Solved my journey to 3D print a JAS 39 gripen

Houndpup Rc

Well-known member
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.
I believe their are programs that will allow you to wind tunnel test your model.
 

Piotrsko

Master member
Orville and Wilbur tested on a bicycle. Your "fences" will stop the tip losses, but a bigger wing works better even without the fences. Look up aspect ratio. Use software to assist your design questions, not to do the design. (Steps off soapbox). I asked if you have flown because until you see a stall, you dont know if the wing is stalled or flying funny and how to fix that. Aero engineers have a saying " everything will fly if you have enough thrust" thats true, look at helicopters, quad drones, and rockets.
 

NeonGreen

New member
Orville and Wilbur tested on a bicycle. Your "fences" will stop the tip losses, but a bigger wing works better even without the fences. Look up aspect ratio. Use software to assist your design questions, not to do the design. (Steps off soapbox). I asked if you have flown because until you see a stall, you dont know if the wing is stalled or flying funny and how to fix that. Aero engineers have a saying " everything will fly if you have enough thrust" thats true, look at helicopters, quad drones, and rockets.
do you have any suggestions on sowftware to help me design a better wing?
 

Piotrsko

Master member
Software, no because there are hundreds of them and most are platform specific. Most are google searchable, find one you like. Like and easy to use is more important than anything else, they all more or less do acceptable aerodynamics for what you need. 98 mph instead of 100 even is close enough unless you are racing someone for money.

Fwiw, for models a SWAG design is as good as anything because our buddies at NASA discovered stuff doesn't scale up or down accurately. My experiences with airfoils agrees. Do stay away from triangle airfoils with an aspect ratio of 1.
 

L Edge

Master member
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.

That's won't help you solve the problem. You need to look at all the aspects when designing a plane. The Wart-Hog, DarkStar , F-117 are a few. If you look at HH SR-71, they use a FC to get it to go and still wobbles in flight.
 

Mr Man

Elite member
That's won't help you solve the problem. You need to look at all the aspects when designing a plane. The Wart-Hog, DarkStar , F-117 are a few. If you look at HH SR-71, they use a FC to get it to go and still wobbles in flight.
Yeah, but I think it would've been better if it was bigger.
 

Houndpup Rc

Well-known member
That's won't help you solve the problem. You need to look at all the aspects when designing a plane. The Wart-Hog, DarkStar , F-117 are a few. If you look at HH SR-71, they use a FC to get it to go and still wobbles in flight.
whats not going to help me solve the problem?
 

L Edge

Master member
whats not going to help me solve the problem?
Things like this. I gave you the answer, so build it. Remember no rudders allowed.

Look at entry number 10 and 12

 

telnar1236

Elite member
do you have any suggestions on sowftware to help me design a better wing?
In general, you need a good understanding of the theory before software will be much use. You can get software to give you an answer, but unless you know the theory the answer it gives you will most likely be fairly wrong. Probably the easiest software to use is XFLR5 since it fully integrates every step of the analysis and has a relatively clean and easy to use GUI. It's also free which counts for a fair bit.
That said, the best way to learn is trial and error. Something to keep in mind is that engineering tools only answer the questions you use them to answer. If you have an aerodynamically amazing plane that can't hold itself together structurally, you don't have a working plane. So, learning rules of thumb is very important if you don't want to get stuck in analysis paralysis. As far as a first jet goes, the Gripen isn't a bad planform. The combination of the canard and delta wing should give it pretty gentle stall. That said, jets aren't good first planes as others have mentioned. Definitely get the guy you mentioned to try and fly it first.
 

NeonGreen

New member
In general, you need a good understanding of the theory before software will be much use. You can get software to give you an answer, but unless you know the theory the answer it gives you will most likely be fairly wrong. Probably the easiest software to use is XFLR5 since it fully integrates every step of the analysis and has a relatively clean and easy to use GUI. It's also free which counts for a fair bit.
That said, the best way to learn is trial and error. Something to keep in mind is that engineering tools only answer the questions you use them to answer. If you have an aerodynamically amazing plane that can't hold itself together structurally, you don't have a working plane. So, learning rules of thumb is very important if you don't want to get stuck in analysis paralysis. As far as a first jet goes, the Gripen isn't a bad planform. The combination of the canard and delta wing should give it pretty gentle stall. That said, jets aren't good first planes as others have mentioned. Definitely get the guy you mentioned to try and fly it first.
yeah, tried to use XFLR5 but it just kept giving me errors and so i kinda gave up on it because i think what i have is good enough just.
currently im working on integrating the servos to get the movment that i want and such. I will upload some picutres of them later today probably.
 

quorneng

Master member
NeonGreen
I do agree it is is all to easy to get tied up in the theory to determine what is best and loose sight of what practically works.
My first true EDF was a true scale Fairey Delta 2
FD2.jpg

As a model there are many features that would be questionable. Its wing is a pure delta and is very thin (4% thickness/chord) and a symmetrical section giving a low drag but also low lift. Compared to today the air intakes look almost crude.
On this basis it should not make a good first RC EDF plane, however it does have some advantages. A simple delta does produce lift over a wide range of angles of attack and does not really stall in the conventional sense. The crude air intakes do have an adequate area to feed an EDF.
The main objective was to ensure is would be light enough for the thrust available from the EDF.
The completed EDF painted in the colours of the restored original.
14Jun19.JPG

No undercarriage so a hand launch and belly land.
Despite its apparent drawbacks as a model it actually flies pretty well.
No direct science involved. Its more about knowing what features are likely to work at the plane size involved and how to actually build it.
 

NeonGreen

New member
In terms of control surfaces, I'm going to have canards and elevons.

For the elevons, I just used a simple setup I found online

Screenshot_23.png

which gave me about 32° of deflection down and 50° degrees up. I'm happy with the deflection down, but I think the max deflection upwards should be at most about 20°.
Screenshot_21.png Screenshot_22.png
The canard movement is pretty similar but a bit more advanced. This setup gives me way more deflection than I want, but I’m hoping to tune it within the transmitter. I'll probably end up using a DX6e radio transmitter, and from what I’ve found online, it should allow for some advanced tuning.
Screenshot_24.png

I don’t really know how to plan out moving parts, and I’m sure there’s some fine-tuning to be done, but for now, I’m pretty happy with how it looks. I’m currently printing out a prototype of the canard area just to test it in person, so we’ll see how that goes.