BC12 ribs vs F21 ribs

[n6346m]

Do the BC12D and F21 use the sane wing ribs.

[astjp2]

I have not seen any difference, are you looking at the wings up at Sutton’s?

[n6346m]

Not really. I was wondering that with the gross weight of the F-19 and F-21 being 1500 lbs, if they have heftier ribs than the B series or if all ribs are created equal. It's hard to tell from reading the available parts manual in this forum if the ribs all carry the same part number.
Bob

[astjp2]

Spars would have been the limiting factor...but I do not have actual drawings to prove that.

[3Dreaming]

Like Tim said I think the spars are thicker for the F-21. One of the tell tail signs is the letter that the parts number starts with. If it starts with an "A", then the part started with the A model, like the struts. If it starts with a "B" it started as a B model part. If it starts with an "F", it is a F model part. There may be some that start with "D" as well.

[n6346m]

What is the spar thickness of the F-21?

[astjp2]

Contact Sutton aircraft salvage 907-715-6998 and see if he will measure the ones from the wreck he just brought in. Tim

[Jim Herpst]

Interesting discussion. Remember that the original spars were spruce. So... the current spars one can purchase from Wag Aero are Douglas fur.. heavier but stronger...how much I wonder do they help with additional gross weight?

[astjp2]

15% heavier, 15% stronger but you are not allowed to reduce size unless engineered and approved to do so. Also no gross weight increase either. If you get an engineer to approve it, anything is possible.

[woodmw]

How much does a set of spars weigh?

[Jim Herpst]

Not looking for official STC approval. I know insurance and legality of over gross..it would be interesting to figure what the limits would be from a piece of mind stand point..I will let someone else load a set of new set fir wings to the breaking point in tests....we already have a good margin...nice to know when we are slammed about in thermals.

[Hank Jarrett]

Doug Fir and Stitca Spruce have different strength AND different weight (actually density) The differences ARE NOT consistent! Unlike metals wood is not the same everywhere in the tree and different pieces have different weights per cubic inch and different strength depending on the tree and even which end of the board you measure. The design numbers have changed with time since recently harvested wood is weaker than wood from the 30s or 40s. Aircraft wood is still selected to get pieces that meet the design numbers and there are restrictions on how many growth rings per inch are required. Any aircraft grade wood "should" be stronger than the standards used for design.
As for which is stronger and heavier for our uses Fir is STRONGER in bending per square inch than Spruce. It is also HEAVIER per cubic inch than spruce (notice the different units of measure....square inches for strength and cubic for density).

To simplify, if you take a Spruce spar and Fir spar of the SAME DIMENSIONS, the Fir will be HEAVIER AND STRONGER.
If you make a Spruce spar and compare to a Fir spar of the same WEIGHT (smaller dimension Fir) the Spruce spar will be STRONGER and LARGER
If you make two spars of the same STRENGTH the Spruce spar will be LARGER and weigh less.

Spruce has a higher strength to weight ratio. Fir has a higher strength to VOLUME ratio. If you want the lightest spar, use Spruce (if you have room for it). If you are short of room for the spar, Fir will let you fit the same strength spar in a smaller space. The differences are NOT huge for our purposes and what is a major factor for us is cost. Fir is a LOT less expensive than Stitca Spruce! If you build a spar from a good piece of Fir THE SAME DIMENSIONS as was used originally it will be stronger and heavier, but not enough to be a huge problem. I calculated it one time years ago and it was only a few pounds for all four spars. I decided to go on a diet. I feel better and the take off weight will be even lower plus I will probably live longer.

Hank

[Jim Herpst]

Understand all the info you have brought out Hank. All this newer stuff folks pile on their airframes does make a difference. Spars are now avail from Wag Aero in Fir. ya only : "a few " pounds more, then we have the ne (" thicker and probably stronger struts "struts -and only a little more weight,.. then of course to finish off that restoration we need airtex interior or the like with door panels, Kick panels ,carpet only a few pounds more...then there is that high solids shiny paint ...just a few ponds more perhaps then heaven forbid we add the large baggage area just a little more ..thicker side windows, cannot forget al the flight gear we need (not) to take with us for a trip around the county shees we have added A LOT of weight in the end not to mention as you have the largest item OURSELVES bigger and unfortunately again the averages here fatter (I drive this average up!) sure leads me anyway more committed that ever to discipline my self in what goes in the plane under restoration and well above all else myself!

I know a light BC12 D flies so well .Really looking forward to it being light with the needle bearings in all the pulleys and yes just a little A65 pulling it !

[Hank Jarrett]

Jim, I know what you mean. I am really pretty careful about ANY weight. When I was building free flight models we said always think in grams to save pounds. One more lick with the sand paper would add seconds to the flight.
There were only a few areas where I just gave up on saving weight. One was after trying to find original spec 2 ply tires. I don't think they exist any more! Struts we were pretty much forced into (thanks a lot Harry, you jerk). I was REALLY careful with my interior to pick really light materials (the upholsterer must have thought I was crazy). Ounces add up. The single biggest way I found to save weight was to trim myself. Took off 25 pounds and not only is my take off weight down but my knees thank me. When I recover the 41 (when the 45 is flying, and who knows when that will happen) I hope to drop some significant weight. There is a LOT of dope on the 41!
I even go so far as to take the battery out of the box when I won't be needing it (which is most of the time) and not taking up the extra headsets when I am alone. I also like the wood prop. Slower, but I'm not in a hurry anyway. ;-)

Oh yea, can't remember the last time I put fuel in the wing tank! I need a bathroom long before I need fuel even with just 12 gallons!

Hank

[3Dreaming]

There were only a few areas where I just gave up on saving weight. One was after trying to find original spec 2 ply tires.

Hank

Just a note on tires. When our airplanes were built, tires were rated in plies. The more plies, the stronger the tire. Quite some time ago aircraft tires switched from plies to ply rating. A 4 ply rating tire may only have two plies, but with the newer stronger material it is as strong as an old 4 ply tire. If you choose wisely you can find a fairly light 4 ply rated tire that is fairly light weight. You can save even more weight by having it polished smooth, but I wouldn't suggest that unless you will be primarily on grass.