Bungee landing gear design

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Dana, May 16, 2017.

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  1. May 16, 2017 #1

    Dana

    Dana

    Dana

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    Who’s an expert (or even reasonably knowledgeable) on bungee style landing gears?

    I am trying to work out the design details for the new landing gear on my Starduster, combining three different designs. The original Starduster design had the bungees mounted vertically inside the fuselage, as on a Taylorcraft. The later design has the same bungee arrangement, but the legs are longer and the design is improved to eliminate the weak spot where mine broke. My plane has the original shorter legs but Cub style external bungee struts. My new gear will be the longer length, but retaining the Cub style strut arrangement… except that I want to replace the bungee struts with die spring struts from Wag Aero, since I would have to remake the bungee struts longer anyway.

    What I’m trying to get a handle on is how the bungee gear works. Am I correct in understanding that with the airplane (e.g. a Cub) at rest on the ground, the shock struts are fully compressed by the bungees against a hard stop, and start to extend only when the load exceeds the initial bungee tension? In this case the landing gear wouldn’t extend any further when the plane takes off. I know that on some older planes the gear is already in mid travel when the plane is sitting on the ground, and droops down quite a bit further in flight.

    The Wag Aero struts are rated 2200# at mid travel. 2200# is not the wheel load but the load in the strut, which is a function of its geometry. My plane uses two 1080HD bungees per side, as does the Super Cub. A 1080HD bungee is rated 900# for a total of 1800# per side, but I don’t know what that number is based on, i.e. what amount of stretch. But the die spring strut is made for Wag Aero’s Cub clone and the geometry is similar, so it should work (?) I don’t know what the total travel is of the Wag Aero struts, but I don’t know what the available travel is of a Cub strut, either.

    Does anybody here know these things?

    Dana
     
  2. May 16, 2017 #2

    Dana

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    This is a similar installation on an experimental PA-14

    100_7345.jpg
     
  3. May 16, 2017 #3

    BJC

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    That is the way a stock Pitts is.


    BJC
     
  4. May 16, 2017 #4

    Rockiedog2

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    I've built I dunno probably 8 or 10 shock strut setups on gear like yours. I never used bungees always springs like what you're talking about. On my first set I was able to find a math dude who was willing to run the loads on the shock struts under the various conditions so I could get an idea what capacity spring to try...all my stuff is trial and error but all the springs came out fine. Not only Wag and Spruce but Mcmaster Carr has a buncha springs. The loads on the Legal Eagle came out to 431# per strut at MGW and I forget what G we used but I was able to find a spring that matced in McMaster Carr and after that I think most all the LEUs used that or a similar spring. After building all those sets I've decided the spring capacity selected isn't all that critical, within reason. I know some(most?) guys preload their spring. I don't like trying to squeeze those things down so I never did it. I really didn't see why they did it tell the truth...I always just seated the spring against the stop and put a dummy strut in it and adjusted the hole center length til it looked about right on the camber; then got in it and checked it and adjusted again til found the length for the strut. The spring ends up a little off the stop when loaded and building it like that but that doesn't seem to make any difference as far as I can tell. I call that slight compression my preload. After flying it a while I've sometimes readjusted the strut length to get it just so. Not real scientific but that's the way I like things.
    I did things different on SS1 and put the springs inside only I didn't use springs I used NAPA rubber shock bushings which weigh way less than springs and we can adjust things by how many we use. I put thin AL wafers and grease between the bushings so they could compress w/o fighting the neighbor. I like the way that worked out. Dirt simple and much easier than building the draggy cabane vee between the gear legs. And much cleaner. After 100 hours in it the gear has turned out to be about the most successful guess on the whole plane. I wouldn't change a thing on that setup. There's a description and pics on the Spencer Original Design thread if you're interested.
    I dunno why anybody would use bungees when we got springs/shock bushings. Maybe a slight weight advantage to the bungees(not over the bushings tho) but I say that's canceled out by the extra drag. And who wants to change out bungees...a PITA. Once the springs/bushings are done we can forget em other than occasionally checking the cross bolts for marking
     
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  5. May 16, 2017 #5

    Pops

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    Its easy to work out the mechanical advantage by the geometry of the gear. On the gear in the picture of my SSSC there is a 1.87 to 1 advantage. Using the die spring chart from the manufacture I was able to find the spring that fit my needs. I worked out the compression of the spring by the average weight on each wheel with me in the seat and full fuel and designed the strut to be compressed that amount when assembled. I used the same adjustable rod end at the top of the shock strut to find tune the length of the strut ( same rod end used on the Bearhawk landing gear for the same reason. The gear on the SSSC had a side load of 3200 lbs on each side, ( 810 lb GW). Use Euler's Column formula for the strut to get the dia and wall thickness to use. Remember if using a streamline strut for the lower end, use the minor axis dia. For the lightest weight per strength its better to use round 4130 tubing instead of streamline strut and streamline the strut with balsa or foam,etc.
    Link to get the die springs. Go on-line and get the list of springs and charts. I have used their die springs on 4 airplanes.
    Wag-Aero's 2200 lb die spring will just be correct for the certain landing gear geometry and weight. I prefer die springs over bungee's any day.
    http://www.daytonlamina.com/node/831 Get catalog and chart for Maxlife Die Springs.
     

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  6. May 16, 2017 #6

    Mad MAC

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    Have you tried working backwards from the Mil Spec MIL-C-5651, particularity if you have a ring to carve up to get the type and number of cords. The Mil Spec gives the load ratings for 4 extension positions, That should be sufficient to give you force verse stroke for your gear.
    http://quicksearch.dla.mil/qsDocDetails.aspx?ident_number=5576

    Warning the following is only slightly better than pondering aloud.
    As for preload, I would consider it to be a combination of strut deflection / tire stroke / tire scrub. The higher the preload, the more rolling drag over small bumps / harder ride but with less scrubbing action. So for soft field ops, that would drive one to a low tire pressure combined with low strut preload but limited by acceptable tire scrub.
     
  7. May 16, 2017 #7

    Dana

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    Dan, how do you get 3200# each side? How much travel do your springs provide (either at the spring or at the wheel)?

    I've worked out some rough numbers the load in the shock struts currently on the plane (based on scaling a photograph, I'll take precise measurements later), and it works out to about 1.36 mechanical advantage or 700# tension in each strut when the plane is sitting on the ground at gross weight. With the new longer gear it'll be more like 1.5 or 786#. Using FAR 23's number of 8 ft/s vertical at touchdown 4" of wheel travel would be 3G... I would have 2.25" of wheel travel (1.5X the 1.5" compression of the spring) but there's also tire compression?

    Oops, wife says gotta run, more later.

    Dana
     
  8. May 17, 2017 #8

    Pops

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    My lower shock strut length is 21.5" ( looking at my plans the side load is rated at 3600 lbs not 3200 lbs as I posted) and I wanted at least a 4G side load. Its works out that a 4130 steel tube of 3/4" dia X .049" wall thickness X 21.5" long will give me 3600 lbs. If I used 3/4" dia x .035 the column strength would be 2900 lbs, less than the 4 G's that I wanted.
    For the SSSC with a 810 lb GW and the landing gear with a 1.875 advantage I used a Dayton MH-200-500 die spring.
    Length 5"
    OD-2"
    ID-1"
    Travel to solid - 2.258"
    Deflection .1" at 53 lbs
    Deflection @ 20% -- 1" @ 530 lbs
    Deflection @ 25% -- 1.25" @ 663 lbs.
    Deflection @ 37% -- 1.85" @ 981 lbs.

    This die spring worked out just right for the weight, travel with the geometry of my gear.
    The landing gear on the JMR is completely different with a GW of 1050 lbs and required a completely different dia spring.


    There are several plans and kit airplanes on the market that are very marginal in the landing gear side load. About the time all the weight gets on one wheel in the beginning of a ground loop and the increase side load on the wheel from the resulting swerve to the right or left the side load can easily get to the max column strength.
     
  9. May 17, 2017 #9

    Dana

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    Oh, OK. I thought you meant 3200# was the tension in the spring strut, and that didn't make sense.

    I believe the Wag Aero struts are 1" for the outer tube and 7/8 for the inner, I don't know what the wall thickness is.

    How much preload did you have on the springs, and how much working travel?

    Tell me more about the adjustable rod ends you used.

    Thanks,
    Dana
     
  10. May 17, 2017 #10

    TFF

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    I know you already have a design drawn, but here is the AcroSport 1 gear. Its bungee, the AS2 is spring. I think you have to reverse the sliding tubes.
     

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  11. May 17, 2017 #11

    Dana

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    Now I see why you said my gear looked like the AS gear.

    Dana
     
  12. May 17, 2017 #12

    Pops

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    As you can see in my post the die spring I used has a 2.58" travel to solid. That will be the total travel in your strut but the total travel at the wheel will change by the geometry of your Landing Gear.
    With the geometry of the LG on the SSSC, I preloaded the die spring .75" which is 397.5 lbs per wheel .
     
  13. May 17, 2017 #13

    Dana

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    Right now I'm inclined to buy the Wag Aero struts to save the work (and time!) of fabricating them, once I have them in hand I can evaluate whether I need to change to a different (probably lighter) spring. The spring looks to be 6" long as installed, 2" OD, 1" ID, don't know the free length or rate. I confirmed that the tubes are 1" and 7/8", both 058 wall. They come 45" long for the builder to cut and finish (they make another version prefinished for their J-3 clone with presumably the same springs).

    Dana
     
  14. May 17, 2017 #14

    Pops

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    The die springs used on the SSSC are a little on the soft side but the next step up is a large jump where they would be to stiff. Just a small increase would be perfect. The present spring has a 53 lb load for a .10" deflection, if the spring was 63 lb for the same .10" it would be where I would want it. On off field operation the gear is really working good along with the larger tires.

    So the bottom tube is 7/8" x .058" , can you tell me how long the tube will be when installed. Just guessing, but if your bottom 7/8"x .058" tube was 29.5" long the column buckling strength would be about 4100 lbs. I don't know your GW, but if 1600 lbs, that would be a side load of 2.56 G's. The round tube that I designed for the Bearhawk to replace the streamline tube is rated at 3.64 G's. The round tube that I used on my scratch built Bearhawk is rated for 5.16 G's and is lighter than the old plans streamline tube of 1.25 G's.

    So if you have a less GW of 1600 lbs or a shorter length of the lower tube of 29.5" its going to be stronger than the 2.56 G's.
    I agree, the Wag-Aero spring is going to be stiffer for your lower GW.
    Isn't this fun :)
    Dan
     
    Last edited: May 17, 2017
  15. May 17, 2017 #15

    Dana

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    My gross is 1082 and the total installed length of the strut assembly will be around 29", so side load shouldn't be a problem. The spring rate is questionable; nobody I talk to at Wag Aero seems to have any real information, I gather the guy who designed it doesn't work there any more. But as I said I'll probably just buy a pair, I can measure the spring force and if it's not right I'm only put the cost of return shipping.

    Dana
     
  16. May 17, 2017 #16

    Pops

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    You are right, side load will not be a problem, (3.87 G's). If the dimensions is 6" long by 2"OD and 1" ID , Dayton Die Spring has a MH200-600 of 46.8 lbs for a .10" deflection, 2.817" travel to solid. ( seems a little soft for the 1082 GW).
    H200-600 of 91.2 lbs for .10 deflection, 2.627" to solid.
    EH200-600 for 154.0 lbs for .10 deflection, 1.770" to solid. ( really be a stiff gear).
    See what I mean by a large jump in rates between springs of the same size.

    On the JMR with a GW of 1050 lbs, I am using the H200-500 for my LG. That is a 2" OD x 1" ID x 5" long. 105 lbs for .10 deflection, 2.050 travel to solid. I believe its going to be correct for the JMR.

    If the spring in the Wag-Aero strut is to stiff, maybe the H200-600 with the 91.2 lbs for .10 deflection and 2.627" travel might work. Looks like you will not know until you try it and then decide. You could always measure the Wag-Aero Spring to find out the rate.
    I know you will get it right. Dan
     
  17. May 17, 2017 #17

    Dana

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    I made an error in my earlier calculations... with my current setup I calculate there is 1061 lbs tension in the strut just sitting on the ground (neglecting friction, of course). If I make the gear 6" taller with the same wheel track the strut tension is 963. Assuming 1.5" strut travel I have 3" vertical wheel travel now and would have 2.8" with the longer gear.

    I need to make better measurements, what I have now is rough calculations based on importing a photo into the CAD and scaling it:

    front view with angles.jpg

    Here's also a 3D model of the old (brown) and new (green) gear from the Starduster drawings, yet to be reconciled with what I actually have. The new Starduster design is actually 8" taller, but I don't plan to go that far.

    lg comparison.jpg

    Dana
     
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  18. May 24, 2017 #18

    Dana

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    I have better numbers now.

    First, I received the Wag Aero struts yesterday. The spring is 6" long as installed, the actual free length is about 6 1/8. The slot in the tube is 1.75" long. Using a hydraulic press and load cell at work, I measured the spring rate at roughly 200# per 1/8" travel, so the spring rate is 1600#/in and the preload is 200#.

    The closest spring I can match it to is an Associated Spring 106-824 which is 6" long, 4.5" solid, 1473#/inch.

    My original bungee setup has a 1.9:1 mechanical advantage and 519# per wheel at full gross. Sitting on the ground there is no extension of the strut (it doesn't move when I hoist the plane).

    With my new planned geometry (5" longer legs rather than the 8" on the later Starduster drawings), the mechanical advantage is 1.75:1. That gives a vertical travel of 3" and an initial compression of 0.56", which doesn't sound unreasonable. If it is indeed the Associated spring the actual solid height is 4.5 or 1.5" travel which means 2.6" wheel travel... but I rough measured the gap between the coils and I come up with more like 2" to solid height, so I don't really know.

    It looks like the plane originally had 1.25" axles, and somebody used a conversion kit to 1.5" axles, slipped over and screwed onto the original axles. Nothing in the logs, of course, but it's clear a lot of used parts were used on the original build, the wheels have a 1972 date and the plane was built in 1982, so who knows. I considered doing the same and reusing the conversion axles, but I think it's better to just buy new 1.5" weld on axles, $35 each from ACS.

    IMG_20170523_150740051.jpg

    Tonight I'll finish making a form block to make the upper bracket for the front legs, and bend the brackets. Since the plane's in an open hangar, I want to get as much done as possible before actually taking the old gear off.

    Dana

    Dana
     
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  19. May 24, 2017 #19

    Pops

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    Dayton Die Spring has die spring # EH200-600 , 1540 lb for 1" and 2726 lb to solid length of 4.23" If you needed a stiffer spring in that size.
    Agree on the new axles.
    Bet you will love the new gear along with the peace of mine when landing.
     
  20. May 27, 2017 #20

    Seanxair

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    Hello all,
    This is a really interesting topic albeit too technical for my brain! However I am trying to source some spring loaded struts to replace the bungee struts on my Easy Raider tandem ultralight which is a Sky Raider II derivative. I have seen some that are used on Belite aircraft and now see the picture in Dana's post above. I've tried contacting Belite in every form to ask where they have them manufactured or if they make their own but can't get a reply. The geometry looks very similar to mine. My bungee struts are 660mm at rest and made from 15mm tubing.
    Can anyone point me to a manufacturer of these spring struts? I will have to get them approved before I can fit due to UK rules. I've put pictures of my plane from which you can see the bungees, the picture from Dana's post of the struts on an experimental PA 14 and the Belite ones.
    Hope someone can help!
    Thanks,
    Sean
    G-OEZI 2.jpg 100_7345.jpg Spring Steel Landing Gear.jpg
     

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