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My team is designing a glider and I am unsure about whether or not to include a vertical stabiliser. The task the glider must complete is to achieve the largest possible displacement in a straight line.

Our glider will not be able to automatically adjust the vertical stabiliser's position so it would stay upright and inline with the glider's fuselage for the duration of the flight. Since our glider does not require any changes in it's yaw angle (because it is flying in a straight line), I don't think there is a need for it.

Should we include a vertical stabiliser or not?

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    $\begingroup$ I'm assuming this is unmanned. $\endgroup$
    – GdD
    Commented May 9, 2019 at 14:27
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    $\begingroup$ Yep just a casual project. $\endgroup$
    – Tom Finet
    Commented May 9, 2019 at 14:28
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    $\begingroup$ Aren't you going to tell us whether the wings are swept or not? Google "Zagi glider". Those fly ok even if you take the fins off. Lots of info missing here. If the vertical fin fell off a typical sailplane like say a Glassflugel Libelle the results would be catastrophic. This is not a good question sorry to say. $\endgroup$
    – quiet flyer
    Commented May 9, 2019 at 14:34
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    $\begingroup$ I don't know much about aircraft design, but if you told me that your boat didn't need a rudder because you only needed it to go in a straight line, I would ask how you proposed to keep it on that straight line if you don't have a rudder to correct the course? $\endgroup$
    – Solomon Slow
    Commented May 9, 2019 at 21:55
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    $\begingroup$ @SolomonSlow Most boats, particularly older wooden designs, have a pronounced keel along their entire length, which will have a significant effect. $\endgroup$
    – MikeB
    Commented May 10, 2019 at 15:00

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The stabilizer is meant to keep the nose pointing into the wind.

Any asymmetry in he wings/fuselage will create a tendency to yaw, you will need to compensate for that. The easiest is a fixed vertical stabilizer at the rear.

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The underlying function of a VTP (not rudder) is mitigating the lateral-directional eigenmodes of the aircraft (Dutch roll, spiral divergence and roll subsidience). Unless you can guarantee a perfect launch and no crosswind, you will need some kind of yaw damping.

For a conventionally-shaped aircraft, Dutch Roll will likely be the worst offender, as roll subsidience is convergent and spiral divergence is slow.

A vertical tailplane is just one solution, however. Other things that can contribute to yaw control are positive wing sweep and active control via spoilerons or differential engine thrust. The latter is commonly employed in flying wings, but since your airfoil is not a reflex one, I take it you are going the conventional route.

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It's unlikely that your project will succeed without a vertical fin. Are you aware that there are countless radio-controlled model airplanes and gliders that feature a fixed vertical fin but no rudder? The fin is much more than just a thing to attach the rudder to. The rudder itself is not very necessary if the aspect ratio of the wing is not too high. (High-aspect-ratio designs tend to suffer from severe "adverse yaw".)

But, the birds seem to be able to pull it off, so you never know. The distribution of the "twist" or washout in the wing seems to be a key factor for them.

The literal answer to the question in the title is definitely "no". Many successful gliders have been flown without vertical fins. and they can do much more than just fly in a straight line. But they don't typically look like a "normal" glider with the fin chopped off.

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    $\begingroup$ The birds have two things model (or real) airplanes lack: Extremely flexible wings which can generate large yawing moments (adverse and proverse, as required), and semi-automatic stability augmentation by the bird's brain. That explains why they don't need a vertical. $\endgroup$
    – Peter Kämpf
    Commented May 9, 2019 at 18:28
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    $\begingroup$ @PeterKämpf Also I have observed bird tailfeathers be articulated in many ways, including angling them as a group to seemingly create (and presumably cancel) yaw as well as bank. $\endgroup$
    – Todd Wilcox
    Commented May 9, 2019 at 20:05
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    $\begingroup$ Absolutely to the last, they definitely twist their tails to vector the lift to create a rudderlike effect. If you watch carefully it is not obvious that the resulting "inputs" are the same ones a sailplane pilot would make with the rudder while entering a turn or exiting a turn, etc. $\endgroup$
    – quiet flyer
    Commented May 9, 2019 at 20:11
  • $\begingroup$ The V-tail Bonanza had no fin. $\endgroup$
    – copper.hat
    Commented May 9, 2019 at 20:49
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    $\begingroup$ @copper.hat yes it does, in fact, it has two. $\endgroup$
    – AEhere supports Monica
    Commented May 10, 2019 at 8:45
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Coming from a very different background here - I am a keen archer.

It is normal practice (as with aircraft) to have fins (fletches) on the back of the arrow to keep it straight, but it is also a normal part of "tuning" to remove those fletches from an arrow or two, and shoot them 'bareshaft'.

The point of this test is that they are less forgiving, but because all arrows have a heavy point, the centre of mass is forward of the geometric centre, which means that the whole of the shaft acts as a stabiliser, and given good conditions, the arrow will still fly straight (and this IS largely aerodynamic flight, quite different from the ballistic flight of a bullet).

Going back to what I was saying earlier, it would be difficult to predict how much disturbance it could survive, but a simple, long straight fuselage, with no stabilising fins at all, IS going to fly fairly straight unless/until tubulance goes above a critical threshold. To be fair, this is also true of a glider with 'normal' fins, its just that the critical amount of turbulance is much higher.

One question that doesn't seem to have been mentioned is 'what is the penalty for failure?' - In a manned vehicle, failing to fly straight is considered 'a bad thing' but if your model is allowed multiple flights, then a few failures may be worthwhile if the trade-off is reduced drag.

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  • $\begingroup$ fascinating stuff !! $\endgroup$
    – Fattie
    Commented May 10, 2019 at 12:40
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Some swept-wing designs have no vertical surfaces e.g. "Horten" or "flying wing" or "Alsomitra". NASA's Albion Bowers presented recent advances in "Why Birds Don't Have Vertical Tails" and "On the Minimum Inducted Drag of Wings".

Designing an optimal washout for a swept-wing is an advanced subject. The simplest design is a conventional glider with a straight wing and a vertical stabilizer on the tail. Find some easy-to-build gliders by searching: free-flight glider plans

A longer tail and more dihedral will make the glider fly straighter.

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It may not be whether you need one or not, it will probably be getting the right size.

Theoretically, one can design without one like a delta wing dihedralled paper airplane. The sides of the fuselage aft of CG act as a vertical stabilizer (make them flat).

However, in practice, for free flight (uncontrolled), it is almost impossible to get the wing drag to perfectly match for long efficient high aspect ratio wings, so on go the fin(s), but where?

Many gliders have wings longer than the fuselage. A fin can go at each wing tip, or conventionally on top of the fuselage. The wing tip fins will have a longer torque arm, there for can be smaller. But the conventional tail placement has a drag saving trick up its sleeve, putting the vertical stabilizer on top. This uses the drag of the T tail for down force rather than deflecting the elevator. And, of course, you can combine the Hstab and V stab and make a "Vee" tail.

So design for a vertical fin, and test in in various sizes. If it works without one great! If not put it (them) on there. That is exactly what the Air Force did when they converted the propeller driven B35 flying wing to that jet powered B49.

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In reference to radio control models, there have been many that don't have a vertical fin of any kind. The Klingberg Wing (from the 1980s/1990s) is a fine example; it's a pure wing R/C sailplane. There have also been human-carrying sailplanes built along similar lines (generally, a Horten derived layout) that flew well without any sort of vertical surface.

If the glider is built for free flight (i.e. uncontrolled), it will generally need some kind of vertical in order to maintain spiral stability, but this is largely because free flight models are generally set up to fly in circles, both so they can catch thermal lift, and so they don't just fly away as soon as they're launched. In your case, however, where you want the glider to fly a maximum straight line distance, a setup similar to a Klingberg wing (swept wing, longitudinally stabilized by tip twist) is likely to be one of the most efficient possible.

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    $\begingroup$ The Klingberg wing is actually identical to a Horten wing, and its yaw stability is achieved by sweep. Sweep generally costs performance, as does sideslip to which both are very prone, especially at high speed. $\endgroup$
    – Peter Kämpf
    Commented May 9, 2019 at 18:33
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Yes, but you will need some alternate means of directional control in yaw. Differential drag can work with split ailerons, spoilers or spoilerons. Other options can be engineered but they will likely add a lot more weight, which you should avoid on a glider.

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  • $\begingroup$ Weight is not a fundamental issue with a glider. It just means you have to fly faster to achieve the optimum glide angle. In fact in manned gliding competitions, the regulations specify the maximum permitted weight in a class, and competitors usually carry ballast (sandbags) to max out the weight and fly faster. $\endgroup$
    – alephzero
    Commented May 10, 2019 at 15:12
  • $\begingroup$ Ballast is one thing, permanent added weight is another. $\endgroup$
    – Juan Jimenez
    Commented May 11, 2019 at 16:21
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Hang gliders and paragliders use a pendulum effect to mitigate yaw disturbances. So much so in the case of paragliders, that a marked anhedral is needed to allow the pilot to steer

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