Item 0744
DESIGN: SynchroLite ~
Fuselage - Tail - Forces on Stabilator
Drawing:
Notes:
Calculation parameters: Stabilator chord is 12". Downwash from rotor dist is 3 lbs. per foot [see
0719]. 67% of downwash acts on stabilator and 33% is lost over the edges. This give a force of 2 lbs per foot of span on the stabilator. On the drawing 10" of force line = 1 pound of force. (i.e. 2.36" is 0.236 pounds of force).No Yaw: Span is 24" on both stabilators.
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Span distance from centerline |
Vertical resultant force |
Horizontal resultant force |
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6" |
1.99 lbs. |
0.24 lbs. |
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18" |
1.89 lbs. |
0.66 lbs. |
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Total = 2 x sum of above |
7.75 lbs. |
0.0 * |
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* Left and right stabilators cancel each other. This results in a loss of lift of 2 x 0.896 = 1.792 lbs. The 1.792 loss is probably high since the greatest horizontal component is at the tip of the stabilators and here there will be tip loss. Also this loss will be much less in forward flight because of the reduced downwash from the center of the disk.
Full Yaw: ( 25 degrees of rotation) Span is 20.48" on high stabilator and 23.03" on lower stabilator.
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|
Span distance from centerline |
Vertical resultant force |
Horizontal resultant force |
Torque about X-axis |
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-18" |
2.0 x 0.71 = 1.42 lbs. * |
0.13 x 0.71 = 0.09 lbs. * |
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-6" |
1.92 lbs. |
0.54 lbs. |
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6" |
1.78 lbs |
0.9 lbs |
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18" |
1.48 x 0.92 = 1.36 lbs ** |
1.35 x 0.92 = 1.24 lbs ** |
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Total |
6.48 lbs |
2.77 lbs |
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* Fraction of square foot. 20.48" - 12" = 8.48" | 8.48 / 12 = 0.71
* Fraction of square foot. 23.03" - 12" = 11.03" | 8.48 / 12 = 0.92
Conclusions:
The yaw force from the stabilator at full yaw is 2.77 lbs x 6.25 feet = 17.31 ft-lbs.
The loss of downward force on the tail at full yaw is (7.75 lbs - 6.48 lbs) = 1.27 x 6.25 feet = 7.94 ft-lbs..
The resultant torque about the X-axis =
This will depend somewhat on the stabilators height above the X-axis. At present the torque looks pretty well neutral at full yaw. GOOD.
Date:
September 16, 2000 02:00 PMThe most appropriate stab airfoil, Scott, is the NACA 0012. Symmetrical, 12% thick, maximum thickness at 30% of chord and a nose radius 1.58% of chord. Gives plenty of depth for a spar and is an easy shape to fabricate.
The lift of an airfoil is strongly affected by the aspect ratio, the ratio of span to chord. Wind tunnel data of most airfoils is corrected to infinite aspect ratio; an actual stab might have an aspect ratio of 3 which would reduce maximum lift and lift slope to about 2/3 of the published value.
The lift you can expect per square foot for an 0012 stab with an aspect ratio of 3 for each degree of angle of attack is about:
40 mph----.27 lb
60 mph----.6 lb
80 mph----1.06 lb
And of course, a stab with an area of 10 ft² and a 10º angle of attack would give 100 x as much lift as indicated above.
Tip plates that prevent air from flowing around the end of the stab from high pressure side to low pressure side would increase the lift by about 1/3.
One of the more important parameters of an airfoil is the nose radius. A sharp nose will make an airfoil perform no better than a flat plate, cutting the lift in half.
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Last Revised: September 30, 2000