Empennage

\label{sec:trade_emp} This section gives an overview over the trade-off for the empennage subsystem for the three configuration options.

Trade-Off Criteria and Weights

For the empennage subsystem there are three criteria defined; Performance, Weight and Complexity. The performance of the different empennage design options is decided with keeping in mind drag, efficiency and the placement of the control surfaces. A low drag will result in a higher speed. The efficiency is important since a larger surface is needed if it is not efficient, resulting in a higher drag. The placement of the tail control surfaces should be kept in mind, but, since the control surfaces are traded of after the empennage, only the complexity of placing it is taken into account. An estimation of the weight is done by keeping in mind the number of connections and the (respective) strength needed for each option. Complexity is based on the number of parts and the number of connections.

The weights that are given to each of the criteria are given in Table \ref{tab:emp_weights}. Since the aircraft is aerobatic, performance is very important and has the highest weight. The weight is also very important as a heavier design leads to all other subsystems being bigger and heavier. Complexity is deemed the least important as all of the options for the empennage (and canard) are not very complex and will not be an issue to implement.

\label{tab:emp_weights}

The DOT of the empennage from Section \ref{sec:dot_emp} is used as a guideline, starting at the top and going down. For each choice first an analysis of weaknesses is done to possibly cross off certain options.

The first choice in the DOT is the choice between a single or a double tail boom. This choice is not traded off as the twin boom structure is eliminated by analysing its weaknesses. A twin-boom is only used if the tail structure would otherwise become too large. For example, the vertical tail has to become so large and high that the aircraft would no longer fit in a standard hangar \cite{torenbeek2013synthesis}. It is not expected that the tail would become too large as a similar tail size as current aerobatic aircraft is expected and a single tail-boom is used on these.

The next choice is the placement of the vertical tailplane. The option to have no vertical tailplane is eliminated as a rudder will be needed for yaw control. This leaves four options, mounting the tailplane; on top of the boom, on bottom of the boom, on both sides or on the horizontal tailplane. These four options were traded off with the trade-off program resulting in the vertical tail on top of the tail-boom to be the best option. This option scored high as for example the bottom mounted options may need a longer landing gear adding complexity and weight, the horizontal tailplane mounted option is less efficient as it will need more than one vertical tail surface thus scoring lower on performance. When looking at all of the criteria it makes sense that the top-mounted vertical tail won this trade-off.

The next level is choosing to use a horizontal tailplane or not, due to the high manoeuvring requirements a horizontal tailplane will be needed to trim the aircraft and to mount some control surfaces to. The next choice is again a trade off, this time for the mounting location of the horizontal tailplane. There were four options, mounting the horizontal tail; on the tail boom, low on the vertical tailplane, on the middle of the vertical tailplane or high on the vertical tailplane. Mounting the horizontal tail on the tail boom won the trade-off. This design scored well on both complexity as weight as mounting it on the tail boom would not effect the strength requirements of the vertical tail, which, if the horizontal tail was mounted on it, would have to be much stronger.

Since the aircraft is aerobatic and has to fly upside down the option of having dihedral is eliminated by analysing the weaknesses. The same is the case for twist, as was already explained for the wing in Section \ref{sec:trade_wing}. As was also explained for the wing sweep is only necessary for high speeds, this is also the case for the tail, therefore the option of having sweep is removed.

A trade off was done for taper, trading-off between no taper, a low taper ratio and a high taper ratio. Having low taper won this trade-off, this is mainly because with a high taper ratio the lift distribution is closest to a elliptical loading  and it therefore scored well in the trade-off.

\label{tab:trade_emp}

Table \ref{tab:trade_emp} shows the outcomes of the trade-offs that were performed for the empennage subsystem.

Canard

\label{sec:trade_can}

For the canard aircraft a separate trade-off is performed. The canard aircraft will, by definition, not have a horizontal tail but it does need some form of vertical stabilizer for yaw-control. From the trade-off of the empennage it became clear that a vertical stabilizer on top is preferred over a tail mounted on the bottom or on both sides. However, an option that was added for the canard aircraft is using wing-tips as vertical stabilizers.

By analysing the weaknesses of the two surviving options, the vertical tail mounted on top and the winglets, the winglets were dropped due to the increase in moment of inertia and the added structural wing weight since sweep would be needed to move the rudders far enough back.

The canard DOT is used as a guideline for the canard trade-off. The criteria and their weights are the same as for the empennage and can be found in Table \ref{tab:emp_weights}.

The first trade-off for the canard is the mounting location; low, middle or high on the fuselage. The trade-off resulted in the mid-fuselage mounting configuration as the best option. This was mostly due to the fact that it is simpler and lighter to mount the canard in the middle of the fuselage.

The next decision was between sweep, no sweep or variable sweep, but as was explained in both Section \ref{sec:trade_wing} and \ref{sec:trade_emp} no sweep is required for the relatively low speeds that are flown in the races and competitions, for this reason, by analysing the weaknesses of the other options, no sweep was the chosen option.

The same trade-off as for the empennage was done for taper (Section \ref{sec:trade_emp}), the outcome is therefore also the same, having a low taper ratio won the trade-off.

Since the aerobatic aircraft should be able to fly upside down, having dihedral and twist can be eliminated by analysing their weaknesses. Again, just as for the wing and empennage, no dihedral nor twist is applied on the canard surface.

\label{tab:trade_can}