Engineering Change - Patrol Only - Flap Pulley Mount Reinforcement

[jaredyates]

Flap Pulley Mount Reinforcement at Fuselage Station F

There have been two Bearhawk Patrols that have experienced a failure of the weld that attaches the small tube used to mount the flap pulley to the T7 fuselage tube near where it joins the longeron, along with the T10 tube, at station F.

The fix is weld back in place and add a 1/4 x .035 tube from the top of the bushing (3/8x .058) to the cross tube (F 3/4) at about 45 degrees.

While this failure does not constitute a safety threat, it does create an operational inconvenience. Bob says that airplanes that have already been covered are fine to continue operating as-built, but he recommends that builders who can still weld in this area should add a short length of T1 tube as shown in the drawing. This is a precaution that is easy to add in the early stages of construction, and this change has been incorporated into production kits.

http://bearhawksafety.com/Patrol/july2021.html

[m.mooney]

Because my Patrol (400 hrs tt plans built) is the most recent of the two aircraft to experience this failure, I’ll chime in with my 2 cents. I land full flaps 95% of the time, my flap handle is a couple of inches longer than spec and it’s easy to pull full flaps at the common 50-55 mph that I use. This day I pulled the last notch at around 60-62 and got an immediate “clunk” and flap retraction to less than half, symmetrical. I retracted the handle further and landed uneventfully with flaps at approximately 10 degrees.

The pulley mount partially separated from the T7 tube that it was welded to. This resulted in the uncommanded retraction and the flap cable buried itself into my rear bulkhead, which is aluminum. The fact that the pulley mount didn’t completely separate from the tube, and I have an aluminum rear bulkhead which grabbed the cable as it tore into it, limited the uncommanded retraction to around 40% flaps. Imagine a scenario where you’re at full flaps, power on and slow, landing somewhere short and challenging. If this pulley mount were to experience a failure that had it completely separate from the T7 tube…..you just lost all of your flaps and might experience something unpleasant, to say the least.

The Engineering Change describes a very logical fix, which I’m getting ready to do. Worst case will be the removal of some belly fabric to access the underside of the tube for welding. If this is necessary, I’ll just make it an inspection cover. I feel very confident in the integrity of this new structural arrangement. Of note, is a similar tube that the builder (not me) welded into the other flap pulley mount to provide a similar level of strength to that mount. He didn’t like the amount of flex that the V tube was experiencing when deploying flaps. You can see this brace tube in the first two photos, the second photo shows the proposed fix drawn in red. Also, Mark tells me that the kit factory has already incorporated this fix as described in the Engineering Change.

As for compliance with the Engineering Change, I obviously have no choice; however, I can absolutely guarantee that I would also incorporate this fix with a healthy, flying airplane. This is just my opinion and it has many contributing factors; to include, frequent full flap, slow speed off airport operations. If you’re not dabbling in that….that’s your decision to make.

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[noema]

Thanks m.mooney. I really appreciate your explanation. It's interesting how the tube failed next to the weld, not the weld itself. Do you know was your frame TIG or oxy-acetylene-welded?

[m.mooney]

Agree that it doesn’t appear to be an actual failure of the weld, as mentioned in the Engineering Change. Fuselage was gas welded.
Mike

[Chewie]

Is the detail shown above a new detail or a revision somewhere in the plans? I was preparing to markup a drawing but could not find it, except for a zoomed out view of the fuselage station on sheet 31.

[m.mooney]

The sheet 31 drawing is the only one that I could find in the plans. This Engineering Change will be the most detail of the area in question, as far as I know.
Mike

[Pbruce]

I’m arranging to find a welder to this for me. I do like the idea of strengthening the other pulley mount (the one on one leg of the”V”) It also carries a combined load from two flaps. Is there any reason not to do it exactly the same as the fix on the longeron? ie. with the brace from the center of the pulley mount across to the other side of the “V”?

[m.mooney]

Collin built mine and he added the brace from the upper pulley mount to the T10 tubing at station F, see photos in my post above. His method seems more appropriate than what you suggest; but, I just fly em, not design em.

[alaskabearhawk]

The cable force looks like it’s in a completely different direction. By joining the V tubes you are altering load paths. One thing I have learned while building my fuselage is that I don’t know squat about engineering, even though it meets the “TLAR” of my Mark 1 eyeball. Definitely a question for Bob.

[Pbruce]

That’s a very good point. I see what you are saying: if a brace was put straight across the “v” using the minimal amount of tubing, it would in fact be misaligned with the load. If it were angled downward though, it might be bang on. In tension though-not in compression.

[jaredyates]

Bob has designed an alternate solution that doesn't require welding, and I have updated the notice to reflect that:

http://bearhawksafety.com/Patrol/july2021.html 2021c2.jpg

[Pbruce]

Super! Great for guys that can’t weld, and probably easier for anyone with a covered plane. Thanks Jared.

[spinningwrench]

I agree with some comments previously mentioned. If you look at the central flap pulley mounted on the V in the middle of the fuselage the forces are just as large if not larger than those at the lower longeron. Because the moment arm is slightly bigger at the V than at the longeron the bending moment will be even larger. Can Bob comment on this, I think the central pulley needs to be reinforced as well if we have already experienced failures at the longeron pulley.

[Mark Goldberg]

I spoke with Bob about this. First of all - no failures of this in thousands of hours in many Patrols. And Bob says steel is meant to flex. He says sometimes making things rigid just adds stress. However, if you do this - weld the reinforcing tube to the bushing on the tube. Not the tube itself. Mark