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 racers and the present day Unlimited Class racer basically incorporate the same aircraft design. The defining requirement of an aircraft to enter the modern Unlimited Class racing is the utilization of a piston engine (Anon., 2013). This has made it more appropriate for the racing mechanics to remodel the old World War birds such as the P15 Mustangs into racers.

The race gives flight mechanics an opportunity to incorporate innovation in the design of the propeller and propeller systems of their aircraft. For instance, 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 (Anon., 2014). This ensures that the little old birds are able to accommodate forces greater than 6g. This kind of acceleration was not a common thing in the 1930s single piston aircraft since the propulsion systems during the period could not keep up with such accelerations. In fact, even the modern day fighter jets require the pilots to wear special suits before performing accelerations which push their aircraft to such forces. 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 propeller system that is capable of such stress while racing these old planes.

Another major design difference between the two birds is 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. These 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 massive effect of the direction of rotation of a propeller is most evident in helicopters. This is the single point in which the effects of rotation are best demonstrated and how catastrophic things can go if and when the normal rotation is affected. Therefore, the modern racing aircraft are a demonstration of pure innovation and the achievements that have been made this far to incorporate propellers in fixed wing light aircraft.

The two birds 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 but this is not a standard deployment (Welch, 1989). 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 birds are mostly 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 clipped partly 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 stems 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 both the 1930 –which was 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) feature 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 is an important propeller design consideration for such an aircraft. The fact that the plane is fitted with four engines is also an important precursor of the possible propeller systems that can be fitted unto the system. Indigenously, STOL operations require that an aircraft should be able to land and take off over short runways (Haulman, 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 propellers. This is due to the fact that it has to perform STOL operations and yet it 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 pilot to tilt them in such a way that reduces drag most hence have the capability of developing more thrust and lift necessary for taking off from a short runway. Most aircraft whose propeller pitch is fixed are either optimized for cruising or STOL operations (Kinney, 2000). 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 which the aircraft is most probably designed to accomplish.

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

The aircraft’s engines which house the propeller systems 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 areas as possible is available in order to develop lift much faster and more efficiently (Byers, 2004). Although the propellers are expected to send a thrust of air over the wings for extra lift, the fact that the craft will be cruising for 420 knots for most of the distance means that there will be enough lift from the airflow hence further warranting the position of the engines.

Each individual engine is expected to have as many propellers as possible per 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 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 fauna diversity. This will also make the control surfaces more maneuverable and responsive at lower speeds.

The fact that the aircraft is supposed to fly in dry deserts and cold arctic regions of the world presents a major expectation for 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 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. Water is likely to thaw around the engine areas while the aircraft is grounded in the arctic regions (Pavelec, 2004). Therefore, it would be necessary for the engine and propeller compartments to accommodate for this possible eventuality which might destroy the engine system.

Desert and arctic regions have markedly different atmospheric pressures which is a very important consideration for designing propeller systems. The hot air in desert 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 horsepower to offset the drag experienced on the propellers which might need to have greater pitches in order to perform a STOLL operation in deserts (Stevenson, 1990). 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.

The propeller theory gives mathematical references towards designing systems that are either focused on blade element or momentum of the forces acting on the propeller per unit time of 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 (pavalec, 2005). However, the same will not be expected in a modern small 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 incorporated including as much control for the pilot as possible in controlling the propeller of the plane.

Most of the small aircrafts tend to be trainers which are usually handled by inexperience pilot trainees, and who ‘love’ their lives very much. 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, 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 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.

The tail-wheel design of the First World fighters increased the angle of attack of the wings while 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 cancelled as soon as the aircraft got airborne (Harvey, 1992). 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. This is makes it necessary to include aesthetic design considerations in the building of propeller systems.

A modern lightweight aircraft is expected to incorporate the principles of blade element more that First World War fighter aircraft. 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 (Thompson, 2004). 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 are basically the same airframes and engine technologies being used in the modern Unlimited Class racing. The modern racing birds 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 endeavors have ensured that each racing pilot gets as much edge as possible over the 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 propeller based on the moment created the blast of air pushed by the propellers per unit time, blade theory and vortex theory. 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 as otherwise could be necessary. General expectations dictate that modern small aircrafts make a combination of the lessons learnt over the period of aviation design and as such make as much flight optimizations as possible for the best flying experiences.

 

 

 

 

 

 

Anon., 2013. YouTube. [Online] Available at:           https://www.youtube.com/watch?v=_HWD3iJtTpU

Anon., 2014. You Tube. [Online] Available at:          https://www.youtube.com/watch?v=o7EIxneD4f8

Byers, R., 2004. Dreams of Flight: General Aviation in the United States (review). Technology     and Culture, pp. 628-630.

Harvey, J. R., 1992. Regional Ballistic Missiles and Advanced and Strike Aircraft: Comparing     Military Effectiveness. International Security, pp. 41-83.

Haulman, D., 2011. The Tuskegee Airmen and teh “Never Lost a Bomber” Myth. Alabama           REview, pp. 30-60.

Kinney, J. R., 2000. Curtis-Wright: Greatness and Decline (review). Technology and Culture, pp. 147-149.

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

pavalec, S. M., 2005. Hitler’s Jet Plane: the Me 262 Story (review). The Journal of Military            History, pp. 877-890.

Pavelec, S. M., 2004. 100 Years of Air Power & Aviation (review). The Journal of Military           History, pp. 1318-1321.

Stevenson, G., 1990. Canada and International Civial Aviation, 1932-1948 by David MacKenzie (review). The Canadian Historical review, pp. 412-414.

Thompson, W., 2004. Chasing the Silver Bullet: U.S. Air Force Weapons Development from        Vietnam to Desert Storm. The Journal of Military History, pp. 1014-1017.

Welch, J., 1989. Assessing the Value of Stealthy Aircraft and Cruise Missiles. International          Security, pp. 47-63.

 

 

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