thundertiger76

I have a Spitfire with a Super Tigre 90 in it and it's almost ready for testing but before I do I wanted to know the easiest way to get a rough idea of the C of G. Can anyone help with this?

Txmustangflyer

That small of a wingspan, the fingertip method is simplest.

Mark your cg point on top of wing then pock it up with your fingers on the tape, have someone else handy to keep it from falling if tail heavy..When it balances level on your fingertips you are very close. The tape lets you feel where to put fingertips. This is done while plane is inverted.

If it balances tail down, add nose weight etc.

thundertiger76

I understand but how far back from the leading edge do I go?

speedracerntrixie

My Feedback: (29)

I would go 25% to 28% of the wing chord at the root. This is a little on the forward side for that wing planform but the spitty has a slightly smaller horizontal stab provided scale outline was somewhat maintained.

Txmustangflyer

It shpuld also be stated in the manual or on the plans. What plane/kit is it?

As in, a great planes, etc.

thundertiger76

I got what you're saying and especially about the tail, I know it's better to be at least slightly nose heavy rather than it be the other way round, thanks for your help buddy :-)

thundertiger76

I don't know as it was given to me through a friend of a friend :-(

Txmustangflyer

Post pics, someone might be able to ID it and then its just down to finding the info online, or someone will have a copy if its not available online.

If thats a no go, liik at the wing from the tip in. The thickest part of the wing is typically where the spar is..Usually cg is at or right near the spar. If you can get it to balance slightly nose heavy with your fingers at the spar, it should be close enough to try and be controllable. If you need a bunch of trim on the levator to keep it level at half to 3/4 throttle then adjust until its flying sweet at maybe 1 to 2 notches of trim. Knowing what it called for doesn't just get you in the ball park, but in the infield. Depending on the quality of plane, the buildet, etc, being in the ballpark might be enough to then know where to adjust.
Since we are dealing with a bit of an unknown, having someone on a buddy box to lend a hand during a balance check flight might be a good idea..3d pilots are used to high alpha flying so they are actually the best "right seat" on stuff like this.

spitfire66

Kyosho made a spit that was 90 size. Compare the wingspans and compare.

My preference is for aft cg around 30% range. When I do this I reduce the throws so it is not twitchy to fly. I wouldn’t recommend forward of 24%. Forward cg requires higher airspeed to land and increases the chance for nose overs.

as recommended earlier, measure from leading edge to trailing edge at the fuselage and multiply by the percentage of cg you like. Don’t worry about the wing taper, this will be close enough. 25% is close to most manufacturer cg

thundertiger76

Well after all the help from you guys I decided to go balance it 127mm back from the leading edge. The wing from the leading edge to the trailing edge measure roughly 355mm in total. I have had to add 675g of lead in total to the nose.
I am a competent flyer so if you think this is close enough I should be fine.
What do you guys think about this?

ForcesR

You can launch the Spitfire at 127mm but it will not be in condition to fly again, it will be too tail heavy at that CG, 127mm is 36% of the wing cord, that's far too much!! Set the CG at 100mm which is 28.2% of the wing cord, the Spitfire will fly at that CG like it's on rails.

Roger

spitfire66

I agree. Get the cg around 110mm or 28-30%

thundertiger76

You reckon? Okay if you think that I may have to add more lead to the nose then in that case...

spitfire66

I am not there to see how the cg is working out, but I think the measured cg you provided is a little aft. Yes, more nose weight to get it in the range.

speedracerntrixie

My Feedback: (29)

I’d be interested in knowing the size and weight of the Spitty. Could be the ST.90 is not going to provide enough power. It looks a bit small in the picture. A bigger engine could solve a couple of issues in this case.

thundertiger76

Just weighed the whole thing including all the gear in it and it's coming out at about 10lb and the wing span is 64"
As I say it has a Super Tigre 90 in it.

ForcesR

Your Spitfire is similar to the Top Flite kit built Spitfire, the wing span is 63" and mine weighs 10.5lbs. The CG is set at 4-1/8" or 101.6mm IAW the manual and it flies like it's on rails with a Saito FA-100 engine. See page # 56 for CG in attached PDF TF Spitfire build manual. https://www.greathobbies.com/manuals...140-manual.pdf

thundertiger76

Oh that's good to know. Wow I really did go back to far with the C of G then, I'll have to bring it forward more for sure and get some more lead. Thanks for the manual I will have a look at that as well...

thundertiger76

So I wanted to try the Spitfire out today but I couldn't find anymore weight except for 100g more to add to the nose and I went with that.
So with 775g of lead to the nose in total at about 120mm from the leading edge and it seem to fly well although I had to trim a fair bit of up elevator in so perhaps it's still a little bit too heavy on the nose?

speedracerntrixie

My Feedback: (29)

Maybe. It’s difficult to determine CG location based off trim without first knowing your wing and stab incidences are. Example, if your stab is at zero incidence and your wing is between zero to 2 degrees positive, then I would agree that your CG is a bit forward. If your stab has some positive incidence then the up trim could be to compensate for that. Another tell would be the landing. A nose heavy airplane will want to land fast and require lots of up elevator.

thundertiger76

I can certainly see what you're saying. The landing wasn't too fast although there was a strong head wind, I'll have to have a few more goes first to decide what else to do, I may just try slightly less weight on the nose next time and see how it handles then...

speedracerntrixie

My Feedback: (29)

I would suggest measuring your incidences before making any changes.

thundertiger76

How do I go about that?

speedracerntrixie

My Feedback: (29)

The easy way is to block up the tail until you get the same measurements for the leading and trailing edges. Then check to see if the propeller is 90 degrees to the table surface. Then measure the distance from the table surface to the leading edge and trailing edge of the wing. Post the measurements and we cab go from there.

Txmustangflyer

If its taking a lot of trim..(how much we talking..more than two clicks?) Just slowly..ounce at a time max...back off the nose weight..c of g is tunable..kinda like tire pressures on a race car. Small changes until its comfortable for you in all aspects..take off, flight, and landing.

Incidences=setting control throws. Your control surfaces should move a certain amount in either direction. Usually measured in degrees..there are incidence meters you can purchase. Again..this is tunable..
If I'm thinkibg right, based on engine size, the scale of your plane is about a 1/6. So should be pretty similar to other 1/6 spitfires.
Hanger 9 markets a 1/6..so download the manual for it from tower hobbies..conyrol throw will be detailed as to how much. Its a starting point. You may find it seems too sensative..in which case..back them off..not sensative enough..nudge in a little more..but it will get you in the ball park...some popsical sticks and a protractor (measures angles) can be cut to make a quick incidence meter..just cut it in half, 90 degree line to the hole for the corner of the angle..make it a goid, straight, cut. Line that uo with your hinge line and rig a way to hold it to the unmoving part of stab, wing, or rudder...tape the popsicle stick so it points back over the trailing edge of the surface to act as a pointer..and use your radio to move the surface...

If you are way off the book..just move the servo end points in your transmitter until max stick is close to the hanger 9's manual...say 5 degrees...

Robart makes an incidence meter thats basically the above..go take a look at the picture of it to get a visual on how to set up a home made.

Again, this is tunable..if other than trim, you like how it flies..don't "fix" what ain't broke. Tune your cg..and enjoy it..but you may find as you tune the cg you have to dial back how much your elevator can move..the closer to perfect balance you get..the more sensitive its going to get to the elevator..and also..somewhat...to the ailerons..so go slow..when you get close to how you want it to fly..go even slower..like tenth or a quarter of an ounce at a time..
Its not going to take much..you'll be close on 3..maybe 4 oz of weight removed.