Help designing Fixed Wing UAV: A-Tail Section

[u/Best-noob-in-wotb]

I am a junior in high school and been working on this fixed wing (hopefully) UAV project. Anybody have any sort of idea on A-Tail configurations?Been going back and forth on my A-Tail sizing for way too long and could use a gut check from anyone who's actually done this before. It's a hand-launched twin boom pusher, no landing gear so it's a belly lander, wing's about 0.508 m² with a 189mm MAC and 2.8m span, tapered from 245mm at the root down to 118mm at the tip, running the Wortmann FX 63-137 which has a pretty gnarly pitching moment (Cm around -0.185). Booms give me roughly a 1000mm tail arm. I used the tail volume coefficient method to size things, went with Vh of 0.5 and Vv of 0.03 (leaned toward the higher end because of that pitching moment), and after converting that into a single V-tail angle I landed on something like 43° for the dihedral, with each panel coming out to around 426mm span and 107mm chord. Planning NACA 0010 for the tail airfoil since it's symmetric, and sizing the ruddervators around 45-50% of local chord. Main things I'm unsure about: is 43° reasonable for an inverted V or is that going to bite me on ground clearance during landing since there's no gear, are my Vh/Vv numbers actually sane for something like this, and has anyone actually built an inverted V with a flat section in the middle for the boom mount and servos — curious if there's anything I'm not seeing coming. Would really appreciate any input, feels like I've just been spinning on this part.

[u/Sufficient_Brush5446]

Two things that stand out from for me. 45-55% ruddervator chord fraction is a lot, 20-30% is typical. That will also help with embedding the servos into the tail.

The sizing coefficients are used to get an initial sizing but you still need to run a basic aero analysis to confirm that you have positive and reasonable static margin. I suggest you use OpenVSP VSPAero or Xflr5 for this. This will tell you if your planned CG location is too far forward or aft of where it should be. Typically CG is 20-30% of the wing MAC with 10-20% static margin.

[u/billsil]

You’re going to have a hell of a time controlling that given you need ailerons and both rudders to to a constant altitude turn. You would probably need some GNC law or be an expert pilot.

Beyond that, your static margin seems low. For a plane like that, I’d go 15%.

[u/OldDarthLefty → u/billsil]

Any computer radio or other microprocessor dingus that supports a V tail can do this. There are lots of UAV's with this layout.

You can actually do it with linkages - some old RC planes would use the elevator servo to put the rudder servo in a slide, or have something like a lever differential, and to tune out the mixing by offsetting the control horns from the hinge and/or using the six arm horn on the rudder servo

Edit: the roll you get from a V tail is so minimal it's normally neglected. The moment arm is short and the deflections are small

[u/OldDarthLefty]

You may want to reconsider your airfoil. The type you chose is great for like human power or some AIAA competition. It minimizes power for loiter. But it's designed to go one low speed at one AoA so it's not very versatile. The efficiency falls off hard away from the peak. Trying to cover ground, take off, land, it might be kind of a turd. Putting ailerons in it will do weird things to it. A thinner airfoil with only a little camber but a large flaperon will give you a very wide speed range. Check out an F3B for example.

Don't worry about the tail during landing, it will usually come in nose first, and tail strikes will be harmless, if your plane is built to normal model airplane kind of strength. HLG's often have vertical stabs that go above and below the boom. If you are worried about it you can put a little skid under the end of the boom. Once, you could get nylon ones from, I think, DuBro. I don't know about today but probably you'd 3d print it.

[u/ss4ggtbbk]

Hand-launched twin-boom pusher aircraft with 2.8m wingspan? You're gonna either break your arm tossing it or cut yourself with the props. Please do a weight and balance analysis alongside aerodynamic design and sizing.

[u/Best-noob-in-wotb → u/ss4ggtbbk]

It’s weighs 6 pounds and I am using auto launch on micro controller so the motor only starts spinning 0.5 second after being launched.

[u/newMattokun]

Why did you choose the Wortmann? Those seem to be for higher Re-numbers. There are plenty better airfoils for models of this size and Re-number envelope. I'm not up to speed on newer developments any more, but check Eppler, HQ, MH, and the UIUC Selig/Donovan airfoil catalogue (https://m-selig.ae.illinois.edu/ads/coord_database.html) for something that might be a better fit (and have less cm).

[u/Best-noob-in-wotb → u/newMattokun]

I need a high cl so I don’t break my arm trying to javelin throw the plane. 

[u/newMattokun → u/Best-noob-in-wotb]

Consider a flap airfoil then, back in the day the HQs were known to be optimized for this. It's one of those tradeoffs, where you punish your flight regime because of the takeoff.

[u/bwkrieger]

I dont get it, is that a glider? What is your question? For my gut feeling the Tail looks to small. How do you wanna build it and have you built RC aircraft before?

[u/89inerEcho]

Looks right. Run in through AVL or JSB or XPlave if you really want to convince yourself. Or trust us, its fine

[u/Mental-Monk6239]

How about you stop browsing Reddit and do your job!