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Module 1: Aircraft Structural Design

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Features of Landing Gear

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Lecture – 36
Landing Gear Layout – Part - 01

(Refer Slide Time: 00:21)

Let us have a look at landing gear layout and how it impacts aircraft conceptual design. The landing gear is typically considered to be the most difficult subsystem of an aircraft to be designed and to be configured and it has to meet a host of very difficult and sometimes conflictive requirements. It has to bear the static loads when it is on the ground. Because that is the item on which the whole aircraft is resting it has to allow smooth transition of the aircraft from the ground to air during take-off.

When it comes into land, it has to withstand the landing loads and at the same time, it has to also provide a comfortable ride to the passengers both during the take-off as well as during the landing. Now, what are the desired features of a good landing gear? This is a wish list of a designer smallest possible size lowest possible weight, least possible drag, least complexity
during operation and maintenance and the lowest operating cost.

Now many of these requirements are conflicting with each other so may not be possible to meet all of them together and that is a challenge in landing gear design.
(Refer Slide Time: 01:37)

But another problem is that many people view landing gear as a necessary evil. Because, the landing gear is used only for nearly half per cent of its operational life. Of course, when the aircraft is parked and not being used landing gear is essential because the aircraft stands on the landing gear, but if you look at the take-off and the landing phases of the aircraft, they
occupy nearly half percent of the total mission.

So, something that is used only for half percent of its actual mission, but it has to be carried is called as a necessary evil. And therefore, there have been many attempts by designers to this landing gear its work is over. So, if you go back in history, you come up with a very interesting plane called the Levasseur PL-8 designed in France and the Germans had designed something called as Junkers Ju EF126 Elli.

The B-52 in the initial design stages was also being considered as an aircraft that would be without the landing gear, but then during the detailed design studies, it was decided that landing gear was unfortunately essential.
(Refer Slide Time: 02:48)

The Levasseur PL-8 is a very interesting aircraft. This aircraft, as you can notice, is a very old aircraft, it is a biplane and you know, the white bird aircraft was designed with a very simple and a single mission in mind. These were the early days of aviation, when we were interested to explore the increase in the range. So, the aim of this particular aircraft was just to cross the sea between France and England that was the only aim.

So, it was the first aircraft, it was the aircraft which was designed to fly across the Atlantic Ocean nonstop and very simple aircraft you can notice there is an open cockpit aircraft. The canopy is open and it is a biplane as you can see there is there are 2 wings 1 on the top, 1 on the bottom. You can see here also there are 2 wings. It had a single piston engine which was
mounted right in the nose somewhere here, you can see it here.

And it was supposed to have a watertight fuselage because there was a chance that it would not be able to make up make meet the requirement and hence it had to be dished on the in the ocean. So it should be able to float. So, what was done in that aircraft is that the landing gear was actually droppable? So, the moment the aircraft leaves the ground, they would drop the landing gear. And at the end, it would just be designed to go and land on the ground.
(Refer Slide Time: 04:40)

The Junkers Ju EF126 Elli was another aircraft which was taken up by the Germans during the war. This was the first aircraft to have a very unique engine called as a pulse jet engine, which you can see is mounted above the fuselage. So, the idea of this aircraft was to come up with a rapidly producible inexpensive small fighter aircraft during the war. It was a pulsed jet powered, but it had a very small propeller in the front to give start up.

It had rocket assisted take-off using detachable solid rocket fuel motors and the landing gear was droppable. So droppable take-off dolly, and a retractable landing skid. So this particular skid, the landing skid was retractable type and not very clear here. But there is a dolly kind of
a structure here, which is supposed to be detached from this point after the take-off is achieved. So once you take off from this from the ground, you drop the landing gear. And when you come into land, you bring out the skid and then try to land with that skid.
(Refer Slide Time: 05:59)

These were very unique designs, because the focus of the designers was to somehow get rid of the landing gear. Let us look at the various layouts for landing gear. And the sketches for this particular section have been borrowed from the book by Daniel Raymer. There are many
possible landing gear arrangements, 6 of them which are quite popular are shown here. You can have a single main landing gear where there is 1 main wheel and then there is 1 auxiliary wheel and then there are 2 wheels on the sides to support and to give the lateral stability.

You can have the most common ones are the tail dragger or the tail wheel type in which auxiliary wheel is in the front. There are 2 of them and on the back you have these are the 2 main wheels and then you have an auxiliary wheel on the back, then you have a tricycle type, which is the most common type today we call it as a conventional type. At one time this was conventional today this is conventional.

In which you have the main legs are behind the center of gravity and you have one in the nose. And then there are some variation there is a bicycle type which is the single mean when type with 2 wheels. In other words, you can say that the auxiliary wheel is made strong and mode in the front you have the quadricycle type, which is like a car 4 wheels. And you have a
multi bogey which is very common in very large transport aircraft. Where you have not to but may be a third main leg in the center and a pair of wheels in the nosewheel.
(Refer Slide Time: 07:40)

So, the most common ones are the tail wheel in the nose wheel types. And you know the most common one today, but also we also got another tricycle type in which the main landing gear struts or struts are behind the center of gravity and the auxiliary wheel is far ahead in the nose. Earlier the tail wheel type was the most common configuration. This is also called the
tail dragger type, because the tail is being dragged literally the tail is being dragged behind the aircraft.

In this case, the main landing gear struts are actually ahead of the center of gravity and the auxiliary struts are mounted on the rear near the rear end.
(Refer Slide Time: 08:25)

Let us have a look at some examples of nosewheel and table type aircraft. So, this is one you know you can see this is the tail wheel type and this is the nosewheel type. This is again the nosewheel type and we have the tailwheel type. So, these as you can see are the old generation aircraft, most of the aircraft that you see today are the nosewheel type and the
nosewheel can have single wheel or multiple wheels, the main wheel can have single wheel or multiple wheels as seen here.
(Refer Slide Time: 09:03)

There are many advantages of nosewheel type because of which has become very popular on ground. The main advantage of a nosewheel type is that when the aircraft is on ground, the fuselage and the cabin floor are roughly horizontal. So therefore, it is very convenient for the pilots and for the passengers because the seating position is natural and horizontal. During take-off, the nosewheel type gives a good view for the pilot again because it is horizontal and during the take-off, you do not have very high angle.

So, therefore, the drag acting on the aircraft is lower. During landing you land on the main wheel and then actually you bring the nosewheel down and when you land on the main with the nosewheel down, the tendency of the aircraft to overturn is minimized because the nose will act like a prop. Secondly, after landing on the main wheels, since the nose wheel is being
brought down the angle of attack is being reduced.

Hence the lift is going to be reduced and that is what is beneficial when you are coming in for landing. Because of these reasons, the nosewheel type is considered to be the most common and hence the conventional landing gear you will find it in most of the aircraft.
(Refer Slide Time: 10:30)

And this diagram tells you how the layout of the nosewheel type is carried out. So, generally, the main wheel is located some distance behind the center of gravity. And if you look at this view, you have this overturn angle this angle is normally to be kept around 60 degrees if you keep it you know if the back of this angle is less than more than 60 degrees, then there is a
tendency for the aircraft to have a problem during lateral stability during landing.
(Refer Slide Time: 11:11)

So, there are many advantages of a tailwheel type which are listed in this particular slide. The most common one is simplicity; you have a small and the light tailwheel. So the net weight of the landing gear is lesser. You can easily streamline the main landing gear because it is the one that is facing the flow so you can put nice nacelles or coverings on that. So during the
take-off, it gives a very high angle of attack.

So, naturally you have shorter take-off and during the take-off and landing, because the nose wheel has got a main wheel in the front there is a large height between the propeller and the ground. So, this gives good propeller clearance. It is said that the ground handling when you move the aircraft is easier if you have a tail wheel type and when you apply brakes on the
main landing gear, when you apply brakes the aircraft tends to pitch nose down.

And it gives you a general tendency for the skidding to be reduced and location of the main landing gear is such that it is easy to mount on the structure. So, these are the advantages of the tail wheel type.
(Refer Slide Time: 12:26)

But there are also severe demerits of the tail wheel type because of which it is not very popular now, especially in larger aircraft, because the center of gravity is behind the main landing gear, there is instability in landing. So, for example, if it comes into land and by chance, let us says the port or the left leg hits the ground first, then the aircraft tends to swing in that direction.

Because the weight of the aircraft is on the CG mounted behind and if there is instability in landing and if the left or the port wheel touches the start the aircraft tends to go forward on the center of gravity. So it will tend to swing in that direction. So, in the case of a tail wheel type, you need good piloting skill to ensure that you have a symmetric landing, if you have a bad unsymmetric landing there can be serious problems.

The second problem is that a tail wheel type aircraft sits with a fuselage inclined at an angle to the ground. So during taxiing it gives a very poor visibility to the pilot because the pilot is looking up whereas the ground is below loading and unloading of cargo and passengers is a problem because due to gravity, the cargo tends to roll down towards the ground and because there is an inclination, the fuel pumps are going to be under larger workload because not only they have to pump the fuel at a pressure, they have to also overcome some amount of gravity in pumping the fuel.
(Refer Slide Time: 14:10)

Let us look at the tail will type layout. So, when you do the layout of the tailwheel type, you can notice here that it is mounted slightly ahead of centre of gravity. And this particular angle between 13 to 17 degrees is to ensure that the angle at which you come and land during landing does not lead to hitting of the tail on the ground. Similarly, you have this particular
figure here, where this angle is to be maintained around 60 degree maximum. If you increase it to beyond 60 degrees, you can have tipping. Thanks for your attention. We will now move to the next section.

Lecture – 37

Landing Gear Layout – Part - 02

(Refer Slide Time: 00:17)

Here is another picture that shows the layout of the tailwheel type which has to be kept in mind while designing a tail dragger or a tail wheel type aircraft. Notice that the 10 to 15 degree angle that might be encountered during take-off has to be considered in locating the landing gear ahead of the centre of gravity with a little bit of margin.
(Refer Slide Time: 00:43)

Let us have a look at what happens when there is a problem in the lateral direction with the tip over margin as shown in the previous slides is not considered this incidents took place at the Eielson Air Force Base in Alaska in 2003, in which the starboard wing of the aircraft tipped and hit the ground because of lateral imbalance. It is worth it is important for us to
note the angle between the point of contact of the nose landing gear and the main landing gear. If this particular angle is not sufficient is not appropriately designed, one can have a problem of lateral tip over.
(Refer Slide Time: 01:26)

Also it is important to realize that when you design the landing gear, there is a ground clearance criteria. So as per the FAR regulations, if FAR part 25 regulations, when an aircraft comes into land with one engine in operational, we have to allow for a bank angle of 5 degrees. So, the, the landing gear should be located so that even when there is a bank angle of
5 degrees, that wheels do not touch the ground, the engine does not touch the ground.

Now, this happened in one case at the Narita Airport in Tokyo when the A380 actually dangerously brushed. So, this engine actually brushed the ground while coming into land. But interestingly, it was not a very serious problem. So, the aircraft continued in the flight.
(Refer Slide Time: 02:20)

Effect of rear CG location on the landing gear stability is very important as this picture from Buffalo airport shows because of collection of iec on the rear part of the aircraft and loading of the aircraft, you know the tail of the aircraft has hit the ground.
(Refer Slide Time: 02:45)

Similarly, there is something called as tail sitting when you exceed aft c.g. limit, while loading or unloading of cargo, you can have this problem in which you know, the rear of the aircraft hits the ground. The one that you saw earlier was because of weather conditions here it is because of improper procedure followed or not following the SOP during the loading and unloading of the cargo. So, as a result, the rear part of the aircraft became heavy and so, the tail has sit on the ground.

Another example of tail sitting is here you can see the nosewheel is up in the air above the ground and not visible but the tail has actually hit the ground. This is on a DHL aircraft while loading, unloading cargo.
(Refer Slide Time: 03:31)

Let us look at the bicycle type landing gear which is a landing gear that has 2 main landing gear wheels and a small outrigger wheel. This is used when the central fuselage is occupied with something else more important or when a very large lateral CG movement is expected.
Here is a sketch of the bicycle type landing gear you notice that there are actually 2 main gears one behind the other and there are these outrigger wheels just to provide lateral stability of the aircraft while it is moving on the ground.
(Refer Slide Time: 04:07)

There are many merits and demerits of bicycle type layout. The advantages are that the main landing gear is located after the CG and the wheels are located on the aircraft center line. So they get the entire load of the aircraft. But the demerits are that you need outrigger wheels.
And because the aircraft is on 2 main wheels, you have to be very careful in you know while landing, it is very difficult to bring an aircraft down so that both the main wheels touch the ground at the same time. Secondly, the elevator power that is required for nosewheel liftoff is very large, because effectively both the wheels are actually the main wheels.
(Refer Slide Time: 04:52)

There are some examples of bicycle type. This is one example B 47 aircraft in which the central part of the fuselage where you normally mount the landing gear is occupied in carrying bombs a large amount of bombs and hence the landing gear had to be moved outside. So that is there are 2 main wheels in the front, 2 on the back. And there are outrigger wheels on the sides.

Another example is the Harrier aircraft, in which the central portion of the aircraft is occupied to mount the swivelling nozzle for vertical take-off. So, therefore you end up having 1 wheel in the front and 2 wheels on the back. And you then have these outrigger wheels on the sides.
So this is the same aircraft B 47. You can see there are 2 wheels here there are 2 wheels here.
And this whole area in the centre is occupied in carrying the bombs.
(Refer Slide Time: 05:53)

In the bicycle type landing gear, the geometry is such that to allow a little bit easy in the rotation, you keep the centre of gravity more than half the length. So this distance is generally around 35 to 40% compared to these distance. This allows ease in rotation.
(Refer Slide Time: 06:15)

We also have a single wheel type which is basically a subcategory of the bicycle type in which you have just a single auxiliary single main landing gear wheel and a small auxiliary wheel on the back. Again you need outriggers for stability as shown in this particular figure.
So, 1 main wheel, 2 outrigger wheels and a small auxiliary wheel for longitudinal stability of the aircraft during operation. Such consideration is very common in gliders.
(Refer Slide Time: 06:45)

And in many cases, the single wheel type allows the gliders. So these are simple lightweight and low drag aircraft because there is only one main leg, but instead of introducing the wheels you may also have skids and very commonly seen on light planes like gliders and sailplanes. But generally such configurations are impractical for larger aircraft. One example
of this particular type on a large aircraft was the U-2 reconnaissance aircraft.

As you can see, you have just 1 main wheel inside there are a couple of wheels here and there are small auxiliary wheels. But in this case, since it was meant for very long endurance flying and you know you cannot have these outrigger wheels during the flight of the aircraft. So what they have done is in this airport are designed in such a way that during take-off, these
outrigger wheels, they fall off once the aircraft attains a little bit of climbing attitude, the 2 outrigger wheels are thrown out, and when the aircraft comes into land.

And you need very good piloting skill to bring the aircraft down very smoothly. So, therefore this particular aircraft is a very unique aircraft in which a car always travels by its side to guide the aircraft during landing. There is a communication between the pilot in the cockpit and the driver in the car and the driver of the car continuously drives along the aircraft and
communicates with the pilot and at the end of the landing run.

To avoid these outriggers hitting the ground you know 2 people actually on each side they run and they attach this dolly. So, these outrigger the removal outriggers are called as Pogos.
So, it is a very unique it is a very unique aircraft and hence there is a very unique solution for its requirements.

(Refer Slide Time: 08:41)

We also see in many military aircraft, the quadricycle type, which is also a subset of the bicycle type where you have main landing gear at the side of the fuselage. So, it is like a car there are essentially 4 points on which you load the aircraft. So, you can see the main legs there are now 4 of them, there could be pairs as you can see in this case, there are pairs of 2
tyres each such configuration allows you lateral stability without any outrigger wheels, and also it gives you a low cargo floor for easy loading and unloading. So, you see this mostly in some cargo type aircraft.
(Refer Slide Time: 09:23)

And then you can go for multibogey type aircraft which is seen normally in very large aircraft and the numbers of wheels per strut are a function of the take-off weight of the aircraft. So, all large transport aircraft, you know, that we use for travel Boeing 747, Airbus A 380 etc.
They all have a multi bogey type.

(Refer Slide Time: 09:48)

Then when we are want to operate at very unprepared and remote locations, where you have very rough field, you know, we have to provide very special features in the aircraft. This is one example of tundra tyres for rough landing. And these are basically oversized tires with very low pressure and they allow you to operate.
(Video Starts: 10:21)
So, I have a very interesting video clip of the Alaskan bush pilots, how they land their aircraft in very adverse conditions. You see this aircraft is now coming into land almost over a mountain river.
(Video Ends: 11:15)
So, using such kind of landing gears, you can actually land the aircraft on very rough ground.
They do not require a properly paved runway because there is not one available where they operate. So, when you have rough field running requirements you go for these oversized low pressure, tundra tyres.
(Refer Slide Time: 11:36)

Then taking this particular thing to the extreme, we have track type layout, which reduces loading on the runway and allows the aircraft to operate from soft ground and unprepared strips, just like we have tanks, which have these tracks, you can actually you know load you can actually create a landing gear where you can have track tank like tracks on the ground.
(Video Starts: 12:08)
Here is an example of a testing of a Piper Aircraft mounted with a track type landing gear.
(Video Starts: 13:57)
So, as you saw, by providing a track landing gear, you are able to operate the aircraft from almost any type of ground that you come across. But there are of course, several limitations.
The landing gear of this type cannot be retractable type, they will be fixed type and they will create tremendous amount of drag. But when there is a requirement to operate at extremely remote locations, unprepared grounds, you have no other option. Thanks for your attention;
We will now move to the next section.

Lecture – 38

Landing Gear of some Famous Aircraft

(Refer Slide Time: 00:22)

Let us end by looking at some landing gear of famous aircraft and try to familiar ourselves with their functioning. This is the landing gear of the F-22 aircraft. You can notice here that the landing gear is short and compact and as it retracts inside, you have this conformal cover that will fit into the geometry and make the whole aircraft fuselage very much smooth to avoid drag.
(Refer Slide Time: 00:45)

Here is the main landing gear of the Fokker F-27 aircraft which is now obsolete. It had long main landing gear struts with no side braces. So, these were the landing gear struts which are quite long because it was mounted on the wing and the wing was a high wing. So on a high wing aircraft, where the wing is above the fuselage when you mount the landing gear of course they mounted it below the engines, but still it was an example of a long main landing
gear stuck with no side braces.
(Refer Slide Time: 01:18)

On a BAe 146 aircraft, you have to provide landing gear on the landing gear on the fuselage because it is again a hiding aircraft and when you have turbo fan engines mounted on the wing, then mounting the landing gear on the wing becomes a serious problem what we saw in is an exception, so in this case, you have the landing gear mounted on the side of the fuselage, but you need to have minimum some minimum wheel tracks so that you can get the when you can avoid the tip off.

So, therefore, you have to have landing gear which is coming out of the fuselage and then when you retract it inside then you require a small blister. So, you can see this is a blister in which the landing gear gets compacted and this is the covering for the blister. So, that the drag is minimized after the landing gear is retracted the same covering can be seen here also.
(Refer Slide Time: 02:19)

Let us look at the landing gear of an aircraft called as a Vought F4U Corsair this particular aircraft had engines embedded in the root of the you can see there is some intake here and in this case the main landing gear retracts rearward into the wing so it goes back.
(Refer Slide Time: 02:42)

If you look at the De Havilland caribou aircraft, this has got a Gull wing configuration and on the Gull wing configuration because the main landing gear is mounted below the engine and it is a gull wing. So therefore, you tend to reduce the strength the strut length of the olio strut on with landing gear is mounted.
(Refer Slide Time: 03:01)

This is the landing gear of Airbus 380. You can see that there are 4 main legs there are 2 on the fuselage on the sides and there are 2 on the wings. So there are 20 wheels in total. There are 6 wheels on each of the main legs on the fuselage, so 12 wheels here and then there are 4 wheels on the main landing gear leg which is in the wing. So, on each side you have 6 + 4.
That means you have total of 20 main landing gear legs, and then you have the nose landing gear also which has 4 wheels.
(Refer Slide Time: 03:35)

If you look at the landing gear of AN-125 Ruslan aircraft, this is a very interesting aircraft. It is a cargo aircraft for transporting cargo over long distances. So the nose of the aircraft completely bends up. And you have as you can see 5 pairs of landing gear legs on the main landing gear and on the nose wheel; you have 2 pairs on 2 nose wheels. Now, this particular
landing gear, we can adjust the length of the struts so that the aircraft can be made to kneel and that allows easy loading of cargo. You can see in this configuration there is a ramp here and you just drive in with the cargo.
(Refer Slide Time: 04:24)

(Refer Slide Time: 04:38)

Then let us look at the monster the largest aircraft that we have seen AN-225 Mriya. It is one step ahead from the Ruslan it has got 7 pairs of main landing gear, you can see there are 7 pairs. And you know, in the nose also you have several landing gears. So, 7 pairs of main landing gear 2 pairs on the nose wheels, 2 pairs on the nose wheel, and you can adjust the pressure of each tire on the main landing gear. And you can also retract or extend those each individual strut which allows you to change the tyre.
(Refer Slide Time: 05:07)

Finally, we will look at the landing gear of Boeing 787. And you can get an idea about the size by comparing with the person here. And this particular video will show you how the testing is done.
(Video Starts: 05:19)
(Video Ends: 06:18)
Thanks for your attention.