AIRCRAFT PROPELLERS

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Question 1: Comparison of the design features of the propeller and propeller systems on single 1930s single piston racing aircraft to a single piston Unlimited Class racer.

Both the 1930s single piston aircraft racers and the present day Unlimited Class airplanes are built on the same design. The most important requirement of an aircraft to enter the modern Unlimited Class racing is the utilization of a piston engine among other requirements which ensure that only the very old aircraft enter the race (Ahlstrom, 2000). This has made it more appropriate for the racing ground crew to remodel the old World War aircrafts such as the P15 Mustangs into racers.

There is a major design difference between 1930s racing aircraft and the airplanes participating in Unlimited Class races. This is particularly true in the design and direction of rotation of the propellers. This is mostly due to the nature of the pylons used at the Unlimited Class racing which makes provisions for left turns only. From a pilot’s view, a propeller which has an anticlockwise rotation tends to raise the right wing of the aircraft while negotiating these corners. This has resulted in a major design change whereby the propellers of the racing aircraft are refitted to rotate clockwise- which is a major difference from the 1930s single piston engines. The effects of the direction of rotation of a propeller are most evident in turbo shaft aircrafts such as helicopters. This is the single point in which the effects of rotation are best demonstrated and the adverse effects when the desired propeller rotation is affected (Aerospace.org, 2015). Therefore, the modern racing aircraft are a demonstration of the achievements that have been made this far in incorporating propellers in fixed wing light aircraft while taking into account the effects and forces associated with their rotation.

The Unlimited Class event gives flight mechanics an opportunity to incorporate new innovative designs into the propeller and propeller systems of their aircraft. For example, some of the single piston aircrafts which participate in the race have specialized combustion engines which burn aluminum instead of the general purpose jet fuel. Aluminum, when properly used as a source of fuel can burn with a higher density and yield more power (Massachussets Institute of Technology, 2017). This ensures that the relatively old aircraft are able to accommodate forces greater than 6g. This kind of acceleration was not a common achievement in the 1930s for single piston aircraft since the propulsion systems during the period could not keep up with the resultant stress. In fact, even the modern day fighter jets require the pilots to wear special suits before performing accelerations which push their aircraft to such limits. With the kind of air frame technology that has been used in the design of the piston engine aircraft, it only leaves one wondering the kind of ingenuity that goes into the design of a propeller system that is capable of such stress while racing these old planes.

The aircraft from both eras share a basic engine propulsion system. This is characterized by the single crank shaft system responsible for rotating fixed pitch propellers. However, the Unlimited Class racers feature various adjustable pitch propellers although this is usually not a standardized requirement (Hassel, 2012). Instead, the ground crew and designers are always given the liberty and freedom to tweak these features in order to ensure optimum aircraft performance and the assurance of finishing the race in spite of the extra stress that the aircraft is subject to.

The Unlimited Class aircraft are much faster and develop more horsepower than their 1930s counterparts. This is partly due to the freedom of the ground crew in choosing the propellant material that is used in the engines. However, some design differences have also led to marked difference in these aircrafts’ propellant system. For instance, most of the participating airframes have their wings partly clipped in order to reduce the overall drag exerted on them (Lawless & Shaheen, 1988). This has contributed to their remarkable speeds which can easily exceed 500mph while utilizing engines that develop nearly the same amount of power as their old counterparts.

Whereas both airframes utilize the blade element theory for their propellers, the Unlimited Class racers seem to utilize this design concept much more than their 1930s racing counterparts. The logic behind using this design concept arises from the fact that these planes have always been required to perform extraordinary maneuvers. These include sudden and steep climbs, descents and turns which characterized the racing environments of the 1930s –which were partly influence by military use and the present day sharp cornered turns over the pylons a the race field.

Frank Monroe Hawks Miller HM. Obtained from Air racers

Question 2: Analysis of the Design Features and Systems Expected to be found on a propeller system to be fitted on a large four engine turbo-prop aircraft flying up to 420 knots and performing STOL operations from jungles, deserts and over the arctic regions.

Turbine propellers (turbo-props) work by means of hot compressed gases rotating turbines which in turn rotate the propeller systems. The aircraft in question shall need to fly at relatively fast speeds which are an important propeller design consideration for such an airplane. The fact that the plane is fitted with four engines is also an important consideration of the possible propeller systems that can be incorporated into its body. Indigenously, STOL operations require that an aircraft should be able to land and take off over short runways (Daniel Hallman, 2011)- another important consideration for the propeller system that should be used.

It is expected that the aircraft has an adjustable pitch as opposed to a fixed-pitch propeller system. This is due to the fact that STOL operations require optimized power generation from the propellers. Moreover, this aircraft will be required to fly over long distances: – a fact that is implied by the number of engines that it has. The adjustable pitch of the propellers will make it possible for the pilot to tilt them in such a way that reduces drag to the fullest which will enable them to develop the capability of developing more thrust and lift necessary for taking off from a short runway. The pitch for which an aircraft’s propeller is mounted is either designed to accommodate cruise speeds or other STOL operations (University of Southampton, 2018). Therefore, the fact that the propeller on this system is adjustable will give the pilot the opportunity to perform STOL but still find pitch optimizations while cruising over long distances for which the aircraft is most probably designed to accomplish.

Image of turbo-prop engine. Obtained from Glen center, NASA

Propellers are usually mounted in or on the engine system and are expected to be positioned as close to the fuselage as is possible. As the propellers develop thrust at their front surfaces, being close to the airframes will ensure that as much wing surface area as possible is available in order to develop lift much faster and more efficiently without affecting the wing’s design dynamics(Airlines.net, 2002). Moreover, the propellers are expected to send a thrust of air over the wings for extra lift. Therefore, the fact that the aircraft will be cruising for 420 knots for most of the distance will mean that there will be enough lift from the airflow hence further warranting the position of the engines being near the fuselage.

It is desirable to mount as many propellers as possible per engine shaft. These could be more than four. Although this design reduces the general efficiency of each propeller, it makes it possible for propellers tips to rotate just below the speed of sound (Mach 1). This will make the aircraft less noisy which is a general environmental concern especially in the jungles where it could disturb the existing wildlife diversity. In addition, more propellers per shaft will also make the control surfaces more maneuverable and responsive at lower speeds.

Flying aircraft in extreme climatic conditions such as dry deserts and cold arctic regions of the world presents a major expectation in the design of the plane’s cooling system. Firstly, it must be able to accommodate overheating problems rampant in desert regions while at the same time avoiding the freezing of propeller systems in the arctic regions while the craft has stayed on the ground for elongated periods of time. Therefore, it will be expected that each engine incorporates a hybrid cooling system that will make it possible for the propeller system to be cooled by either water in hot conditions or air in the arctic regions. Ice is likely to form around the engine regions while the aircraft is grounded in the arctic regions (Technische Universitat Darmstadt, 2016). Therefore, it would be necessary for the engine and propeller compartments to be designed in such a way that accommodates this factor.

Atmospheric pressures in desert and arctic regions are markedly different which create a very important consideration for designing propeller systems. The hot air in deserts is mostly characterized by low pressure while the arctic regions have relatively higher pressures. This affects the amount of air speed above the wings that is required to generate a lift for take-off. This means that the aircraft’s propeller system should yield enough lifting power to offset the drag experienced on the propellers which might need to have greater pitches in order to perform a STOLL operation in extreme and rugged conditions (NASA, 1997). In addition, the propellers should be able to accommodate reverse thrusts which will serve as brakes on landing in the arctic. This is especially true given the fact that the already short runway might be snowy hence the need to apply extra breaking via the propellers.

 

 

Image obtained from Scriebel.com

Question 3: Analysis of design features and propeller theory expected on a propeller fitted to a single engine tail wheel fighter aircraft developing less than 100 hp compared to a modern light aircraft with tricycle undercarriage.

It is important that the various propeller theories are applied in designing systems that are either focused on blade element or momentum of the forces acting on the propeller per unit time in order to create lift for flight. It would be expected that the First Word War fighters are fitted with non-adjustable propeller pitches. This could partly be attributed to the fact that the warring countries needed to produce a very large quantity of aircrafts which were often destroyed just as fast as they were being produced from the factory lines hence the need for fast advancements such as developing the B-17 bomber (ethw.org, 2015) which scrapped any need to fit them with the relatively expensive pitch adjustable propellers. However, the same will not be expected in a modern small fighter aircraft. There is much more competition from commercial aircraft manufacturers such as Cessna who have mastered the art of producing such low cost effective aircraft. Due to this kind of competition, it has become necessary to optimize the functionalities of the individual planes and this will include fitting them with as much control for the pilot as possible for the pilot’s control systems.

Small fighter aircrafts are usually handled by inexperienced pilot trainees. During the First World War, it was common for war time pilots to sacrifice their lives by flying right into other aircraft during combat missions. This necessitated the need for a propeller system that could maintain a steady course as much as possible, and indeed maintaining a steady course is something that is still admired in today’s aviation industry. However, the extensive use of smaller airplanes in training activities has necessitated the need to incorporate as much control and automatic systems as possible in order to ensure that the students can complete their flying classes without a major incident. Among others, the modern propeller systems are designed in such a way that they can even recover from a stall which can save a panicked and shocked pilot trainee- something that could not be possible with the First World War aircrafts.

First World War fighters had a shorter tail wheel which gave the wings an increased angle of attack against the air while the plane is taking off. This is different from the modern tricycle undercarriage aircrafts which are almost lateral to the ground surface while on the ground. However, the fact that the wheels of the old fighters could never be retracted meant that any propeller advantage gained over the modern design was reversed as soon as the aircraft took into the sky (Century of Flight, 2018). The modern light aircrafts could tap into this advantage if they got their undercarriage slightly elongated. This would in turn provide a natural increased angle of attack for the mass of air released over the surface of the wings by the rotating propeller. Consequently, this kind of design would be able to server STOL operations much better and efficiently. However, the general aesthetic design of this system would not be as appealing and this could reduce the market share of a company trying out such an innovation.

Blade element theory would be an important consideration in a modern lightweight aircraft whereas First World War fighter aircrafts might not incorporate this principle. The blade element theory utilizes the principles behind the functioning of a normal wing, by increasing the curvature at the front more than the one at the back side (Aerodynamics for Students, 2016). This, in effect, creates a kind of lift towards the front which is in essence the generated thrust for the aircraft. Consequently, the modern light aircraft is capable of deriving more thrust from the action of the propeller. This means that a smaller engine is capable of developing as much power as did a larger engine fitted on a fighter during World War I.

 

Conclusion

Aircrafts with a single piston have been around since 1903 and are likely to stay around for quite some time. The single piston racing aircraft from the 1930s basically have the same airframes and engine technologies being used in the modern Unlimited Class race. The modern racing airplanes are however much more modified than their old counterparts. Some of these modifications include the freedom of the ground crew to use fuels of their choices, tweaking the propeller rotation systems and reductions gears and redesigning the wing control surfaces among others. These changes have ensured that each racing pilot gets as much edge as possible over other competitors and harnessing higher speeds and more resilience on the g force being exerted on the aircraft.

A turbo-prop four engine aircraft used in STOL operations has a set of unique propeller requirements that could enable it perform its job optimally. Among others, the pitch of the propellers will need to be adjustable so that the pilot can continuously optimize the performance of the propellers while flying under different air speeds and conditions. These are primarily dictated by speed requirements while taking off and those that come into the picture while the plane is cruising. The propeller theory is a comprehensive mathematical design consideration which is used in propellers. This theory makes a designation for designing propellers based on the moment created by the blast of air pushed by the propellers per unit time. The first two approaches are the most commonly design approaches while the vortex theory is rather mathematically complex hence does not receive as much attention by designers. General expectations dictate that modern small aircrafts make a combination of the lessons learnt over the period of aviation design and as such make flight optimizations as possible in order to harness the best flying experiences from the propeller system.

 

 

 

 

 

 

 

 

Aerodynamics for Students, 2016. Blade Element Theory for Propellers. [Online] Available at:            http://s6.aeromech.usyd.edu.au/aerodynamics/index.php/sample-page/propulsion/blade      element-propeller-theory/ [Accessed January 2018].

Aerospace.org, 2015. Helicopter Rotation Conventions. [Online] Available at:            http://www.aerospaceweb.org/question/helicopters/q0212b.shtml     [Accessed 28 January 2018].

Ahlstrom, E., 2000. The Design of an Unlimted Class Reno Air Racer. [Online] Available at:         aero-comlab.stanford.edu/Papers/AIAA-2000-4341-839.pdf [Accessed 30 January 2018].

Airlines.net, 2002. Under Wing Engines Vs Tail Mounted Engines. [Online] Available at:            http://www.airliners.net/forum/viewtopic.php?t=138213      [Accessed January 2018].

Century of Flight, 2018. Development of Aviation Technology. [Online]      Available at:    http://www.century-of            flight.net/Aviation%20history/evolution%20of%20technology/Variable      Pitch%20Propellers.htm            [Accessed January 2018].

Daniel Hallman, 2011. The Tuskegee Airmen and the “Never Lost a Bomber” Myth. [Online]         Available at: http://www.redtail.org/wp-content/uploads/2013/03/The-Tuskegee-Airmen and-the-Never-Lost-a-Bomber-Myth.pdf            [Accessed 30 January 2018].

ethw.org, 2015. World War II Aircrat. [Online]         Available at:    http://ethw.org/World_War_II_Aircraft            [Accessed January 2018].

Hassel, P., 2012. A History of Development of the Variablle Pitch Propeller. [Online] Available     at: hamburg.de/pers/Scholz/dglr/hh/text_2012_04_26_VariablePitchPropellor.pdf       [Accessed 30 January 2018].

Lawless, R. & Shaheen, T., 1988. Airplanes and Airports: The Subtle Skill of Japanese      Protectionism. SAIS Review, pp. 101-120.

Massachussets Institute of Technology, 2017. Aluminium as a Fues. [Online] Available at:            https://www.ll.mit.edu/publications/technotes/TechNote_AIH20_Fuel.pdf [Accessed        January 2018].

NASA, 1997. Boundary Layer Control, STOL, V/STOL Aircraft Research. [Online]           Available at: https://history.nasa.gov/SP-3300/ch8.htm [Accessed January 2018].

Technische Universitat Darmstadt, 2016. What causes ice formation on aircrafts during flight?.     [Online] Available at: https://phys.org/news/2016-01-ice-formation-aircrafts-flight.html            [Accessed January 2018].

University of Southampton, 2018. Curtis-Wright: Greatness and Decline (review). [Online]           Available at:            https://www.southampton.ac.uk/~jps7/Aircraft%20Design%20Resources/aerodynamics    Bristol%20University%20Breguet%20range%20eqn.pdf        [Accessed January 2018].

 

 

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