Measuring cable tension without a tensionometer

FastEddieB

FastEddieB

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Fast Eddie B
I'm in the middle of an annual condition inspection on my Sky Arrow.

The only flight control cables are for the rudder.

The maintenance checklist says the rudder cable spring tensioning should be "1.5 daN - 3.5 daN". That's apparently "dekanewtons".

Searching the web,that appears that converts to about 3.4 to 7.9 "pound-force".

Without a tensionometer, is there a way to measure this with just a spring scale?

Thanks - headed to the hangar shortly and I'll be checking this periodically for ideas.
 
I'm assuming the tension comes from springs holding the pedals forward. With the rudder held in place, can you pull the rudder pedals back against the spring (and slack the cable)? If you pull where the spring is attached you get a direct measure of the tension.
 
First you wrote "cable tension" in the title, I'd say "no" to that. Then you specified "cable spring tension". Yes, you may be able to use a push pull spring scale for that. Is there an adjustable spring, or spring pushrod? Of course this could be one of those areas where an A&P that has done it before is required for oversight.
 
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Thanks for the replies so far.

Yes, I have the maintenance manual for the Sky Arrow. I'm just a little confused on this one part.

This is all the checklist says:

26689392121_cb61bb9b8b_z.jpg


That seems to be addressing the "spring tensioning of the rudder cables".

Elsewhere is this:

26689392081_20614e0fa6_z.jpg


And the associated diagram:

26662676172_77a8631cde_z.jpg


Try though I might, I can't quite figure that second one out.
 
The % elongation?

Say the spring is 5 inches long. You stretch it to 7.5 inches. delta L = (7.5-5)/5 = 0.5 or 50%. If it took less than 3 kg or more than 5.5 kg (approx), then your spring is out of spec per the diagram.

My ride has a couple bungee cords between the pedals and the firewall and no spec at all.
 
It looks like there isn't enough info. To me the spec looks like a pre load number, i.e. the amount of tension in the cable at rest that you calculate by measuring the spring length unloaded, then the length under tension using the spring constant to calculate the load per the chart. There must be more verbiage associated with the chart, load up the full pages if you can, maybe we can help. The three parallel slanted lines need to be defined.
 
Capt. Geoffrey Thorpe said:
The % elongation?

Say the spring is 5 inches long. You stretch it to 7.5 inches. delta L = (7.5-5)/5 = 0.5 or 50%. If it took less than 3 kg or more than 5.5 kg (approx), then your spring is out of spec per the diagram.
Click to expand...

Something doesn't seem right there.

It says to apply a force (not to exceed 5kg), the checklist specifies a range of 1.5 daN - 3.5 daN (daN is used in some fields as an approximation to the kilogram-force). The force is applied to the rudder pedal with a spring scale. That is your vertical scale number (1.5 kg - 3.5 kg) left side of the graph. You record the spring length: 1. loaded and 2. unloaded, to use in the equation to compute the Delta L % number.

For example: at 3 kg force applied, the Delta L% has to fall between 21 and 50 (horizontal on the graph) or the spring is out of limits.
 
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Two consecutive pages unedited:

26667011282_52e3cf0f78_c.jpg


26667011402_9c9d635d06_c.jpg


On the plus side, an A & P stopped by the hangar, felt the cables, and said they felt "about right"! :confused:
 
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That's a little far for me, sorry.
It looks quite simple actually.
You measure the springs "unloaded".
You apply a force between 1.5 Kg and 3.5 Kg to the rudder pedals and measure the springs "loaded".
You use the measurements to come up with the Delta L %, which must fall between the corresponding range for the force you used.
 
I'll see if I can clear my mental block and try some measurements tomorrow.

I have a digital fish scale that I think can read in kg, so no conversion will be necessary.
 
Clear as mud, I tend to agree with Glenn, if you apply 3kg of force, then the percent elongation should be between 21 and 50 %. Might be worth a call to the factory for clarification though, cheap insurance.
 
FastEddieB said:
I'll see if I can clear my mental block and try some measurements tomorrow.

I have a digital fish scale that I think can read in kg, so no conversion will be necessary.
Click to expand...
How'd it go? I suspect you had an access problem with a fish scale. You may have to adapt it to make a push tool.
 
GlennAB1 said:
How'd it go? I suspect you had an access problem with a fish scale. You may have to adapt it to make a push tool.
Click to expand...

Actually a regular bathroom scale might work. Just put it on the pedal and push until you get 6 or 7 pounds.
 
Does every one realize that a cable tentionmeter can be calibrated in either scale ? These tentionmeters each have calibration sheet that converts units on these meters to pounds/ or ? of tension. there needs to be no conversion.
 
What I'm wondering is if the checklist and AMM are referring to two different tests.

One for cable tension, expressed in a daN range, and one for rudder pedal spring tension, with the formula involving that spring length delta under load.
 
you need one of these.....measure the spring tension force "and" displacement of the spring.

d07848b022bc4e0f217fe252575215c81.jpg
 
FastEddieB said:
What I'm wondering is if the checklist and AMM are referring to two different tests.

One for cable tension, expressed in a daN range, and one for rudder pedal spring tension, with the formula involving that spring length delta under load.
Click to expand...
Where's the cable tension check in the AMM?
 
Seriously, a phone call or email to the factory would probably answer this quickly.
 
Here's what I think...

The concern is that the rudder pedal springs have not lost their "springiness" - they are what hold the rudder cables in tension.

Here they are in real life:

26197291173_03ef0d0d8a.jpg


So, I pulled them out and measured them at rest - "lo" in the formula:

26734070821_8d02914b7b.jpg


I then applied 4kg to them and measured them again:

26195880374_288579d944.jpg


If I'm doing the math right, both points plot in the "acceptable" range:

26195878494_bf097b61b3.jpg


So I should be fine. The only thing I don't quite get is the spec on the checklist - the 1.5 to 3.5 daN.
 
Obviously, the same writer or engineer wasn't used. Springs "work harden" and eventually break, that is what you want to prevent by doing this "springiness" check. Is the removed length the same as the installed unloaded length? And, why did you use 4 kg when the checklist specified 1.5 kg - 3.5 kg (daN = kg)
 
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