HAPPOMATOX

Hello Sirs,
anybody can help me to calculate with good accuracy the C.G. of my Spitfire that I have homebuilt from my personal project 1:5.5 scale and has the following wing dimensions:
Wingspan 80.71inches ( 2050 millimeters)
Width of the rib in the center between the two half wings 18.7inches ( 470 millimeters)
I have approximately calculated a C.G. to 5.5inches (140 millimeters) from the leading edge.
It might go well?I
Somebody told me that its position is about 20% the main rib, but it seems to me a bit exaggerated and maybe the CG is more advanced towards the nose.
Also you have to measure the center of gravity with the model upside down and main gears retracted into the wings?
Many thank for all good advices will give me.

Capt G

My Feedback: (8)

Use this: http://adamone.rchomepage.com/cg_calc.htm do it inverted with the gear retracted for best results

BobFE

OK, I'm not sure where on the wing you want to set the balance point, but I can talk about the rest. When you balance your plane you want it to be in the most aft CG condition the plane will ever be in while flying. If the landing gear retracts to the rear, then you want the gear up. The best way to tell if you should have the gear up or down is to balance it, then put the gear up or down and see if it changes the balance. Which ever configuration makes the CG move to the rear, then that is the configuration you want the plane in when you balance. If the fuel tank is aft of the wing then you want it full, if it is forward, then empty. It is better to have the balance forward than aft. A forward CG plane will be hard to fly or even get off the ground, but a plane with the CG too far aft will crash. I've always been told if the plane is a high wing then balance the plane right side up, and if the plane is a low wing balance the plane upside down.

All Day Dan

My Feedback: (5)

Here is another one. Dan.
http://adamone.rchomepage.com/index5.htm

Capt G

My Feedback: (8)

here's how it works http://www.youtube.com/watch?v=TnKRQhXlMgE

BMatthews

When you use those CG location calculators with the elliptical wing of a Spit you want to use numbers for a tapered wing that are a simple taper along the 1/3 chord line with the tip being 0.6 times the root. This gives a wing area that matches the elliptical planform closely enough for the calculator to work for you.

HAPPOMATOX

Thank you for your advices. I used a couple of formulas that you have reported me and in fact the distance from the wing leading edge and the CG point is more or less the same.
This is my first all fiberglass warbird by my own plans and buit from scratch.
Next Saturday I'll do the first take-off. It seems all right.
During landing I have to worry about not to overthrow the model?
What can you tell me about this?
Saturday on the runway we would have absence of wind, therefore 75°full flaps down and then ......?
The wing carriage wheels are 4.50 inches Dia. and they are out of the leading edge for the entire tread. Approx. 1 in.
Weight is 33 pounds, powered by DLE 55 RA gas engine with 23x8" Fiala" wood prop.
The max. static traction on the ground by dinamometer is 28.65 pounds.
Can I go to takeoff?

rmh

Plane should fly -
be sure the plane is flying ON THE GROUND , before you gently rotate to the air.
for landing - the same - fly level to the ground and touch wheels before letting tail settle Your wing loading is high -by any standards and keeping speed up to retain solid control, is necessary -
Those scale warbirds I have done in that size were in the 20-25 pound weight range and still required care on take off and landing.

buzzard bait

By the way, when you have curved wings and stabs and you want to use the calculators (which I strongly recommend) then the way to get the area is to take a piece of cardboard, rectangular, weigh it, cut out the wing or stab outline, or half of it, and then weigh it again. Of course it's proportional so you can use small sizes.

Want the location of the MAC? Take your cardboard wing half and make a guess; then cut it there, perpendicular to span. Weigh the two pieces. One will weigh a little more, so trim a slice off that one. Pretty quickly, you'll have your MAC.

Lnewqban

Any update?

alasdair

OR you can just balance your carefully cut scale plan outline of the wing shape on the edge of a ruler.
The chordline on which it balances is the MAC, and for a scale Spitfire (or similar) you want the CG at about 25% of the MAC from its LE.

alasdair

Sorry, this is the post that I meant to quote above, but I don't know how to undo it.

Incidentally, the cardboard cutout idea also works for wings with an unusual planform. The cardboard shape will always balance along the chordline whereat lies the MAC of the wing. Turn it round 90 degrees relative to the ruler and balance again and it balances at 50% of the MAC.
In the accompanying photos the MAC is marked in red, after being calculated using formulae in "Basic Aeronautics for Modellers", published by Traplet (who publish RCJI and Model World in the UK)

Notice that this wing has two panels, the outer panel having much more sweepback than the inner panel. The result is that the MAC starts behind the LE of the panel and extends behind the TE. The MAC does not all lie on the wing.