arcticflyer

I am constructing the P-51D Double Trouble. I am getting ready to mount the engine and noticed that the marks placed by the company on the firewall (that I ASSUME is the center point and thrust line) appear to be off center by nearly 3/16th. In the picture below the mark through the center is mine and the factory marks are to the right. I am assuming there is a reason that they are off center but with no directions I question the location. It appears that it is setup for a Horizontal (side) mounted engine because the supplied engine mounts do not fit between the triangle stock on the firewall for vertical mounting. When holding the mounts in the horizontal position everything lines up. I am mounting my engine inverted with an incowl exhaust! I do not under stand why anyone would mount an engine horizontal on a P-51. Any help is greatly appreciated. This is the first time I have built one of these SEMI BUILT KITS (not an ARF by any stretch) with little to no directions. Thanks Skip

Rube Goldberg

This is one of those situations where the big Hobbico "Builders cutting mat" with the 1" square grid lines can be really handy.

First off, I would suggest the possibility that the firewall is NOT "square" or perpendicular to the centerline of the fuselage, but has right hand thrust built in. Therefore, the marks being to the left of center will allow the engine, out at the crankshaft end, be centered on the centerline of the fuselage. This is where the Hobbico mat with the grid lines is easily used to line up the aft end of the fuselage on the grid as well as say the wing saddle areas on each side of the fuselage. It would be readily apparent, probably wouldn't even need a drafting triangle to determine that the firewall is indeed built on a skew to allow for the proper right thrust.

Of course you could also stand the fuselage up on the firewall (assuming the triangle stock is the same on both sides) on a flat surface and use a builders square to see if both sides of the fuselage are square to the surface, or if one is more than 90 degrees and the other is less.

Rube

arcticflyer

Rube, thanks for the info. The firewall indeed has a bit of right thrust added to it as it can be seen while holding the fuselage up. So I gather that with this right thrust built in I should use the marks to mount the engine. Thanks Alot! Skip

Rube Goldberg

Glad I could help!

Now that we have a possible explanation for the vertical mark being left of center, I highly recommend we check the horizontal mark before we drill the holes and install the blind nuts to mount the engine mount to the firewall.

Fit the cowling to the fuselage to where it "looks right". i.e. not tilted up or down, but "flows" with the lines of the fuselage. Then using something to measure with such as a metric tape or a balsa stick, measure the depth from the face of the firewall out to the front of the cowling, and depending on what spinner you are going to use, you'll need to add a bit more to that measurement, as some spinners have a flat back plate while others extend rearward behind the face of the prop drive. You'll also want to add a bit for clearence between the spinner and the cowl.

Use your measurements to locate the engine on the mount fore and aft. Drill the mount(s) and then fasten the engine to the mounts. Now we will confirm the horizontal line.....

With the cowling removed, and the fuselage held in an upright position, place the engine and engine mount assembly on top of the firewall where the marks are, then still holding the fuselage upright fit the cowl. Does the center of the crankshaft fit the center of the cowl opening? If not, then you'll have to move the engine up and down a bit (that would be horizontally since you are holding the fuselage upright) to get the proper fit, again, with the cowling in place so it looks right.

Rube

arcticflyer

Hopefully I'll have the motor and antivibe mounts in the next couple of days. I just wanted to get straight what I needed to do prior to its arrival. These are great tips thanks. Skip