Toe-In Degrees?

[robcaldwell]

Today I checked my gear alignment using a set of instructions Mark sent me. These are the same instructions also at BH Tips: https://bearhawk.tips/1002

Following this guide resulted in these measurements:

• Inside top of left brake disc, to inside top of right brake disc = 63 3/4"
• 4' angles clamped to discs and measurement taken from 42" BEHIND = 64 3/4"
• Centerline measurement = 32 3/8"

So... My gear legs are spread equally. Good.

Toe-In exists and is proven with 1" wider at 42" behind brake discs. Again, measurement was taken from behind the discs.

The "Main Landing Gear Wheel Alignment" states that TOE-IN should be 0 to 1/2 degrees, but nowhere does it define how to calculate degrees. These instructions demonstrate how to check toe-in / toe-out but nothing about how to adjust.

Tomorrow I will call Bob to discuss. But for now I just wanted to see if anyone has a formula for understanding the toe-in degrees? (I'm lousy at math...)

My guess for remedying the toe-in to zero is to turn a few threads out on the gear leg struts?

[jaredyates]

Keep in mind the spread of the gear will change the toe-in, so you want to take the measurements at the prescribed setting. To figure the angle, use trigonometry. The length of the piece clamped to the brake disc is the hypotenuse. The lateral measurement between the end of that piece and the (straight ahead) line is a leg. Sine of the smaller angle equals the little number divided by the big number. If I understand your measurements, sin(angle)=1/42 when the angle is 1.36 degrees or so.

[svyolo]

You can calculate what you have, but how could you adjust it, other than gear spread? What is more important, max and min gear spread, or toe in? There is no built in way to adjust toe in, other than cutting and welding. I suppose you could rig up a jig and bend the gear with a winch or come along or hydraulics.

[jaredyates]

The erbman method is to put a piece of pipe inside the axle, heat the cluster, and bend it. But that isn't going to allow for preserving the covering, which is why we do it early in the build.

[robcaldwell]

I've heard about this before and read of another on here that did the same with a rosebud to heat the cluster, as Jared mentioned. I hope I never get to that point.

[whee]

See the second to last post in this thread: https://backcountrypilot.org/forum/p...16912?start=20

I can’t remember the degrees but I set my mine at 1/8”. It works on a Cessna, Pacer, Maule and my BH.

[robcaldwell]

This is an interesting method to accomplish a near neutral toe setting. Seems like you would need to completely suspend the front of the airplane with only the tail supported to make this measurement.

[whee]

I measured it with the tail up with the mains on the ground and spread 72” as Bob specified. I just like Steve’s Aircraft method of measurement because it’s easy.

[robcaldwell]

Keep in mind the spread of the gear will change the toe-in, so you want to take the measurements at the prescribed setting. To figure the angle, use trigonometry. The length of the piece clamped to the brake disc is the hypotenuse. The lateral measurement between the end of that piece and the (straight ahead) line is a leg. Sine of the smaller angle equals the little number divided by the big number. If I understand your measurements, sin(angle)=1/42 when the angle is 1.36 degrees or so.

Thank you for the math, Jared!

I did set the distance at the prescribed setting of 68 - 70" center of tire to center of tire. As the instructions discuss, I do see that the gear legs move in as I roll the airplane forward, and the gear legs move or spread open as roll the plane back ALL in less then 10 feet.. Indicative of TOE-IN. And it is easy to roll the airplane until I get the desired 68 - 70" I need to perform the check. So this has me thinking... If I turn the rod ends of both strut legs out equally should reduce the amount of TOE-IN making it harder for the gear to behave with these in and out movements in static ground rolls.

[jaredyates]

The only reason to adjust the shock strut length is to get the spread within spec: https://bearhawk.tips/717
74 inches at the heaviest weight, 72 being the ideal target.

[jaredyates]

Here is the text from that link: In the service bulletin concerning the replacement of the rod end bearings it was mentioned that the landing gear geometry must be such at the tread is 72” (measured center of wheel to center of wheel) when at normal weight and maximum 74” at gross. These dimensions are critical because the loads on landing gear components increase dramatically as the wheels are allowed to splay out. They absolutely must be held.
To check the spread, you can load the airplane then roll it forward and back, giving the gear time to equalize. Another method is to take two pieces of old Formica counter top, place them face-to-face with grease between them. Then this homemade “slip plate” is put under one wheel and the airplane rocked by the wing tip a few times until everything settles out.
DO NOT UNDERESTIMATE THE IMPORTANCE OF KEEPING THE GEAR TREAD WITHIN THESE LIMITS. ALSO REMEMBER THAT THE ONLY ACCEPTABLE ROD END BEARING FOR THE SHOCK STRUT IS THE AURORA XAM-7M. This magnafluxed bearing can be purchased from AviPro.

[robcaldwell]

This is great information and I will be using the Formica method to check the "real" spread of my gear legs today. Another option that you mentioned to me awhile back Jared, is to taxi the airplane under power and let it roll to a stop without any brake application, shut down and check spread. I will also use this as a final check. Thanks!

[Bcone1381]

I see inconsistencies in this discussion that could lead to mis-alignment of the gear.

The Bearhawk TIPS article referenced in post 1 says The main gear axles should be checked for proper alignment with the plane in the three point position, and with the tread at 68”-70”. But when I the Backcountrypilot.org reference and the text in post 4.0 - 4.2 I see Tail up in Flight Attitude is instructed.

I'm a black and white kind of guy and need a single answer to things like this. Today the Bearhawk Tips article my is authority on this. Part of the reason I like it is that if the tail in a three point attitude when I land and slow down and rudder authority is decreasing then aircraft wheel alignment must help me go straight down the runway. Zero toe in will do that. If Toe In changes away from Zero as the tail comes down especially in a cross wind then it seems like directional instability might increase.

[robcaldwell]

I too have found the same inconsistencies as well as a need for emphasis or priority about certain information on this subject. I now realize that setting the gear leg distance should be a critical task in the pre-first flight checklist. I would rank it right up there with drilling the wings and setting the angle of incidence (Which I believe this is now done for you at the factory on new kits).

For me, this was about measuring the gear leg spread. I've known for quite some time that I need to look for 72" center of tread to center of tread. With my airplane in the hangar I would measure the tire distance, and there it was.... 72". WOW! I hit my distance during the installation and this is one task I certainly will not have to go back and adjust. How lucky of me!

Well... Not so fast.

TOE-IN and TOE-OUT was not introduced to me until recently. Yes I am aware what these positions are, but for some reason I had it in my mind that those angles were accounted for when the gear legs were built. And it is, but I hadn't thought through the geometry of the varying gear leg positions.

So, with the airplane in the back of the hangar means it was rolled backwards to get it there. If the gear legs have any TOE-IN, the gear legs will naturally splay OUT as it rolls backwards. Naive me slaps a tape measure on the tire centers while the airplane is napping in the back of the hangar, hence my 72" measurement. (I now see that If I roll the airplane forward, as little as 5 feet, the TOE-IN causes the legs to suck in and I measure 67” between center treads.)

Mark viewed my 1st Flight video and asked if I had measured my gear leg spread. (I guess it was that evident.) I replied, "Oh yeah, I have 72" Mark!". But after a couple of hours of flying my APP (Additional Pilot Program - Part 91.116) says, "Rob, why are you taxiing with so much right rudder?". Little did I know, my airplane is pigeon toed! (Who knows why it is pulling to the left, let's get the TOE-IN issued solved first and see if that helps the directional tracking.)

I spoke to Mark again about this and he said, "But I thought you said you had 72" and everything was fine". Apparently I convinced myself that everything was fine, until I performed the TOE-IN / TOE-OUT check. https://bearhawk.tips/1002

I'm no good at math, and Jared helped me with the angle measurement that came in at 1.36 degrees TOE-IN. Not Good. Should be 0 to .5 degrees. 0 is best!

So here I am in today and haven't flown in a week. Other factors have contributed to the non-flight activity, including the to TOE-IN / TOE-OUT check. The good news is that my gear legs distance are equal so it's now a matter of turning my rod ends out far enough to remove/ reduce the TOE-IN for a goal of ZERO in a normal flight weight. 74" at gross, as Jared referenced above

I need to stop typing here and get out to the hangar and start adjusting the rod ends (out). How much out, I guess trial and error will make that determination...

Stay tuned...

[Bcone1381]

The good news is that my gear legs distance are equal so it's now a matter of turning my rod ends out far enough to remove/ reduce the TOE-IN for a goal of ZERO in a normal flight weight. 74" at gross, as Jared referenced above

I need to stop typing here and get out to the hangar and start adjusting the rod ends (out). How much out, I guess trial and error will make that determination...

Stay tuned...

Ummm.....My understanding from reading (no experience) is Rods ends adjust gear spread. Heat and a long pipe adjust toe in. This would impact the fabric and and finish on your gear of course. Have you called Bob? It would be goo to hear from experience, like N92VT or Battson on this topic. Here is what I've read.

https://bearhawkforums.com/forum/bearhawk-4-place-plans-build/25186-landing-gear-alienment

[robcaldwell]

Hey Brooks. Yes I am aware of those remedies and have read the post you are referring to. Let's see what spreading the gear legs does first as this is a must do task based on the current gear leg stance.

I did speak to Mark and Jared (here) and will call Bob after I make the rod end adjustments and I see no relief. I'm sure he would have me do that anyway before proceeding with heat.

I don't even want to think about rosebudding the gear legs right now. I have ZERO time for that. My summer is booked.

[jaredyates]

Rob, the half degree toe-in spec applies only at a single gear spread measurement. Toe in varies depending on gear spread, so if you are measuring the angle at a different spread, the angular spec would need to change correspondingly. Adjusting out the struts is important for other reasons but it doesn't relate to toe-in. I'll try and get in touch to help shortly.

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[whee]

I apologize for providing conflicting information. Doing things Bob’s way is best for most builders. Just follow the instructions! 😁

[jaredyates]

I've just been on the phone with Bob and Rob to clear up some questions, and have some updates.

First, the bearhawk.tips article gave a spec of 0-1/2 degree of toe-in at 68 inches. This needs to be updated to 72 inches of spread. Adjusting the shock strut length does not fix a misalignment of toe in, but it does change the toe in. So it is important that we have an apples to apples comparison of degrees only at a specified spread, in this case 72 inches. Rob was at 1.3 degrees per wheel at 68, and Bob suspects that once Rob checks it at 72, he will be in spec. This is based in part on how the airplane is handling, which is "not bad" in scientific terms. Bob also said another check for toe in is to push the airplane back on pavement at least 10', then push the airplane forward at least 10 feet. Measure the spread at each place and look for no more than 2 inches of difference between the two.

Regarding the spread, Bob wants no more than 74 inches at maximum flying weight, with 72 being the target for typical flying weight, yielding around 68 when empty.

All of these measurements are in the 3-point attitude, and center to center on the tire. It doesn't have to be the center, but does need to be the same place on each tire. The point being it's not a distance between tires or between brake disks.

Absolute minimum thread engagement of the rod end in the shock strut is 1/2, and he'd rather see 9/16 just to be sure.

I'll update the article shortly.

[jaredyates]

The atiicle is updated, can someone take a look and let me know if there is any remaining ambiguity or room for improvement please?

https://bearhawk.tips/1002

This particular article should be viewable by anyone, not requiring access.

[Aero_tango]

The toe does vary with the gear leg spread. I went through the trouble of measuring it at different spread, before I did my first flight. Refer to my results in the post below. I ended up doing nothing, my airplane track very well on the ground, no bad habit.
Mike