Project-1: Powertrain for aircraft in runways

part A

1.ans

Aim: to Search and list out the total weight of various types of aircrafts.

Typical Airplane Weights

MTOW = Maximum take-off weight,

MLW = Maximum landing weight,

TOR = Take-off run (SL, ISA+15°, MTOW),

LR = Landing run (SL, ISA+15°, MLW)

Helicopters

MTOW = Maximum take-off weight

Convertiplanes

MTOW = Maximum take-off weight

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2nd ans.

Ground Speed

For a reference point picked on the ground, the aircraft moves relative to the reference point at the ground speed. Ground speed is also a vector quantity so a comparison of the ground speed to the wind speed must be done according to rules for vector comparisons.

Airspeed

The important quantity in the generation of lift is the relative velocity between the object and the air, which is called the airspeed. Airspeed cannot be directly measured from a ground position, but must be computed from the ground speed and the wind speed. Airspeed is the vector difference between the ground speed and the wind speed.

On a perfectly still day, the airspeed is equal to the ground speed. But if the wind is blowing in the same direction that the aircraft is moving, the airspeed will be less than the ground speed.

Examples

Suppose we had an airplane that could take off on a windless day at 100 mph (liftoff airspeed is 100 mph). We are at an airport with an east-west runway that is 1 mile long. The wind is blowing 20 mph towards the west and the airplane takes off going east. The wind is blowing towards the aircraft which we call a headwind. Since we have defined a positive velocity to be in the direction of the aircraft's motion, a headwind is a negative velocity. While the plane is sitting still on the runway, it has a ground speed of 0 and an airspeed of 20 mph:

The airplane starts its take off roll and has a constant acceleration a. From Newton's second law of motion, the ground speed V at any time t is:

and the distance d down the runway at any time is:

For a fixed length runway, this specifies the time to be used in the velocity equation. Let's assume that at 5000 feet down the runway, the velocity is 80 mph. Then the airspeed is given by

and the airplane begins to fly. Now another pilot, with exactly the same airplane decides to take off to the west. The wind is now in the same direction as the motion and this is called a tailwind. The sign on the wind speed is now positive, not negative as with the headwind. The acceleration along the ground is the same, so at 5000 feet down the runway, the ground speed is again 80 mph. The airspeed is then given by:

This airplane doesn't have enough airspeed to fly. It runs off the end of the runway!

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3rd ans.

For movement on the ground the aircraft uses it own engine power, which is called taxing.

Although many aircrafts are capable of moving in backward using reverse thrust, as result the jet blast or prop wash might cause damage to the terminal building or equipment. Engine close to ground may also blow sand and debris forward and then suck them into the engine, causing damage to the engine. Therefore there are pushback trucks in the airport. A pushback truck lifts the front weel of the aircraft, their role in most airports is to move the aircraft away from the parking spot on to the taxiways, which is called towing.

Unwanted consequences mainly arising from the unintended movement of the aircraft during engine running.

i) Effectively a loss of control can happen, especially during high power engine running. Damage can occur to the aircraft itself, other aircraft nearby or to airside structures.

ii) There is a risk of injury to ground support personnel who may be in relatively close proximity to the aircraft.

iii) It is easier for truck driver to see the surroundings better than the pilot sitting in the cockpit with limited window. The pushback driver knows the airport better than the pilot.

iv) It also saves fuel. virgin Atlantic had proposed a new system in which pushback truck will move the aircraft all the way to runway to save fuel and reduce impact of environment.

v) The plane engines are very loud. When more than one plane will add up their sound, than it will not be pleasent for passangers and others near airport.

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4th ans.

Pushback is when an aircraft is pushed backwards away from the airport gate by vehicles called tugs or tractors. Closer to departure, an aircraft tug will park right in front of the nose wheel. The tug might be directly attached to the plane's nose gear with a tow bar, or could be a "wheel-lift" tug.

Pushback is when an aircraft is pushed backwards away from the airport gate by vehicles called tugs or tractors.

Closer to departure, an aircraft tug will park right in front of the nose wheel.

The tug might be directly attached to the plane’s nose gear with a tow bar, or could be a “wheel-lift” tug. These tugs cradle the nose gear, then lift it up before moving the plane. That gives the tug driver control over the plane’s direction during pushback.

New taxi technologies are appearing, like pilot-controlled tugs, and electric motors mounted to the plane’s landing gear. Both promise to save fuel, and reduce airport noise.

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5th ans.

The wheel dynamometer system used in the experiment consisted of a sensor, modified wheel and tire, wireless data transfer unit, and software. The sensor core was made of steel, and strain gages were used as sensing elements. The instrumentation amplifiers, which prepare the outgoing signals for data acquisition, were placed outside the sensor in a small box, together with a board of the microprocessor data analysis and transfer system. Figure 1 shows a schematic of the sensor, but without the electronics. This schematic also includes the orientation of forces and moments.

Figure 1. A schematic of the wheel sensor together with axes orientation and forces/moments system. The X, Y, and Z axes are mutually orthogonal.

As mentioned earlier, the sensor was designed based on the strain gage measurement technique. The sensing element was made of spring steel and linear type strain gages were glued onto it.

The in-wheel transducer measures two forces, F_Z and F_X, and two moments, M_Z and M_X. Due to the restricted space mentioned above, a braking moment sensor was made as a separate, external device. The M_Y moment was measured with the strain gages sealed on the brake jig, which is bending proportionally to braking moment. Figure 2 shows the complete sensor system, embedded in an aircraft wheel.

Figure 2. The complete wheel force sensor.

The wireless data transfer unit was built as a microprocessor system (see Figure 3), with an 8-channel A/D converter and a Bluetooth radio device to transfer measured data wirelessly.

Figure 3. The heart of the microprocessor data transfer system, used in the wheel sensor system.

The complete and detailed description of the wheel force sensor system is given in the reference by Pytka et al. [3].

2.2. Airplane Used in the Experiment

The PZL 104 Wilga 35 multipurpose airplane was used as a test aircraft. This is a light, four-place, multipurpose aircraft, powered by a 192 kW radial piston engine. The Wilga has a good reputation as a STOL (Short Takeoff and Landing) or even the so-called “bush-plane”, thanks to its landing gear. The aircraft has a tail-dragger-type landing gear and the main gears are rocker type with oleo-pneumatic shock absorbers. The landing gear wheels have low-pressure 500 × 200 mm tires with hydraulic brakes. The wheels are castored and have a positive rake angle of 18°; the axle offset is 400 mm. The airplane is 8.10 m in overall length, with a wingspan of 11.12 m, a wing area of 15.5 m², and an empty mass, equipped, of 900 kg. In the landing tests, there were four persons on board and the take-off weight was 1150 kg. The test airplane is depicted in Figure 4.

Figure 4. The PZL 104 Wilga 35A, used for the tests.

The wheel force sensor was installed on the test aircraft before measurements. Installation did not require any modification to the airframe or landing gear, since the sensor system was developed specially for the given aircraft. Simply, leveling up the aircraft, removing the wheel with its axle, and placing the sensor. Also, the brake hydraulic system had to be re-switched to the sensor, since the F_Y wheel moment was measured by means of a transducer embedded in the wheel brake.

Another element of the measuring system was the electronic unit, which consisted of five differential signal amplifiers, each for one of the five channels (two forces and three moments), as well as of a microcontroller-based, wireless data transfer unit. The electronic box was mounted on the landing gear with glue tape and the sensor was connected with the electronics with signal cables. The whole system, the wheel sensor, and the electronics power supply (7.2V 800 mAh NiMH accumulator) was placed in the cockpit. Figure 5 and Figure 6 show the details of the test airplane with the installed measuring equipment.

Figure 5. The wheel force sensor installed on the test airplane, ready for the tests.

Figure 6. The electronics: Signal amplifiers and wireless data transfer system in the box, installed on the landing gear leg (a silver tape was used for quick installation).

2.3. Grassy Airfield Conditions

Measurements of wheel forces and moments were performed on a grassy surface of the Rzeszów Jasionka aerodrome, located in southeast Poland. The measurements were carried out in the area adjacent to the runway, with the surface and soil conditions identical to those on the grassy runway. The height of the grass was approximately 10 cm each time of the measurements.

The days in the autumn and spring months were selected, when there is a high probability of increasing humidity and lowering the traction properties of the grassland due to

Before the test runs, we also measured soil moisture using a handheld TDR moisture meter. TDR is an acronym for time-domain reflectometry, and this is a technique that utilizes the phenomenon of different electrical permeability of porous, three-phase media, such as soil. The TDR meter is very easy to use. Measurement requires simply inserting test electrodes into soil and the instrument quickly gives results, which are collected in Table 1. Figure 7 shows the test engineer performing soil moisture measurement on the grass field. See also the reference by Pytka et al. for more details on the use the TDR sensor in terramechanics studies [24].

Figure 7. Measurement of soil moisture with the use of a handheld time-domain reflectometry (TDR) sensor.

2.4. Field Procedures

Since the sensor system has not been certified for flight tests yet, we could only manage ground tests. In this study, it was aimed to determine wheel forces and moments acting on the aircraft’s wheel at low speed of taxiing over the grassy runway. We used an all-terrain vehicle (ATV) as a tractor vehicle and the aircraft was connected to this vehicle by means of cables. As mentioned earlier, four adult persons were in the test aircraft during all tests. An approximate speed of taxiing was 5 km/h. The aircraft performed the following maneuvers:

• Accelerating from stop to the taxi speed;
• Taxiing with a constant speed (about 20–30 m);
• Turning left or right;
• Braking to a full stop or to block the wheels.

We performed a minimum of five repetitions of all the maneuvers for every measurement time. Figure 8 shows a sample test run on the grassy surface.

Figure 8. Starting a test run. The test airplane is pulled by an all-terrain vehicle (ATV). The test engineer is walking by the test airplane, collecting data wirelessly. Four persons onboard the airplane.

The wireless data transfer system enabled that the test engineer could be anywhere, within a 100 m range of the airplane. Since the measurements were performed with all four passengers on board, the test engineer was in the airplane during measurements. We tried to collect data on a laptop computer, but it was more comfortable to gather the results on a handheld smartphone.

3. Results

Results obtained in the field experiments include courses of wheel forces and moments acting on the left wheel of the Wilga airplane, and we collected data from four days of measurements: 17 October, 5 November, and 5 December 2018, as well as 13 March 2019. The following is a presentation and analysis of the results obtained in the autumn–winter part of the measurements campaign (a so-called “dead season” in sport aviation) that was performed as a full-season measurement for the verification of the wheel–turf interaction model of the GARFIELD information system [1,2,23,24].

3.1. A General Presentation of the Results

Figure 9 shows time courses of the vertical F_Z and horizontal F_X forces that act on the landing gear wheel. The F_Z shows almost constant values with low fluctuations, probably caused by vertical vibrations of the entire aircraft rolling over the grassy surface, and those values are close to the aircraft weight component on the wheel. The horizontal force F_X exhibits negative values since the sensor has a defined orientation and a force of the direction backwards is indicated as negative. Moreover, the course of the horizontal force F_X reveals effect of wheel function modes: Free rolling (the first second of the course), acceleration increasing of the force’s values (between 1 and approximately 4 s of the course), braking to a full stop (5–9 s), and, again, acceleration (from approximately 9 s of the course). The peak maximum value of the horizontal force F_X reaches 2.7 kN, but the average value is much lower.

Figure 9. Sample result of the measurements: Time courses of wheel forces acting on the landing gear wheel.

Another sample result is shown in Figure 10, where we can see time courses of the three moments acting on the aircraft wheel. The character of these courses is very similar to those of Figure 9. These is a short period of free rolling, then acceleration, braking and stop, and finally acceleration again. The peak maximum values reach 150 Nm for the M_Z moment, 280 Nm for the M_X, and 250 for the M_Y moment. The values of the moments acting on the landing gear wheel changes their sign at a moment of a change between two function modes, for example, accelerating–braking.

Figure 10. Sample result of the measurements: Time courses of three moments acting on the landing gear wheel.

In the next sections, an analysis of the measured forces and moments is presented in order to determine coefficients that describe wheel–surface interactions, which are of importance for the analysis and modeling of airfield performance of an aircraft.

3.2. Rolling Resistance Coefficient

Rolling resistance is a property of the tire–surface system and depends on many factors. In automotive technology, rolling resistance is determined for a tire rolling on a drum test stand and its coefficient takes essentially constant values, on the order of 0.010–0.012. In the case of a tire interacting with a deformable surface, such as grassy surface, rolling resistance is difficult to determine, mainly due to the changing conditions of the surface, which was mentioned in the introduction. In addition, the values of the rolling resistance coefficient of the tire–grass surface system can vary by as much as an order of magnitude, taking into account, for example, changes in soil moisture. The presence of vegetation (grass, roots), as well as the size and mass of green parts of vegetation, are also significant.

The rolling resistance coefficient, k_RR, is defined as the ratio of horizontal force during free turning to vertical force [25]:

k_RR = F_X/F_Z

(1)

Figure 11 shows an example of horizontal force, F_X, which is input data for calculations (upper graph) and the rolling resistance coefficient, k_RR (lower graph), value determined using Equation (1). Average values are marked on the graphs (F_X = 1719 N and k_RR = 0.275). When determining the k_RR coefficient, it is important to correctly select a portion of the F_X force waveform. Namely, it is a fragment where the waveform is determined (in Figure 11, up, this fragment begins after the first second of the wave).

Figure 11. Sample courses of the measured horizontal force, F_X, and calculated rolling resistance coefficient, k_RR.

The exemplary value of the k_RR coefficient was determined to be 0.275, which in comparison to the aforementioned value for a car tire on a paved road is almost 25 times [25]. This is the effect of a soft surface and a low-pressure aviation tire. The impact of weather conditions on the determined values of the k_RR coefficient will be presented in the next section.

3.3. Braking Friction Coefficient

The braking friction coefficient μ is another important factor. It determines the course of the braking process and the landing distance of the aircraft. In the case of the tire–paved system (asphalt, concrete), the friction coefficient usually reaches values between 0.6–0.9. For unpaved surfaces such as grass turf, the friction coefficient is usually lower and may be 0.1–0.5. Factors affecting the μ value are grass length and humidity: The longer the grass and the more water it contains, the lower the friction coefficient. By definition, the friction coefficient is determined on the basis of the following relationship:

μ = M_Y/F_Z × r_d

(2)

where r_d is the dynamic radius of the tire.

Figure 12 shows an example of the braking moment, M_Y, course and the braking friction coefficient determined. As it can be seen, μ values oscillate around 0.22.

Figure 12. Sample courses of the measured braking moment, M_Y, and braking friction coefficient, μ.

3.4. Aligning Moment

When driving on a curved track, the road wheel moves in such a way that the longitudinal axis of the wheel is deflected from the direction of the instantaneous speed vector by an angle, called the side slip angle. The reaction of a pneumatic tire to side slip is to generate a moment that tends to reduce this angle. This moment is called the stabilizing or aligning moment.

The sensor used in the tests directly measures the value of the aligning moment, it is the moment marked as M_Z. Figure 13 shows the M_Z course recorded during one of the turning maneuvers. The importance of the M_Z moment is revealed during taxiing, especially at high speed. The higher the M_Z value, the easier it is to maintain a straight direction, while too high a stabilizing moment can cause significant steering forces when cornering, especially when taxiing at low speed.

Figure 13. Sample course of the aligning moment, M_Z.

3.5. Effect of Weather Condidtions

As already mentioned, the study was carried out over four day—in October, November, December, and March. The purpose of such a research program was to check to what extent the worsening deterioration of weather conditions, in terms of lowering the external temperature and increasing humidity of air and soil, affects the measured values of forces and moments, as well as the resulting coefficients. Based on the measurements carried out, mean values of repetitions (minimum of five repetitions for each day) of individual measurements were determined. A collection of final data in the form of bar charts is presented in Figure 14, Figure 15 and Figure 16, where average values of rolling resistance coefficient, k_RR, friction coefficient, μ, and stabilizing moment, M_Z, are shown for four days, representative for particular weather periods.

Figure 14. Average peak values of rolling resistance coefficient, k_RR, for the four measurements.

Figure 15. Average peak values of braking friction coefficient, μ, for the four measurements.

Figure 16. Average peak values of aligning moment, M_Z, for the four measurements.

The rolling resistance coefficient increases, and this is a significant increase. The explanation for that is quite obvious: In autumn, late autumn, winter, and early spring, there is a gradual increase in humidity of both air and soil, which causes increasing deformation of the surface and, as a result, an increase in rolling resistance. Although we did not construct any relationship between the rolling resistance coefficient and soil moisture (see Table 1), these parameters correspond well to each other.

The second term, braking friction coefficient shows an inverse relationship, i.e., it decreases during the period considered. In this case, the cause is similar, although the increasing humidity works slightly differently, namely, lower grip (=more slipperiness), especially in the case of grass-covered surfaces. A slight increase in the braking friction coefficient was observed for the measurement carried out in March. Perhaps the reason was a stronger solar operation, which dried the surface, especially grass.

The last of the parameters analyzed, the aligning moment, shows an interesting tendency (Figure 16). No significant impact of the weather period is observed during the months October–November, while a significant decrease in the M_Z moment value in December was recorded. In March, the M_Z value increased significantly compared to the December measurements.

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part B

6th ans

b ans.

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