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Rhondas
02-19-2006, 10:41 AM
I wanted to convert a gp supersport arf into a electric, the write up in the tower catalog states its cg should be at 3.1/8 from the leading edge to the back of the wing, It also says if you want good stunts move the cg forward a bit, now isnt that going to make the plane more nose heavy,I thought for stunts you would favor the cgs back making it tail heavy, [what does moving the cg forward actually mean? and when I convert to electric will it make a difference because it will be much more heavier with the nimh cells Im using.

flyranger
02-19-2006, 12:40 PM
In my experience that would have to be a typo. Moving CG back makes the plane more responsive to control inputs. Moving CG forward makes it less responsive and tamer to fly. Since you already have the plane (?), why not run the numbers through the "Center of Gravity Calculator" at the link below. I do this for all my planes, especially the ones I modify.

http://sky.prohosting.com/air2/cg_calc.htm

Dereck
02-19-2006, 07:50 PM
I wanted to convert a gp supersport arf into a electric, the write up in the tower catalog states its cg should be at 3.1/8 from the leading edge to the back of the wing, It also says if you want good stunts move the cg forward a bit, now isnt that going to make the plane more nose heavy,I thought for stunts you would favor the cgs back making it tail heavy, [what does moving the cg forward actually mean? and when I convert to electric will it make a difference because it will be much more heavier with the nimh cells Im using.

This is going to hurt

It's going to be very, very painful

Go read some books!

Find all the books you can on model aerodynamics ( the full sized ones tell you similar things, but they really will make your head hurt :eek: ) and read them. Martin Simmons is real good, as is the Canadian model aircraft designer (who's name I will wake up screaming at 2AM after I remember it ...)

There might be web-based info on model aerodynamics, but I don't know of any off-hand. If you find any, be wary of what they cost you to obtain - the information might be worth every pennycent you paid for it...

A CG placement is a compromise - sometimes between a flying style, sometimes between the number of pieces you take the model home in...

Basically, forwards leans to safer, backwards leans to un-safer (???) - though it happens in degrees (or at least until you enter what the full-sized boys call a 'departure from controlled flight' - which we usually call a crash).

To make matters worse - maybe they hired a proofreader from Outer Patagonia who's learning English from the guy who rents a room to him and what they really meant was 'backwards' not "forwards". Even the mightly Tower Hobbies conglomerate has failings now and then.

If you want trimming for aerobatics in close to sound bites, there's a column on pattern flying in "Flying Models" magazines (like an RC Mag, but for intelligent people ;) ) that covers the incredibly complex game of trimming for FAI standard aerobatic flying.

Moving the CG means you move the position of where the model would hang level from if you screwed a hook into the top of the fuselage and hung it off the ceiling. The reference line is usually close enough to having the tailplane perfectly level. Most of us substitute holding the model on two finger-ends and eyeballing that it looks about level. Amazingly enough, this works for a lot of us...

If you add weight to the tail end, that balance point moves backwards, if you add noseweight, it moves forwards. This phenomina doesn't care if you move the battery, add lead or hang a sneaker off the prop, BTW.

Adding the weight of a battery pack causes a bunch of other things to happen, mostly in the wing loading department. The principle Bad Thing that can happen is that your stall speed increases, alongside of which your spin entry speed and snap roll characteristics will change - add enough weight and you won't like the changes much either.

Last idle thought to ponder on - the laws of aerodynamics have no appeal court. Break them at your peril.

For practical level advice - build the model as per plan and start with the CG accurately placed 1/4" ahead of the foremost design position shown on the plan . DO NOT fly with the CG aft of the design position shown on the plan, not the website, table-napkins or other odd places found on the web.

If you think it's real risky - you might be one of those folk the computer game was created for. The biggest kick in this hobby is not the arrival of a big, shiny shrinkwrapped box, it's opening the throttle on a completely untested own design model for its first flight

Regards

Dereck

bassmanh
02-19-2006, 11:39 PM
hi,
could they be saying moving the CG forward as in from 3 1/8 to 3 in ? (torward the front of plane) wich would make the plane more tail heavy ? just a thought ;)

bassman

Matt Kirsch
02-21-2006, 06:37 PM
Actually, bassman, you've got it backwards. When you move the balance point (aka CG) toward the front of the wing, you have to add weight to the nose of the plane, and/or shift onboard equipment forward. This makes the plane more NOSE heavy.

It was a simple mistake in the documentation.

A good starting point on the CG after a conversion is the same point as it was before the conversion. From there, adjust the CG as necessary to bring out the desired flying characteristics of the plane.

Remember that on a glow plane, we normally balance tank-empty, and the tank is in the front of the plane. Any fuel load will actually make the plane more nose heavy than what we balanced for. The idea is that the plane is still flyable if you accidentally run the tank dry. It actually might not be a bad idea to balance the plane a little nose-heavy for the first flights after a conversion, just in case the designer was pushing the envelope with the CG range.

bassmanh
02-21-2006, 07:07 PM
matt,
a simple way to show what i mean is to take a pencil lead side as the nose of the plane, balance it on your finger now measure then move the balance point TOWARD the front of the pencil (plane)

what happens ??? the tail end drops. i dont mean add weight or move things to the front. just a way to make the plane more tail heavy by moving the CG point towards the front of the plane as indecated by the first poster and what the instrutions say. IE... moving the CG forward on the wing from 3 1/8 to 3 in.


bassman

inconel710
02-21-2006, 10:54 PM
I don't follow that logic, Bassman.

The Center of Gravity (CG) is determined by the distribution of mass in the plane. If you haven't moved any mass in the plane (battery, lead weights, whatever) you have NOT changed the CG.

Matt and Dereck have the correct answers.

bassmanh
02-21-2006, 11:46 PM
;) ok im not TRYING TO CHANGE THE CG im simply saying that there manual might be SAYING TO change the CG location to get better airobatic performance. hope this clarifies what im trying to say. ;)

most instruction manuals give a CG range 3 to 3 1/4 in or what ever it may be, if you move it to the 3 1/4 in mark it will be more nose heavy (more stable)
if you move it to the 3 in mark it will make it less stable , IE more snappy lets call it , or more airobatic. make sense that way ? i hope LOL


bassman

DIVER DON
02-24-2006, 11:19 AM
Don't think so.
Bassman you have it backwards.
If you move your CG Mark to the 3" point ,and then balance the model level, you will have added more weight, thus the model will be on the nose heavy side.
This will make the bird more stable.
You have to pick the point and then move things so you balance to it.
Try it with a piece of wood and a string and you'll see what I mean.
Hope this helps.
DD

Matt Kirsch
02-24-2006, 01:53 PM
bassman,

When they say, "change the CG," they mean change the point where the plane actually BALANCES, not the point where you check the balance.

If you simply move your fingers toward the front of the plane, and do not add/remove/relocate any mass, you aren't changing where the plane balances. It still balances in the same location. It's no more tail heavy than it was before.

bassmanh
02-24-2006, 03:07 PM
NEVER MIND MY FEEBILE MIND NOW HAS AN IDEA OF WHAT YOUR SAYING LOL thanks for the help and clarification.



the lost at the moment BASSMAN LOL

cyclops2
02-25-2006, 09:11 PM
The COG of the WING.

COG is the SPOT on any wing where it flies the most stable.

It can not be changed. There is only one correct spot for each shape.

You can put the same wing all the way foward, or at the very rear.

The COG spot remains in the same place.

The " balance point " of the plane can be moved around to change the flying characteristics of the plane. It is usually close to the wings COG.

That is how I was told.