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Lightening holes in seat backs

M

mcencula

Well Known Member
Darwin's post on his neighbor who is putting in lightening holes everywhere got me thinking (again) about something I've been considering for a while...cutting lightening holes in the seat backs to reduce some weight. I first thought of this after seeing a picture of the RV-12 seat backs which already come with a bunch of lightening holes.

Based on the corrugated design of the sheet, much of the flat portion could be removed without compromising the bending strength of the seat back. They way they are currently designed, the tip of the V bends would fail way before the flat portions of the sheet anyway. Cutting lightening holes would just make the stresses more uniform between the tips of the V bends and the flat portions. Yes, I'm an engineer, but I haven't done any calculations to back this up. I'd probably just use the TLAR method.

The weight savings seems like it could be considerable. Say I cut 5 holes of 1.5" diameter in each flat section:

.75 rad ^2 * pi * .025 thick * 5 holes per zone * 6 zones * .094 lb/in^3 aluminum density * 2 seats * 16 oz/lb = 4 oz weight savings

Another nice aspect of this modification is that since it's close to the center of the CG range, it would have negligible effect on the CG.

So the questions I've got for the group are:
What do you guys think about the idea?
Is it worth the effort?
Has anyone done this before?
Can anyone think of any downside that I haven't yet?

Thanks!
 
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I'm sure you won't be the first to do this Mike...I say go for it! I didn't do lightening holes, but did make my own lightweight seatbacks...

7-15-07-004w.jpg


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I the seatbacks are an area that are well worth the effort...I saved 6oz with mine...
 
why make the seat back out of metal at all? It can't be for strength (it isn't a structural part in any way, is it?) I wonder if a piece of composite or even a stretched piece of nylon fabric of some sort (even as strong as trampoline material)would do the same thing and save even more weight?
 
Have you thought about what your seat cushions will do when pressed against all those lightening holes? I think you'll definitely shorten the life of a typical seat cushion. Of course, they could be strengthened, but then that would add the weight back.
 
Danny7 said:
why make the seat back out of metal at all? It can't be for strength (it isn't a structural part in any way, is it?) I wonder if a piece of composite or even a stretched piece of nylon fabric of some sort (even as strong as trampoline material)would do the same thing and save even more weight?
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I think that some get a bit to hung up on the weight issue. My RV is fast, agile, and not exactly light! In fact, it has a heavier C/S prop, the older heavy starter motor, and oxygen tank as standard; not to mention a heavier six pac panel. Thing is, I've flown RV6's such as mine that are quite a bit lighter, but there isn't much difference in handleing at all. In fact, due to my heavier C/S prop, it's much more peppy on the takeoff roll, than lighter weight F/P planes of the same model.

L.Adamson --- RV6A
 
L.Adamson said:
I think that some get a bit to hung up on the weight issue. My RV is fast, agile, and not exactly light! In fact, it has a heavier C/S prop, the older heavy starter motor, and oxygen tank as standard; not to mention a heavier six pac panel. Thing is, I've flown RV6's such as mine that are quite a bit lighter, but there isn't much difference in handleing at all. In fact, due to my heavier C/S prop, it's much more peppy on the takeoff roll, than lighter weight F/P planes of the same model.

L.Adamson --- RV6A
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Amen, brotha! No offense intended, but I think it's kind of funny when I hear someone say that they saved a few ounces here and there. I didn't cut any lightening holes that I can remember, and my -7A weighs 1043 lbs. Granted, it still needs paint and wheel pants, but if it's under 1100 lbs after that, I'll be more than happy (and I'm sure it will be).

My point with saying this is that even if your RV isn't light, it will fly just fine. If your argument is that of getting the most Utility out of your airplane, then I'd say make sure your W&B is far enough forward as to not limit your baggage carrying capacity.
 
Don't take this the wrong way, But....

Hmm...6oz. Yea...that is 1 cup of fuel. So, if you put lightening holes in every piece that you can, does anyone think they will save 1 lb? Ok, maybe 2lbs total. My body weight fluctuates about this on a daily basis based on if I drank one or two home brews the night before. I doubt any RV7 would fly at max gross weight differently at 1800 lbs or 1810 lbs so utility is not a good argument.

During my build I did not concern myself too much with all the work and time necessary to drill lightening holes and my empty weight came in at 1108 lbs. If it had come in at 1106 lbs I would have been kicking myself for all the extra hours of work required to save a measly 2 lbs. Oh, and that is equivalent to only 1 1/2 pints of fuel.....or a good RV breakfast on a weekend morning with the guys.
 
What also needs to be realized........

.....is that the -6, -7 and -8's are designed to withstand 6 G's with an ultimate load of 9 G's. The seatbacks are somewhat reclined so there would be more than 1 G during a 6 G pull...exactly how much, I'd leave to the engineers on here to calculate.

However, if you decide to make a battery support in the rear of the airplane, for example, keep this into consideration....make the tray able to withstand 6 G's, like the rest of the structure....kind of a "total engineering" concept. If you take a good look at our engine mounts compared to a Cessna or Mooney, you'll see a very substantial difference, that ours are more robust. The weight of the engine X 6 G's explains why. The others are either in the Normal, or Utility category, 3.8 and 4.4 G design limits, respectively.

Regards,
 
L.Adamson said:
I think that some get a bit to hung up on the weight issue. My RV is fast, agile, and not exactly light! In fact, it has a heavier C/S prop, the older heavy starter motor, and oxygen tank as standard; not to mention a heavier six pac panel. Thing is, I've flown RV6's such as mine that are quite a bit lighter, but there isn't much difference in handleing at all. In fact, due to my heavier C/S prop, it's much more peppy on the takeoff roll, than lighter weight F/P planes of the same model.

L.Adamson --- RV6A
Click to expand...

lostpilot28 said:
Amen, brotha! No offense intended, but I think it's kind of funny when I hear someone say that they saved a few ounces here and there. I didn't cut any lightening holes that I can remember, and my -7A weighs 1043 lbs. Granted, it still needs paint and wheel pants, but if it's under 1100 lbs after that, I'll be more than happy (and I'm sure it will be).

My point with saying this is that even if your RV isn't light, it will fly just fine. If your argument is that of getting the most Utility out of your airplane, then I'd say make sure your W&B is far enough forward as to not limit your baggage carrying capacity.
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Capflyer said:
Hmm...6oz. Yea...that is 1 cup of fuel. So, if you put lightening holes in every piece that you can, does anyone think they will save 1 lb? Ok, maybe 2lbs total. My body weight fluctuates about this on a daily basis based on if I drank one or two home brews the night before. I doubt any RV7 would fly at max gross weight differently at 1800 lbs or 1810 lbs so utility is not a good argument.

During my build I did not concern myself too much with all the work and time necessary to drill lightening holes and my empty weight came in at 1108 lbs. If it had come in at 1106 lbs I would have been kicking myself for all the extra hours of work required to save a measly 2 lbs. Oh, and that is equivalent to only 1 1/2 pints of fuel.....or a good RV breakfast on a weekend morning with the guys.
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I respectfully disagree with anyone that says that 6oz here, a pound there...makes no difference in the end. Doesn't really matter one way or the other though. Since this is my airplane that I'm building, I have a goal of 1,050lbs empty, FINISHED, and when cutting out 6oz here, and a pound there...it all figures in to my goal at the end.

Van's mantra is light, light, light, and KISS. I posted pics of my seat backs as and example of another way to do it. It may not work in the long run, but I had fun building them, and yep, they are lighter than Van's seat backs.

If I didn't pay any attention to weight during the build and didn't care, and I ended up at 1100 or 1102, you're right I wouldn't know the difference. But with a goal in mind, I am working to add lightness to mine.

:)
 
cjensen said:
Van's mantra is light, light, light, and KISS.
Click to expand...

It was in the beginning, but not exactly that way now. You see a lot of Van's demos with C/S props and added navigation equipment in the cockpit. Of course with today's glass panels; some extras can actually weigh less than the simple panels of the past.

Personally, if I want a light, light, light plane, I'd build the RV3. But then it would be almost worthless for long cross country's, which I very much enjoy.
All in all, though, building lighter is fine, as long as it's good structurally. I just don't find the differences in performance to be much at all. I'll throw the largest engine on that I can, and a C/S to go with it; even if it weighs more. Because at the end of the day, raw horsepower counts! :D

L.Adamson --- RV6A
 
another consideration

The seat back support gets sandwiched on the cross brace. It takes quite a bit of force to slide them out. Be careful that you do not bend your seat back when you take out your seats or move the support. At least mine are very tight built to plans and I have to pound on them with my palm to get them out.
I have a neighbor who designed his own composite interior including seats. His airplane is very heavy, but very quiet, plush, and comfy.
 
A light plane climbs faster accelerates faster and cruises the same on less fuel. Every pound saved in weight requires less horsepower to operate.
 
Lightening

Speaking of making cuts to acheive lightness...

AFAIK a pound of body fat is about 3500 calories. Cut 200 calories a day (not too much, really), and you lose about 6 oz per week, a pound every three weeks or so. If your build time is a year and a half, you could cut your gross flying weight more dramatically by cutting calories than by cutting lots of lightening holes.

Or, to use another approach. For someone my size, 100 calories is worth about 1 mile walking or running. Two miles per day is a different way to cut net calories. Of course, that would cut into your build time.

;)(Not meant to be taken completely seriously)
 
Wieght count for fuel savings

Norman CYYJ said:
A light plane climbs faster accelerates faster and cruises the same on less fuel. Every pound saved in weight requires less horsepower to operate.
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To tell you how inportant weight can be for fuel savings, at our airline we realized we did not need to carry the full load of water every flight. Most wide body aircraft carry water for 12 hours of flight time and a full load of passangers. If the flight is only 5 hours you only 1/2 the water. Past practice was to fill er up, not any more. The fuel savings with the number of flights will be substantial. Mybe it will help pay the executive bonuses! :mad:

As for my airplane I trying to build light but I realize I will save the most in fuel if I can mange to shed 30 lbs from my waist.
 
Whether lightening the seat back is structurally advisable or not (I'd hate to have it buckle at the wrong time, say, like on climb-out :( ), I view "building light" by making all the small cuts and lightening holes and whatnot more of a philosophical thing...this is an *airplane*, and whether it's 6 ounces or 6 pounds, extra weight is extra weight. It's also more aesthetically pleasing, to me, to see parts that have been engineered to include lightening holes and such...as opposed to just big slabs of aluminum, those things speak to me of *thought* and design.

Plus, my neighbor/advisor has drilled it into my head...it's a plane, build it light wherever you can! :) Thanks, Alex! LOL!
 
I think it may be important to think about what your body weight, in a 3 g turn, will due to the seat back (forget about 6 G's). If you don't ever plan on pulling G's, then lighting up the seat back is not an issue. If you plan on yanking and banking, you'll want to make sure your seat is nice and strong.
 
Bad, bad idea...

For crash safety is making seat backs out of window screen a good idea ? I personally have been thinking that the stock seat back design is a little weak vertically. I am sure that I have read about seat backs failing in some RV crashes (RV8's?)and the pilot falling back. Also, those triangular transverse folds provide a massive amount of rigidity. I would think carefully before replacing that rigidity with the structural equvalent of wet paper.

Just my .02
 
minute2 said:
For crash safety is making seat backs out of window screen a good idea ? I personally have been thinking that the stock seat back design is a little weak vertically. I am sure that I have read about seat backs failing in some RV crashes (RV8's?)and the pilot falling back. Also, those triangular transverse folds provide a massive amount of rigidity. I would think carefully before replacing that rigidity with the structural equvalent of wet paper.

Just my .02
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Well, first of all, the corrugated triangular folds will do nothing for the seat vertically...if they were vertically oriented, they would, but not the way they are. A horizontal fold will just, well, fold if vertical pressure is put on it.

Second, I have three 1/8" angles running across the back of the seat back (top, middle, and bottom-in addition to the standard angles that make up the side frame), and a two sided strap along the top 1/4 of the seat for the seat supports to make the frame extremely rigid. I have no worries about the screen mesh...the frame is there for the structure, not the mesh. Even if the screen failed completely, the frame will still support the cushion because of the cross angles.

This was not an attempt to improve upon Van's seats...just weight.

:cool:
 
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LIghtening strikes

I bought a finished RV-9a, so anything I do borders on cosmetic, but I did start to drill holes in the seat back while they were out this winter.
I then realized that 10 holes just weren't going to add up to much weight, and due to the type of foam seat cushion, would not add the ventilation I was also hoping for!
I agree the rear bulkhead failing in a crash is more of a concern, right now I am also looking at a seat belt mod that will help keep the bodies down in their seat during 'maneuvers', and keep one from flipping over the bulkhead in an overturn-type impact.( basically a shoulder-harness retaining bar)

pics to follow when done!
 
weight reduction

I found a way to cut 15 pounds out of the plane design.

- I got on the treadmill every day for 16 weeks;)

I lost 15 lbs.

I think most of us have a bit of that kind of "ballast" that we can lose.
 
Good info

Lots of great replies here. Thanks everyone for sharing their opinion.

FWIW, cutting lightening holes should do essentially nothing to the strength of the seat and it should be able to withstand 6Gs just as well as the stock seat as long as the hole diameter is slightly smaller than the width of the flat area.

The effect on the seat foam is something I hadn't considered and sounds like a valid concern. When doing aerobatics, I plan on wearing a softie chute, so I wouldn't have a seat cushion in place, but the question still seems a good one whether applied to the cushion or a parachute.

I think for now I'm going to wait on the lightening holes for a few reasons:
  1. It's a lot of work and, frankly, I've done a bunch of other things that have already made this project take too long.
  2. If I decide to cut holes at some point in the future, it should be easy to just remove the seats, cut the holes, and touch up the paint.
  3. As others have mentioned, probably the best thing I could do to cut weight in the plane is to get off my fat keester and get some exercise. It'll help avoid issues with the medical and the frequent walks will make Snickers happy as well:

20081207-08-tn.jpg


Thanks again for the replies!
 
mcencula said:
Lots of great replies here. Thanks everyone for sharing their opinion.

FWIW, cutting lightening holes should do essentially nothing to the strength of the seat and it should be able to withstand 6Gs just as well as the stock seat as long as the hole diameter is slightly smaller than the width of the flat area.

The effect on the seat foam is something I hadn't considered and sounds like a valid concern. When doing aerobatics, I plan on wearing a softie chute, so I wouldn't have a seat cushion in place, but the question still seems a good one whether applied to the cushion or a parachute.

I think for now I'm going to wait on the lightening holes for a few reasons:
  1. It's a lot of work and, frankly, I've done a bunch of other things that have already made this project take too long.
  2. If I decide to cut holes at some point in the future, it should be easy to just remove the seats, cut the holes, and touch up the paint.
  3. As others have mentioned, probably the best thing I could do to cut weight in the plane is to get off my fat keester and get some exercise. It'll help avoid issues with the medical and the frequent walks will make Snickers happy as well:

20081207-08-tn.jpg


Thanks again for the replies!
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Snickers looks to be in great shape. If I ever have a dog though, I think I might name him Keester. Thanks for the great idea.
 
I would read this as saving 6oz by itself as hardly worthwhile... But, if you can take that philosophy forward, and save it 20+ times, you are looking at 10lbs+ and that is worthwhile :)

Adding to that, the easiest way to save weight is avoid adding things you don't need in the first place ;)

Andy & Ellie Hill
RV-8 G-HILZ
 
Here is another idea for seat backs. I have been flying with this pilot seat in my rocket for about five years now. I call it the Sling Seat. There is a weight saving over the standard metal seat back. Also there is a bit of "give" in the system and it takes the jolt out of turbulence. I purchased seat belt material and had a local awning manufacturer sew them up. The seat is adjustable up and I find it very comfortable for long trips. There is not one piece of furniture that you own, or a vehicle that you drive that has a rigid piece of aluminium as the seat and back rest. I am much more refreshed after long flights then with the standard seat structure. And yes, I have tried many different types of cushions with the standard seating arrangement. Currently I have a 3? layered temper-foam cushion that I use with my sling seat and after three years I am going to switch, at least the top part to a softer type of foam. This should not be too difficult to modify for any RV.


 
mcencula said:
Lots of great replies here. Thanks everyone for sharing their opinion.

FWIW, cutting lightening holes should do essentially nothing to the strength of the seat and it should be able to withstand 6Gs just as well as the stock seat as long as the hole diameter is slightly smaller than the width of the flat area.
Click to expand...

Dimple dies would make it even stronger. :D

And a Suspension seat would probably be the lightest, most comfortable option.

mcencula said:
[*]As others have mentioned, probably the best thing I could do to cut weight in the plane is to get off my fat keester and get some exercise. It'll help avoid issues with the medical and the frequent walks will make Snickers happy as well:
[/LIST]

20081207-08-tn.jpg
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I thought my family was the only ones silly enough to name a dog after a candy bar. Here's our snickers, Hershey (our black lab) was too busy chasing feild mice to stop and get his picture taken:

 
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Old thread...still building

At the risk of outing myself for having still not finished this airplane :eek:, I thought I'd post a follow up...

Since I've done a bit of work that added some weight (click for a possibly long, boring video of the popup tip-up modifications), I decided to go ahead and remove some weight from the seat backs. (140 g or about 5 oz).

Here's one seat back to show how they came out. In addition to the 5 oz savings, I think they look cool, even if the holes won't be visible most of the time. :)
20221015-03-tn.jpg
 
Seat backs don't need to be any wider than those in an RV 4. 12". And only tall enough to rest on the cross brace..... Do that and you will save a LOT of weight.

Shop for seat cushion foam and save more weight.
 
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