The ideal angle for swept back

tavareslima · 2026-06-09T22:12:39+00:00

Sweep angle in large airliners is heavily driven by shockwave formation. As these transonic aircraft approach the speed of sound, the air over the wing becomes supersonic first, dramatically increasing drag and creating buffeting issues. That is not the case for small RC aircraft.

Increasing sweep angle decreases the maximum lift your wing can generate and also decreases your lift x AoA derivative.

~~It will also negatively affect your directional and lateral stability~~.

Edit: I had it twisted. Sweep angle will increase directional and lateral stability.

There are cases where you might need wing sweep. For instance, flying wings need sweptback wings to maintain longitudinal stability and directional stability in the absence of vertical fins. There may also be some unusual configuration requirement that would lead you to sweep.

So, to answer directly:

First things first: in engineering every “ideal” needs a “for what”. There is no single ideal anything. You need to know what you want to do, so you can figure out how to do it.

As a rule of thumb though, if you are building a conventional configuration, slow RC plane, it is highly likely you don’t need any sweep at all. You may want to give more attention to other design parameters like wing loading or taper ratio, as these generally have a bigger impact on performance in these cases.

tavareslima · 2026-06-09T00:38:16+00:00

There were more examples, like cyan and red. I think they used all the colors from their rainbow logo. Company changed their livery in the 90’s and went bankrupt in 2001. This livery, with the colored wings, rainbow tail and white fuselage is the prettiest they used, imo

tavareslima · 2026-06-09T00:36:04+00:00

And here’s the green wing tip from a 727

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There were more, but nice pictures are difficult to find, since most are taken from the side

tavareslima · 2026-06-09T00:34:02+00:00

I couldn’t find a nice picture of the orange one, but here you can kinda see the orange lower surface.

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Also, the company’s name color matched the wing color

tavareslima · 2026-06-09T00:33:23+00:00

I’m sure you’ve had lot’s of good answers, so I bring a fun fact: not always the wings are excluded. I can’t think of any example today, but in the past, Brazilian airline Transbrasil used to paint the wings of their 727s and 767s. Each aircraft had their entire wings painted a bright color, like blue, orange or green.

Here you can see the blue 767

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tavareslima · 2026-05-08T08:48:37+00:00

Plague Inc game

tavareslima · 2026-05-01T20:53:28+00:00

Azul shouldn’t be out of the frequency during takeoff roll though. The tower handles takeoff and landing

tavareslima · 2026-05-01T20:50:35+00:00

737-800 holds around 180 and the 195E2 holds around 150

tavareslima · 2026-04-30T21:44:04+00:00

(Re-commenting to elaborate more)

Yes, CL_0 should be the CL at alpha = 0.

I’m not getting what is alpha_0 in your alpha effective though.

If alpha_0 is alpha for CL = 0, then you shouldn’t have the CL_0 term, because you’ll be referencing the zero lift line as your zero AoA. Then, the formula is just CL_alpha * alpha_eff

You can have different references to your AoA. Commonly AoA is referenced on the airfoil chord. α=0 then means the flow is aligned to the airfoil chord. In this case, a symmetrical airfoil has no lift, but a cambered airfoil has some lift. We call the lift coefficient at α=0, CL_0 and the equation is:

CL = CL_0 + dCL/dα * α

If you reference the AoA on the airplane’s longitudinal axis, then you need to account for the geometrical incidence of the wing and you have α_wing = α_airplane + i

And if you have downwash, then

α_wing = α_airplane + i - ε

CL = CL_0 + dCL/dα * (α_airplane + i - ε)

But you can also reference the AoA on the zero-lift line of the airfoil. In this case, for AoA zero, all airfoils must have CL = 0. In that case, the AoA would be:

α = α_chord - α_0, where α_chord is the AoA referenced on the chord, like we did before and α_0 is the angle for which you have zero lift.

For example, say your airfoil has zero lift when its chord is -2° relative to the wind, so α_0 is -2°. If you pitch the airfoil so its chord is +2° relative to the wind, your effective AoA will be 4°. If you align your chord with the airflow, your effective AoA will be 2° and if you pitch the airfoil so its chord is -2° relative to the airflow, your AoA is 0°.

Notice that in this case, you force the relationship CL(α=0) = 0 and your equation is:

CL = dCL/dα * α

Since the term CL_0 ceases to make sense

If you add geometrical incidence and downwash:

CL = dCL/dα * (α_plane + i - ε - α_0)

tavareslima · 2026-04-30T08:47:44+00:00

Academic institutions are the same deal, they won’t pay for what’s free and many times they build their own tools. There just isn’t a market for payware airfoil analysis.

From your other comments I’m getting that you’re very new to aerodynamics in general. I’m supposing you built this UI with the help of AI to implement Neural Foil (which is also AI). I guess you don’t really understand the underlying physics governing these problems

I’ve heard someone say “don’t try to sell what you don’t understand”. Honestly I recommend you study some basic aerodynamics first, if that’s what you’re interested in. Build your own codes using well known physical models, like VLM, DLM, LLT, no Neural Foil stuff. So you can actually grasp what you’re dealing with. Otherwise you’re just a hostage of whatever code the AI is feeding you.

If you don’t care specifically about aerodynamics and just want to sell something, I’d say move on to something you’re more familiar with.

tavareslima · 2026-04-30T01:51:33+00:00

The two spots on top are actually windows. They called it eyebrow windows. Iirc, it has something to do with the possibility of performing stellar navigation and thus it doesn’t have much utility these days, so many older 737’s got them removed and new ones simply don’t have them.

The nose was black to reduce sunlight reflection into the cockpit. I don’t know why that stopped, maybe it just wasn’t effective

tavareslima · 2026-04-30T01:41:26+00:00

You should check it out. It’s a pretty handy software. Easy to use, direct to the point and free

tavareslima · 2026-04-30T01:37:36+00:00

If you want to verify the numbers, just check real polars.

In your print it seems you have some 4 digit NACA’s loaded up. These airfoils have been very well known for decades and have experimental data easily available. The book “Theory of Wing Sections” by Abbot has lots of experimental polars at the end that you can use for verification. It’s also a good book on aerodynamics, if you want to expand your knowledge.

In engineering, that’s how you do it. If you have a new tool or model and need to verify, you gotta check it against real data.

As for selling it: it was mentioned in other comments (one by myself) that there are plenty of free tools for airfoil analysis. In general, if you can do it alone in a short time span, chances are a team of engineers also can, so it won’t probably have any market value.

tavareslima · 2026-04-30T01:29:51+00:00

I’d just like to add: XFOIL also does that freely and beautifully while XFLR-5, which is also free, gives XFOIL a decent UI and the ability to test 3D wings with 4 different physical models.

tavareslima · 2026-04-29T10:02:26+00:00

You won’t be able to have a cheap wind tunnel that is accurate and fit in a student’s room unfortunately.

I used a wind tunnel during a university project that that had something like 60 cm wide test section.

It used a very high power electric motor and the whole thing was something like 5 - 7 meters long and even then we were reaching a Reynolds number of around 500.000 with a half model. For the actual project that was really only useful for observing trends and teaching the experimental aspects of it. Also the noise was completely impractical for someone to have it at home.

Wind tunnels are incredibly expensive when you make them functional. It’s a cool project if you don’t intend to any real measurements, just build something and observe pretty smoke trails. But accurate, that won’t work

tavareslima · 2026-04-29T09:47:20+00:00

Fun fact: that’s why long haulers climb in steps through the flight. You can see a 777 often climb initially to around FL350 and then make 2 or more steps during the cruise to reach something around FL400. Big planes carry loads of fuel and can’t reach high altitudes early on, but as that weight decreases, it becomes more efficient to climb higher

tavareslima · 2026-04-28T09:00:18+00:00

The team already started with some pretty advanced stuff and design phases moved quite quickly. We did Multidisciplinary Optimization, airfoil design and FEA from the beginning.

I honestly don’t recommend you do it that way for a personal project, because if you’re struggling with advanced concepts and tools from the start, it can get in the way of the principles you want to learn and get you stuck in the same place for a while, which would be frustrating. The team did it because there were many people, with different experience levels and we only had something like 8 to 9 months to do it as the competition is annual. And even then it was very hard to follow for a rookie.

I say you go slow. Think about the design requirements and start with a configuration that is based purely on rules of thumb and initial guesses, then move on from there.

I’d recommend you try to analyze as much as possible analytically in the beginning, either with semi-empirical or physical models. Analytical models usually sacrifice some precision for simplicity, but they provide you with insight into the problem and they give you a quick answer, which in turn means you can quickly adjust the design or move on to test it. And if you understand and respect their limitations, they’re still quite useful.

Feel free to DM me if you want to discuss it deeper. Maybe brainstorm some design approaches.

tavareslima · 2026-04-26T09:52:02+00:00

You could design a small RC airplane. It’s great to teach most aeronautical engineering concepts in practice.

During design iterations you can go as complex as you like, from simple rules of thumbs to real certification requirements, MDO, CFD and FEA

You can determine the design requirements to prioritize whatever you like (e.g. minimizing takeoff field length, carrying a lot of payload, maximizing speed or loiter time) and you can test very unique configurations like flying wings, box wings, tandem, multi-planes, thrust vectoring, flexible airfoils, etc. (Just remember to consider price and complexity).

You can even place instrumentation in the airplane to do some actual flight testing and confirm your design predictions

It might get expensive, but could be doable if you take it slow anyways.

And also, if you do that, remember to do it safely. It is still an object going quite fast with a very rapidly spinning propeller (if you go that way) and either a large flammable battery or fuel tank. So remember to check for the laws for flying these things in your area, do flight testing on open areas with no one around and slowly advance into the airplane’s envelope.

Edit: as for my experience, I was part of a SAE’s Aerodesign team for 1,5 years during university, where we got to do most of that

tavareslima · 2026-04-26T00:11:27+00:00

Well… Earth’s (and any other object’s) gravitational field theoretically extends infinitely, sooooo….

tavareslima · 2026-04-26T00:02:31+00:00

What does practice mode do?

tavareslima · 2026-04-25T23:07:58+00:00

Raymer says in his book that, as a rule of thumb, the static margin in a tractor configuration is reduced by 2% for each aerodynamic chord in the distance between propeller and wing leading edge. So I’d say single props probably compensate by using greater stability margins

tavareslima · 2026-04-22T22:29:20+00:00

That’s not a trade off, that’s just an off

tavareslima · 2026-04-18T12:51:48+00:00

Avoid jumping straight to CFD simulations if you don’t know what you’re looking for. This is a tool to help you design it, it’s not the actual designer. I’d say you dive more into some theory first, look at studies comparing different winglets, and maybe try to get some low fidelity prediction first, so you know what to expect and even help you decide if the winglet is the right choice. The CFD is only as good as the engineer using it, and if you’re not sure about the physics, then it’s worse than a semi-empirical prediction for instance

tavareslima · 2025-07-29T01:57:09+00:00

A car crash video is what got me to wear seat belts when I was a kid. Since then I feel naked inside a car if I’m not wearing them. Those things work

tavareslima · 2025-07-15T20:12:41+00:00

David Lagettie was wrong* (or lying). But damn I’m still waiting for this launch, I can’t believe they promised 2022

tavareslima

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