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lateral balancing

Old 03-25-2010, 08:34 AM
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1fasthitman
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Default lateral balancing

I laterally balance all of my airplanes. The last time I did it, I got to thinking. Why doesn't the right thrust angle of the engine throw off the lateral balancing? The crank is not exactly on the center line of the airplane.
Old 03-25-2010, 11:03 AM
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Jetdesign
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Default RE: lateral balancing

Laterally balancing the plane using the propeller is not really correct as you came to understand; there are methods to do so in the air
Old 03-25-2010, 02:37 PM
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ARUP
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Default RE: lateral balancing

There is a difference in 'static' vs 'dynamic' balancing.
Old 03-25-2010, 02:55 PM
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landeck
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Default RE: lateral balancing

If you look at where the engine is mounted on the fire wall, it is shifted to the left of the center line so that the crank shaft exits the cowling on the center line because of the right thrust. I have been balancing this way for years and it has always produced a good approximation of the lateral balance point.

Bruce
Old 03-25-2010, 04:19 PM
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Default RE: lateral balancing


ORIGINAL: 1fasthitman

Why doesn't the right thrust angle of the engine throw off the lateral balancing?
The crank is not exactly on the center line of the airplane.
1fasthitman,

Short response:

Because the right thrust angle of the engine and the lateral balancing are acting on two different axis that cross each other at the CG.

Long response:

In level flight, and with no control input, a properly balanced (static balance) and trimmed (dynamic balance) airplane is in perfect balance or equilibrium respect to its center of gravity (CG).
All the static and dynamic forces and moments that are acting on the structure and surfaces become equal and opposite, just canceling each other.
It is as if the whole aircraft were hanging from that single point (CG) in a horizontal and steady position.

By convention, three imaginary lines cross the CG, which are called axes:

Lateral Axis: Extends crosswise, wing tip to wing tip. Rotation around the lateral axis is called pitch and it is produced by the deflection of the elevator at the trailing edge of the horizontal tail.

Longitudinal Axis: Extends lengthwise through the fuselage, from nose to tail. Rotation around the longitudinal axis is called roll and it is produced by the deflection of the ailerons located at the trailing edges of the wings.
When the pilot deflects the ailerons (control surfaces), the reaction of the airstream over the control surface creates a force.
That force is applied far from the CG (where the central point of the area of the control surface is located on purpose), which creates a moment or torsion force around the CG, and, as a result, that previous perfect balance is altered, rolling the model.

What happens when a model is not laterally balanced?
Any lateral imbalance locates the CG and hence, the longitudinal axis, off the central line of the fuselage.
That asymmetry makes different the distances between the axis and the center of lift of each half-wing, producing imbalanced torques or moments around the longitudinal axis that induce an undesired roll.

The magnitude of that torque depends of the linear force and the length of the arm.
Linear force = Torque / Arm length

If that induced and undesired roll is compensated by simply trimming the ailerons at cruise velocity to create a compensating roll, then the roll will reappear when the model slows down and the ailerons become less effective.
That is the reason for achieving lateral static balance before flying.

Vertical Axis: Passes vertically through the center of gravity. Rotation around the vertical axis is called yaw and it is controlled by the deflection of the rudder located at the trailing edge of the vertical tail.
Some forces created by the propeller can upset that balance around the vertical axis, inducing undesired yaw.
The principal unbalancing force is naturally created by the combined effect of the rotating propeller and the wing AOA: the P-factor.

P-factor is the asymmetry of the thrust force between the right and left halves of the propeller.
Increasing with the AOA of the wing, the right half produces more thrust than the left half.
The resulting total thrust force moves over toward the right side of the central line of the fuselage.
The combination of the thrust force and that distance of its extension to the vertical axis that crosses the CG, creates a moment or torsion force around the CG, trying to yaw the model counterclockwise.

What happens when a model has right thrust angle?
The designer has specified that right thrust angle in order to purposely create a clockwise moment or torque around the vertical axis that compensates for the counterclockwise moment or torque that is created by the P-factor.

Any thrust angle locates the prolongation of the thrust force away from the CG and the vertical axis.
The bigger the angle, the bigger the distance that is measured perpendicularly between the prolongation of the thrust force and the vertical axis.
The combination of the thrust force and that distance to the vertical axis that crosses the CG, creates a moment or torsion force around the CG, trying to yaw the model clockwise.

Ideally, both dynamic moments (from the thrust angle and from the thrust asymmetry), that are acting on the vertical axis, become equal and opposite, just canceling each other.
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Old 03-25-2010, 05:24 PM
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Default RE: lateral balancing

wellll alllrighty then
Old 07-11-2011, 06:53 PM
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Default RE: lateral balancing

I have a seagull cap232 with a zenoah 20 in it and it rolls real hard to the right on takeoff. I was told that if I add weight to the right wing, not left, it will fix the problem. This sounds backwards to me, but is it right ?
Old 07-12-2011, 02:47 AM
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airbusdrvr
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Default RE: lateral balancing

ORIGINAL: ralphp

I have a seagull cap232 with a zenoah 20 in it and it rolls real hard to the right on takeoff. I was told that if I add weight to the right wing, not left, it will fix the problem. This sounds backwards to me, but is it right ?
I would ask one of these "friends" to demonstrate that on one "their" airplanes before you try it.
Old 07-12-2011, 04:28 AM
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jbdismukes
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Default RE: lateral balancing

Ditto! And just for grins and giggles, videotape the demo.
Old 07-12-2011, 04:55 PM
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Default RE: lateral balancing

Where do you put the weight?
Old 07-12-2011, 05:43 PM
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Default RE: lateral balancing


ORIGINAL: ralphp

I have a seagull cap232 with a zenoah 20 in it and it rolls real hard to the right on takeoff. I was told that if I add weight to the right wing, not left, it will fix the problem. This sounds backwards to me, but is it right ?
That is not wrong;.......it is absurd.

Do you know how to balance the plane laterally?

Is it balanced now that it rolls hard to right on take off?

If so, you may have more left than right wing area.
Old 07-13-2011, 06:18 AM
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LesUyeda
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Default RE: lateral balancing

"If so, you may have more left than right wing area. "

Or, more likely, twist in the wing.

Les
Old 07-13-2011, 07:31 AM
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Default RE: lateral balancing

Hi!
Yeah!
If it rolls to the right it has either a twisted wing (check it with a Robart meter or just eye balling it) or... is not latterally balanced (ad weight as far out on the tip as possible

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