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Airframe weight: foamboard vs balsa/monokote?

Monte.C

Well-known member
#1
It's an unanswered question that comes up for me now & again. Sure there's a lot of different types of designs that we build from foamboard - some heavier than others - but in the most general terms how does the airframe weight compare to something comparable in balsa/monokote?

I'm designing a little foamboard trainer inspired by the Lazy-Bee or Speedy-Bee (which I'm sure was a lot of the inspiration for Josh's Sportster).
Sure a balsa skeleton is stupid light, but it includes plywood components and stuff...
And I know we have loads more power available than we need for these planes, but for this one I'm wanting it to just float. I'm building it from foam either way, but I still want to know the weight comparison.

It might be something that scales funny too. Like, for a mini the weight to wingspan ratio might increase as you decrease plane size. I have a feeling FB scales this ratio harder than balsa.
Anyone have any thoughts on the topic?
Thanks.
 

Hai-Lee

Old and Bold RC PILOT
#2
It's an unanswered question that comes up for me now & again. Sure there's a lot of different types of designs that we build from foamboard - some heavier than others - but in the most general terms how does the airframe weight compare to something comparable in balsa/monokote?

I'm designing a little foamboard trainer inspired by the Lazy-Bee or Speedy-Bee (which I'm sure was a lot of the inspiration for Josh's Sportster).
Sure a balsa skeleton is stupid light, but it includes plywood components and stuff...
And I know we have loads more power available than we need for these planes, but for this one I'm wanting it to just float. I'm building it from foam either way, but I still want to know the weight comparison.

It might be something that scales funny too. Like, for a mini the weight to wingspan ratio might increase as you decrease plane size. I have a feeling FB scales this ratio harder than balsa.
Anyone have any thoughts on the topic?
Thanks.
Balsa is good and so is FB! A combination of both is also something worth investigating. Structure and structural loads are the key.

I often use the example of comparing the Hurricane and the spitfire! The hurricane has a strong structure where the skeleton is required to handle all of the forces and stresses of flight whereas the Spitfire actually used a lighter internal structure and the Stressed metal skin carried a large proportion of the flight loads and stresses. Both did the job for which they were designed very well.

The only real advantage of the Balsa is that it can be obtained in a wide variety of thicknesses and so the weight can be managed far easier than the FB which has a limited number of sizes. In addition FB is considerably heavier with its dense paper covering.

Balsa can be strong and light BUT it can fracture easily and even have the fractures be undetectable without disassembly. FB has great resistance to breakage but it is far less rigid. If you combine the properties of both Balsa and the FB in the same build you end up with a lightweight flying tank!

Just what works for me!
 

Monte.C

Well-known member
#3
Balsa is good and so is FB! A combination of both is also something worth investigating. Structure and structural loads are the key.

I often use the example of comparing the Hurricane and the spitfire! The hurricane has a strong structure where the skeleton is required to handle all of the forces and stresses of flight whereas the Spitfire actually used a lighter internal structure and the Stressed metal skin carried a large proportion of the flight loads and stresses. Both did the job for which they were designed very well.

The only real advantage of the Balsa is that it can be obtained in a wide variety of thicknesses and so the weight can be managed far easier than the FB which has a limited number of sizes. In addition FB is considerably heavier with its dense paper covering.

Balsa can be strong and light BUT it can fracture easily and even have the fractures be undetectable without disassembly. FB has great resistance to breakage but it is far less rigid. If you combine the properties of both Balsa and the FB in the same build you end up with a lightweight flying tank!

Just what works for me!
Dude you've been around! I understand about balsa builds, but I don't have 1/10th the experience you do.
I do what I can to lighten my FB builds as much as possible. (I'm new to flight and trying to build slow and pretty small.) It's my understanding that removing one side of paper reduces the weight by 30%+. Or was that both sides of paper?
In the end both FB and balsa are darn light, but I still wonder which is lighter. I bet the slope soarer guys would prefer balsa...
Thanks for the input!
 

Hai-Lee

Old and Bold RC PILOT
#4
Dude you've been around! I understand about balsa builds, but I don't have 1/10th the experience you do.
I do what I can to lighten my FB builds as much as possible. (I'm new to flight and trying to build slow and pretty small.) It's my understanding that removing one side of paper reduces the weight by 30%+. Or was that both sides of paper?
In the end both FB and balsa are darn light, but I still wonder which is lighter. I bet the slope soarer guys would prefer balsa...
Thanks for the input!
The weight ratio is approximately 30% PER paper side. There is a slight variation in some FBs where there is a dress side and a rough side.
As for the lightening you could try using balsa internally for bulkhead replacement or even as the wing spar.
Here is an early experiment of using both Balsa and FB together in the building of a FB Spitfire. The only damage to date is a cartwheel which ripped the wing from the fusleage and the covering has sagged a little. A little glue and the bird is still flying. (I gave it away as I got bored with it). https://forum.flitetest.com/index.php?threads/balsa-foamboard-test-build-ft-spitfire.32734/
It may give you some insight into techniques though it is an early version and further improvement is possible. The paperless fuselage was reinforced with 1mm FB and the whole bird finished in covering film!

Have fun!
 

Monte.C

Well-known member
#5
The weight ratio is approximately 30% PER paper side. There is a slight variation in some FBs where there is a dress side and a rough side.
As for the lightening you could try using balsa internally for bulkhead replacement or even as the wing spar.
Here is an early experiment of using both Balsa and FB together in the building of a FB Spitfire. The only damage to date is a cartwheel which ripped the wing from the fusleage and the covering has sagged a little. A little glue and the bird is still flying. (I gave it away as I got bored with it). https://forum.flitetest.com/index.php?threads/balsa-foamboard-test-build-ft-spitfire.32734/
It may give you some insight into techniques though it is an early version and further improvement is possible. The paperless fuselage was reinforced with 1mm FB and the whole bird finished in covering film!

Have fun!
Dude.

Man oh man. Now I've got some interesting reading to do! I won't be getting myself into a balsa build, but you're a man who does his research! I always toy with the idea of "lightening panels" in my wings & fuse - which could serve a purpose for some sort of ultralight like a slope soarer I suppose. It would be to remove circles of material from flat portions like you did. But then I'd need to cover with monokote or something. So then instead you could remove the material but leave the outside skin of paper intact. But then I imagine you'd want to reinforce that paper panel somehow, like by gluing another piece of paper to the inside of it. Now we're adding the glue and reinforcement paper. For me it real quick gets to be too complicated for what might be a negligible weight loss and significant reduction in strength/rigidity. Gee wiz. And then I tell myself to stop breaking my head over this, stop being a knucklehead, build something simple and FLY IT. Whatever you build it's infinitely cooler than playing video games.
 

Hai-Lee

Old and Bold RC PILOT
#6
Dude.

Man oh man. Now I've got some interesting reading to do! I won't be getting myself into a balsa build, but you're a man who does his research! I always toy with the idea of "lightening panels" in my wings & fuse - which could serve a purpose for some sort of ultralight like a slope soarer I suppose. It would be to remove circles of material from flat portions like you did. But then I'd need to cover with monokote or something. So then instead you could remove the material but leave the outside skin of paper intact. But then I imagine you'd want to reinforce that paper panel somehow, like by gluing another piece of paper to the inside of it. Now we're adding the glue and reinforcement paper. For me it real quick gets to be too complicated for what might be a negligible weight loss and significant reduction in strength/rigidity. Gee wiz. And then I tell myself to stop breaking my head over this, stop being a knucklehead, build something simple and FLY IT. Whatever you build it's infinitely cooler than playing video games.
You do not need to add paper and glue if you select what is to be removed and what is to remain!
If the plan is to build light to the same strength the weight savings can be large. My effort was far too strong than required and it was a learning exercise only.

The correct covering is lighter than the paper ever was and it removes the need ot water proof and paint, (all of which add weight).

Just remember the basic principles of frame strength Vs shell strength and to consider the lines and direction of stress and whatever you build, (regardless of the materials you chose) will be amazing!

have fun and never be afraid to experiment!
 

Merv

Well-known member
#7
I do what I can to lighten my FB builds as much as possible.
I agree with @Hai-Lee, removing the paper will make a lighter plane, especially if you are using the heavy fb. Adams & Ross foamboard are lightweight to begin, not much of a need to lighten them. Removing the paper form the other, heaver brands of fb will be a huge benefit.

Using less glue will also lighten a plane. Hot glue is heavy, my goal is no more than one stick of glue per plane.
 
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rockyboy

Skill Collector
Mentor
#8
I agree with @Hai-Lee, removing the paper will make a lighter plane, especially if you are using the heavy fb. Adams & Ross foamboard are lightweight to begin, not much of a need to lighten them. Removing the paper form the other, heaver brands of fb will be a huge benefit.

Using less glue will also lighten a plane. Hot glue is heavy, my goal is no more than one stick of glue per plane.
Another way to reduce glue weight is switch from hot glue to White Gorilla Glue for most of the assembly. Way lighter, just as strong, and as a bonus, it won't sag and let your plane fall apart on a hot day!
 

Monte.C

Well-known member
#9
Another way to reduce glue weight is switch from hot glue to White Gorilla Glue for most of the assembly. Way lighter, just as strong, and as a bonus, it won't sag and let your plane fall apart on a hot day!
You bet. My builds lately are getting no hot glue at all. Not just about weight - it's just such a bother to do this with hot glue.
I was still wondering the weight comparison of a balsa build vs FB though...
Anyway I'm finding these builds to be stupid light. I guess that's all that matters.
 

Piotrsko

Well-known member
#10
A proper balsa build can be as light as a penny, aka pennyplane which is an indoor class. Haven't seen an equivalent FB design. Have seen carbon fiber pennyplanes.

A 2 meter slope glider will tend to be heavier in balsa because FB does stressed skins easier. IF you do stressed balsa skins, it will be much stronger. Back in the day, you built only one balsa covered Ace wing for a dozen airplane destroying crashes.

There is no one right answer to this since it depends on what and how you are building.
 

Monte.C

Well-known member
#11
A proper balsa build can be as light as a penny, aka pennyplane which is an indoor class. Haven't seen an equivalent FB design. Have seen carbon fiber pennyplanes.

A 2 meter slope glider will tend to be heavier in balsa because FB does stressed skins easier. IF you do stressed balsa skins, it will be much stronger. Back in the day, you built only one balsa covered Ace wing for a dozen airplane destroying crashes.

There is no one right answer to this since it depends on what and how you are building.
Nailed it. Gotcha. Completely understood. Thank you @Piotrsko!
 

Jackson T

Well-known member
#12
My 4 metre wingspan balsa glider's wing weighs about 805 grams (29 oz), and it has a wing area of 93 dm2 (1441 square inches). To make a wing that size out of foamboard FT style, without any extra reinforcement, would probably weigh in the vicinity of 500-600grams (18-21 oz). Obviously it would most likely need extra reinforcement, making it heavier. Another balsa glider I built a while back has the same wingspan as the Simple Soarer, but my balsa glider weighed 240grams without a battery, while the SS, according to the FT website, weighs 308 grams (11 oz) without a battery. I guess the second example is pretty good proof that balsa can be built lighter than foamboard if built right. In saying that, I'm currently building a balsa Pilatus PC 21 that's turning out to be a bit of a brick. It would probably be lighter if I built it out of foamboard or if I "built it right" :p.
 

Monte.C

Well-known member
#13
My 4 metre wingspan balsa glider's wing weighs about 805 grams (29 oz), and it has a wing area of 93 dm2 (1441 square inches). To make a wing that size out of foamboard FT style, without any extra reinforcement, would probably weigh in the vicinity of 500-600grams (18-21 oz). Obviously it would most likely need extra reinforcement, making it heavier. Another balsa glider I built a while back has the same wingspan as the Simple Soarer, but my balsa glider weighed 240grams without a battery, while the SS, according to the FT website, weighs 308 grams (11 oz) without a battery. I guess the second example is pretty good proof that balsa can be built lighter than foamboard if built right. In saying that, I'm currently building a balsa Pilatus PC 21 that's turning out to be a bit of a brick. It would probably be lighter if I built it out of foamboard or if I "built it right" :p.
Gotcha @Jackson T. I think the answer I'm hearing is "Comparable, but can vary significantly depending on a range of factors." You and @Piotrsko have really clarified this for me. Many thanks.
 

Bricks

Well-known member
#15
Talk about building lite one of the most memorable planes I have seen in person looked like a Gentle Lady it was a balsa build had roughly an 84 inch wingspan powered by a 2812 1100 KV motor and all up weight I don`t think it hit 3.5 pounds. Deffinetlly not a windy day flier it was impressive the shear size for the weight.