Nick Rigg Posted May 19, 2008 Share Posted May 19, 2008 Hi - I've just more or less finished a Hangar 9 Pulse XT40 and have a Dualsky motor up front, Turnigy 60 UBEC and 4S Lipo. I ran this up to full throttle using an 11x7 APCE prop and got about 515 watts at 38 amps. I suspect I should really be using a 12x6 prop but would I get the same thrust for possibly less speed and would I be able to maintain the watts output? Ideally, I'd like to be drawing slightly fewer amps as my motor has a 40 amp limit Quote Link to comment Share on other sites More sharing options...
winchweight Posted May 19, 2008 Share Posted May 19, 2008 Don't forget that when it's in the air, the prop unloads a bit and the current will drop a few amps. Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted May 19, 2008 Share Posted May 19, 2008 well why do you suspect you need to change it ? - if you change to a 12 x 6 the amps will probably be pretty much the same , and you will get more thrust but less speed. As shaun says, the motor will unload in the air ( often as much as 25% or more, so you you should be fine at 38A static. Quote Link to comment Share on other sites More sharing options...
Mike Rolls Posted May 19, 2008 Share Posted May 19, 2008 According to the Hobson site, which a lot of folk have found useful - to turn a 12x6 at the same revs as an 11x7 needs around 20% more power, and the 12x6 will give around 40% more static thrust. The 6" pitch will, of course, result in a lower pitch speed, which may or may not be important to you dependig on the styleo f flying involved.HTHMike Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted May 19, 2008 Share Posted May 19, 2008 But I doubt the revs will stay quite the same, despite the motors attempts to maintain its Kv.Of course without a wattmeter or similar, it is not possible to accurately say what effecxt the other prop may have. What is the Hobson site Mike, sounds interesting ? Quote Link to comment Share on other sites More sharing options...
Nick Rigg Posted May 19, 2008 Author Share Posted May 19, 2008 Thanks all - I suspect my style of flying will be pretty sedate - the Pulse is a follow on from my trainer. The 12x6 sounds like it's the way to go, I guess I'll put one on the wattmeter and just make sure it doesn't fry my motor. Once airborne I would think I could throttle back quite a way and still get reasonable performance - running the 11x7 at full throttle was a pretty scary experience (uprooting trees, causing the tides to change etc...) Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted May 19, 2008 Share Posted May 19, 2008 Better keep away from my house then when I spin up the 90 mm EDF at circa 1200 Watts and 32000 RPM Quote Link to comment Share on other sites More sharing options...
Former Member Posted May 19, 2008 Share Posted May 19, 2008 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted May 19, 2008 Share Posted May 19, 2008 Yeh, I tell you "watt" it dont half do a great job of blowing all the dust and dirt out from under the workbenches ! Quote Link to comment Share on other sites More sharing options...
Mike Rolls Posted May 20, 2008 Share Posted May 20, 2008 TimboIf you Google 'Thrusthp' that should find it. It is a downloadable (free) utility which will calculate the power required to turn a given prop at whatever revs, expressed in BHP and also gives the static thrust for the combination. I have checked it against a variety of combinations in th epast and in terms of power required it is pretty accurate. I don't have a set-up for measuring static thrust so I take that element on trust.HTHMike Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted May 20, 2008 Share Posted May 20, 2008 Cheers Mike. Quote Link to comment Share on other sites More sharing options...
Sean Eltringham Posted May 21, 2008 Share Posted May 21, 2008 The link from google doesn't work, it looks like the Hobson site is no longer available.But I did find a useful spreadsheet (thanks to PaulF on rcuniverse) that will do the same thing .http://www.flindt.us/planes/prop_performance.xlsSean Quote Link to comment Share on other sites More sharing options...
Nick Rigg Posted May 21, 2008 Author Share Posted May 21, 2008 Hi Sean - have you checked this for accuracy? I input diameter of 11, pitch of 7 and rpm of 12580 (motor kv multiplied by voltage?) and got a wattage figure of 910 on the s/sheet. I only measured 515 on my meter.Maybe I need to input more parametres? Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted May 21, 2008 Share Posted May 21, 2008 I am not sure this relates to electric flight at all ? Plus of course his calc are for power out, as opposed to our normal electric power claculations of power in ( Volts X Amps ).I inputted the data from one of my own s/sheets and records , and the power was way off. My Alienator on a 9 X 6 prop revved at 11000 and the wattage input was 488 as measured by my whattmeter, and 486 watts by maths ( Amps were 47 and volts were 10.4 ) This linked s/sheet with a 9 x 6 prop and 11000 rpm gave 233 Watts as power output. I know my Mega motor is better than 48% efficient ! Quote Link to comment Share on other sites More sharing options...
Nick Rigg Posted May 21, 2008 Author Share Posted May 21, 2008 Apologies - it seems I failed to take account of ohm's law in calculating my motor's rpm - it would be nearer 11300 and the wattage then works out at 660 which is closer to what I read (but no cigar)Interestingly, the calculation says a 12 x 6 prop should give 800 watts... Quote Link to comment Share on other sites More sharing options...
Sean Eltringham Posted May 21, 2008 Share Posted May 21, 2008 Sorry, I hadn't tested it when I posted the link, but your right it seems way off! Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.