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Why is my motor pointed to the left and up ?

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Why is my motor pointed to the left and up ?

Old 07-20-2021, 10:35 AM
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Default Why is my motor pointed to the left and up ?

Hi - my first post here - looking to better understand some trim dynamics .....

I have an Albatross (46" flat bottom wing) from crashtesthobby with a pylon mounted motor.
(sorry - not allowed to post a picture yet)

In the nose mounted configuration - the motor is angled to the right and down. This makes sense to me.

In the pylon configuration the motor is angled to the left and 2deg up (from the fuselage line).
This configuration was apparently arrived at, empirically, by crashtesthobby.
The pylon is forward of the CG and the motor is about 4" above the top of the fuselage.

From my reading - I imagine the upward angle of the motor is directing the prop wash down toward the horizontal stabilizer and aiding in preventing down pitch due to the cantilever of the pylon. Is this a reasonable / likely explanation ?

I cannot comprehend why the motor is angled to the left. Can someone assist in my understanding by explaining the dynamics of what is going on here ?
In both the nose mount and pylon mount - prop rotation is clockwise when seen from the rear and the motors are in the same forward facing orientation.

Thanks in advance.


Old 07-20-2021, 12:38 PM
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Just guessing here:

Up thrust makes for an upwards component of thrust, and then the propeller being in front of the C/G makes for an up-pitching moment, cancelling the down-pitching moment by the cantilever.

If the propeller is so far above the fuselage, the fin may be hit only by the swirl's lower half. That would push the fin/tail to the left side, letting the airplane veer to the right side. That is cancelled by left thrust, making for a left component of the thrust force in front of the C/G, giving a left-yawing moment.



Old 07-21-2021, 08:57 AM
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That sounds reasonable - basically, you are proposing that the dominant force here is the slipstream - overpowering other effects that would tend to produce left yaw.
In other words the directional effect of slipstream mostly depends on where it hits the vertical stabilizer (ignoring other parts). In a nose mount, all of the vertical stabilizer is usually above the thrust line and produces left yaw. With the thrust line sufficiently above the middle of the vertical stabilizer (more-or-less) - the directional effect is reverse ?
Old Yesterday, 03:40 AM
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Yes, at least that's the simplest explanation I can think of, and I don't see other effects here, either. P-factor should be small with the small prop and there would have to be down-thrust for a right-yawing moment, but you say it's 2 deg up. On the other hand, even not that much swirl in the slipstream may have a noticeable effect on the big (lots of area) vertical tail. And the lever arm prop-to-C/G isn't that long so some side-angle is needed to compensate the tail's longer lever arm.

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