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Weight and balance on floats

Dan Gervae

FRIEND
Upper Peninsula of Michigan
What method is correct to do Wright and balance on floats? Do you use 4 scales? And where are they placed? Or, do you find center of buoyancy and place at empty cg? Looking for a little guidance
 
Easiest way (in my opinion) is to hang it from one scale and place another scale under the tail. You could also place one scale under each keel and one under the tail. Doesn't really matter as long as you have accurate measurements from the datum to each scale location.

Web
 
I helped do it once for a C-180 on EDO 2870's. Four electronic scales...two at the step and two further forward towards the bows...equal distances each side to the scale contact points from a lateral line on the hangar floor dropped from the firewall datum. Plane was previously leveled of course. Sum the weights minus any tare (blocks between scales and floats) x their arms and divide total moment by total weight = CG. That might be close to what happened but I'm no expert.

Gary
 
Easiest way is to put it on wheels and weigh it. Do you not have a wheels W/B? If so, you can calculate the new E/W on floats.

MTV
 
I'm helping a friend....I have a weight and balance on wheels...and one for this particular airplane on both edo 1320 and Baumann 1500s...he's putting Aqua floats on it and wants me to launch it off a trailer and give him instruction after the install and so forth...I'm concerned about flying a plane off a trailer that has not been previously flight tested in this config with these floats....and I want to fully understand the cg calculations before setting out....I asked him (and his A&P)if he would be willing to pull the wings and trailer it to the water and re assemble for test flights....seems a littler safer albeit a fair amount of work:) I may be too conservative....the Floats were on an F19 and are going on a converted Model 19...so its very similar. but....I'm the feller at the controls:)
 
He is submitting a 337 as I understand it...I do not know the details yet...still in conception.. Not sure on FAA feelings on this, but again, I'm not up to speed on his paperwork yet.
 
I'd like to know more if and when it's Field Approved or STC'd....I'm supportive but there's some issues. The original Model 19 with C-85 was certified by Taylorcraft under CAR 3 and TCDS 1A9 but floats weren't on the approved equipment list or required Flight Manual. Later Baumann added the Model 19 to their 1420/1500's but Aqua did not as far as I can determine.

Another issue is STC SA1-210 that developed via airframe and engine changes the "equivalent" of a Model 19 to be constructed from earlier Taylorcraft models certified under CAR 4 and TCDS A-696, 699, or 700. Gross weight on conventional gear was 1500# but there was no allowance for floats via that STC at that weight that I can determine. Under the same STC lower GW's were permitted like 1280 conventional gear and 1351 floats for EDO 1320 and 1400.

I've asked the current STC holder (https://dc65stc.blogspot.com/2018/12/stc-sa1-210-explained.html) about floats at 1500# for the Model 19 "equivalent" but haven't had a reply. Do your homework before flying.

Gary
 
Just put a piece of heavy wall PVC pipe under the floats, roll it back and forth until you find the balance point. That's the CG. Go fly.
 
What would the paperwork look like after doing that?

Gary
That would depend on whether you buy it by the sheet or the roll. 8)

Put the floats on a scale, what is the number?
Take off the landing gear, put them on the scale. What is the number?
Simple addition and subtraction, do the math.
 
Sky's roller technique takes off. Marks the contact points with the floats and distances to datum. Puts scales out of business.

Gary
 
Easiest way is to put it on wheels and weigh it.....

Unless it's going back on wheels, that doesn't seem so easy.
I'd say 4 scales, 2 under each float.
Measure the arms, record the weights, do the math.
Just like on wheels.
After possibly multiple calculated W&B changes,
IMHO actually weighing the airplane is a good idea.
 
Unless it's going back on wheels, that doesn't seem so easy.
I'd say 4 scales, 2 under each float.
Measure the arms, record the weights, do the math.
Just like on wheels.
After possibly multiple calculated W&B changes,
IMHO actually weighing the airplane is a good idea.

Sounds easy.....try it sometime.....

MTV
 
Just put a piece of heavy wall PVC pipe under the floats, roll it back and forth until you find the balance point. That's the CG. Go fly.

Not only will that find the CG, you can also get the weight. Just mark the balance point and move the rollers some fixed amount backwards, like a foot or so. Then hang weights on the tail until balanced again. Compare lever arm between cg and new pivot to the length from the new pivot and the tail weights.

Just remember to subtract the tail weights when done. I've weighed boats using this method and come to around 1-2% scale weight.
 
Using scales isn't difficult providing they are electric and connected to a central data collector-controller. It takes an overhead hoist and a means of leveling the fuselage. That's the time consuming part but four small scissor jacks on the scales would speed that up. A hard markable floor helps with establishing arm data. It's admittedly a rare event for most, but companies or owners that never fly conventional gear would find it useful when equipment changes.

Gary
 
Actually weighing the aircraft is something that needs to be done from time to time. But don't make it into something bad as it's not difficult when you make a game plan first.

Things I'm assuming: You have the log books and TCDS present. You have an A&P present. You have certified scales present.

OK. I'm going to just address an airplane on float gear AND out of the water. The easiest way is to have the aircraft under a hoist or a forklift. Hang a scale from the hoist and connect to the lifting tabs on the cabin roof. Raise the aircraft, by the lifting tabs just until the keels clear the ground. Go back to the tail and set up a stand,such as a barrel and enough blocks, so that you can level the aircraft, AS PER TCDS, with a scale on top of the blocks and under a spot on the tail. Note the weights on the scale on the hoist and the scale under the tail. If you need to, measure the distance from the Lift tabs to the datum and from the location of the tail scale to the datum. Now, let the aircraft down before it can fall.

Subtract the weight of any chains or lifting bars from the scale weight at the hoist. Calculate the CG by using; weight X arm = moment. And to make it an easy example we'll assume a datum forward of the spinner. Lets use made up numbers:
Weight at hoist = 1125 lbs and the lift tabs are 61" from the datum. 1125 X 61 = 68625
Weight at tail scale = 34 lbs and it is located 182" from the datum. 34 X 182 = 6188

Now add weights together and moments together 1125+34 = 1159 and 68625+6188 = 74813

And to get the actual CG (arm) divide the total moment by the total weight. 74813÷1159 = 64.55"

Check in the TCDS to see if 64.55" is within the min/max CG limits.

This is the CG of that aircraft, at that time. There are other details that will need to be addressed such as the fuel is assumed to be unusable only and oil is assumed to be at max. But these can be mathematically added or subtracted as necessary IF THE CG IS KNOWN AND ACCURATE.

Another detail that gets overlooked is that you can place the scales anywhere handy as long as you can get an accurate distance measured from the datum to the scale location. The datum is key as the TCDS lists CG measurements from that datum location.

Web
 
Most shops would have certified floor scales for weighing versus meat hanging scales. Actually we did what Web suggested to a Cub I owned. Hung it from a certified butcher scale borrowed from a local natural resource agency attached to a float plane bar, and also weighed the tail when level on top of a ladder (maybe on a bathroom scale). Took very little time but that was before the advent of electronic scales.

Gary
 
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