Tuesday, April 23, 2019

NAMC Mig-35B motor mount modification

Hi Everyone -

I have experimented with moving the motor around on several park jets over the years to help with better balance and performance in my planes.  I wrote this article awhile back about some of my thoughts and experiences at that time about motor location in a prop and slot park jet.

I also recently shot this video as part of my intermediate park jet video series.
In this article, I will cover how I go about moving the motor mount forward in the Mig-35B, I have done this a few times now, primarily to account for the extra weight of keeping the paper on the Dollar Tree foam in the rear part of the plane.  The Mig-35B is a bit more tricky than a lot of park jets to move the motor forward, but hopefully you will get the idea if you want to experiment with other planes.  Also, I will hopefully demonstrate how I modify the prop slot and other aspects of the plane to accommodate a smaller, shorter quad motor which I use pretty much exclusively in my park jets now with some excellent results.

So some of the modifications that need to be made to this or any other park jet are a function of just moving the motor forward and some are a function of using the shorter quad motor.  In this write up I will try to identify which is which depending on what motor you want to put in your park jet.  You might want to do it all so that if you want to experiment later with a shorter quad motor, your jet is all ready to go😎

I should also mention at this point that the plans I am using for all these measurements in this article are for a plane that will be built at 95% of scale, so I will end up with a 25.5"/648mm wingspan.  Depending on the size you build your Mig-35B or any other park jet, your measurements will more than likely be different.

On most previous Mig-35B builds where I have moved the motor forward, even those not made of DTF, I has moved the motor forward by about 1".  For most of the DTF planes, I found this was not quite enough to really tighten things up the way that I like them, so I am going to experiment with moving the motor and servos forward a bit more😉

One of the first things you want to is identify where the trailing edge of your KF airfoil is going to be.  Even if you are not going to use them, this area is probably a good limit to set for moving the motor forward.  You can move the motor too far forward and then the plane can have almost "gyroscopic torque roll" which can make launching a real challenge.  I simply trace a line where the trailing edge of the KF will be so that I know the maximum I can move the prop slot forward which needs to be modified for using a shorter quad motor. 

I normally start off making all the measurements and mods to the wing plate so I know where things can go, this has a domino affect on other parts of the plane, so I do everything in pencil.  I have marked many of the lines in highlighter simply for this article so that they might be easier to see.

In the picture below I have marked the trailing edge of my KF and also moved the leading edge of the prop slot forward so that it will only be about 5 mm deep to allow best clearance for the quad motor.  Now I know how much room I have to play with for moving my motor forward.  In this picture, I have currently moved the prop slot leading edge forward about 14 mm which is how far I would move it forward if I was using a quad motor in this park jet.
I find using clear plastic rulers is easiest for this type of work as I can see through it to help measure from existing lines to the new lines.  This ruler happens to be metric, so I make all measurements in mm.  I have about 35 mm with which to play here after marking the leading edge of the prop slot (this is without moving the motor just yet, but since I will be moving the prop slot forward also, this is key to ensuring I don't move things too far forward).
In order to give myself a little "leeway", I decided to go with 30 mm for the motor mount.  I know that I have already moved my prop slot forward by 14 mm to accommodate the shorter quad motor, so that will factor in when I look at moving servos and other air frame parts forward.  So as you can see in the picture below, I essentially move everything to do with the motor mount forward by 30 mm.
Also, there are two slots which will accept tabs from the bottom part of the fuselage when it is "mated" to the wing plate, so those also need to be moved forward 30 mm as well.  These are affected by the new motor mount placement, but not the new prop slot placement.
Since the leading edge of the prop slot will be moving forward 44 mm from original position, I move the rudder servo slot forward that amount also.  Moving the servos forward also helps shift some weight forward to keep things tighter around the CG.  This will also affect the aileron and elevon servo locations which we will move later when we get to that part.
On the Mig-35B and many other park jets for that matter, when you move the motor mount forward, it will affect a wing spar, so you will want to remeasure this to see what size the new wing spar will be.  I like to leave at least 1 cm or maybe a little bit more between the end of the spar in this location and the leading edge of the wing as I will be angling/trimming the wing leading edge back after the KF airfoils are installed.

Now it is time to move on to adjusting the fuselage, this is a bit tricky, but taking your time and ensuring you measure everything correctly will pay off later in the build.  In the centre part of the fuselage, I move this forward 30 mm as it is affected by moving the motor forward, but not the leading edge of the prop slot.
I then adjust the slot which will slide onto the wing plate and move that forward by 30 mm also.
This is one area where it gets a little tricky, normally the area where it says "cut out and save" would still be attached as part of the fuselage if built stock, however now it is not after moving some lines for the modification.  I cut this out and save it as it is critical to support the bottom of the fuselage after "mating" it to the wing plate.
Next, it is time to adjust the top parts of the fuselage that will be on the top of the wing plate after "mating" the fuselage and wing plate.  These are primarily affected by moving the leading edge of the prop slot forward to accept a shorter quad racing motor, so 44 mm is the key measurement here. 
This also affects the tab and slot to align the electronics bay door.  I measured it to 30 mm, you can measure to whatever size you want.  In the picture immediately below, I measured the "slot" on the side piece of the fuselage to accept the tab from the electronics bay door.
Here I moved the back of the electronics bay door forward by 44 mm to accept the new leading edge of the prop slot. 
Then I adjusted the tab to 30 mm to fit into the slot on the side of the fuselage. 
Since I moved the leading edge of the motor mount forward so much, I also added some to the back plate/trailing edge of the motor mount to maintain some structure.  I like to keep my prop slots a little larger to allow for best airflow to and from the prop, so I split the difference between 30 and 44 mm and moved the trailing edge of the prop slot forward by 37 mm.
Next, time to move on to the nacelles which need to be shortened for both the motor mount mod and the quad motor in this situation.  First I normally move the servos forward as shown in the pictures below.  Since I have already moved the rudder servo slot forward 44 mm in the wing plate, I do the same for the aileron and elevon servos.
You will note below that moving the aileron servo forward interferes with a tab that goes into the wing plate, so I simply measured off 15 mm in this situation, remembering to carry this over to the tab on the wing plate as you can see in the second picture.



In these following pictures, I modified the inside part of the nacelle so that the tab is moved forward to fit into the slot I moved forward by 30 mm when moving the motor mount forward.
Then I moved the trailing edge of the nacelle forward by 44 mm to allow for the fact the leading edge of the prop slot is also moved forward by 44 mm.  This is something to check after the motor is installed by turning the motor by hand with the prop installed first to ensure it does not contact anything before giving it power and chopping up your plane😮
One last thing to check is to remove a bit from the top of the motor mount (I start with about 5 mm and then adjust as needed).  If the top of the motor mount is left the way it is, the electronics bay door won't lay flush.
Hopefully this has been somewhat helpful.  Moving the motor forward on the Mig-35B is a little trickier than with some other park jets, but I have done it on three Mig-35Bs thus far and it has worked out well.  I have moved the motor forward on several different park jets, the principles are basically the same, just remember it can create a bit of a "domino affect", so always best to figure things out and double check before cutting out the foam.

#ParkJetnoise
#ParkJetpilot

Cheers,

Scott











































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