Aluminum Plate Engineering/Forming Question

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proppastie

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Minimum of 2.67 g's at peak, and generally maximum of 3.67 g's
3.67 x 1.5 FOS = 5.5 G....Glider criteria calls out ultimate of 6 G....

now is the 3/4 6061 strong enough... the 7075 for the same weight is stronger, but if you have a budget, can bend the 6061 and not the 7075 and the 6061 is strong enough, might have to decide what is most important....design is lots of compromises....and budget is part of the compromise.....

I would wounder how many of the 6061 gear have "not held up" I would guess a good pilot on pavement should be able to use it.....often homebuilts save weight in the gear but if the plans vendor or kit supplier has not made a change perhaps one might consider the less expensive option.
 

flyboy2160

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.....might have to decide what is most important....design is lots of compromises....and budget is part of the compromise.....
Yes. Far too many many forget this. Is it worth it to spend $5000 to save 5 lbs? Only the owner, builder,or designer can make that choice on a case-by-case basis.
 

wsimpso1

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3.67 x 1.5 FOS = 5.5 G....Glider criteria calls out ultimate of 6 G....

now is the 3/4 6061 strong enough... the 7075 for the same weight is stronger, but if you have a budget, can bend the 6061 and not the 7075 and the 6061 is strong enough, might have to decide what is most important....design is lots of compromises....and budget is part of the compromise.....

I would wounder how many of the 6061 gear have "not held up" I would guess a good pilot on pavement should be able to use it.....often homebuilts save weight in the gear but if the plans vendor or kit supplier has not made a change perhaps one might consider the less expensive option.
Hey, I am known around here as a guy who says "stick to the plans". Right? Actually I say "stick to the plans unless you have known fixes to known issues". The factory gear is one of those "known issues" and the Grove or other equivalent gear are a "known fix". So let's see if we would expect an issue with the plans gear. Yeah, you guys ought to know better than to push me for data and calculations... Let's do some math.

Anchors are 43" apart, the axle mount faces on the gear legs are 62"apart. So the axle mount faces are 9.5 inches out from the anchors. The center of the tread patch is maybe another 5" further out. That makes the arm 14.5". A landing with the nose wheel off the ground gives a bending moment on the gear leg at the anchors of 1200/2*14.5 = 8700 in-lb per g.

6061 gear leg is 3.25x0.75. I = bh^3/12 = 3.25*0.75^3/12 = 0.114 in^4. Axial stress at top and bottom fibers = sigma = M*y/I = 8700*0.375/0.114 = 28,600 psi per g. Yield for 6061-T6 is typically 35,000 psi, so this gear will yield in a nose high landing when ground contact g exceeds about 1.22 g for the ground reaction or about 1.89 g's when you include the wing pulling upwards at 0.667 g per the regs. I reckon that most of us are worse than that once in a while. Then there are other stressors on the gear: The gear splays outward as the gear deforms, increasing the moment significantly; The gear leg gets a backward tug as the tires and wheels spinup that puts a bending moment aft on the gear and twists the gear. So the gear as designed is barely capable of handling anything above a greaser landing. No wonder the folks flying these guys say the 6061 gear performs poorly.

How about 7/8"? 1.67 g ground reaction at yield with the 0.667 wing making it a 2.33 g landing. Some of us may NEVER hit the gear that hard, but I would not count on anyone being able to operate in the backcountry without getting one once in a while. I would hate to be way out on some desert mountain strip out of cell phone range with a bent gear and broken prop and it is getting dark and cold and you are listening to unseen critters around your improvised campground while that jetliner relays a message on guard freq that S&R will be out on the morning...

The old version of FAR Part 23 said 2.667 g's on the airplane and 2.0 times airplane mass for ground reaction MINIMUM. We do not have to live by them, but they are pretty darned good advice anyway. This airplane should have a significantly better gear than the plans one.

Billski
 
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BBerson

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The thicker Grove leg is listed in post 29 at 3.25" wide.
But the stock 6061 leg width isn't listed here (as far as I know) (3.5 to 4" was mentioned)
 

blane.c

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I am a reasonably good pilot, but this airplane is going to be spending most of the time out on surfaces that are not paved. And short. And soft. And rocks.
Tires are going to be important as well then, but the thing most often forlorned is the tailwheel. Lord knows I've seen some gnarly looking tailwheels. They can get serious ugmo fast.
 

Victor Bravo

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The 2003 Revision of the Zenith 701 plans calls out a main gear spring made from 3.25" wide x .750 inch 6061-T6. Part number 7L2-1J.

The length is listed at 84", which I am assuming is the flat layout "blank".

So any main gear spring made out of any material, that you want to fit in the Zenair 701 fuselage (LG clamps, fuselage "channel"), will have to be 3.25" wide.

The thickness can be greater or less than .750, all this will require is some different spacers and rubber blocks for the clamps to sandwich tightly.

So a carbon fiber gear might be one inch thick, and a 4340 steel gear may be 3/8 inch thick, and a 7075 gear may be .750 or .875 thick.... and they'd all fit in the same mounts.

However, if it turns out that a 5 or 6 inch wide gear can work better from an energy absorption standpoint, it would require a significant redesign to get it all to fit. And it would likely not be worth it to anyone.... and definitely not worth it to me.
 
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BBerson

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If .75" 6061 is standard, then .875" 6061 would be significantly stiffer and stronger, but harder to form.
I don't see a need for 7075 at all. In fact, I prefer the 6061 bend a bit if needed in a crash and absorb the energy.

Does anyone add another short and thin helper leaf spring doubler for heavy duty use? Finding .875" bar or sheet might be hard to find.
 

pictsidhe

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If you're going to redesign the gear, do it properly, not, 'hey, let's add a bit here and see what happens'.
Of course, we can see what happens mathematically...

1. Decide how much travel you want.
2. Decide how much energy you want to absorb which determines how stiff you need it.
3. Size the gear

If you allow more travel, you can absorb the same landing energy with a softer spring. People tend to like this, it reduces broken teeth or worse, their back. Too much travel, you run the risk of broken airplane teeth: the prop.

Without changing leg length or tapers, a 7/8 7075 gear is going to deflect roughly about 60% more than 3/4 6061 before it yields.

Where does that put the prop? If it is 12" into the dirt, you might want to go to even thicker gear and save a bit more weight. Thickness and yield strain define deflection before yield. Stiffness can be adjusted with the width. E is is a wash between 6061 and 7075, so we can ignore it.

If the width is the same, 7/8 gear will be 60% stiffer. Will your teeth be OK with that?

7/8 7075 gear will absorb about 4 times the energy that 3/4 6061 gear will, with width staying the same. That's double the vertical velocity.

You could build a 6061 gear that absorbs the same energy over the same stroke as the 7075 gear, but it will be much heavier and wider. I get 1/2" thick and 6x the width of our 'perfect' 7/8 7075 gear. That is significantly different, and much heavier.

If you are going to compare the materials cost of 6061 and 7075 gear, you really ought to be comparing gear that deflects the same and absorbs the same energy. This makes 7075 over treble the value of 6061.

Is 7075 > 3x the price of 6061? No? Then 7075 gear would be cheaper as well as lighter. What's not to like? Oh yeah, finding the stock...

If yes, well, are the weight gains worth it? Only you can answer that question.

If you want to compare ease of forming, compare apples to apples again, not apples to marshmallows. You'll find that once you have sized the gear to suit whichever material you are using, the forming force is the same!
 

BBerson

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Midwest Steel has 6061 bar 3/4" X 3.5" X 84" at $50 (.875" not available in bar)
For 6061 .875" thick they have plate. A cut piece is $90
Shipping 84" or 96" for extra forming length is about $50, I suppose.

The 7075 would need about twice the radius.
 

pictsidhe

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If .75" 6061 is standard, then .875" 6061 would be significantly stiffer and stronger, but harder to form.
I don't see a need for 7075 at all. In fact, I prefer the 6061 bend a bit if needed in a crash and absorb the energy.

Does anyone add another short and thin helper leaf spring doubler for heavy duty use? Finding .875" bar or sheet might be hard to find.
optimised 7075 gear of the same stiffness and energy absorbtion would be 1 3/8 thick, 1 5/16 wide. Radius would be rather big
 

wktaylor

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Anyone here ever consider the BEDE-4 design for aluminum leaf-spring MLGs?

Also the following SAE documents might be useful for this discussion...
J788 Manual of Design and Application of Leaf Springs
HS-796 Manual on Design and Manufacture of Torsion Bar Springs and Stabilizer Bars
 
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