Landing Gear Lessons

[Jackal]

I've just built my Landing Gear for the CB-1 and here's a few observations/lessons for those who have it in their future.

NOTE: These are my observations and ideas, I welcome any further info or rebuttal to be posted to better help those in the future. Don't be shy if you think something below is wrong - but please be polite : )

Page 13 shows a 5 degree angle cut on the fuselage end of the main landing gear leg. At first this makes sense as the lower longeron is bent upward 5 degrees at the front spar/aft landing gear attach point. This 5 degree angle makes the landing gear leg perpendicular to the longitudinal axis of the airplane (upper longeron) when the gear leg is hanging (or standing since at this point of construction the fuse is likely upside down on the floor) vertical. However, when the gear leg is horizontal (pointing straight OUT from the fuselage) it will be "swept back" this 5 degrees since the lower longeron does not also BEND INWARD at the same point it bends upward. Even for those who do bend their longeron in (per plans) it is bent forward of the front landing gear attach point. I would think the ideal would be to "aim" for no sweep back in the main gear leg in the "normal operating position" of the gear leg. It hardly matters what its angle is when its vertical or horizontal, as it will never be there unless something breaks. Since the normal gear position is closer to vertical than it is horizontal the angle should be closer to 5 degrees than to zero degrees. It seems and looks to me that a 3 degree cut instead of the 5 degree cut on the main gear leg where it attaches to the fuse would be about right.

If you do have "sweep back" (or sweep forward) of the main gear leg it causes a couple of things: First, the spring strut assembly must be built to "point" at the main gear leg which means it must also "point back" which means it must have angles on the ends where it attaches to the longeron and the gear leg near the axle. This is easy enough to do if you know you need to.

Secondly - sweep back will cause "toe out" of the axles (and sweep forward will cause toe in) unless you cut or grind a "compound angle" on the end of the main gear leg. This is possible if you have not yet welded the axle mounting plate on.

Of course if you do like some folks and don't bend the longeron up until forward of the fwd gear attach point, all of this is nothing. Just cut a zero degree angle on the main gear leg and it will be no sweep at all positions. But, it won't look as pretty (to some).

Using the 5 degree angle on the end of the main gear leg will cause your main gear leg to sweep back slightly in the "normal" position. I don't see this being a problem, but it will complicate the construction of the spring strut assembly and a compound angle is harder to cut than a straight one (with most tools) on the end of the main gear leg. Even if you use a 3 degree angle it will still be true that this "sweep" will change as the gear goes through its motion, and there will be limits to its travel before the entire assembly "binds" but this limit is well outside of the normal gear travel range.

As far as the order of construction, I'd recommend first building the main gear leg to fuse attach end (with whatever angle - I'd use 3 degrees next time instead of 5) with the 52 degrees cut on the axle end only lightly tack the two .125 plates in the slots of the main gear leg. Then build the aft brace to fit to the main gear leg and be sure both gear legs are in line with each other when viewed from the side. Then build the "V-brace" in the center making sure it "points" at the main gear leg. Then the spring struts making sure that when building the second spring strut its length is such that it puts the second gear in the same position as the first one. Ideally the landing gear ends will be the same distance out from the fuselage and the same distance above (or below if you're not building with the fuse upside down) the fuse at the same time. Mine came out exact, but if they don't "match up" at the same time its probably better to have them the same length as opposed to the same width. A plane that looks like its always setting on unlevel ground is going to be more noticeable than one with one wheel slightly closer to the centerline.

Last I'd mount the actual axle bolting plate after being satisfied it is placing the axles with the proper toe out/in that you want (I used zero, but some people love a little toe in or out - I won't take up that argument except to say that this is the point to make it just what you want). There will likely be a bit of adjusting the gear leg end and the positioning of the other two (tacked) plates at this point. The key is to get the plate setting in such a position that the toe and caster (or is it camber), anyway the tilt, of the axles is what you want, and the same as the other side. Of course slight adjustments can be made later using shims between the axle and its mounting plate, but it should be close to start with.

I may be missing the reason why the 5 degrees is cut on the end of the main gear leg instead of 3 degrees, but it definitely does sweep the leg back. That may be the goal, and its the way mine ended up. I've not been to school so be warned about taking my advice - I'm NOT an engineer......... or any other credentials.

One other thing - tacking the main gear to the aft brace is probably a good idea until all is finished. The welding of the joint will likely cause it to "pull" and make the "V" very tight in the mounts, even if you do weld it in place on the plane. If you don't weld it on the plane be sure you bolt it to something to keep it from pulling closed, thus preventing it from fitting on the fuse. This is also true of the center V support for the spring struts. Even if you do weld it together in place on the fuse (and re-heat to stress relieve), it will probably be forever more difficult to remove and re-install and this will likely be done several times during construction. Might as well only "fight" it once at final assembly.

[mtaylor]

I have a Lauzon frame and gear. I have logged 1 landing on pavement since first flight (20 hrs logged) and I noticed some extra attention was needed with rudder work on roll out. I checked the gear and found out I have a toe in condition. The axles are welded, not bolted on, so the easy fix with shims is not an option. My idea is to remove the wheel, rest the inner portion of the gear axle on a 4X4, slide a 10' long heavy walled pipe over the axle and apply some pressure rearward. Just a slight tweak should be enough to make a difference in the right direction. Too much will distort the gear symmetry and possibly weaken weldments and/or bolts. I'm still thinking this one out and would appreciate some sage input from the gaggle. My options, as I see them, are (1) the above, (2) Stay on the grass (ok with me mostly), (3) do wheel landings when on pavement, (4) build a new gear (ughhh!). Bucker Jungmann's have a toe in condition (in their original configuration) and it's recommended to do wheel landings when on pavement because the tail high position nuetralizes toe in. I think this would apply to mine, too. I can prop my tail up and check this to be sure.

All this said, I do love this little biplane!

[mtaylor]

Another possible option that I forgot to mention...my bungees are for a heavier airplane than what my Hatz is. I wonder if a softer set of bungees might allow a bit of spread for the gear (none as it is now) and change the symmetry.

Just a (another) thought...

Comments,thoughts, ideas welcome and encouraged.

[M Lightsey]

Another possible option that I forgot to mention...my bungees are for a heavier airplane than what my Hatz is. I wonder if a softer set of bungees might allow a bit of spread for the gear (none as it is now) and change the symmetry.

Just a (another) thought...

Comments,thoughts, ideas welcome and encouraged.

I find that NX1279 is very docile on pavement as long as I make absolutely sure to stay off the brakes. If I start getting happy with the brakes though, not so docile.
Something you might try...
Mark