ESSAR

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.

JimCasey

My Feedback: (1)

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.

ESSAR

Great reply Jim, Thanks for the info that clarifies alot.
Do you think that 3 degress is a reasonable amount of upward thrust?
Essar.

JimCasey

My Feedback: (1)

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

jmb52760

Hello,

I am a lifelong model and full-scale aircraft person who is mostly into Classic Pattern. After a bit of a layoff from RC for, reasons, I am jumping back in. I'm assembling the Sebart Macchi Castoldi MC 72 ARF. I am a boater (21 ft Glastron with 5 liter Volvo Penta) and it will be fun to fly out of the coves while we are out having fun with the grandkids this summer.

I noticed a significant up thrust line built into the engine mount frames. Not being a Seaplane person, I suspected that this was due to the reasons cited in this thread but I wanted to make sure. It didn't seem likely that there was an error as the ARF is outstanding in design and build quality. Thank You Jim for providing that answer. It has been here waiting for me for ten years. I hope you are doing well.

If there is interest here, I can post details of the assembly and the equipment chosen and then provide updates when I get her flying. Sharing the experience on RCU has been enjoyable for me in the past as I have done a few build threads in Classic Pattern. It keeps me motivated.

If no interest, then farewell and enjoy the hobby.

Mitch

scale only 4 me

My Feedback: (158)

Mitch, Jim hasn't been active on the forums for about 8 years, but go ahead and post a thread when you do your plane,, I'm sure there are plenty still lurking to help answer questions you may have

jmb52760

Thank You for the reply!

easystar123

My Multiplex Mentor landing at the local lake.

allanflowers

I would love to see a thread on this project!