

Dec 13, 2010, 01:09 PM  

Quote:
OTOH, that small prop diameter does keep the thrust line low and reduces your thrust/pitch coupling problem. However, I suspect you're paying a pretty high price for it in prop efficiency losses. The "States" is a pretty big place. Where on this side of the pond are you going to be? 

Dec 13, 2010, 03:58 PM  

Don
Quote:
OTOH gentle acceleration is good because it gives the canard a chance to respond to the extra thrust. What I mean is, thrust comes first, then extra speed. Canard lift isn't interested in thrust, only speed. So too much thrust too quickly would just push the nose down. Now that I have a smaller prop with it's line running closer to the CG I have very little of that kind of problem. Quote:
cheers Nick 

Dec 13, 2010, 08:13 PM  

Nick, I had to estimate a few parameters, but it appears that your pitch speed for that prop is around 65 MPH. My guess is that's at least twice your airspeed in climb, which means your prop efficiency is something less (perhaps a lot less) than 50%.
The probable reason you aren't getting any lowspeed acceleration is because your prop is stalled until you get to maybe 50 MPH or so at full throttle. You might actually get a faster climb rate at reduced throttle than at full throttle. It's doing the aerodynamic equivalent of what's referred to in cars as "burning rubber". 
Dec 14, 2010, 03:20 PM  

with regard to my 5x5 prop...
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In any case I've ordered a 6x4 prop now, so we'll see, come January. Cheers Nick 

Dec 15, 2010, 12:29 PM  
Joined Jun 2005
2,567 Posts

My favorite prop pitch is a 4. It provides lots of pull and enough speed. The 3 pitch is just too slow but pulls well. Figuring pitch speed for a given motor is helpful. My guess is that a PS of 50 to 60 mph would be suitable for my two pound models.
Charles 
Dec 15, 2010, 09:13 PM  

Charles, the "best" prop pitch is another of the parameters that is meaningless if taken out of the context of the other parameters. The qualities you attribute to "4" only apply to certain combinations of airspeed, altitude, Kv and voltage (and therefore RPM), etc.. For example, if you installed a motor with twice the Kv (all other parameters unchanged), you might have a problem with a stalled prop and/or overloaded motor at low speeds. OTOH, a motor with half the KV on that same pitch = 4" prop might not pull enough watts to taxi well, much less actually take off and fly.
The reason you like 4" pitch is because the particular combinations of motor, battery and airplane you commonly use just happen to work well with that pitch. Coincidence, nothing more. I've found that pitch speed is a good starting point, provided that you allow for the effects of prop efficiency. Take the plane's airspeed at the operating point you want to design for, then divide by about 0.70 to 0.80 for cruise, 0.60 to 0.70 for takeoff and climb. The speed you get is a good starting point for pitch speed. For example, if you want to cruise at 40 MPH, divide that by about 0.75, and you get a pitch speed of about 53 MPH. We can convert that to feet per second if we multiply by 22, then divide by 15, which gives us a pitch speed of 78 feet per second. Multipy that by 12 and we get 939 inches per second. If our motor runs at 5000 RPM at cruise power, divide by 60 and we get 83 revolutions per second. Divide our pitch speed in inches per second by the revs per second, (939 / 83) = about 11 inches pitch. If our motor had twice the KV and ran twice as fast (10,000 RPM), we would need a pitch of about 5.5 inches. Note that each of these examples is appropriate for their operating characteristics (airspeed and RPM in particular), and that neither is equal to 4. 
Dec 16, 2010, 07:38 AM  
Joined Jun 2005
2,567 Posts

Thanks, Don for the great discussion on propeller pitch and pitch speed requirements. It would be interesting to hear of those requirements on a Long EZ.
A few years ago when the "Calc" programs were used, my motors were set up to run at maximum efficiency and nothing else. They are now set up to consume a conservative 100 watts per pound with a prop size which looks good to me on the model. Ampere draw is never allowed to exceed 30 amps with speed controllers at double the amp draw. Roughly speaking, the details on models from 1.5 pounds to 5 pounds use from 65 props at 18k rpm to 118 props at 8k rpm. the motors are from 1800 kv to about 900 kv and the batteries are from 3 to 5 cells respectively. The prop pitch is selected for amp draw below the motor's maximum with a minimum of 4. I try to limit battery amps to the 1/10th hour or 6 minute depletion rate. That would be 22 amps on a 2200 mah battery. Charles 
Dec 17, 2010, 10:32 AM  

I've just been looking back at my LongEZ thread. That's powered by a 2200 Kv motor, with a 9 x 4" prop and 2S1P battery drawing up to 170 watts quite steadily. For 25 oz, that arrangement is pretty successful. I've used the same 2 cell arrangement with other models quite successfully. I think the prop on Stuntcat is 8 x 5.
Just arrived in Georgia, how I hate Atlanta airport! We landed at 5pm and we didn't get out until 8pm. Here's a little video I assembled whilst travelling, hopefully to reassure those who arrive at the kneetrembling moment, about to launch a treasured new model. 9 different canards getting airborn, all different configurations and launching styles.
cheers Nick 
