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Old Jun 09, 2005, 03:38 AM
philipnow is offline
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Mega 16/7/6, 16/7/10, or 16/15/5?


I've spent several evenings reading through many threads in this forum and playing with MotoCalc, trying to decide on a brushless setup to replace the speed 400 in my T-Hawk. Although it might be reasonable to question putting a brushless in this sort of model, I do enjoy flying it and want to use the experience of upgrading my T-Hawk's power system to prepare me for work with future models.

Anyway, the T-Hawk, like the Easy Star and other pusher models, is limited to a propeller of no greater diameter than 6 inches. I want good solid performance, but am not trying to break any speed records. I think I want to use a can type non-outrunner motor. I like the looks of the Mega's.

Experimenting with newly acquired MotoCalc has raised several questions, the answers of which I seek from the many experts in this forum:

1.) Within a given motor series, the Mega 16/07/x series, for example, and given an approximately constant propeller size and pitch (due to my model's constraints), is it best to choose A.) a motor with less turns/higher Kv run at lower voltage (e.g. Mega 16/07/06, 2s Lipo) or B.) a motor with more turns/lower Kv at higher voltage (e.g. Mega 16/07/10, 3s Lipo), and why?

MotoCalc Details:

A.)
Motor: Mega ACn 16-7-6; 2850rpm/V; 0.9A no-load; 0.041 Ohms.
Battery: Apogee 1570; 2 cells; 1570mAh @ 3.7V; 0.012 Ohms/cell.
Speed Control: Castle Creations Phoenix 25; 0.0065 Ohms; High rate.
Drive System: 6x3 Propeller; 6x3 (Pconst=1.31; Tconst=0.95) direct drive.
Airframe: T-Hawk; 217sq.in; 16.1oz; 10.7oz/sq.ft; Cd=0.05; Cl=0.42; Clopt=0.72; Clmax=1.05.
Stats: 113 W/lb in; 95 W/lb out; 17mph stall; 21mph opt @ 57% (47:27, 90F); 27mph level @ 71% (33:32, 93F); 1109ft/min @ 37.5; -172ft/min @ -5.4.

B.)
Motor: Mega ACn 16-7-10; 1700rpm/V; 0.45A no-load; 0.0975 Ohms.
Battery: Apogee 1570; 3 cells; 1570mAh @ 3.7V; 0.012 Ohms/cell.
Speed Control: Castle Creations Phoenix 25; 0.0065 Ohms; High rate.
Drive System: 6x3 Propeller; 6x3 (Pconst=1.31; Tconst=0.95) direct drive.
Airframe: T-Hawk; 217sq.in; 17.4oz; 11.5oz/sq.ft; Cd=0.05; Cl=0.42; Clopt=0.72; Clmax=1.05.
Stats: 88 W/lb in; 76 W/lb out; 18mph stall; 22mph opt @ 65% (68:05, 89F); 28mph level @ 81% (48:07, 91F); 853ft/min @ 26.8; -179ft/min @ -5.4.

2.) Or would it be better to accept the slight weight penalty and chose a motor from the 16-15-x series, such as the 16-15-5, and why?

MotoCalc Details:

Motor: Mega ACn 16-15-5; 1800rpm/V; 0.8A no-load; 0.068 Ohms.
Battery: Apogee 1570; 3 cells; 1570mAh @ 3.7V; 0.012 Ohms/cell.
Speed Control: Castle Creations Phoenix 25; 0.0065 Ohms; High rate.
Drive System: 6x3 Propeller; 6x3 (Pconst=1.31; Tconst=0.95) direct drive.
Airframe: T-Hawk; 217sq.in; 18.4oz; 12.2oz/sq.ft; Cd=0.05; Cl=0.42; Clopt=0.72; Clmax=1.05.
Stats: 101 W/lb in; 86 W/lb out; 18mph stall; 22mph opt @ 63% (56:39, 91F); 29mph level @ 79% (38:49, 93F); 982ft/min @ 30.3; -184ft/min @ -5.4.

3.) For some scenarios in the MotOpinion readout it says "The static pitch speed is greater than 3 times the model's stall speed..." How important is that?

4.) When I plug in the numbers for certain propellers, such as a 5.5 X 4.5, the MotOpinion indicates that "The diameter (5.5in) to pitch (4.5in) ratio is less than 1.5:1, which will result in reduced propeller efficiency at low speeds (the propeller is stalled). Although this is not likely to affect flying characteristics, it may make take-off or hand launching difficult." How important is this? I see many individuals describing successful setups with propellers with ratios less than 1.5:1.

Thank you for any advice!
Last edited by philipnow; Jun 10, 2005 at 11:50 AM.
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Old Jun 10, 2005, 11:52 AM
philipnow is offline
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Any opinions on possible setups?
Old Sep 19, 2005, 11:07 PM
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Since my query didn't seem to generate a great deal of interest in this subforum, I have posted my experiences upgrading the power system of my T-hawk here (post 69).
Old Sep 20, 2005, 08:17 PM
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There are other motors of the "Speed 400" form factor besides the Megas - RCer Warp4 and Medusa 028-032-xxxx to name two. The two ourunner-in-a-can motors from Ultrafly also - one of these is probably the right Kv to work with the prop size limitation in the T-hawk.
Old Sep 21, 2005, 12:12 AM
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Thanks for your suggestions BEC... I was strongly considering the Medusa MR-028-032-1900. I ended up getting a Mega 16-7-10, because the MotoCalc numbers gave it the edge (I don't yet have enough experience to know how much credance to give to the Motocalc results). It gives good power -- all the power I want -- with a PQ 1800 lipo and a Aero-naut 6.5 X 4 prop I managed to squeeze in with a little Dremeling.


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