Help! Trainer Aircraft Design Questions - Seeking Designer Input (School Project)

[DesignStudent_RC]

Hi everyone,

I’m a year 12 student working on my school Extended Project Qualification in the UK where I'm developing design plans for a remote-controlled trainer aircraft inspired by the Cessna 172 Skyhawk and Piper J-3 Cub. The project is design only, creating detailed design documentation and architectural plans. I'm trying to understand how experienced RC aircraft designers actually make decisions about aerodynamics and aircraft configuration, rather than just relying on textbooks.

I've got 6 questions I'm hoping you can help with about what makes a good trainer aircraft, common design mistakes, aerodynamic features, airfoil choice, wing characteristics, and stability versus manoeuvrability. I'd really appreciate any responses based on your experience, whether that's building, flying, or designing.

Thanks in advance, and here are my questions:

1. What makes a good beginner/trainer RC aircraft?

2. In your experience, what are the most common design mistakes made when creating RC aircraft?

3. What aerodynamic features make an RC aircraft more stable and easier to control?

4. How important is the choice of airfoil for a trainer aircraft and why?

5. What wing characteristics would you prioritise for a trainer aircraft: larger wing area, higher aspect ratio, or something else? Why?

6. Is it better for a trainer aircraft to be more stable or more manoeuvrable? Why?

 

[telnar1236]

Any design is a trade-off between a number of different requirements. As an aerospace engineer who had to complete a couple design projects like this during the course of my education, my advice would be to rely on the textbooks as much as possible if you don't actually need to build the plane. Teachers put far more weight on concrete numbers from reliable sources than on feedback from an online forum, so a lot of the time it's better to end up with a thoroughly analyzed design that would never fly than a design that would fly great but relies on rules of thumb more than actual math from a grading perspective.
That said, let me try and answer your questions:

1. A good beginner RC plane should be stable and recover quickly if you get it into a situation you don't want it in (gentle stall characteristics and impossible to enter a spin). It should fly reasonably slow so it doesn't get away too fast and you have more time to respond to anything that goes wrong. It should also be durable, especially in the landing gear if it has it, since you will likely crash or at least land hard a few times. And when you do crash, it should be easy to repair or replace damaged parts. Some degree of self-leveling is a good idea since it keeps you from putting the plane inverted (upside-down) and makes it easier to focus on keeping in the air. In my opinion, a gyro with something like SAFE (Horizon Hobby's self-leveling software) is better for this than using aerodynamic tricks like significant dihedral since it protects you in both pitch and roll, where a dihedrdal wing only prevents rolling the plane inverted.
2. CG, too weak a wing spar, and things related to directional stability. Even if you know you need a stable CG, it's easy to put it in the wrong place or design yourself into a situation where you can't get the right center of gravity. Especially with foam board planes, it's also easy not to have enough structure in the wing and the wings can fold. Finally directional stability is yaw stability. You need a big enough vertical stabilizer and not too much area in front of it. A lot of the time you see people with less experience trying a wild design that looks really cool, but has way too small of a vertical surface or one far too far forwards.
3. Correct CG is the biggest one. Correct dihedral is another, especially for a foam board plane since it gives it better lateral (roll) stability. Washout in the wingtips or another way to make the wing tips stall after the wing roots also helps a lot but isn't strictly necessary. In foam board planes this is typically achieved by not continuing the skin on the bottom of the wing all the way to the tip which turns the tip into a more stall resistant under cambered airfoil.
4. Fairly important but it depends on the medium you use to build the plane. With a 3D printed or balsa design, you can get a fairly precise airfoil and having a good stall resistant airfoil is very useful to making the plane easy to fly. With a foam board design, you're not going to get a super precise airfoil ever, so having a thicker airfoil is helpful, but it's less important overall.
5. In general, you want a moderate aspect ratio wing with light wing loading (plenty of area for the airplane weight) and no sweep or a very slight forward sweep. It's a tradeoff always - you're not prioritizing any one of these so much as designing a wing that best meets all your requirements which tends to result in something in the middle
6. Again, it's a balance. If you're plane is too maneuverable it will be a handful for someone just starting off, but if it turns too slowly, it requires more thinking in advance on the part of the pilot which is also something that beginners need to develop. Trainers probably lean more towards the stable side though. In general, you want a stable, but nimble airplane that can't necessarily get to high angles of attack like a 3D aerobatic design, but that can still turn somewhat tightly without needing to pull too hard on the sticks.

 

[DesignStudent_RC]

Hi telnar1236, thank you so much for taking the time to give such detailed answers. This is really helpful for my project. I chose this topic because I'm genuinely interested in aircraft design. Your explanation of CG placement and dihedral effects, and the balance between stability and manoeuvrability has given me a much better understanding of how designers actually think about these problems. I really appreciate you explaining the reasoning behind each choice. Thanks again!

 

[HappyRCFlyer]

A neat way to go beef up what you are learning is to get into making simple chuck gliders out of foam and/or other materials. It's great to have things memorized, seeing it applied in the real world is very satisfying.