What are the simplest and most accurate ways of making one ?

I could draw round the mounting posts with the engine on a sheet of paper but that then would need translating to something that has the cross hairs to match up with those on the firewall.

In case it's relevant, the engine is a CRRC Pro GF45i which needs to be mounted inverted.

If I had the official mounting bolt pitches documented I could make one on the computer. But I don't [:@]

Thanks

If I am understanding your question correctly, use a sheet of paper or cardboard to make an exact copy of you firewall....dimensions and shape. Once you have accurately cut the template from the paper, lay it on the firewall and copy your crosshair lines, or measure them and draw them on the paper. Mark your mounting holes from the motor to the paper. Accurately tape the template with the mounting hole marks onto the firewall and drill your holes. Am I interpreting your question correctly?

Not quite. Have a look at this - #37

http://www.rcgroups.com/forums/showt...=773195&pp=100

See what he made ?

I can draw around the standoffs but how did he work out the exact centres of the 'holes' ? Once you have them it looks easy to draw the lines which will end up mating with the ones on the firewall.

Looks me he used some geometry using measurements taken as accurately as possible. If you know the diameter of the standoff, 1/2 of that distance (radius) would be the center point of the standoff markes that he drew. If you draw 2 lines at different spots accross the diamteter of that circle, where they intersect inside the circle is exactly the center point.

Using the drawing r2champion posted, along with a compass, protractor and ruler you should be able to figure it out.
Remeber, however, most airplanes require some offset thrust and the engine in alot of cases are not centered on the firewall but are slightly off to compensate for the offset and still have the engine exiting the cowl properly.
An example: I am curently building a Norseman. It requires 2* right and 2* down thrust. In order to have the engine centered in the cowl it has to be 1/8" above center and 1/8" to the left of center on the firewall.

Very true....I guess I looked at it as if the crosshairs marked on his firewall took this into consideration.

It most likely does in an airplane with a box on front like his, in fact I would bet thrust offset is already built in also.
I put out that info in case another beginner read this thread that has a different plane.

My motor box has the sidethrust built in and has similar lines to the one in the photo.

I have a digital measure so getting the diameter of the standoffs is easy but 'measuring' to the centres' on the paper still seems to be a hit and miss affair. The circles seem to be too small for going about drawing lines and using a square to draw perpendicular lines around the circles. As in the method, for example

http://www.coateshomecentre.com.au/t..._of_circle.htm

It would work on the outside of the circles as well but they're still only small ones.

Please take this as only my opinion, others may disagree. I'm not trying to discredit your attempt to make things perfect, but it seems like you are putting a LOT of effort into making the holes perfectly, where in return there is no way that you will be able to drill them to the precision of your measurements. The last thing I want to do is insult your abilities and concern, but you will never drill the hole perfectly, even with the proper tools. I am all about getting things made as close as possible, but I make myself draw the line somewhere before I kill over worrying about something that I will never notice the difference between "better than good enough" and perfection. Yes, in this case close alignment is the only way to go, and guessing the center just isn't good enough, but I think your taking it to the next level. Again, I'm not trying to be a turd, only trying to help and maybe ease your mind on the matter. Anyone disagree?

Nope. No disagreement here. In fact, I've often wondered about placement of the engine mount in ARF or kits.

I have a Seagull Super Star 120. The engine mount came bolted to the firewall, however, I had to remove it. I forgot why, but I did. Then after that, I put it back on, then put the engine on the mount. It (the engine) was able to slide front to rear in this particular mount (it's a slotted hole that allows the engine to slide front to rear) and unfortunately, with some slop, from side to side. So I eye-balled the thing and tightened it down a bit. When I finally put the cowl on, I will do a final alignment on this and we will see how it flys.

At least, I have the ability to loosten the engine and move it to provide more up-down thrust as necessary (it's mounted sideways).

I am not that much of a precision pilot, at least not yet anyway, so I can't say if I will ever notice if there is a difference in a degree or two in the thrust angles. We will see, though.

CGr.

I'm mature and will take your comments as constructive. I know that I'm a perfectionist. People tell me that. And psychometric tests confirm it too. I know that this can be a good thing but I also know that too much attention to detail can spoil your view of the bigger picture. It takes me ages to build but I promise you that for me it's been worth it to see pretty damned perfect results when I've done. My friend rushes things and ends up with rips in coverings, extra holes, servo ply doublers pushed inside the fuse, huge holes in cowls, etc, etc.

The new bigger E-Flite electric kits come with sliding blind nuts to allow virtually any mounting spacings. Totally flexible. In this case, I have blind nuts to go behind the firewall and the holes in the engine have no slack so once the holes are drilled in the firewall, that's where the engine sits. Precisely there. I've got a template with the holes positions. I'm sure that I can get four screws to go through the firewall in one attempt but whether they are in the correct place is another thing entirely.

Good point.
I have been known to stand a plane on its tail, set the engine on the firewall, slide the cowl over it as a guide. No fancy measuremants and works just fine. (Of course this method will only work on engines that will fit inside the cowl, with needle valve and muffler removed)

Thats great to hear and nice feedback as well.
Thats great to hear, I guess there really was no way of saying it (that I can think of) without sounding like a turd...so I worry. Another reason for my approach was that you posted this in the beginners forum. No problem with that, but most of the time a question like this posted by an inexperienced builder are digging too deep into the matter...I had no way of knowing your experience level. For the record, I have always used a mounting system that ivolves drilling holes and installing blind nuts the same as you, so I was referring to your specific situation based on my experience. Considering that you do have experience, might I suggest asking this question in the kit building or arf forum which might be more specific to your situation to get a better answer. Good luck with the installation!

I think someone invented washers/shim a long time ago. Then someone came up with an adjustable engine mount becuase
people don't drill perfectly or have a bench press drill.

there's marks on the side of those adjustable mounts.

I'm building one of these

http://www.greatplanes.com/airplanes/gpma1305.html

I've already got about $1400 equivalent in it so I really want to get this one right

Maybe the more experienced 'proper' builders frequent the Beginners forum ready to offer solutions to 'proper' issues rather than just sliding a mount up and down the firewall ?

As it happens, I asked the guy who did that build how he got from having four circles on some paper to having it all sketched out. It's on page 2 near the end but he's not replied.
And I've e-mailed CRRC to ask them if they have a template. And what size the gaps are. I'm told I'm like a dog with a bone. I'll get it right one way or another.

Thanks

This plane, and it's assembly, is WAY more advanced than the Beginner's Forum. I am moving this thread over to the ARF Forum.

Ken

Fine by me. Thanks

I sorted this out last night by making my own template. It seems to fit perfectly. This morning CRRC send me a picture with the dimensions on

I've asked them if I can publish it in the GF45i thread.

I'm not sure but it looks a little crooked to me? Just kidding, glad you got it sorted out!

I had to make a new firewall for an arf I damaged in a crash. I decided to mount the engine inverted this time, so I had to do everything from scratch. First I found the vertical center by measuring the midway point, then drew vertically with a right triangle/square. . . I then determined the distance from the firewall to the opening in the front of the cowl. Using a contractor's (home depot) protracter, I determined I had built in 3 degrees of right thrust in order to center the crankshaft in the opening of the cowl. . .I multiplied the tangent of 3 degrees (scientific calculator) by the distance from the firewall to the cowl opening (say 6") this told me how far to the right (looking at the front of firewall) to make my vertical cross hair. As far as finding how high to mount the engine on the bracket, I put the cowl on the fuselage temporarily and taped a piece of straight cardboard along what would be the centerline of the cowl, I then moved the engine up or down as necessary, and drilled holes.

With the template made earlier he would have used a circle template (you can get them at office depot) used for drafting. . . the circle templates have crosshairs on them, so basically find a circle that matches, lay a straight edge over your crosshairs on the template, and draw them inside the circle. Taddaaa. Hope it helps