Why are 3- or 4-seat gliders so rare?

Question

Sailplanes capable of climbing in rising air (not troop transports, which can only descend after the towrope releases) have one or two seats. I know of only one type with more, the Schweizer SGS 2-32 "2​½ seater." Even experimental one-offs like the Perlan series have at most two seats.

Ignoring social aspects such as marketing, class associations, and racing leagues, what are the technical difficulties in making/flying/maintaining a sailplane with more than two seats? Frontal area, if seated abreast? CG range, if seated fore-aft? Something special about mass or wingspan that doesn't apply to powered airplanes?

Answer 1

Because much of the fun in being in a sailplane, is in actually flying the sailplane. It's not a means to get from point A to point B, it's a means to enjoy the magic of flight, much of which involves seeing the sailplane respond to your control inputs.

Once you start getting more than two occupants, most of the occupants won't get to do much of the flying.

It's kind of like asking, "why aren't there more saddles for horses that can handle 3 or 4 people?"

As an aside, raw novice passengers tend to be at high risk for airsickness when subjected to prolonged thermal turns in turbulent conditions, especially if they get to do little or none of the actual flying. Three or four non-flying passenger seats would potentially equate to a lot of clean-up afterwards.

I guess it's all a little subjective-- a commercial pilot friend of mine regularly gives 15-to-60 minute semi-aerobatic rides in a glider with room for two passengers squeezed in side-by-side on a seat in the back (Schweizer 2-32), with controls for the pilot only, and most of his passengers love it (and most don't get sick). But being seated tucked in close to a friend for a little while feels different than riding on a "bus" with, say, four or more strangers-- and in that particular case the very same glider can have the second control stick re-installed in the back and be used for student instruction with one rear-seat occupant and one front-seat occupant. A bigger "bus" with more ample seating for multiple passengers probably would be a poor candidate for a training glider.

Answer 2

As you suspected, the reasons boil down to performance and cost. CG management can be handled by using a 1-2-1 seating arrangement, with little drag penalty (if the cockpit shape is optimized for this) compared to a tandem -- but when flying light, that would leave the passenger in the back seat and conversation in the cockpit would be difficult with the wind noise at high speed.

Overall, however, everything in a sailplane is generally minimized. Dry mass (if you want to fly heavy, you fill ballast tanks with water), drag, and so forth. At a minimum a cockpit design that keeps the CG position optimal will result in some extra front area, producing more drag, and the seats, even empty, will comprise mass that can't be dumped by opening a valve as would be the case with ballast added for speed.

Further, the main reason to want to carry more than two (pilot and copilot or instructor and student) is transport from point to point, which isn't, in general, what sailplanes are built to do.

There are motor gliders with side by side layout -- but they don't have the soaring performance of a proper sailplane due to the compromises that make them more like a light power plane than even a self-launched sailplane (larger engine, smaller wing, heavier construction, conventional gear rather than monowheel, usually) -- compromises which also mean losing the lift at low altitude doesn't mean an out landing, because you can just start the engine.

Answer 3

Another concern is ground handling. All ground operations for gliders tend to be pretty labor intensive. Gliders can't taxi under power. So, they are maneuvered using human muscle and tow vehicles. They are launched while a person holds up the wing. They taxi the same way. Many can be (fairly) easily disassembled for storage or when they land out (in a field away from an airport). Many can just be put into a trailer by pulling off the wings. But, if two people can't carry each wing, it becomes much more difficult. While each of these things could be designed around, they do add extra limitations on the design.

Answer 4

There are technical limitations.

When I fly a two-seater, my thermalling technique is different from flying 15m single seaters. With the single seater it is no problem to center the thermal while observing the variations in climb speed. Tighten the turn when climb speed drops and vice versa.

But with two-seaters I wait for a three quarter turn and only then apply my correction. That means I have to wait with a correction and have fewer chances for them. If I wanted to fly like with a single seater, my corrections would come too late and shift me further from the center of the thermal. Two seaters are heavier and have more wingspan, so both inertial moments and roll damping are greater and make all responses more sluggish, requiring me to fly corrections more deliberately.

Now extrapolate that to a three- or fourseater! While thermalling a single seater (Discus, ASW-24) is fun, doing so in a large two seater (ASH-25 or SB-10) is real work. I cannot imagine doing this successfully in a glider that is again a lot heavier.

The only gliders with more than two seats were designed for gliding only.

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More detailed explanation:

After two or three turns in a thermal you have already a good mental picture where you are relative the core of the thermal. A single seater is agile enough to make corrections immediately, using the change in climb speed as a cue.

A two seater (and an open class glider) is more sluggish and requires to think more ahead. Now those immediate changes become counter-productive because they will be out of phase with what you intend. You have to wait your turn (literally) for the next change, so you will be slower to optimize the flight path.

On top of that: On weak and especially cloudless days thermals are narrow and the higher wing loading of the two seater forces you to fly wider circles, so you will stay in a weaker thermal than the single seater which can fly tighter circles. Going to more seats will drive up wing loading and inertias more and force you to fly even larger circles which give you even less climb speed.

Answer 5

There are no technical limitations.

Only market forces.

Other answers point toward design issues, but I disagree that those are relevant to 3 or 4 vs 1 or 2, since they are the identical design issues which have to be met for any airplane. And since we have known for almost a century how to build 4-seaters that can fly at 200+kts, it's very easy to build one which only has to fly at 50kts.

Technical questions

Weight, frontal area, and C.G. are not issues:

• the side-by-side Stemme S12 achieves 53:1 L/D and its MTOW is 1860lbs. This is one of the highest performance gliders currently available.
• A good-sized tow plane can haul up heavily ballasted gliders with 1-3 people's worth of water stored in the wing.
• Modern design techniques and materials allow side-by-side craft such as the Pipistrel Taurus to completely outperform the 1940's design Schweizer 2-33, which itself is still in daily use in American clubs.
• Any regular four-seater can be flown solo, so there is no C.G. limitation which cannot be accounted for through time-tested design.

Practicality

There's never been any demand for a four seater glider. @quiet flyer has it exactly right, it's pretty boring to be in a sailplane and not be flying it. If you're flying the plane, it's mentally very engaging and thus the time flies by, but when you're a passenger you'd just be spending hours looking out the window before winding back up where you started.

So long as you're not looking to exceed, say, 30:1 L/D, it would be very easy to build an experimental 4-person glider. However, when the pilot is alone it would not perform anywhere near as well as a single-person glider, and there will only be a few times a year-- if that many-- that the pilot would have a plane full of people. So why build it and maintain it?

To take paying passengers would require regulatory certification, which would be very expensive. It would only make sense if the designer intended to sell enough to make back the development and certification costs. Again, market forces dominate here.