teamgrumman(at)yahoo.com
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 Posted: Fri Nov 05, 2010 7:54 pm Post subject: Engine "THRUST" angle: Page one. |
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I have reread the information in this email a couple of times. On the surface, there appears to be a lot of valuable information. However . . . . I have a few questions.
At the factory the engine mount is installed on the engine while the engine is on a stand before being mounted to the airframe. Typically one shim / washer is installed on each of the 4 mounts.
- - - - Typically. What does that mean, exactly? How does the installer make that decision? Why not always install one shim?
Initially, the engine mount is received from the mount maker and set on a granite surface that is certified within certain limits to be 'flat'. Certified, calibrated height gauges are used to measure the height of components of the mount from the flat surface and it is determined that the mount conforms within a certain tolerance to the drawings. All the mounts are to be the same within that close tolerance.
- - - - I can see that this would be part of the conformity check. This makes total sense.
At station 10(,) the engine / mount assembly is installed on the airframe. As I understand it, Station 10 determines / ensures the proper alignment of the engine. Station 5 determines / ensures proper alignment of the cowl to the engine that has already been proven to be in correct alignment at station 10.
- - - - Is station 5 after station 10? If not, is the cowling removed after shimming to correct? then checked again? How is the alignment determined at station 10? Is there a jig/fixture to align the engine to the airframe? What is the alignment goal? Is the alignment goal: For the engine to be the at zero degrees vertical thrust line with respect to the airframe? If so, how is the alignment of the engine mount brackets on the airframe aligned?
Field procedure for the verification of the proper alignment of the engine is to align the back of the spinner to the front of the nose bowl. Adjustments are made by shimming the engine to the nose bowl. The cowling nose bowl is the determining factor for alignment of the engine after the airplane leaves the factory, this is what I was trying to communicate to Gary recently and why I ask him repeatedly to adjust my engine to fit the original cowling before he installed the Jaguar cowling.
- - - - This statement tells me that it is known and expected for the engine/engine mount/airframe attach points to have quite a degree of variation and that the final determining factor is the position/location of the nosebowl/cowling with respect to the airframe. When I install a cowling (any cowling, any plane) I put the screws in all of the holes (without tightening any of the screws), I then close the cowling, I check the alignment of the spinner to the nosebowl and, if it's close, I tighten the screws in a special sequence. If not, I will force the entire cowling assembly in the direction needed to align it the best and then begin my sequence of screw tightening that maintains that alignment. It is possible to move the alignment 1/4 of an inch.
This alignment is made at the factory to be within .020" of center between the two. With 1 shim in each mount the front of the nose bowl on the cowling should be at station 10.775 i.e. 39.225" from the firewall (50- 10.775 = 39.225)
- - - - This dimension is really esoteric. One coat of paint is typically .005 thick. Plus, the number of shims does not affect the location of the nosebowl. Perhaps I don't understand just what dimension is being scrutinized within .020 inches.
When Gary and I were discussing the misalignment of his Jaguar cowl on my Tiger I probably misspoke about zero thrust. I was talking in the context of the relationship between the nose bowl and the spinner where as above the difference in alignment is required to be less than .020 when the plane leaves station 5 at the factory.
- - - - Again, I guess I don't understand this dimension and why it's so critical. There isn't much on the entire plane that could be built to that tolerance.
As I understand it, the thrust angle of the engine is 0 in the vertical plane and 2 deg right in the horizontal plane.
- - - - THis I understand.
If one were to build 4 jigs for the engine mount in place of the 4 rubber isolation mounts and such jigs would hold "dowel rods" and those dowels were extended forward they would intersect at a focal point. This focal point would be 28.72" forward of the firewall and be .032" left from butt line zero and sit .23" above the Waterline i.e. Wl +45.230. The waterline is spec'd to 3 decimal places so this position specification has 3 digit accuracy.
[b] - - - - Not significant to the discussion of zero degrees thrust line offset in the vertical direction.
[/b]
If one were to look at the backbone of the engine (as I understand this it is the seam at the back of the engine) and compare this with a hole in the tab on the engine mount where the ground strap is attached one should see these two points in alignment. These two aligned points are offset .085" left of the butt line.
- - - - Not significant to the discussion of zero degrees thrust line offset in the vertical direction.
If one were to bisect the distance between the upper engine mounts, center to center of bolt and extend that point parallel with the longitudinal axis/ water line/ butt line, into the firewall one would measure a 1.316" offset left of the butt line.
- - - - Not significant to the discussion of zero degrees thrust line offset in the vertical direction.
These figures were given me during a telecon with someone who knows this stuff and I believe them to reflect accurately, however take them at your own risk as I cannot guarantee that I understood properly or heard accurateley.
Now, if I can get back to the hangar to start the extensive fiberglass work required to "repair" and "refit" the Jaguar cowl to the correct position then I may someday give you some good flight test data on the really cool looking Jaguar cowl.....
Fit of Jaguar AFTER aligning engine with factory cowl:
- - - - This assumes the factory cowl is correct.
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