I'll preface this by saying I know very little about aircraft design or RC design. After I learn to fly EDF jets, I want to start a ridiculous custom-designed twin-EDF build. It'll probably be a few years down the road, so I'll have plenty of time to save money for it, but I still want to avoid some of the more expensive parts, like all-metal EDFs, if possible. Here's my very limited plan/ideas as it stands:
- Twin EDFs
- Composite skin/body (I should have plenty of composites experience by the time I get around to making this thing)
- Damped retractable landing gear
- Filtered ducting for each EDF (likely 3D printed or composite inlets with race car brake duct tubing)
- Ideally reasonably stable at a wide range of speeds and capable of some aerobatics and high speeds
My main concern/question to start is just the general design basics of the whole thing. What wing shape and profile do I want, do I want canards, how do I determine wing, stabilizer and control surface dimensions, etc. Also, if it makes a difference, the tentative plan is to use two 120mm EDFs.
Sounds like an interesting project. In terms of design basics, you need to somewhat nail down what you want the plane to do before you can get all-that in depth. Pretty much, so long as there is enough wing and stabilizer area, you have enough power, and you have a good CG, the plane will fly.
Wing shape:
- Straight wing: efficient and simple, produces good lift, but cannot produce lift at high angles of attack w/o specialized devices (e.g. vortex generators)
- Swept wing: Increased stability/shifts center of pressure back, but worse stall characteristics and less lift
- Forward swept wing: Structurally difficult and can cause issues with stability but great at pretty much everything
- Delta wing: Structurally the simplest, lightweight, and good at high angles of attack, but inefficient
In general, high aspect wings (long and thin) wings are more efficient but stall sooner and worse while low aspect wings are the opposite and are less efficient but stall better. Personally, I like the way low aspect trapezoidal wings and delta wings perform on jets, but others will differ. Once you have a wing shape selected you can choose your airfoil to suit that wing. For non-delta and non-flying wing designs, a Clark-Y airfoil is great for most things, but is a bit draggy at high speeds.
Canards:
Canards are great for planes capable of high angles of attack since they stall before the wing. Personally, I don't like the way planes with them fly, but most people like canards. They do prevent you from having flaps on your wing unless you also have a horizontal stabilizer behind the wing.
Flying surface dimensions:
Again, very dependent on what you want and your wing design. You can calculate stall speed and lift generation, or just Google what acceptable wing-loading is for various types of RC plane. If you Google pictures of real aircraft and have similar ratios of dimensions for your flying surfaces you should be ok. If you want to experiment with crazy designs, you either need to try modeling them in something cheap like foam board and to be prepared to crash a lot, or you need a ton of math.
Here are a couple resources:
CG calculator:
https://rcplanes.online/cg_calc.htm
- You can do it by hand, but why would you when there are easy tools like this available?
XFLR5:
https://www.xflr5.tech/xflr5.htm
- This is a fairly easy to use tool that lets you model the aerodynamics of wings and flying surfaces. If you have components weights and locations it can also give you both static and dynamic stability. There is a fair bit if good documentation on it out there.
You can also use XFOIL and AVL to get similar outputs, but they are much harder pieces of software to use.
If you want to get more in depth than these tools let you, you need a ton of math and conceptual understanding. There are a bunch of good tutorials on aerodynamics and CFD out there, but you don't really need any of that at an RC level.
A couple final notes. When you say filtered ducting to your EDFs that immediately sets off alarm bells. EDFs are very vulnerable to pressure losses in their ducting and so anything other than a straight through duct with minimal obstructions will kill your thrust and speed. And lastly, when designing an airplane that big and complex, it's very important not to forget the non-airplane parts of the airplane. 90% of the time when my projects have issues, it's not because I messed up the wing or the airplane can't fly, but because of something dumb like weak landing gear, or landing gear in the wrong spot, or a very hard to access battery tray.
Like Merv was saying, your best bet is to learn to build and fly with simple foamboard aircraft where you don't need to deal with a ton of complexity. It will give you an intuitive sense of what works and what doesn't that you will need anyway, even if you decide to use a ton of analysis later. Having built a composite/3D printed 80mm EDF, composite planes are insanely time consuming to build and a single crash can take months to fix. Better to experiment with something cheap like foam, or eventually 3D printed designs (they are heavier and break worse in a crash).
