First Scratch Build

Minton

New member
About a month and a half ago I ordered the FT Trainer kit along with everything else I needed to get started with RC aircraft. I had such a blast with it that I decided to try and design and build my very own plane. I'm new to flying planes but had previously modeled an RC plane for my school's Design Build Fly club, so I already had lots of experience with CAD software and the structural aspect of designing planes.

unknown.jpg


However, we ran out of time and never got to fly that one so I never really found out if I was capable of designing an airworthy plane. Compared to that 8-foot span, 20-pound monster, my little 4-foot trainer was pretty easy to design.

unknown-19.jpg


For the previous plane, I didn't have any part in the aerodynamic aspects of the design, so doing that for my own plane was a bit intimidating. I basically just designed the wing first, then figured out what the proportions of everything else had to be to match.

Since I don't own a laser cutter, I decided to hand cut all of the parts out of balsa with an Exacto knife. To ensure that I cut them accurately, I first made paper templates of all the parts and then used them as a guide when cutting the balsa. In case you were wondering, this process took about 2 weeks and was extremely tedious. I would not recommend doing this unless you have no other choice.

20220705_153506.jpg
20220710_001232.jpg
20220714_224426.jpg


To my surprise, the parts came out rather nicely and required minimal sanding to fit together. The biggest issue that I encountered during this process was that I discovered part way through that the 4"x36" sheets of balsa that I bought from Hobby Lobby were rather inconsistent. Some were extremely dense and felt almost like plywood, while others felt more like foam. Knowing this, I tried to cut any load-bearing parts from the harder balsa, and use the softer stuff for wing ribs and whatnot.

After all the parts were made, the plane came together in about a week. I decided to use 1/16" balsa sheets for the wing surface instead of Monokote because I don't have an iron and thought it would be easier. It turned out to be rather tedious but it gave the wing an incredible amount of strength. The wingtips are rather delicate due to how I designed the dihedral, but the straight part of the wing is so sturdy that you could probably beat somebody to death with it.

20220716_191734.jpg
20220717_141205.jpg
20220717_145835.jpg
20220719_194156.jpg
20220720_171641.jpg
20220721_220554.jpg
20220721_220617.jpg


Unfortunately, I discovered that the CG was very far back, nearly at the trailing edge of the wing. I could have avoided this by checking the CG in Solidworks before finalizing the design but as you'll come to see, I'm a bit impatient when it comes to double-checking things. I tried to counteract this by adding about a half pound of random stuff into the nose, which was a lot considering that the plane only weighed about 1.5 pounds prior. The CG moved to the front half of the wing which I deemed to be good enough, thinking that even if it was a little far back I could counteract it easily enough. As it turns out, it wasn't good enough. The plane took off and immediately nosed up. I tried to push it back down which caused it to start turning. At that point, I decided to try and land it as quickly as possible. I throttled down and tried to set it down but it was flying too slow and it rolled over and nosedived into the ground from about 4 feet up.

20220722_111746.jpg


Luckily, the only things that were damaged were the left wingtip and the nose. The ribs in the wingtip were all completely destroyed, but all the spars and the root of the wing were completely fine. All the panels on the nose were gone, and the nose itself was a bit mangled, but nothing too serious. A week later, I had everything repaired and ready to fly again.

20220727_135625.jpg


This time I took the extra 2 minutes to google where the CG should be, and then mark it on the wing. I ended up having to move the wing back about 2.5 inches to get the CG in the right place, but that's just the price for not planning thoroughly.

For the first flight, I attempted to fly on the golf course that is right behind my house. It's quite long but a bit narrow and has trees and houses lining either side. Following the pattern of doing the bare minimum, I found a nice big park on google maps to fly at for my second flight. However, once I showed up I realized that there were power lines going over one-half of the field, and huge light poles all over the other half. I found an area that seemed big enough to fly and went for it.

Screenshot_20220729-103637_Video_Player.jpg


This time it flew wonderfully! However, I quickly realized that it flew rather fast and I didn't really feel comfortable flying it with the space I had so I turned around and came in for a landing. Unfortunately, the grass was a bit long and the approach was a bit steep so I ended up ripping the landing gear off and destroying the bottom panel of the fuselage.

That last flight was yesterday, and I plan on repairing and flying it again but I'm going to wait until I'm back at school first. My college is located in Middle of Nowhere, California, and has a huge open area perfect for flying right next to it.

Overall, despite having about 20 seconds of total flight time between 2 test flites that both ended in crashes, I'm incredibly happy with how it came out and I certainly don't regret doing it. In the future, unless I can get my hands on a laser cutter or cnc machine, I'm definitely going to stick to foamboard because I do not have the mental strength to go through that cutting process all over again. I hope that this encourages anyone that might have been on the fence about trying to scratch build a plane to go for it. If I, someone with very little designing and flying experience, basically no power tools, and a bedroom closet for a workshop can build a working balsa plane, I'm certain that you can too. I would love to answer any and all questions that anyone has so feel free to ask away.
 

Tench745

Master member
It sounds like you learned a lot about balsa in a short period of time. And you made a nice looking plane in the process!

I wanted to throw in some information which might connect some of the things you've learned to some of the common balsa building practices.

Builders grade their balsa based on density and how straight the grain is. Typically structural things like firewalls and load-bearing formers will be made from the denser wood and things like wing ribs and sheeting can be made from the really light stuff, as you discovered. Really straight grain makes sheeting around curves easier than pieces with a more meandering grain pattern.
See the article here if you want to learn more about grading and choosing balsa: http://www.airfieldmodels.com/information_source/construction_materials_for_model_building/balsa.htm

Just looking at your CAD model I said to myself "That's going to be tail heavy." Often times if a fuselage is built from sheets of balsa it will have large cutouts in the tail cone to reduce weight there. The tab-and-slot style of construction grew out of the advent of laser-cutting kits. It's quick and easy to punch some pieces out of a sheet of balsa and have them slot together perfectly, but it is not well suited (or necessary) for hand cutting. The traditional, built up truss fuselage of balsa sticks is often lighter and stronger with the added advantage of being easier to cut out yourself with just a razor blade.
Likewise, stick-built tail surfaces (Rudder, elevator, vertical and horizontal stabilizers) can also significantly reduce tail weight if you can't move your wing around for one reason or another.

You say your plane "flew rather fast". Does this mean that it stalled if you flew it any slower, or just that you had too much power?
The lighter you can build a plane, the slower it will be able to fly before stalling, but it's a balancing act because if you build too light the plane is easily damaged. What thickness balsa did you use for the various parts?


Out of curiosity, I looked up a plan for a 48" wingspan "trainer" with slab sides and plank tail surfaces for a comparison.
https://outerzone.co.uk/plan_details.asp?ID=10389
According to one pilot it was built heavy and was a real handful, so maybe not the best comparison, but I just did a quick search to find something similar to yours in construction.
It's estimated weight with a .40 size nitro engine, four large "standard size" servos, and covered in Monokote was 4lbs with a wing area of 504 square inches. (18.3oz/sqft wing loading). Compared to your 1.5lb plane, it looks like a bit of a porker.
Do you know what your wing area is? Wing loading can make all the difference.

If you want to keep learning and progressing in balsa and you like to read everything you can get you hands on like I do, I definitely recommend reading the rest of Airfield Models how-to articles as well. http://www.airfieldmodels.com/information_source/how_to_articles_for_model_builders/index.htm
 
Last edited:

Minton

New member
It sounds like you learned a lot about balsa in a short period of time. And you made a nice looking plane in the process!

I wanted to throw in some information which might connect some of the things you've learned to some of the common balsa building practices.

Builders grade their balsa based on density and how straight the grain is. Typically structural things like firewalls and load-bearing formers will be made from the denser wood and things like wing ribs and sheeting can be made from the really light stuff, as you discovered. Really straight grain makes sheeting around curves easier than pieces with a more meandering grain pattern.
See the article here if you want to learn more about grading and choosing balsa: http://www.airfieldmodels.com/information_source/construction_materials_for_model_building/balsa.htm

Just looking at your CAD model I said to myself "That's going to be tail heavy." Often times if a fuselage is built from sheets of balsa it will have large cutouts in the tail cone to reduce weight there. The tab-and-slot style of construction grew out of the advent of laser-cutting kits. It's quick and easy to punch some pieces out of a sheet of balsa and have them slot together perfectly, but it is not well suited (or necessary) for hand cutting. The traditional, built up truss fuselage of balsa sticks is often lighter and stronger with the added advantage of being easier to cut out yourself with just a razor blade.
Likewise, stick-built tail surfaces (Rudder, elevator, vertical and horizontal stabilizers) can also significantly reduce tail weight if you can't move your wing around for one reason or another.

You say your plane "flew rather fast". Does this mean that it stalled if you flew it any slower, or just that you had too much power?
The lighter you can build a plane, the slower it will be able to fly before stalling, but it's a balancing act because if you build too light the plane is easily damaged. What thickness balsa did you use for the various parts?


Out of curiosity, I looked up a plan for a 48" wingspan "trainer" with slab sides and plank tail surfaces for a comparison.
https://outerzone.co.uk/plan_details.asp?ID=10389
According to one pilot it was built heavy and was a real handful, so maybe not the best comparison, but I just did a quick search to find something similar to yours in construction.
It's estimated weight with a .40 size nitro engine, four large "standard size" servos, and covered in Monokote was 4lbs with a wing area of 504 square inches. (18.3oz/sqft wing loading). Compared to your 1.5lb plane, it looks like a bit of a porker.
Do you know what your wing area is? Wing loading can make all the difference.

If you want to keep learning and progressing in balsa and you like to read everything you can get you hands on like I do, I definitely recommend reading the rest of Airfield Models how-to articles as well. http://www.airfieldmodels.com/information_source/how_to_articles_for_model_builders/index.htm

The wing area is about 366 sq in and I didn't mention it in the original post but after the first crash I swapped out the motor and battery for bigger ones that weighed a bit more bringing the weight up to a little over 2 lbs. So the wing loading should be about 14.16 oz/sqft. I don't really know what a typical wing loading for a trainer is but a quick google search seems to indicate that 10 - 15 oz/sqft is pretty standard for something like this.

When I said it flew a bit fast, I meant that it took off and maintained flight at what seemed to be a rather fast speed. I only had it in the air for a couple of seconds so I didn't really get a full feel for its characteristics but as soon as I let off the throttle a bit it seemed to lose altitude pretty quickly. The FT Trainer is the only other plane that I've flown so I'm used to being able to have it at about 15-20% throttle and just float above the ground so maybe my view is a bit biased.

I used 1/8" balsa for everything except for the wing and aileron covering which was 1/16". I had some trouble finding balsa because most of the hobby/craft stores near me either didn't have any or had really bad quality balsa with weird seams down the middle. Is there a good place to get model aircraft quality balsa?
 

Tench745

Master member
The wing area is about 366 sq in and I didn't mention it in the original post but after the first crash I swapped out the motor and battery for bigger ones that weighed a bit more bringing the weight up to a little over 2 lbs. So the wing loading should be about 14.16 oz/sqft. I don't really know what a typical wing loading for a trainer is but a quick google search seems to indicate that 10 - 15 oz/sqft is pretty standard for something like this.

When I said it flew a bit fast, I meant that it took off and maintained flight at what seemed to be a rather fast speed. I only had it in the air for a couple of seconds so I didn't really get a full feel for its characteristics but as soon as I let off the throttle a bit it seemed to lose altitude pretty quickly. The FT Trainer is the only other plane that I've flown so I'm used to being able to have it at about 15-20% throttle and just float above the ground so maybe my view is a bit biased.

I used 1/8" balsa for everything except for the wing and aileron covering which was 1/16". I had some trouble finding balsa because most of the hobby/craft stores near me either didn't have any or had really bad quality balsa with weird seams down the middle. Is there a good place to get model aircraft quality balsa?

Yeah, it is hard to feel out a model when it's only up for a few seconds at a time. More practice and experimentation will improve the experience.

A better way to compare wing loading across different sizes of model is something called cubic wing loading. It's a unit-less number that will scale across all different sizes of airplanes. Here's a calculator if you're interested: http://www.ef-uk.net/data/wcl.htm

In the case of the trainer I linked to it had a Wing-Cube Load of 9.8. This is within the aerobatic range of cubic wing load

At 2 lbs and a wing area of 366sqin yours would have a WCL of 7.9. Trainers tend to have a WCL of 6 to 7, so yours is maybe a little heavy for a trainer, but not unreasonable.

The Tiny Trainer at 10oz with a wing area of 194sqin will have a WCL of 6.4.
So yours will need to fly a bit faster and will sink faster than the Tiny Trainer.

If you could find it, you probably could have gone with 3/32" instead of 1/8" for most pieces and saved a little weight. When sheeting a wing the sheeting will take some of the load so you can actually afford to go lighter on the internal structure; 1/16" ribs with 1/16" sheeting might have been sufficient. I don't have enough balsa building experience to say for sure though.
Most of the big-box craft stores near me have pretty iffy quality balsa as well. Some don't even carry balsa, they only have basswood. I am lucky that there is a good hobby store near me that carries higher quality balsa. Bud Nosen is their supplier; I'm not sure if they sell to individuals or just businesses. https://budnosenmodels.com/
I do know that Balsa USA has an online store and will sell to individuals. I haven't used them personally, but they're well known in the hobby. https://shop.balsausa.com/category_s/1005.htm