Spikey Posted August 22, 2014 Share Posted August 22, 2014 I know these props are only a few quid each (plus pee 'n' pee), but my box of wrong props is almost full now and The Lady Wife keeps asking me how come I can't just buy the right one in the first place, so ... The motor I'll be using on my next Lazy Bee is drawing 74W with an APC slow flyer 10 x 7 on it. Experience tells me that for the type of flying I do, 90-100W is ideal for a Bee - so do I need an 11 x 4.7SF or an 11 x 7SF with this motor? Quote Link to comment Share on other sites More sharing options...
Mike Smith Posted August 22, 2014 Share Posted August 22, 2014 Had to laugh at your post Spikey... I too have an ever increasing box of props... never seem to get it right first time!! Even using the "correct" prop size as recommended by some of the motor manufacturers ends up overloading the motor and/or ESC. It all seems a bit hit and miss, so will watch the replies to this question with interest. Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted August 22, 2014 Share Posted August 22, 2014 No such thing as a wrong prop lads, you just have not bought the motor and model to suit it yet John Quote Link to comment Share on other sites More sharing options...
Mike Smith Posted August 22, 2014 Share Posted August 22, 2014 Ah... so by your logic John, I must be due about 20 new models!! ...now I wonder how that idea will go down with the wife Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted August 22, 2014 Share Posted August 22, 2014 Spikey, fun that I've just been looking at what prop I need to go from 1100 watts (on a 17 x 10) to 1400+ watts and used drivecalc, www.drivecalc.de to run a few different props. I've said it before on here, but it's a easy to use program which helps with motor and prop selections. If you motor isn't in the database just pick one with the same KV and dia and that will be pretty close. Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted August 22, 2014 Share Posted August 22, 2014 eCalc does the job as well. Quote Link to comment Share on other sites More sharing options...
andyh Posted August 23, 2014 Share Posted August 23, 2014 Posted by john stones 1 on 22/08/2014 15:18:52: No such thing as a wrong prop lads, you just have not bought the motor and model to suit it yet or the wrong wife Quote Link to comment Share on other sites More sharing options...
John Olsen 1 Posted August 23, 2014 Share Posted August 23, 2014 This has probably been covered before, and there are probably others more qualified than me as well, but anyway... The pitch is mainly concerned with how fast the aircraft should be going. Raising the pitch would raise the angle of attack and so would tend to make the prop absorb more power, but if the pitch is about right is not the way to go. If the pitch is too fine, the initial acceleration will be good but the speed in the air might be a bit lacking. Raising the diameter will make the prop absorb more power, but remember that the increase in power is something like the fourth power of the increase in diameter, so a very small increase in diameter will cause a large increase in the power. (Not sure if it is third or fourth power, but it is quite high.) So my inclination would probably be to try the 11 by 7, but check with a wattmeter on the ground to make sure that the maximum current taken has not increased too much. If it is a little more than you want, you can trim the tips and re-balance. John Quote Link to comment Share on other sites More sharing options...
Spikey Posted August 23, 2014 Author Share Posted August 23, 2014 Thanks for that John. You know, it's so obvious now, but it had never dawned on me that cropping the tips and re-balancing is always an option if the prop's a bit too much ... Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted August 23, 2014 Share Posted August 23, 2014 Spikey The IC boys may crop prop tips (they tend to have power to spare) but with electric "every little helps" and it is the outer part of the prop blade that does most of the work so altering the tips can cost efficiency. Or to put it another way cropping the tip will reduce the power required but it could also reduce the effective thrust by a bigger percentage. Matching prop to motor to airframe for maximum 'performance' is complex and it depends what you are after. I recall a 'economy' competition for full size Tiger Moths where the winner achieved nearly 20% more 'mpg' by using a special prop matched exactly to the planes most economical cruising speed. What was interesting was that in other respects like take off and climb performance it was not as good as the standard prop. Quote Link to comment Share on other sites More sharing options...
Spikey Posted August 23, 2014 Author Share Posted August 23, 2014 Cheers Simon. I realised that there's always a tradeoff, but you make a good point there. I'm probably ploughing my own furrow somewhat here, as I like what I can only call "scale-like" flying. I'm restricted to the playing field up the lane, and I'm quite happy pootling slowly around the sky with my Lazy Bee trying to perfect the art of what passes in my book for "precision" flying eg trying to do a textbook landing exactly where I intended to, or flying figures of eight with one straight leg right up the middle of the cricket practice wicket. It doesn't take much to make me happy! If it was "performance" I was after, I know the answer by now for a Lazy Bee weighing 765gm with a 2S LiPo in it - a 1350kv motor and 9x4.7SF draws 18.3A (135W) and goes like the proverbial. Not what I'm after though ... 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.