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Old Dec 17, 2005, 04:47 AM
Molded Hotliner Destroyer
Magnum9's Avatar
Perth, Australia
Joined Jul 2004
1,720 Posts
Opinions on small prop/high rpm vs larger prop/lower rpm

I am looking at a power system for a Twin Jet. After running some numbers through Motocalc on 2 parallel Warp 4 motors, I get the following:

2 turn APC 4.5x4.1 - 71.4 amps, 34101rpm, 44.6oz thrust, 132.4mph pitch speed

3 turn APC 6x5 - 69.2 amps, 21335rpm, 55.7oz thrust, 101mph pitch speed

With an AUW on 38-40oz both combos are going to be good on vertical. As the twinjet is a fairly draggy design, I think it would be hard pressed to go over 100mph anyway, so the question is, screaming small prop on the 2 turn or more thrust at less rpm with the 3 turn? Also would one motor be more comfortable with the 35 amps than the other?

Also bear in mind that these currents won't be sustained very long. The 3 turn setup would have a vertical climb rate of 2800ft/min so a 20 second burst would get almost 1000 feet, much too high for a plane that small.
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Old Dec 17, 2005, 05:05 AM
Registered User
Indio, CA.
Joined Sep 2005
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Magnum,

You will be much happier with the larger prop. Pitch speed and thrust readings are deceiving in MHO. I have been doing some testing with an AeroCat, and have tried many set-ups, for both speed and performance.

The best set-up I have found was a 6x4 APC on my JustGoFly 450TH running a 2100, 3s battery. I got a level flight speed verified by radar at 74 MPH and out of sight vertical in 5 seconds.

The AUW of the plane as tested is 15.8 oz.

I found with the smaller props you have the pitch speed and great static thrust, but in real life application, there isn't enough thrust to perform anywhere near what the numbers say it will and nowhere near the speed.

My 6x4 set-up ran 74 MPH and had a pitch speed of 98 MPH. The 4.1x4.1 had a pitch speed of 114 MPH but in reality the plane only flew at 56 MPH and had very little vertical performance.

You need to find a happy balance of prop speed and thrust to take advantage of the speed and performance.
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Old Dec 17, 2005, 09:45 AM
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Racine, WI AMA# 809291
Joined Nov 2003
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Amp draw

Why is the Amp Draw so High on these Motors?
Quote:
2 turn APC 4.5x4.1 - 71.4 amps, 34101rpm, 44.6oz thrust, 132.4mph pitch speed

3 turn APC 6x5 - 69.2 amps, 21335rpm, 55.7oz thrust, 101mph pitch speed
Can you step-up the voltage, so you don't have to draw as many amps?
What was the pack voltage in the above comparison?

I have a friend who used a Mega 16/15/2 4600kv, 35 Amp (recomonded) in a Foamy F-16, and it fly's great, but sounds like a dentists drill!! He's not burning up the sky with it though. The Parkjet is over 1 mt. long, and quite draggy (for a F-16) He's probably lucky if he hits 50 mph with it. Forgot to mention that I think he is using a 4 X 3 Prop.

http://www.parkjets.com/F-16C_with_landinggear.wmv
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Old Dec 17, 2005, 11:14 AM
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Southwest Florida
Joined Mar 2004
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I have a Warp 4 and there is no way you can push that many amps thru them. Get your amp draw down or you are going to have one very fast, very brief flight. Jim
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Old Dec 17, 2005, 12:02 PM
I'd rather be Flying
davecee's Avatar
Nashville, NC, USA
Joined Mar 1999
2,803 Posts
Quote:
Originally Posted by Magnum9
I am looking at a power system for a Twin Jet. After running some numbers through Motocalc on 2 parallel Warp 4 motors, I get the following:

2 turn APC 4.5x4.1 - 71.4 amps, 34101rpm, 44.6oz thrust, 132.4mph pitch speed

3 turn APC 6x5 - 69.2 amps, 21335rpm, 55.7oz thrust, 101mph pitch speed
Guys, he's talking about TWO motors running in parallel from the same battery pack. That halves the amperage seen per motor as shown in the calc program (both motors).
Thus: 2 turn= about 35.7 amps per motor
3 turn= about 34.6 amps per motor
This seems to be OK for these motors.
And yes, the bigger props will provide better real world performance.
Dave
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Old Dec 17, 2005, 12:58 PM
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Indio, CA.
Joined Sep 2005
511 Posts
Magnum,

If you go to the JustGoFly web site you can see a video of an actual twin just with 2 JGF 400DF's on it, I think he has a video pass at 114 MPH. I think he is also running the APC 5x2.5 props.

The 400 DF like the 450Th is a 3000Kv motor. My 450 pulls 37 amps static with the 6x4, and 42 static amps with the 6x5.5. The 6x4 works best with that motor on my plane.

Check out JGF to get an idea.

Mitchj
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Old Dec 17, 2005, 09:27 PM
Molded Hotliner Destroyer
Magnum9's Avatar
Perth, Australia
Joined Jul 2004
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Thanks everyone for the replies. The JGF motors are all outrunners right? It would be difficult to mount outrunners on the twinjet because they glue into a half circle of foam, although a ply back mount could work.

I maidened it today with the racer edition Permax 480 motors on 3S lipos. After landing I discovered one prop was on backwards (I can't remember the number of times I have read that here and though "Are you dumb or what?" ) and the performance was okay, but I want it to be the plane you pull out and watch peoples jaw drop when it goes vertical. I guess the 6 inch prop would be the best choice for that.
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Old Dec 20, 2005, 02:30 AM
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Indio, CA.
Joined Sep 2005
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Magnum,

Yes the JGF motors are outrunners, They also have aa aluminum mount that is glued to the wing which enables you to mount the outrunner. Check out the video and still pix he has on his site.

For vertical performance the longer prop you can safely turn is better, but for speed you need a shorter prop with more pitch. My motors are very happy turning the 6x4 prop, this gives me a "happy medium" of speed and vertical performance. With judicial throttle management I can safely fly my AeroCat without burning anything up and still awe the crowd.

Mitch
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