What are the design disadvantages of a monowheel landing gear setup which might have led to its historic rejection?

Question

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:

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

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...

Answer 1

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.

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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!

Answer 2

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.

Answer 3

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.