Aluminum Plate Engineering/Forming Question

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Winginitt

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The thing I'm curious about is where do all the landing forces go when you increase the strength/resistance of the landing gear spring (thats what Zenith calls it) so that it doesn't flex/spring ?
 

rotax618

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This is interesting conjecture, but really the 701 is a very simple easy to build STOL aircraft. There are thousands of them and their like flying, sure some pilots bend the UC because of accidents, overload or bad technique.
The solution is simple, if you are going to be flying into unmade airstrips and/or flying heavy, then fit tundra tyres and increase the the leg thickness by 1/8” - large tundra tyres/tires allow the UC leg to be shortened by an inch and the track to be narrowed by 1-1/2 to 2 inches. Tundra tyres absorb a large amount of the landing shock.
If you want to fly fast, remove the slats fit VGs, fit smaller wheels and increase the leg thickness by 1/8”.
This is not the thread to discuss the shortcomings and improvements to the 701 (apart from the UC) but the 701 is much improved by increasing the span of the wing and tail, removing the slats and fitting VGs to the top of the wing and underside of the tailplane.https://stolspeed.com/
 

flyboy2160

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The thing I'm curious about is where do all the landing forces go when you increase the strength/resistance of the landing gear spring (thats what Zenith calls it) so that it doesn't flex/spring ?
If the one piece leg is free to slide cross-wise at one of the the attach points, the increased bending load remains in the leg. The fuselage will just see the increased vertical load. The larger tires will act as springs in series with the leg. (I'm assuming the leg is free to pivot where it attaches to the fuselage either via a real pivot or by rolling around on a radius block.)
 
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12notes

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Please do not put words in my mouth. I said what I said in standard English. For those of you who still did not follow it, I will elaborate.

By the time you make your gear leg durable in 6061-T6, it will be heavier than a similarly durable gear leg for the same airplane in 7075-T6. Why does this happen? First let's understand that WEIGHT IS THE ENEMY. When we design landing gear, we are trying to suck up the kinetic energy of the airplane sinking towards the pavement. Since we do not want it to weigh any more than it has to do the job, we will usually design our parts to be absorb enough energy and be strong enough and stop there.

Our figure of merit for spring leg materials is how much energy we can store in a unit weight of spring. If we can put more energy in a unit weight of one material than another, then we expect that we can make a lighter spring with the higher energy storage material. You can store a lot more elastic deformation energy in a unit of 7075-T6 than in that same unit of 6061-T6. 7075-T6 nearly twice as strong with about the same Young's modulus and about the same density as 6061-T6.

Want math? Energy stored elastically in a spring (any spring) is 1/2*k*dx^2, where k is the spring rate, dx is the spring deflection. Do this with a unit mass of material and you have your figure of merit for comparing materials.

Let's take a 1" cube of each, and apply a force just shy of the yield strength of the material squeezing two opposite faces towards each other. The stress is just shy of the yield strength, and the change in cube length is the strain just shy of yield strain. Divide the whole thing by density and you are figures.

FromASM's Matweb:

6061-T6 is 0.0975 lb/in^3, yield is 40 kpsi, and modulus is 10.0Mpsi. Strain at yield is 40e3/10e6 = 0.0040 = dx. K is 10e6. 1/2*k*dx^2 = 80 in-lb. Divide by density to get 820 in-lb of energy per pound of aluminum.

7075-T6 is 0.102 lb/in^3, yield is 73 kpsi, and modulus is 10.4Mpsi. Strain at yield is 73e3/10.4e6 = 0.0070 = dx. K is 10.4e6. 1/2*k*dx^2 = 256 in-lb. Divide by density to get 2512 in-lb of energy per pound of aluminum.

Yeah, you can store a little more than three times as much energy in a pound of 7075-T6 than in a pound of 6061-T6. Can you make 6061 hold up under 10 fps sink rates? Sure. But you can make a leg out of 7075 that can do the same thing with similar durability at significant weight savings. And since WEIGHT IS THE ENEMY in flying machines, you should do this where you can.

Billski
Thank you for this, it explains something I've been wondering about. The Hummelbird calls for 3 pieces of 1.5" x 15" x .125" of aluminum for a tailspring, some people have no problem with it, others replace it rather quickly. The plans call for these pieces to be either 2024-T3 or 7075-T6, the 2024-T3, using your math, has half the spring energy at 118 in-lb and 1179 in-lb of energy per pound. I'm guessing those that replace the tailspring used the 2024. I used 7075-T6 on a recommendation without knowing why, this explains it very well.
 

Victor Bravo

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The 701 project that I was talking about in creating this thread arrived at my hangar today. It turns out it is a much bigger repair project than I had thought, even after having seen it in perosn.

(I was not able to spend a lot of time looking at it in person, and it was already packed up on a trailer.)

Today we took it off the trailer and I had the opportunity to see just how much work is involved in repairing this. The landing gear fabrication discussion in this thread can go on for several weeks... while I make a new firewall, perhaps a new tail section, patch a bunch of un-necessary holes in the skin, perhaps replace a longeron or two....

One of the options that I had been willing to discuss, the Pilatus Porter taildragger configuration, is pretty much off the table. It looks like there is simply not enough structure in the areas where there needs to be structure. A more formal engineering study and some added structure would have to be in place before I pursued this idea.
 

flyboy2160

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Want math? Energy stored elastically in a spring (any spring) is 1/2*k*dx^2... Billski
Sometimes it's easier to grasp or to remember this by seeing a picture( :) or a pitcher, if you do beer math like me!)

The math part above with the 1/2 is like taking the area under the stress strain curve - which represents the energy used to deform the material. In the stress strain curve below, imagine that the lower curve is 6061 and the upper curve is 7075. The area under the curve to yield is triangular, so the area is 1/2 x the base x the height. The 7075 curve goes higher because it's stronger. The base of its triangle is wider. Thus, its area is greater.

 
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wsimpso1

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Compare the torsional stiffness of a box section and and an I section of equal bending properties.
Landing gear legs as the OP is using are solid rectangles. Given a fixed cross section area: the rectangle has more J than a square; The wider the rectangle, the more J it has. Torsional stiffness goes up - not down - as we widen this type of beam.

A discussion of flanged and box section beams is thread drift in multiple ways. Best if we not...

Billski
 

wsimpso1

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View attachment 88848 I think I would assemble the gear into a 1" x 4" I beam.
Using 1/4" x 4" caps that are easy to bend. And 1/2" bar for the web. With bolts or solid rivets about 1" apart.
I am still trying to figure out why we would want to do this. It will make for a strong gear leg and at less weight, but let's remember that gear leg has TWO major tasks that must be met together:
  • It must suck up both the airplane's vertical kinetic energy and the potential energy of the airplane setting onto the gear while all the energy is absorbed;
  • It - and the rest of the airframe - must be strong enough to not get damaged while doing all this.
The old FAR Part 23 (which does not apply to homebuilts and LSA's, but is still a pretty good idea) specified:
  • Minimum sink rate based upon wing loading which gives the kinetic energy;
  • Minimum of 2.67 g's at peak, and generally maximum of 3.67 g's;
  • Allowed the wing to carry 2/3 of the airplane's weight during the landing stroke of the gear, so the gear has to stand whatever the peak is minus 2/3 times the airplane weight at max stroke;
  • The gear system and the rest of the airframe have to take whatever vertical load that results.
In homebuilts and in LSA, we have no such restriction on the g's or the sink rate. But we do have to suck up energy (force over travel) and not damage the leg or the airframe. The tires suck up some of that energy and the gear leg has to be springy to suck up the rest.

It appears that the plans leg is prone to failing in use, while the Grove part seems to be fine in the field. The Grove part is thus a decent target for strength and energy absorption in landings. I am skeptical that the wide flange beam, while no doubt strong, will be springy enough...

Billski
 

wsimpso1

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The 701 project that I was talking about in creating this thread arrived at my hangar today. It turns out it is a much bigger repair project than I had thought, even after having seen it in perosn.

(I was not able to spend a lot of time looking at it in person, and it was already packed up on a trailer.)

Today we took it off the trailer and I had the opportunity to see just how much work is involved in repairing this. The landing gear fabrication discussion in this thread can go on for several weeks... while I make a new firewall, perhaps a new tail section, patch a bunch of un-necessary holes in the skin, perhaps replace a longeron or two....

One of the options that I had been willing to discuss, the Pilatus Porter taildragger configuration, is pretty much off the table. It looks like there is simply not enough structure in the areas where there needs to be structure. A more formal engineering study and some added structure would have to be in place before I pursued this idea.
Bummer on the effort needed. But then everyone needs a hobby - ;-). My work on the truck for the wife's airplane fuel just got deeper, so I have one too.

Billski
 

TFF

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I hope you document your repairs here. I have seen Rockiedog fly his and it’s impressive. Into a good wind takeoff was about 75 ft.
 

wsimpso1

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The thing I'm curious about is where do all the landing forces go when you increase the strength/resistance of the landing gear spring (thats what Zenith calls it) so that it doesn't flex/spring ?
First off, there is no such thing as "completely rigid". Everything has an inherent springiness. Young's Modulus (E)and its closely related Torsional Modulus (G) are present in all solids and determine their elastic behaviour under load.

Now if you do make the gear really stiff (triangulated steel tubes and the like), you still have the tires. For some airplanes, that is enough, but for most of us, well, we better have some sort of suspension.

The stiffer you make the springs, the higher the reaction loads we get in sucking up a given amount of energy. See post 128. Little airplanes do happen to be in a region where leaf spring gear legs can work.

Billski
 

BBerson

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I am still trying to figure out why we would want to do this. It will make for a strong gear leg and at less weight, but let's remember that gear leg has TWO major tasks that must be met together:
What's wrong with stronger gear leg and at less weight?
If too stiff, then just reduce the leg thickness. I was only suggesting a method of home fabrication without heat treating. Not an exact drawing.
 

wsimpso1

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What's wrong with stronger gear leg and at less weight?
If too stiff, then just reduce the leg thickness. I was only suggesting a method of home fabrication without heat treating. Not an exact drawing.
It does sound like it could be an advantage for homebuilding, but the usual problem in getting a design that works IS getting deflections high enough - too much stiffness is in the way when doing this work.
 

BBerson

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I was thinking the caps could even be 1/8" thick. Another advantage is splicing thin sheet isn't that difficult. So ordering and band sawing shorter lengths is convenient. I have a piece of 1/8" X 3" 7075...
 

BBerson

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An I beam with the same moment of inertia should deflect the same as a flat bar of the same material.
 

Winginitt

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Winginitt said:
The thing I'm curious about is where do all the landing forces go when you increase the strength/resistance of the landing gear spring (thats what Zenith calls it) so that it doesn't flex/spring ?

Bilski reply:
First off, there is no such thing as "completely rigid". Everything has an inherent springiness. Young's Modulus (E)and its closely related Torsional Modulus (G) are present in all solids and determine their elastic behaviour under load.

Winginitt: Agree, there is no such thing as completely ridgid. My concern is that the new gear might be "so ridgid" that it transfers sufficient additional forces to the fuselage which might then suffer damage. As I mentioned, Zenith refers to it as a "landing gear spring" which implies they designed/calculated it to have a certain amount of give. Its common sense that virtually all landing gears have a certain amount of movement, so I'm not questioning or asserting their gear is somehow different from other gears, only pointing out that they took the trouble to call it a spring. Changing the "spring rate" may save the gear but could harm the fuselage.

I posted several alternative ideas (videos) that show ways to absorb the energy and dissipate it in a way that is less harmful and in my mind a more desirable solution. I realize that VB is trying to improve his plane and keep costs to a minimum. Since he's a bright guy, I suggested he look at those designs and see if he might originate his own ideas and possibly come up with a better solution. Maybe the thicker/stronger gear works well and doesn't cause any fuselage problems....I don't know. I did however find the alternative solutions interesting. Anyone who ever raced dirt bikes can attest to the fact that as the bikes evolved, the solution to good "landings" and travel over rough terrain became increased travel with adjustable/controlled dampening. Since VB is virtually starting from scratch so to speak, I hoped he would consider other things. Apparently it is not something he wishes to consider, but he should at least be aware that if his landing gear proves to be too ridgid, he may redamage the fuselage while the gear remains unharmed.


Billski:
Now if you do make the gear really stiff (triangulated steel tubes and the like), you still have the tires. For some airplanes, that is enough, but for most of us, well, we better have some sort of suspension.

Winginitt: The trend today seems to be toward gear that has lots of dampened travel. As the pictures showed, there are even modifications to adapt longer gear to Cubs. Personally, I'll take all the advantages I can get.

Added note: Watch the gear compress on the Just Superstol. After it plays, another video pops up that can be watched. Its a Zenith and it lands at about the 0:50 mark. Watch how ridgid the landing is and how little travel occurs.



Bilski:
The stiffer you make the springs, the higher the reaction loads we get in sucking up a given amount of energy. See post 128. Little airplanes do happen to be in a region where leaf spring gear legs can work.
Billski

Winginitt: Yes, obviously leaf springs have worked for many years on many airplanes. Apparently the Zenith design is having more than its share of problems causing builders to search for more durable gear. Inadequate design or overstressed by poor landings....maybe both ? Again, I don't know how well the thicker gear actually works, but apparently there are some builders using it successfully.
 
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BoKu

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Thats probably the best way to use two part gear.
I don't know, but I get the sense that braking loads are going to result in much more toe-out than if the gear were secured across the bottom of the fuselage as usual.
 

BBerson

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Gear on bush planes is normally ripped off from hitting stumps and snow berms. I repaired a Citabria that hit a snow berm on skis. It's much more difficult to repair the fuselage (especially in the bush) than replace a bent gear.
So making the gear stronger than the fuselage might not be optimal. (as winginit said)
 
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