Tail Spring

[Vince Carucci]

I need some input from this forum.

I laminated up a tale spring (3 layers of maple, 2 layers fiberglass alternating). The final piece is about 3/8  inch thick. The weight is 10.4 oz. before any trimming. I am extremely pleased with how it came out. Now I need to load test it. This is where I need some input. I need to establish an acceptable load limit.

I read somewhere that there is 25 lb. static load on the tail wheel. Is that with or without a pilot? Is that value even correct?
I found a recommended safety factor of 1.5 minimum over maximum calculated load (circa 1930s). But I have no idea what max load might be. Twice the static weight during a hard landing?
If I go with these numbers, it should handle a load of >75 lbs. I'd prefer to draw an arbitrary line in the sand at 100 lbs. Is this enough?

I'm going to test it to failure to find my starting point. If anyone can answer any of these questions, I'd sure appreciate it.

Vince

[Dan_]

I asked an aeronautical engineer about needed tail spring strength once.  I told him I thought it might have 30 pounds on it with me in it and told him I was guessing 10 times that number would be a good test for it.  He did not answer the question directly, probably for legal exposure reasons.  He did say I was "on the right track"...

Steve Kiblinger's weight and balance shows 32 lbs at the bottom of the linked page...

[Vince Carucci]

I stacked 125 lbs. and bent the heck out of the spring but didn't break it. I removed the weight and stood on it. Broke it! The spring weighed 7.9 oz. and was .25 inch thick measured by caliper. I think the next one will increase to 4 ply wood, 3 ply fiberglass. I'm shooting for 10 oz. I have enough material left over to make several more.

[scottiniowa]

You need to apply loads to the test spring in the fashion that the spring is being used.  

 i.e.
     1) hold the aircraft end the way it is held in the aircraft
       2) hold the tail wheel end with a Tail wheel in place

Then either apply weight at the aircraft end or raise the tail wheel end.  The second method being slightly easier to do, but harder to measure.
Hint- a super duty bathroom scale and floor jack goes a long ways here.

From your photos, it looks like your supporting both ends randomly (not in the way it would be on your aircraft)  and applying weight in the middle.
  This maybe telling you something, but it is not telling you how strong the ends are or how it holds up in aircraft landings. Which I think was your goal.

Load test can not simulate the real world, if not applied  in real world application setup pattern.  You may find exactly the same
results, but am guessing, done as above, you will get results that make you feel warm and fuzzy (good results)
or results that make you reconsider the end set ups or build ways---   But when done, you will be confident that it works on real aircraft and real landings...

I do know your results will differ from your present load testing your doing now,  as presently it would appear your entire load is being placed at one
point, and your holding spring differently than on the aircraft.

Related test that I have done-  much larger aircraft-
a 7.3# triple leaf spring, failed first in the attach point of the tail wheel, then a bending of the spring
vs a 2.3# taper rod spring that simply did not fail at any point under the same loads.  
What was learned from this, was the fact we could hold a tapered rod so much better, with less weight than any flat spring.  Again, this was for a 2000# gross aircraft but the point is the same.

if we had simulated strength with rod and flat spring held on the ends, with weight in the middle, it would not have shown failure at the weakest point. Which of course was unknown until testing.

Your spring now could easily be stronger than both ends, but your testing will never show that presently.

thank you for considering my suggestions on your fine work.

[Vince Carucci]

Scott your right. The position in the previous photos makes it appear incorrectly mounted. But I think that's because the bending spring distorts the look of the whole test rig. The two photos in this post show the rig before loading, and the graphic shows what I was attempting to do.  Let me know if you still think I'm approaching it wrong. I too considered that an actual tail wheel attached to the end of the spring might change how the forces are distributed. But this was a quick test that I thought would give me a reasonable starting point. I'm going to make another spring today (should be ready tomorrow) and see how it performs.

[scottiniowa]

   Let me know if you still think I'm approaching it wrong. I too considered that an actual tail wheel attached to the end of the spring might change how the forces are distributed. But this was a quick test that I thought would give me a reasonable starting point. I'm going to make another spring today (should be ready tomorrow) and see how it performs.

Vince
Your third photo, the line drawing,   I would hold the left end so it cannot move, and apply lifting (floor jack/scale) and have this last piece (tail wheel end) attached just like you show...or in other words, just like you intend your tail wheel installed.  But I would NOT apply weight  down at your middle arrow... Only lift from the tail wheel location, and holding solid at your first big arrow. 

This will test your WHOLE system rather than a more directed point. which could never happen flying...  Your on the right track!

[Kamcoman77]

What thickness maple strips did you use, and how many layers of what weight fiberglass was each fiberglass section?

[okawbow]

What is the length of the tail spring? I have many pieces of .050” thick x 2” wide x17” long fiberglass strip from longbow making cut offs. I can also provide parallel or tapered ash laminations of 17”-36” lengths.

Chuck

[Vince Carucci]

I purchased a Skateboard Laminate kit from Woodcraft. (Click on the link to see it) The fiberglass is fairly thick. It was left over from a cut demo I did for a German windmill mfgr. (Enercon) The first tail spring in the pictures was 3 layers of wood alternating around 2 layers of fiberglass. The second tail spring I made was 4 layers of wood and 3 layers of FG. It increased from approx 1/4 in. to 3/8 in. thick. That should almost double the strength of the tail-spring. I used the West System from Aircraft Spruce. As you start to price this stuff you'll see it wasn't cheap if you purchase it just for this project. I have a ton left that I hope to use one day. That might help mitigate the cost of the tail spring.

[Vince Carucci]

The tail spring is about 17.5 inches long by 2.75 inches wide where it mounts to the frame. It tapers down to 1.5 inches where the tail wheel will mount. I think the wood was 1/16 in. thick and so might be the FG.

[okawbow]

I use Smooth on EA 40epoxy to laminate longbow and recurve limbs. It is not quite as strong as West System but has better shock and bending strength. Holds up better than West System in archery bows. Seems it would work well for tail springs. Can be bought in pint kits for less than the West System, and is easier to use, with a one to one mix, and 1 hour pot life.

[Kamcoman77]

Vince. Thank you for the rapid response.

[jrbirdman47]

I was considering using replacement limbs for takedown recurve bows. Very light and length/ width/ base thickness, etc seemed about right. I've only seen one XL in person, and lifting the tail off the ground, I would estimate the 30 lb figure to be about right for static load with pilot aboard. Don't know if you really need 10 G's worth of strength, but was wondering if the bow limbs could handle that kind of shock.

[Poorman2]

Vince I used 10 laminations of maple 1/16th inch thick glued with T-88 for a final thickness of about 11/16th x 1 1/2 inchs wide. It has not been tested on my XL yet though. I did not use any fiberglass. I have not weighed it yet. It feels a lot lighter than a steel spring. I posted a picture of it back in 2016 on my build log on this fourm (Randy's build log). You asked Joe how much it weighed. I did not answer at that time because I had not weighed it and I had to go to work. I will go to the post office and weigh it today on their scales. Sorry I could not post at that time. Randy Q.

[Poorman2]

I just weighed it. I made it extra long so I could cut it to length to fit my fuselage. As it is it weighs 10 0z.