Power to hover a 60kg drone? - Page 1
Forum section: electronics / general

author: zenerbjt
author_rank: Regular Contributor
timestamp: August 22, 2020, 02:48:35 pm
content: Dear Engineers,For a battery powered drone of 60kg total , how much electrical power will be drawn from the battery to hover it?There is an equation: P = K. m^(1.5)/r (from the following website)https://justdrones.com.au/how-much-power-is-needed-to-hover/Where:P =powerK=22.35m = mass in kgr = propeller radius (length) in metres.However, I am speaking of a drone with 4 motors and each motor drives dual stacked propellors which rotate in opposite directions. (so eight sets of propellers in 4 "pairs")

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 22, 2020, 03:45:10 pm
content: thats seems obvious... divide weight by 8 (m = 7.5kg), calculate for one propeller, and then multiply back the power by 8 (its in Gamera II - Can I build a human powered helicopter? example down below the linked website). do not add air turbulence effect or how much energy loss factor etc, we can discuss to no end... ymmv.

author: zenerbjt
author_rank: Regular Contributor
timestamp: August 22, 2020, 05:26:24 pm
content: Thanks, i think those "dual stacked drone in opposite rotation" type propellers arent as good as two separate propellers....there's some physical phenomenon that stops it being that good....i mean, they are good in that it means they take up less space, but thats all.I think "dual stacked propellers in opposite rotation" only give the thrust of something like 1.5 times "two single propellors in different places, spinning together" ...but i'm not sure?

author: IanB
author_rank: Super Contributor
timestamp: August 22, 2020, 06:10:16 pm
content: You linked to an article that explains it all.I'm not sure how anyone in this post can give anything more detailed than the article you linked? That article is already several screens long, and includes both formulas and derivation.To summarize:There is a theoretical minimum power required in the ideal case for a given propeller diameter.This theoretical minimum power is inversely proportional to the propeller diameter, so the power increases for smaller propellers and decreases for larger propellers.[Note: This is why modern aircraft jet engines have large diameter bypass fans to increase the effective diameter and improve the fuel efficiency. This is also why helicopters are much more efficient at hovering than VTOL aircraft like the Harrier jump jet. Helicopters have a way bigger "fan" to produce downward thrust than jet engines.]Now, the theoretical minimum power is a law of physics, you can't do better than this. But you can obviously do worse. This will depend on the specific characteristics of the motor and fan. To learn about any specific case it will be necessary to look at the characteristic performance charts of a given fan or propeller arrangement, if such is available. Or maybe do your own experiments and measure some results?But the theoretical minimum should certainly get you in the ball park.

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 22, 2020, 06:12:42 pm
content: oo ok i missed the stacked part if you mean stacked vertically one on top of the other sorry

author: IanB
author_rank: Super Contributor
timestamp: August 22, 2020, 06:17:38 pm
content: Quote from: zenerbjt on August 22, 2020, 05:26:24 pmWith dual stacked propellers you still have a propeller diameter to put into the minimum power formula. Stacking propellers doesn't change the diameter.Now, suppose someone decides to stack two smaller propellers to achieve the same thrust as one larger propeller. Then the diameter just went down and the power requirement went up. So the basic formula still applies.In every case there is an efficiency to consider when applying to the real world. The best way to find out what kind of efficiency to expect is to survey the writing on the subject. There must be other articles like the one you linked that give an idea of the numbers?

author: IanB
author_rank: Super Contributor
timestamp: August 22, 2020, 08:24:30 pm
content: Incidentally, after scratching my head over why the formulas given on that page linked didn't match my own derivation, I found another page with similar content but differently wrong formulas. It seems to be a case of the blind leading the blind, or "don't believe everything you read on the internet".Page containing wrong information:https://justdrones.com.au/how-much-power-is-needed-to-hover/Another page containing wrong information:http://www.starlino.com/power2thrust.htmlBoth of the above pages introduce some mysterious quantity called "Qair" which they use in formulas without providing a definition of how to calculate its value. It's as if they copied someone's homework without properly understanding it.A page containing accurate information is here:https://en.wikipedia.org/wiki/Thrust

author: zenerbjt
author_rank: Regular Contributor
timestamp: August 22, 2020, 09:10:42 pm
content: Thanks yes, and the wiki page on Thrust it confirms that Thrust is Force, with units of Newtons.The question of how much power is needed to hover a drone of x Kg still tantalises......and this when using coaxial contra-rotating (CCR) propellors.It turns out ...https://en.wikipedia.org/wiki/Contra-rotating_propellers...from this website, a coax contra rotating prop is only slightly more efficient than a single prop of the same radius.....only about 10% more efficient.This seems counter intuitive, but then is probably why CCR props are so rare in aircraft/helicopters etc...so it appears that it takes some 21kW (taking inefficiency into account) of power for a 0.5metre radius CCR prop to hover a 60Kg mass.....but in truth, to handle wind etc, a power capability of double that is needed.

author: IanB
author_rank: Super Contributor
timestamp: August 22, 2020, 09:53:08 pm
content: Quote from: zenerbjt on August 22, 2020, 09:10:42 pmYou might want to check your calculations.

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 23, 2020, 05:38:29 am
content: I'm not going to go into the details while i have a book about aeronautic engineering with lots of heavy math and diagrams. But i recon the efficiency of the system is partly affected by the blade/wing angle and how much turbulent or wake it generates. So this alone implies that input power is more or less can only be figured out accurately from empirical study and then work way back to get system efficiency. Fwiw.

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 23, 2020, 05:57:26 am
content: One simpler way to test propeller's efficiency is to put it on a rig with sensor to read thrust. Power meter to battery.. i remember back in school there is one discipline called parametric study in fluid dynamic iirc (but its been rotten out of my mind due to prolonged unusage) when you want to build very large and expensive system, you can build smaller model and test and then use parametric study to estimate the performance of full scale system. But even the largest minds in aerodynamic design such as the america, russia or china will have their own millions dollar propulsion lab to test full scale propeller.. so this is obviously not the solvable solution on paper with numbers and letters alone..https://www.machinedesign.com/3d-printing-cad/fea-and-simulation/article/21819304/the-role-of-parametric-cfd-analysis-in-design