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UAV eVTOLs

Issue No 36, 24 July 2023

By: Anthony O. Ives

An eVTOL is an electric vertical take off and landing aircraft, a small scale UAV (Unmanned Air Vehicle) version is more or less a hybrid quadcopter and fixed wing UAV. The hybrid quadcopter and fixed wing UAV works well as the additional weight of having four electric motors for vertical flight as well as a separate larger motor for forward flight is fairly small. Compared to the complexity of a rotor tilting mechanicism which will probably not really save on the weight of the four motors for vertical flight given the simple design.

Using some simple maths equations as seen in some of the previous articles, [1], [2], [3], [4], [5], [6] and [7] its fairly straightforward to size a small scale fixed wing aircraft. An eVTOL can be sized in a similar way with the exception that you must consider the additional weight from adding the quadcopter elements and you have to calculate the thrust required from the quadcopter element. The vertical thrust can be calculated using the same method as in reference [5] except the required thrust is not the drag of forward flight but the estimated weight of the UAV eVTOL, and of course it is divided by four as there are four vertical rotors. So using the same equation from Reference [5]:

\[R=\left( \frac{T}{\rho \pi \omega^2 C_T} \right)^{\frac{1}{4}}\]

Where R is the radius of the rotor, CT is thrust coefficient, ρ is air density, π is the mathematical constant [8], ω is the angular frequency of the rotor

ω = 2 π (1/60) × RPM

T is thrust which is given by the following expression:

\[T=\frac{m g}{N_{rotor}}\]

Where m is mass, g is gravity and Nrotor is the number of vertical rotors which is 4 in this case. Calculating the radius of the vertical rotors using the values given in reference [1] to [7] as below:

\[R=\left( \frac{2.07 \times 9.81}{4} \frac{1}{1.2256 \pi (1047.2^2) 0.002} \right)^{\frac{1}{4}}\]

=0.157 meters

A typical eVTOL UAV design in plan view would look something like the following design drawing:

Design Drawing of UAV eVTOL

In hover the UAV eVTOL operates in the same way as a quadcopter, the quadcopter element is described in more detail in Reference [9]. In forward flight the UAV eVTOL operates like a fixed wing aircraft using its control surfaces such aileron for roll, elevator for pitch, etc. Generally the fixed wing control always operates hence the aileron, elevator and rudder will always move even if the UAV eVTOL is hovering. This keeps the control system function simple, ensures that there is always control over the aircraft in forward flight and the aileron, elevator and rudder have no adverse effects in hover hence it is more convenient to keep them operating all the time.

The quadcopter controls on the other hand only operate in hover flight because their operation could cause problems in forward flight and they may consume battery power unnecessarily. The transition to forward flight would start at hover obivously and then the large forward flight propeller would start the aircraft moving in horizontal direction. The quadcopter controls would still remain active until sufficient speed (which could be described as the transition speed) has been reached that the aerodynamic control surfaces (such as the aileron, elevator, etc) become effective. Transition from forward flight to hover the reverse procedure would happen, the forward flight propeller would be stopped and the quadcopter controls would take over once the aircraft speed drops below the transition speed. This is similar to the approach used for tilt rotors, in that hover control is only active during hover phases where as the aerodynamic controls (elevator, aileron, etc) are active all the time [10] but are only really effective during the forward flight phase.

Future articles will look at transition from hover to forward flight of UAV eVTOLs and tiltrotor aircraft in more detail. As a UAV eVTOL becomes larger using a hybrid quadcopter fixed wing design becomes more inefficient and a tiltrotor design [10] becomes more necessary. Tiltrotors will also be introduced separately in a future article.

Please leave a comment on my facebook page or via email and let me know if you found this blog article useful and if you would like to see more on this topic. Most of my blog articles are on:

  1. Mathematics

  2. Helicopters

  3. VTOL UAVs (RC Helicopters)

  4. Sailing and Sailboat Design

If there is one or more of these topics that you are specifically interested in please also let me know in your comments this will help me to write blog articles that are more helpful.

References:

[1] http://www.eiteog.com/EiteogBLOG/No5EiteogBlogStability.html

[2] http://www.eiteog.com/EiteogBLOG/No1EiteogBlogLiftCL.html

[3] http://www.eiteog.com/EiteogBLOG/No2EiteogBlogDragCD.html

[4] http://www.eiteog.com/EiteogBLOG/No3EiteogBlogMass.html

[5] http://www.eiteog.com/EiteogBLOG/No4EiteogBlogThrust.html

[6] http://www.eiteog.com/EiteogBLOG/No6EiteogBlogRange.html

[7] http://www.eiteog.com/EiteogBLOG/No7EiteogBlogEndurance.html

[8] http://www.eiteog.com/EiteogBLOG/No10EiteogBlogCircle.html

[9] http://www.eiteog.com/EiteogBLOG/No33EiteogBlogUAV.html

[10]The History of the XV-15 Tilt Rotor Research Aircraft:From Concept to Flight, NASA SP-2000-4517, 2000, Martin D. Maisel, Demo J. Giulianetti, Daniel C. Dugan, https://history.nasa.gov/monograph17.pdf

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