If you took two identical aircraft kits and built one up with primarily flush rivets on the surfaces and the other with primarily pop rivets on the surfaces, how much difference would you experience in top speed. Every other aspect of the build is the same. This is a hypothetical question of course, but I'm curious exactly how much effect the aerodynamic differences between the rivet types has on speed.
Van's Air Force
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Effect of rivet type on airspeed
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If you have access...
...to "27 Years of the RVator", there is an article about the WW II Spitfire aircraft and the investigation of flush vs universal head rivets. IIRC, leading edge airfoil was critical for flush rivets, the rest did not matter.
It would be interesting to do the experiment on an RV, but it seems that being able to precisely measure and correlate air speeds is the limiting task.
...to "27 Years of the RVator", there is an article about the WW II Spitfire aircraft and the investigation of flush vs universal head rivets. IIRC, leading edge airfoil was critical for flush rivets, the rest did not matter.
It would be interesting to do the experiment on an RV, but it seems that being able to precisely measure and correlate air speeds is the limiting task.
Depends on what airplane you were talking about.
The speed difference of an RV-12 flush riveted or not would probably be hardly measurable. The Speed difference on a Questair Venture would be much more significant.
With "primarily flush rivets" do you mean "driven rivets"?
With "primarily pop rivets" do you mean "flush pop rivets"? Pop rivets come in "round head" and "flush" just like driven rivets do.
If so there would be relatively no difference in performance.
If you are talking "round head" vs. "flush", it depends on What speed you are talking about. the lower the airspeed, the less effect drag has.
At 130 mph I don't think you will see a difference. At 200, you might see a small difference.
As an example, I had a T-Craft with 3 venturies that cruised just over 100 mph. Without the venturies cruise would be about the same. Put those 3 venturies on an RV and you would see a noticeable decrease in cruise speed.
As an example, I had a T-Craft with 3 venturies that cruised just over 100 mph. Without the venturies cruise would be about the same. Put those 3 venturies on an RV and you would see a noticeable decrease in cruise speed.
jjconstant
Well Known Member
split peas
Didn't the spitfire program also glue split peas on top of the flush rivets to find out exactly where it was critical and where it wasn't. You could do the same experiment with your RV....if it was an itch you REALLY wanted to scratch!
Jeremy
Didn't the spitfire program also glue split peas on top of the flush rivets to find out exactly where it was critical and where it wasn't. You could do the same experiment with your RV....if it was an itch you REALLY wanted to scratch!
Jeremy
Dave_Boxall
Well Known Member
Split pea Spitfire
Using a tube of proprietary cellulose adhesive bought from the local ironmongers - there were no DIY shops in those days- we stuck split peas on the heads of all the rivets on one of the development aircraft. It is reported that the machine with all the split peas was 22mph sloweron flight test, so flush rivets continued to be used except in a few places where it was considered that dome heads would not affect the performance.
From Supermarine Engineer Harry Griffith's book "Testing Times"
Using a tube of proprietary cellulose adhesive bought from the local ironmongers - there were no DIY shops in those days- we stuck split peas on the heads of all the rivets on one of the development aircraft. It is reported that the machine with all the split peas was 22mph sloweron flight test, so flush rivets continued to be used except in a few places where it was considered that dome heads would not affect the performance.
From Supermarine Engineer Harry Griffith's book "Testing Times"
Mooneys
FWIW, Mooneys have long been built with flush rivets on leading edges and round head rivets everywhere else. I guess they figured that was the best compromise when you put economy/ease of construction vs aerodynamic performance....and for Mooneys, speed was always a big selling point.
FWIW, Mooneys have long been built with flush rivets on leading edges and round head rivets everywhere else. I guess they figured that was the best compromise when you put economy/ease of construction vs aerodynamic performance....and for Mooneys, speed was always a big selling point.
Bob Kuykendall
Well Known Member
Another thing you sometimes see in factory riveting schemes is that lines of rivets parallel to the airflow (such as along wing ribs) might have protruding heads, while rivet lines perpendicular to the airflow (such as on spanwise wing skin stiffeners) are flush.
The thinking seems to be that in the rivet lines parallel to the airflow, the forwardmost rivet in the line creates a turbulent wake that engulfs the rivets behind it, so the drag increase of the entire rivet line is (relatively, of course) not that much greater than the drag increase of just that one foremost rivet. With rivet lines perpendicular to the airflow, each rivet creates its own wake, so the drag penalty increases linearly with the number of rivets in the line.
The thinking seems to be that in the rivet lines parallel to the airflow, the forwardmost rivet in the line creates a turbulent wake that engulfs the rivets behind it, so the drag increase of the entire rivet line is (relatively, of course) not that much greater than the drag increase of just that one foremost rivet. With rivet lines perpendicular to the airflow, each rivet creates its own wake, so the drag penalty increases linearly with the number of rivets in the line.
I seriously doubt it makes any difference. At osh, I overheard a designer from one of the experimental kit manufacturers (forgot which) stated they had no evidence to show a difference for the speeds we fly. In addition, at an airshow, I looked very carefully over the Boeing Dreamlifter - many thousands of large round head rivets all over the nose and tail area, doubler plates galore stacked into the airstream, let alone a giant fuselage enlargement, etc. I spoke with one of the Evergreen contracted 747 pilots for the Dreamlifter, and he said they can and routinely fly at almost the exact Mach speeds (eg 0.85) as the 747-400!!!!
John Clark
Well Known Member
The Aviator
All you have to do is go to the movies, Leonardo DiCaprio (as Howard Hughes) explained the whole rivet thing in "The Aviator." Made a huge difference for him
John Clark
RV8 N18U "Sunshine"
KSBA
All you have to do is go to the movies, Leonardo DiCaprio (as Howard Hughes) explained the whole rivet thing in "The Aviator." Made a huge difference for him
John Clark
RV8 N18U "Sunshine"
KSBA
Auburntsts
Well Known Member
IMO it's mainly an aestethic thing, plus for solid rivets I'd rather shoot 426's over 470's any day of the week. YMMV....
The two biggest advantages of driven rivets over pulled is cost and weight, both of which are significant.
John Clark
Well Known Member
Quick calculation...
On the question of cost, I took some averages and did a quick calculation on the back of an envelope. The average price of an aluminum rivet is about $.02, the average price of a structural pulled rivet is $.83. So, again averaging, 15000 aluminum rivets would cost about $300. and the pulled rivets would cost $12,450.
John Clark ATP, CFI
FAA FAAST Team Member
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA
On the question of cost, I took some averages and did a quick calculation on the back of an envelope. The average price of an aluminum rivet is about $.02, the average price of a structural pulled rivet is $.83. So, again averaging, 15000 aluminum rivets would cost about $300. and the pulled rivets would cost $12,450.
John Clark ATP, CFI
FAA FAAST Team Member
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA
Kevin Horton
Well Known Member
The difference in resale value is probably even more than the difference in cost of the rivets. I think you would have a very hard time selling such an aircraft.
pierre smith
Well Known Member
This is a misconception....
Much more time is spent on other things, like wiring, plumbing FWF and so on.
Best,
....because with practise, a driven rivet can be set just about as fast as a pulled one. I doubt if you could measure the total time difference in the build.
Much more time is spent on other things, like wiring, plumbing FWF and so on.
Best,
Geico266
Well Known Member
The cost of the LP4-3 (primary skin rivet for the -12) is about .03 each.
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Geico266
Well Known Member
There is no dimpling with pulled rivets, and no helper needed. IMHO they are faster than bucking, and the time savings is fairly significant. You do have to allow time for picking up the stems though, they go everywhere!
I agree with Mel, painting is problematic.
Resale, that remains to be seen, but there are a lot of "certified aicraft" drivers looking at experimentals due to rising costs that fly "bumpy" planes now that would not care one way of the other.
I'm not trying to argue, just adding a difference view point.
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There is no dimpling with pulled rivets, and no helper needed. IMHO they are faster than bucking, and the time savings is fairly significant. You do have to allow time for picking up the stems though, they go everywhere!
I agree with Mel, painting is problematic.
Actually flush pulled rivets require the same dimpling and prep as solid rivets.
Another point that has not been addressed is that pulled rivets require a more perfect "round" hole. They do not expand to fill an irregular hole like a driven rivet does.
I've built at least 2 airplanes with flush pulled rivets. A Moni Motorglider and a Zenair Ch601HDS. It does save time, but not as much as you might think.
I take it you are curious about this because you have an idea this might be a little more attractive to you than driving rivets. If so, i kind of think you are going about it the wrong way- You should be calling Vans and trying to figure this out with their information and things you pull up from the plans, not advice from strangers, even well established ones with numerous RV experience. They've given you plenty of general guidelines, but the "modification" you are describing is something no one (that i know of) has ever done before, and anytime you are going to do that you need to check it over with Van's Aircraft.
so while older kits might have used pulled rivets for certain sections of wings, to try and use pulled rivets for as much of the airplane as possible is a huge change. the rivet schedule needs to be looked at for probably the whole plane, because as has already been discussed, there is nothing inherently wrong with using pulled rivets, just different characteristics. they generally are not as strong as driven, so the schedule (spacing) needs to be looked at and maybe made denser.
If you really think you might do this don't rely on the internet. Call Van's A/C.
John Clark
Well Known Member
Apples and Oranges
True on the price of an LP4-3, but the original question was posed regarding an RV9. To make my point I quoted the price of pulled rivets that could work in an aircraft designed for standard rivets.
John Clark
RV8 N18U "Sunshine"
KSBA
True on the price of an LP4-3, but the original question was posed regarding an RV9. To make my point I quoted the price of pulled rivets that could work in an aircraft designed for standard rivets.
John Clark
RV8 N18U "Sunshine"
KSBA
couldn't they just increase the rivet density to use LP4-3's to make them usable on an RV9 if there was a worry of strength? I mean if we're talking about going crazy like this why not look at the RV9 with rivets like the RV12- If that was a drawback to people wanting to build, maybe the RV9.2 designed with pulled rivets would sell 50% more kits?
I remember a thread a while back "what should Van's design next?" maybe someone should have thrown out an RV9.2 - the RV9 with all pulled rivets (where realistically possible) Van's already has the main engineering done for the airframe, just rework the spacing and "Ta Dah!" a plane anyone can build with less noise and time and work (maybe).
Kevin Horton
Well Known Member
But, the rivet spacing on the RV-12 was selected knowing that the LP4-3 rivet was going to be used. If someone wants to use pulled rivets on an RV with prepunched skins, or matched hole construction, they can't reduce the rivet spacing. So, they need to use a rivet that is at least as strong as a solid rivet. That drives you to a more expensive pulled rivet.
If you were building an RV-3, you could reduce the rivet spacing to compensate for a lower strength pulled rivet. Then you could use a lower cost pulled rivet.
This is great discussion guys. There's seems to be some difference of opinion here on how much build time could be saved by using pulled rivets on the -9, but based on my recent experience in a sheet metal class, I think it's safe to say that for a first timer, pulled rivets would be considerably faster and more consistent in the delivery. If the -9 could be built safely with pulled rivets in considerably less time than driven ones, I would be interested. I'm at a stage of life where I don't think I can spend 2000 hours on a build, but if I could knock 500+ hours off by changing rivet type, without compromising the integrity and function of the plane, then the -9 warrants serious consideration for me. Don't get me wrong, I'm not a maverick type and would never do this unless blessed by Van's.
Flying Scotsman
Well Known Member
12,000 or so rivets sounds like a lot, but when it's a matter of a few seconds per rivet, it just doesn't add up to much overall...assume 15s/rivet average...
12,000 rivets * 15 s/rivet = 180,000 s = 3000 min = 50 hours
You'd have to be ungodly slow at pounding rivets to save 500 hours...150 s/rivet, or one rivet every 2 and 1/2 minutes!
12,000 rivets * 15 s/rivet = 180,000 s = 3000 min = 50 hours
You'd have to be ungodly slow at pounding rivets to save 500 hours...150 s/rivet, or one rivet every 2 and 1/2 minutes!
I guess the time that it takes to actually place the rivets is half the equation for me. The only time I will consistently have to build will be late night after the family is in bed. If driving/bucking rivets takes 2 people, as some have suggested, then that will set me back having to wait for opportunities when I can get someone to help.
DakotaHawk
Well Known Member
The majority of the RV can be driven and bucked by yourself. There are only a few areas that require a second person to assist. Closing out the last wing skin and closing out a few of the fuselage areas are areas that you'll need help on.
I had helpers come by my shop about ten times during my entire build to give "required" help.
One thing that seems to scare a lot of new builders is learning how to rivet well. I never drove a single rivet until I started my RV7. Van's starts you out on some easy areas, easy to screw up, easy to repair, not visible after you start closing out systems, cheap to order replacement parts. By the time you graduate to the visible and important stuff, you'll find that you are bucking and driving and chewing gum, all at the same time!
I don't think there's a single RV builder who had a helper available for every rivet.
Bill Wightman
Well Known Member
Drag hit will be pretty small
There has been considerable research done on the effect of surface roughness variation in fluid flow. Much of the data I've seen talks about laminar to turbulent transition characteristics; the effects of Reynolds Number and Mach number on skin friction coefficient (C_f), but I haven't seen anything showing how much C_f might change from one type of construction to the next.
As a general rule, the higher the airspeed and Reynolds Number, the less effect roughness has on C_f. Perhaps this is why the transport jets can get away with more than, say, a sailplane or a GA type airplane.
Even with all the testing in hand, it still looks almost impossible for one to perfectly predict the effects of standard construction "flaws" such as protruding rivets, skin seams, imperfect skin geometry (bulges, waves, etc), leakage through seams, etc etc.
Perfect test articles can tell us what theoretically perfectly smooth shapes can produce, and well built composite airplanes probably can approximate that. But from one airplane to the next - ???? You got me.
Standard C_f values for metal construction show increases of about 10% above the totally smooth values. Perhaps we do a bit better with our flush rivets and attention to detail on how our skins fit. Those things do matter.
A really quick and dirty analysis:
Suppose the standard flush riveted RV has an average C_f of .002. If we build one with protruding rivets, keeping all else equal, then C_f might go up by .0002 on those areas affected, which would be much less than 100% of wetted area.
Assuming the total wetted area affected by the roughness change is approximately 200 ft^2 (total wetted area being about 500 ft^2: WAG), we can calculate the increase in drag due to skin friction. At 8000' and 200 mph (290 fps), this would result in a drag increase of about 3 pounds.
How much speed loss? The 3 pound drag rise would require a speed reduction of 3.1 mph (4.5 fps) with no change in thrust, or other drag variables to get back to equilibrium.
Anybody have a bag of split peas?
There has been considerable research done on the effect of surface roughness variation in fluid flow. Much of the data I've seen talks about laminar to turbulent transition characteristics; the effects of Reynolds Number and Mach number on skin friction coefficient (C_f), but I haven't seen anything showing how much C_f might change from one type of construction to the next.
As a general rule, the higher the airspeed and Reynolds Number, the less effect roughness has on C_f. Perhaps this is why the transport jets can get away with more than, say, a sailplane or a GA type airplane.
Even with all the testing in hand, it still looks almost impossible for one to perfectly predict the effects of standard construction "flaws" such as protruding rivets, skin seams, imperfect skin geometry (bulges, waves, etc), leakage through seams, etc etc.
Perfect test articles can tell us what theoretically perfectly smooth shapes can produce, and well built composite airplanes probably can approximate that. But from one airplane to the next - ???? You got me.
Standard C_f values for metal construction show increases of about 10% above the totally smooth values. Perhaps we do a bit better with our flush rivets and attention to detail on how our skins fit. Those things do matter.
A really quick and dirty analysis:
Suppose the standard flush riveted RV has an average C_f of .002. If we build one with protruding rivets, keeping all else equal, then C_f might go up by .0002 on those areas affected, which would be much less than 100% of wetted area.
Assuming the total wetted area affected by the roughness change is approximately 200 ft^2 (total wetted area being about 500 ft^2: WAG), we can calculate the increase in drag due to skin friction. At 8000' and 200 mph (290 fps), this would result in a drag increase of about 3 pounds.
How much speed loss? The 3 pound drag rise would require a speed reduction of 3.1 mph (4.5 fps) with no change in thrust, or other drag variables to get back to equilibrium.
Anybody have a bag of split peas?
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Kevin Horton
Well Known Member
The time spent riveting is only a tiny percentage of the total time spent building. Don't let this aspect drive the decision as to which model to build. Decide what the mission of the aircraft will be, and then see which model best fits that mission. You'll be able to find a way to deal with the riveting no matter which model you eventually chose to build.
You might find it instructive to visit with some local builders. Learn what is involved to drive solid rivets, and the fear will go away. Over 6,000 RV builders have learned how to drive solid rivets, and you can do it too.
Bill Wightman
Well Known Member
Right on, Kevin.
I just gotta say, about our flush riveted airplanes - the thing I REALLY like about it is how it looks sitting on the ramp. These are hand built airplanes: built with our own sweat, blood and money.
To me, building and flying a sport plane is all about two things: Looking good and going fast. The flush rivets help in both areas. I don't even think twice about the work involved.
I just gotta say, about our flush riveted airplanes - the thing I REALLY like about it is how it looks sitting on the ramp. These are hand built airplanes: built with our own sweat, blood and money.
To me, building and flying a sport plane is all about two things: Looking good and going fast. The flush rivets help in both areas. I don't even think twice about the work involved.
