Propulsion System

\label{sec:trade_propulsion}

This section gives an overview over the trade-off for the propulsion system of the three different configurations.

Trade-off Criteria and Weights

This subsection sets out to present the chosen criteria and their weights. The three different configurations lead to three separate trade-offs. For each trade-off, the criteria and weights are almost the same. The “clearance” criterion however, is the only criterion that is weighted differently for the configurations. This represents the fact that propeller clearance is only a minor issue for the traditional and the biplane configuration but can lead to major problems during take-off for the canard configuration. Therefore, although the clearance rating for the different propeller options stays the same for the three trade-offs, the “clearance” criterion is rated significantly higher for the canard than for the other two configurations. The criteria and their weights are presented in Table \ref{tab:tradeoff_weights}.

• Maneuverability: The propulsion system has an influence on the maneuverability of the whole aircraft by changing the airflow around the airplane. The slipstream of conventional single rotor propellers for example causes a significant yaw moment at low speeds and high power settings that has to be counter-acted using the rudder \cite{swirl}. This can be partly counteracted by a propeller with contra-rotating rotors, making control easier for the pilot. Also gyroscopic effects and the effect of the propeller position on maneuverability effects are taken into account. For a pusher configuration, for example, the slipstream has no effect on the maneuvering characteristics. As a trade-off criterion, maneuverability is rated the highest, because the aircraft needs to perform in aerobatic races and any influence of the propulsion subsystem on the overall maneuverability is of major importance.

• Complexity: The complexity of the propulsion system greatly affects its development, construction and maintenance cost that can be significant for a maintenance intensive component such as the propulsion subsystem. The system should therefore be as simple as possible to build and maintain. It is rated slightly lower than maneuverability.

• Clearance: Propeller clearance is crucial for safe operation on the ground and for rotation during take-off. Especially for pusher configurations the size of the propeller and it’s clearance can lead to a very high and heavy landing gear to allow for rotation during take-off. The importance of having a propeller with a small diameter for the required clearance is weighted very low for the traditional and biplane configurations which provide enough space to house a large propeller. For the canard configuration this is much more important, resulting in a high weight for the clearance criterion.

• Propulsive efficiency: Designing an electric plane requires to make the plane as energy efficient as possible to allow for a lighter accumulator. The propulsion system has a great influence on the overall energy efficiency of the airplane and sizes the battery. It determines how sustainable the aircraft will be. The first design goal however, is an aircraft for the Red Bull Air Race, so that energy efficiency is rated less than the capability to compete in the aerobatic races.

• Power density: Apart from the efficiency of the whole propulsive system, the power density of the motor and transmission depend on the electric motor layout and the necessity of a gear box. Contra-rotating propellers for example need a more complex and thus heavier transmission. This, however is less complex for an electric motor than for a piston engine. Electric motors are usually quite light, so that the power density is rated rather low, although the weight could increase with additional cooling systems needed or more energy efficient and advanced electric motor designs \cite{siemens}.

0.48

\label{tab:prop_conv_weights}

0.48

\label{tab:prop_can_weights}

\label{tab:tradeoff_weights}

Trade-Off Propulsion

After the weights are determined, the trade-off is done. Shrouded propellers have a rather low efficiency at the air speeds considered, so the added complexity and weight let them score poorly in the trade-off \cite{shrouds}. The main trade-off was therefore done between a single open rotor and a contra-rotating open rotor propeller.

The higher maneuverability for contra-rotating propellers due to swirl reduction in tractor configuration (not beneficial in pusher configuration) was found to be of only minor importance. The situations where the yaw from a single rotor propeller slipstream would require rudder input are mainly at low speeds and high power settings. Most of the maneuvers during the Red Bull Air Race are flown at moderate speeds and it was concluded and confirmed from talking to an aerobatic pilot that the extra rudder input for a single rotor propeller would not be an issue for the pilot. The added complexity for the contra-rotating propeller however is limited for an electric motor as well, due to only a few rotating parts. An electric motor even offers the possibility to have both a rotating rotor and stator, connecting them to one of the contra-rotating propellers each. This can allow for a construction of high simplicity and very little maintenance \cite{contramotor}. One of the main advantages of contra-rotating propeller is their improved efficiency due to the straightening of the slipstream from the first propeller. Extra thrust can be generated from the transformation of the rotational flow into axial flow, allowing for approximately 6-16% higher efficiency \cite{contraeff}. Higher efficiency means less energy needed for the mission and therefore a lighter battery. This is traded off against the higher weight of the contra-rotating propeller solution. A preliminary estimate for the battery weight of 98kg would mean a battery weight reduction of 9.8kg for 10% higher propulsive efficiency at the same total power and disregarding the small snow-ball effect resulting from the weight reduction. With a single open rotor constant speed propeller weighting about 35kg\cite{propweight}, adding a second propeller (although both propellers can be smaller) was estimated to increase the total rotor weight by 30%. The electric engine could make use of the contra-rotating rotor-stator solution and increase only 10% in weight. An extra 5kg are assumed for the more complex solutions for the shaft and the system for adjusting the pitch angle. With an engine weight of about 30kg (derived from an existing Siemens engine \cite{siemens} and adjusted for less power) the extra weight added up to 18.5kg, significantly more than the saved battery weight of 9.8kg. With the maneuverability and clearance being of minor concern, the power density, propulsive efficiency and complexity therefore ranked the single open rotor solution highest for the biplane and conventional configuration.

Although the canard would not benefit from contra-rotating propellers in terms of maneuverability at all because the slipstream has no effect on the aircraft in a pusher-configuration, enough clearance (smaller rotor diameter), is important for this pusher configuration to allow for rotation during take-off. The higher weight on the clearance lead to the contra-rotating propeller option for this configuration, even though the airplane gets slightly heavier (18.5kg added for complexity, 9.8kg less battery weight). The difference in score to a single open rotor however, is very small for the canard. The final scores are presented in Table \ref{tab:tradeoff_results_prop} as fractions.

An alternative to the contra-rotating propeller is making the second rotor stationary, becoming a ring of stator vanes. This would still slightly increase the swirl reduction, but add less to the complexity. For the canard however, the swirl reduction has no influence on the flight performance, so that the extra complexity without much performance increase would only have a negative effect on the design. The option of a fixed rotor was therefore discarded.

\label{tab:trade_prop_conv}

\label{tab:trade_prop_can}

\label{tab:tradeoff_results_prop}