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Tiger Tim

Well-Known Member
I’m looking at making up a set of tube truss landing gear legs and want to make sure I take everything (or at least enough) into account. Right now I’m at the most basic sketching stage but long story short it will be fabric covered and bungee sprung within the fuselage. I think the gear and associated attach structure needs to bear the following loads:

-landing
-taxiing
-groundlooping?
-aero drag
-braking drag
-braking torque

Sound about right? Is there anything I missed?

dino

Well-Known Member
HBA Supporter
Everything you wanted to know on the subject. Landing Gear Design for Light Aircraft, Ladislao Pazmany

Dan Thomas

Well-Known Member
I’m looking at making up a set of tube truss landing gear legs and want to make sure I take everything (or at least enough) into account. Right now I’m at the most basic sketching stage but long story short it will be fabric covered and bungee sprung within the fuselage. I think the gear and associated attach structure needs to bear the following loads:

-landing
-taxiing
-groundlooping?
-aero drag
-braking drag
-braking torque

Sound about right? Is there anything I missed?
Figure out how you will adjust camber and toe-in or how it will end up exactly right once built. You don't want to end up with a gear like the old Champs had. To make alignment changes you had to take the leg off and bend it. Then maybe do that two or three times until it was right.

Tiger Tim

Well-Known Member
Figure out how you will adjust camber and toe-in or how it will end up exactly right once built.
Light and simple would be to weld in the axles with the landing gear installed on the fuselage and one long temporary tube running through both of them to give zero camber and tow in. That ought to handle okay, it’s not like this needs to be a trainer or anything... I think.

I suppose a set of bolt-on axle stubs would work too as they would allow shimming to whatever is needed. This is all yet TBD anyways.

Dana

Super Moderator
Staff member
Light and simple would be to weld in the axles with the landing gear installed on the fuselage and one long temporary tube running through both of them to give zero camber and tow in.
That's what I did when I built a new gear for my Starduster, at least as far as toe in. Getting the camber right under spring tension was some educated guessing, but it came out pretty good. Of course that's simpler with a bungee gear which is pulled against a stop when sitting on the ground at 1G, unlike a die spring gear where the springs are always somewhat compressed.

For a leaf spring gear, you want to talk to the people making the gear.

wsimpso1

Super Moderator
Staff member
Log Member
I’m looking at making up a set of tube truss landing gear legs and want to make sure I take everything (or at least enough) into account. Right now I’m at the most basic sketching stage but long story short it will be fabric covered and bungee sprung within the fuselage. I think the gear and associated attach structure needs to bear the following loads:

-landing
-taxiing
-groundlooping?
-aero drag
-braking drag
-braking torque

Sound about right? Is there anything I missed?
+1 on Paz' book on landing gear.

Lateral load, which would include turning, landing in a crab, and groundloops;
Tire/wheel/brake spin up reaction load;
Level landings;
Tail low landings;
One wheel landings;
Your gear has to keep the prop and airframe off the pavement in the face of flat tires, flat struts,

Paz covered 'em all.

Billski

Topaz

Super Moderator
Staff member
Log Member
Another enthusiastic vote for the Pazmany book. It's a gem. "The" go-to book for light aircraft landing gear design. Don't be fooled by the "Volume I" in the subtitle. Volume 2 was intended to cover retraction mechanisms and suchlike, but he didn't finish it before he passed away.

And yes, it covers all styles of gear and is comprehensive with loading cases.

Pops

Well-Known Member
HBA Supporter
Log Member
The side load (lateral load) is marginal on a lot of homebuilt designs. I agree, Ladislao Pazmany's book is very good.

Tiger Tim

Well-Known Member
The side load (lateral load) is marginal on a lot of homebuilt designs
What kind of side loads would they see in the worst case scenario? I figure it would be the weight of the airplane times the coefficient of friction of rubber on dry pavement at an angle where the wingtip hits the ground, plus some extra margin. Maybe Paz lays it out.

wsimpso1

Super Moderator
Staff member
Log Member
What kind of side loads would they see in the worst case scenario? I figure it would be the weight of the airplane times the coefficient of friction of rubber on dry pavement at an angle where the wingtip hits the ground, plus some extra margin. Maybe Paz lays it out.
It's all in there. Usually, Paz just covers the regulations. For lateral and braking loads it is usually 25% of the vertical load on the wheel... Vertical load on the wheel can be very large at peak downstroke from a big sink rate, which is also defined by the regs and presented with realistic coefficients for everything. Way more stuff than we can get into on a forum thread.

Billski

Pops

Well-Known Member
HBA Supporter
Log Member
Picture of the Landing Gear Jig that Dallas and I built for welding the LG attach brackets on the lower longerons. Fuselage upside down, bolt the LG brackets to the LG legs. Bolt the LG legs to the jig with the correct tread and set down on the longerons and use a long straight edge ran across the lower longerons and get the same measurement up to the axles. Also same measurement from axles to the lower tail post. Installed LG on 4 Bearhawks. LG legs were make in steel jigs.
The Bearhawk has a large HD rod-end on the upper end of the oil dampened, die spring, shock strut to vary the length of the strut for your average load. Works very good.

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Pops

Well-Known Member
HBA Supporter
Log Member
Well known taildragger had a side load limit of 3150 lbs with a GW if 2500 lbs. Large percentage of fleet had the streamline shock strut fail with a small swerve upon landing on payment. Failure at about 13.5" up from the bottom shock strut attach bolt. Hard wing tip strike and wheel and LG leg folds up under the fuselage, hard prop strike.
My little 800 lb GW SSSC has a side load limit of 3500 lbs.

Tiger Tim

Anyone have a cheap source for the book? ACS lists it at $133USD which is probably north of 200 for me after shipping and exchange. Topaz Super Moderator Staff member Log Member Anyone have a cheap source for the book? ACS lists it at$133USD which is probably north of 200 for me after shipping and exchange.
It's a long-shot, but see if your local library can get a loan of one from another somewhere in the system. I've seen some extremely "niche" books this way. Other than that, start hitting the used-book sites and whatever used-book stores are in your area. Put in a special request at one of the latter. They might be able to dig one up for a reasonable price.

Keep watching the Amazon.com link for the book. Used book prices go up and down a lot over time. I've seen books that were over $500 drop to under$50 in less than three months. Right now, it's about $300 there. In six months it could be half that. Or double. The Kindle edition of the book seems to be free, but make sure you've got a decent screen (the Kindle Reader app on your laptop, for example) or the diagrams are going to be awfully small. Put a thread in the For Sale/Wanted portion of HBA. Someone may part with a copy for less than ACS' rate. If you're really serious about designing an airplane, even the$133 is worth it, IMHO. You're not going to find a better resource for this material.

pictsidhe

Well-Known Member
FAR 23 specifies landing gear loads if you don't want to cough up for Paz's book. Not got mine here, but I think you want the 2007 or earlier? My FAR AMT was about \$5 from Amazon...

HBA Supporter
Log Member

Vigilant1

Well-Known Member
Well known taildragger had a side load limit of 3150 lbs with a GW if 2500 lbs. Large percentage of fleet had the streamline shock strut fail with a small swerve upon landing on payment. Failure at about 13.5" up from the bottom shock strut attach bolt. Hard wing tip strike and wheel and LG leg folds up under the fuselage, hard prop strike.
It seems to me that "barely good enough" landing gear is one of the most expensive things you can put on an airplane. OTOH, gear strong enough to keep smoothly absorbing significant energy until the plane's belly touches the dirt can save a lot of pain in a high sink rate landing. That 12-18" of deceleration space can be priceless.

Pops

Well-Known Member
HBA Supporter
Log Member
It seems to me that "barely good enough" landing gear is one of the most expensive things you can put on an airplane. OTOH, gear strong enough to keep smoothly absorbing significant energy until the plane's belly touches the dirt can save a lot of pain in a high sink rate landing. That 12-18" of deceleration space can be priceless.
Truer words were never spoken.