Tri-cycle undercarriage

[Genghis the Engineer]

I just thought I should share this, I was looking something up in a very old (1939) textbook on aeroplane design, and came across the following...

The present interest is in the so-called "tri-cycle" landing gear. This reverses the location of the single wheel which in the conventional landing gear is the tail wheel, and now becomes the nose wheel for the new type.

...

Various tests have been made by the Army, by the National Advisory Committee for Aeronautics, and by a few commercial companies.

.....

There should be greater passenger comfort since the passengers are sitting in a cabin which is level not only when flying but also on the take-off, and in landing. With conventional gear of today, the three-point attitude of the airplane on landing and at the start of the take-off may be decidedly uncomfortable.

...

Since the airplane cannot nose over, there si the possibility of landing at almost any angle of attack. For the private flyer who may be a "dub" pilot, this is a very good feature since the landing technique need not be well-nigh letter perfect. For transport airplanes, the possibility of landing at almost any angle is advantageous in blind flying when the ground is not visible.

However it also says...


The front or nose wheel may "shimmy" unless there is friction damping.

...

Difficulty may be encountered with the nose wheel in riding over obstacles. The tail wheel seem to behave better under such circumstances.

...

The tail wheel has the advantage of protecting the tail surfaces. Unless a skid or special "crash pad" is provided for the rear portion of the fuselage when the nose wheel type of landing gear is used, the rear portion of the fuselage may be damaged in case of an unusual "tail low" landing.

(Airplane Design Manual, Frederick K Teichmann, Pitman, 1939)

G

[Jan Olieslagers]

Thanks for sharing! It is nice to look at things "the other way around" - I was taught to do the same in chess, many years ago.

As for
Unless a skid or special "crash pad" is provided for the rear portion of the fuselage when the nose wheel type of landing gear is used, the rear portion of the fuselage may be damaged in case of an unusual "tail low" landing.
yes, my little pride has such a skid, and a mighty relieving idea that is when landing at (in my beginner's eyes) absurdly high AoA on short runways, the stick all the way back and almost loosing sight of the horizon - or so it feels, at least.

But your nice text doesn't mention what was mentioned in my training as the main advantage of the tricycle configuration: avoiding ground loops just after touchdown, because now the centre of gravity is BEFORE the "point of attack" of the braking power. Whereas a taildragger has it the other way round, making the momentum seek a way of overtaking the braking power of the main wheels.

[BackPacker]

No mention at all of the inherent stability of having the steering wheel(s) in front of the CofG and the non-steering wheel(s) behind, vs. the inherent instability of having the steering wheel(s) behind the CofG and the non-steering wheel(s) in front?

[John Farley]

To be fair to the aticle from which the quotes were taken, it might be worth reading the whole thing rather than assuming your very valid points are not covered elswhere...............

[Jan Olieslagers]

Fair enough, John Farley. I reckon we can assume GtE saw no need to quote what must be obvious to anyone with a basic understanding of the fundamentals of aviation.

[Genghis the Engineer]

There was three pages of it, I cherry picked.

G

[BackPacker]

Well, given the tone of those quotes, I was half expecting the authors to actually see the directional instability of the conventional (tailwheel) configuration to be a good thing. To prevent pilots from getting lazy feet or something.:*

[Genghis the Engineer]

http://a8.sphotos.ak.fbcdn.net/hphotos-ak-ash4/396222_360986640587046_100000271394563_1381327_1850514797_n. jpg

http://a1.sphotos.ak.fbcdn.net/hphotos-ak-ash4/418448_360986760587034_100000271394563_1381329_185277152_n.j pg

http://a6.sphotos.ak.fbcdn.net/hphotos-ak-snc7/407151_360986917253685_100000271394563_1381330_337010106_n.j pg

[Pilot DAR]

So Genghis, If I'm understanding correctly, we land a tricycle aircraft on the main wheels first?

Best Landing Ever...Nose Landing Gear First - YouTube

[foxmoth]

Interesting that in point 6 of advantages they talk about braking on all three (sets of) wheels, does anyone know any aircraft this was used on?

[Shaggy Sheep Driver]

But for ab-initio training, the tailwheel is best precisely because it will not tolerate poor footwork (because of directional instability) or a not-correctly-held-off landing (becuase any bounce will throw the nose skywards, where the pilot has to sort it out, rather than forwards where a nose wheel catches it).

When nosewheel-trained pilots convert to tailwheel, most of their conversion training is simply to correct basic errors in technique that the nose wheel configuration has allowed them to get away with.

[patowalker]

One advantage of the tri-cycle undercarriage is that the lower AoA makes the aircraft less vulnerable to high winds when tied-down. I wonder whether this is reflected in insurance premiums.

[John Farley]

I guess whether it is better to use a tailwheel or nosewheel aircraft for initial training depends on whether you want to give the student the most comprehensive training or whether you want to allow them the greatest chance to gain a licence should they turn out to have limited natural ability or money.

[Shaggy Sheep Driver]

Indeed John. My comments were a tad tongue-in-cheek knowing full well that the world wide training fleet is almost entirely nosewheel, and indeed I did my PPL back in the 1970s in C150s, for precisely the reasons you state.

[Pilot DAR]

Perhaps John would offer us some observations on bicycle undercarriage.;)

[Shaggy Sheep Driver]

Now those are just wierd (a la B52). An aeroplane that can't rotate to land or take off.

Most un-natural!

[John Farley]

About as tricky as a tailwheel but in different ways. Very intolerant of much rate of descent at touchdown. Can be very prone to porpoising. Rather depends how far behind the CG the mainwheel is. Could need to touch with nosewheel very close to ground or even at the same time. Tolerance of poor touchdown likely to be more type dependant than tailwheel or tricycle.

['India-Mike]

If prone to porpoising does that make the mechanics similar to porpoising in a floatplane? And hence the cure similar too?

[Pilot DAR]

I speak with no experience relative to bicycle landing gear aircraft, but porpoising water borne aircraft is something I have some experience with.

Interestingly, a floatplane and a flying boat have differing tendancies in this regard.

An aircraft will porpoise (mostly on landing) because it can. Pilots have some opportunity to prevent it, or at least "check" it. My experience has been that aircraft whose first contact wheels are very close to the C of G is less likely to porpoise. It' a bit different with a hull.

Consider the basic C-150/152/172. You would probably agree that if you load that aircraft to an aft C of G, and gently push the tail to the ground, it'll darn near stay there on it's own. Thus, in that pitch attitiude, and loading, the C of G is very close to the mainwheels. This is in part because the aircraft is very nose high, and with the aircraft C of G higher than the mainwheels, it moves back as you raise the nose.

So if the ground reacts back to the aircraft on landing, it reacts to the aircraft through the mainwheels (for a "good" landing) and the aircraft reacts back through it's C of G. If one is vertically in line with the other, there is no tendancy for the reaction of the ground and the aircraft to combine to change the pitch attitude - no porpoise.

You can probably relate that if you land that Cessna very nose high, you might drop it on, and bounce a bit, but the pitch attitude remains pretty well what the pilot has set. If you fly that Cessna on at higher speed, it's going to try to porpoise. The plane still wants to fly, and the ground has reacted back up through a line which is aft of the main wheels. It's gonna force the nose down a bit, the pilot will pull up a bit, and the porpoise begins. In my experience, Aztec "F's" are touchy this way.

If you land a taildragger sloppily, the same will happen in reverse. If you bounce on a wheel landing, it's going to pitch the aircraft up, as well as reacting it up, the comination is going to put you back in the air nose up. You'll push the nose down, it will hit again, and the porpoise begins. If you contact the tailwheel first, you're okay for the one moment it takes for the mains to hit, and then the same cycle begins. As long as the tail wheel is on the ground, you do not have pitch control, and you're just along for the ride.

That's why a good "three pointer" is satisfying, if you get it right! A wheel landing can also work very well, as long as you keep the nose low to prevent a return to flight.

So the planes that float... The difference lies in the fact that the point of support is fluid, and thus the reaction of the water up to the plane can change rapidly. Waves, and the fact that water cannot be compressed contribute to the point of support moving relative to the C of G. If it's moving, and the pilot allows a pitch change to result, the porpoise begins. This is worsre than on wheels, as the affect of the water on the hulls(s) is to react back up at a position reltive to the aircraft C of G which can change a little, but quite rapidly.

In the floatplane, you can land fully stalled in the water, get the aft end of the floats in, and still have some pitch control, as the tail is yet further aft, and still able the change the pitch attitude of the plane a little, but enough for th epilot to overcome a porpoise. I very happily land floatplanes fully stalled. Generally you can touch the steps and heels at the same time, and get a pretty gentle landing, certainly with no porpoising.

In a flying boat, a full stall landing is going to give you a real porpoise, and once it starts that way, there is no pilot action to correct it, other than to hold the controls nose up and wait it out. It's going to be too late to put the power on and go around, you're already off the step, so you're not flying back out. Like a taildragger, once a flying boat's aft hull is in the water, the pilot has very little pitch control, and can only hold the attitude nose high to slow it down as fast as possible. Failure to hold the nose high is likely to result in a smashing the nose into the water during a porpoise.

On the other hand, step landing a flying boat, is very likely to result in a great landing, and litle chance of porpoise. It is possible to touch calm water, and hardly feel it, just a tiny tug of the slight drag from contact
with the water.

Taking off either form of water aircraft is where the porpoise can easily happen. You start off pulling to get onto the step, then pushing over on the step. This pull to a push, combined with the moving center of bouyancy can set up the porpoise. The pilot has to react fairly quickly with the un natural motion of pushing the nose down while accelerating during takeoff. Many a Lake Amphibian has been wrecked because it got airborne early off the water during a porpoise, the pilot lowered the nose too far too fast, and smashed back into the water. Lakes got a bad reputation for the high thrust line and difficult to handle on the water, but this was only with pilots who had not received proper training in them. A Lake will generally climb away safely once airborne, if you have the nerve to try it. It's better than the land back hard and porpoise though!

[Mixed Up]

For transport airplanes, the possibility of landing at almost any angle is advantageous in blind flying when the ground is not visible.


:confused:

[Pilot DAR]

'Reminds me of an old saying:

"The chances of survival are inversly proportional to the angle of arrival."

[FREDAcheck]

...some observations on bicycle undercarriage
In a crosswind I sometimes touch down on a unicycle undercarriage, but I keep another couple of wheels around in case I lose my balance as I slow down. :O