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thrust line on sea planes

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Old 07-04-2012, 08:06 AM
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ESSAR
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Default thrust line on sea planes



Hi I am building a seaplane similar to the ones attached. Could someone please explain to me the importance of an upwards thrust line on a seaplane? The plans I have call for 3 degree upwards thust line and I can't figure out what the purpose of it is. I am used to building downwards thrust lines on conventional prop driven planes so they don't climb when throttle is applied.
Any clarification on the sublect would be appreciated, Ilike to understand why I am building something a certain way.
Thanks,

Essar.

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Old 07-04-2012, 09:24 AM
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Default RE: thrust line on sea planes

The thrust line is above both the center of gravity and the center of drag. A thrust-line set at zero degrees will get leverage and push the nose down. This can be really scary if you abort a landing and try to go around because the plane will dive when you add power.

If you tip the thrust line up a bit, it has a lift component that helps keep the nose from plunging, AND the prop blast puts a little downforce on the horizontal stab when you add throttle, als tending to lift the nose and countering the pure thrust's (zero degree thrust line) tendency to push the nose down. When you have it right, there should be no pitch response when you add or reduce power.

A little extra right thrust is good, too, because the prop disc is running at more of a positive angle to the relative wind, so the right blade makes more thrust than the left. If you get the right thrust correct you won't have to punch in right rudder just as you lift off.
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Old 07-04-2012, 10:36 AM
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Default RE: thrust line on sea planes

Great reply Jim, Thanks for the info that clarifies alot.
Do you think that 3 degress is a reasonable amount of upward thrust?
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Old 07-04-2012, 05:07 PM
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Default RE: thrust line on sea planes

Yep.  3 degrees up is common for pod-mounted engines. 
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