Engineering Change for Bearhawk 4-Place and Patrol

[JimParker256]

Do we have any statistics on how many failures of this type there have been? If it's one or two out of the fleet, that's one thing. If it's 10% of the fleet, that's something else...

[Mark Moyle]

Perhaps......we could ask The Bob....about cutting the stream line tubing somewhere along its span as if it were a repair to a longeron. Insert a tube and rosette welds four at each end of the inserted tube.


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[DRLPatrol]

It would also make a difference knowing what led up to the failure. If it was a full blown, high speed ground loop then I would agree with Bob and rather the gear failed than taking out the lower longerons.

Doug

[whee]

The failure mode on there struts is buckling. When you side load the gear during a ground loop or a turning takeoff/landing it exerts a side loading the strut, a quite massive one because of the steep angle between the shock strut and the axle. To beef up the strut enough to handle more side load you start exceeding the strength of the attachment fittings and associated tubing.

This is side loading is something I have thought a lot about because I used to fly a luscombe which has a similar gear geometry but uses a center oleo which is attached to the gear legs using flying wire. Obviously a wire can handle zero compressive loads and some Luscombes have folded their gear under because of side loading.

I know Bob has received many calls from people saying he needed to redesign the strut to handle more compressive loads. It would also be easier and cheaper to manufacture one type of strut out of round tube.

All that being said when I ordered my gear from Mark I requested the round tube strut. My BH will be my off-road SUV and will have to endure turning takeoff/landings and off canter rough terrain. If I didn't plan of abusing my airplane I would have been totally fine with the strut and Bob originally designed it.

[jaredyates]

Jim and Doug, I don't know if anyone has figured any solid statistics. Anecdotally, there have been a handful of planes that had the streamlined struts fail. There were a couple with round tubes that had ground loops and stayed on the gear. Is there a way to know if the failed streamlined tubes would have failed even if they were round? Dan R. or Dallas (I can't remember which one) told me once about some calculations that they did, and maybe he can remind us of what those numbers were, but the streamlined struts were significantly weaker than a similar round tube.

My impression is that Bob has specified a round tube that still mitigates his concerns for damage to the rest of the airplane, though his wording in the 2010 shock strut notice was this: "This change is much stronger in resistance to side load which can cause buckling of the shock strut. Hard ground loops on black top still will cause the wing to impact the ground and be damaged, possibly even more so if the gear does not fold under."

The 2010 update said "The landing gear shock strut is now being offered with an alternate design..." in contrast to the 2016 update that is eliminating the streamlined tube option entirely.

The 2010 update is here: http://bearhawksafety.com/4-place/noticejan10.html

[marcusofcotton]

When the streamline LG strut fails it will at about 12"-13" above the lower end of the strut.

Thanks for the feedback Dan! I put a lot of value on your input.

[marcusofcotton]

Perhaps......we could ask The Bob....about cutting the stream line tubing somewhere along its span as if it were a repair to a longeron. Insert a tube and rosette welds four at each end of the inserted tube.


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Precisely what I'm thinking Mark. Leaning towards 3/4 x .065.

[marcusofcotton]

Jim and Doug, I don't know if anyone has figured any solid statistics. Anecdotally, there have been a handful of planes that had the streamlined struts fail. There were a couple with round tubes that had ground loops and stayed on the gear. Is there a way to know if the failed streamlined tubes would have failed even if they were round? Dan R. or Dallas (I can't remember which one) told me once about some calculations that they did, and maybe he can remind us of what those numbers were, but the streamlined struts were significantly weaker than a similar round tube.

My impression is that Bob has specified a round tube that still mitigates his concerns for damage to the rest of the airplane, though his wording in the 2010 shock strut notice was this: "This change is much stronger in resistance to side load which can cause buckling of the shock strut. Hard ground loops on black top still will cause the wing to impact the ground and be damaged, possibly even more so if the gear does not fold under."

The 2010 update said "The landing gear shock strut is now being offered with an alternate design..." in contrast to the 2016 update that is eliminating the streamlined tube option entirely.

The 2010 update is here: http://bearhawksafety.com/4-place/noticejan10.html

I see Bob increased the round tube wall thickness to .065" from the 2010 figure of .058"

Hmmm, might be good to design some energy absorption into the wingtips.

Thanks everybody for discussion!

[Bdflies]

No doubt, the folded gear strut wouldn't be a good thing. Can anyone elaborate on what other damage was done to the plane, when the struts folded? Catching a wingtip isn't great, but a bent lower longeron sounds really bad! If the bulk of the damage, with the streamlined strut, is the bent strut and a piece of wrinkled sheet aluminum, I think I'd rather go that route. Or, was there a prop strike involved???

[Mark Moyle]

Precisely what I'm thinking Mark. Leaning towards 3/4 x .065.

I'll have to add this to my to do list. Like Wee my Bearhawk will be an off airport SUV. One thing I did when building the fuselage was to increase the lower longeron wall thickness to .049"

[Dan R.]

Increasing the wall thickness by adding a tube inside the SL strut will be almost no help in increasing the sideload. You will be just adding weight. Its tube dia, not wall thickness. If you just increase the dia to a 1" round tube you will be increasing the side load from 3200 lbs to 6100 lbs. Do it Bobs way by increasing the round tube to 1 1/8" for a side load of about 9500 lbs. 3 times as strong as the SL strut, and be happy

[Battson]

The background:

Bending strength is related to an engineering term called '2nd moment of area'. Basically how much metal exists, and how far away it is from the centre-line axis of bending.

Both a thicker wall and a larger diameter tube would increase the 2nd moment of area of the tube, which increase bending strength in both cases. Notwithstanding that, you are right, the lightest solution is a larger diameter hollow tube. That is because it has more cross-sectional area further from the centre-line of the tube, and this has a greater effect on the 2nd moment of area than a smaller thicker tube. Comparatively, with a thicker tube of lesser diameter, you need a lot more metal to get the same bending strength.

[marcusofcotton]

Good to ruminate some more on this. Bob's two versions weigh essentially the same. Adding the 3/4 x .065 will add two pounds - my building goals are for a STOL light plane, not high speed (but do love efficiency). Already have new Avipro gear struts (actually just the outer portion - bought 2nd hand).

[alaskabearhawk]

Ask Russ Erb about the strength of the gear with streamlined tubing. He did a ground-loop with Three Sigma and it was severe enough that the tire was pushed off the rim and the wheel dug a deep groove in the pavement. Very tight arcing skid mark. I saw it in person and have pictures...no gear failure. I'm going against all here, but IMHO, fixing a problem that doesn't exist.

[Glenn Patterson]

We had concerns with the streamline tube after reading some discussion and of Pat Fagan's & another incident. I cannot tell what we used but we installed a snug fitting round tube inside the streamline tube for reinforcing. There was a discussion years ago on this tube being designed to buckle to save the fuselage. Some commented that it would be less expensive to repair the fuselage than fix an engine & prop strike. My gut instinct was that this tube needed to be beefed up & this confirmed it. We chose back then to stiffen the streamline tube to protect the engine & the CS prop reasoning that fixing the fuselage would cost less. We drilled & bolted the wing up yesterday & will install the wing struts today. Shock struts hopefully will serve us well.

[marcusofcotton]

Thanks for chiming in Paul and Glenn! I had forgotten about Russ's experience.