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Balancing low-wing plane

Old 10-07-2005, 05:59 PM
  #1  
LeeHop
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Default Balancing low-wing plane

Greetings -

I balanced my Sky Raider Mach II. Wound up adding 8.5 oz to the nose. I know that's a lot but that's what it took. I did this with the plane rightside up.

A couple of old field geezers told me today I did it wrong. They say you have to balance a low-wing plane *upside down*.

How could this possibly be? Wouldn't the balance point be the same whether rightside up or upside down?

Tnx -

LeeH
Rightside up at the moment...
Old 10-07-2005, 06:17 PM
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Oregon Craig
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Default RE: Balancing low-wing plane

Upsidedown is EASIER, given the pendulum effect. Seems like it would be the same, though.
Old 10-07-2005, 06:22 PM
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Default RE: Balancing low-wing plane

Well you can answer that question yourself easily by turning the plane upside down and using the same CG measured points (3.5" from the leading edge but don't change what you have already done to see the difference). I think you'll find that it no longer balances out.
The old field geezers are CORRECT with their recommendations, I had to balance every Mach II our club has put together in the upside down position to get rid of all the extra weight needed compared to rightside up. Matter of fact, we're running o.s. .46 engines and most are balancing out real nice by just moving the battery pack around. I'm running a ringed o.s. 50 on mine and it's scary fast, no added lead to get good CG either when balanced upside down and the planes fly great.
8.5 oz. of lead is way to much in my opinion for such a lite weight plane.
Good luck

Old 10-07-2005, 06:25 PM
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BillS
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Default RE: Balancing low-wing plane

Likely your aging friends have reasons for the recommendations that are outside the realm of basic scientific principles. The CG does not change from right side up to upside down.

Bill
Old 10-07-2005, 06:58 PM
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Default RE: Balancing low-wing plane

Balance the plane upside down???

YES!!

That is the correct way to do it.Oh and near the fuse not from the wing tips!!

tailspin90

www.aircrafthobbies.com
Old 10-07-2005, 07:00 PM
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Default RE: Balancing low-wing plane

What about a mid wing?
Jeff
Old 10-07-2005, 07:39 PM
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Default RE: Balancing low-wing plane


ORIGINAL: BillS

Likely your aging friends have reasons for the recommendations that are outside the realm of basic scientific principles. The CG does not change from right side up to upside down.

Bill
Sure it does .. on this plane anyway. I have 2 sitting right here in the room with me and neither one will stay balanced when turned rightside up ( CG was done with the plane upside down ) after several attempts at rightside up. Rightside up required several ounces of weight to get proper CG and upside down required VERY LITTLE to none to get proper CG.
I noticed right away that if your fingers or stand is off the recommended CG marks by just a 1/2" this particular plane can be quite a bit off than when dead on the marks. Balanced close to the fuse, not way out at the wing tips ... just like tailspin90 posted.
Old 10-07-2005, 08:19 PM
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Default RE: Balancing low-wing plane

Bull.

Turn the space shuttle upside down and the center of gravity changes. Not likely. The damn thing would not fly with an indeterminate center of gravity. Your methods of measuring may change but the center of gravity does not vary from right side up to upside down. Think about it.

Bill
Old 10-08-2005, 11:51 AM
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Fubar-One
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Default RE: Balancing low-wing plane

The CG front to rear does not change. The CG top to bottom changes quite a lot.
Balance a low wing plane upsided down. Anyone tells you different is full of it.
I believe a mid wing can be balanced either right side up or upside down. Canopy, pilot, landing gear would be the variables. I have tried both and settle on whichever orientation gives me the most steady balance.
Old 10-08-2005, 12:20 PM
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Default RE: Balancing low-wing plane

Fubar has hit it spot on ... its about accuracy. Low wings need to be balanced upside down because the 'heavy top' causes the plane to be unstable as you are trying to get the CG checked. I balance my mid wing planes like the Laser 200 right side up as there is no difference. Of course try balancing a high wing trainer unside down and see what happens.
Old 10-08-2005, 08:05 PM
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BillS
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Default RE: Balancing low-wing plane

I prefer to balance right side up in a string cradle (front to back) with a suspended pendulum simply because it is convent, safe for the airplane (two inches off the floor) and allows laying parts on the airframe to check various conditions. Instability doesn’t exist using a cradle.

Bill
Old 10-08-2005, 09:41 PM
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LeeHop
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Default RE: Balancing low-wing plane

Bill -

Could you describe the string cradle a bit more? - sounds interesting -

LeeHop
Old 10-08-2005, 09:59 PM
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BillS
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Default RE: Balancing low-wing plane

LeeHop,

Here is a picture of a hand me down that was being checked for CG. The servos are laying on top of the airplane for a rough balance check and the airplane remained in the cradle for 24 hours while alternatives were being considered.

Bill
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Old 10-08-2005, 10:30 PM
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LeeHop
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Default RE: Balancing low-wing plane

Bill -

Thanks for the pics - they are very helpful.

Does the string run thru a nearly frictionless pulley at the top?

Does the pendulum hang from the axle the pulley is on?

If the plane is, say, nose-heavy, does the pendulum point to a location behind the desired COG?

Is the rear hanging point actually the leading edge of the horizontal stab? How does this affect the measurements?

Thanks for your help -

LeeHop
Old 10-08-2005, 10:52 PM
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Default RE: Balancing low-wing plane

Just a little something for 2 cents ... I only balance my planes for the maiden flight also its usually set slightly nose heavy, after that all balancing is done by flight trimming. I will get the plane into inverted flight and KE to see where the best compromose is. What I will do is to then move the RX batt pack about or to add some weight if necessary. This is still the best way to set your CG, again each of us like the plane in a certain manner. I remember when I first flew my F90, I had something like 8 oz on the nose, once I got used to the plane all that weight was removed. I guess the best way of setting a CG is still to fly her and not just taking measurements.
Old 10-09-2005, 05:56 AM
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Barry Cazier
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Default RE: Balancing low-wing plane

tIANci...You fly your planes?!?!?!
Thanks
Barry
Old 10-09-2005, 08:08 AM
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BillS
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Default RE: Balancing low-wing plane

LeeHop,

Suspension is from an open hook. The string is looped once or twice to prevent the string from sliding. The airplane should be at approximate flight attitude when picked up, which means that when the airplane is suspended it will be level.

The pendulum will always point to the GC.

The cradle is looped on both ends and the attach point (to the airplane) is relatively unimportant. A big loop goes around the tail feathers easily and doesn’t need to be changed for the next airplane. In the picture the loop went to the tail wheel bracket otherwise the loop slides up the fuselage.

The cradle works for upside down balancing for those who prefer. The airplane doesn’t know the difference.

Think about the process like a crane operator picking up an I-beam. The choke point (pick point) is important but the cable attachment points to the I-beam simply need to be towards the ends.

I use a string ratchet for suspension for convenience. Prevents tying so many knots.

Bill
Old 10-09-2005, 01:13 PM
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Default RE: Balancing low-wing plane

Barry - OOooiii!!! What are you trying to insinuate???!!! hahahahaha ...
Old 10-10-2005, 10:15 AM
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Default RE: Balancing low-wing plane

tIANci...Just trying to add a little levity to a serious topic.

Thanks
Barry
Old 10-10-2005, 03:07 PM
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Default RE: Balancing low-wing plane

The question of balancing a plane Right-Side-Up, or Up-Side-Down is really a matter of the design of the plane.

Aside from Center of Gravity, you need to consider Center of Weight (CW).

In the first figure below, if you were needing to balance the Red bar, you would hold the ends of the blue bar with the red bar hanging below (as pictured). But if you were balancing the Blue bar, you would invert it and hold it by the Red bar, wouldn't you? That's due to the "Pendulum Effect" that N70R mentioned in post #2.

Now if you tried to balance the Top bar by holding the ends of the bottom bar, it would try to flip over to get the CW below the pivot point.

So depending on where the aircraft's Center of Weight is you might find it's easier to balance it either Right-Side-Up, or Up-Side-Down

A High-Wing plane will (Except in rare cases) always balance better Right-Side-Up, because whether you balance at the wingtips, or next to the Fuse, the CW is always below the wing.

But with a Low-Wing plane, the CW will always be above the wing at the Wing Root, but at the tips, it may be below, depending on several factors, including the amount of dihedral.

So the bottom line is, You can't just say one way is better than the other, but with a little common sense and maybe some "Trial and Error" it's not too difficult to figure out which way works best for whatever plane you are working with
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Old 10-10-2005, 06:42 PM
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LeeHop
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Default RE: Balancing low-wing plane

Glide Balancing -

Heard of it? A friend at the field told me today to put your plane in a power-off glide with controls neutralized. If the nose goes *down* it's tail-heavy; it the nose climbs, it's nose-heavy. "Just the opposite of what you'd think," he said.

Gentlemen: your considered opinions please -

LeeHop
Looking for a balanced view...
Old 10-10-2005, 09:07 PM
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Default RE: Balancing low-wing plane

That test must be done after the airplane is trimmed for high speed level flight, and the nose must be pointed straight down. Putting the airplane vertical takes the horizontal CG out of the mix and any elevator trim that was needed to compensate for horizontal balance problems will cause the airplane to deviate from a straight down course. Elevator down trim needed to compensate for a heavy tail will cause the airplane to pull to the belly ( nose "down" ) and vise-versa.

It also only works with an airplane that is set up without up or down-thrust. Throttling back for the vertical downline takes the thrust line out of the mix as well and in this test you wouldn't know whether the elevator trim was needed to compensate for balance or thrust line.

It's a pretty crude test that only really tells you whether you have a problem or not.

Jim
Old 10-10-2005, 11:57 PM
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Default RE: Balancing low-wing plane

On your little 40 size plane, turn the plane upside down and screw a tiny eyebolt into the very center of the wing, and on the recommended CG. The teeny eyebolts are available at arts and craft stores for picture frames I think. Hang the plane from a string. It will amaze you as it appears to be flying upside down. You can check the lateral balance at the same time this way!

Ernie
Old 10-12-2005, 08:18 AM
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Default RE: Balancing low-wing plane

I've heard arguments from both sides for ever. I think the CG doesn't change, but balancing a low wing inverted is easier that rightside up. Because of more instability right side up, you're CG has to be right on. Within a mm tolerance or it will spill out of balance. That just my take. But here is my dilema. I have a 1/3 scale SIG Spacewalker. It is technically a low-wing, but it has such a deep dihedral that balancing inverted is actually less stable. I haven't measured the dihedral but if you were to draw a line from wingtip to wingtip, it would pass through the pilots head!! So, how would you balance it?
Old 10-12-2005, 08:54 AM
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Default RE: Balancing low-wing plane

garcay, the bottom line is: Balance it wherever it sits most comfortably (See post #20)

If I had to guess, I would say to balance it either right-side-up at the wingtips, or up-side-down next to the fuse.

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