Cg & all

[HansLab]

All, I have a question:
Now that the PH-FOX has flown some 10 hours, it is time for me to solve this queeste. The CG is pretty forward. The plane is build per manual. Trim with the flapperons.
Now when I fly on my own, the flapperons are some 8+ degrees UP.....And she's in perfect harmony: she flies hands-off. But there is much drag from the flapperons that way. I lowered the elevator one hole (one to go), and it made some difference (not much). Flying two pob I trim more aft, and the flapperrons are almost 0- degrees, hands-off.
Now I wonder: shouldn't it make difference to move weight aft? And if so, what to choose.....the plane is build per manual, you guys don't have the same, have you? And there's added weight in the back: rudderhorns and elevator and stabilo all have profiles, the ACL techniques are in the back - what can I do?

[Av8r3400]

Hans, is your plane a tail dragger or tricycle gear?

[Danzer1]

His photo album shows taildragger, model IV, speedster wings and with Grove gear and a 912 UL - assuming we are talking about that aircraft.

Hans - please clarifly. And it would also be helpful if you posted your empty weight and balance numbers.

[HighWing]

Hans,

Lots of thoughts here. Where is your battery? This might not be the simplest solution, but if your battery is forward of the forewall, moving it aft would help some. You can check the results by manipulating the CG spreadsheet.

When I built my recently completed Model IV, I moved the engine forward 2-1/2 inches to help compensate for the fully ribbed and faired empennage with ruddder and elevator trim - I did the math. My final CG calculations put me pretty much where you are and like you I wasn't quite comfortable with it for a number of reasons. My battery was already behind the firewall and I didn't want to mess with a new mounting tray and the run of the wire so left it there. What I did is prepare some lead that I could bolt to the tailwheel spring bolt. With an arm of 11 feet, it was the best location to put the least amount of "dead" weight. Future plans call for a pneumatic tailwheel which should eliminate most of that weight.

A friend who built a Series V with O235 decided to fix a tool box to the aft edge of his baggage floor and filled it with tools he thought he might need. It was placarded as mandatory when not carrying baggage. Then when he was on a cross country with significant weight back there, he felt free to lighten that according to his CG needs. Just some ideas.

[HansLab]

Thank you for thinking ahead!
The FOX is a taildragger, the battery is behind the FW: cockpit-side. Enclosed the drawing we made during W&B, before I attached the wheel pants...(behind CG).
Measures in millimeters and kilograms (!)
Adding lead is an option - but I consider it as one of the last: firstly this bird isn't one of the most light-build, and secondly I can't stand having build an airplane that isn't balanced whilst building per manual.

[t j]

Hans, am I correct that your Empty weight is 327 kg and empty CG is 285 mm? For us that is 719 pounds and 11.2 inches.

For a reference point, My Classic four is 547 (249 Kg) empty and empty CG is 12.5 inches (317 mm). First flight with horizontal stab in top hole required slight forward stick pressure for level flight. I have no + (up) flaperon rigged. Moved horizontal down one hole and level flight was hands off.

Everything that is put in the airplane (passenger or baggage or fuel) moves the CG back. I have no need for trim solo up and only slight down flaperon trim for level flight at all combinations of loading weight.

I would do some test flights with increasing weight in the baggage sack to get an idea of how much weight you need and where. When you find the weight needed in the baggage to fly hands off compute the CG arm with only that weight in the airplane. That will give you a new target empty CG arm to shoot for.

For what its worth. A friend built an RV 7 with same problem you have. His solution was a 5 gallon collapsible plastic water jug behind the back seat for solo. When he picked up a passenger or cargo he dumped the water out of the jug.

[jtpitkin06]

Hans,

Let's say you decide to move a 13 kg battery from above your knees to behind the seat... about one meter. That will shift the empty weight CG 40 mm aft. It will also add weight to the aircraft because you will need longer battery cables and other wiring. It's a lot of work.

Adding weight to the tail spring bolt is 13 times more effective. So if moving the battery added only 1 kg of new cables you could accomplish the same CG shift without moving the battery and adding the same one kg to the tail.

Adding weight to the tail is simple and you can easily change the weight after a flight test to fine tune the amount needed.

John Pitkin
Greenville, Texas

[HansLab]

Thank you all for thinking with me.
Adding weight was something I simply did not think about..;-(
Looking at the CG drawing I calculate that there should be some 2,5 - 3 KG's added on the tailwheel......;-( to get the flapperons more or less 0 degrees per manual.

Does that much sound familiar to you? And some practical question: how do I get it there - and hold it there...?

And another one: how did the nose-weight get there? There's a quite common 912 with oil cooler and Grove gear in the front, but that's about it. No IFA stuff, no thermostats (yet), battery behind the FW....:confused:

[t j]

In need to change my post (#6) from First flight with horizontal stab in top hole required slight forward stick pressure for level flight. To required slight back stick pressure for level flight.

[t j]

I calculate 3kg on the tail wheel will bring the empty CG back to 319 mm (12.5 in.). That sounds good to me.

When people were putting Subaru engines in model 4's I remember some reports of empty CG arms in front of the forward CG limit. Of course they would sweep wings forward and install the battery on the tail to get the loaded plane within CG limits.

[HighWing]

Does that much sound familiar to you? And some practical question: how do I get it there - and hold it there...?

Hans,

I made a box out of thins steel sheet and filled it sequentially with lead. I welded a piece of tubing over a drilled hole for the bolt and then added sides that fit within the fuselage structure. You can make the vertical height as tall as you think you may need. A thought on the lead: Another approach rather then melting and pouring would be to use some
fine buckshot from the hunting supply store and after finding how much you need, pour some epoxy resin in to saturate it and bind it to the walls of the box.

Diagram

[mr bill]

I did exactly that (lead bird shot and epoxy adhesive) on my first homebuilt in 1974. I worked very well. Airplane was in service until 2011.

[SkyPirate]

I did the same thing with lead buck shot but I put it in the aft vertical tube of the stabilizer

[vixenrookie]

ItemWt. (Lbs.)Arm, (in.)MomentLeft Main, (- tare)355.0024.008520.00Right Main (- tare)338.0024.008112.00Tail Wheel (+ tare)0.000.000.00Nose Wheel (-tare)245.8038.509463.30TOTALS938.80 ^ ^ ^ ^ ^ ^ ^ ^26095.30 EMPTYWEIGHT CG, (in.) =27.80

couple questions this is vixen 5.
the wings are sweep 3 degrees foward, how do i add that factor in?
if the cg are over the factory limit will it affect my airworthiness cert?
do i need to make the added weight adjustment before inspection?

[jiott]

I believe you must have a valid W&B document when you apply for your AW inspection.

[t j]

vixenrookie, Just trying to double check your math. In can't figure out your nose wheel arm and moment. Did you forget the nose wheel arm and moment are negative numbers?

The 3 degree foreward sweep will move both the foreward and rear cg limits foreward. You may need to contact the factory to find out how far forward they moved.

[vixenrookie]

ItemWt. (Lbs.)Arm, (in.)Moment
Left Main, (- tare)355.0024.008520.00
Right Main (- tare)338.0024.008112.00
Nose Wheel (-tare)245.8038.509463.30
TOTALS938.80 ^ ^ ^ ^ ^ ^ ^ ^26095.30 EMPTYWEIGHT CG, (in.) =27.80

see if thats easier to read, that factory sheet shows 1 degree, foward or backward.

[t j]

I interpret it as:

left main: 355 pounds X 24 inches = 8520
Right main: 338 pounds X 24 inches = 8112
Nose wheel: 245 pounds
Total: 938 pounds

The numbers for the nose wheel arm and moment are 8038 and 509463. Something's wrong there.

[t j]

Okay, working the math backwards off the total moment I figure your nose wheel moment is 9463.

I would do the math as (8520 + 8112 - 9463) divided by 938 = empty weight CG of 7.64 inches.

[vixenrookie]

i got my numbers off the kitfox site where automates the numbers for you. im not real familar in understanding the weight and balance,

heres where i measured and weight
center of measurement was front of the wing to center front wheel was 38.5 inches
front of the wing to center right and left wheel was 24"
front of the wing to back of prop hub 53 5/8
right 338 lb
left 355 lb
front 245.8 lb

the wing sweep is 3 dregrees foward

little history: my father-law gave this and learning as go ,getting to the point of finally assemble now the math comes in and its like teaching a rock to roll over. i greatly appciate the help on this.

[HighWing]

A question for all. My understanding is that an airplane is measured for W/B with the airframe in level flight attitude. In conventional gear airplanes this is accomplished by raising the tail so that the longerons below the cockpit are level.

The question - is a tricycle gear airplane in level flight attitude while on the ground or are shims placed under the tires to achieve level attitude before measuring for wheel weight and arms?

[t j]

You need to understand how to calculate weight and balance before you use the automated spread sheets to do it. Find one and only one person to show you how. Any two experts will start arguing about what is correct.

Here's how the empty weight CG is calculated. Don't feel bad about it, I know at least one major airline captain and one A&P mechanic that can't calculate a weight and balance for a single engine airplane by hand. When an error occurs using a spread sheet they can't figure out where the mistake is.

[vixenrookie]

i think i figured out where i went wrong, no negitive arm on the nose! that what was throwing me for a loop. thanks so very much for smacking my marbles around.

[desertfox1]

Hi Lowell, Yes I shim the tires to obtain level for weighing.
Sometimes it's the nose wheel and sometimes the mains.

Phil

[RobS]

This is a good document from the FAA on all things regarding "W&B" - http://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/media/faa-h-8083-1a.pdf .

The doc suggests reducing the pressure in the nose wheel strut to level a plane. I would suspect if there is no oleo strut, shims would work well.