Alternatives to spruce in wooden aircraft construction?

[John wadman]

I always thought that the grain in spars/spar caps should run parallel to the ground too. At Sun n Fun I was looking at a kit aircraft and the spars were running with the grain vertical to the ground and I questioned the representative. He told me that was the correct orientation. On the plans to my first aircraft the cross sectional drawing of the spars showed the spar caps (webbed spars) with the grain running horizontal. I asked two other designers/suppliers of wood kit aircraft via email and neither of them answered my question. I got on a forum and asked the same question and what an argument that started! Then I got on the website of a very popular aerobatic biplane manufacturer and looked at photos of spar tests and many of the spars were laminated of 1/4" thick pieces but some of the tests showed spars laminated with layers of 1/4" with the laminations in the horizontal to ground plane BUT the individual pieces in the laminated spar had the grain lines running vertical. When I tested 3/4" x 3/4" pieces to failure, some solid, some scarfed, some laminated, I tested them in every configuration and found that the pieces made of three 1/4" layers glued in the horizontal but with the grain running vertical to the ground in each stick were right up there as far as strength as the ones that had three layers in the horizontal with grain also in the horizontal. What I don't know is bending strength. I may be wrong but in theory it seems to me that the best would be the later, horizontal laminates of horizontal grain as far as the wings being able to flex and bend. I'm not an aeronautical engineer though so i have to rely on what the pros say. I guess the hardest thing is to figure out just who the real pros are!

 

[PTAirco]

I know there is an N.A.C.A report on the subject, bu I haven't found it yet. I had some wartime spars of a Fairchild 24 that were made of vertical laminations, but I don't recall the grain orientation on those.

For general advice on wood construction, find a copy of ANC-18.

 

[mcrae0104]

ANC-19 has a section on vertical laminations following the snip I posted yesterday (2.401).

 

[SpaceRat]

I was gifted enough clear red maple wood (as it turns out, red maple wood is actually white - I guess it's the leaves that are red).
Anyway, I decided to do a bit of research to determine if this wood would be strong enough to built an airplane... and I came across a very interesting pamphlet from 1950 - "Aircraft Woods: Their Properties, Selection, and Characteristics" in PDF format. Wow, what a treasure trove of information!
As it turns out, this type of maple shares a lot of the same characteristics of Douglass Fir as far as weight and strength. I checked with the folks selling the plans, and he agreed, it would make a strong fuselage and wing ribs.
I'm going to build some structures typical in the airplane and stress test to destruction - Ought to be educational!
By the way, I say I was "gifted" this wood - I actually pulled it out of a burn pile moments before they were going to set it on fire !
What is your opinion on using maple instead of fir or spruce?

 

[SpaceRat]

By the way, if any of you want a copy of the above mentioned pamphlet, I will be happy to send to your email.

 

[Brünner]

By the way, if any of you want a copy of the above mentioned pamphlet, I will be happy to send to your email.

Thank you SR, would it be possible to upload it here for everyone? If not let me know and I'll shoot you a PM.

 

[mcrae0104]

Pound-for-pound, it appears that maple has 95% of the strength of spruce. If the member sizes are designed appropriately, the maple airframe would only be marginally heavier to meet the same load requirements. The "gotcha" that may play into this is that many parts of an airplane are already quite slender, and you may find that buckling or practical constructabily will dictate minimum sizes that do not allow you to fully exploit the higher flexural strength of the maple.

If, instead of modifying the design to take advantage of the maple's higher strength, and just build per plans, it will be 68% heavier than spruce (if that is what the designer intended) and and significantly stronger than it needs to be--probably not a great option to go this way.

Probably the ideal thing to do would be to take advantage of the free material anywhere that "minimum gauge" does not carry a penalty--i.e. spar caps and relatively larger member sections--and stick with the designer's intended species for the rest.

https://ntrs.nasa.gov/api/citations/19930091423/downloads/19930091423.pdf

 

[103]

I was gifted enough clear red maple wood (as it turns out, red maple wood is actually white - I guess it's the leaves that are red).
Anyway, I decided to do a bit of research to determine if this wood would be strong enough to built an airplane... and I came across a very interesting pamphlet from 1950 - "Aircraft Woods: Their Properties, Selection, and Characteristics" in PDF format. Wow, what a treasure trove of information!
As it turns out, this type of maple shares a lot of the same characteristics of Douglass Fir as far as weight and strength. I checked with the folks selling the plans, and he agreed, it would make a strong fuselage and wing ribs.
I'm going to build some structures typical in the airplane and stress test to destruction - Ought to be educational!
By the way, I say I was "gifted" this wood - I actually pulled it out of a burn pile moments before they were going to set it on fire !
What is your opinion on using maple instead of fir or spruce?View attachment 106654 View attachment 106655

Wooden Aviation- Home is promoting ash in structural areas. Weight penalty can be taken or mitigated with traditional engineering and test.

 

[blane.c]

https://www.homebuiltairplanes.com/forums/search/257939/

 

[ragflyer]

On grain direction, having the edge grain be parallel to ground is driven not by strength (directly) but shrinkage/swelling due to change in moisture. Wood will shrink most tangential to growrh rings. This means a spar aligned with the edge grain parallel to the ground will shrink in thickness while the other way around it will shrink in depth of spar. As the bending strength of the spar is very sensitive to depth( varies by cube) you want to make sure the edge grain is parallel to the ground.

 

[TFF]

I don’t think maple is good for bending. I would skip making ribs from it. It is good for blocking, usually preferred. Depending on size and hardness, you could laminate it together and make a nice prop out of it.

 

[GeeZee]

Ok, Pops has me considering fisher flying products geodetic wood construction. Doing some research I discovered FFP is putting out weekly YouTube vids on various topics related to their aircraft. This one talks about their decision to replace the Sitka spruce spars on the FP202 with northern white pine. The wood discussion starts at about the 4 minute mark. They hired an engineer to run the numbers for most of the wood that’s been considered a viable substitute for Sitka. Then they redid the spar calculations using the norther white pine. Looks like less than 1% loss of strength. They have an earlier vid that shows someone loading various wood species samples until failure. I thought the description of the failures were interesting. For example they noted that when Douglass Fir failed it was “explosive” with complete separation of the pieces and shards of wood flying out. That certainly made me rethink Douglass fir as the best substitute.

 

[Jimboagogo]

Ok, Pops has me considering fisher flying products geodetic wood construction. Doing some research I discovered FFP is putting out weekly YouTube vids on various topics related to their aircraft. This one talks about their decision to replace the Sitka spruce spars on the FP202 with northern white pine. The wood discussion starts at about the 4 minute mark. They hired an engineer to run the numbers for most of the wood that’s been considered a viable substitute for Sitka. Then they redid the spar calculations using the norther white pine. Looks like less than 1% loss of strength. They have an earlier vid that shows someone loading various wood species samples until failure. I thought the description of the failures were interesting. For example they noted that when Douglass Fir failed it was “explosive” with complete separation of the pieces and shards of wood flying out. That certainly made me rethink Douglass fir as the best substitute.

I viewed the FFP video, and interestingly enough I am in the process of preparing a slide presentation on use of wood for aircraft for our EAA chapter meeting. Looks like this is a timeless topic! LAA in the UK published their recommendations for Sitka substitutes as did the NACA and there are many alternatives. Sitka is not magic and as you have noted, only marginally better than some alternatives. I'll note that the Cavalier line sa 102.5 and variants all use white ash as it is stronger than sitka. Peter Sripol is building his ultralights/microlights with polar spars, and at this point, at least in Canada, the price of clear sitka really isn't worth the effort of tracking down, in my opinion. The NACA study looked at dozens of species of hardwoods and dozens of softwoods and they concluded that you can use at least 15 of those for wood aircraft.
As for weight penalty, unless you are engaged in mass production and can afford to throw away all the aircraft grade sitka at the top of the density range (more growth rings per cm) and only accepting the lightest sitka, then you are paying a weight penalty anyway. Improperly cleaning all the glue drips adds up as do a thousand other decisions. Accept that you might come out a kg or two over the ideal material, and that this is equivalent to a couple of radios. Plans for wood aircraft were pretty much all drawn decades ago when radios weighed 2kg each, today a new transceiver comes in at 400 g. Modern fabrics are lighter as well and I imagine most of us pilots could stand to lose 4-5 kg like nothing and that would improve the performance of any aircraft we fly. Don't be concerned about the spar weight if you want to stay sane.

 

[fly2kads]

They have an earlier vid that shows someone loading various wood species samples until failure. I thought the description of the failures were interesting. For example they noted that when Douglass Fir failed it was “explosive” with complete separation of the pieces and shards of wood flying out. That certainly made me rethink Douglass fir as the best substitute.

I wouldn't get too worried about differences in failure modes between different wood species. They're all going to fracture. Heck, carbon fiber fails catastrophically, too, and it is used in spars all the time. Here's a representative stress-strain diagram from ANC-18:

This is for bending, as you would get in a wing spar. The spar will be sized to hold the limit load below the point marked as the proportional limit. The ultimate limit will normally be 1.5 times that amount. For most wood species, the point of maximum load is roughly 2 times the proportional limit. This means that the spar's breaking point will more than likely be well above even the aircraft's ultimate limit. Since douglas fir is stronger than spruce in the first place, building the spar to the dimensions given for spruce will give an even larger reserve factor.

 

[John J]

Hi!
I just retired from a 42 year career as a framing carpenter. My main goal in life now is to build an aircraft. I've had a set of plans for Bud Evan's VP1 but have also considered scratch building a RV3 because I thought 2024 aluminum would make for a stronger airframe.
I live in Louisiana and we had a once in a hundred year snow and ice event in February. Six inches of powder snow followed up with seven inches of ice pellets. Hundreds of flat roof structures collapsed. Most of the failures we're on steel buildings and not wood framed roof systems.
The wood frames deflected and the metal frames snapped. It was upon making this observation that I finally decided to build the VP1. I have cut my bulkheads and I am seeking alternatives to Sitka spruce.
I personally don't want to utilize lumber from a 500 year old tree. My plan is to use Douglas fir for the longerons and stiffeners.
Down here ,we use yellow pine for rafters up to 20 feet and then move to d.fir for 24-28 feet lengths. My plan is to hand pick a couple of 2x6 at the local lumber yard and rip the pieces that I need.
My spars will be built from Sitka spruce because of it's flexibility.
Aviation grade lumber and even marine grade d.fir plywood is unavailable locally. Gathering materials is going to be my biggest challenge for my project.
My fuselage will be slightly heavier but I wiegh about 160 on a good day and I feel like I can stay under gross weight.

 

[BJC]

because I thought 2024 aluminum would make for a stronger airframe.

Either material is suitable for an airplane structure. A proper design uses the material in a way that keeps the maximum stresses, plus a safety factor, below the values that result in significant deformation or failure.

There is lots of good advice here at HBA about choice of materials. Learn to use the search function. A summation: after consideration of environmental factors (salt water corrosion, high humidity, etc.) decide which materials you would like to build with.

Welcome to HBA.


BJC

 

[TFF]

When it comes to airplanes, one material is not better than another. It’s how good the engineer is. An RV3 is much tougher than a VP1 but they are also so different on philosophy. One not better than the other. What part of Louisiana are you in?

 

[Pops]

I have built 3 sets of geodetic wings. On each I used spruce for the spar (JMR) and spar caps (SSSC) and yellow poplar for the geodetic strips on all the wings.
Local high end wood molding mill has an outlet store and will let you pick out the boards you want. They sale yellow poplar, oak, cherry, walnut. I usually buy 1"x 8" x 8-12' long boards and cut to size on my table saw. Prices are very good. When I was building my house I bought all of the molding from there.

 

[John J]

When it comes to airplanes, one material is not better than another. It’s how good the engineer is. An RV3 is much tougher than a VP1 but they are also so different on philosophy. One not better than the other. What part of Louisiana are you in?

I live north of Shreveport near the Arkansas state line.
I totally agree with you. Two totally different types of single seat aircraft. I get an adrenaline rush whenever I see an rv3 but it may be too quick for me and I think it would cost about 40k to build.
The VP1 should cost between 10and20k and I would end up with a plane that I could keep up with.
All my time is in Cessna 172's, so I'm not sure if I am ready for a 200 mph high performance airplane.
I do get tired of flying a four seat aircraft that weighs about a ton . I think a single seat light airplane would be much more fun for the occasional100$ hamburger run.

Either material is suitable for an airplane structure. A proper design uses the material in a way that keeps the maximum stresses, plus a safety factor, below the values that result in significant deformation or failure.

There is lots of good advice here at HBA about choice of materials. Learn to use the search function. A summation: after consideration of environmental factors (salt water corrosion, high humidity, etc.) decide which materials you would like to build with.

Welcome to HBA.


BJC

Thanks for the welcome!
In Louisiana, 2024 would be the logical material to work with. We have high humdity, moisture and rot. I have looked at the Hummel Ultra cruiser (6061)
as an alternative to a rv3 but I believe the VP1 to be a more well designed plane.
My VP1 will stay indoors. I rode Harley Davidson motorcycles for years and kept them in the house. My bikes were always shining and without the least bit of corrosion even on the machine screw heads.
I'm excited to be building a plane. My friends and family don't understand.
It's not about flying so much as it is an intellectual and craftsmanship challenge I can accomplish.

 

[TFF]

Not as much as a looker but the Teenie Too is the grand dad of the Hummels and is probably a little tougher. As for wood, something like a Corby Starlet would be close to RV.