HS Premount sanity check

[ArVeeNiner]

Hi:

I'm at one of those you know what puckering parts in the assembly of my RV-9A. One of those places where you don't want to screw up.

I'm about to drill my HS to the rear of the fuselage and I just want to make sure I've got everything in the right place before I drill.

I measured the distance from the tips of the HS to the same points on either side of the firewall and both dimensions are within 1/16" of each other.

I drew a line between the tooling hole vertically on the rear most fuselage bulkhead just before the HS. Then I laid a straight edge across the forward and aft spars on the HS and lined it up with the mid points of the HS. It ended up lining up perfectly with the line I drew on the bulkhead.

So far I think I'm good to go but I have a question with the 0 degree incidence angle. First of all, 0 degrees to what? I'm thinking that we're trying to get the plane which lies across the forward and aft HS spars to be parallel to the Aft Deck. I used 11/32" drill bits under the rear spar as directed. I then attempted the measurement from the Aft Deck to the spar tooling holes as directed. THIS IS A VERY DIFFICULT MEASUREMENT TO MAKE! Do you have any tips on how to make this measurement? Using a couple of methods (including a long distance eyeball) I measured that these measurements for the fore and aft tooling holes on the right side of the fuselage are the same (1 30/32" each). The left side measurements also agree fore and aft but with a slightly different value from the right (1 29/32" each). This seems good enough to me but like I said, I'm not comfortable with the accuracy of these measurements.

To verify my numbers I measured the distance between the bottom of the rear spar and the Aft Deck and the bottom of the front spar and the aft deck. These distances agree within 1/32". I'd still say everything is looking pretty good.

As a final check, I measured the angle of the plane between the front and rear spar and compared it with the angle of the Aft Deck. This seems to be the most straight forward measurement and easiest to make using my digital level. The trouble is, these measurements do not make sense. I measured the Aft Deck in front of the front spar and got 3.8 degrees. I measured the Aft Deck between the spars and got 4.0 degrees. I placed a level between the spars and measured 4.6 degrees. In order to make this number agree with the number I got on the Aft Deck I'd have to raise the rear spar by 0.8 degrees. This doesn't seem right.

So, should I ignore the angle measurements and drill away? Why would everything else check out fairly well while the angle measurement doesn't?

I tried to attach some photos but I had no luck.

Thanks again!

Kelly Johnson
San Jose, CA

 

[n2prise]

Drilling the HS to the fuselage.

You are making this way too hard on yourself.

http://www.n2prise.org/rv9a031.htm

I just looked at the photo I took of the work you are doing and I realized I did not say anything about the process I used. I measured from the same rivet on the end rib at both ends of the HS to the aft edge of the air scoop inlet on each side of the fuselage. Since ALL the holes and the scoop cutout are CNC machined at Van's factory, those are valid reference points.

Here is what makes this go easy for you.

1. Is the fuselage LEVEL laterally and longitudinally?
2. Is the aft fuselage deck LEVEL at the same time as the longeron at the seat back bulkhead? If not, you have a slight twist in the aft section of the fuselage.
3. The HS must also be level. If it is not, you may need a very thin shim to augment the two 1/8" spacers called out in the plans.
4. The drill bits under the aft spar is the easy way to go.
5. When drilling the holes to secure the front spar, be sure that you will have enough room to turn the bolts or nuts for the outside bolts that go through the longerons.

The bottom line, N2PZ flies straight and true at most cruising speeds. When I look over my shoulder at the elevator and horizontal stabilizer when cruising at 12,500 MSL (160 MPH TAS), the elevator is perfectly aligned with the HS for minimum drag.

I learned early on when I was building RC models, the wings and tail must be square with each other to get the airplane to fly right. I had one RC model that had a small twist to the fuselage. The fix for that one was to put the HS on the aft end of the fuselage with just a little balsa shim and the appropriate super glue to insure the HS was squared with the wings. That one flew nicely as well.

Jerry K. Thorne
RV-9A N2PZ 189.6 hours on the Hobbs meter.

 

[ArVeeNiner]

Thanks

Jerry, I think you're right. I'm making this harder than it is.

What I found is:

1-The easiest method is to measure from the tooling holes to the aft deck like the instructions say. The way to do is to place a dowel in the left and right tooling holes and measure the distance at the rear spare with a dial caliper or depth gauge and with a depth gauge at the front spar. I don't have a depth gauge presently (but I will tomorrow) but it seems that the fore and aft measurements are within a few thousands of each other. I used my dial caliper and a 6" rule to get a pretty close approximation.

2-Laying a straight edge across the plane that intersects the top of the front and rear spars will give you an incorrect measurement. If you measure down from the top of each spar to the aft deck you'll see that the tops are at different levels. All I have to say to myself is DUH. Of course this is the case, I have drill bits under the rear spar! If a digital level is preferred place one dowel across the rear tooling holes and another across the forward tooling holes. The dowels span from the left side of the HS to the right. Lay a straight edge across these dowels, then place the digital level along the longitudinal axis of the fuselage. Finally, you will get numbers that compare with the Aft Deck.

3-The best bet is to forget about item 2 above and just measure per item 1. That will work just fine.

I ran out of time tonight but tomorrow, I drill!!!

Thanks again!

Kelly Johnson
RV-9A
San Jose, CA