[rv8ch]

In order to get my airplane registered here in Switzerland, I need to prove to the local FAA-type people that it won't fall out of the sky. Unfortunately, hundreds flying elsewhere don't seem to make much difference to them. Fortunately, 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". I don't really want to do a 9g drop test on my airplane, and while the suppliers of the engine mount and gear legs (Eggenfellner and Grove) claim that their stuff will pass this test, they don't have any documentation to back up this claim.

So, I'm left with the finite element analysis. I have a rough idea of what FEM is, but I was wondering if there is anyone on the list that could help me with the following questions:

1) do I have any chance of downloading software, figuring it out, and performing the calculations myself? I have an engineering degree, but we never studied this. I doubt I could still solve a partial differential equation to save my life, but I do remember how to add and subtract, and I *can* work a mouse.

2) if it is possible do do it myself, do you have any suggestions for good software to do it?

3) are there companies or freelancers that could do this for me, and if so, about what would I expect to pay for this service? Any recommendations?

Many thanks for any hints or tips.

 

[Radomir]

4) N-register it, fly Mel over and come out cheaper?

<I'd say something else about those regs, but I'll refrain to keep it PG rated>

 

[AlexPeterson]

Fea

Mickey,

Bummer about the rediculous requirements there... can't fight city hall though.

FEA is generally not something that can be done casually, even by engineers, as operating the software to do it is not simple. Most FEA software is quite expensive ($10k and up), as there are usually very few users. If there are any free programs, I suspect they are worth what is paid.

I do some of it, and can certainly have a look at the gear legs, as they should be quite straightforward once you send me the geometry. The motor mount may be another matter altogether, as the geometry and loading conditions could be challenging. Have they been at all specific with what they expect with an FEA analysis? Do you impart some load to the main gear, and look for peak stresses?

If you have to pay someone, take a deep breath. Those guys do not go cheaply.

 

[jcoloccia]

My guess is that it would be cheaper to build another RV with dead weight for your engine and stuff, and drop test THAT than do an FEA on your airframe. I can't imagine that their requirements are so stringent. There's gotta be an easier way.

 

[Kahuna]

My good friend, fellow formation junkie, retired F-15, now lyco powered rv-4, Bob "Subob" Goodman of Atlanta Georgia does this for a living and has done this on his custom Subaru engine mount (back in the ole days when he was running a Subaru in his RV-4, hense Subaru Bob is not "Subob"). I dont think it would be that expensive if the designer would provide the drawings.

You can try him here [email protected] and I have sent him the thread.
Best

 

[jtrusso]

I agree with the above posts that it's not really feasible to do this yourself if you don't already have the software and are very familiar with it. I'm a mech engineer and the two software packages I'd use to do this type of analysis, NASTRAN or ProEngineer Mechanica both run upwards of $20K a license. I've been doing this for 10 years and have no idea how to run NASTRAN, and a fairly good understanding of ProE, but I've got over 12,000 hours of ProE work under my belt.

I'd question the authorities by exaclty what they mean when they say they want an FEA. If you have a background in mechanical engineering I would think you could perform a basic analysis of both the gear legs and engine mount using excel. Does it really have to be a finite element analysis? You could do a static analysis and then an dynamic impact analysis, simulating the engine as a point mass on the frame. After all, a drop test is really a impact test afterall. There are several good text books out there that cost an order of magnitude less than the software that could explain how you'd set up such an analysis (http://tinyurl.com/mtob4, http://tinyurl.com/h6vme, http://tinyurl.com/pu44r) if "the man" will accept it as a substitute for the FEA.

 

[mlw450802]

In order to get my airplane registered here in Switzerland, I need to prove to the local FAA-type people that it won't fall out of the sky. Unfortunately, hundreds flying elsewhere don't seem to make much difference to them. Fortunately, 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". I don't really want to do a 9g drop test on my airplane, and while the suppliers of the engine mount and gear legs (Eggenfellner and Grove) claim that their stuff will pass this test, they don't have any documentation to back up this claim.

So, I'm left with the finite element analysis. I have a rough idea of what FEM is, but I was wondering if there is anyone on the list that could help me with the following questions:

1) do I have any chance of downloading software, figuring it out, and performing the calculations myself? I have an engineering degree, but we never studied this. I doubt I could still solve a partial differential equation to save my life, but I do remember how to add and subtract, and I *can* work a mouse.

2) if it is possible do do it myself, do you have any suggestions for good software to do it?

3) are there companies or freelancers that could do this for me, and if so, about what would I expect to pay for this service? Any recommendations?

Many thanks for any hints or tips.

Mickey,
I don't know what your time frame is here but the short answer to your question is that it is unlikely that any free software will give you realistic answers and the learning curve is pretty steep to get reasonable results.

The major effort is in getting a nice representative solid model of your system then meshing it appropriately for the results you want.

There are several packages that will solve drop/impact type problems and the real world answers are different from the static 9g test that you can probably do with your calculator and a statics textbook. The differences are the inertial loads that are hard to handle because the truss geometry of the engine frame changes as it deflects and the dynamic 'impulse' is hard to estimate due to the cushioning effect of the inflated tires etc. That is where the FEA comes in; all those individual components are calculated in concert with one another.
I use LS-Dyna and ALGOR MES in my work with the nod to LS-Dyna in terms of capability.

Solving your problem would be a piece of cake once the correct model was built (true of any FEA problem)and it would only take some cpu time to get there.

I would be willing to work the problem if I could get someone else to provide the model but unfortunately, I just don't have time to do the model creation. I have followed your site for quite some time and would be willing to give back some of what I have received from all your input and experiences.

Generally, a rough estimate of modeling the system might be 40+- hours (if all dimensions are readily available) plus another 20 debugging.

-mike

 

[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. Any FEA results have to be verified by loading a test article and measuring the strains with strain guages to verify the model predictions. Takes alot of tweeking. I don't think you should attempt the FEA approach.

 

[Ironflight]

....I have an engineering degree, but we never studied this. I doubt I could still solve a partial differential equation to save my life, but I do remember how to add and subtract, and I *can* work a mouse.

.

I feel your pain Mickey...yet I'm afraid I can't give you anything but encouragement. I too am allegidly an engineer, but school was a long, long, time ago! I came across a folder of some space pointing software I wrote when I was a young flight controller - not only don't I understand the code I wrote, but the spherical geometry stuff I used also looks pretty mysterious...and I'm not even sure I coudl do the arithmetic anymore!

On the plus side...the young Engineering Coop Students I had working with me on my flight test data were whizzes at running the latest Mathlib, curve-fitting software and stuff, and could really understand C++. But when they had to run Carl Morgan's early versions of the GRT Decode program, they were stumped - they didn't know how to run programs from a command line in DOS! So maybe there is hope for us old guys after all...

Now there's an idea! Have you got any connections with your old engineering department? Coops or Interns where you work? They might very well have the skills you need, you have a project to complete, they'd get real-world experience....mutual benefit?!

Or just hoist it to the rafters and drop the thing...

Paul

 

[David_Nelson]

Hi Mickey,

I did a search on Freshmeat.net and came up w/ a couple of s/w packages.

http://freshmeat.net/search/?section=projects&Go.x=0&Go.y=0&q=finite element analysis

HTH,

 

[rv8ch]

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!

 

[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.

 

[mlw450802]

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

 

[flyeyes]

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...

 

[rv8ch]

Gear legs

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.

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!

 

[Neil]

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

 

[gmcjetpilot]

Doable but you might need an engineer

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

 

[fodrv7]

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.

 

[John C]

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.

 

[rv8ch]

Drop test

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.

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.

 

[rv8ch]

Humor

After a while I realised that the Swiss were just like the Germans..............................without their sense of humour.

That's the stereotype. Here's a quote from an official Swiss website "Jokes are optional in Latin Switzerland (Italian-and French-speaking parts), but they are better avoided in the Swiss German-speaking areas unless you are familiar with both culture and language."

The Swiss say that they don't seem like they have a sense of humor because they're keeping it all for themselves. That's actually a joke - it gets lost in translation.

It's a very closely guarded secret that the Swiss are very funny, love to laugh, and have a good time, but they're trying to maintain the image of "seriousness" and "sobriety". Don't tell anyone I told you!

 

[rv8ch]

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?

Van's provided them with all the details they need to be happy about all the structures of the aircraft. The reason I need help is that I chose to install different gear legs (Grove) and a different engine (Eggenfellner). Don't start!

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?

Thanks for your offer, George. These calculations only need to be done for the first of any type of aircraft. Future RV8 builders in Switzerland won't need to do much more than you do in the US. It's only the first builder that has to jump through the hoops. I've only got two more to go! (hopefully)

Thankfully I'm not building a plastic airplane. Every one of those has to pass a wing loading test.

 

[jcoloccia]

It's a very closely guarded secret that the Swiss are very funny, love to laugh, and have a good time, but they're trying to maintain the image of "seriousness" and "sobriety". Don't tell anyone I told you!

Ellen comes from a Finnish family. Once upon a time, in her corporate years, she was called upon to help a Finnish company's executives interact more effectively with their American counterparts. For some reason, they seemed to be having a great deal of difficulty communicating with the Americans and frankly, felt extrememly uncomfortable anytime they had to deal with them.

Her long presentation on the differences between American and Finnish culture was met with deafening silence. She wanted to help but simply could not get any sort of dialogue going that gave any insight into their problems and exactly what was making it so difficult to deal with Americans. Was it language? No. Business practices? No. Morals? No. Power struggles? No.

Finally, one lone voice in the crowd, a rather well dressed man and clearly a top executive, had the courage to speak out and tell her what their real problem was that was making them so uncomfortable as to call in a consultant....

"Make us funny."



You can't make this stuff up...

 

[Chickenlips]

One thing that may influence your decision is that the actual drop test requirement is only 3g not 9g. My advice is to not get funny and try riding through one of the tests... at least if you do get a rubber bite plate to save your teeth...

 

[gmcjetpilot]

I got it.

I chose to install different gear legs (Grove) and a different engine (Eggenfellner). Don't start!

Thankfully I'm not building a plastic airplane. Every one of those has to pass a wing loading test.

OH I GOT IT! You config is just a little different. May be they are not that ridged after all, so once you go thru the process all RV8, Egg, Grove gear are good to go. I forgot you had the RV8, which has those nice flat gear legs. Too bad you told them you had Grove gear legs? You think they would have known?

BUT WHAT A HEY!!!!!
http://www.groveaircraft.com/rv8.html

This says the GROVE GEAR meets Part 23 drop test!! Sweet!!!

How heavy is your Egg / prop set up? I guess its very similar to a Lyc IO360 (angle valve) and Hartzell prop (50 lbs). IF its the same or less than you are golden as far as support structure. If Van says it is GOOD for the IO360/Hartz than it's good for you. I also venture to guess Egg has a special engine mount with his engine kit? Right. Call Eggy. You want the engine mount analysis or test report or what ever they have. I absence of analysis you may be able to compare to Van's engine mount? Same Same. Less or same load? GOOD TO GO. ("bon pour aller")


DONE DEAL, HECK WITH ANALYSIS OR TEST, GET GROVE AND EGGY TO DO IT FOR YOU. There's no doubt it's over kill and way more than strong enough. I would go for the logic in the following paragraph. You can't argue with 1+1=2 .

THIS COULD BE YOUR SOLUTION WITH least work!!!!
Another goodness is to compare what the "STOCK" config is and what you have. If you can say the GROVE is thicker, stronger and so on than stock you may be good to go by similarity. That is a very common approach the FAA buys off or approves all the time. As far as the engine mount that's just a matter of saying your engine compared to an IO360 Angle valve and Hartzell prop (50 lbs), weight and (fwd) CG, is same or less. If the gage of tubing of your mount is the same as Van's stock engine mount, it's not critical. The words are GOOD BY INSPECTION! No engineer can argue with, If X configuration is good, than if configuration Y is stronger or the loads are less, than Y is also good based on similarity. You don't need to know the details of why X is good, just that its a given, it's good and Y is the same or better. If there are any IO360/RV-8's flying in your country you could say, my structure is as good. Its just a matter of getting the data and making a case for similarity. It's been done many times. I know I did it all the time with the FAA.

The idea is to play down the difference and play up the "Mo Better".

I agree 9.2" to 18" is not terrible, but to suspend the plane and than DROP is a little challenge. There is risk of damage. If that buys them off it could be worth the effort but I think we agree it would be avoided if possible.

Your plan to get clarification on exactly what he (the FAA.engineer guy) wants is an excellent idea. Be sure if you discuss analysis what FEM software is approved? Also ask if you can do hand or classic stress calculations? It really is simple; its just a cantilevered beam. The engine mount is just a truss. The gear towers are just a shear box and any structures engineer could show you have enough fasteners to get the load into the fuselage and local structure is good enough. I don't know what the official language, but know they speak German. So I hope they say "es ist gut. wir genehmigen".

G

 

[rv8ch]

DONE DEAL, HECK WITH ANALYSIS OR TEST, GET GROVE AND EGGY TO DO IT FOR YOU. There's no doubt it's over kill and way more than strong enough. I would go for the logic in the following paragraph. You can't argue with 1+1=2 .

That was my first stop. They both said "go take a flying leap at a rolling donut". Actually, they said that they had tested them, but couldn't provide any written documentation. There is a bit more to the story, but I don't really want to go into all the details here. Both suppliers did do a lot to help, but it was not quite as much as the locals wanted.

Another goodness is to compare what the "STOCK" config is and what you have. ...

Another good suggestion. The problem here is that the Eggenfellner engine mount is very different from a standard lycoming mount. Here's a picture to show you what I mean. Obviously the white tubing is the engine mount. It attaches to a large flat plate at the front of the engine with rubber engine mounts.

The Van's gear is made of plate steel, and the Grove gear is 7075-T651 aluminum. I can't really make the case that it's stronger, even if it is a bit thicker.

Thanks for your suggestions!

 

[gmcjetpilot]

Don't give up

Both suppliers did do a lot to help, but it was not quite as much as the locals wanted.

The Van's gear is made of plate steel, and the Grove gear is 7075-T651 aluminum. I can't really make the case that it's stronger, even if it is a bit thicker.

Thanks for your suggestions!

OK, I thought about it after I asked and figured you did that all ready.

The Engine mount: Oh I see there is little that is the same as a Lyc mount. The good news is the engine mount structure is super simple. I assume they want engine mount analysis for flight loads not just a landing gear drop test?

If you can provide me the:
Geometry
Gage of tubing, plate, fasteners
engine weight/cg
location and prop data (geometry, weight of blades)

I can write a one page report for the engine mount. The support structure will be good by some hand waving similarity. I would just compare the loads at the 4 corners to a heavy IO360 (angle valve) Hartzell prop.

Find out what load cases they want for the engine mount. 9g vert (here is where the 9g comes in) is just one of many for a part like the engine mount. If they are not sure or just say Part 23 than we can make something up. I would like to know what the background is of the person who is approving? Sometimes it is better to have a sharp person who knows what they are doing. The incompetent ones don't know what is important, so they run you through the hoops for no reason. Of course that would never happen at the FAA.

Engine mount loads are both static and dynamic, like prop precession (gyroscope), yaw/pitch acceleration, engine torque and gust loads. Gust loads are based on Vno maximum structural cruise speed. That's a little more tricky but doable. The engine mount takes a whole step or steps to just figure out what the loads are before you can calculate strength. At big aircraft companies they have load groups. Their sole job is to come up with the load cases, and loads. It's a bit of an art. It would be nice to see a previous report that they think is acceptable? Any chance of getting one. It does not have to be for a RV. It could be almost any high performance single, even with a Lyc. We just want to know what load cases they expect?

Since they don't allow aerobatics in Switzerland (I recall you said?) we don't need to show the full acro loads and accelerations. We can fake something up. I have a feeling they don't really know what THEY want. However no doubt they will take your report and some other analysis and compare it? The question will be why did you not check for XYZ? They have no idea why you do the check, but they know it was done on this another report. It is like taking a test in school. It kind of helps to know the "MO" of the teacher's who giving the test.

So bottom line is there are several combined load cases for the engine mount, but once you have the structure data figured out, its not too hard to recalculate the critical stress. This exercise is not pointless. At the end, you will know the critical condition and part of the structure with the lowest margin. That's all I THINK they want. They want the worst case load and lowest margin detail and to say GOOD ENOUGH. They my just want +9g's and -6g's vertical and some side loads.

I am guessing at what they want? I never certified a small plane engine mount, a B767 wing, yes, Subaru engine mount no.

A drop test will not prove an engine mount for flight loads obviously. You will need analysis in my opinion. If I was "THEM" that is what I would ask for, if I was validating an engine mount. I would not need a FEM, just plan old statics and mechanics of materials.

I can guide you thru it. Have you do most of the leg work, than I'll turn the crank and crunch the numbers which is a trivial exercise at that point. The real work is data gathering.

There are no engine mount test like gear drop test, just analysis. The fact you have a RV8 helps the engine mount analysis, since there are no gear loads to worry about.


Landing gear, Don't give up on the Steel verses 7075-T6 (about 73ksi limit 83ksi ultimate) comparison. What is the temper and alloy of the steel? If the aluminum is thick enough the comparison could still be valid. For example assume the steel has say twice the yield strength (150 ksi), the equivalent (flat rectangle) gear leg cross section requires only a 41% thicker (not twice as thick) leg for aluminum. Over simplified but the aluminum gear may equivalent. Van's steel can be very low temper. It's worth a look see. We don't know until we do a quick comparison. It would take just the geometry and material properties of the gear legs. Of course we have to say we have the same overall Gross weight and CG. I am assuming there is an "approved RV8 flying already.

If they will not accept the Grove is same as stock, than the calculation is a simple beam calculation with the landing loads. Again no FEM. Since the support structure is the same, we are good to go. Just need to show sufficient strength for hand full of landing conditions (vertical, side and drag loads). Very simple. Because the gear legs are flat cross section, tapered and there is compression, shear and torsion applied all at the same time. This complicates it a little but there are off the shelf (in the book) formulas to "plug and chug".

Still need to find out what they really want, like spacific FAR's, not just "part 23". Ask around the experimental comunity and see of you can get a report to look at. That will give us a clue to what you need to show. We can basically copy and just change a few of the details.

I am sure they are sharp but my guess is they would be happy with some "engineering analysis" to CYA and theirs. How rigorous they need is a question. With just a little analysis they may just go, OK. I played this with the FAA before. They can be reasonable and they can dig their heels in. They can jut keep asking for more and more. The game is do what is needed but no more.

Cheers George

 

[Kevin Horton]

Another goodness is to compare what the "STOCK" config is and what you have. If you can say the GROVE is thicker, stronger and so on than stock you may be good to go by similarity.

George - what happens if the Grove gear is stiffer than the stock gear? Couldn't a stiffer gear put higher loads into the support structure from the same drop? It seems to me that changing the gear implicates the support structure, not just the new gear.