Blanton gear vs bushmaster vs pacer ???

[danP]

HI, Hope you all are doing well. I'm refurbishing a blanton conversion and considering rebuilding the landing gear as original builder did a not so professional job. Anyway, I was looking at the pacer gear drawing from the CD, I also have the bushmaster drawings and Blanton drawings. Here's what I noticed, the location of the axle on the pacer is 12" forward of the strut attach point (i used this reference as its was consistent with all three). The Blanton axle position is 10.5" forward of the strut attach fitting while the Bushmaster is 6" forward of that point.

That seems like a huge amount of variation. I didn't see the CG changing on these three. What Am I missing??

Thanks.

[Stephen]

A six inch variation in the axel location will have little effect on the CG, since there is little moment arm difference. I assume the numbers are based on the plane being in level position. The different axel locations will have effect on the planes tendency for the tail to come up when applying brakes during landing. The different gear may have a small effect on the sitting deck angle of the plane and may have a slight effect on take off distance. The main differences I would look at is which gear is more rugged. Wider V, generally is generally stronger. There might also be a slight weight difference in the gear. The Bushmaster gear is easy to install since it uses the T Pacer gear attachment locations. It is also strong. Some people don't like the looks.

[danP]

Hi Stephan, thanks for your insight. Your comment on the axel location is what I found concerning, all things equal the the Bushmaster gear would be the most likely to allow a nose over although I also noted the Bushmaster gear do sit higher and, as you say, will affect the deck angle.

I like the Bushmaster gear better than the Blanton and they are stronger than the pacer gear. One option I considered as well is using the rear attach point and the forward attach point creating a very wide 31" at the top "V". I have not seen this done with the existing hydrosorb though.

But, whatever I do, I want the axel in the best position.

[Steve Pierce]

I would think the further forward the axle the more weight on the tail and the less likely to nose over. I belive the axle being inline with the forward landing gear tube would be stronger than if it were canted forward or aft, load would be straight. Cub Crafters sell a 3x3 gear for the Super Cub which is 3" extended and 3" forward. The pireps I have heard are it is harder to lift the tail and you can get on the brakes harder when wanting to land short.

[Rick]

Hi DanP.

I would not recommend using the rear TriPacer gear fitting and the front Pacer gear fitting to achieve a 31-inch wide gear mount on the fuselage for a tail dragger. The rear TriPacer gear mount is only 35.5-inches wide and the Pacer front mount is 36.0-inches wide. This dimensional difference would create a toe-out condition during landing bump which is not acceptable under any condition for tail dragger. Toe-out will promote darting a very unstable condition in tail draggers leading to loss of control and ground loops. To reinforce this statement read note “8” on the PA-20 fuselage fitting drawing 12509 and note “6” on drawing 13938, it states.

Quote:

The lateral dimension between the front landing gear fitting hole centerlines (36) is never to exceed the lateral dimension between the rear landing gear fitting hole centerlines (36).


The above requirement would only allow no toe change during landing (which could only happen in a perfect world” or only “Toe-in” condition during “Bump” on landing.

I have had people challenge me on my position stating that the TriPacer is very stable with a built-in toe-out design so toe out must be the way to go. I don’t know about the way to go but the Tripacer design works well and the reason why is the main landing gear is “behind the center of gravity” . When the mains on a Tripacer touch down with the center of gravity/mass forward of the wheels it will force the alignment of the airplane to the direction of travel and the wheels follow. This is not the case with a tail dragger. Everyone know the center of gravity/mass is behind the wheel centerline in a tail dragger and that is where the loss of control and the proverbial ground loop comes from when the center of gravity/mass tries to go by the wheel centerline.

Tail dragger landing gear must never promote toe out during landing, only toe-in, and for those who say they set their gear at “0” toe-in are living in a perfect world that doesn’t exist, just sayin.

OK guys, start shootin.

Rick Lach

[Stephen]

Rick,

You're the last person I would challenge about Pacer gear knowledge. Maybe, you could explain something about Trimmer Gear STC. Apparently, it calls for the gear to be set at zero toe in. Why zero? They accomplish this by running a straight pipe through both Axel's. I would think zero toe in would mean slight toe out on landing due to resistance of the tires. I asked Eddie and he didn't know why. The gear I've installed all had toe in.

Steve is correct about axel position and nose over. Bushmasters axels are in the most aft position. It makes it easier to lift the tail. I don't know how Trimmers gear compare.

[Rick]

Hi Stephen,

I have asked Eddie the same question and gotten the same response. I have never analyzed the Cub gear that Eddie’s STC used but I have studied many Pacer gear legs and Univar’s Pacer gear leg. All the straight ones were built to match the Piper PA-20 drawing 13189, with the exception of axle size. 13189 states that the axle is welded on the same center-line as the main gear leg with a tolerance of “0” degrees to a minus “1” degrees off center. Another way of putting it is the axle center-line window is either on center or canted forward of center by one degree. This again only allows for a perfect “0” toe condition or toe in.

So mounting the gear up at the rear strut mount and locking the axles with a beam into a “0” toe condition and then welding the front mounts in place would appear to create an acceptable “0” toe environment. But when you apply the above information what you have just done is create a fixed toe out installation. Because the gear legs are never perfect. In fact, everyone I have studied that was not bent had a toe in cant, well within the drawing tolerance. So, by forcing a “0” toe in at the axle you have now moved the forward mount outward making the front mounts wider than the rear. When I tried this type of installation, I ended up with the forward mounts wider then the rear mounts by 3/32 to 5/16. I then went out and bought a four-foot pair of calipers and built fixtures to locate the front mount .020 to .025 narrower than the rear mounts without regard to the axle toe in. In other words I build it to the Piper Drawings.

One last thing, don’t assume that the rectangle of the four mounts is square to itself, measure it and check its center to the center line of the fuselage.

One more last thing. Don’t forget Piper drawing 21242 calling out the proper installation of the front gear mount and distance from the center of the fuselage longeron tube to the center of the mounting bolt of the landing gear. All these issue must be considered before anything is welded.

Hope this helps.

Rick

[danP]

Wow Rick, that is great informatiom. A couple questions based on your information. Isn't the solution what you did, simply make sure the forward attach points are equal and not wider the the aft attach pointsa whe you weld them on? Also, the blanton and bush master use the tripacer attach points which you note are toe out. So with those the angle has to be addressed to move them toe in, no? By the way, Mike patey made the comment that with larger bush tires there is play in the barrings so strting out with 0 toe can end up a slight toe out.

[Stephen]

Rick,

Thanks for these details. Using a large micrometer is a great idea, I've only used tapes and trammel points.

It seems that finding a center line to within a few thousands of an inch would be difficult, especially with our old tubing. I was thinking of measuring diagonals with a large micrometer. Also, is there any reason to check the alignment to the tail? Is the tail on a centerline between between the landing gear? Finally, does .20 to .25 come out to 1 degree toe in?

[Rick]

Dan I have not personally seen an conversion by Blanton. So I’m not sure how his landing gear is designed. As for the Bush Master, of the aircraft I have seen, they were both using the Piper layout which says the Piper dimensions and theory of operation applies. I’m not trying to avoid your question and thoughts Dan, I just don’t have a good educated on those conversions.

Stephen, I check the fuselage alignment by first leveling with a plumb bob per Piper instructions. Then I check the leveling of the main tube the seats rest on. Then I check the level of the two wing mounts. As always non are perfect so I just average everything out. Then I drop a plumb bob from the center of the rudder tail post (had to make a fixture for that) and two more plumb bobs from the gear fittings at the strut mount and start measuring. I have found more than one fuselage that measures within 1/16 of an inch when triangulation measurements are taken. Because the concrete floor are not very smooth a clear pencil mark is hard to get, I tape a piece of paper under the fuselage big enough to plot out the landing gear mount rectangle, 36 x 12. Then I set up the front gear mounts with fixtures I have made to hold them in place and drop two more plumb bobs off the front mounts and start measuring again. Then taking the width of the existing gear mounts at the strut I set the front gear mounts .020 to .025 narrower than the rear gear mount, even if the rear gear mount is never then the perfect 36-inches. Triangulate the gear mount box again and if everything is good start welding.

This is the perfect world situation. There are many variables to complicate the process. The biggest variable is what if the rudder post is not on center with the strut gear mounts. Meaning the measurement from the rudder post to the left strut gear mount is off by a significant amount as compared to the right strut gear mount measurement. That problem must be addressed before the gear conversion can continue.

Hope this helps. It’s two pounds of words for a ten-pound issue.

Rick