. Everyone seems to like how it flies.

If it were me I would simply set it up to balance as per the instructions and enjoy it.

Usually models of this size give a pretty good range for the CofG

You obviously have a pretty good handle on how you intend to accomplish the balancing act so to speak so I will sign off and wish you the best of luck.

Dennis

abufletcher

Dennis, I appreciate all your input on this thread and may well end up using your "cradle" method in the future (for example with my Albatros CI with it's thin airfoil and wire TE).

MajorTomski

Now that you know the CG, it would be an interesting exercise to do the weighing and run the numbers to see what numbers the equipment you had would have given you and to see how far the CG would have moved if you do the +/- 0.1 KG recalculations based on the tolerance in your bathroom scale

abufletcher

Neat diagram! You're right, it might indeed make for an interesting comparison to do the weigh-and-measure calculation using both my bathroom scale and the more accurate HF digital scale. If nothing else, it will give me some practice using this method in the 10 days or so before my trip to the US.

Again, being a stickler for words (after all I am a professor of linguistics), I always "knew" the CG...because it was printed right there in the manual. Someone somewhere decided that the CG for this model should be at 124mm back from the LE. All I've done here is see how the model hangs when I prop it up at that location. It doesn't quite "balance" at the indicated CG, specifically, the nose is heavy ("down") and the tail seems too high the air. It would probably be flyable like this.

If I want the model to hang level (or at least what I perceive as level), then I need to move the fulcrum point forward about one inch...or I can achieve the same thing if I leave the fulcrum point at 124mm and place a 50g weight on the tail. To be honest, I'm a bit torn between just using the CG given in the manual (124mm) and doing the maiden with the model in its current nose-down attitude or adding a bit of weight to the tail to get a better visual balance on the bench, which is in effect shifting the model more towards a tail-heavy state. Due to a couple of really nasty tail-heavy maidens in the past, I tend to be perhaps overly cautious and set up my models with a distinct nose-down "balance."

abufletcher

Hmm...unfortunately, the bathroom scale has an "automatic" tare feature such that if the model is already sitting on the scale when I turn it on, it displays as 0.0. So I need to turn the scale on (with the board in place) then get the model positioned on the scale (not entirely easy to do by myself) before the scale automatically turns off again (after about 30 seconds). I did manage to get a main wheel weight of 9.8kg and a tail weight of 730g, but I'm not at all confident of these initial weights. I'll try again and see if I come up with the same numbers.

(This is with the model propped up so that the stab is level.)

*****

The mains come out at 9.8kg each time. The weight on the tail wheel (with the stab level) seems to vary randomly between 728g and 731g. Three grams is not a lot but it's annoying that the weight changes even slightly from moment to moment (raising the tail, turning off the scale, turning the scale back on, then setting the tail down). This is giving me a total weight of about 10.5kg which is one kilo less than I was getting with the crude method of holding the model in my arms while I stepped on the scale (then subtract).

Anyway, I'll go ahead and make the length measurement and then using the spreadsheets to see how numbers work out.

abufletcher

Hmm...doing the length measurements isn't easy. Lots of room for mistakes to creep in. In my case, there's no way to put the nose of the model against a wall in my shop. What I did was align the end of the engine shaft with the edge of the building table using a large metal T-square. Getting the model to stay precisely in this location wasn't easy. Marking the location of the main wheel(s) was simple enough. Marking/measuring the location of the tail wheel, however, was harder since the total length of the model is longer than my building table. I had to have the tail resting on a board that was duct taped to a tripod head, which of course is also raised quite a bit above the level of the table. Getting an accurate measurement from the end of the building table to the middle of the wheel involved some eye-balling with a ruler. I measured it as 27mm extra, giving a total length of 1,847mm. Similarly, it's not easy measuring from the CG (on the underside of the wing) down to the table. Using a nut on a thread close in to the fuselage side won't work because the fairings on the undercarriage get in the way. If I measure from a point out on the wings, that point is no longer above the building table.

It seems like there's no truly simple way of getting a large model balanced. Both methods, either a visual method or a calculation, are in principle simple enough. But it's the practical details of doing either than make either one harder.

With the numbers I've got at the moments (numbers I'm not entirely confident of) the spreadsheet calculates a CG of 519mm back from the datum point. Very crudely judged just holding up a tape measure, that comes in at about the position I would expect on the wing.

abufletcher

I found a space (on the floor) where I could put the nose of the model against the wall. I've redone all the measurements and here they are, as close as I can get them.

Lengths:

Datum to main wheel (D1 on diagram above): 397mm/15.63in
Datum to recommended CG (124mm behind the LE): 532mm/20.945in
Datum to tail wheel (total length): 1862mm/73.30in
Main wheel to tail wheel (D2 on diagram above): 1465mm/57.67in

Weights:

At main wheels weighed together: 9.800g/345.68oz
At tail wheel: 730g/25.75oz

******

Result with spreadsheet #1 (which outputs a CG position from the datum): 499mm/19.646in

Result with spreadsheet #2 (which outputs a recommended tail-wheel weight for a given CG): 970.38g/34.229oz

*****

So the results with the first spreadsheet give a CG that is a bit more than an inch in front of the recommended CG (19.646in vs. 20.9.45in). The second spreadsheet calculates that the tail-wheel should weigh 970.38g instead of the 730g as I measured it. I need to ponder these results.

Propworn

My Feedback: (3)

dang double post again

Propworn

My Feedback: (3)

With the addition of pilot, passenger and luggage most of the weight is behind the empty CofG calculation in the full size. A forward CofG empty insures even with max gross weight the CofG will remain within design constraints. This is not needed with our models not much changes except weight of fuel if the tank is other than on the CofG so the model can be balanced at the rearward tolerance for a workable CofG. This is why they tell us to balance our models with an empty tank. Usually being in front of the balance point the weight of the fuel moves the CofG forward. Its another reason I have never considered using full scale balancing methods. We don't run into the same payload variables as you would in a full size.