Bearhawk 4-Place Off-Airport Steering Technique and Tail Reinforcement

[Battson]

From an airmanship point of view coming to a stop on the airstrip and pushing the aircraft off is far better than taxiing on rough ground or doing a tight turn that has the potential to side load the stinger excessively.

For the benefit of others reading this (and none of this is Bearhawk specific):

Shutting down and getting out to push is a great idea if someone taxis into "no man's land". It's embarrassing but worth the price.
Taxiing straight ahead or making slow sweeping turns is extremely unlikely to cause any damage.

Particularly when small groups of aircraft are flying together though, shutting down to line each plane up on the end of the airstrip is impractical, so we usually swing the tail around under power with great care. Planning the turn before getting into the plane is important.

For those who are wondering how this damage happened:
In this case we wanted full length on this short airstrip. We had to taxi between two spinning props so needed to continue a little further than planned, to make room for the turn - taking us off the end of the airstrip. Then we were forced to taxi a little further still, to miss a hole we could see (getting into no man's land at this point). Finally we swung the tail around rather quickly and hit a different hole / obstacle we could not see. I had a good look at the area before the turn.... This is always the risk with grass airstrips - grass hides things very effectively, even snow-flattened dead grass. Each airstrip has so many hazards.

It's preferable to walk the airstrip (or know the airstrip intimately) and decide where to turn / taxi before starting the engine.

Off airport operations are all about ground handling, as boring as that sounds. Until you touch the ground, you aren't off airport.

[Bissetg]

That is correct, it’s important to understand that beefing up the aeroplane is not a substitute for good airmanship. Side loads on the tail stinger should be kept to a minimum.

[Wyo Johnson]

The first Super Cub I ever restored was a 1957 PA-18A. I was literally putting on the last wing inspection panel when a local rancher heard about it and came into the hangar...he purchased the airplane and flew it back to his grass strip in front of his house in western Nebraska. Upon landing, with family and friends there to see the new plane, he spun the aircraft around to tie it down but caught a metal "T" post that had been cut off and was hiding in the grass. He just caught the lower longerons about 12" ahead of the tail post and the fence post sliced through about 75% of the tubes...the Rancher just thought it was a bump...until he got out and the tail was bowed severely because the lowers buckled and the uppers bowed. He was hot rodding it or doing anything he hadn't done 1,000 times but just forgot that the stupid post was there. My takeaway was that of the above comments that airmanship and ground handling are equally important! Back country flying also should change the interval and type of inspections you do...in other words a pre and post flight might look at the tail wheel area a little differently...trying to detect damage before it propagates.

Andy

[svyolo]

I am having a hard time with this one. I will look at my fuselage in the morning. At the moment, the only way I can think this can happen is with repeated, massive side loading. With vertical loading the lower longeron should be in tension. I can't see a way to load the tailwheel negative in the vertical to put the lower longeron in compression.

[Battson]

When the tailwheel is loaded with a large side force, a bending force (torque) is applied to the lower longeron. Tension and compression are not the whole picture. You need to consider the three axis of torque applied, as well as the three axis of forces, and then buckling calculations, rather than just linear force calculations, to resolve the stresses in each member.

[Bissetg]

If you haven’t yet covered your fuselage, support the rear fuse so the tail wheel is off the ground and give the tail stinger a wiggle from side to side and you’ll see how it works. Repeated massive side loading is not required.

[Nev]

I am having a hard time with this one.

You're not alone here, it took me a while to get my head around this. When the tailwheel is at 90° as in a tight turn, the normal weight of the aircraft is acting vertically downward on the stinger, but because the wheel is now off to the side it is inducing a torsional (rotation) force through the stinger up into the longerons.

In addition, if the wheel then hits a bump, it induces a lateral force pulling the stinger in one direction and transferring the opposite force into the longerons. However a lot of what would be a lateral force is also turned into a torsional force and added to that caused by the weight. Particularly if it's a sharper "bump" the resulting loading could be quite high. Whereas the leaf spring may have dissipated some of the torsional stress, the stinger doesn't do that so well.

Heres a novice drawing (not to scale and possibly with the authors new 29" ABW's on ) of a Bearhawk viewed from behind, with the tailwheel sitting 90° off to the left side to illustrate and about to run over a bump. The result is a strong torsional force on the stinger being transmitted into the aft structure.

A731D58C-4179-4DA6-A3D9-1922C5EF289A.jpg

[Utah-Jay]

I think Nev had the same art class as I did

Joking aside, the drawing and explanation make sense. I have been trying to figure out how this happened. Certainly a case can be made for lightening the tail when making tightish turns on unimproved strips.

It also seems from Battson that it was a bit of a perfect storm that created this one off issue

[Battson]

I think Nev ertainly a case can be made for lightening the tail when making tightish turns on unimproved strips.

One thing I would say about this, it totally depends on the surface you're working on. Different approaches are needed for sand, gravel, rocks, and grass.

I never like pushing forward on the stick while taxiing off-airport, you're asking to damage the elevator or trim drive - this can happen in several ways. I prefer to turn slowly if possible, but it is not always possible.

[Bissetg]

I think Battson is right, forward stick while taxiing is fraught with risks of other sorts. I think if in doubt jump out and man handle it. It’s not like we are burning $$$ on start cycles. We all want to look out for ourselves and others so taking a little longer in order to protect our airframes shouldn’t be too onerous.

[500AGL]

In case it helps anyone, I just welded in the gusset as cut to Bob’s specs for the 4 place. As I have a patrol kit the angle is slightly different but it works fine as is.

note : I cut two because I owe a guy. Only one per tail.
pb

[DBeaulieu]

If I understand Mark Goldbergs' msg about his recent factory visit, this reinforcement wll be on all the kits shipped out this upcoming February...

[500AGL]

If I understand Mark Goldbergs' msg about his recent factory visit, this reinforcement wll be on all the kits shipped out this upcoming February...

Yes that’s correct. This is just for those in process that plan to add.

[Mark Goldberg]

Actually, that tail longeron reinforcement has been on kits in the last shipment and the one around the end of this tear. What will not be on the kits until February is the boot cowl improvement. Mark