Landing Gear Upgrades

Task Description

Current landing gear is wobbly due to hoop stress on the CF tubes, we need to modify/redesign to solve this problem.
Below from @Conall Kingshott :
We have three main point where the slop comes from right now. From the connection of the tubes to the center block, from the connection of the arm to the leg of the landing gear, and from the arm to the motor. This slop is principally coming from the clearance holes in the carbon fiber for the bolts. The typical ISO “standard” fit is 5.5mm for a bolt which is nominally 5mm, this is because bolts aren’t meant to be locating features, they are only really meant to apply a compressive force. The more common approach in industry to locating things is using dowel pins, but given the nature of carbon fiber and its lack of resistance to hoop stress, there isn’t a clear way of doing this.

The least concerning of the 3 sources of slop at the moment seems to be the arm to the motor. First off, this joint gets the least load, secondly the holes are M4, not M5, and finally, the thrust of the motors actually has a “restorative effect” on the rotation of the motors.

The joint connecting the center block to the arms isn’t great and I am very open to something that would help fix it, but it doesn’t seem to be causing the same amount of issues as the joint connecting the arm to the leg. I think this is because the part of the center block that goes inside the tube is aluminum and well machined to it holds its shape and position well.

The part that seems to be causing the biggest issue is the joint from the arm to the leg. It is a 3D print that goes around the carbon fiber tubing and has bolts passing through the whole assembly. When I look at the drone landing gear being misaligned, this joint being askew seems to be the biggest culprit. Our hands are also somewhat tied here as we can’t really over-tighten the bolts without crushing the 3D prints

One important thing to keep in mind is that the majority of our issues come from rotation around the tube, so anything that connects and supports the leg from bending along the axis of the tube is unlikely to solve our issues.

Constraints

Assignees

Task Progression/Updates

Author: @Alison Thompson Date: 2024/01/12

INITIAL IDEAS

I don’t have a concrete idea for this so it’s pretty open ended.

@Megan Spee 's ideas

Pic	things of note

Pic	things of note
Open	these look lightweight, probably made of injection molded plastic. this structure wouldn’t work well on 3D printed plastic. Good points: lightweight taper based on stress concentration easy attachment (though flimsy) - zip ties   Bad points: design is for injection molded material, which we likely do not have access to unless we buy COTS parts flimsy for our drone weight
Open	curved nicely. this can have suspension effects depending on the material - and if you zoom in near the top of the legs, there is a little gap in the plastic where the plastic is allowed to flex into. shows that there is suspension in this landing gear.   Good points: suspension bounce effect looks sturdy while also being cheese-holed out to minimize weight has little feet on the end, more stable   bad points: attaches under the body of the drone - could be good, depends how it interacts. listed as negative because it would likely need more overhead for interactions with different parts injection molded plastic looks like the feet could catch on the ground and tip
	nice and chunky homemade landing gear   Good points: clamps on fully to the tube, similar to current landing gear sturdy feet for heavy drone arm-to-leg joints have specialized shape to accommodate more stress when legs are pushed outwards Wider feet can help with self leveling - Conall   bad points: probably need to be mounted before all motors are secured - like, slid on from the end of the arm 3D prints not structurally good all stiff materials give pretty much no suspension

General thoughts

• look at different materials to 3D prints and CF

  • TPU prints, maybe?

• these legs should break before the drone does - but not too easily

• think about sensor view to ground - do the legs block anything?

• picture the drone landing, skidding, and tipping over. try to avoid this at all costs

• picture the drone landing hard on one leg, and the leg pulling out the arm, wires coming out, breaking the CF plates. avoid this too

• this will be fun!

• springs may be hard to deal with if you make something custom.

• look at integrating COTS items with your own designs.

Author: @Conall Kingshott Date: 2024/01/22

Some Additional Thoughts

We are sadly somewhat limited on what we can do manufacturing ie: No injection molding, so we likely will need to stick to carbon fiber, 3D prints, and Aluminum. On the 2023 competition aircraft, Icarus, we tried a lot of stuff with clamping around carbon fiber tubes and it proved to be very problematic, so I think we would be well advised to try and avoid that.

In the timeframe we have, I am not inclined to pursue a design with a lot of “flex” with the concept that it can absorb impact or anything. I would be very concerned about it breaking or moving in a way we don’t want and causing a prop strike or other mechanical failure.

Possible Solutions:

• Since we can’t dowel pin straight through the carbon fiber, if we wanted to use dowel pins, we would need to glue bushings into the tubes first, likely with a doweled jig to make sure everything aligns. Then you could pin bushings in the arms to something, possibly a thin aluminum plate on the top of the leg section of the landing gear. This is a lot of effort, and if it failed, any tubes we have glued would subsequently bee unusable.

• We could also consider adhering the legs to the arms and using some sort of gusset to set their position. This would create a joint without slop, but one that wouldn’t have any room for adjustment or a good “break” point.

• We could look at whether “nut plates” that distribute the load more evenly across the 3D print would help, or otherwise reinforcing the joints with metal.

• The wider feet on the bottom photo Megan sent are interesting. Could help us set everything more level and would be really easy.

Hao’s Ideas: based on comments from Tuesday meeting, original sketches are being revisted/rethought.

Ideas generated on 24th of Jan based on previous Mech Meetings and Comments.

From what I can understand, each leg seems to have a different degree of wobbling that comes from the joint between the arm and the leg (what @Conall Kingshott said in the description). As discussed with Benjamin Lovegrove (the @ doesn't work) after the mech meeting, adding bushings [1] to the holes in the arm or the addition of a beam [2] between the battery panel clips and the arm might help stabilize/dampen the wobbling. Finally, an additional idea would be to use thin double shafts that would extend from the bottom of the center block to either side of the arm joint, thus offering lateral stabilization [3].

Preliminary Pros and Cons:

Bushings [1]:

Pros: Cheap, light, easily manufacturable/sourceable that will patch up the clearance holes in the arms to a sliding fit, thus significantly reducing the lateral movement range of the leg overall.

Cons: A significant amount of hoop stress on the arm could lead to cracking in the carbon fiber arms.

Possible Solutions to Cons: Addition of rubber rings/sleeves may help with hoop stress.

Stabilizing beams [2]:

By redesigning the area around where the landing gear attaches to the arm, an area can be made where a small beam (aluminum or plastic) could be put. This would attach to the connecting joint and the main carbon fiber outer shell.

Pros: Lightweight, cheap, easy to manufacture

Cons: Involves reprinting already existing pieces.

Center block to Arm joint rods [3]:

Pros: Creates a general solution to wobbling on all 4 arms, can be made to be very stable and durable, and can also be used to reduce back-and-forth rocking of the leg (if any).

Cons: Possible extra composite work may be required ( although I was thinking of using 2050 or 6061 Alloy, both lightweight and easily machinable aluminum). Modifications will have to be done to the cabin to accommodate the extra piece.

Solutions to Cons: The “rods” can be turned into slats as they only need to resist concentric “around the arm” wobbling (will be more clear with sketch)

Author: @Conall Kingshott Date: 2024/02/01

Resin Printing Design Review:
- I think we may get better results if we make the clearance of these holes a “close” fit instead of a normal fit (For M5, this is 5.3mm vs 5.5, but make sure you edit it in the hole wizard hole specifications, not manually). Please do it for both sets of M5 holes.

• We need some level of clearance on the holes for the tube, they are 25mm exactly rn. Maybe we can try 25.05mm or 25.1mm? I don’t know how precisely we can actually get IDs on resin prints. Maybe worth some research.

• I think we’ll want to remove the groove at the intersection point of the horizontal and vertical parts of this print. I’m not convinced it is giving us the benefits we had originally hoped for.
  
  

Author: @Hao Gao Date: 2024/02/02

Material Choice for reprint:

Choice to be made between Nylon, Tough and Nylon Carbon Fiber. I imagine that the nylon carbon fiber is quite light compared to the tough filament. The tough filament offers good overall performance in my opinion (ie it will flex easily and resist impact, so it won’t shatter).

Author: @Alison Thompson Date: 2024/03/19

CAD REVIEW OF PROTOTYPE:

Your tapped holes are done weirdly, instead of doing cosmetic thread like you are doing, you should be using hole wizard’s tapped holes. This will make nice callouts in the drawing. They should also be tapped for M6, not M5 because the shoulder bolts are M5. Otherwise, this looks good to me.

Author: @Alison Thompson Date: 2024/03/20

CAD CLEANUP AND DRAWING

Changes made to sleeve part:

• Width of part to 1in to match purchased stock

• Size of slot from 25mm to 1in to match cutter

• Depth of slot to a nice number (6.7mm from top of part instead of random decimal caused by dimensioning edges of part to edges of circle)

• Hole wizard features to be defined off of the top face so accurate depth callouts are given in the drawing

• Cosmetic threads to M5 tapped hole

• Removed chamfer for ease of manufacturing prototype (necessary on real part to reduce weight, just add extra manufacturing time for prototype)

Final drawing (reviewed irl by @Conall Kingshott ) :

Author: @Alison Thompson Date: 2024/04/01

DESIGN UPDATES (these weren’t added earlier so here they are):

General ideas/why this is better:

• Main problem was wobbliness due to hoop stress on CF tube holes and lack of a good way to locate things in carbon fiber tubes

• Solved with epoxying bushing into tube holes (located with a jig so they line up correctly

• Bushing flanges slot into aluminum part that distributes load and allows for mounting of vertical tubes

• 3D printed sleeve/adaptor to be epoxied to aluminum part with same break point as current design

Author: @Alison Thompson Date: 2024/04/22

DRAWING FOR REAL LANDING GEAR:

The previous drawing was for the prototype part, this is a similar drawing for the real size/geometry.

Author: @Alison Thompson Date: 2024/04/22

REVIEW OF REV 1 from @Conall Kingshott :

• 8.70 on the right view and the chamfer callout are redundant

• Hard to tell what the 8 and 8.2 are on the top view, add reference dimension to depth of slot