Why round tubes for landing gear?

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rtfm

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Hi,
I'm sure there is a perfectly obvious reason all landing gear are constructed from round tubing, but I don't know what it is. Anyone care to enlighten me? Likewise for wing struts.

Duncan
 

wsimpso1

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Duncan,

Well, not "all" gear are constructed of round tubing. Yes, a lot of slow airplane gear are, but a quick perusal of these sites will show that some Wittman type gear are rectangular section. C-150 and fixed gear C-180-182 have steel rectangular section. Many Van's taildragger models use rectangular steel.

The round tube sections used on C-172 are a thick wall tube, turned to a taper, tailoring them to reduce weight and increase stored energy per pound. They are all bent, then heat treated, then shot peened.

Robbie Grove makes rectangular section aluminum: Grove Landing Gear

Then Harmon Lange makes solid round section steel, also machined, bent, and heat treated. His big volume is Van's tricycle gear and he does a lot of straighten and re-heat treat on them: Gear Leg Drawings

Then you get into bungee type gear, and yes, round tube dominates, mostly because round tube is available in so many sizes that you can get really close to min weight.

Nothing keeping any of us from using teardrop tubing instead from a bunch of sites and cutting drag in half or fairing over the round tube with fabric. It is a trade of weight for lower drag.

As for wing struts, most use streamline tubes to cut drag way down.

Round tubes are way draggy things, and should really not be out in the airstream if you can help it. Lots written on the topic. Barnaby Wainfan has even presented at OSH on the topic of how a little wire is more drag than a big wing section.

Billski
 

Pops

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When using streamline tubing for less drag the column strength is the minor axes , so its a lot weaker in column strength than the round tubing. Better to used the round tubing and streamline it with lightweigh materials. Also for equal column strength the streamline tube will be heavier.

Added--- There is a HUGE price difference between round tubing and streamline tubing.
 

challenger_II

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As for round tube vs square tube in a gear leg application, weight is a key factor. Also, add that a comparably sized square tube will have additional stress risers, due to the corners, where-as the round tube would transmit the stresses circumferentially.
 

TFF

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Like round spars, they are easy way out. They are not necessarily the best. If it is strong enough on the most demanding direction, it’s strong enough everywhere else so it doesn’t take as much thinking as designing the most elegant solution. Still you have to get it right and it has to be made right.
 

Pops

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Lot of airplanes with the streamline tube bungee struct has a low sideload limit when the strut is in compression as when happens in a swerve. One 4 seat taildragger built by plans with streamline bungee strut has a side load of 3150 lbs on a 2500 lb GW. So, a little swerve were most of the weight comes off the inside wheel and with the centrifugal side load on the bungee strut in compression and the most of the weight of the airplane, the streamline strut buckles about 13.5" from the bottom bolt. With that the LG, V-strut and wheel folds under the fuselage , you get a hard prop strike and a destroyed wing from the strut attach point to the tip. If this happens at a higher speed the elevator and stab will also hit the ground on that side and if on the right side with the cargo door the fuselage will be twisted. I changed to a round tube that increase the side load from 3150 lbs to 12900 lbs in column strength and it was 1.5 lbs lighter weight than the streamline strut. Then use the saved weight to use a little hot wired foam and fiber glass to streamline the strut. So the weight comes out a wash.
Also added two tubes on the cargo side of the fuselage to bring that side up to almost equal with the other side. Especially important if you have the aircraft on floats.

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David L. Downey

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I believe a fair answer to the OPQ is that a few chunks of blue foam and a layer of fiberglass (or even plastic sheet) glued on to the round tubes will dramatically reduce the drag, while dramatically NOT increasing the cost.
a "today's tech" solution would be to simulate the low drag profile with a single sheet of 0.012"-0.016" aluminum "folded" around the constant diameter round tube and attach it to the tube using one of the very capable foam mounting tapes at the point of contact with the tube to fixture it and a similar narrow strip of the foam tape along the trailing edge where the two edges come together face to face. No holes drilled into the round tube to cripple it, extremely weatherable and plenty secure (skyscrapers and spacecraft, buss, truck, and train exteriors are built that way, have been for decades).
The tapes allow for eyes on inspection of the structural component if needed and then replacement; a high note guitar string can be dragged along the length of the section to separate it from the structure after the trailing edge is slit when that is needed. Note that if you dont trust the tape only along the trailing edge, please add a 960PDL washer instead of the foam at each cheater rivet location to minimize the dimpling and ripples!
It takes carefull planning and execution though! A minor mis-positioning error results in remove, scrap and start over with the sheet! (use copious amounts of applique installation fluid to make it less critical at first contact then let it age for at least 48 hopurs before loading it with a slipstream)

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David L. Downey

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a "today's tech" solution would be to simulate the low drag profile with a single sheet of 0.012"-0.016" aluminum "folded" around the constant diameter round tube and attach it to the tube using one of the very capable foam mounting tapes at the point of contact with the tube to fixture it and a similar narrow strip of the foam tape along the trailing edge where the two edges come together face to face. No holes drilled into the round tube to cripple it, extremely weatherable and plenty secure (skyscrapers and spacecraft, buss, truck, and train exteriors are built that way, have been for decades).
The tapes allow for eyes on inspection of the structural component if needed and then replacement; a high note guitar string can be dragged along the length of the section to separate it from the structure after the trailing edge is slit when that is needed. Note that if you dont trust the tape only along the trailing edge, please add a 960PDL washer instead of the foam at each cheater rivet location to minimize the dimpling and ripples!
It takes carefull planning and execution though! A minor mis-positioning error results in remove, scrap and start over with the sheet! (use copious amounts of applique installation fluid to make it less critical at first contact then let it age for at least 48 hopurs before loading it with a slipstream)

View attachment 115923
forgot to add that thin PET sheet could also be used at a weight savings while double the thickness. If clear, then visual inspection can be done through it in most cases.
 

TFF

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I think the original question is really lined up as, is there anything simpler I can build? I don’t think streamlined is even first runner up here. I think low cost might really be number one, easy to make, number two, materials off the shelf in Australia, number three. Streamlined is probably tenth on the list.
 

Pops

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A streamlined faring coming lose in flight can be nu-nerving. Wife and I was flying from Huntsville Al to WV in the Falconar F-12 and was up high with a good tailwind and a high groundspeed, about 1/2 way on the trip I started to hear a light vibration at times. The vibration was getting more frequent, but no louder. So decended and landed at an airport to checked it out. The F-12 had streamline aluminum covering over the round C-150 round LG legs. Some screws had come loose and at times the streamline alum covering would flutter on the LG strut. Screwdriver from the tool kit and all was good. However you make the streamline covering, do a good job of attaching.
 

WonderousMountain

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Switching from the Guppy single tube to a streamlined section,
would require replacing brackets & new attachment. The alternative, Ed Fisher 404 style X braced V bungee solves one issue at the price of several imperfections. However, one can avoid cabin build difficulties.
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Riggerrob

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Luscombe *a Silvaire was originally built with streamlined wires transmitting loads from the wheels to the shock absorber hidden in the center fuselage. This worked great as long as the wires were only in tension.
After a few student pilots collapsed landing gear - during clumsy cross wind landings - many flying schools replace wires with steel tubes. Steel tubes were slightly stronger in tension, but vastly stronger in compression .... and survived far more un-coordinated landings.
 
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