klovning

klovning

Well Known Member
We have a couple of DAR's around here so I thought I'd ask... Forgive me if this is a dumb question (yes I can be an inquisitive idiot) from a new guy but here goes....

A lot of people do modifications on Van's original plans (From what I gather a full electrical system with lights was not even part of the original design) A lot of these get posted here for others to consider/improve on/talk about.

How much do modifications on a kit's plan matter when issuing the airworthiness certificate?

For example some of the things I've seen pictures of:

-Showplane's fastback mod

-Putting a IO-390 (210hp) engine in a RV-8 instead of the suggested 200hp limit

-Putting a subaru engine in

-Adding retractable gear

-adding tip/rear seat tanks

-having a full electric ignition

Do these type of things affect weather or not a plane will be given permission to enter phase 1??

I'm guessing that there's quite a bit of leeway, but what is considered "crazy" or dangerous? What would be considered a "no go" modification?

-thanks

P.S. any suggestions as to where I can put the flux capacitor?
 
Good question Jason. But not so easily answered. The inspector can deny the certificate for anything he considers unsafe. Most common modifications can be easily evaluated. Others can not. If the inspector wants, he may require engineering support data.
Modifications involving changes to the fuel system design and/or an increase in gross weight will usually trigger a request for engineering support data.
 
adequate support data

Would "somebody else did it and lived" be considered adequate engineering support data?

That is - empiric testing done on a previous airplane without known failure/catastrophe - would likley be considered adequate ... correct?
 
Modifications

Jason,
Now, before you get started, is the time to find a DAR and get them involved in the project. The DAR will left you know what they expect and what documentation would be required for the engineering changes or deviations from accepted aviation standards. Having a plan also allows the DAR to informally examine the project during construction. Also, a good working relationship with a DAR allows for continuing advice about the project even after it is flying. (Thanks for the email advice last weekend Mel)

Jim Lewman
 
klovning said:
Would "somebody else did it and lived" be considered adequate engineering support data?

That is - empiric testing done on a previous airplane without known failure/catastrophe - would likley be considered adequate ... correct?
Click to expand...

You cracked me up. This is as saying the Wright brother's aircraft is air worthy. :D

Seriously, I really don't think so. Making it a retractable definitely will add some weight to the aircraft.
 
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Develope a good working relationship with your inspector.

Like Jim says, get with your inspector early if you are planning drastic mods. Ask what kind of documentation he requires and neither of you will have surprises.
 
If you're doing any major changes to an airframe or major system, then my opinion is that you don't just need a DAR, you should look for a DER. No disrespect to the DAR's on this forum, but the engineering knowlege requirements/experience to be an AB-DAR are relatively lax compared to the requirements for a DER. Most DAR's are very smart people that know a bit about a lot of things, but aren't necessarily the guys who I'd use for significant structural changes and their relative impact on airworthiness/structural integrity.

Like I said, no disrespect to the DAR's. Most of them are extremely sharp and intelligent folks - but - we all know some that are, well.....less than superbly qualified. I mean, the sad truth is if you look hard enough, you can find a DAR to sign off a ham sandwich! :)

If you do choose to use a DAR, make sure they are well respected (Like Mel or Vic or Tim or whomever) and know their stuff.

That's just my 2 cents!

Cheers,
Stein
 
Remember though, these airplanes are experimental. You don't really need engineering approval to do anything. The whole point of experimentals is you DON'T need a DER to do stuff. Mind as well buy a grumman tiger if you are gonna go that route.
 
osxuser said:
Remember though, these airplanes are experimental. You don't really need engineering approval to do anything. The whole point of experimentals is you DON'T need a DER to do stuff. Mind as well buy a grumman tiger if you are gonna go that route.
Click to expand...

True....but, I have dead friends to prove that not getting any qualified engineering assistance (doesn't necessarily have to be a DER) to look over their "experimental" modifications is really a dumb idea. VERY few homebuilders are really qualified to decide what is good and what is bad when it comes to major modifications.

Just because you can doesn't mean you should. While many of the certified rules are outdated and dumb, remember that almost all of them are written in blood.

Cheers,
Stein
 
thanks for the input

Thanks for all the responses.

I don't intend to do anything drastic.

I was going to do the fastback mod from showplanes (personal prefrence)

I was thinking of getting a IO 390 (depending on budget)

I was toying with the idea of enclosing another bay or 2 for fuel. KATL to KGRB is at the edge of range, I'm #339 on the season ticket waiting list. (I don't really care about acro capability)

As far as engineering support:

My father was a civil engineer and I was a math major in college (surgeon now). I've ordered a few books on design that hopefully will have formulas for load bearing, shear force, wing loading, spin/stall characteristics etc... Between he and I - we should be able to do the math ;)

All the modifications described above have been done by someone I know that is currently flying...

I just couldn't find much info on what was required so the DAR didn't just start lauhging at my face.
 
Rodger - Wilco---

Who is the bigger fool? The fool who leads or the fool that follows?

If the numbers come up nominal or shaky on anything - the drawings become liners for the bird cage.

-Jason
 
I think my only point was, engineering=good, Paying insane amounts of money for an "FAA Approved" DER... not necessary.
 
Experimenting with specific goals is not a bad thing

If you have things that you want to do that deviate from Van's plain vanilla design of a particular RV it probably will not be noticed. I doubt that any two are exactly alike. You obviously have some good ideas that you want to incorporate and you should not be guided by fear of the bureaucracy away from building the airplane you want. Fear of failure or reduction in function or performance is a healthy thing to have in making your design decisions and implementing them. Everything you mentioned has been done and you and your father should be able to work out a safe solution for your airplane. If you ever think about racing the airplane, the IO-390 takes you out of the RV classes but that may not be a concern to you. Dick Martin has one in his RV-8 and he is very fast but he has to fly in a faster class in the Air Venture Cup Race for example.

I have four individually selectable fuel tanks on my RV-6A in cruise configuration and the span is increased by 1.5 ft. The system has worked very well for more than 500 flight hours but the wing loads are changed and the plane is slower because of this difference. Increasing the fuel capacity in the existing physical envelope is a better way to go. Van personally discouraged my tip tanks and suggested a header tank or converting the existing tips to fuel tanks. When I convert to race configuration I remove the tanks and stock tips and replace them with fiberglass tips of my own design. This decreases my fuel capacity by 17 gallons but increases my top speed by 3 kts. When the DAR inspected my airplane he inspected the fuel system and it was not a problem.

I have well over 1,000 hours in modification work on my airplane since I first flew it in 2004 and the work continues.

The thing that bothers me more than an innovative spirit is poor workmanship.

Bob Axsom
 
Airworthy?

Even though you get an "Airworthines Certificate", no homebuilt is "Airworthy". That is reserved for normally certificated products. Homebuilts are in a "condition for safe flight". That's my story and I'm sticking to it.
 
Say it ain't so....

...Dave!! You mean I've been flying a non-airworthy -6A for 3 1/2 years AND giving lessons in it....Egads:eek::eek::)

Regards,
 
UTS --- Not

klovning said:
If the numbers come up nominal or shaky on anything - the drawings become liners for the bird cage.
-Jason
Click to expand...

In your reading, please include Fatigue Life Prediction and Endurance Limits for the frozen bauxite mush we build our toys with.
 
SteinAir said:
If you're doing any major changes to an airframe or major system, then my opinion is that you don't just need a DAR, you should look for a DER.
Click to expand...

I agree. Major alterations require more than pretty craftsmanship. If you skip the engineering, that pretty modification just might tear off the airplane in flight.

A DAR is mostly concerned with whether your modification "looks" safe. A DER can help you with the analysis and testing to verify that it really "is" safe.

-David
 
I guess Cessna should hire a DER

Don't kid yourself into believing that an engineering blessing is going to make a modification safe. You can follow the discipline, use good judgement, good workmanship, base changes on experience, design with style, go through preliminary and critical design reviews, consult with experts, have independent assesment of the design and the product, perform failure mode and mitigation analysis, construct detailed fault trees, have the materials and processes approved, perform static and dynamic test, subject the product to destructive and nondestructive testing, etc. and the risk will remain. That is after all part of the appeal isn't it. As Richard Collins said years ago "We wouldn't have it any other way."

Bob Axsom
 
My 7A was inspected about 10 days ago with 5 total tanks (65 + usable), and there was no problem.
The DARs responsibility includes many things, but not to be an engineer. The DAR looks for obvious unsafe conditions. And, as to "51% approved kits" to ensure that variations from the kit were done in consulation with the kit manufacturer or another authority (which I think the literature says can be a DAR, DER, or an IA).
For my inspection, I prepared a single page statement that listed the 5 items that varied significantly from the RV7A kit/plans. I described them, mentioned to whom I had spoken to about them, and mentioned what if any written materials I referenced (e.g. 43-13.1A). Note: I don't think Van's ever approves any modifications; the best you get is "no objection."
Two of my tanks were the SafeAir kit, so I included their liturature in the builders records and adhered (pun, sort of) to their placards. The fifth was a removable, bottom drainable, vented, 15 gallon auto tank made by RCI. It was installed in the baggage compartment, with slightly beefed up floor ribs, all AN hardware for the floor attach, and aeroquip hoses. The locking external filler cap, now that I think about it, was non-aviation. So, I included several pictures, a listing of the parts used, and the weight and balance computation, and my DAR didn't spend much time looking it over.
Its experimental, folks, and you are the builder. Remember, you aren't supposed to fly over populated areas! This is why. I don't like fuselage fuel for a variety of reasons, but this removable tank will meet my ocassional LR travel needs, so it it went. NOTE: It pays to get aux fuel systems approved at the initial inspection since later changes may be deemed a major modification to the aircraft and necessitate another inspection and renewed (though shorter) phase 1 test period.

Mike Coster
RV7A N92MB
Virginia
 
I did the math and the loading stresses on the wing spar/fuse bulkhead are still well within design limits with full fuel in the extra tanks if you can keep the plane in utilitary category (+4.4 to -1.76 g) (Wing shear stress and torque in normal flight and +g maneuvers is actually lower with more weight in the wings)

To exceed design limits of the original spar with full extra fuel I'd have to hit on landing and have the fuse experience a negative 2.5G (actually a bit ore than 2.5) force on landing... I think most people would refer to this as a crash

Actually looking at the forces these little planes can take is rather humbling.... 4700 lbs to -2656 lbs shear and 277300 inch pounds to -153447 inch pounds torque

That much torque and shear is like putting a 1/2 ton pickup on the other end of a 10ft teeter totter.... :0
 
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Load re-computations

I am surprised that your two large tip tanks don't require more changes. The hangar owner where I built is an A&P/IA building her second RV -- this one an RV6A. She is adding two tanks, one in each wing, so as to have 38 (original factory design) + about 22 or so gallons (in the two new tanks) just outboard of the design main tanks. She obtained drawings from an aeronautical engineer (customer of hers) that were discussed with Vans, which required that the wing spar be made thicker, and planned skins located outside the new tanks were replaced the same thickness as the two original tanks had.

Not that it makes a difference on load, but her L and R aux tanks will feed directly to the fuel valve, not into the L&R mains. Sounds like yours (like my SafeAir tanks) feed into the mains.

Mike
 
If you ask for outside approval or evaluation

If you ask for outside approval or evaluation you will get CMA layers of the reviewers self protection.

Bob Axsom
 
RE: Load Re-Computations

Mike, I'm very interested in what your hanger partner is doing with her aux tanks. Would love to have some more details if she's willing to share. Assume she's running 4 lines to a 4-way fuel selector with gravity flow. I've wondered why this method isn't used/discussed more--seems much simpler.
 
I have 4 tanks and two 4-way valves

I have 4 tanks and two 4-way valves for individual selection of one tank or no tank at a time. After 5 years and 500+ hours of flight there has never been a fuel control problem. I spent a lot of time evaluating the various combinations that would give me the control I needed. All four tanks have 3/8" aluminum line gravity feed to the central fuel selection location. One valve is used for the tip tanks - one input port for each tank, two input ports plugged and are physically not selectable by the design of the console - the output port is connected to one input port of the second valve. The other valve uses all four input ports - one for the tips, one for each main tank and one plugged for all fuel off - the output port is connected to the aux pump and supplies all fuel to the engine. The valves all point to the tank selected and there are no ambiguous positions. Both valves are mounted in the center and they are rotated 45 degrees to minimize the necessary width of the console. The tip tank valve uses the two selectable adjacent ports 45 degrees to the front right selects the right tip tank and 45 degrees to the front left selects the left tip tank but no fuel flows to the engine from either tip tank unless the tip tanks are selected by the second valve. The tip tank valve is in front of the other "main" fuel valve and when the tip tanks are selected by this valve it is pointing directly forward toward the tip tank valve. When the main fuel valve is pointing directly to the right only the right main tank is supplying fuel; when it is pointing directly left only the left main tank is supplying fuel; and when it is pointing back to the tail of the airplane all fuel flow from the tanks is shut off. Since the main fuel valve is also rotated 45 degrees off of the aircraft centerline the shaft for the selector lever on this valve has a reindexing flat filed in it to provide the intuitive selection orientation of the lever. The operational choices are (M=main valve, 1,2,3 and 4 are the selectable ports beginning on the left, T=tip valve, 1 and 2 are the selectable ports beginning on the left - rotation to the other two are physically blocked by the console):

M-1 T-1 = left main
M-1 T-2 = left main
M-2 T-1 = left tip
M-2 T-2 = right tip
M-3 T-1 = right main
M-3 T-2 = right main
M-4 T-1 = Fuel off
M-4 T-2 = Fuel off

Bob Axsom
 
plans

I'd build it per plans, get the "condition not unsafe for flight" then add the aux tanks, power recovery turbines, thrust augmenters, etc etc. there are already enough reasons for faa not to want to issue CofA. don't give them more.
 
changing fuel tanks post-inspection

James: I recall that there was a violation found where a pilot/builder made several "major" changes to his fuel system after (over the course of a few years) the DAR inspection. He added an aux tank, then removed it, then made some lesser change. Unfortunately, he later had fuel starvation and an accident wherein the plane (not sure about the person/people on board) were injured. The finding was that the fuel system mods were "major" and that his original experimental airworthiness cert was invalid once he made the first modification. Maybe someone remembers the details. I think that John Yodice wrote an article about it.

The recommendation was to get your fuel system mods approved at the first inspection so that you did not have to go back into phase one testing later. For that reason, I had my plane inspected with the removable baggage compartment tank installed and plumbed. After flying with it for two flights to test fuel flow during phase one, I removed it.

Mike
 
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