Bicycle landing gear

[PR19_Kit]

I'm working on a Whiff which hopefully will have bicycle landing gear, but there seems to be two or three schools of thought about how they must work in the real aviation world.

Some aircraft have the main, larger wheel, on or around the CG, and a smaller nose wheel with outriggers. I'm thinkiing here of the Yak-25 series, and they rotate around the main wheel to take-off.

Others, and the obvious candidate is the B-47 Stratojet, have two sets of equal sized wheels with outriggers. Strats seem to levitate rather than rotate, the whole aircraft lifts off at the same angle and vanishes into the clouds. The B-52 does the same but it is rather a special case with it's umpteen wheels.

The third sort have their main wheel well forward, and a smaller tail wheel with outriggers. The best example of this is the U-2, and they don't so much rotate or levitate, they just propel the Earth downward so eager are they to get airborne!

The question is, why are there so many different arrangements and what's better about one setup over the others?

Looking back, the Yak isn't that good an example, as the later Yak-28s had an even crazier arrangement with a large main wheel WELL aft. Quite how they rotated that lot I have no idea.......

[rallymodeller]

I think it has more to do with the aircraft's all-up weight and how it is distributed. The Myasischev M-4 Bison has two four-wheel bogies as well as outriggers. More wheels distribute the weight better. The later Yak designs may have had something to do with rough-field performance, although I agree the design is weird. I don't have any films of the Yaks taking off, but I suspect that it rotates around the rear bogie like a conventional aircraft.

[jcf]

The bicycle gear as used on the B-47 was originally a Martin development, their XB-48 was the first aircraft
specifically designed with the gear, and it was tested in 1945 on a modified B-26 called
the Middle River Stump Jumper.



As rally points out choosing the gear layout does seem to involve matters of weight distribution and load.

The Vautour is another example of a layout similar to the B-47, XB-48 etc.


Jon

[Daryl J.]

The Marauder pilots must have been praying those external stiffners worked!   

Daryl J.

[B777LR]

Cant forget the harrier, its almost a tricycle gear with the 2 side-by-side main bogey

And the B-52, that had 4? sets of wheels on the center (2 next to each others), with the wingtip wheels working more like struts keeping the wings off the ground...

[PR19_Kit]

Thanks for the thoughts guys. I knew about the XB-48 and I assumed it levitated like a B-47, and I've seen a Vautour fly myself, and it does the same. I'd forgotten about the Bison, it seems to follow the B-47 setup with two large bogies spread equally around the CG.

As for the Harrier, I'm not sure it fits this scenario really. After all the very last thing a Harrier does is 'rotate', of all the others it really DOES levitate!

It does seem as if bicycle landing gear aircraft don't need to rotate in the (almost) conventional manner, and tend to be 'flat risers', so I'll assume my invention works like that and build it accordingly/

[Mossie]

Kit, there's an unusual VG design in BSP Hypersonics (pg 42), the APD.1017 appears to be some kind Tomcat-alike.  Not quite bicycle undercarriage, it has an unusual reversed tricycle layout, with single main undercarriage on a single twin bogie.  It appears to give provision to rotation, as it has a tail bumper wheel.  However, the datum line seems to intersect all the wheels, so it may have a tail down attitude on the ground to assist take off.

[kitnut617]

Having seen a few B-52's land and take-off at Calgary International, I was facinated by the way they did take off, the whole plane seemed to lift off all at the same time like Kit says.  When they did leave the ground it was only after a very short run too but the pilots would hug the runway as far as they were allowed and then climb out, but they had an unusual tail up attitude when they did this.  Anyone care to explain this for me?

Robert

[Ed S]

Having seen a few B-52's land and take-off at Calgary International, I was facinated by the way they did take off, the whole plane seemed to lift off all at the same time like Kit says.  When they did leave the ground it was only after a very short run too but the pilots would hug the runway as far as they were allowed and then climb out, but they had an unusual tail up attitude when they did this.  Anyone care to explain this for me?

Robert

Big FLAPS.  The flaps on the B-52 change the wing angle of attack so much that even with a slight nose down attitude, the wing is still moving up.

Ed

[kitnut617]

Big FLAPS.  The flaps on the B-52 change the wing angle of attack so much that even with a slight nose down attitude, the wing is still moving up.

Ed

Well that would explain it, cheers Ed    That would suggest then that the rear wheels would leave the ground first, if only momentarily.

[PR19_Kit]

When the BUFFs were regulars at RAF Fairford (dropping explosive things on people who'd rubbed various Bushes up the wrong way) they would take-off pretty much over my office. (when I was there of course) They'd still be 3-4000 ft up then, with the gear retracted but they still had at least some flap, and would still be in that odd nose-down mode.

When required, such as a RIAT display, they could emphasize the nose-down climb bit to a ridiculous degree, sometimes seeming to want to dive into the gravel pits on the other side of the main road. Perhaps they used landing flap to take-off with for that? Certainly was impressive though. specially with LOTS of smoke!

Mossie,

I'll check out that F-14-alike when I get home, I just bought that Hypersonics book and haven't looked all through it yet, thanks for the pointer.

[lancer]

If you want a decent bicycle undercarridge, why don't you try one of those all singing, all dancing, 10000 geared, gas suspention mountain bikes??.... Oh sorry, wrong kind of bike...



No, I didn't have a coat, I'll just go quietly....

[tinlail]

The main thing to determine is the angle of the plane need to climb or descend.
The B-52 has it's wings canted up so that it can climb when the fuselage is level, this gives it the nose down appearance in flight, when the wing is level. The other choice is to have the front gear longer that the back gear to give the entire plane a nose up attitude. Plane that have that setup tend to have large nose gears to distribute the landing forces of a nose first touchdown. I suppose a bicycle geared plane could have it's back wheel as far forward as most tricycle geared planes, but I am not aware of any that are arranged that way. So they can't rotate, or flare as much as tricycle geared planes.

I suppose a variable incident wing like the F-8 could be helpful in such a situation.

[The Rat]

Another thing that will affect whether an aircraft needs much rotation is simply how much lift the wings are generating at low angles of attack. Spend some time in sailplanes and you'll find that those very high aspect ratio wings just pull the rest of the contraption straight up with no rotation whatsoever, and sailplanes don't have flaps. The U2 is the same, they don't really rotate until they're already off the ground. Both the B-47 and 52 had high aspect ratio wings, that would explain a lot.

[PR19_Kit]

If you want a decent bicycle undercarridge, why don't you try one of those all singing, all dancing, 10000 geared, gas suspention mountain bikes??.... Oh sorry, wrong kind of bike...

I have six bikes with suspension already thanks, but they are 1960s Moultons, built LONG before mountain bikes were even thought of......

I like the ideas about the wing incidence being the deciding factor. Checking through the lists of bike landing gear a/c it does seem as if most sit very nose high on the ground or they have the wing at a hefty incidence angle relative to the fuselage. The one that goes against that is the U-2 and its variants, in that although it sits at a slight angle on the gorund, its wing is at 0 degrees pretty much. I do take the point about it not rotating until it's airborne though, it has so much grunt that the wing generates oodles of lift quite quickly.

The same is true of a sailplane, in that either the winch is doing all the work, or the tow plane. I contest that sailplanes don't have flaps though, many high performance sailplanes do have them, to minimise their flying speed when in lift, and to minimise the drag when flying between lift areas, although they tend not to use them for take-off.

It looks like my planned aircraft will have to sit on the ground a bit nose-up as I've already got the wings set at around 0 deg incidence.