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.

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.

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 ?

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.

When your engine is mounted high above the CG of the plane the engine thrust will tend to pull the airplane down when you advance throttle. So the upthrust will compensate for the nose diving. When the engine is mounted on the datum/center line of the plane the torque of the engine will tend to roll the plane to the left thus you use right thrust. However if the engine is mounted high above the CG the torque of the engine will move the plane to the right so you have left thrust to compensate. We are not even talking about the P factor and how the air flow hit the plane.

I am hoping that someone can explain why "the torque of the engine will move the plane to the right" . It seems to me that the torque of the motor will act on the pylon in exactly the same direction as if it were mounted on the nose i.e. a reaction torque will be in the anti-clockwise direction. Given the length of the pylon this would seem to (potentially) make a left rotation of the fuselage in an arc centered on the motor axis causing a turn to the left. If this is not the case - then what is happening and why ?

When you have a propeller on the center line or "datum" of the plane and the propeller spins, it grabs air and torque the plane and the left wing dips. Then the plane will veer to the left. so you add right thrust.

When you have the engine mounted on a pylon then you have to think of the plane as a pendulum. When the propeller spins and grab air the plane is suspended below the engine and the pendulum will swing to the right. Granted the left wing will still dips and the plane may still veer to the left eventually. So it is really a design with some empirical tests and the designer must have found out that a little left thrust helps to "straighten" the plane. There are a lot of factors acting on the plane. A lot can be explain with theory and some will come from empirical data from testing.

Thanks and YES ! ... or as I like to say "There is no difference between theory and practice ....... except in practise"

That's a nice and often heard saying but I think it's actually a humorous corruption of "There is nothing more practical than a good theory", attributed to Kurt Lewin. And you bet that the designer of the Albatross found out by trial or knew by experience, respectively, that a bit left-thrust is needed here. I agree with Hansen that a lot of factors is acting here, but I thought you asked especially for a simple explanation.

A closer look is even fun, though, so I tried to sketch (by hand) what Hansen explained (see attached pdf). It's still simple (no math) and maybe even naive but it should sum up which forces and moments (torques) are acting on the C/G. It turns out that the side thrust slightly increases the "pendulum effect" (by 1.6 Ncm) around the roll axis and by far not compensates this effect (by 1.0 Ncm) around the yaw axis. Hence there must be more factors acting - as expected.

Hope you don't mind metric units. In the middle of drawing I decided to use Ncm (Newton * centimeter) and not Nm (Newton * meter) for moments (torques) so the units are not quite consistent.

Assumed are 15 cm vertical distance between datum line and propeller axis and 10 cm longitudinal distance between propeller disc and C/G. That comes pretty close to the real values. The drive figures are from a drive I have at hand and which is quite similar to the proposed drive, so thrust and torque are realistic (as well as 11 m/s flight speed).

>>A closer look is even fun, though, so I tried to sketch (by hand) what Hansen explained (see attached pdf). It's still simple (no math) and maybe even naive but it should sum up which forces and moments (torques) are acting on the C/G. It turns out that the side thrust slightly increases the "pendulum effect" (by 1.6 Ncm) around the roll axis and by far not compensates this effect (by 1.0 Ncm) around the yaw axis. Hence there must be more factors acting - as expected.<<

It is a lot of fun to try and figure out, especially when using a diagram. Thank you. We will never know what is "really" going on when the plane is in the air.:rolleyes:

Yes (sigh), our theories are just not good enough. :cool:
(Couldn't resist - my post #1000)

Yet another guess on second (or third?) thought, to add to the confusion, just because it's so much fun (a fairy tale from One Thousand and One Nights, or my post #1001 :cool: ):

Remembered that in 2004 I had calculated the prop with JavaProp (by Martin Hepperle) and there is even information about swirl. (Meanwhile there are even measurements by UIUC/Michael Selig but without swirl information.) The diagram is calculated for the assumed operating point (11 m/s, 6450 rpm, thrust and torque like above) and shows the swirl angle over the blade radius (r/R=0 is centerline, r/R=1 is blade tip). Shown are the angles at the propeller disc and far behind (far field ff):

https://cimg8.ibsrv.net/gimg/www.rcu...fa99cba57f.png

This latter swirl angle varies between 6 and 9 degrees and is here reduced by 2 degrees left-thrust to between 4 and 7 degrees. That's the angle at which the prop-wash hits the vertical tail's right side what makes for a right-yawing moment, adding to the right-yawing moment from propeller thrust and torque. Since the tail is above the datum line, there is a smaller left-rolling moment adding to the left-rolling moment from propeller side-thrust and torque.

Obviously, left-rolling and right-yawing effects cancel each other now, with 2 degrees left-thrust. To find that out theoretically, elaborate CFD calculations could be needed which might be fun but which are surely overkill for a cheap model airplane. There's nothing better than just finding out practically, by trial and error (or, put differently, by experience). Theory is too complicated and inefficient here.

P.S.: Forgot to mention that the airplane will probably sideslip a bit, its nose a bit turned right from flight direction (by the right-yawing moment) so dihedral produces a right-rolling moment cancelling the left-rolling moment. That throws the angle calculations out of the window, though.

Folks --
Thanks for these insights. I am (or more accurately was) an engineer and am always curious about the "why". Usually - I can figure it out but lacked a proper grounding for this one!

I remember talking with a helicopter test pilot (military) many years ago. We were looking at a helicopter he had helped develop and I asked "What does that do?". It was a piece of angled bar that seemed to have been placed in an arbitrary place. His answer was basically "We found that we could correct some inconsistent behavior with it - something that only showed up in actual flight" so it became part of the official build.

I hope it's not dangerous and won't cause a breakdown.

It really depends on where that angled bar is located. If the bar is on the top of the tail boom a lot of times that is for deflecting the main rotor blade away from severing the tail boom in case of a boom strike. If the angle bar is somewhere else in the fuselage it may or may not be aerodynamic. All depends on the size of the bar and where it is located.