Solved my journey to 3D print a JAS 39 gripen

[telnar1236]

I’m trying to model and 3D print a roughly 1-meter-long JAS 39 Gripen RC model for a school project. I’ve been researching online and working on the modeling process, but now that I’m trying to determine if it can actually fly, I’m struggling.

According to the simulations I’m running in SolidWorks, the model has no chance of getting off the ground. However, when I look at similar models on YouTube, it seems like mine should be able to fly. When I import the model into Bambu Studio, it estimates a weight of about 710g at 5% gyroid infill. Based on that, I’ve been roughly estimating the final weight to be between 1–1.5 kg.

The issue is that my flow simulations indicate almost no chance of achieving lift. With a convex airfoil, I’m only getting about 0.8N of lift at 15 m/s, which I doubt will be enough. To improve this, I redesigned the wings to have roughly twice the surface area, using a thicker and more aggressive airfoil. With this new design, I achieved 2.7N of lift under the same conditions. The original wing has a total area of 0.098 m², while the larger-winged version has 0.18 m².

I’m mainly looking for feedback from more experienced individuals on whether this model seems flyable. Additionally, I’d appreciate any suggestions on airfoil selection—what type to use or how to design one that suits my needs. Any advice, suggestions, or feedback would be greatly appreciated!

You might need to look at your boundary conditions, mesh, and assumptions for the simulation. This plane does look a bit small for a 750g weight, but it should still be producing more than 275g of lift. One big thing to consider is that the plane will fly at a positive angle of attack. At 0 AOA, a symmetrical airfoil will also produce 0 lift. However, at a higher AOA, a delta will produce a ton of lift as it generates a powerful leading-edge vortex keeping the flow attached. It's also almost impossible to get good results for a delta wing without control surface deflection since that deflection kills part of the lift, or with canards adds to it.

For preliminary work, xflr5 is a good resource that can give you reliable results without needing to go the whole CFD route.

xflr5

www.xflr5.tech

In terms of CFD, Solidworks simulation is also pretty poor. Solidworks is mostly a piece of CAD software, so any results you get should be taken with a grain of salt.

 

[telnar1236]

Thanks for the feedback! Unfortunately, for my chances of succeeding, the two main building blocks of this project are 3D printing and form factor, so I’ll have to work with the extra weight and reduced wing area.

I looked around for some airfoils on AirfoilTools and found a few options. The first picture shows the airfoil I originally used since I had heard that the Gripen used a convex airfoil, so I tried to come up with something similar. The second picture is of a NACA 2408 airfoil, which I thought looked similar to the one used by a guy who built his own 3D-printed Gripen. His model seems to be roughly the same size as what I want to make. However, I think the airfoil looks a bit thick, so I might adjust the thickness slightly to see how it looks. The third picture is of a NACA 0006 airfoil, which I like for its thin profile and symmetrical shape, as you suggested for higher speeds. However, I believe the extra lift from the one with the flatter bottom might be necessary.

When looking at my model, I started to doubt the size I had chosen because the walls are quite thin—around 1 cm. I believe this will make it quite fragile, making it difficult to support landing gear, and I won’t be able to create channels for wire management.
View attachment 247823
At first, I considered using a smaller EDF, but the only ones I found were 50mm, and according to the manufacturer’s specs,gave a significantly less amount of thrust. So, I’m thinking about scaling up the model, which would increase the wing area but also make it heavier. That said, I don’t think the weight increase will be too drastic since I still need to hollow out the front for the electronics, and I plan to make the model more hollow when adding the wire management channels.

I plan on reaching out to @telnar12346 about the problem as you suggested, but any insight into the matter would be greatly appreciated!

The last airfoil picture would probably be your best bet. On a delta, the wing should generally have a pretty small pitching moment, since the elevons need to counteract that. For the second airfoil, the flat-bottomed one, the elevon deflection required would mostly cancel out the gain in lift, so it would probably actually require more speed to fly. The first airfoil won't be very efficient, but it's probably still better than the second one. However, in general, the airfoil choice, so long as it's not something absurdly thick, isn't as important for a delta, as the leading edge vortex very quickly starts to dominate lift generation.
In terms of speed vs. lift, in general thin airfoils have lower zero-lift drag coefficients compared to thick ones so go faster, as others have mentioned. However, there's a point past which making an airfoil thinner/pointier doesn't really help that much. Much thinner than 6% isn't really that good for an RC plane since we aren't trying to fly supersonic (I hope). For this plane, I'd lean towards the simplicity of a NACA 0008 or 0009 airfoil since they're generally pretty good for deltas. Alternatively, airfoil tools also has a bunch of low pitching moment flying wing airfoils that are pretty good.

 

[Houndpup Rc]

The last airfoil picture would probably be your best bet. On a delta, the wing should generally have a pretty small pitching moment, since the elevons need to counteract that. For the second airfoil, the flat-bottomed one, the elevon deflection required would mostly cancel out the gain in lift, so it would probably actually require more speed to fly. The first airfoil won't be very efficient, but it's probably still better than the second one. However, in general, the airfoil choice, so long as it's not something absurdly thick, isn't as important for a delta, as the leading edge vortex very quickly starts to dominate lift generation.
In terms of speed vs. lift, in general thin airfoils have lower zero-lift drag coefficients compared to thick ones so go faster, as others have mentioned. However, there's a point past which making an airfoil thinner/pointier doesn't really help that much. Much thinner than 6% isn't really that good for an RC plane since we aren't trying to fly supersonic (I hope). For this plane, I'd lean towards the simplicity of a NACA 0008 or 0009 airfoil since they're generally pretty good for deltas. Alternatively, airfoil tools also has a bunch of low pitching moment flying wing airfoils that are pretty good.

Wow! Seems complicated to design a jet!

 

[telnar1236]

Wow! Seems complicated to design a jet!

Honestly, it's not that bad - I tend to get way too into the science, but lots of people on the forums here wing it and make amazing projects that fly beautifully. From an aerodynamics perspective, model planes are every bit as complex as real planes, and so you can go way down the rabbit hole, but a bit of common sense will carry you as far as you need to go.

 

[Houndpup Rc]

Honestly, it's not that bad - I tend to get way too into the science, but lots of people on the forums here wing it and make amazing projects that fly beautifully. From an aerodynamics perspective, model planes are every bit as complex as real planes, and so you can go way down the rabbit hole, but a bit of common sense will carry you as far as you need to go.

I suppose with help on the forum it easier too. Just been getting into FreeCAD lately and have been thinking of designing a jet!

 

[Houndpup Rc]

The last airfoil picture would probably be your best bet. On a delta, the wing should generally have a pretty small pitching moment, since the elevons need to counteract that. For the second airfoil, the flat-bottomed one, the elevon deflection required would mostly cancel out the gain in lift, so it would probably actually require more speed to fly. The first airfoil won't be very efficient, but it's probably still better than the second one. However, in general, the airfoil choice, so long as it's not something absurdly thick, isn't as important for a delta, as the leading edge vortex very quickly starts to dominate lift generation.
In terms of speed vs. lift, in general thin airfoils have lower zero-lift drag coefficients compared to thick ones so go faster, as others have mentioned. However, there's a point past which making an airfoil thinner/pointier doesn't really help that much. Much thinner than 6% isn't really that good for an RC plane since we aren't trying to fly supersonic (I hope). For this plane, I'd lean towards the simplicity of a NACA 0008 or 0009 airfoil since they're generally pretty good for deltas. Alternatively, airfoil tools also has a bunch of low pitching moment flying wing airfoils that are pretty good.

"Much thinner than 6% isn't really that good for an RC plane since we aren't trying to fly supersonic (I hope)." Why wouldn't we want to?😂😂

 

[NeonGreen]

im thinking i want to post updates and such since i have to document it either way so might aswell do it here. does anyone know if i should just keep posting in this thread or if there is a specific place or way to do it

 

[Mr Man]

im thinking i want to post updates and such since i have to document it either way so might aswell do it here. does anyone know if i should just keep posting in this thread or if there is a specific place or way to do it

You can just edit thread title.

 

[AIRFORGE]

im thinking i want to post updates and such since i have to document it either way so might aswell do it here. does anyone know if i should just keep posting in this thread or if there is a specific place or way to do it

If you want to make it separate from this thread, perhaps start it in General RC Topics - Lessons Learned - Flite School. Then name it appropriately, of course. There may be a better section, but that looks good to me for now.
EDIT: You can link each of the threads to one another also.

 

[NeonGreen]

Hello everyone. a little update for now is that i have cut the model into smaller parts that i then put togheter again inside of a asssembly. Probably not the most effective way to do things but i wanted to leave all of the jank of the original model behind. Putting it all in a assembly is going to be useful for simulating the movements of the elevators and canards. i also removed the canards and the veritical stabalkiser because i didnt like the look of them so im going to redesign and add them back later. I also hollowed out the plane in the way I plan on it being in the final version. this landed us on a model weight of about 750g.

I also felt that i wanted to see some of it in person so i printed out one of the wings and made all of the finnishing details of it by making the elevators making place for the servo and all suchyou can see it in action here.

The plan is to put the wing with the wooden structure on a scale and drive around with it at different AoA's and deflections just to see how it effects the lift. mostly just for the final report but also will be interesting to see how it will act in person.

 

[telnar1236]

Hello everyone. a little update for now is that i have cut the model into smaller parts that i then put togheter again inside of a asssembly. Probably not the most effective way to do things but i wanted to leave all of the jank of the original model behind. Putting it all in a assembly is going to be useful for simulating the movements of the elevators and canards. i also removed the canards and the veritical stabalkiser because i didnt like the look of them so im going to redesign and add them back later. I also hollowed out the plane in the way I plan on it being in the final version. this landed us on a model weight of about 750g.

I also felt that i wanted to see some of it in person so i printed out one of the wings and made all of the finnishing details of it by making the elevators making place for the servo and all suchyou can see it in action here.

The plan is to put the wing with the wooden structure on a scale and drive around with it at different AoA's and deflections just to see how it effects the lift. mostly just for the final report but also will be interesting to see how it will act in person.

Something to watch out for is that the wing will behave quite differently off the plane vs. on the plane. Having the other wing gives a surface that prevents the low pressure on top of the wing from spilling off the side, and the fuselage itself also experiences the low pressure from the wing which is why generally for aerodynamic calculations the wing area with the wing projected to the airplane center line is used. Therefore, the wing will both produce a smaller pressure difference (less lift) and have less area to produce lift, also resulting in less lift if tested on its own.

Also, any structure connected to the wing will change its performance. If you look at wind tunnel models, they generally have an airfoil shaped strut to prevent that from happening. And finally, driving it in a car, the wing will most likely be in the car's boundary layer which will also change performance unpredictably.

This all isn't to say that it's a bad idea to try driving around. Adding a large flat plate to the inboard side of the wing won't necessarily give you back the full amount of lift the installed wing would get since you're still losing the fuselage area, but it will go a long ways towards giving a more realistic value. And the structure and turbulence from the car will change your answer, but you should still get a ballpark number you can use.

 

[quorneng]

NeonGreen
Whilst I do appreciate taking a scientific approach to the aerodynamics remember that aerodynamics do not scale. This makes the results of any testing very hard to compare. Even getting meaningful results from a properly engineered wind tunnel is not easy and from physically moving the test through the air is even worse.
This is why previous experience,and the more the better, in model flying is so important.
Most models do not suffer from a lack of lift but more a lack of power to fly at the speed required to achieve the lift to overcome their weight. It follows that limiting weight or to be more exact the wing loading will reduce the speed and thus the power required to fly. In addition kinetic energy effects "impact with the ground" and kinetic energy is proportional to the velocity of impact squared!

A Gripen like all modern fighter jets has an very high wing loading and needs a very high power loading to achieve flight. These are both characteristic hard to emulate in a small scale model particularly if using an EDF for propulsion.
Are you sure that by selecting it along with using the printing process you have given yourself a much reduced chance of success?

 

[Piotrsko]

At this location, without seeing actual evidence in the conversation: has the OP actually flown something more than once? Cad simulation is nice, theory works until it meets reality, and Nasa says models dont necessarily follow the rules in flight. Hate to be the wet rag here, but....btdt got the ribbon after wasting a forest of balsa trees

 

[Mr Man]

@NeonGreen have you ever flown a plane before?

 

[NeonGreen]

At this location, without seeing actual evidence in the conversation: has the OP actually flown something more than once? Cad simulation is nice, theory works until it meets reality, and Nasa says models dont necessarily follow the rules in flight. Hate to be the wet rag here, but....btdt got the ribbon after wasting a forest of balsa trees

@NeonGreen have you ever flown a plane before?

I have indeed not. Luckily there is a rc flight hobby club close to me and i just today got in contact with a member who said he could fly the plane if i felt it was needed. He also said that i could probably train on some of his easier planes and i have a rc simulator that i plan on flying some in.

 

[NeonGreen]

NeonGreen
Whilst I do appreciate taking a scientific approach to the aerodynamics remember that aerodynamics do not scale. This makes the results of any testing very hard to compare. Even getting meaningful results from a properly engineered wind tunnel is not easy and from physically moving the test through the air is even worse.
This is why previous experience,and the more the better, in model flying is so important.
Most models do not suffer from a lack of lift but more a lack of power to fly at the speed required to achieve the lift to overcome their weight. It follows that limiting weight or to be more exact the wing loading will reduce the speed and thus the power required to fly. In addition kinetic energy effects "impact with the ground" and kinetic energy is proportional to the velocity of impact squared!

A Gripen like all modern fighter jets has an very high wing loading and needs a very high power loading to achieve flight. These are both characteristic hard to emulate in a small scale model particularly if using an EDF for propulsion.
Are you sure that by selecting it along with using the printing process you have given yourself a much reduced chance of success?

I wanted to bring together some things that interest me for this project. I've always found 3D printing fascinating because you can create something on a computer and then bring it to life in the real world. Since my craftsmanship skills are somewhat lacking, this technology amazes me even more.

For the past few years, I’ve also been interested in airplanes, especially fighter jets. So, I decided to combine these two passions. I don’t know how well it’s going to fly, but my confidence in it is increasing by the day, and I’m getting close to the finish line.

As for the scientific approach, it’s because this is a school project that will culminate in a scientific report. I wanted to conduct some testing before building the final plane.

 

[Houndpup Rc]

I wanted to bring together some things that interest me for this project. I've always found 3D printing fascinating because you can create something on a computer and then bring it to life in the real world. Since my craftsmanship skills are somewhat lacking, this technology amazes me even more.

For the past few years, I’ve also been interested in airplanes, especially fighter jets. So, I decided to combine these two passions. I don’t know how well it’s going to fly, but my confidence in it is increasing by the day, and I’m getting close to the finish line.

As for the scientific approach, it’s because this is a school project that will culminate in a scientific report. I wanted to conduct some testing before building the final plane.

You said it! 3D is so cool! Put enough thrust and she will fly! lol

 

[Houndpup Rc]

You said it! 3D is so cool! Put enough thrust and she will fly! lol

When I get good enough at FreeCAD my big goal is the SR-71 Blackbird! I want it to be like the FT master series though.

 

[NeonGreen]

as for updates i tried to do the lift test but i realised i had no way of checking the results so i will try again tomorrow. I also added some really low walls out of sheetmetal in an attempt to atleast hinder the pressure from going around the side altough honest i doubt it will do much.
Im also looking at the final decision for esc and edf and im currently heavily leaning towards this one with the 3500kv and an included esc just to make life easy. altough im not a big fan of the start ducting of the fan but i think ill either let it be or sand if off.

 

[NeonGreen]

When I get good enough at FreeCAD my big goal is the SR-71 Blackbird! I want it to be like the FT master series though.

looks like a really fun project that net a really cool plane.