I was so focused on the hole that goes through the HS-405 thru the 810 angle I did not watch the top and bottom flanges. When I drilled the holes for the top and bottom of the HS-405 using the skin as a guide the last two holes on the forward end of the flange came out very close to the edge. One is right at the limit the other is 1/32- over the limit.
It's fine I think. 1/8th protruding head fasteners? Last hole? Thick or sheet part?
A 1/8" rivet needs 0.25 for e/D=2.0; You're 1/32= 0.03125 short?
Your new edge over Dia or e/D is:
1/8th rivet > (0.25-0.03125)/.125 = 1.75 (not really that short)
3/32" rivet needs 0.1875 for e/D=2.0. Your new e/D:
3/32th rivet > (.1875-0.03125)/3.32 = 1.25
(getting short, anything under 1.5 needs a little more analysis, but it could still can be totally OK)
Questions that engineers ask: How is the fastener loaded, in tension or shear? What size and type of fastener? How thick are the parts? What other fasteners are in the joint? There are so many things to consider and ways to evaluate the issue problem w/ logic.
The Fix? (Use as is/Fix/Scrap)
How hard is it to fix? Frankly sometimes leaving it the heck alone is best. You can make it worse by monkeying with it. Some typical fixes might be a doubler riveted to the part with short e/D. (note: repair rivets go though just repaired part and doubler usually, not through the whole joint; flush rivets are sometimes needed or possible; just an extra piece of metal in there can just act like a spacer not a doubler if not done properly attached.) If the e/d is REAL SHORT, some times not using the hole and relocation of another hole/fastener can work or additional fasteners. Sometimes a new part is the best way to go. Van is good about selling parts at reasonable cost. There are too many scenarios. Your case, I don't have my RV-7 dwg in front of me which should be like the -8. I can't picture off the top of my head with out a visual aid.
You have two main issues, ultimate strength & fatigue life, one or both can be affected by short e/D, or these things may not be affected at all. Often engineers are worried about fatigue life. On a large commercial jetliners that fly more in a year than some GA planes fly in their total life, its often more a factor than ultimate strength. Fatigue is based on the GAG or ground-air-ground mission and utilization. Lets skip that but suffice to say GA planes have it a little easier simply because they fly so little, relatively speaking. Plus airliners are pressurized which puts major cyclic stresses on the fuselage every flight, smooth or rough air.
Is there any fudge factor here or is this edge distance thing. If there were only two holes in this area I would say get some new parts, but this is one of multiple holes that run along the long edge of the HS-405 rib. Just looking for suggestions. Like many things there are guidelines and hard and fast rules, just wonder what edge distance fudge factors might be available.
I am talking about the last hole in the upper flange - forward side of the HS-405, this is the most inboard of the HS ribs.
e/D=2D is plenty, center of hole to edge. Call it the rule of thumb. Some times in critical areas that should not be violated or e/D greater than 2D is needed (not usually but possible). Some times you just can't get 2D and have to accept it. Often 1.5D will be fine, depending on location. As Rick says below even 1D can even be enough. It's ain't pretty but GOOD ENOUGH is good enough. Also because you have sweated over this hole/fastener it has been scrutinized, so we assume its perfect in every other way. The hole will be lightly and properly deburred (not too much of couse because that will aggravate the short e/D). You will make sure the rivet is set perfect. Your care and inspection really makes that joint statistical better because its perfect in all other ways, save the short e/D. You may have problems un-noticed in other joints that you're not aware of. That is why structure in airplanes is somewhat redundant with multi fasterners. Now if its an "Oh Gosh" bolt, a single load path than greater care and less tolerance is given.
liaison and stress engineers who get paid to consider such matters. Sometimes an answer can take WEEKS before filtering down to the shop floor. Most times though, the answer comes within minutes or hours.
At one time I wore the title of stress engineer and liaison engineer. If your stress guys take weeks, they either forgot about you or its marginal and scrapping the part or assembly would cost WAY TOO MUCH money, so people are fighting or the customer is complaining.
It all depends upon the area in which the specific short e.d. occurs. I've been okayed to install a fastener in a certain area with barely more than 1D.
That is true especially if its an end fastener and it's not a hard point issue. Yes it can be complex and it's a case by case deal as you said, based on actual loads and stresses. However the "over built" aspect of a structure some times is apparent by inspection sometimes, which allows the engineer to make a on the spot acceptance of short edge with out much effort; it takes some experience, knowledge and training of course. It could be the engineer has seen that case before and already researched it.
Structural analysis is an arcane art. You can ask Van's for their opinion, and I say this with sincere respect....unless the numbers have been crunched to simulate a specific condition being subjected to the maximum anticipated loads over the life of the structure, the otherwise qualified and well-meaning staff may just as well stick their wetted finger to the wind.
I agree 100%, you need to know the exact "stress field" or critical load case or cases to analyze. Analysis is a bit of art or arcane, but let me dust off some of the mystery. The math of structural or stress analysis hasn't changed in a half of century. In the last few decades there's been a explosion in computer based tools to crunch numbers, but the theory is the same. With more computer power engineers can just run more load cases in a shorter amount of time than with slide-rulers & hand cranking. Techniques like FEA (finite element analysis) is easily done today w/ the power of a PC's & low priced (for engineering) software. More detailed information is great, but edge margin is still edge margin. The "tribal knowledge" may be viewed by the layman as arcane, but it's just collected experience & history of that type of part or structure/joint, observed, documented & correlated with mathematical analysis. Arcane or art, it really is engineering common sense, phsyics & math.
Engineers armed with (arcane) tribal knowledge can extrapolate past experience to other like structures and components with similar operating environments, which than becomes a "rule of thumb". One rule is the "standard" edge distance. From experience, it works over a wide range of structures & loading and fits empirical & experimental data.
If you are looking for a boilerplate, definitive answer to your most reasonable of questions....sorry....it just doesn't exist.
True. Van should know where the super-critical fastener edge margins are, if any. I hope the good folks at Van's answering the help line have guidance, not just a wet finger in the air to OK shorter e/D.
One last factor in the scrap/use/repair decision, HOW EASY IS IT TO INSPECT AFTER CONSTRUCTION? If its in a dark corner never to see light of day again or viewed by an eyeball, than less tolerance should be given. Also what if it cracks? Some times nothing, but if it's something like the wing spar cap or rear spar attach, its a bad thing.
