# Retractable Landing Gear Design and Material

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#### WARPilot

##### Well-Known Member
Is there acceptable material like Aluminum tube that can be substituted for steel tube in retractable gear? Aircraft gross weight will be 900lbs and tail wheel.
I am looking for simple, reliable, and lighter gear design. The gear will need to retract inward. This will be to replace the WAR design which uses a worm gear system.

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#### wsimpso1

##### Super Moderator
Staff member
Log Member
Detail design of structures is always a matter of trading volume, density, and strength to find the lowest mass system with enough strength that also can be fit in the bird and be built in your shop.

There are a variety of aluminum alloys, but decent strengths come with heat treatment. Weld them and the heat treat is either given up or must be redone. For instance, 6061-T6 after welding can be easily taken back to -T3 or -T4, but T-6 means a trip to the heat treater. Then you also need to make joints for fasteners work. Then there is 7075 forgings and machined-from-solids.

4130 steel is basically the same before and after welding, and makes more compact assemblies. Fastener joints are also simpler and more compact.

As the design engineer you get to do the design studies, decide what you need, and choose. Welcome to the Monkey House.

Billski

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#### Jay Kempf

##### Curmudgeon in Training (CIT)
It gets hard to justify aluminum in retractable landing gear primary structures as you move up the MTOW scale. For experimental light weight stuff like Billski says 4130 tubing weldments are hard to beat. As you move up to fighter weights Titanium forgings become prevalent. Move up to transport and grades of exotic stainless forgings and hog outs become common.

Aluminum hog outs can be done if you have ready access to CNC and don't have to pay full price for it. Landing gear is about reducing the size of the parts until the material can just stand each landing case and then adding back enough FOS to make sure it is robust. Hog outs end up being optimized in the directions that the milling machine has access. So you cut I beams and C channels out of everything and run boring bars through long distances to get torsion tubes in the middle of open structure to hang pivot lugs off of. Lots of good resources out there for designing. Lots of YT videos showing well evolved designs for plagiarism, I mean flattery.

Been doing quite a bit of heavy and severe duty retractable gear design lately. It is not an easy task even when you have a very experienced team to break down the loads and then to do stress/strain on the parts you design.

#### Jay Kempf

##### Curmudgeon in Training (CIT)
And I forgot: hard to beat the Mooney Mite design.

#### WARPilot

##### Well-Known Member
Ok, so it looks like \$ to redesign to use different material. Weigh reduction has to come from analysis of the steel material and getting down to the minimum plus safety factors.

#### rv6ejguy

##### Well-Known Member
6061T6 has roughly half the strength of 4130 so you'll have to double the wall thickness roughly for the same strength. Aluminum weighs about 35% as much as 4130. The fatigue strength is about 1/3rd that of 4130. I think a well designed 6061T6 gear could weigh 10-15% less than a 4130 one with similar strength properties. An unoptimized design won't save much if any.

Anything welded has to be heat treated again with aluminum.

4130 is just easier all around.

#### Jay Kempf

##### Curmudgeon in Training (CIT)
The big unknown in 6061 is fatigue. If you leave an area that has a lot of strain you have to deal with fatigue. Big strains even if within stress limits can have short fatigue lives. So you have to beef up anything you are worried about. And that goes on a shopping spree in that that weight budget that you think you are saving. Landing gear gets cycled more than you might think.

What you want in retract gear parts is stiffness against buckling and then pure tensile. If you arrange all the sticks right you can get dead straight load paths. But it takes time, experience, and a really good stress guy. Loads are pretty straight forward normally unless you are dealing with something weird or you are way in an obscure corner of the FARs for some reason.

Don't be intimidated if you want to design your own. It can be done. It just takes time and you have to do a lot of research. Like I said: look at anything Al Mooney did before say 1980 that was light. Incredible light, simple mechanical, manually actuated gear. Look at the BD5. Simple double over center manual mechanism with cable actuated nose gear. Very clever people out there to follow. Both the Mooney Mite and BD5 took advantage of low wing and high thrust line to make the gear short, stout and light. It can be done. One load at a time, one feature at a time, isolate the axis', trudge through the math, size the structure, back check, trudge through....

#### Jay Kempf

##### Curmudgeon in Training (CIT)
Oh, forgot, the mountain bike world is welding a lot of 5000 series aluminum alloys. These are super thin walled, hydro formed, robotically welded frames and rear suspension and have impressive strength to weight and fatigue performance. Those 5000 series alloys have been around for a long time but are only really now having a day in the sun in lesser than aerospace applications. Worth a look. But the math is the same regardless of the material. Carbon is another possibility if you have the space for some open box structures.

#### Gregory Perkins

##### Well-Known Member
For thirty years I was on the lookout for ideas leading to a feasable 103 legal UL
retract gear certainly to use in a land configuration but expecially for the 103 seaplane
configurations. There were some interesting retracts for the float UL planes and the hull based UL planes and I remember a few for the amphib Trikes as well. In the museums there were some old old amphib seaplanes that allowed close up shots of their amphib gear. I also remember a Trike that looked like a streamlined bullet when its gear was retracted. One of the funniest retracts was on the UL Buchaneer where the pilot reached down and plugged in or unplugged the gear leg on each side and
tossed the gear leg and wheel into the fuselage when airborne. Somewhere there is a folder with the name "retracts" and maybe I can find it. I never looked at the Mooney Mite or the BD-5 that I can remember but I will be looking them up. Thanks.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
If you have not been through it yet, Pazmany wrote a book on Landing Gear. Good one to have or at least see if you can get it through your local library. Inter-library loans are nice.

#### rv6ejguy

##### Well-Known Member
Dark Aero just uploaded a video about their gear design which uses carbon and aluminum. It looks incredibly complex with many parts and doesn't look light either, which I think was the main intent. It won't be cheap for sure. It's a total work of art but not well suited for production IMO. I offered that a single titanium spring leg as Sonex, Rocket, Sukoi uses would be a better way to go in almost every respect. We'll see what they end up with for the kits.

What price light weight?

#### WARPilot

##### Well-Known Member
Current WAR landing gear, spring type, design. The trunnions are not depicted but need reinforcement and or a sacrificial bearing added. The steel tubing seems well overbuilt where it goes into the trunnion.
Then there is the retract system itself. It needs to be smooth, lighter, and simpler.

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#### TFF

##### Well-Known Member
I think you are wanting magic. That is probably the lightest gear that won’t collapse. There is not much there. I would be more inclined to add. Probably look at Mooney, Swift and Bellanca gear actuation. There still might be free plans for the Falco on the web. Look at those too.

#### Riggerrob

##### Well-Known Member
If you have not been through it yet, Pazmany wrote a book on Landing Gear. Good one to have or at least see if you can get it through your local library. Inter-library loans are nice.
Ladzlo Pazmany only wrote one book on Landing Gear. I have read the first volume and understood significant chucks thanks to all the diagrams. Unfortunately, Pazmany never completed his second book which was intended to cover retractable landing gear.

#### Riggerrob

##### Well-Known Member
Is there acceptable material like Aluminum tube that can be substituted for steel tube in retractable gear? Aircraft gross weight will be 900lbs and tail wheel.
I am looking for simple, reliable, and lighter gear design. The gear will need to retract inward. This will be to replace the WAR design which uses a worm gear system.
How wide is your wings' center section?
If it is less than the 8 foot trailer limit, you might want to extend its span to include the landing retraction mechanism.
That change will eliminate several variables and potential for jamming. Moving the wing joint farther outboard will also reduce bending loads on the joint, so may allow you to reduce the weight of the straps that span the joint. Again, ask an engineer to review your wing spar changes before cutting any metal.

#### Helton

##### New Member
Has anyone made a recent purchase on the CVJETKOVIC website? I sent doubts to the available e-mail and I didn't get a return.

#### Jay Kempf

##### Curmudgeon in Training (CIT)
That one is super simple. I seem to remember a 3/8" ratchet and a socket welded to the input shaft as the entire mechanism. Ratchet the wrench handle in the cockpit to retract. Release the ratchet to extend.

BD-5 extended

BD-5 in transit

BD-5 retractedin