Birch vs Poplar plywood vs Balsa wood sheet

[woundedbear]

One thing I have been meaning to ask. How does Poplar plywood stack up to Birch plywood for model airplane construction. I know that Birch is for firewalls, but what about Poplar plywood (AKA lite ply) for Formers and wing ribs for enlarged Guillows builds. I know that Bill Parker has done some large and incharge builds from Guillow's plans, Bill what do you think of this so called "lite ply" ? You have done a lot of enlarged Guillows builds, if this lite plywood is cut to shape and then preforated. Can it be better than just using medium weight (9 to 12 pound) balsa wood sheet? I know this sounds like another one of my "receding hair brained ideas", and I may have ask this question already But I have been thinking about some future build with plans enlarged up from 1/16 scale from 1/8 scale that would include 6 channels with retractable landing gear and flaps I have also posted a question about carbon fiber tubing. How would carbon fiber tubing stack up agents aluminum tubing for scale oleo struts Not that I'm up for such a build yet but when I've done some basic 4 channel builds I would like to try my hand at something bigger in 1/8 scale and think a Guillow's plan suteiblly modified could work Maybe or Maybe not Any thoughts?

[Bill Gaylord]

I try to avoid using anything heavier than hard balsa grades, unless I really have to. Hardwoods weigh. For electrics, the firewall is one of the few uses I have for ply, where I'll often use only a partial ply firewall, laminated over hard balsa, only for the motor mounting area. Similar notion for servo mounting, where you don't need a heavy, ply tray. Maybe a few ply plates only for the mounting screw area, but even then, two layers of glue laminated hard balsa will provide a strong threading surface. When you get into larger sizes there are more uses for hardwood, such as basswood which is not terribly heavy. Efficient designs can eliminate much of the heavier wood however, and still be strong. One area I have become a fan of hardwood, is for wing LEs. In that case, 1/8" dowel rod still isn't much heavier, if any heavier, than a sculpted balsa LE. The rod creates a consistent LE, also adding robustness in a critical area. I used 1/8" dowel rod for the wing LE in a Comet C150 that I just finished yesterday. The wing struts are also hardwood, but the 8-3/4 oz AUW with battery is still reasonable, as those two areas are about the only areas that use heavy wood, other than a the motor mount. Note the wing main spars are nearly contest grade, as you won't get these models light using ironwood.

[woundedbear]

Thanks for your reply Bill, That's one more sharp Cessna you've built there. Would the Guillow's 800 series Cessna 172 Sky Hawk make a good first time 4 channel conversion?

[RocketRobin]

I never use ply. I use ply composite.

1/64" birch ply is in fact 5 layers of hardwood. If you use 1/64" + balsa core + 1/64" ply you get an eleven layer composite that is very strong. The thicker the core, the stronger it gets.
This "ole po' boy" composite is even stronger than CF plate by weight.

If you use this type of structure for a motor mount/firewall, add basswood compression blocks (end grain cut) to the balsa core where the blind nuts will attach. If you're lacking end grain cut basswood, wooden furniture legs could work. Don't tell Mom!

To make that stuff you need some high tech stuff called aliphatic resin. Also known as white glue, carpenter's glue, Elmer's glue, space age wood glue, what ever happened to that nice old horse in the pasture down the road that we fed apples to on Saturday mornings, etc.
And... some books with waxed paper to lay down on top of your composite structure, and a day, or so. to dry to dry. And a prayer for "Good Old Silver" for making strong glue and never once biting off your fingers even once when you fed him apples.

What is the world coming to when we have to use radical modern technologies developed back in the early 20th century just to build a proper model airplane in the 21st century? When I was a young man back in the 11th century, BC by the way, all you needed to make a good plane was a rock, a stick and a leather strap. Those were the heady days of model airplane building. The full scale versions didn't fly either and cost many lives during the scheduled airshows.

Cheers!
Rocket

[Bill Gaylord]

Thanks for your reply Bill, That's one more sharp Cessna you've built there. Would the Guillow's 800 series Cessna 172 Sky Hawk make a good first time 4 channel conversion?

I have one. It's not a bad flyer, but flies more like a sport plane than a trainer. The wing construction requires a bit of extra effort to build without warps, versus a wing with a single piece main spar and leading edge. The molded cabin is actually quite strong, where the rubber band attachment is not necessary. I reinforced the strut mounts, and use them to hold the wing down.

[David Lewis]

The strength-to-weight ratio of Lite Ply is lower than birch ply, and much lower than balsa. That is the biggest drawback. However, it is cheap and good for concentrated bearing loads, e.g. blind nuts, wood screws, motor mount and strut attachments.

Plywood should be considered over plain wood when you want a material that is strong in all directions. Plain wood is only strong when the load is parallel to the grain. The load carrying ability perpendicular to the grain (such as the top and bottom of fuselage formers) is low.