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Horizontal Stabilizer Triangulation Discrepancies

mfleming

Well Known Member
Patron
I decided to start a new thread on my -7 horizontal Stabilizer mounting efforts. My previous thread talked about my difficulties getting the HS sitting far enough forward to mount properly. That's resolved.

The issue now is triangulation of the HS.

I figured this would apply to many more models than just the -7 so I'm posting it here in the General Forums area.

The plans call for clamping the HS on the aft deck along the fuselage centerline. Then measuring from a common rivet on the HS tip to a common rivet on the firewall. Those numbers should be the same if the HS is perpendicular (square) to the fuselage.

So I have a quick build fuselage and the darn things not square. I was getting a pretty good error when trying to measure to the firewall. The HS was obviously not square when the firewall measurement were the same. So I used the method shown in the Les Bourne Series videos. He uses a rivet in the center of the fuselage for a reference.

When I measure to the fuselage center, all looks well. The HS is only slightly off when viewing the aft deck.

I think the fuselage center rivet is the way to go...I'm just wanting to make sure before I start drilling holes :eek:

Here's what I'm talking about.

stab14.jpg


stab15.jpg


stab16.jpg
 
Measuring

I took a 6' piece of aluminum angle and centered it to the fuse across the longerons and basically over the center section so a single screw in the angle served as the point of reference. Easy to hook a tape measure to the screw head. It was very useful for all the tail measurements. Just make sure that screw is centered to the fuse. :D
I do recall my HS was a tiny bit off center like your photo but the tips were exactly the same distance from the reference screw so I called it good.
Wait till you get to the VS! :D
Tip. Dont cut that mounting plate per plan. In fact, don't cut it at all till you measure and get exact amount to trim. I had to fab a new one.
 
1/8" difference in the HS tip to firewall measurement is insignificant in the grand scheme of things. Build on!!!

You're getting to a critical step of drilling the holes for HS mounting. Take a ton of time here and really nail those edge distances on the longerons.
 
You can put the hs on center and then get it square by rotating about your centerline. Since your bars needed to be tweaked you could be fighting a little difference in the way you bent them. It wouldnt take much difference in the bars to be 1/8? out at the tips
 
I took a 6' piece of aluminum angle and centered it to the fuse across the longerons and basically over the center section so a single screw in the angle served as the point of reference. Easy to hook a tape measure to the screw head. It was very useful for all the tail measurements. Just make sure that screw is centered to the fuse. :D
I do recall my HS was a tiny bit off center like your photo but the tips were exactly the same distance from the reference screw so I called it good.
Wait till you get to the VS! :D
Tip. Dont cut that mounting plate per plan. In fact, don't cut it at all till you measure and get exact amount to trim. I had to fab a new one.

Thanks for the tip about using the aluminum angle across the fuselage. I looked that up in your log...I'm doing that tomorrow :D

As an aside, did you have your 3/16" aft spacers (drill bits) and your F-798 forward shims in place when you drilled your pilot holes through the HS-714 angle? The plans say no but the Les Bourne video says yes.

About the tip on not cutting the mounting plate...are we talking about the F-781 attach plate for the vert stab??
 
You can put the hs on center and then get it square by rotating about your centerline. Since your bars needed to be tweaked you could be fighting a little difference in the way you bent them. It wouldnt take much difference in the bars to be 1/8? out at the tips

Yes I agree, the vertical bars are probably throwing things off a bit.

I was really caught off guard on how a very small change at the centerline made such a huge change at the HS tips :eek:
 
Centerline Reference

Yes I agree, the vertical bars are probably throwing things off a bit.

I was really caught off guard on how a very small change at the centerline made such a huge change at the HS tips :eek:

As others have said, 1/8? error is quite insignificant in the overall scope of things. However, I noticed that you are using several different centerline references for your HS with different results. What?s important here is that whichever references you use for your HS stabilizer rigging should also be used for centerline reference when you install and rig the wings. Frankly, I would use a centerline point at the firewall as one of the references for both.

Skylor
RV-8
 
Tips

Thanks for the tip about using the aluminum angle across the fuselage. I looked that up in your log...I'm doing that tomorrow :D

As an aside, did you have your 3/16" aft spacers (drill bits) and your F-798 forward shims in place when you drilled your pilot holes through the HS-714 angle? The plans say no but the Les Bourne video says yes.

About the tip on not cutting the mounting plate...are we talking about the F-781 attach plate for the vert stab??

Yes on the spacers. I had it exactly where it needed to be before drilling.
Yes, F781. Van's plan shows a dimension to cut. I cut less and still ended up fabricating a new one.
 
As others have said, 1/8” error is quite insignificant in the overall scope of things. However, I noticed that you are using several different centerline references for your HS with different results. What’s important here is that whichever references you use for your HS stabilizer rigging should also be used for centerline reference when you install and rig the wings. Frankly, I would use a centerline point at the firewall as one of the references for both.

Skylor
RV-8

I just reviewed the wing rigging plan. The plans say this: "Then square the wing with the fuselage. This is done by measuring from corresponding points on the wing tips to a common centerline point of the aft fuselage"

I'm not sure how to equate the HS being square with a point on the firewall and the wing being square using a point on the aft fuselage, being that the firewall is off by ⅛" :confused:

The only centerline on my QB fuselage that lines up is from the aft deck to the top of the aft fuselage.

Here's a photo with the laser lined up on the fuselage centerline.

stab17.jpg


I suppose I could use the most forward centered rivet on the top of the fuselage for squaring the HS. Then use the very aft centerline point on the fuselage for squaring the wing.

This would square everything to the same reference line!?
 
Keep following the plans in sequence as you have been. As mentioned 1/8? is nothing in the scheme of things here. It?s been a long time, But I don?t remember any references to square the HS to the wings?
My QB center section was out of square 1/4? to the firewall. Everything still worked out with one wing zero sweep and the other having 1/2?. It made no difference and she flew straight and true. There is no way my HS is square to the end of the wing. That would be impossible on my build.
Your asking good questions, but don?t get hung up on it. QB fuselages may not necessarily be perfectly square as you are finding out.
 
Sqare?

I have worked on a lot of zillion dollar aircraft
Civil and government and have never seen a
?squared? one....ever, 1/8 inch don?t worry
About.
 
Sqare?

I have worked on a lot of zillion dollar aircraft
Civil and government and have never seen a
?squared? one....ever, 1/8 inch don?t worry
About.
 
Keep following the plans in sequence as you have been. As mentioned 1/8? is nothing in the scheme of things here. It?s been a long time, But I don?t remember any references to square the HS to the wings?
My QB center section was out of square 1/4? to the firewall. Everything still worked out with one wing zero sweep and the other having 1/2?. It made no difference and she flew straight and true. There is no way my HS is square to the end of the wing. That would be impossible on my build.
Your asking good questions, but don?t get hung up on it. QB fuselages may not necessarily be perfectly square as you are finding out.

Thanks for the encouragement. Sometimes analysis paralysis can sure slow things down ;)
 
I just reviewed the wing rigging plan. The plans say this: "Then square the wing with the fuselage. This is done by measuring from corresponding points on the wing tips to a common centerline point of the aft fuselage"

I'm not sure how to equate the HS being square with a point on the firewall and the wing being square using a point on the aft fuselage, being that the firewall is off by ⅛" :confused:

Yes, you’re correct about the plans. My bad. It’s been a while since I rigged mine.

The comment I made about using the point on the firewall is due to the fact that the point furthest away from the HS should in theory yield a more accurate measurement because of the better angle.

Skylor
 
Last edited:
Thanks for all the help - VAF is great!

I'm sure this is boring by now but thanks for everyone who helped get my head around this.

Today I finally mounted the HS. It came out perfect.

stab18.jpg
 
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