


Discussion
thrust angle on motor mount  a bit more scientific approach to offsets etc
Hi Guys,
I'm currently doing my first electric conversion of a glow plane and as many of you know, electric motors unlike their glow counterparts need a down and left thrust angle (around 2deg) to work properly. This did kind of yield a simple yet interesting problem  if you get a rigid metal motor mount which doesn't have the thrust angle built in you will have to mount the whole metal motor mount at an angle. Now with electric conversions motor mount will be quite long so this means you will have to offset the whole motor mount in order for the shaft to come out in the middle! Now I'm not a person that likes to do it based 'on the feel and looks'  I'd rather properly calculate the offsets and number of spacers I need to put in (I'm an math whore!) There is plenty of engineers here so they might share an better approach  anyways  my motor mount is a bit funky in a sense it's not square based (rectangle) and that the motor is not mounted in the smack middle but on the upper (picture attached). The approach presented here can be generalized to any kind of firewall (starfish, pentagram, circle, euclidean geometry of Cthluhu stronghold) The idea is  we measure (in this case) 5 points of interests of the firewall  4 base points where the screws come in and the point where the shaft should come out of the cowling. With those 5 points we construct a simple mesh within 3d euclidean space which we then transform with two rotational matrices around the X and Y axis. This yields new point coordinates  and voila  we got X, Y offsets for the whole motor mount and thinness of spacers we need to put in 3 of the base screws to get the thrust angles right. So: step 1)  measure the motor mount (between the screws): In my case  width wise the base screws are 80mm apart  height wise the base screws 60mm apart  the shaft from base to where it comes out of the firewall is 130mm deep  the center section of the motor shaft from the top screws is 14.5mm apart (as mention and seen on picture  it's not central) step 2)  create the mesh In this case it's just going to be 5 points. Because we are rotating around the base points will be at the Z = 0 depth. Because we want the offset of the center piece the point on the shaft will be at (X,Y) = (0,0) So in my case we have the following (X, Y, Z) coordinates: P1  upper left corner of the motor mount P1 = (40, 14.5, 0) P2  upper right corner of the motor mount P2 = (40, 14.5, 0) P3  lower right corner of the motor mount P3 = (40, 45.5, 0) P4  lower left corner of the motor mount P4 = (40, 45.5, 0) P5  point on the shaft where the whole thing is coming out P5 = (0, 0, 130) As seen on screen (I'm bad with pictures sorry): Now let's rotate this little construct to get the right thrust angle  first rotate it right around Y axis with Alpha degrees to get the right thrust angle and then against the X axis with Beta degrees to get the down thrust angle. (The formulas here are for counterclockwise rotation). So a bit math here: The transformation rotation matrix around Axis Y (Ry) and X (Rx) have the following formulas. In order to get the transformation matrix for rotation against Y axis first (for left thrust) and then X axis (for down thrust) we multiple Matrix Ry by matrix Ry and get the following product (keep in mind Ry * Rx is different from Rx * Ry) To get the coordinates of the new point we multiple the old coordinates by the transformation matrix (formula bellow): So let's put some numbers in (because now it's easy) I'm going for standard 2 deg down and left thrust (So both Alpha and Beta are 2 deg) sin(2deg) = 0.034899497 cos(2deg) = 0.999390827 So the formula roughly looks like: Now putting in the numbers I get P1 = (40, 14.5, 0) P1' = (39.95, 14.49, 1.9) P2 = (40, 14.5, 0) P2' = (39.99, 14.49, 0.89) P3 = (40, 45.5, 0) P3' = (39.92, 45.57, 2.98) P4 = (40, 45.5, 0) P4' = (40.03, 45.47, 0.019) P5 = (0, 0, 130) P5' = (4.53, 4.53, 129.84) Rounded it up to two digits. Ok so those are nice numbers but what do they tell us? The important one is P5  it tells us that after applying the 2 deg down and 2 deg left thrust the point will move by 4.53mm left and 4.53mm down from the original location: This means that I have to offset the whole motor mount by 4.53mm to the top and 4.53mm to the left in order for the shaft to still come out of the middle of the cowling. Another important information is how many spacers I need to add to each of the point on the mount to get the 2 deg left and 2 deg down downthrust!. If we look at the P3 which is the 'lowest one'  if we treat it as a zero (touching the firewall) It means that:  on the upper left corner (P1) I need to add 4.88mm of spacers (1.9mm + 2.98mm)  on the upper right corner (P2) I need to add 2.09mm of spacers (0.89mm + 2.98mm)  on the lower left corner (P4) I need to add 2.961mm of spacers (0.019mm + 2.98mm) Hope this helps someone OK  mistake on my side  it should be right side thrust so it should be Ry(2deg) not Ry(2deg)  case is symmetric so no problems there but will amend! 




ITT: hardcore matrix and trig. be warned
nice one  BrotherBloat London RC Project www.rcforc3.com 



"electric motors unlike their glow counterparts need a down and left thrust angle (around 2deg) to work properly."
Based on what, I may ask. My scratch designed and built Jungmeister (my avatar) used neither, and flies like it is on rails. Les 



I've never noticed that electric power models need any more or less down and right thrust compared to glow (there is no logical reason why they would).. Both types often benifit from thrust offset but it's not necassarilly two degrees, it could be more it could be less, it could be nothing.
The calculations look ridiculously overly complex it you ask me.. It's just simple schoolboy trig (or it should be): The amount you need to pack between mount and firewall is: Packing = sine(thust offset angle) x distance between mounting bolts The amount you need to offset the mounting points upwards or to the left: Offset = sine(thust offset angle) x firewall to prop length That's a whole load simpler than a page of calculation Steve 



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For example.. say you want 3 deg down thrust and 2 right thrust and lets say the bolts are 50mm apart. The two top bolts get packed by: sine 3Deg x 50 = 2.6mm The two left bolts get packed by: sine 2Deg x 50 = 1.7mm The top left bolt gets both packings added together: 2.6 + 1.7 = 4.3mm Simple as it gets..... PS... unless your prop is spinning the opposite way to normal your calculations are wrong because you would need rightthrust, not left thrust, which would mean P4 should be flat against the firewall?? 




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Using my simple method to check your calculations.....
I confirm your up/left offset exactly (or within a micron or two): sine 2Deg x 130mm = 4.53mm (up and left) However your spacer calcs are wrong: P4 should be flat on the firewall. P3 should have 2.8mm spacer P1 should have 2.1mm spacer P2 should have 4.9mm spacer Steve 



Yeah and I noticed the mistake thx
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The need for side thrust is to counteract the asymmetrical thrust caused by the prop rotation. The descending blade produces more thrust than the ascending one. This is why helicopters have cyclic. Because our props rotate counterclockwise (when viewed from the front) the descending blade is on the right side of the prop disc. Rightthrust is needed to neutralize this asymmetric condition. If the prop rotated the other way, then left thrust would be needed. Dan 



Joined Jan 2014
530 Posts

A few comments. Most of my glo and gas R/Cs from 20+ years ago, had offset thrust. A whole lot of full size airplanes have offset thrust. Some full size aerobatic planes use side thrust, while some don't.
The real advantage of side thrust in a full size, is that you won't be constantly stepping on the right rudder pedal, or trimming..............to fly straight. Otherwise, the plane will be in a constant side ways flight (yaw), while at cruise speeds. The forces that create this yaw, vary with power output. More power, and more yaw. Side thrust is just more automatic, than relying on foot pressure or trims. Side thrust can be used, as well as an offset vertical stabilizer or rudder trim tab. The disadvantage of side thrust, is the way it can effect some aerobatic manuvers. As an example, an Extra 300 has it, while the Pitt's does not. Flying a Pitts across country is no fun. It wears your leg out! 



OK! I went out and looked at what was in my hanger (garage). Out of 10 planes hanging ( 8 have been flown). Only one has right thrust, none have down; and they do just fine.
I probably include that these range from .049 fuel up through, and including .91 fuel, and up to 120 size (equivalent??), electric. Les 


Joined Dec 2008
536 Posts

I have 11 planes and all are electric powered. All of them are balsa, ply and no foamies. 48" SBach, assorted Sticks from 40 size to 160 size, 74" Extra, 76" Sukhoi, 96" Storch, etc.
Eight have between 1.5 and 2 degrees of right thrust and ZERO down thrust and three have 1.5 to 2 degrees down thrust and ZERO side thrust. All fly fine. There is no predetermined required amount of side and down thrust for a model plane. Each plane is different and will be determined by the design and flying characteristics. 
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