Lecture – 32
Wing Geometry Definitions
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Let us have a look at some wing geometry parameters. Now, the basic geometrical configuration of a wing is supposed to be a trapezoidal wing and these are some of the important geometrical parameters. The first parameter is the root chord, which is the chord of the wing not at the place where it attaches with the fuselage, but at the location of the
extended center line of the fuselage.
So, this is a very common mistake, sometimes many people take the root chord as the location where the wing and the fuselage are physically meeting, but, the root chord is defined theoretically as the chord of the wing when it meets the theoretical extended center line of the fuselage. You also have tip chord, which as the name suggests is the chord of the wing when it is at the tip. The distance between the 2 ends of the wing is called as the span band half of it is called as the semi span s. S is normally reserved as the parameter wing reference area, which is the area of the wing as
viewed in the top view, including the part that is submerged inside the fuselage. Though the hatched area in this figure is the definition of wing reference area, it is a reference. So, as long as everyone understands what it stands for, there will be no confusion. So, it is important to remember the definition of the wing reference area.
You then have thickness t, which as you can see in this figure is the maximum distance between the upper and the lower surface of the aerofoil you have the chord C, which is the distance between the leading edge and trailing edge of the aerofoil or the wing. And angle of attack is the angle that is made by the ambient wind vector with a reference line on to the
There are some derived parameters like the taper ratio, which is a ratio of the tip chord to the root chord. There is an aspect ratio which is an indication of its slenderness is defined as the square of the span upon the wing reference area. And we have a thickness to chord ratio or the t/c ratio, which is the ratio of the maximum thickness divided by the mean aerodynamic chord of the wing.
(Refer Slide Time 03:20)
Now, the wing geometrical parameters like the camber of the aerofoil, the thickness ratio of the airfoil aspect ratio, taper ratio and the leading edge sweep angle, they all affect the aerodynamic characteristics and the weight quite substantially. And it is summarized in this particular chart. But let us have a look at each of these elements one by one to have a better
understanding. The first parameter that affects is the camber. Camber of the airfoil essentially is an indication of its curvature.
So, the black line is for the base aircraft baseline geometry and the red line is for the effect of change or increase in a particular parameter. So, we notice here that as you increase the camber then the lift coefficient increases. In fact, you have a line which is almost parallel to
the original line. So, the lift coefficient increases, but the drag coefficient also increases as far as the weight is concerned effect of camber on the aircraft weight is not that substantial.
And the typical values of camber that you see are between 0 % which is a symmetric aerofoil to around 6% of the chord. The airfoil thickness ratio is another parameter which affects the CLα curve, mainly it actually increases the angle at which it stalls. So, it increases the CLmax, but it also increases the drag coefficient. However, when you have a higher thickness to chord
ratio, you generally can come up with a lower wing weight.
This is not very intuitive because many people think that a thicker wing should actually weight more because they think it larger in size. However, please remember that one of the main component of the aircraft that is heavy is the main and the rear spar or the spar which are present and the spars themselves consists of a flange and a web. Now, in a wing with
higher thickness to chord ratio, these spar flanges are farther away because the web is larger in size.
And because they are larger in there, because they are far away they have a it gives us. It gives a higher moment of inertia and higher moment of inertia gives you a smaller value of the bending moment, which is the principal load that a spar has to carry. So, up to a point, increasing t/c can actually lead to a reduction in the aircraft weight.
Wing weight the range of values for subsonic aircraft is between around 5 to 18%. And for supersonic aircraft, the thickness to chord ratio is kept low because of the high drag between 3 to 7%. The next important parameter is the aspect ratio, the aspect ratio is a very important aerodynamic parameter as I mention, it is an indication of the slenderness of the wing. It improves the induced drag coefficient K and actually it reduces the induced drag because of that.
And another problem with the increasing aspect ratio is the more you increase the aspect ratio the more slender the wing becomes and a slender wing is going to be more prone to aeroelastic problems because of flexibility. And also, to make it sufficiently rigid, we need to provide huge support and that leads to an increase in the wing weight. In fact, increasing
aspect ratio of the wing leads to a very large increase in the wing weight.
And the wing is approximately 12% of the aircraft weight in most cases between 10 to 12%. So, it substantially affects the weight of the aircraft. So, the recommend value aspect ratio is between 7 to 9 or maybe 10 for subsonic aircraft, except those which are designed for very long endurance and for supersonic aircraft, the value recommended is between 2 and 4. Taper ratio is another parameter that is very important in reducing both the weight of the wing as well as the induced drag of the aircraft.
However, giving large taper is going to create a problem with the lift distribution. So, the taper ratio generally is between 4 to 14 for subsonic aircraft and 2 to 5 for supersonic aircraft.
And lastly, we look at the leading edge sweep angle. Sweep angle definitely reduces C D0 but increases the induced drag coefficient and it makes the wing heavy. So, subsonic aircraft normally we do not see them to be strapped more than 35 degrees.
But for supersonic aircraft you normally see the sweep from 35 to 70 degrees or even more sometimes, it may be noted that the only advantage of riding wing sweep is to reduce the drag at high speeds and for all other considerations, the wing sweep is actually detrimental. So, wing sweep should not be provided unless it is essential from pure aerodynamic reasons.
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The reference wing planform is always considered as trapezoidal and we have already seen the effect of aspect ratio. If you increase aspect ratio we see that the induced drag reduces.
But another good thing is that the angle at which the aircraft will stall is also going to reduce.
So, the subsonic L/D of the aircraft increases because of the reduced induced drag. But as I already mention, there is a substantial increase in the wing weight sweep twist incidents dihedral are other parameters which are very important as far as the geometrical choice of an aircraft is concerned.
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So, I did not mention to you that taper ratio of the aircraft the benefit of that is that it givesyou easy construction, but it makes the wing heavy if you do not give taper then you have a rectangular wing and it gives you a heavier wing. When you have a lower taper ratio, then you have a lighter wing because the wing root bending moment is reduced. However, the concentration of the lift moves towards the tip.
So, therefore, as the value of taper ratio reduces that tip start getting loaded and that means the tips will start stalling first and that is not desirable as far as controllability is concerned.
For a good controllability in the post stall scenario, we do not want the tips to stall first we want the route to stall first, because when the route stalls first it gives some kind of vibration and physical feel to the tail.
And also, if the route stalls before the tip, then the aileron which are normally outboard are in unstalled wing. So, they are still providing the required moment for controllability. Whereas, if the tip stalls first, then the ailerons which are at the tips will also be in the stalled condition and it will be ineffective. So, it will be difficult to recover from a disturbance specially in roll. So, the compromise value of paper ratio is normally between; 0.4 to 0.6 in most aircraft. Thanks for your attention we will now move to the next section.
Lecture – 33
Options for Wing Layout
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Let us look at the options available for the layout of the wing. Broadly speaking, we have 3 choices; it can be a high wing, a mid wing or a low wing. Each position has its own advantages and disadvantages which we will now look into.
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So, these are the images of some famous aircraft which have these configurations. The high wing configuration shown here is the Dornier 228 dash 212 aircraft which was produced under license by HAL Kanpur.
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Under the low wings, we have a look here at the Airbus A 380 dash 100 aircraft. And for the mid wing the example chosen is that of the RAFALE aircraft which is being procured. Let us first start by looking at the benefits of high wing layout.
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The first and foremost benefit of a high wing layout is to permit ease in loading and unloading of cargo because the wing is mounted above the fuselage; hence the fuselage height tends to be low. This is a very good example of a military cargo aircraft.
(Video Starts: 02:00)
This is the C 5 galaxy aircraft and let us have a look at how it is used to load and unload heavy cargo. In this clip, we see a large number of items being loaded onto the aircraft. We see a helicopter, with its rotor blades, C5 galaxy aircraft can handle 2 large abrams tanks, several of these jeeps and a huge amount of cargo. All of it can be easily loaded because of
the ramp configuration given on the rear of the fuselage and you can see when you move in these heavy items, we tie them down onto the floor of the aircraft with the some anchors which prevent relative motion of these items when the aircraft is operating.
(Video Ends: 03:10)
So, all military aircraft, which are heavy cargo, generally have a high wing configuration and a very low floor of the fuselage.
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One more advantage of a high wing layout is that you can build the whole wing in one piece and then put it literally over the fuselage, attach it with some bolts and you can externally support it by using braces such as the one seen in this aircraft. So, this gives a huge structural advantage and results in a light weight structure.
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The third benefit of a high wing layout is to allow better short take-off and landing performance, because we have a huge uninterrupted wing mounted over the fuselage, you can have flaps over a much larger span and since you are away from the ground, you will also be able to have lesser ground effect. Ground effect is useful when you have take-off, but it can be detrimental when you come for land. So, in a high wing, since the wing is far away from the ground, you have a lower ground effect and hence you have a better stall performance.
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There is a fourth benefit of high wing layout which is to give better rough field performance by rough field performance we mean operating the aircraft from runways and airports which are not paved and properly maintained from grassy lands etc.
(Video Starts: 05:10)
Let us have a look at a video which shows us the versatility of this aircraft called Twin Otter.
This aircraft is being operated from a farm from a field in a farm. And as you can see, as the aircraft comes into land, there are a large number of bumps which the landing gear has to encounter. And because of that, a lot of mud and dirt is thrown. A high wing is very useful, because in a high wing configuration, the wing is far away from all this dirt, clouds which are
being thrown up.
(Video Ends: 05:10)
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This is an example of the mounting of the high wing, you can see that you can have a through and through spar. As you can see here, the spar can be through the rear spar and this is the front spar. Both can be through and through. And that leads to tremendous structural advantages.
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Now high wing also has some drawbacks. One drawback is visible in this aircraft, you can see the landing gear of the aircraft is mounted below the wing mounted engine Nacelle, but you can see the landing gear is very slender. In most high wing aircraft, you do not have the option of mounting landing gear like this; we will have to mount them on the side of the fuselage, which leads to its own problems.
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One serious issue of concern for a high wing is poor visibility of the pilots especially when they go into turning and climbing flight. As you can see here, the port side view of this pilot is obstructed to a large extent by the presence of this particular wing structure. Now, special features have to be provided in the aircraft. For example, you can see in this aircraft, they have actually created a cut in the wing at the root and on the top of the fuselage canopy.
That is actually a perspex covering and this perspex covering is enabling good view. So, it is claimed that in this particular aircraft called Zenith 701 there is a very good forward and downward visibility available to the pilot and also a little bit of sideward visibility because the wing is having a special kind of a configuration. But what it shows us that certain special
features have to be provided in the aircraft to increase its visibility during turning and climbing flight if it is a high wing layout.
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Another drawback of the high wing layout, which I discussed a few minutes ago is also due to the landing gear being heavy, the landing gear is heavy because the landing gear cannot be mounted always on the wing, you can see the landing gear is mounted here along the side of the fuselage, you need to have a sufficient amount of wheel track for stability in the ground
And therefore, you need to take the main landing gear wheels little bit out and that leads to the need for providing this particular blister in the landing gear. So, these blisters are the ones that you know create a lot of drag. So, while they are on the ground, you can see that the landing gear is visible.
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And when the aircraft takes off and the landing gear is retracted, there is a requirement for these large blisters on the fuselage and these blisters create additional drag and also additional weight.
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Let us move on to the mid wing layout. The mid wing layout is a layout in which as we can see in this Piper PA-60-600 aerostar the wing is mounted in the center of the fuselage.
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Now, the most important advantage of a mid wing layout is the aerodynamic advantage. A mid wing aircraft has the least interference drag and also it has neutral stability. These parameters make it the configuration of choice for all aerobatic aircraft in most fighters and aerobatic aircraft, we observed that we go for a mid wing layout.
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However, a mid wing layout leads to a restriction on the design of the carry through structure and the spar because in normal circumstances in a mid wing aircraft, the spar carried through structure will go through the fuselage and that will create a lot of trouble or the disturbance in the passenger cabin. One solution for such a problem is to have a configuration in which the entire passenger cabin is ahead of the wing such as the one in this Hansa jet aircraft.
In this aircraft as we can notice, the mounting of the fuselage and the wings structure is behind the passenger cabin. This is a 10 seat business jet aircraft. So the passengers are seating ahead of the wing.
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Another example of this configuration is the Piaggio P 180 avanti aircraft, where you can see clearly that the passengers are sitting in front of the wing and you have a carry through structure of the wing giving it sufficient structural rigidity. So, when you can go for such kind of carry through structure, then mid wing is possible. But apart from these 2 examples, there
are very few aircraft which actually have a mid-wing configuration for a passenger aircraft.
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Finally, we come to the low wing configuration such as the one in this spit fire aircraft. The first benefit of a low wing layout is that it leads to lower structural mass and this becomes extremely important and relevant, when we look at large transport aircraft such as the airbus A 380 or the Boeing 787. In these aircraft, it has been shown that lowing configuration leads
to the least possible structural weight.
Here is a photograph of the attachment area of the wing onto the fuselage for an airliner you can notice that there is this huge structure and a low wing configuration actually allows you to have an uninterrupted passenger cabin located above the wing.
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This is another photograph of the same aircraft which shows the 2 wings being brought in for assembly at the location.
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In case of a loading structure as I mentioned, you are able to provide a carry through structure and through a carry through structure for the spar, you have a better load transfer and you have a lighter structure. As you can see in this figure, the whole spar actually passes through the wing and the fuselage joint uninterrupted.
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One more benefit of the low wing layout is it gives you additional safety during accidents. In case there is a bad landing, in which let us say the landing gear is crumpled or there is a tyre burst. Or as in this case, when you see for a JetBlue aircraft there was in a locked nosewheel
and it was actually turned sideways. And when the aircraft came in to land, the nosewheel tyre actually was so hot that the nosewheel caught fire.
Now, if we were looking at a high wing configuration for similar accident, the fire would have immediately reached the passenger cabin and caused grievous harm and injury to the passengers. We see in this figure, how the wing of the aircraft is actually coming as a protective layer in between the disturbed and the damaged portion below and the passengers in the cabin. So, from passenger safety point of view, low wing is something that comes in between you and the mother earth in case of an accident during landing.
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A low wing configuration also allows mounting of engines in the proximity of the ground, and hence for the maintenance crew. It is very easy for them to look after the engines or to do the inspection and the maintenance activities such as the case in this photograph of a Boeing 737 dash 100 aircraft.
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But then, when you go for changes in the fuselage length, when you go for upgrades of the aircraft, the same low wing location which gave an advantage of engines nearby become a problem because as in this case, this is an example of the same Boeing 737 aircraft, but now this is dash 600 version the 600 version had more the CFM 36 engines, which were more efficient than the GTA DS which are used earlier in dash 100 and dash 200 engines, but these engines were larger in size.
Now, we cannot change the location of the engine mount so easily because that would require major structural reworking and redesign. So, to ensure that the structural modifications are minimal, the location where the engine is mounted is undisturbed. And hence, if you have to put a larger diameter engine, the only option that you have is to create a non-circular entry you will have to create a flattish bottom so that the minimum distance between the bottom of the nacelle and the ground which is typically 18 inches by requirement can be maintained. So, in a low wing configuration, the benefit of having engines near the ground for servicing can soon become a disadvantage, if you are looking at the higher growth versions of the same aircraft, in which you have to provide such special features and shapes to the engine nacelle to accommodate at the same location.
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Let us look at some other drawback of low wing layout. Since the wing is nearer to the ground, there is a much larger chance of the foreign object being ingested by the engines on the wing and also creating a problem?
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Here is a video of Vampire aircraft which is taking off from a runway and you see very soon when the aircraft engine is Wrapped up to the highest RPM they are you see the exhaust of the jet is actually creating damage to the runway. Now, in the low wing aircraft, this debris from the runway or from any foreign object can create a lot of problem with the ingestion of
the debris by the engines or damage.
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So, in the low wing configuration, this is a problem high wing is better you are expecting such kind of disturbances from the ground. Thanks for your attention; we will now move to the next section.