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The Aerodynamics of Multirotors

Disclaimer: At Drone Insider we are not aeronautical engineers, however we have an understanding of mechanics and basic aerodynamics. Enjoy!

A great scientific paper was published a few days ago; it discussed a very important topic in the design of mini quad frames, aerodynamics.

Aerodynamics are often neglected by frame designs, who instead try to focus on strength especially. Whilst we are not saying that strength shouldn’t be an important consideration, we feel aerodynamics need to be factored in more, especially as quads keep on getting faster!

The paper itself contains large amounts of information, so we are going to look at some of the more interesting results.

In total, 9 different frames and a total of 25 different configurations were tested in the wind tunnel. The frames ranged in size from 122mm from motor to motor up to 450mm motor to motor. Some of the configurations that were tested were:

  • Bare frame vs. ready to fly frame
  • 122mm to 450mm ready to fly frames
  • Ready to fly frame props off vs. props on and free wheeling in the flow
  • Frames using top plates vs. aerodynamic canopy
  • GoPro on board vs. no HD recording camera onboard
  • Small racing X frame vs. larger freestyle H frame
  • Covered 5.8gHz video transmitter antenna vs. bare 5.8gHz antenna

First some explanation what the graphs are:

Y-Axis:

Drag coefficient:

This is simply how much the frame resists the flow of air. The higher this number, the more the air will resist your quadcopter moving forward, this will limit your top speed. We are therefore trying to get this number as low as possible.

Lift Coefficient:

This is how much lift the quadcopter provides, excluding the power generated by the motors.  It is going to be negative most of the time in this article, as the quadcopter body isn’t generating any lift itself and is in fact detrimental! We are trying to get this number as close to zero as possible, or even above zero.

X-Axis:

On the X axis lies the Angle of attack. In layman’s terms this is how far forward the quadcopter is pointing. It is safe to assume that for (in this case) angles of attack with a greater negative value will cause there to be a higher drag co-efficient.

Canopy or no Canopy?

A question asked by many pilots is whether to run a canopy, some believe it decreases the drag coefficient. However this data, from use of the RotorX Atom with and without canopy, does not agree.

Canopy

As you can see the canopy increases the drag on the frame, however it does increase the lift coefficient (this is a good thing, we know this is a bit of a brain bender), especially at the extreme angle of attacks.

Racing Frame v Alien

One graph we found extremely interesting was the comparison between an ImpulseRC Alien and a Shrike. The Graph shows, a HUGE advantage to the shrike in terms of the drag coefficient, 44% to be exact. Comparing the true lift and drag of each frame side by side, the Shrike shows an average 44% less parasitic and 39% less overall drag, which is a very large amount and makes a large difference in how the vehicle performs at higher speeds. This data really shows how much of an advantage racing optimised frames have over freestyle optimised frames in terms of pure speed.

Racing VS Alien

GoPro v No GoPro

This is the piece of data that really shocked us. We expected a GoPro to add huge amounts of drag to a frame. Apparently not, so FGA and Steel you’re safe for now.

Gopro graph

As you can see, the GoPro makes hardly any difference in terms of the drag coefficient over running no GoPro. Even more interestingly though, it increases the lift coefficient, making it in fact BETTER than running no GoPro for purely aerodynamic reasons. Be warned though, the extra weight of a GoPro is obviously going to detriment your handling in the air.

Does all the electronics on your quad change the aerodynamics significantly?

Indeed it makes a clear and noticeable difference to both coefficients, however this result is not very applicable to the real world as you can’t do much with a bare frame.

The results for a 5” Alien: Adding all of the electronics to the bare 5” Alien frame increased its drag by an average of 60% but decreased parasitic lift by a whopping 98%. Though we cannot objectively say how neat his builds were, considering how much the electronics impacted performance, it may be interesting to note that a clean build will likely fly much better than a very bad build.

Electronics and effiicency

The results of the Alien of very interesting when compared to the standard H quad of the ZMR 250: Drag was actually reduced on the RTF compared to the bare frame by a very small amount (~.5%) but parasitic lift was decreased by a whole 81% on the RTF frame versus the bare frame itself. This is very interesting as it suggests that maybe H quads produce less drag when electronics are applied compared with an X shaped Quads.

ZMR drap coefficient diagram

What’s better, cased or de-cased antennas?

There have been many debates about this very question of the years, with most people believing that a cased antenna will provide the least drag and parasitic lift.

Antenna Quality

There is not a huge difference here, but we can see that a cased antenna decreased drag by 4%, but increased the parasitic lift by around 10%.

Conclusion:

In conclusion, this paper is extremely interesting and has raised some light on the issue of aerodynamics. Indeed there were some very  unexpected results, especially the GoPro VS No GoPro. In the future i would be nice to see more comparisons like this between different frames, so there is proper evidence to back up manufacturers claims that their frame is better than all the rest. Currently the drone community thrives on subjective hype, backed up by little to no data. It will be nice to see the day when proper aeronautical studies become common place.

Credit:

All of the data expressed this article is quoted from Multirotor Aerodynamics – Nicholas Willard. Big Thank you for all his work!

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