Vertical Stab Measurements

[Rupester]

The 9A 'structions say, "Take measurements from the tip of the VS to both tips of the horizontal. Adjust the VS until the measurements are equal and the stabilizer is truly vertical."
Well,heck ... the F-981 VS bracket gives the VS a 1/4" offset to the left. Why would I want the measurements to the tips of the HS equal when the VS is not on centerline? And when they say "tip" are they talking the forward-most tip of the VS (which is a few inches above the rear deck), or do they mean the very top of the VS?

 

[Greg Arehart]

Terry,

I measured from the "top" of the VS at the back to each of the HS tips at the back. Although the front of the VS is offset, the back is not, only slightly twisted (rotated about the yaw axis) so it **should** be truly vertical and centered.

greg

 

[rrd1_99]

I am just getting ready for this myself. My understanding (don't have plans in front of me) was that you plumb the VS with the measuring method to drill the rear spar holes, then insert the F-981 to get the desired offset. Measure from the upper tip to the outboard edge of the HS. I would use a string from one of the open tip attach holes on each side of the tip to an identical location on each outboard HS edge.

Hope this helps!

Roger
-9A slow QB
La Grande Oregon

 

[Rupester]

Thanks! ... that all makes good sense. Sure do like getting knowledgable responses in a couple of minutes!

 

[MauiLvrs]

... then insert the F-981 to get the desired offset. ...

The F-912 bulkhead is not perpendicular to the centerline and creates most of the off-set... so the offset will already be in place. Center the F-981 with the VS-802.

NOTE: Don't forget to trim the bottom of VS-802 (DWG 27) like the guy we got our project from did. Having had to fix ours, you don't want to have to fix yours. (We had to fabricate a new F-981 from stock and then back drill the spar to the F-981.)

In addition to measuring near the rear VS spar to the aft HS a dozen or two times with a metal tape, we double checked the angle numerous times with a digital level to make sure that in addition to the measurements being the same the angle was the same on both sides. So, it should be "truly vertical" within 0.1 deg.

 

[rrd1_99]

That doesn't sound right...

The F-912 bulkhead is not perpendicular to the centerline and creates most of the off-set... so the offset will already be in place.

I am at my day job and don't have my plans in front of me, but I certainly don't remember ANY of the bulkheads being offset. My fuse is a QB, so it's possible I missed that but I am CERTAIN that the left deflection of the VS leading edge is a direct result of the "joggle" built into the F-981 bracket that attaches the VS front spar to the HS front spar and fuselage deck. Am I missing something? (Wouldn't be the first time!)

Roger
-9A slow QB

 

[MauiLvrs]

It's off-set, that's why you have to add a washer between the F-912D and the rear spar. Not much, but enough to displace the front of the VS about 1/4 inch to the left.

 

[Bugsy]

Why a 1/4 inch

Why is the VS Stab offset a 1/4 inch to the left? Engine torque or something else?

 

[rrd1_99]

Left-turning tendencies

It simply helps offset the left-turning tendencies inherent with single-engine tractor airplanes, torque being one of the causes.

Roger

 

[C-J]

Off Set

Yes, it can be done in many ways like right thrust in the enginie mount etc, but Van uses the Vert stab offset. Don't forget the washer between the VS and the bulkhead to compensate. Cheers C-J

 

[Bugsy]

Thanks

Thanks for the responses and I just put a sticky on the VS to remember the washer when I put this back together for final assembly.

 

[captainron]

Thanks for the responses and I just put a sticky on the VS to remember the washer when I put this back together for final assembly.

Sorry, I didn't like the washer idea. I built the dimension into the part. Since you have to fabricate the up-elevator stop anyway, why not just factor the washer thickness into it and skip the washer? This shot was taken with the aid of a mirror, so the image is reversed.

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