Mavic Drag Coefficient?

[Andrew F]

Does anyone know or have a very good guess at what the drag coefficient is for the Mavic? I'm doing a school science project with one of my kids and need this number to calculate air resistance at different speeds. I have a pretty good idea but want to verify for reasonableness. I will post the project results here as an added incentive (so exciting.)

 

[quick_dry]

Drag coefficient would depend on the AOA to the apparent wind.

For a different science experiment, you could calculate it using a fan, an anemometer (a few bucks from hobby king), some fishing line and a small measuring scale (in lieu of an actual device to give you Nm directly)

The cross sectional areas could be calculated using a photo of the front, side and top/bottom then a little trig.

 

[Andrew F]

Yeah. I'm OK with the surface area measurement. I'm also OK with using a Cd of 1.0 to make it easier but I'd just see if anyone had seen an actual calc.

 

[Andrew F]

So I take it that the deafening roar of silence means no one has ever calculated or know the Cd? Bummer but understandable.

 

[JoostGT3]

Andrew, I love the idea, but I'm going to burst your bubble here: Aerodynamic calculations around rotorcraft are largely useless.
I have worked for a couple of years on the design of the tail of a modern day military transport helicopter. After 10 years, a gurney flap was introduced to the horizontal stabilizer, which was increasing the downforce provided by that stabilizer. I was surprized to see that with all our modern day technology, we weren't able to properly design a stabilizer for a helicopter. What do you think? Turns out that the airflow around the tail of a helicopter is so d@mn complex, with the downwash from the main rotor, the wash from the tail rotor and the normal airflow around the helicopter, that we are not able to properly analyze the airflow and therefore the effectiveness of the stabilizer... Long story short; I'm not sure of the use of airflow calculations around a quad, and even if you could calculate it, chances are big that your results won't be verifiable with real life results! :-(

 

[Andrew F]

Andrew, I love the idea, but I'm going to burst your bubble here: Aerodynamic calculations around rotorcraft are largely useless.
I have worked for a couple of years on the design of the tail of a modern day military transport helicopter. After 10 years, a gurney flap was introduced to the horizontal stabilizer, which was increasing the downforce provided by that stabilizer. I was surprized to see that with all our modern day technology, we weren't able to properly design a stabilizer for a helicopter. What do you think? Turns out that the airflow around the tail of a helicopter is so d@mn complex, with the downwash from the main rotor, the wash from the tail rotor and the normal airflow around the helicopter, that we are not able to properly analyze the airflow and therefore the effectiveness of the stabilizer... Long story short; I'm not sure of the use of airflow calculations around a quad, and even if you could calculate it, chances are big that your results won't be verifiable with real life results! :-(

Haha! I'm laughing with you not at you. Yes. That sounds about right. Also, the changing posture of the Mavic (much like a helicopter) makes the Cd dynamic at best. This is for a grade school science project so I'll just use a Cd of 1.00 and call it good. Regardless, we will have a nice graph afterward that shows battery usage at 4 mph increments all other factors being equal. That might be interesting for folks on this forum. I'm also getting to teach him fairly complex formulas and concepts - even if I assume-out most of the variables. Thanks for getting back to me.

 

[Wombat55]

Does anyone know or have a very good guess at what the drag coefficient is for the Mavic? I'm doing a school science project with one of my kids and need this number to calculate air resistance at different speeds. I have a pretty good idea but want to verify for reasonableness. I will post the project results here as an added incentive (so exciting.)

I think that your question sounds a bit confusing because (as JoostGT3 pointed out) the airflow around a quadcopter is pretty complicated and not just a function of its forward movement. Also, speaking as a physicist with many years of research experience, I think that even if you did have some effective drag coefficient for the horizontal motion of a Mavic, I'm not sure how you would use that information as the foundation for a school science project. If your intention is to compare a calculated effective drag coefficient with an experimentally measured effective drag coefficient, then offhand I don't see any simple way of experimentally measuring the effective drag coefficient of a Mavic by monitoring any of its diagnostic information (i.e., ground speed, prop speed, body pitch, etc.) while it is in flight. Precisely what is your overall project goal here?

 

[Wombat55]

Haha! I'm laughing with you not at you. Yes. That sounds about right. Also, the changing posture of the Mavic (much like a helicopter) makes the Cd dynamic at best. This is for a grade school science project so I'll just use a Cd of 1.00 and call it good. Regardless, we will have a nice graph afterward that shows battery usage at 4 mph increments all other factors being equal. That might be interesting for folks on this forum. I'm also getting to teach him fairly complex formulas and concepts - even if I assume-out most of the variables. Thanks for getting back to me.

Oh, OK, so while I was writing my previous post I see that you mentioned how you plan to generate some effective drag coefficient data: By monitoring the (battery) power versus speed info. One immediate problem with that that I see is that the Mavic has the curious property that it actually uses less power when moving forward in flight than it does when it is stationary in position and hovering. (There are threads here discussing this effect and in fact on the DJI website you can find a spec page where DJI notes that the flight time of the Mavic is longer when moving than when stationary.) If you just directly plugged this information into your drag calculations, then you would come up with the bizarre conclusion that the coefficient of drag of a Mavic is negative! Why does a Mavic use less power when it is moving than when it is stationary? Because when it is hovering in one fixed position, it is hovering on a downward moving column of air, whereas when it is moving it moves off that downward moving column of air into new air that has not yet been affected by the downwash of the Mavic's propellers. That means the Mavic doesn't have to work quite as hard to stay aloft. I think that your battery power versus ground speed measurements may be dominated by this effect rather than the coefficient of drag effect that you are trying to study.

 

[Cut.Aussie]

Cd is not the only major component of flight efficiency. Unlike simple objects like cars, bikes or fixed wing planes any rotary wing aircraft also has other weird effects like transitional lift which charges with forward speed, eg the faster you go, the less power is needed to support the weight. I'm glad I flunked advanced maths, screws with you mind to much hahahah.

 

[Andrew F]

Good points all, and yes to most. Clearly, I can't control many of the variables but again this is just a grade school project. I'm going to assume an average front facing surface area, consistent wind (measuring 1,500 feet out and then back), equal battery discharge rate (e.g. discharge measurement is equivalent whether fully charged or at 50%), straight line motor efficiency (probably my worst assumption), etc. It will still be fun and at least a partial indication of the effects of increased air resistance / drag. If nothing else, we'll have fun together!

 

[Wombat55]

Good points all, and yes to most. Clearly, I can't control many of the variables but again this is just a grade school project. I'm going to assume an average front facing surface area, consistent wind (measuring 1,500 feet out and then back), equal battery discharge rate (e.g. discharge measurement is equivalent whether fully charged or at 50%), straight line motor efficiency (probably my worst assumption), etc. It will still be fun and at least a partial indication of the effects of increased air resistance / drag. If nothing else, we'll have fun together!

Actually, despite the complication introduced by the "air downwash effect" I mentioned earlier, this project of yours may nonetheless be a very good learning exercise. It might be a good idea to not tell your students about the "air downwash effect" and watch how they respond when they calculate that the apparent coefficient of drag is negative from the power-versus-velocity data. Encountering surprises because you've overlooked some effect that you hadn't considered when originally designing an experiment is something that every experimental scientist occasionally encounters. When that happens, one then has to backtrack and re-examine all of the numerous assumptions that went into designing the experiment and figure out what went wrong with one's reasoning. Again, a very good learning exercise.

Don't know how much information one can gain about the coefficient of drag from the experiment, but at least it might be possible to turn the experiment into one studying the "air downwash effect" on flight characteristics such as the rate of battery power drain versus forward velocity. Knowing how to turn apparent lemons into lemonade is also a valuable thing to learn. There have been many scientific breakthroughs which have resulted from scientists trying to study one thing and then frustrated in their attempts because of something that they later found out to be a new, previously undiscovered, important physical phenomenon. From apparent failure into blazing success.