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Old Dec 13, 2010, 10:12 AM
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Steve Merrill's Avatar
USA, IL, St Charles
Joined Nov 2002
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Make sure you water proof your ESC. I have fried 2, flying from water, they are like cats and water, don't mix very well
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Old Dec 13, 2010, 10:35 AM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
3,696 Posts
Don
Quote:
How many watts are you asking that little 5" prop to absorb?
It's an 1850Kv motor and it draws about 210watts at full power, which I rarely use. Mine is an APC. The 7x5 prop, recommended for the motor, is more like 280 or 300watts.

I was out flying with my 1600MaH battery today, instead of 2200. It moves the CG back at least 1/2" and reduces the AUW by 1.5oz to 29oz. Much better pitch control and tighter turns now. If I point it straight up and reduce the power, it still stalls with a flop over forwards rather than slipping backwards. Flying inverted requires just a light touch on the down-elevator to keep the nose up.

Steve

Waterproof - yes! I'm coming to the States over Christmas so I'll buy some Corrosion X that you guys always recommend. The stuff I know of in England is not very user friendly. I tried Vaseline once and burnt out an ESC with it.

cheers

Nick
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Old Dec 13, 2010, 01:09 PM
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Don Stackhouse's Avatar
United States, OH, Bradford
Joined Jun 2005
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Quote:
Originally Posted by nickchud View Post
Don
It's an 1850Kv motor and it draws about 210watts at full power, which I rarely use. Mine is an APC. The 7x5 prop, recommended for the motor, is more like 280 or 300watts...
You can get away with that on a pylon racer or other very high airspeed model, but on a relativley slow-speed airplane that sounds like an awful lot of watts for so few inches. Of course with that high Kv you don't have a lot of choice, but that in turn suggests a mismatch between the plane and the motor, like installing a Lemans or Indy motor in an SUV or pickup truck.

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?
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Old Dec 13, 2010, 03:58 PM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
3,696 Posts
Don
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.
Yes, I understood that. In fact, now that I've amended that rear pylon I couldn't fit a 7" prop anyway. The way I like to fly usually suits the 1400 or 1100Kv motors. On the Polaris thread they talk about a 2200Kv motor I think, so I went for the compromise. In fact, I'm very pleased with this setup. I only occasionally use full throttle and then I can get a nearly vertical climb, if I give it a couple of seconds to "engage". The slow, flat turns I've been doing work alright at 50% throttle. I do have to be careful about using the throttle to "get out of trouble". My feeling is that, with this arrangement, I need to feed in the extra power gently. Nothing happens at first. Does that sound right?

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:
The "States" is a pretty big place. Where on this side of the pond are you going to be?
I usually go to California 'cos that's where my wife's family live, mostly. This time it's Atlanta, she also has family there. It's a big family, lucky me!

cheers

Nick
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Old Dec 13, 2010, 04:40 PM
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United States, CA, Fresno
Joined Sep 2007
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Barking Dog

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Originally Posted by nickchud View Post
The only thing worse than a yapping dog is the sound of an angry owner shouting at him.
Or your wife chewing on your butt.
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Old Dec 13, 2010, 08:13 PM
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Don Stackhouse's Avatar
United States, OH, Bradford
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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 low-speed 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".
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Old Dec 14, 2010, 03:20 PM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
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with regard to my 5x5 prop...
Quote:
It's doing the aerodynamic equivalent of what's referred to in cars as "burning rubber".
I'm sure you're right Don. That's why I feed in the power gently. And it's no bad thing. I don't want too much thrust too quickly 'cos it causes a pitch down.

In any case I've ordered a 6x4 prop now, so we'll see, come January.

Cheers

Nick
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Old Dec 15, 2010, 12:29 PM
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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
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Old Dec 15, 2010, 09:13 PM
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Don Stackhouse's Avatar
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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.
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Old Dec 16, 2010, 07:38 AM
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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 6-5 props at 18k rpm to 11-8 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
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Old Dec 17, 2010, 10:32 AM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
3,696 Posts
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 knee-trembling moment, about to launch a treasured new model. 9 different canards getting airborn, all different configurations and launching styles.
Canards Taking off.wmv (3 min 54 sec)

cheers

Nick
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Old Dec 17, 2010, 03:21 PM
Jets are for kids-of all ages
Florida, USA
Joined Jun 2008
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My latest venture in canards....

http://www.youtube.com/watch?v=eIbHmmlRBiI

Roger
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Old Dec 17, 2010, 05:18 PM
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Thanks for the videos Nick and Roger. The wind is not your friend sometimes.
The new Duck has it's fuselage tail properly aligned this time. A 1/16" plus error was found in the drawing which caused a problem on the last build.
Charles
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Old Dec 17, 2010, 06:04 PM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
3,696 Posts
Roger! .... That's why I showed the take-offs and not the landings!

There's more of them.

Nick
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Old Dec 17, 2010, 09:17 PM
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Herndon, VA
Joined Oct 2005
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Aw man, Roger. What do you think happened?
RE
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