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Horizontal Tail Proof Load

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GESchwarz

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I'm looking for guidance and references on how to go about proof load testing a horizontal stabilizer. What are the relative merits of applying the load by sand or hydraulic jacks?

I'm also looking for a second opinion on the load I should apply using the following numbers: Gross weight 1400 lb, Wing area 110 sq ft, Stabilizer area 15 sq ft, 150 hp, 150 mph cruise, 14' Tail moment arm, Tricycle gear, Low wing.

Thank you,
 

Jeremy

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Gary,

You need to calculate two numbers, the tail balancing load and the tail manoeuvring load, and then take a look at the code you want to use (most probably CS-VLA for the EU, or maybe FAR 23 for the US) to determine some reasonable factors to take into account.

The tail balancing load is a function of the tail area, wing area, moment arm length, MTOW, main wing aerofoil pitching moment coefficient and desired C of G range primarily, but there are some simplified methods you might be able to use to get an estimate, depending on the code your using (here in Europe we usually have to calculate and include everything!).

The manoeuvring load is calculated from the maximum forces that the control surfaces can apply to the stab.

Sand bags, or lead shot bags, are the easiest way to apply the load, in my view, as they allow the load distribution (both spanwise and chordwise) to be approximated fairly easily. There are some simplified graphs and diagrams in the annex to most codes showing the distributions.

Your best bet is to download something like FAR23 and take a look at these, as they will give you a good steer as to how to go about it.

I ended up writing a custom Excel spreadsheet to work out the loads on the wing, control surfaces, tail etc, using the formulae in CS-VLA. It's all in metric units, but if I can figure out a way to post it here you might find it useful.

Jeremy
 

wsimpso1

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FAR Part 23 does not rigorously apply to homebuilts, but we would be foolish indeed to ignore the issues it discusses. In it are loading recommendations for horizontal and vertical tails, ailerons, flaps, and trim tabs. All of the values are relative to design wing loading (n*W/S) and Cn. Additionally, the design FoS is 1.5 for metal and usually 2.0 for composites.

As was recommended above, if you calculate higher loading, use that. And yes, if you are designing airplanes stuff, downloading and reading FAR Part 23 is a great plan.

Billski
 

Jeremy

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FAR Part 23 does not rigorously apply to homebuilts,
That depends very much on where you live, I'm afraid. The US is lucky, in that it has an Experimental category that places the onus for airworthiness primarily on the builder, but many (perhaps most?) countries in the world aren't quite so obliging.

Here in the UK, for example, a homebuilt with an empty weight over 115kg has to go through a full design approval process, that requires demonstration of airframe integrity (independently witnessed) to ultimate loads in one of the acceptable codes, such as FAR Part 23, CS-VLA, CS-23 or whatever. Even our microlights (sub-450kg MTOW) have to go though this rigorous process, albeit to a cut down national code, BCAR Section S.

It's common to write-off airframes or components during test, so one-offs aren't too practical a proposition here!

As you rightly say, knowledge of the relevant code is valuable, as, in my experience, most of the requirements have been put in there for a good reason, often based on some historical failing found after an accident or incident.

There is a downside to the not having to use any design code for US kit planes, at least when it comes to importing them to Europe. I've assisted on two or three different US kit compliance demonstration tasks now and every single airframe I've worked on has failed in one area or another when subjected to normal testing to the supposed design code used. This was more common with two seat microlight kits, which were presumably sold in the US under the old "fat" ultralight exemption. I've seen a main wing spar fail at +2.8g when actually load tested - we later calculated that it would have failed at about -1.1g, due to lack of a jury strut brace. One of the scariest things I've seen, especially as quite a few of these things were flying in the US, supposedly giving ultralight flying tuition.

Thankfully, the big guys, like Vans, for example, not only use the design code as intended, but actually test and analyse the structures they design. This almost certainly explains why we have so many of them flying here, despite our tough regulations!

Jeremy
 

Tom Nalevanko

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Getting back to the original question; "What are the relative merits of applying the load by sand or hydraulic jacks?" Some of my European friends seem to favor the latter. Inquiring minds, you know? Anybody know?
 

Jeremy

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It's a bit more work, in my view, to build a whiffle tree and use hydralic jacks, as the loads are pretty high, so substantial bits of test structure need to be built.

Sand bags (or better, in my view, lead shot bags) are certainly simpler to use for most amateurs. I use gravel, rather than sand, as it is easier to clean up when a bag splits (as it will.............).

The advantage of a whiffle tree and jacks is that the load can be applied more gradually, so the deflection with load can be monitored more readily. This may save damaging a structure that can be recovered, undamaged and modified to increase it's strength/stiffness if required.

Sand bags are a bit tedious to fill and place, but have the advantage of being cheap, reusable and flexible in that they can be easily used to apply complex load distributions, with only simple calculations.

I've used jacks to load test structure that doesn't need to have distributed test loads applied, like the control system and engine mount. I've also used sand bags to load test items that need distributed loads applied, like wings and control surfaces.

Jeremy
 

orion

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Sorry for the short answer here (short on time) but Appendix A of Part 23 has a series of simplified tail load diagrams that are applicable to all aircraft under 6,000#. We use this section quite often - hasn't let me down yet.

Regarding the load, I prefer sand bags or even better if you can find a place to rent them, bags of lead shot.
 

Jeremy

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That simplified method in Appendix A is so useful that it's been copied pretty much directly into the Euopean codes, as is. I found this out the first time I used it, as the author of CS-VLA (the sub-750kg MTOW code over here) translated the formulae into metric units, but only changed the titles on the graphs. The result was that the formulae didn't match the curves!

A bit of head scratching led me to discover that Appendix A of CS-VLA was really Appendix A of Part 23, with errors added...............

I've used it a fair bit, as it is pretty simple and easy to use.

Jeremy
 

wsimpso1

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The simplified loading charts are what I had in mind from Part 23. Design the elements for bending and shear with that level of load and a elliptical distribution, and you are nearly there. Then you load build up the load with a elliptical distribution.

Billski
 

PTAirco

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I believe the original Section S microlight rules in Britain also had a simplified method for determining tail loads: Assume for the tail surfaces:

1: Cl of 0.8 at Vd

and

2: Cl of 1.0 at Va.
 

wsimpso1

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Hydraulics require a wiffletree.

Shotbags can do all sorts of stuff, like different load distributions. IF all that you are trying to do is establish a proof load and test for it, they work great.

If you are trying to establish a load-deflection curve and demonstrate first fiber failure and then ultimate strength, the wiffletree and hydraulics works better.

Billski
 

GESchwarz

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I'm very close to completing my entire tail section and so I'm planning the proof load. Here are some additional questions:

Do I load the entire stabilizer and elevator as one assembly, using the assembly square footage of the load calculation?

I am confused by Figure A7, Chordwise Loading, in Part 23.11. What is P1 and P2? The position of P1 at the leading edge is confusing to me.

A23.11.a states that the control surfaces shouldn’t be loaded more than can be applied by the pilot through the control linkage. Might that value be used in place of what is derived from Figure A7?

Do I just lock the elevator in the neutral position, and what should I lock the control horn to, the test base?

I also want to confirm that I must load the surface twice, right side up and up side down…it that true?

I'm planning to proof load it to 6 G's. Does that sound reasonable for those of you who are familiar with my design? I believe that the RV-8 is tested to 9 G's. I'm a pretty conservative guy so I don't see myself getting too crazy with the stick. The control panel will have a G-meter.
 

wsimpso1

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You only have to do what is required by your regulatory situation and your conscience...

In the States, our Homebuilts require none of this, and quite frankly, we have not had problems with tails flying off of airplanes... Here, I would go for a slightly conservative design, build with integrity, and if you really want to be sure, proof load it only once. Tails are typically built symmetrically, and what works one way will work equally well the other way. The exceptions are braced tails and non-symmetric tails, and then I would think that your would test relative to the positive and negative limits you designed to, with the tail pulling down for positive limits and up for negative limits.

If you are really concerned that the design be checked out, yeah, you should lock the elevator with the control linkage at a pretty high fraction of travel, and apply a proof load, then reset for negative G and do it again...

I have written elsewhere of the limitations of sandbag and whiffle tree testing and the bad path that trial and error design can be. The skin in flight carries additional loads due to aeroloading that are not included in this type of testing. Ultimately, you really build in robustness by either over-building beyond the basics or by performing thorough analysis with all of the loadings. Thorough analysis is time consuming and over-building is chancey (you are guessing at the amount of over-building needed). Most of us do a bit of inbetween...

My look at FAR A23.11 figure 7 shows an idealization of the loading the tail at high positive deck angle (High AOA), and up elevator (back stick). High angle of attack, would shift the loading way forward, while the big back stick would then drive big opposite loading in the elevator. This is what you would do to the tail during a hard positive G pull. Loading will tend to be huge at the leading edge of both the stab and the elevator... While the air will give this loading, and you should design to this loading, I am hard pressed to figure out how you will achieve this loading with bags of lead shot or a whiffletree. The load is up at one-end and down at the other... On small airplanes, this is tough to even get the load applied, which is why we have the simplified rules.

As to the panel having a G meter, well that is fine for controlled and planned maneuvers. But tell me, are you really going to be looking at the FunMeter while the airplane falls out of a manuever, enters a spin, you neutralize the stick, stop the spin with rudder, and then are pulling out of the ensuing dive with rapidly building airspeed? Most of us would have to be cool indeed to do better than spin stop, Valsalva manuever, pull (or push) with our eyes on the horizon. You may be a better prepared pilot, able to read the G-meter during recovery from an inadvertant spin, but the rest of us, well, that is why 6 G is the min for aerobatic planes.

Billski
 

BBerson

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(quote) "I'm planning to proof load it to 6 G's. Does that sound reasonable for those of you who are familiar with my design? I believe that the RV-8 is tested to 9 G's. I'm a pretty conservative guy so I don't see myself getting too crazy with the stick. The control panel will have a G-meter".
Gary Schwarz


A 6 G's limit load test would be 9 G's ultimate ( for metal). The test must be done to ultimate if you want the test to include the additional 1.5 safety factor. But this will likely make the part unfit for flight because of yielding or buckling that can happen near ultimate.
 

Topaz

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...I'm planning to proof load it to 6 G's. Does that sound reasonable for those of you who are familiar with my design? I believe that the RV-8 is tested to 9 G's. I'm a pretty conservative guy so I don't see myself getting too crazy with the stick. The control panel will have a G-meter.
Well, what did you design the structure to withstand? That would be the G-value you'd test for. What someone else did is completely irrelevant.

I'll second what Bill said about the G-meter. They're useful for planned maneuvers, but the G-loads that take wings off of airplanes are generally unexpected. I'll probably have one on my airplane, too, but a G-meter is an information device, not a safety device.
 

PTAirco

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As a further consideration, I believe the old CAP 23 code in the UK required a chordwise distribution, ranging from the centre of pressure right at the leading edge to 50% m.a.c.
 

rtfm

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I ended up writing a custom Excel spreadsheet to work out the loads on the wing, control surfaces, tail etc, using the formulae in CS-VLA. It's all in metric units, but if I can figure out a way to post it here you might find it useful.

Jeremy
Hi,
I would very much like to take a look at your spreadsheet. You can upload it as follows:

  1. Click on the "Post Reply" button
  2. Scroll down to the bottom of the screen, till you see the "Additional Options"section, and click on the "Manage Attachments" button
  3. This presents you with a dialog box which allows you to browse for the file on your computer. When located, hit the "Upload" button
And that's it.

Regards,
Duncan
 

GESchwarz

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I have a question concerning the application of load by way of sand bags. I'm concerned about damaging the stabilizer. I don't imagine there is a way to know what that load is until you have actually done damage. I would imagine that each structure will behave a bit differently as it approaches it's limit.

I am planning on resting the stabilizer across two tables that are separated enough to extend a hydraulic jack in between. The jack will be attached to the attach fuselage attach points of the stabilizer. The plan is to apply sand bags in increments. Between increments, I will lift the stabilizer with the jack and measure for deflection. Then lower it back to the table. The data will be plotted in real time on a graph. I would imagine that the slope of that curve will begin to show a change as it approaches it's limit.

The stabilizer will have to be held in position while it's off the table, as it will want to teter around.

Any thoughts on this process?
 
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autoreply

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I would imagine that the slope of that curve will begin to show a change as it approaches it's limit.
I'm afraid you'll only notice that limit once you're already in the plastic stadium:

Any thoughts on this process?
How about strain gauges? You need to know where the heaviest load will be (and it doesn't help you with buckling), but it might give you an indication about how close you're to damaging the material.
 
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