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Measure Right Thrust

Old 03-27-2007, 06:50 AM
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AVGJohn
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Default Measure Right Thrust

Am repairing a crashed P-40 and plans call for 2 degrees right thrust. Can anyone tell me how to check this so I know I'm at least close or a little over? Some place I remember seeing a formula measuring fin tip to prop tip diferential to get a close number.
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Old 03-27-2007, 06:16 PM
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BillS
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Default RE: Measure Right Thrust

I put a pin in the tail and use a wire to measure the difference from right to left to prop tip. Then knowing the prop length I calculate the angle.

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Old 03-27-2007, 06:59 PM
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AVGJohn
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Default RE: Measure Right Thrust

Thanks BillS. This is what I was thinking but wasn't sure. It should get me close enough.
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Old 03-29-2007, 02:47 AM
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hkbii
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Default RE: Measure Right Thrust

Howdie gents. I set the plane on its side then level the vertacle stab then mount a robart incidence meter to the prop shaft.Hkbii
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Old 03-29-2007, 06:35 AM
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Default RE: Measure Right Thrust

Using the prop is certainly the most convenient way. But props very often aren't square to the prop shaft. And they often have their tips cut or cast "in different places".

If you're using the prop, keep in mind that you're not getting really good accuracy. Truth is, you don't need it, but this warning is still worth the time to type it.

Before you rely on the prop, try this:
Block the airplane rigidly, and place a marker that matches up to one of the prop tips. Anything will do, just so long as it and your airplane won't move as you rotate the prop.

I've often shown this trick at the field with a cinderblock as the "immovable pointer".

With the "pointer" rigidly showing where one side of the prop tip is, rotate the prop 180degrees so the other tip comes to the pointer.

Doesn't always come around to the same place the other tip was, does it.
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Old 03-29-2007, 06:39 AM
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Default RE: Measure Right Thrust

BTW, make sure the reference point is exactly in line with the prop shaft if you want this simple technique to show you the truth. The prop should be parallel to the ground for both checks of position, in other words.

Tricycle gear airplanes don't have much problem, but you can stick something under the tailwheel to level the taildraggers so the comparison is more accurate.
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Old 03-29-2007, 03:16 PM
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Default RE: Measure Right Thrust

I like to remove the prop, and slip a one foot length of straight aluminum or brass tubing over the end of the crankshaft, using tubing of an inside diameter that fits snugly on the shaft. Then, I hold a long straightedge over the top of the fuselage, along its centerline, and check to see how the tubing lines up with the fuselage. I find that I can detect thrust line errors as small as about 1/4 of a degree by this method.
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Old 03-31-2007, 11:03 AM
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Default RE: Measure Right Thrust

The right thrust is built into the firewall of that plane. assuming the plane was built straight the first time, the crankshaft at the hub should line up with the center line of the plane. That plane produces alot of torque and p-factor, so even 2 degrees is not really enough but any more the that makes the plane look odd. right thrust is a convenience but not required. The red box P-40's did not have any right thrust and flew fine. You will need to know how to manage the rudder on that plane. Good luck in you rebuild and keep your CG forward.
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