new "misshap" sub-forum and gear strength ?

[fairchild1934]

browsed through the new sub-forum last night. was struck that about 90 % of the incidents all seemed to be the same event----- "after touchdown---
xxxxxx happened and I tried to correct with rudder to no effect and then with brake to no effect and the aircraft exited the runway and xxxxxxxxx........"

I would think this indicates that we have lots of pilots who are taking too much for granted and not staying on top of and ahead of the plane on roll-out.
Perhaps we are lulled into a false sense of comfort because the thing flies so good and lands slow. ( I wonder if everybody is checking toe-in and out on their gear legs ?)

I notice many of these incidents also include--- gear leg collapsed during ground loop. (substantial damage to airframe and wings) But this brings up a question----- are the gear legs strong enough ? I THINK I remember seeing where bob had published something to strengthen the legs. I remember where Univair also added an extra tube to their gear legs.

I assume it would be stupid (impossible) to try to make the gear strong enough to withstand any and all insults of side loading--- but at the same time we dont see pipers and cessnas haveing gears folding frequently. Bent on occasion ---

I would think -- as you make it stronger--- there comes a point where you end up moving the damage from the gear to the tubes where the gear attaches----
which at that point maybe the gear is a tiny bit too rigid. But I get the idea we are not at that point yet. ( like the cessnas that have the flat spring gear---
sometimes that gear remains intact but takes out the sheet metal where it attaches---- )

Where are we currently at- in terms of gear strength ? it should be able to tolerate boo-boos that would not otherwise damage other parts. (yes-no ?)

Tim

PS-- one point the reports seem to drive home ---- if after touchdown--- if its anything other than behaved--- jamb the throttle and get airborne again. No reason to feel committed to a landing---- just go around..... once those mains are
off or even "light" ---- less risk of tire traction forcing you off the runway.

PPS----- and another follow on question would be---- is the rudder large enough ? im not qualified to answer that question---- (or maybe even ask it--) but maybe is there a consensus of what "enough" is in terms of rudder authority at low roll out speeds ?

[Nev]

Tim, I think you're seeing the correlation between incidents that a number of us are seeing.

I notice many of these incidents also include--- gear leg collapsed during ground loop. (substantial damage to airframe and wings) But this brings up a question----- are the gear legs strong enough ? I THINK I remember seeing where bob had published something to strengthen the legs. I remember where Univair also added an extra tube to their gear legs.

I believe the gear leg tubing has been addressed some years ago. In short, I don't think that a loss of control is being caused by a gear leg strength issue, rather it is the result. Like the rest of the aircraft, they're very strong in the direction of intended use. However once the aircraft direction has changed to the point that the CG trajectory lines up with one of the wheels (or exceeds it) then something has to be the first to give.

PPS----- and another follow on question would be---- is the rudder large enough ?

I think most of us agree that the rudder is plenty large enough for full control, in fact once you're used to is it's what gives the Bearhawk really good handling authority in crosswinds.

My own thoughts are that this is more of an "us" problem (tailwheel experience/currency) than a Bearhawk issue.

[svyolo]

I also believe it is an "us" problem. And I would be a poster child for "us".

[zkelley2]

Ya, it's a pilot problem. The gear leg strength issue was fixed, to the extent that it was an issue to begin with. Rudder has plenty of authority.

It may very well be worth while for someone to do a write up on checking gear alignment if that hasn't bee done already.

[fairchild1934]

Yes-I think its 100% a training and awareness issue. The bearhawk may be spoiling its pilots and lulling them into paying less attention that they might on a plane having a more aggressive manners on the ground. but be that as it may---- i was thinking that a stronger gear would be good an any taildragger
so it could put up with surviving the student pilot. ( like a stearman-- ) and the occasional swerve.

so the gear mod for the Bearhawk was done a while back ( before my plans- which are about 4 years old---) and has been incorporated into the plans.
so I dont have to worry about building it in.

I wonder how many builders build in some toe-in. ? and how many check it once the plane is built ?

[jaredyates]

Everyone should check wheel alignment before the first flight, this is crucial. Don't wing it on the specs, Bob is very specific about how to do it. https://bearhawk.tips/1002

[zkelley2]

So given that everything is welded, what's the solution if it's got some toe out? Bend the gear?

[jaredyates]

Yes, with heat.

[Bcone1381]

Bob produced the Engineering change in 2016 and it applies to the Patrol and Four Place models to increase side load capabilities. Originally the gear had stream lined tubing. Now the shock strut uses round tubing.

http://bearhawksafety.com/Patrol/june2016.html

[Mark Goldberg]

For those of you that are perhaps not aware of how planes are designed - the landing gear can for sure be made so strong it would NEVER break. But what would then break in an overload/over stress is the fuselage which is much more difficult and expensive to fix compared to landing gear. Bob thinks about and designs into the plans exactly where something might break when the plane is stressed beyond the design strength. In order to make it safer and more repairable.

To my knowledge Bob is the only design engineer to design to utility category strength at full gross weight. Everyone else designs to standard category at full gross. To achieve utility category strength the other planes have to be several hundred pounds under the gross weight. No plane likes to have side loads on the landing gear. It is our job to keep them going straight down the runway. Although I have done some swerving around on the runway in cross wind conditions on occasion. Mark

[zkelley2]

The utility category thing is just G loading and where you set the gross weight. Cessna could just reduce their gross weight and say the same thing. Or bear hawk could increase the gross weight rather substantially and have a lot more useful load, assuming that the landing gear can take that load, which I think we've discussed it cannot, at least to the standards you want it to where people are dropping it in and having enough margin to not bend things. But for the wing and fuselage structure to do 2500lbs at 4.4G, by definition you can do 2890 at 3.8G, as these are the exact same forces on the aircraft.

[kestrel]

I wonder how many builders build in some toe-in. ?

Why toe-in? If you swerve left, you are transferring weight to a wheel that is pointing further left just when we really need to go right?

[AZBearhawk272]

Well,

I just spent 1/2 hour with Toe-in tutorial and it looks like it did not post.
Jared can you see if the reply is floating around somewhere in the wrong place?

The real nugget is that Toe In is really important and self correcting. Yes, it is pointing left but has more frictional drag and scrub than the left wheel. The drag on the right is correcting and not reinforcing.................
Please stand by as technical difficulties are sorted out...... : )

Thanks

Kevin D
#272

[jaredyates]

Hi Kevin, very sorry for the trouble. I tried logging into the database directly. There is a table where autosave content is stored, but the only entry was for this post. Unfortunately I'd say the last one is lost. I wish I knew more about why or how or what happened though! Did you press post and then it just failed?

[AZBearhawk272]

Thanks Jared


When I posted reply it stopped and asked for my login PW. Then continued as normal with no post.
I’ll get back to it in a day or two.

Kevin D.

[marcusofcotton]

Yes, always best to highlight and "Control C" posts before hitting post, especially if they've taken some time. Looking forward to your always excellent input Kevin!

[AZBearhawk272]

Ok second try, Toe In Why?

In general, slight Toe-In has a stabilizing / self centering capability for small excursions off the CL track.
This is not a statement that Toe-In will eliminate a ground loop or swerve as they are strongly influenced by the inertia of the CG mass.

Many aircraft have Toe-In rigging on the main landing gear. Bob Recommends Toe-In rigging. 1 Deg in per wheel.

Both wheels Toe-In, as the aircraft moves forward the wheels move toward the center of the aircraft, compressing the struts positioning them into position to absorb / damp rebound.

Both wheels Toe-In, as the aircraft moves forward and turns, not swerves to the left. ( for example )

The right wheel begins to have a greater angle inward to the forward motion of the aircraft. It stays loading the strut in compression and picks up a higher scrub angle / friction between the tire and the surface. The tire is resisting the inertial load of the aircraft trying to go straight. Highly loaded, scrubbing, friction, holding the aircraft back.

The left wheel begins to have a lesser angle inward to the forward motion of the aircraft. It unloads compression in the strut and picks up a lower scrub angle / friction between the tire and the surface. The tire left tire is unloaded of weight scrub and rolling friction and is very happy to go straight ahead.

At this snapshot in time, the difference in couple moment between the right wheel forces and the left wheel forces, the drag on the right side is greater than on the left and the result is to restore equilibrium. Right wheel retards right side of aircraft due to higher drag and the left side " Moves Forward " due to less friction. The system is inherently stable and damped.
This does have a limit that is proportional to the Toe-In angle. Turn a couple degrees further that the Toe-In angle and situation changes rapidly...........

Continue the left turn a bit longer, the right wheel is increasing in scrub, load and friction. The left wheel is now angled out from the CL of the surface and is beginning to start a scrub that is
pulling the the left wheel away from the center line of the aircraft, extending the strut creating a situation where the moment arm of the left wheel is greater than the right wheel.

Now we get to the high speed portion where all of a sudden, what happened, we swapped ends...................
The left wheel loses effective ground contact, the inertia of the aircraft CG in motion is heading to catch up with the right wheel which is in effect the center pole on a merry-go-round.


Toe-Out

Both wheels Toe-Out, as the aircraft moves forward the wheels move away from the center of the aircraft, extending the struts positioning them in a position where they provide no damping.

Both wheels Toe-Out, as the aircraft moves forward and turns, not swerves to the left. ( for example )

The right wheel begins to have a lesser angle outward to the forward motion of the aircraft. It unloads tension in the strut and picks up a lower scrub angle / friction between the tire and the surface. The tire right tire is unloaded of weight scrub and rolling friction and is very happy to go straight ahead.

The left wheel begins to have a greater angle outward to the forward motion of the aircraft. It stays loading the strut in tension and picks up a higher scrub angle / friction between the tire and the surface. The tire is aiding the inertial load of the aircraft trying to go tangent pivoting on the left wheel . Lightly loaded, scrubbing, friction, holding the left side of aircraft back.


At this snapshot in time, the difference in couple moment between the right wheel forces and the left wheel forces, the drag on the right side is less than on the left and the result is self reinforcing with no equilibrium. Left wheel retards left side of aircraft due to higher drag and the right side " Moves Forward " due to less friction. The system is inherently UN - stable.
Turn a couple degrees further than the Toe-Out angle and situation changes rapidly........... Both wheels are scrub left...........


Continue the left turn a bit longer, the left wheel is increasing in scrub, load and friction. The left wheel is now angled out from the CL of the surface and is beginning to start a scrub that is
pulling the the left wheel away from the center line of the aircraft, extending the strut creating a situation where the moment arm of the left wheel is greater than the right wheel.

Now we get to the high speed portion where all of a sudden, what happened, we swapped ends...................
The left wheel loses effective ground contact, the inertia of the aircraft CG in motion is heading to catch up with the right wheel which is in effect the center pole on a merry-go-round.


A great self help aid to becoming zen with this situation involves your favorite adult beverage, a chalk outline of your favorite runway centerline in your driveway.

Walk through above observations while Pigeon Toed inward feet under hips. Then Duck Walk Toes Out, feet about 36 in apart. Try both right and left turns. Report your observations.

Kevin D
#272
KCHD

[fairchild1934]

I had experience with toe-in from RC models. You can hardly keep one straight without toe-in. It still tricky even with it.

Mark- yes--- i am aware there is a point when making the failure part stronger and stronger-- you just break the next part down the stress line.... i guess the unfortunate part here is- if you lose a gear leg- you often loose a wing....

makes on want to consider a swiveling gear like you could get for cessnas for a while. Anyone ever had one of those ?