Main Gear Length

[Spaceman]

Hi guys,

I'm working on installing my main gear legs and setting the length and alignment.

First, I might be kind of going about this backwards but I'm committed now so I'm going with my method! What I've done so far is install the axle weldments on the end of each leg, and drilled them in place on the drill press. The legs are both bottomed out in the axles. So that end is fixed. Now I have the top ends of the legs installed in the motor mount, so what I have left to do is set the overall length and toe angle by sliding and rotating the legs in the mount before drilling the upper holes through the legs.

There's a revision to the gear leg drawing which I totally forgot about, that has you drill the upper 1/4" hole through the legs, 1/2" from the top. Keeping that in mind, since I skipped that step, I inserted the legs until the existing 1/4" holes in the engine mount lined up with a mark 1/2" down from the top of the legs. This puts the top of the legs flush with the top of the tubes of the engine mount, so that seemed to make sense.

Now I am trying to confirm I have the correct length set, and it appears to be way off! With the forward fuselage level, the bottom edge of the lower longerons is 617mm off the floor, and the center of the axle at the outboard end is 160mm from the floor. So the end of the axle is 457mm, or 18", below the fuselage floor. The plans call for it to be 16-43/64" (424mm)!

I figured if it was a little too long I could push the legs a bit farther into the motor mount, but this is more than 1.25" too long! Every picture I can find looks like the gear legs are either flush at the top, or maybe like 1/4" sticking out. Could I be missing something?

[Spaceman]

Also, I just did a sanity check and the legs are indeed the correct overall length of 34", and they're definitely inserted all the way into the axle assemblies!

I'm just not sure how this can be correct, since if I followed Sonex's drawing revision and drilled the upper hole in the legs first, this is where they'd be now and then there'd really be no way to fix it. With what I'm working with, I guess I can either stick with what I have, or just insert the legs farther into the engine mount to achieve the proper ride height.

Here's the revision I'm talking about, by the way:
https://www.sonexaircraft.com/documents ... t_Hole.pdf

[sonex892.]

May be something to do with the variation in engine mount from B to A model ? Ask Sonex about that.

With my A model. I ignored the 34" dimension and just aimed for the required perpendicular dimension from the bottom of the fuselage to the axle. Probably best to clamp a straight edge under the fuselage and measure down, than trusting the floor to be both perfectly flat and level.
Steve
Sonex 892

[Rynoth]

Without the plans in front of me, just a couple of questions:

Does the length of the gear leg match the 34" described in the link?
When you say the fuselage is "level", do you mean with the tail raised and lower longerons level in both dimensions? (lateral and longitudinal)?

[T41pilot]

I'm building a "B" model and experienced the exact same thing. First thing I noticed was that the gear legs were not exactly 34". One was a 1/16 and one was an 1/8 too long. I saw that the plans wanted a hole 1/2 inch from the top so my logic was to measure 33 1/2 from the bottom and just let the difference stick out the top of the mount. Well, having drilled the holes and installing the legs, I found the same thing. The measurement from the bottom of the fuselage to the tips of the axles when toed in to the right spot did not meet spec by over an inch. Well now what? So I talked to Kerry at Sonex and sent a bunch of pictures. He seemed surprised by the result but couldn't find anything I had done wrong. He basically agreed with my assessment that I didn't think the plane would fly any differently with longer gear legs but that the picture over the nose when ground handling might be a bit higher. He said since the new tailwheel sits a bit higher it may help offset that difference. We basically agreed that you could cut the gear legs down by the amount of error or just use them as is. Since Titanium is such a "joy" to work with, I opted to use them as is. My personal conclusion is the angle that engine mount gear leg tubes are positioned is slightly more vertical than they should be or that during the cooling process after the welding the angle changes slightly. Bottom line is you can cut them or use as is. A third option just occurred to me. If you haven't drilled the top holes yet, I don't see why you couldn't let the error length stick out the top of the mount if there is room for that. I had already drilled my top holes so that wasn't an option for me.

[Spaceman]

Thanks for the replies guys. I'm in the garage looking at it some more. Gregg I think you're on to something with the angle of the B-model engine mount.

Both of my legs arrived dang near exactly 34"!

I went ahead and beat the gear legs (they fit really tight) further into the mount until I got the correct 424mm floor-to-axle distance. There's a full 50mm of leg sticking out the top on both sides!

I was initially going to try to set the toe angle using the 2x4 method some others have used, but now I think I'll need to keep the fuselage jacked up so I can adjust both the length and angle together. So I guess I'll attempt the factory recommended alignment method.

I might try to get it set tonight but give Kerry a call tomorrow before actually drilling the holes.....

If this is correct I might also try to cut this excess length off the top... 4" length x 1" diameter times titanium is like a solid half pound of dead weight by my rough estimation. That would make up for all the extra bolts and rivets I've had to install to fix my own screwups on this whole plane so far!

[Spaceman]

Also, I grabbed a chunk of angle iron and clamped it to the lower longerons to give me something to measure to from the axle. So the wavy garage floor variable is out... Good thinking

[T41pilot]

I used the factory method with the help of a jig from a friend. I'm sure any solid straight edge for the span would work. Getting the right spot to touch the axle fittings with that straight edge so that my spacer on the back side (the aluminum angle) would be in the right spot took some experimentation. Had to use shims on the floor to take care of my floor variation. Make sure the forward fuselage is level in all directions. You can see the Sonex supplied shim installed in the clamp photo. At this point I'm just assuming their method works. My tires squeak when rolling on the floor so there is definitely some toe in.

[Gordon]

Longer Gear is Better for a Taildragger

Not sure why everyone is so worried about the gear being a touch too long. Longer is better with these airplanes..........the "deck angle" is too shallow....they tend to hit the tail wheel first on landing.

With the early RV-4's that was quite a problem.......they constantly hit the tail wheel first on landing. Van's finally changed the gear length in the early 90's....they increased the gear length by a whooping 4 inches...! When you walk up and down the rows of RV's at OSH you can easily pick out the early '4's' by how low they sit at the nose. Some of the early RV-4 guys bought the longer gear legs and upgraded their landing gear...for the better....... so they could get a "3 point full stall" landing.

My Onex ALSO tends to hit the tail first even with my larger 5:00x5 tires. And yes, I know it's not the same gear as a Sonex.

If your Sonex gear is a 1/2" or 1" longer than spec......don't worry about it.......worry about alignment. That's the real issue to focus on.

Gordon....Hummel 2400.....Onex....(RV repeat offender)

[sonex1374]

(snip) So I talked to Kerry at Sonex... He basically agreed with my assessment that I didn't think the plane would fly any differently with longer gear legs but that the picture over the nose when ground handling might be a bit higher. He said since the new tailwheel sits a bit higher it may help offset that difference. We basically agreed that you could cut the gear legs down by the amount of error or just use them as is. (snip)

The Sonex sits a little flat on the gear with the standard gear legs. You'll notice this on landing if you get really slow and nose-high - the tail wheel will touch first, followed shortly by the mains. This is due to the 3-point attitude of the landing gear being at a shallower angle that the full-stall angle of attack that you can achieve on a slow approach. In those situations a bit longer main gear legs would be a nice addition. As it is, approaches to landing are done at the 3-pt attitude and then held off until the plane settles to the runway. The pilot has to pitch the nose appropriately (e.g. not too high) to make a perfect 3-pointer, otherwise you'll tailhook the landing! Joe Norris used to cover this explicitly in his training, recommending that new pilots mentally "burn-in the attitude of the plane while taxiing" to get a feel for how steeply they could settle in to the runway.

With an extra inch you could be the next Sonex STOL champ with your supper-deep 3-pointers!

Jeff