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VTOL Unmanned Aerial Helicopters – A Concept Study

ABSTRACT

This study deals with the development of a Conceptual Design Tool for unmanned helicopters, so called VTOL UAVs. The goal of the Design Tool is:

  • Quick results
  • Good accuracy
  • Easy to use

The two first points of the goal are actually more or less dependent on each other. In almost all cases a high accuracy gives a slow calculator and vice versa. In order to fulfill the goal a compromise between calculation accuracy and calculation time needs to be done.

To make the Design Tool an easy to use program a graphical user interface is used. The graphical user interface allows the user to systematically work his way thru the program from a fictive mission to a complete design of a helicopter. The pre-requirements on the user have been eliminated to a minimum, but for the advanced user the possibilities to create more specific and complex helicopters are good.

In order to develop a Conceptual Design Tool the entire helicopter needs to be seen as a complete system. To see the helicopter as a system all of the sub parts of a helicopter need to be studied. The sub parts will be compared against each other and some will be higher prioritized than other.

The outline of this thesis is that it is possible to make a user friendly Conceptual Design Tool for VTOL UAVs. The design procedure in the Design Tool is relatively simple and the time from start to a complete concept is relatively short. It will also be shown that the calculation results have a good agreement with real world flight test data.

THEORY

Like every other system the helicopter follows the laws of physics and has one equilibrium state for every flight condition. The following forces and moments acts on the helicopter in flight:

Figure  2-3. The forces and moments acting on the helicopter in longitudinal direction

Figure 2-3. The forces and moments acting on the helicopter in longitudinal direction

In forward flight the flow-conditions starts to be really messy, mainly because the local airspeed depends on where you look at the rotor. To be able to analyze this, the Azimuth angle, ψ, is introduced. The Azimuth angle is zero over the tail-boom and is defined positive in the rotational direction. This will be discussed more in Section 2.3 and for now the flow is assumed to be evenly distributed.

Figure 2-7. The mass flow thru the rotor in forward flight

Figure 2-7. The mass flow thru the rotor in forward flight

Due to the velocity change around the rotor the advancing blade will tend to accelerate upward and the retreating blade will tend to accelerate downward. Because of this the rotor blades will not move in a strictly circular path around the helicopter but they will bend upwards and downwards in a harmonic movement. That motion can be expressed by a local flapping angle, β, defined like this:

Figure 2-10. Blade flapping

Figure 2-10. Blade flapping

CONCEPTUAL DESIGN PROCESS

Figure  3-1. Flowchart of the first stage in conceptual design process

Figure 3-1. Flowchart of the first stage in conceptual design process

When starting the design of a new helicopter the first step is to define the goals of the design. The goals of the design can be mission requirements, performance requirements and/or cost goals. Mission requirements can be payload capacity, endurance, range, speeds and physical size. Performance requirements can in many cases be the same as mission requirements but also other things not necessarily dictated by the mission. In some cases it can be requirements on climb speed, service ceiling, autorotative landing capability, one-engine-out performance etc.

Figure 3-2. Flowchart of the last stage in conceptual design process

Figure 3-2. Flowchart of the last stage in conceptual design process

When the overall helicopter design has been established it is used as input together with the design goals to predict performance of the helicopter. The performance calculations are based on the theories described in chapter two and will give the engineer a good estimation of the performance and if the helicopter is able to fulfill the different requirements.

DESIGN TOOL LAYOUT

Figure 4-2. Close-up view of a BERP rotor blade tip

Figure 4-2. Close-up view of a BERP rotor blade tip

The rotor blades are one of the most important parts of the helicopter regarding the helicopters performance and the shape of the rotor blades can be very complex. Many helicopters in service today have a variety of different airfoils at different locations on the rotor blade and the geometrical shape of the blade tip can be very advanced like the BERP blade tip seen in Figure 4-2.

Figure 4-5. Typical mission appearance

Figure 4-5. Typical mission appearance

The more advanced stage is started off with calculation of SFC. The SFC is selected by finding the rpm at which the ratio between engine power and SFC is the highest. If the engine is a two-stroke engine 2% extra is added to the SFC to compensate for the extra weight from two-stroke oil. The fuel calculator then separately simulates flying the endurance mission and then the range mission with the parameter settings specified as mission requirements. Range and endurance are defined as the distance and time from take-off to landing. Figure 4-5 shows a sketch of a mission. During the calculations the two equations seen below are used to calculate fuel weight for the endurance and the range mission.

RESULTS & DISCUSSION

Figure 5-9. Results & Comparison tab

Figure 5-9. Results & Comparison tab

In this chapter the results will be displayed and discussed. The main result from this thesis is the Conceptual Design Tool and its functions. The results and discussion part will focus on the design and performance of the Design Tool, not on how the program is supposed to be used and not how different pa rameters affects the helicopter performance. The user’s manual for the Design Tool can be found in Appendix. How the input to the Design Tool affects the helicopter performance is for the user to find out on his /her own.

CONCLUSIONS & FUTURE WORK

The outline of this thesis is that it is possible to make a user friendly Conceptual Design Tool for VTOL UAVs. The design procedure in the Design Tool is relatively simple and the time from start to a complete concept is relatively short. The user does not necessarily need to be a helicopter expert in order to create a complete helicopter concept, instead the statistical database can be used to design a concept for a certain mission specification. If however the user is an expert in helicopter design the concept design can be created rather freely.

It has also been shown that the results from the Design Tool and actual performance results from real helicopters match well.

The next step for the Design Tool is to use more sophisticated methods for the calculations and the estimations in the program. The rotor aerodynamics can be modeled by use of CFD. By using CFD the rotor can be made almost as complex as the user wants to and the interactions between the main rotor and the tail rotor can be evaluated, a lso the interaction of the fuselage and the main rotor can be evaluated in a better way.

For a design point of view it is possible to connect a CAD program and a FEM solver to the Design Tool. This would make it possible to optimize the structural design and the shape of the helicopter. Also the weight estimations could be made directly in the CAD program if the geometries and materials are known. Another evolutionary step for the program could also be to include more requirements such as cost goals. Finally if CFD, CAD and FEM are connected together an optimization algorithm could be made to find the best and most efficient helicopter that fulfills the requirements.

Source: Linköping University
Authors: Daniel Moëll | Joachim Nordin

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