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  #11  
Old 05-09-2006, 03:42 PM
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Default Finite Element Analysis

Thanks for all the suggestions, guys. It seems like this is much more complex than I thought! I've got a note into the engineer that is supposed to sign off on the structures to see if he can live with something less than a full FEA. I've also got a note into the guy that Mike suggested. If that doesn't work out, I'll try to get some engineering school kids to help me out.

If they'd let me fly it, I'm sure I could demonstrate some 9 G landings - anything less than 6 Gs is a greaser in my book!
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  #12  
Old 05-09-2006, 08:00 PM
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Are there not other RVs flying in Switzerland? What did they do? Local inspectors? Go to HQ. Are you talking to an engineer or an inspector? Inspectors could not use the data, perhaps they don't realize what they are asking. EAA chapters? There is no reason to require a FEA. Many airplanes have been certified without FEA. Van's has statically tested many of the parts. Just some thoughts.
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  #13  
Old 05-09-2006, 08:59 PM
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Quote:
Originally Posted by John C
Are there not other RVs flying in Switzerland? What did they do? Local inspectors? Go to HQ. Are you talking to an engineer or an inspector? Inspectors could not use the data, perhaps they don't realize what they are asking. EAA chapters? There is no reason to require a FEA. Many airplanes have been certified without FEA. Van's has statically tested many of the parts. Just some thoughts.
FEA is not required as Mickey stated earlier. He could just do the drop test specified in FAR 23. It may be that Switzerland simply recognizes experimental aircraft differently from the US and requires more testing of a certificated nature.
Mickey surely knows the answer here. I'm just speculating.

-mike
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  #14  
Old 05-09-2006, 09:51 PM
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Quote:
Van's has supplied them with almost all the information they need to be happy, with the exception of my engine mount, and the gear legs.

The only things they accept are a "FAR 23 drop test" or a "finite element analysis".
Mickey, are you using stock gearlegs? If so I recall a photo in the RVator of Van himself doing the drop test on an RV-8 fuselage. It was probably '98 or so. I know that doesn't help you with the motor mount but surely Van's has documented the FAR 23 drop test.

James Freeman

I can personally verify that the gear on the RV-8 are quite sturdy, but I refuse to divulge how I came by the information...
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  #15  
Old 05-10-2006, 12:15 AM
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Default Gear legs

Quote:
Originally Posted by flyeyes
Mickey, are you using stock gearlegs? If so I recall a photo in the RVator of Van himself doing the drop test on an RV-8 fuselage. It was probably '98 or so. I know that doesn't help you with the motor mount but surely Van's has documented the FAR 23 drop test.
I chose to use the Grove aluminum gear legs, to save weight, and to be cool. I didn't know about this gear testing requirement at the time.
Quote:
Originally Posted by flyeyes
I can personally verify that the gear on the RV-8 are quite sturdy, but I refuse to divulge how I came by the information...
I saw that guy landing at OSH last year, too!
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  #16  
Old 05-10-2006, 12:30 AM
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Default Fea

At a risk of not getting too deep into this subject. A FEA is only one method of determining forces within the structure. Other methods are available and a 3-D truss analysis would also serve the identical purpose and give effectively the same results. A computer program makes the analysis easier for the experienced user. For given conditions any form of analysis must be acceptable for the "powers that be". If other forms of analysis are not acceptable then unfortunately they would fail the design course and be sacked as they don't understand basic engineering. FEA is just one tool to get a result.

To model the 9G forces would normally be done my using a factor of 9 times in the static weight of the engine at the support points. The boundary conditions would be ok to be simple "pin-joints" and I think the wheel condition could be modelled fairly simply as a straight line force along the line of the strut.

I think a 3rd or 4th year undergraduate engineering student - with access to a simple 3D truss analysis program, (or finite element program) will give results indicating the integrity of the structure.

-Neil
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  #17  
Old 05-10-2006, 02:48 AM
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Default Doable but you might need an engineer

Quote:
Originally Posted by ferret
FEA is more art than science. Getting the boundary conditions are very difficult. You need a pre and post processor like ProE or PATRAN, and then the analysis code like NASTRAN. Takes a lot of tweeking. I don't think you should attempt the FEA approach.
IT is not that bad and you can use simple programs, but is that acceptable? The ironic thing is kit planes don't need to meet FAR's. There is no guarantee it will meet FAR23?

I do agree with you all about FEM. Stress analysis, including FEM was my job for many years. The idea of getting an engineering student to do it, not bad. Still you need to get the data together.


First how bad is the drop test? I think 9 g's is may be over stated. I hope. A quick look at the FAR's says you need at least 9.2 inches but no more than 18.7 inches. Also weight is offset by wing lift, so you are not dropping at full fuel+payload wt. If it was 9g's I would never do that. That with out calculation I think would "LEAVE A MARK" or bend the plane or gear. At 9's at just empty weight is an equivalent of over 1500 lbs on the nose and over 4000 lbs each main gear! Ouch! At gross fwd / aft CG's respectfully that's about 4000 lbs nose - 7000lbs mains and one heck of a large deflection.

Interesting enough the actual G force from the impact is hard to calculate because you don't know how long it takes the airplane mass to decelerate to zero. With some guessing the G force is about 3g's. You have to include the tire deflection and of course the gear deflection (bending). The tires will scrub and absorb the impact. Its a really interesting engineering problem to analyze DYNAMICALLY with the rebound, thus the reason for actual test. That dynamic solution analytically is of course more complex than a static solution. Better find out what they want or need before you do anything. I have doubts about what they are asking?


DROP TEST?

http://www.risingup.com/fars/info/part23-725-FAR.shtml
http://www.risingup.com/fars/info/part23-723-FAR.shtml

other gear FARS:

http://www.risingup.com/fars/info/part23-726-FAR.shtml
http://www.risingup.com/fars/info/part23-727-FAR.shtml
http://www.risingup.com/fars/info/part23-473-FAR.shtml
23.479, 23.481, 23.483, 23.485, 23.493, 23.497, 23.499
http://www.risingup.com/fars/info/part23-499-FAR.shtml


This gives the hight and effective weight. It does not sound too bad. At 18" you would take 0.3 sec or speed 9.5 ft/sec. However I am not crazy about dropping the plane. Also how do you set that rig up to drop it? It would be an effort. I lean to analytical solution.

Hey its experimental why the FAR's? Again there is no guarantee.

I never made a 9g landing in my RV, and I have a g meter (that is my story and I am sticking to it). Also to be correct one drop does not design a gear system, since there are drag, spring back and side loads. Its a start but I have no idea what they want? Is it written down somewhere. To have a full analytical and or scale gear test is way more than I think you want or need to undertake (I hope).


ANALYSIS? FEM

First you need the software to do the FEM. Full bore FEM programmes ones that use NASTRAN or PATRAN code, are many thousands of dollars for just the basic elements. Basic ones can be a few $100. Excellent programs with a basic CAD interface can be inexpensive. Here are a few free one I googled. Have no idea but could do the trick? http://www.lisa-fet.com/index1024.htm (it looks like a full 2D/3D Fem with a basic pre/post processor, even w/ plates, shells and 8-node solid elements. Just downloaded it, never used it. It's a demo limited to 1300 nodes.)

Very simple 2D and 3D truss program: http://www.cuylaerts.net/
Some free/demo FEM programs: http://www.freebyte.com/cad/fea.htm


Not all use NASTRAN code (the Cadillac and widely accepted code by aerospace world wide). You really need to know "mechanics of materials" to use one effectively; as ferret said you need to choose the boundary conditions, element type, properties and how to apply the loads. I have done a lot of FEM in my engineer days, not hard but takes judgement and time. You need to have all the geometry and material properties. There are handbooks with the material properties. The fancy programs fill in the blanks for you with a material data base. "THEY" may not accept non NASTRAN code?

In your case the FEM would be like a 8-year olds "Stick-MAN" drawing compared to Lenardo Da vinci's "Mona lisa". You would just need to model the gear and local support structure. The rest of the plane would be a few dummy elements (truss/beam ) to connect the dots.


How to model?

Elements, Boundary conditions, if not chosen carefully they can throw the results off. The gear has all 6 degrees of freedom fixed at the support. The other end is free or cantilever. I don't know but assume they want some knowledge the support structure will pass mustard. The airframe I assume they are happy with? Not sure what you told them to make them believe than but not the landing gear?

In the case of a RV-7a that is some simple truss (engine mount or main gear leg fittings bolted to the firewall or wing spare structure). The rest of the airplane is modeled with infinitely stiff dummy beam elements, which ties it all together.

To model the gear legs themselves you can do it two ways. One is solid FEM elements of the gear legs, which is simple if you have the definition or geometry and material properties. Most FEM programs can take a CAD drawing of a part and "mesh" it. The second way is represent it with a standard FEM tapered beam elements, which saves you from modeling the gear legs with solid elements.

I would be glad to help, but I can't volunteer or promise to do number crunching. I always thought someone should do a solid FEM model of the RV "A" model's nose gear and see what was going there. I just can't believe someone does not have a FEM model already, even a full airplane model.

FEM software is getting cheaper, more prolific and easier to use with graphical interface than they where 10-20 years ago.

Of course PC computing power is what drove this. FEM analysis once only ran on mainframes. Even today solutions of a larger model can take hours or all night to solve on top of the line PC's working in parallel. The tricky thing is as the gear bends the geometry changes, a lot! That is the issue with the nose gear, it deflects. There are special elements to solve for this "non-linear response" or "large deflections", but you can make some assumptions to simplify this. We assume we remain in the elastic range?



Stress Analysis:

The last way is classic stress analysis, i.e., pencil and paper. This is what they did before FEM and wide spread computer use and is still very accurate, especially with simple structure like this. FEM use to be used only for very large analysis jobs. Now it is common to model simple structure.

Some times its easier to do a quick and dirty model (FEM) even for simple structure. Just depends on how adapt the engineer is at hand calculations using standard stress analysis techniques. Good hand calculations are as good or better than FEM in my opinion many times. WOULD THEY (your FAA) ACCEPT IT? I know the FAA in the US would be fine with it. The thing about hand calculations with "classic" formulas is its easy to check.

If you gave me all the data I could try to do a quick crunch if the number by hand. No promise. It might be acceptable? It would show the max stress (guess). I would compare that to the limit allowable (assuming you gave me the steel alloy and heat treat). If the stress is less than the elastic limit, than you are good to go. I see NO reason why they would not accept that.

FEM will simply returns the stain which is stress when multiplied by the modulus of elasticity for the material (E). This is called Hooke's Law.

Even though the geometry changes under load, i.e., the gear spreads out, it is statically determinate. If they would accept a page of stress analysis I might be able to dust my mechanical pencil and HP41C off. I would have to calculate the stress for side load, drag and vertical per part 23 and just crank the numbers.



DECIDING

If you go analysis regardless who does it you need to give them all the info.

You would assume if the fuselage and wing are good for flight (by their estimation) so this does not need to be shown. What did you show them to make them happy the wings stay on?

Therefore analysis needs to only show the gear(?), engine mount and the main gear fittings. Interesting a hard landing can break wings of some planes well before the gear, especially ones with tip tanks. A guy landed hard in a twin Comanche with tip tanks full and bent the wing. The gear was fine.

Write me off line if you want help. Can't promise to crunch the numbers, but I'll see what I can do. If we can get a FEM analysis document on file with your "controlling agency", with a range of weights, all RV'er could be "gear" approved in your country if they fit the approved weight and CG range. Why does this have to be done over for every plane?

Van is an engineer. He might have detailed stress analysis? At least they should provide you info on the gear material and we have the geometry. Best of luck. Again there is no guarantee our RV's meet any part 23 FAR. I think the case that there are almost 4000 flying, at least 1/2 with your exact gear configuration for at least 10-15 years should say something.

Cheers George

Last edited by gmcjetpilot : 05-10-2006 at 10:54 PM.
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  #18  
Old 05-10-2006, 06:14 AM
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Default Humour

Mickey,
I used to fly into Frankfurt and Zurich.
After a while I realised that the Swiss were just like the Germans..............................without their sense of humour.
Pete.
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  #19  
Old 05-10-2006, 05:50 PM
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Default What is the objective?

Mickey, what regs are they asking you to meet? And for what part of the airplane? The limit drop test would require about a 13 inch drop (I think). The reserve energy drop test would require 1.44 X 13 inches. A 19 inche free drop does not sound like much, but it is a big hit. Is that what they want? That may answer their question about the gear but may not answer their question about the engine mount.

Whatever you do, drop or analyze, make sure that they agree, in writing, what has to be met, and how you are going to meet it. You don't want to spend a lot of time and money to find that what they really meant was........

I am not sure about the 9G load, as George notes. That is usually the basis for a static forward load for belts and seats. Is there a 9G downward load that is considered an equivalent for one of the vertical tests?

Regards, John.
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  #20  
Old 05-11-2006, 01:28 AM
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Default Drop test

Quote:
Originally Posted by John C
Mickey, what regs are they asking you to meet? And for what part of the airplane? The limit drop test would require about a 13 inch drop (I think). The reserve energy drop test would require 1.44 X 13 inches. A 19 inche free drop does not sound like much, but it is a big hit. Is that what they want? That may answer their question about the gear but may not answer their question about the engine mount.
I didn't realize that it would be so small. I think my RV8 fell off a jack once when it was higher than 19"! That really doesn't sound like much at all.
Quote:
Originally Posted by John C
Whatever you do, drop or analyze, make sure that they agree, in writing, what has to be met, and how you are going to meet it. You don't want to spend a lot of time and money to find that what they really meant was........

I am not sure about the 9G load, as George notes. That is usually the basis for a static forward load for belts and seats. Is there a 9G downward load that is considered an equivalent for one of the vertical tests?
I'm still standing by to get more info from the engineer assigned to my project. Hopefully simple "stick men" type drawings and calculations will be enough.
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