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Old 02-12-2018, 08:12 PM   #41
FoxDB
 
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Default Re: Landing gear choices...

You can find the drop test I did on YouTube just search: Kitfox Landing Gear Drop Test
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Old 02-12-2018, 09:15 PM   #42
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Default Re: Landing gear choices...

Here are pics I could find:
Attached Images
File Type: jpg nimergear.jpg (40.2 KB, 266 views)
File Type: jpg post-183-0-40373200-1383789205.jpg (41.6 KB, 264 views)
File Type: jpg post-183-0-42672300-1383789220.jpg (41.4 KB, 251 views)
File Type: jpg post-183-0-73540900-1383789237.jpg (42.3 KB, 251 views)
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Old 02-12-2018, 09:17 PM   #43
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Default Re: Landing gear choices...

Cabane "V"
I did a poor landing to a steep hill and hit HARD No damage to the gear but major damage to the Fuse Longeron twisting and buckling diagonal tubes.[/quote]

I've often thought that if the center of the V of the cabane was connected to the center of the seat truss, it would take a lot of stress and twisting action off the longeron/landing gear mounts. When/if the V flexes downward, it has lots of leverage pulling in on the sides of the fuselage. By tieing the V to the strong seat truss, it wouldn't flex downward and would'nt then be pulling in on the longerons. Never had anyone come out and say they agreed with me, but that's what I think! :-) It would actually be pretty easy to do as well....JImChuk
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Old 02-12-2018, 09:28 PM   #44
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Default Re: Landing gear choices...

There are a number of landing gear drop tests on utube. Here is one with an S7. JImChuk
https://www.youtube.com/watch?v=FILn8civ4UU
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Old 02-13-2018, 05:27 AM   #45
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Default Re: Landing gear choices...

Jim,
I agree there is an inward component to the cabane V loading but I believe the horizontal cross tube would take care of it. Also attachment to the fuse rather than a cabane V puts the tension bars at a steeper angle and increasing there load.
I find the drop tests on YouTube interesting, I intentionally did mine with small tires at high pressure and greased slide plates to see what the gear was doing without deceleration due to tire squish. So my test at 5.5" droop is probably equivalent to squishy tire drops of at least twice that. I designed my gear springs to go solid at 3 Gs I don't think they went solid during the drop test. Also note the lack of rebound which is what I was going for. The Beringer gear test shows rebound at first but I believe that is the squishy tires.
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Old 02-13-2018, 08:30 AM   #46
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Default Re: Landing gear choices...

I wasn't saying to get rid of the V cabane, only to connect it to the seat truss in the center so it couldn't pull down. The crosstube kind of does the same thing, but it's no wheres near as strong as the seat truss. JImChuk
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Old 02-13-2018, 09:12 AM   #47
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Default Re: Landing gear choices...

Jim,
Down is not a concern as the vertical reaction at the ends just apposes the upward force of the gear legs. It's the inward reaction at cabane attach combined with the inward reaction of the gear legs on the attach lugs that causes the effect that failed my fuse tubes. Mine did only fail on the pilot side. I added some bracing during the repair to counter the inward reactions.
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Old 02-13-2018, 09:24 AM   #48
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Default Re: Landing gear choices...

But it is the downward force on the cabane V that causes the inward pull on the longerons. It's a matter of leverage. As an example: If you tie a rope tight between 2 points and then pull at 90 degrees at the center of the rope, you get way more force exerted than if you pulled straight on the rope with the same amount of force. The longer distance between those 2 points and the closer to straight the rope is when you are pulling on it, the more leverage you have. That is why I think tieing to the seat truss would help. If the center of the V can't go down, it can't pull in on the sides. JImChuk
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Old 02-13-2018, 10:27 AM   #49
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Default Re: Landing gear choices...

I see your point Jim, we typically resolve the forces to the x & y to determine the vertical and horizontal reactions needed to resist.It is true it would strengthen it but it is difficult to know exactly how much. The vertical component would be eliminated but there is still a horizontal component to be resolved. Sorry this is getting to deep I guess in my opinion I would rather put in the horizontal tube in the cabane V than to deal with the fusalage connection.
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Old 02-14-2018, 10:50 AM   #50
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Default Re: Landing gear choices...

Here's a chart showing the cabane style landing gear tube forces. The following approximations are made:
  • All tubes are in one plane. I'm ignoring the aft link.
  • The cabane roughly lines up with the lower tube, otherwise you get additional forces in the cabane, as mentioned previously.
  • Landing gear support width 36". (Kitfox IV.)
  • Landing gear height 24" for stock, 27" for bush. (Axle to fuselage support.)
  • Horizontal landing gear force zero. (Perfect landing.)
  • Vertical force = 1. (Forces normalized to 1.)
You can see that as you well know, as the landing gear gets wider, the forces escalate, slightly non-linearly. Load increase to go from 60" width, 24" height, to 72" width, 27" height:
  • Upper strut 21%
  • Lower strut 50%
  • Horizontal compression tube 50%.
Attached Files
File Type: pdf Kitfox Landing Gear Forces.pdf (183.9 KB, 87 views)
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