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Monowheels have a lot to be said for them, which is why we use them in gliders. They save weight, drag, and cost, are tolerant of tire failures, and frequently handle outlandings with aplomb.

However, they have not proven to be popular outside of gliders. There are very few examples of them, with only the Europa XS, the B-52, and the U-2 as modern(-ish) examples I can think of. (And even the Europa XS sales have now shifted largely over to tricycle gear.)

Below is a monowheel variant of the Europa XS: enter image description here

And here's a landing B-52 (technically not a monowheel, but still a centerline gear configuration), with the outrigger supporting the starboard wing: enter image description here

While it can be argued that maybe the relative absence of monowheels is because pilots are a conservative sort and will only fly things that "look right", that's a less compelling argument when someone says "your job is on the line". If the monowheel were better for a broad class of planes, the logical conclusion is that there would be commercial pressures which would lead to their broader introduction.

(This would have been especially true in wartime 1940s when rubber was notoriously hard to come by and minimizing the number of tires would have had instant payoff.)

Perhaps the technology to enable monowheels' theoretical superiority wasn't (and still isn't?) present? With the exception of gliders, outriggers are necessary, and those would be dramatically smaller and less draggy with modern composites as opposed to, say, chunks of wood or steel.

Anyone have any hard data on this? Surely NASA, or NACA, or some early airplane manufacturer must have dabbled in this and written down their findings...

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    $\begingroup$ The U-2 was specialized for one and only one purpose, so its landings are not great examples of successful or failed gear topologies, but boy is this video fun: youtu.be/eamnTyfkUBY?si=bD__Gj58ftYYTQGu&t=70 $\endgroup$ Oct 30, 2023 at 2:15
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    $\begingroup$ B52 is definitely not a monowheel, it has 4 main landing gears (each with two tires) and 2 on the wing tips. $\endgroup$
    – sophit
    Oct 30, 2023 at 10:42
  • $\begingroup$ @sophit yes, see my comment in response to your answer. It's possibly a scaling concern. However, it embraces the monowheel approach, where the main wheels are close together that it is not fully stable while on the ground, and needs outriggers to support the wings. $\endgroup$ Oct 30, 2023 at 10:57
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    $\begingroup$ Building on your point, if we define "monowheel" as anything which requires outriggers (and maybe this isn't a fair and reasonable definition), there are a wide variety of "monowheel" configurations, incl. main-and-tailwheel (most gliders), main-and-nose-and-tailwheel (ASK21), bicycle (Harrier), quad truck (B52), and maybe more? $\endgroup$ Oct 30, 2023 at 11:00
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    $\begingroup$ KennSebesta - I think I see what you are saying. Tail-dragger and tricycle gear have side-by-side main landing gear bogies. They may also have additional center gear under the fuselage (MD-11). You are describing planes with center main landing gear only. In some cases monowheel ( Europa XS), in other cases two-wheel center bogie (U-2), in other cases tandem center bogies (B-47). I think the B-52 may not be a good example however because even though it has bicycle main gear and outriggers and visually has a "centered" look, it does not have center gear, only side-by-side main gear bogies. $\endgroup$ Oct 31, 2023 at 11:54

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If an A380 landed on one wheel only it would leave a groove so deep that the runway should be rebuilt after every landing.

A standard civil tarmac runway can sustain tires with a maximum inflation pressure of 7-8 bar (100-120 psi - a tarmac runway couldn't survive a Grand Tour race 😉). On grass the maximum pressure might be half of that value. Multiplying this maximum inflation pressure by the footprint area of the tire we get the maximum weight that each tire can bear before ruining the landing strip. Given the total weight of the aircraft, we can then decide to spread it either on few big wheels (one single wheel as an extreme case) or many small wheels.

For a jetliner the second option (many small wheels) is the preferred one due to several reasons:

  • it gives a more compact design for the landing gears;
  • the design is redundant; should one wheel get damaged the other ones can sustain its share of weight without damages;
  • smaller wheel implies less inertia to start rotating at touchdown;
  • they can be inflated at a lower pressure increasing their life.

Just for fun, let's see what tire our monowheel A380 would need. According to this NASA report, the footprint area $A_p$ of a tire can be estimated via the following empirical equation:

$A_p=0.45d²$

where $d$ is the nominal diameter (in inches). This is the equation 12 in the report with the additional assumptions that under load the tire gets squashed of 1/3 of its diameter and that its width to diameter ratio is 1/3 too.

Using the maximum 120psi allowed for a civil landing strip and the 1'200'000lbf of an A380 we get:

$A_p=1'200'000/120=10'000=0.45d² \Rightarrow d=150 in$

Well, a very big wheel indeed!

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  • $\begingroup$ Technically, an A380 could still land on only one wheel, you'd just need to make it very very large $\endgroup$
    – MikeB
    Oct 30, 2023 at 10:17
  • $\begingroup$ @MikeB: it would be more practical to simply slide on the belly 😉 $\endgroup$
    – sophit
    Oct 30, 2023 at 10:40
  • $\begingroup$ @sophit, that's an excellent point about scaling effects. Building on your point, the B52 isn't a true monowheel, since it has a series of wheels narrowly grouped together. However, a single main wheel seems to be compatible for lightweight GA and even the U2, so scaling effects alone cannot be the cause. Do you have any data about where the crossover point is, where a wide enough, tall enough tire would no longer fit into the airplane? As you point out, gliders are light enough not to have an issue, and so presumably are many (most?) GA airplanes. $\endgroup$ Oct 30, 2023 at 10:54
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    $\begingroup$ Even the U2 doesn't use a single wheel - both the fore and aft undercarriage has two tyres side by side $\endgroup$
    – MikeB
    Oct 30, 2023 at 17:28
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    $\begingroup$ @Jpe61: done 😉 $\endgroup$
    – sophit
    Nov 6, 2023 at 13:29
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There are some serious show stoppers before you even get started.

The biggest problem for monowheels is, if you want minimalist outriggers near the wing tips to really gain the drag or structural benefit of a single main wheel, you you need a maneuvering path that is wider than the plane's wingspan. And even if you do find a taxiway that is wide enough, unless it's WAY wider, you need a center line to follow to avoid driving your outriggers into the weeds.

And if you move the outriggers inboard to make it able to operate from narrow taxiways, they have to be much more robust, and you are creeping toward regular landing gear now, except you have three units, one big middle one, and two smaller ones outboard. Might as well do it the regular way and do away with those hassles, which is the reason almost nobody does that configuration outside specialized cases.

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  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Aviation Meta, or in Aviation Chat. Comments continuing discussion may be removed. $\endgroup$
    – Ralph J
    Oct 30, 2023 at 23:32
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not proven to be popular outside of gliders

The reason may be the ability to land and take off from less than ideal surfaces.

Two strong wheels and a tail-skid was the norm for the first 50 years of aviation. Many "airfields" of this time were little more than closely cropped stretches of grass.

Paved runways opened the door to not only monowheels, but also nose wheels. But weakly designed landing gear simply won't cut it out in undeveloped areas, where a mechanical breakdown means you're stranded.

Two main wheels also offer the possibility of differential braking to improve ground control.

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  • $\begingroup$ That's an interesting point about ground control. Maybe after much experimentation there was no satisfactory manner to steer on the ground without either a short-coupled nosewheel or two wheels with differential braking? $\endgroup$ Nov 3, 2023 at 20:43
  • $\begingroup$ Taxiing a mono wheel in a cross wind may not be easy, but a weight savings of 100 lbs is of interest. $\endgroup$ Nov 3, 2023 at 23:08

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