Horizontal Stab Incidence and Strut Construction

[lsa140]

I was reading through my plans and didn't see much detail on the horizontal stabilizer strut for the LSA. Someone asked about it a couple years back in the LSA forum, but it didn't seem like they found a clear answer. ( http://bearhawkforums.com/forum/bear...abilizer-strut )

I have both the Patrol and LSA plans. As mentioned in the older thread, there is a drawing of the strut on page 21 of the Patrol plans, with no material/details called out for the sheet that forms the attach points. "Horz. Stab Strut 1.012 x .428 x .035" is called out next to the attach point detail on page 19 of the LSA plans. It looks like both the Patrol and LSA use the same strut tubing.

Looking through Eric's builders manual, he shows the process for building a stab strut for the 4 place. (After the stab incidence is set) It looks like 035 formed into a channel, notched into the strut for the bottom attach point, and the strut flattened, bent and drilled to match the horizontal stab (top)attach point. This seams reasonable for the LSA and Patrol as well? Does anyone have additional information?

I also note that the LSA plans show a 3/4" long front stabilizer attach bushing that can be adjusted "as needed for stab. trim" (pg 17). It looks like the plans call out -2 deg angle of the horizontal stab. I'm assuming that after the horizontal stab is mounted to the rear attach point, it is set to -2 deg, and the attach bushing is fine tuned to hold the stabilizer at that angle. Does the horizontal stab incidence ever require fine tuning after the aircraft is flying?

[lsa140]

Looks like Bob used an adjustable strut end, but I'm guessing he did so on the prototype to fine tune the incedence during flight testing.

[whee]

The 4place kit uses a threaded end on the fuselage side. I plan to do the same.

[lsa140]

Good to know, I'd like to have the ability to make fine adjustments.

[Battson]

You will need that fine tuning ability to make sure there is no twist in the horizontal stabiliser.

[whee]

Goes together something like this.

[Battson]

It's really important to ensure that the threaded section and abutting radius section does not get ANY weldment on it, and that the distance from the bolt eye to the weld is kept as SHORT as possible.

I had an in-flight failure of this part, at the thread, for the reasons stated above.

The part is not critical to flight safety and is not even necessary for flight - the prototype first flew without any strut. But the failure does get your attention in fight.

[lsa140]

Good information guys, thank you. Battson, now I am remembering the service notice Mark posted in the 4 place Bearhawk forum regarding the stab strut: http://bearhawkforums.com/forum/bear...or-stab-struts

I was cringing at heating up, smashing, and bending a piece of airfoil tubing. That's a lot to put the steel through, long before any loads are applied. I see it was discussed in Mark's post, and additional measures were taken to prevent cracking. The tail surfaces on fabric airplanes can shake quite a bit under high power. I could see where there would be some cracking issues.

There wont be a fire breathing 540 blasting back on my LSA tail, so that should help .

I think some of the Maules have dual struts under the horizontal stab, I wonder what the ends look like on those. That's all the excuse I need, time for a trip to the airport.

[whee]

I was cringing at heating up, smashing, and bending a piece of airfoil tubing.

Still have to do that on the H-stab side of the strut. I'm not really excited about it either and that is part of the reason I haven't made those struts yet.

[sbmurphey]

This is an area I have had several questions while looking at the LSA plans. Thanks for all the input. I did snap some pictures of the strut on Bob's LSA to help when I get to that point.

[lsa140]

The Maule is similiar to the 4 place Bearhawk on the top attach point of the front strut. Looks like they bolt up to a tube on the bottom attachment.