


Ireland, Dublin, Dublin City
Joined Apr 2011
319 Posts

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I need a remedial class version of that! Do you mean the tip speed of the props or the "slippage" experienced by the prop moving forward? Can pitch speed be measured? I was expecting that someone would suggest that the figures at half or two thirds throttle would be more important, which is where most flying is done. I'm still amazed at how the R2 motors would seem to be more efficient than the TP ones! 





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When the propellers pitch is measured in inches Pitch Speed in MPH is found by using the formula RPM x Pitch / 63,000 X 60. i.e. At 8000 rpm a propeller with a pitch of 6 inches would have a pitch speed of 8000 x 6 / 63000 x 60 = 45.71 MPH This is the theoretical speed at a given RPM before slippage is taken into account. Ken 



Ireland, Dublin, Dublin City
Joined Apr 2011
319 Posts

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Luke 




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The equation I use is ( RPM x Pitch ) / 1056 1056 is a constant. I see you got the right pitch speed answer, but I don't think you were using the 63000 x 60. Did you mean 63000 / 60? 




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RPM multiplied by pitch gives the distance in one minute measured in inches. (Inches per Minute.) Divide by 63000 converts this to the distance in one minute measured in miles. (Miles per Minute) Multiply by 60 gives the distance in one hour. (Miles per Hour) I did some research and found that there are actually 63360 inches in a mile so the correct formula is: RPM x pitch / 63360 x 60 Ken 



Canada, ON, Toronto
Joined Jun 2010
286 Posts

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So it's (RPM x Pitch) / 63360 = miles travelled in a minute x 60 minutes = MPH EDIT: Never mind, I didn't refresh the page and missed the answer above... 




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I have been using MotoCalc for over 10 years and find that it gives me a good idea of what motor and propeller combination is most suitable. Here is the MotoCalc predictions for my CL215 MotOpinion  Hobby King CL 415 Water Bomber Sea Level, 29.92inHg, 59°F Motor: Turnigy Bell 241012Y; 1000rpm/V; 0.9A noload; 0.205 Ohms. Battery: Zippy1600 (20C); 3 cells; 1600mAh @ 3.7V; 0.0093 Ohms/cell. Speed Control: Hobby King Blue Series12 Amp; 2 controls (separate); 0.0013 Ohms; High rate. Drive System: GWS 9 x 5 DD; 2 motors (parallel); 9x5 (Pconst=0.86; Tconst=1.336) direct drive. Airframe: Hobby King Cl 415; 493sq.in; 38.4oz RTF; 11.2oz/sq.ft; Cd=0.049; Cl=0.47; Clopt=0.68; Clmax=1.24. Stats: 76 W/lb in; 54 W/lb out; 15mph stall; 21mph opt @ 59% (38:52, 85°F); 25mph level @ 69% (31:04, 89°F); 1082ft/min @ 36.2°; 168ft/min @ 5.2°. Power System Notes: The fullthrottle motor current at the best lifttodrag ratio airspeed (8A) falls approximately between the motor's maximum efficiency current (6.8A) and its current at theoretical maximum output (26.4A), thus making effective use of the motor. Aerodynamic Notes: The static pitch speed (40mph) is within the range of approximately 2.5 to 3 times the model's stall speed (15mph), which is considered ideal for good performance. With a wing loading of 11.2oz/sq.ft, a model of this size will have very sedate flying characteristics. It will be suitable for relaxed flying, in calm or very light wind conditions. The static thrust (37oz) to weight (38.4oz) ratio is 0.97:1, which will result in very short takeoff runs, no difficulty taking off from grass surfaces (assuming sufficiently large wheels), and steep climbouts. At the best lifttodrag ratio airspeed, the excessthrust (22.5oz) to weight (38.4oz) ratio is 0.59:1, which will give steep climbs and excellent acceleration. This model should be able to do consecutive loops, and has sufficient inflight thrust for almost any aerobatic maneuver. General Notes: This analysis is based on calculations that take motor heating effects into account. These calculations are based on mathematical models that may not account for all limitations of the components used. Always consult the power system component manufacturers to ensure that no limits (current, rpm, etc.) are being exceeded. Ken 




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Do not hesitate to PM me if you meed help with MotoCalc. Ken 




Had another mishap with my 415 today. I was 3 or 4 minutes into my 3rd battery, landed smooth as can be on glassy water and went to take off again. Got her going on a nice scale takeoff run, slowly feeding in throttle. Once on step, I advanced the throttle a little more to about 3/4, and began to pull up. I was about 4 feet above the water when, without warning, the plane snap rolled to the right and dove into the water inverted at a very steep angle I cut throttle immediately, and thankfully the plane popped back to the surface upright and in one piece. I was able to taxi back to the shore, which was really lucky because the air temp was in the upper 30's(F), and the wind blowing gently away from shore with at least 5 miles of water to the other side of the lake. Damage was limited to a clean crack in the fuse under the wing. Looks to be just a 5 minute repair
But now I am left to wonder what caused this My suspicion is that one of the ESC's went to LVC as I advanced the throttle. I had seen this happen a couple of times on the bench, but only using an older battery at close to full throttle. The battery I was flying with today was once one of my favorites, a Mystery 3S 25c 2450 mAh, but it is probably going on 3 years old now. It does not have the pop that it did when it was new, but I never really worried about it since I mostly fly this plane at 1/2 throttle. I think I will reprogram the ESC's for a lower voltage cutoff point. And I think this particular battery will also be banned from the CL415 


Joined Aug 2004
373 Posts

That is exactly why I use a seperate BEC on my multis. I like the Castle Creations 10 amp. Russ Farris



Ireland, Dublin, Dublin City
Joined Apr 2011
319 Posts

Quote:
Luke 




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I had it happen on my Catalina 2 years ago.. 

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