Multirotor Aerodynamics – Multirotor Design

How can we classify a multirotor airframe?

• Size?
• Number of propellers?
• Style?

Consider comparing them to an aeroplane airframe:

• Wingspan (for symmetric aircraft): Distance from on motor to the opposite motor
  • Wingspan is valid for symmetric designs, and for “cross-frames” is typically the span from one blade to the opposite. In general, it could be said to be the longest distance from one propeller-centre to another.
• Propellers: Quadrotor, Trirotor, Hexarotor, Octorotor, Dodecarotor…
  • The propeller count really depends on the airframe itself, but the examples there are:
    • Trirotor = 3 propellers (and an extra servo to control yaw)
    • Quadrotor = 4 propellers (shown)
    • Hexarotor = 6 propellers
    • Octorotor = 8 properllers
    • Dodecarotor = 12 propellers
• Style: X, +, Y, H, …?
  • Airframes come in many different forms, and each have different benefits:
    • Redundancy (designs with >4 motors)
    • Mechanical Simplicity (designs with minimal motors & arms)
    • Agility (Smaller wingspan)
    • Stability (Larger wingspan)
    • Mounting locations
    • Visibility

Characteristics of weight (in short):

• More weight => less flight time!
• More weight => more stability
• More weight (or more spread-out weight) => less agility

A bit more information:

• More weight (with same amount of power) will reduce our flight time
• Weight in general will stop us getting blown around by the wind, and will make the platform more stable (larger inertia)
• A multirotor with a smaller weight distribution will be more agile than an aircraft with a large weight distribution (as a smaller moment is required to rotate the aircraft)
• Explain the “15 minute” rule (thurst-to-weight ratio of 2:1 (?) typically results in 15-20 minute flights with current battery technology)

Weight balance (in short):

• The location of the Centre of Gravity (CoG) determines the RPA’s stability
• A CoG that is not centred laterally will cause a loss of efficiency (some motors will need to work harder!)

A bit more information:

• The CoG really should be as close to the airframe centre as possible for best performance
• CoG below the prop-line will cause pendulousity to help stabilise the aircraft (assuming it has enough thrust to still be properly controlled)
• CoG above the prop-line is unstable, and may cause control issues (but can still be OK if it’s not significantly high)
• CoG that is not in the roll/pitch centre of the aircraft will cause a mis-match in thrust, meaning some props will have to work harder than others (loss of efficiency, worse control performance)

“Pendulosity” is the term used to describe the position of the centre of gravity:

• The closer the CoG is to the centre of the RPA, the easier the RPA is to control
• A CoG below the prop-line will be naturally stable
• A CoG above the prop-line will be naturally unstable

Dihedral in an aircraft is intended to stabilise the aircraft in ‘roll’. The natural position of the aircraft is roll-centred.

Dihedral on a multirotor has exactly the same effect!

• The multirotor will gain a natural tendency to correct slight rotations around the lateral axes (roll and pitch)
  • A small dihedral (4° – 6°) is typical for most small multirotors. They will gain a slight increase in stability at the cost of a small reduction in efficiency.
  • The multirotor will gain a natural tendency to correct slight shifts in around the horizontal axes – as in this context multirotor aircraft don’t have a ‘nose’ or ‘tail’.
• It may also help with stability during movements, ascent, and descent

If multirotor RPA don’t have wings… can they still stall?

Of course! A quadrotor has four rotating wings!

We call this propeller stall, and it affects all propellers (including aeroplane propellers). Propellers are often designed to avoid stalling:

• Tapering
• Sweep
• Changing AoA

The beginnings of a propeller stall can be seen in the Ring Vortex State (RVS).

This occurs when the aircraft passes through the trailing vortices of the propellers:

• Flying in extremely gusty air
• Rapidly descending into the propeller wash
• Flying close to the ground (in the “ground effect”)