D

David Z

Well Known Member
I was looking ahead in my plans today and I discovered that I have lost/mis-placed/never received the smaller aileron pushrod tube. It's the one that goes through the rear spar to the aileron. So I started to look for a replacement. I can get the same 4130 tube from Aircraft Spruce for quite a bit less than Vans sells them (although not powder coated).

Then I started looking at the aluminum tubes. Here's what I found in my research:
2024-T3
-80% yeild strengh of 4130
-most expensive

6061-T6
-60% yeild strength of 4130
-more corrosion resistant than 2024 or 4130
-quite a bit cheaper

Considering that each end of the pushrod is held together with two 1/8" aluminium rivets, I see that as the weak point, not this monster heavy steel tube. Is there any good reason not to use 2024-T3 tube? The only good reason to use the 6061-T6 is because it is more corrosion resistant.
 
You Might start by checking your delivery ticket from VANs, every now and then they can not fill every item on your order, and they miss filling the back order.
If its a back order item, and they forgot to send it, then a reminder would get it headed your way.
 
Don't be mislead by the material tension strength. The column buckling strength is not related to that. And the local crippling strength is only slightly connected.

Control system stiffness is one of the factors affecting flutter -- also based on other properties than tension strength.

Use the material and size specified and you'll be good.

Dave
 
David Z said:
Considering that each end of the pushrod is held together with two 1/8" aluminium rivets, I see that as the weak point, not this monster heavy steel tube. Is there any good reason not to use 2024-T3 tube?
Click to expand...

David,
The capacity of a control pushrod is limited by buckling failure in compression. Go into the kitchen and get an ordinary plastic drinking straw. First grasp it by the ends and pull so as to try stretching the straw. You'll find it can withstand quite a load. Now stand it on end on your desk, and apply a vertical load to the upper end. It will bow and fail at a very low load. That's buckling failure.

Here's the interesting part. The basic equation for bucking failure of a slender column is:

Load at Failure = pi^2 [(Youngs Modulus x Moment of Inertia) / Length^2]

What is so interesting? Look close. There is no term for material strength in this equation. The only required material property deals with stiffness (modulus), not strength. The rest just describes shape.

When engineers scribble on napkins, the rule-of-thumb for Young's Modulus of aluminum is 10 million, while for all steels it is 30 million. Starting to see the problem with substituting aluminum for steel in that aileron pushrod?

But wait you say, what about our aluminum elevator pushrod, or the big aileron pushrods?

Again consider the buckling equation. For a round, thinwall tube, Moment of Inertia is roughly described by the tube diameter:

Moment of Inertia = [pi (major diameter^4 - minor diameter^4)] / 64

or

I = 0.049 x (D^4 - d^4)

You can greatly increase the buckling strength of a pushrod by increasing the diameter.....which is why the smart guys in the engineering office at Vans gave us fat aluminum pushrods.

No room for large diameters between bellcrank and aileron. Better stick with steel.
 
Last edited:
Yeah, having Dan around enables me to be stupid and just follow along when I need to. Kinda like paint by numbers.
 
You must log in or register to reply here.