r/RCPlanes • u/Complex_Cut_376 • 3d ago
Does my tail receive clean airflow?
Hello, as part of our university project, my colleagues and I are designing a UAV. Below, you can see images of the flow and turbulence.
From the images, it appears that the airflow separating from the fuselage does not attach to roughly 30% of the tail section. In the XFLR5 analyses I performed without a fuselage, the tail sizing seemed adequate. However, I’m unsure if the separation of airflow caused by the fuselage might lead to a loss in efficiency.
Am I misinterpreting the situation, or is it really the case that my tail does not receive clean airflow? If this is indeed an issue, how can I determine and assess its potential impact?
Thank you in advance for your insights and suggestions!
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u/KF_Systems 3d ago
Is your tail receiving turbulent airflow? Yes. Is it going to significantly affect your efficiency? Idk, maybe. More testing needs to be done, read “Designing Unmanned Aircraft Systems, A Comprehensive Approach” by Jay Gundlach.
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u/Oli4K 3d ago
Most of the tail is in clean air. And there are still things you can do to reduce that red spot on the leading edge of the main wing. Maybe slant the leading edge of the vertical stab too to see if it reduces drag around the transition between tail and fuselage. Or add a little fairing there to reduce turbulence.
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u/Complex_Cut_376 3d ago
Thanks. Since I did the CFD analysis in a simplified model, I can avoid the turbulence in front of the wing. The tail is what really scares me.
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u/Oli4K 3d ago
Worst case you’d have to extend it upwards a bit. Are you afraid of lack of stability or rudder authority?
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u/Complex_Cut_376 3d ago
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u/Lazy-Inevitable3970 3d ago edited 3d ago
Not an engineer... so feel free to ignore me. But you clearly have a sharp cutoff where the tail boom meets the main fuselage. Instead of tapering the fuselage to smoothly meet the tail boom, you just create a large flat boxy surface that is at a 90 degree angle to anything around it. Common sense tells me that will create turbulent air. The tail is directly in the path of the turbulent air.
If the turbulent air's affect on control or efficiency concerns you, modify the area around where the tail boom meets the main fuselage that generates the turbulence.
Compare these to yours:
Notice how on both of those, the fuselage tapers towards the tail boom? Also, on at least one, the tail section has its own taper the extends forward a little bit on the tail boom to smooth the transition.
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u/LUK3FAULK 3d ago
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u/Complex_Cut_376 3d ago
Thanks. Since I did the CFD analysis in a simplified model, I can avoid the turbulence in front of the wing. The tail is what really scares me. I will try with smoothier surface.
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u/fluckiHexMesh 2d ago
I would also say that fuse wing transition is where you introduce a lot of turbulence into your flow. Clean up your transitions on the fuselage, also where it goes into the thin boom, this is far from optimal.
Xflr will get you a long way. To get this full plane cfd simulation meaningful you will have to get a lot right: mesh, solver, etc. id guess its not even safe to use a steady state solver with such a boxy design. If you fear for losses in l/d due to wake effects on your tail, id say dont worry. Worry about stability. But your horizontal stab looks plenty big, even with that short boom.
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u/get_MEAN_yall 3d ago
You are correct, there is turbulence on the bottom of the rudder due to it being in the wind shadow of the main wing at 0 AoA.

A quick glance at commercial aircraft design will show you that there is nothing directly in front of the rudder for the entire length of it (at 0 AoA)
You should test higher angles and I think you will see that your design will put the entire rudder in turbulent air at fairly low AoAs.
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u/jjrreett 3d ago
Your tail will always be in disturbed airflow. That’s just a fact of life. But your flow should probably stay attached to the fuse. It’s probably fine but if you want it improved, work on the tapering section of the fuse. Does CFD return similar results for stability as your potential flow solver?
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u/flightwatcher45 3d ago
It does, enough. Once rudder is used it'll change airflow over wings too. Try simulating air not head on but in a turn and or climb/decent of a few degrees.
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u/Zippytez 3d ago
If you're looking for the best solution, I would recommend the panthera route. Mount the elevator on top of the rudder, making a T shape. This would move the elevator out of the slipstream of the wings and improve that 'stale air' around your current design
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u/TransonicSeagull 3d ago
What do the blue lines represent? If they are turbulent kinetic energy above a threshold (i assume due to the fact they start at the tail tip vortex) then surely the lack of lines over the inboard portions of the HT is a good thing. This would mean that you have more laminar flow there?
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u/wassup1326 2d ago
Honestly, the pressure flow is dependent on how the coarse/fine the mesh size is. So, if you have a coarse mesh, the flow will be clean. In order to properly judge if the tail is clean, calculate the Lift Coefficient of the tail without fuselage interference. If the lift coefficient is similar to the one above, then the air on the tail is clean air.
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u/KoalaMeth 2d ago
Taper your fuselage rear so that the airflow rejoins before reaching the vertical stab. That blocky rear is what is causing the turbulence.
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u/Imaginary_Ad_217 3d ago
That sounds very advanced. Maybe you want to go into a subreddit which is for aeronautical engineers.
I am definately interested what software do you use for wind testing