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FT Explorer Thrust Angle Woes--Raise CG?

#1
So, I am new to r/c and just started building an FT Explorer.

I have read several posts and seen a few videos with folks struggling to get their thrust angle sorted out. I read the related articles on FT and understand the thrust angle should ideally line up with the CG...not just the balance point on the wing, but the actual three dimensional CG (somewhere below the wing).

It seems to me one could improve on the need to angle the motor mount by bringing the CG as far up in the fuselage as possible. My idea is to mount the battery on a ramp on top of the platform normally reserved for this use. Also, to mount the ESC and receiver to the roof of the fuselage (though these do not make nearly the contribution as the battery).

Please let me know what you think of the idea.
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#2
Yes you can but you will lose some of the pendulum stability in the model and struggle with thrust vector being too far removed from the centre of drag. Add to this that a lower thrust angle reduces the maximum prop you can fit and the problems just get worse.

Just my opinions I suppose and I encourage you to try. If it works for you all the better.

The original thrust line shown in the construction/plans works well though the up angle of the motor thrust does make it seem to "Sag" on launch especially at low speed.

Have fun!
 
#3
Hai-Lee,

Thanks for the advice. I think I understand about the center of drag...so, ideally the thrust vector would go through both the COG and COD to avoid changes in pitch when throttling up or down. I reckon with this design that COG is considerably lower than the wing line and I see how this causes a pitch-down moment.

Not sure I understand the pendulum effect...is it related to the desirable condition of a stable plane that wants to dampen pitch oscillations? If so, I thought that could be achieved simply by positioning of COG ahead of COP (center of pressure...or lift).

As it turns out, there isn't much room to move the battery. (see pic)
WIN_20171027_07_54_17_Pro.jpg
My hope was just to make the pitch angle as prescribed in the plans as effective as possible. I also intend to install wheels to help with more graceful takeoffs...as many others seem to be doing.
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#4
Pendulum stability is just that a pendulum will always want to hang vertically or return to the vertical position after being upset. Hold a pendulum inverted so that its weight is at the top and upset the balance and it will want to quickly fall and settle up the other way, (inverted).

As for fitting an undercarriage just remember the higher the CG the easier it will nose over on landing or when taxiing. so make sure the wheels are well forward of the CG if taildragger or a strong nose wheel if tricycle.

Have fun!
 
#5
Hai-Lee,
Thank you for the advice. I intend to position the wheels under the leading edge of the wing. I hope that is far enough forward...I will use enough ply on the bottom of the fuselage to allow adjustments if necessary.

For the thrust angle issue I am planning to make the mount adjustable. I considered attaching it to a servo but I am not sure it would handle the strain of the motor. So I will just use some mechanical fastener or screw mechanism. It should be fun messing with the angle to optimize. I know it's overkill but "exploring" is what I like to do. ��
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#6
An old school method of making a variable angle motor mounting was to hinge the motor mounting plate, (firewall/bulkhead),slightly off centre and fit a spring or similar to pull the mount towards the the longest side. The Servo was used to drive a wedge between the motor mounting plate and a fixed point or plate. The servo only needs to push/pull the wedge and motor thrust is not coupled in direct opposition to the servo but rather just a very slight force is evident. As the normal angle changes are not extreme the system is very reliable if maintained with some Lube.

On larger vectoring systems the pivot point is as close to the CG of the rotating parts of the motor/Spinner/Motor rotating parts so that the gyroscopic forces are a little more balanced and the servo would need to be rather beefy and have metal gearing of course. The pivot is best if using ball bearings, can be salvaged from burnt out or damaged brushless motors.

Just some thoughts!

Have fun!
 
#7
Hai-Lee,
Thanks! The wedge method sounds doable. I had something like that in my mind but wasn't able to work out the details. This might be worth trying since I intend to convert the plane in various ways: twin boom with triangular tail, biplane using both wings from the kit, etc.

I would guess the spring would not need to be too strong given that the force of the motor in flight would rotate the mount to push against the wedge. As far as lubricating the wedge this might be a good time to use some HDMW tape...very thin and slick plastic. I use it on drawer runners for the furniture I build and it works very well. I am envisioning putting the wedge under the motor mount...it would be like a "sled" between rails the same width as the cavity the power pod slides into. Can't wait to try it!

Thanks again!
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#8
Just a further note. In order to keep friction under control the wedge is normally a little convex or has a contact point/rail do that the contact point is small in area. But then experiment is what we do!

Have fun!
 
#9
Just a further note. In order to keep friction under control the wedge is normally a little convex or has a contact point/rail do that the contact point is small in area. But then experiment is what we do!

Have fun!
Makes sense. The the wedge could even be topped with a smooth rod or even something like a pair of captured ball bearings.