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trust the thrust line?

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Old 01-28-2013, 09:43 PM
  #1
sparky4lawndart
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Default trust the thrust line?

Hello all... I have just started building (assembling) the .60 size Hangar 9 P-47. I'm following the recommendations in the build thread for that plane here on RCU but I've run into an issue...

The ARF has a pre drilled firewall for their aluminum mount. As I do as I'm told I've glassed the inside of the firewall and I'm getting ready to re-drill the firewall so the YS 120 will be fully inverted instead of the 120degree orientation the ARF had.

Here's the rub... although the build thread talks about re-inforcing, re-drilling for 180 degree inversion of the motor, it does not say whether to believe the drawn lines on the firewall for thrust line...

the firewall is 6 3/4 inches wide at its widest point but the thrust line is drawn at 3 1/4 inches from the left side (fuse is inverted and you're looking at the firewall). should I trust that drawn line or should put the prop shaft at the point on the firewall in the middle of its widest point and split down the middle (vertically)?
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Old 01-28-2013, 11:48 PM
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Default RE: trust the thrust line?

Sparky,
The line is in the correct location, not a mistake. The mount/engine is mounted centered on the line, the line is offset to the left because the engine is going to be angled to the right (right thrust) once the motor is mounted the drive washer of the motor will be centered in the cowl. Angling the motor to the right is to help minimize the prop torque that pulls to the left. To obtain the right thrust angle they may have you adding washers or shims between the left side of the mount and the firewall, the most common way is to build the plane with the firewall angled to the right, some ARF's already have the firewall angled.
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Old 01-29-2013, 04:54 AM
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Default RE: trust the thrust line?

mvallyman is correct trust the thrust line
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Old 01-29-2013, 09:42 AM
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Default RE: trust the thrust line?

Alright... I don't know my next step then... If you look at the pic you can see the factory pre-drilled holes for the motor mount... the dots and corners are mine using the motor mount with the motor bolted to it... I drew those dots and corners by eye-balling the prop shaft to be centered at that point. Imagine me standing there holding the fuse with the rudder and elevator an inch from the floor while trying to balance the motor on the firewall to mark the location of the motor mounts holes... I was using the vertical and horizontal lines to put the prop shaft at the intersection. based on what you just wrote, I would be wrong.

There is no angle in the firewall that I can tell... If I stand the fuse on the firewall, the model is perfectly straight (as much as I can tell anyway given that I just had it standing like that overnight for the epoxy on the inside of the firewall to set...)

so... am i supposed to center the back of the motor at the intersection of those lines and put shims to have the drive washer closer to the vertical centerline?

(mental note: if the fuse at its widest point is 6 3/4 inches wide and the vertical line is only 3 1/4 inches from the side then it stands to reason that the angle of the engines own drive shaft thrust line (through the prop shaft) would be pretty steep...also... the original mount points had the motor at 120 degree angle.. how would they achieve a thrust angle you've described if the factory motor mount pieces are identical, theres no mention of shims in the assembly instructions, and there's no visible angle builtin to the firewall...).

I'm not trying to be a pest!!! I just want to understand the mechanics... I spoke to a club build pro (he actually builds real airplanes for people) and he described exactly what you've described but I don't know how to achieve what so many others have...
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Old 01-29-2013, 11:09 AM
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Default RE: trust the thrust line?

Are you just trying to figure out how far over to move the engine so it is centered to the cowl when you re-mount the engine?? If so I can just show you in a couple photos. Dose the plane call for right thrust in the instructions? If so and there is no built in thrust on the fire wall then you do need to make a shim.
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Old 01-29-2013, 11:15 AM
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Default RE: trust the thrust line?

Gray Beard... thanks

there is no noticeable angle built into the firewall (this is an ARF). There is nothing special in the manual regarding thrust angle. There is nothing special about the motor mounts that came with the kit... I got stumped when I noticed that the lines drawn of the firewall from the factory aren't centered...

in your experience, how much impact will having the thrust line of the engine centered on the firewall (as opposed to having it off kilter a bit) affect flight?

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Old 01-29-2013, 11:15 AM
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Default RE: trust the thrust line?

This Offset Calculator may help.
[link]http://www.lcrcc.net/offset_calc.htm[/link]

Regards,
Bart
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Old 01-29-2013, 11:46 AM
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Gray Beard
 
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Default RE: trust the thrust line?

I just went out and drew up a way to measure how much offset to the left you would need to center the engine in a cowl. Because it's a jug it isn't that important, it's big and opened. I just draw a straight line then measure the degrees of the thrust, in this case I did it at 3 degrees. Drew another line to the 3 degree mark. Then I just measure how far out the front of the thrust washer would be when mounted and measure the distance between the lines and that tells me how far over to the left the engine would need to be moved to have the shaft centered.
When I build a plane from plans I put no thrust in the engine or fire wall at all if it doesn't call for it. Then I test fly the plane to see what it does without any thrust. Usually pulls to the left on the up line a little. Using washers behind the left side of the mount I start adding one washer at a time and keep flying until the plane doesn't pull at all. When I get home I make up a shim to go behind the mount that gives me the thrust required. If any! Depending on the type of plane I may plug the mount holes and move the engine over so it is centered in the cowl. This is important on planes like an Extra that has a cowl with a smaller hole in the center. On something like a Sukhoi I don't bother because the cowl is open like on your jug.
Some planes require thrust and some don't. I never know unless it is stated in the instructions or after I have done the trim flights?
I'm building a small Ultimate right now and it has built in down thrust on the fire wall so the designer of this kit discovered the need for it during his testing.
If it doesn't call for thrust and the fire wall doesn't have it built in I wouldn't worry about it until after I have done my trim flights.
The worst thing that will happen without the thrust will be the plane pulling either up or to the left. Your plane will probably pull a bit left, my Bipe would probably be pulling up so it was built in.
If it doesn't call for it in the instructions I wouldn't worry a lot about it.
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