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Old Feb 26, 2004, 02:29 PM
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Rich Hill Missouri 64779
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Need some help, "Gear drives"

I've notice on the B2 stealth the motor is a geared motor and it goes fast. I built a dealta wing and I was wondering about a gear drive for a 400 motor. Does the gear drives give you more speed or thust. Can you use a gear drive to lift a heavier plane? Is a 3:1 faster than a 1:3? Do you use a smaller prop for higher speed and a larger prop for a slower motor?
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Old Feb 26, 2004, 02:51 PM
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N. Staffs, UK
Joined Jan 1997
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I suggest getting a copy of Motocalc and playing with some numbers. The trouble is that the answer to all of your questions is... "It depends". All of the things you are asking about are affected by the particular combination of motor, battery voltage and propeller as well as the gear ratio.

Typically people use geared motors and larger props to get more carrying capacity or better climb rates but not necessarily higher top speeds. OTOH some of the fastest planes around also use geared drives. Sorry, but there are no simple answers.

Steve
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Old Feb 26, 2004, 06:57 PM
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The 'Wack, BC, Canada
Joined Oct 2002
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The reduction drives are strictly for getting more pulling power for climbing with heavier models. They do not make the model fly faster in any way. In fact they do quite the opposite. The are generally suitable for slower models. Note that all the racers use direct drive motors. That's because to go fast you need to turn the props fast.

You might think that you could use a gear box and turn a very high pitch prop and technically I suppose it would work but that ignores the problems of the prop being stalled. And even with higher pitch props to achieve high speeds the RPM the props have to turn is often beyond many of the motor and reduction gear boxes.

You also ask why gearing UP is not used. I suspect that is because gears introduce their own power losses and have definite speed limitations. It's just easier to design the motor to turn those sorts of speeds directly and add cells until the voltage is high enough to achieve those RPMS.

In the end you can't get something for nothing. Check out some figures in Motocalc like the guys are suggesting and you'll see how a lot of this dovetails together.
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Old Feb 26, 2004, 09:37 PM
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Rich Hill Missouri 64779
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A big thanks!

Thanks to everyone. I figured it was like small truck with overdrive to get good gas mileage but can't pull heavy loads vs a granny low transmission able to pull heavy loads but poor on gas mileage.
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Old Feb 27, 2004, 03:50 AM
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Its moe like dropping a gear to get the thing to climb uphill, versis being in top gear to get the top speed. There is a 'right' gear ratio for every plane and prop.

To an extent, if you drop teh RPM by say two, you will get the same pitch speed if you double the prop pitch, and get the same or more thrust by increasing the diameter. What we should be doing, is looking at the airfarme, and its drag, and working out the most efficient prop and RPM to get that amount of thrust, and then sorting from there backwards.
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Old Feb 27, 2004, 10:37 AM
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Rich Hill Missouri 64779
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OK! I built a 48" delta wing and it came out heavier than I wanted. It's over 2 pounds. Someone sent me a chart on motors and a 400 will lift 1.5 pounds. Tell me, do I need a bigger motor or a gear drive system with a bigger prop and pitch?
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Old Feb 27, 2004, 09:08 PM
Marion
USA, NC, Hillsborough
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Take the recommendations to run MotoCal and see what happens for yourself....
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Old Feb 27, 2004, 09:46 PM
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Rich Hill Missouri 64779
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What is Motocal and where can I locate it to use it? I looked on the internet and all it said was for robotic arms.
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Old Feb 27, 2004, 10:14 PM
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http://www.motocalc.com/
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Old Feb 28, 2004, 06:20 AM
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Quote:
Originally posted by quailbird
OK! I built a 48" delta wing and it came out heavier than I wanted. It's over 2 pounds. Someone sent me a chart on motors and a 400 will lift 1.5 pounds. Tell me, do I need a bigger motor or a gear drive system with a bigger prop and pitch?
IME,both. Cheapest system that will lift that lot would be something like long can 480, mildly geared - say 2.3:1 - and a prop in the 8-9" bracket, and ten cells/3s LIPO. Yup. 8x6 prop looks good according to motocalc. 18oz of thrust and a 56 mph pitch speed. 9x5 also possible, but straing a little. More thrust, less speed.

Expect to draw between 15and 18A.

If thats not hairy enough for you, next step up is speed 600, or a buggy motor. Geared mag mayhem on ten cells 3s LIPO is allegedly good, but not ever tried it.
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Old Feb 29, 2004, 10:34 AM
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Georgia
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I've had the best overall results using direct drive motors in delta wing planes.

What does your 2 lb. weight include?
What is the planform like? Airfoil?
Any pictures?

Bill
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Old Feb 29, 2004, 08:55 PM
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Rich Hill Missouri 64779
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It is a Pibros, only 48". http://gjkool.filternet.nl/s400pibros.html.
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