Jan 06, 2013, 05:22 AM
Registered User
Joined Jun 2012
810 Posts
Quote:
 Originally Posted by yomgui For propellers of the exact same shape (8x4, 9x4.5, 10x5,...) idealy thrust increases with rpm at 2nd power and with diameter at 4th power, like in the formula for thrust : T = rho.Ct.n^2.D^4 Indeed, if you wish to match the same slope of thrust vs airspeed when lowering kv, you'll need to increase both diameter and pitch. Probably the propeller will have a better efficiency. But the motor may suffer more... so in the end... who knows ? might be worst ! might be just the same... I guess in most of the case you'll only know after some flights.
It's hard to theorize, but for those of us with limited funds, it's harder to buy 3 different motore and 3 different props and 2 different cell counts and go test it out.

My issue with the 700kv motor is that according to the HK resistance spec, the 840kv I am using isn't in a terribly bad operating regime, it's actually more efficient than the 700kv and, somehow, despite being well over its max current, isn't drawing so much current that it is wasting lots of power. I just made a thread in the motor design board above this one asking about measuring winding resistance, I'm gonna go measure my 840kv motors spec myself and see how accurate the HK number is.
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 Jan 06, 2013, 06:12 AM Registered User France, RA, Cormaranche-en-Bugey Joined Feb 2011 507 Posts Yes of course I understand this point ! I have to admit that I have several motors, all well known, and several props, also well known... You can measure winding resistance, if you go with U = RI don't forget that if R is 0.05, and U is 5V, then I will be...100A !! so I've reduced voltage down to around 0.1V... But you'll only have winding resistance... which is not enough IMO to have an idea of the motor+ESC efficiency. IMO, "drivecalcing" a motor is a better solution, but needs a tach. In fact, I only use my spreadsheet... it can modelise a motor just like drivecalc do. If the model is good, and the prop is in the UIUC database, I can have good prediction of WOT current (+/- 2%) and rpm (less then +/- 1%) at any speed in flight or on the ground, knowing battery voltage. Torque is so much well predicted that I assume efficiency of motor+ESC to be simply exact or nearly. It has come to the point I'm getting bad habits : launching a plane without even having tested the power system, amps draw, temperature rise, thrust, etc. just a short full throttle before launch that's all... Bad habit, I know.... (I have ESC temperature in the glasses, so...)
 Jan 06, 2013, 06:40 AM Registered User Joined Jun 2012 810 Posts It does let me figure out what % of power is being lost to resistive losses, which if it was, I don't know, more than about 20%, I would say that is an unsatisfactory amount. I will find the leakage current too just because I can, but it isn't very useful since at larger currents it starts to become a smallish % of losses I have no tach so I can't find the Kv. I might get one soon though, I think a large amount of motor specs from manufacturers are out, and possibly there are even motor-to-motor variations, and all this theory is very limited for that reason. How do you get ESC temperature telemetry? Your ESC has an output with it, or you have attached a probe to it or something? Latest blog entry: Some mathematical relationships for...
 Jan 06, 2013, 07:13 AM Registered User France, RA, Cormaranche-en-Bugey Joined Feb 2011 507 Posts I have the temp sensor via the eagle tree OSD
 Jan 06, 2013, 07:54 AM Registered User Joined Jun 2012 810 Posts Ah cool, my friend and I have been looking at the eagle tree system for a couple days. Latest blog entry: Some mathematical relationships for...

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