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Old Dec 11, 2007, 08:45 PM
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mspicela's Avatar
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making a plane fly slow

I have a plane that I've retrofitted with one of the UH brushless motors. I want to know what the prop choosing strategy is for making it slow but have lots of torque. I assume the length of the prop and the pitch both play a factor. It is natural to think that going up in either will increase the work the motor has to do and the current is likely to go up and it is likely to not spin as fast. Can someone give me some education :-)
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Old Dec 11, 2007, 09:07 PM
Honey, I got more planes!
ghee-grose's Avatar
USA, AL, Athens
Joined Jun 2003
4,274 Posts
I remember them talking about that here...

http://www.rcgroups.com/forums/showthread.php?t=577137
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Old Dec 11, 2007, 10:27 PM
Brian
USA, AL, Madison
Joined May 2007
691 Posts
There are several things to consider:

1. What is the stall speed of the aircraft? You can't fly any slower than that.
2. You can get a gearbox to slow down how fast the prop actually turns, then add a bigger prop to keep it in the air
3. Adding weight (evenly distributed) will slow it down

What sort plane are you talking about? Some planes just aren't made to go slow (see #1).
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Old Dec 11, 2007, 11:57 PM
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Brian, that all makes sense. I want to just focus on the prop size and pitch portion though. What do changing those two variables do? When would I want to increase or decrease size and when do I want to increase or decrease pitch. For instance, Rick suggested that I go with a smaller prop with more pitch when I moved from a plane to my stryker. I assume it was to make it spin faster but cut more air for more thrust .... not sure though.
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Old Dec 12, 2007, 06:19 AM
Fly long and land softly
Jim_Marconnet's Avatar
Madison, Alabama USA
Joined Oct 2005
5,391 Posts
Sorry, Brian, but I don't understand your third point.

All else being equal, the heavier the plane, the higher the wing loading and the higher the stall speed.

And about your first point. You really don't want to fly near the stall speed. If so, any manuever you perform or sudden change in the wind can drop your speed below stall speed, and there you are, stalled and falling out of the air!

Jim

Quote:
Originally Posted by brivers
There are several things to consider:

1. What is the stall speed of the aircraft? You can't fly any slower than that.
2. You can get a gearbox to slow down how fast the prop actually turns, then add a bigger prop to keep it in the air
3. Adding weight (evenly distributed) will slow it down

What sort plane are you talking about? Some planes just aren't made to go slow (see #1).
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Old Dec 12, 2007, 07:37 AM
Brian
USA, AL, Madison
Joined May 2007
691 Posts
Quote:
Originally Posted by mspicela
Brian, that all makes sense. I want to just focus on the prop size and pitch portion though. What do changing those two variables do? When would I want to increase or decrease size and when do I want to increase or decrease pitch. For instance, Rick suggested that I go with a smaller prop with more pitch when I moved from a plane to my stryker. I assume it was to make it spin faster but cut more air for more thrust .... not sure though.
See the links below, hopefully they will help. Also, props are cheap - I bought a pack of 2 11x7's at RC Hobbies for ~$3. Go buy a few and see how they change the performance.

http://www.rcgroups.com/forums/showthread.php?t=722579
http://www.rcuniverse.com/forum/m_62...tm.htm#6303262
http://en.wikibooks.org/wiki/RC_Airplane/Propellers
http://en.wikipedia.org/wiki/Model_aircraft#Propellers

Quote:
Originally Posted by Jim_Marconnet
Sorry, Brian, but I don't understand your third point.

All else being equal, the heavier the plane, the higher the wing loading and the higher the stall speed.
OK, maybe I'm wrong about that one.

Quote:
Originally Posted by Jim_Marconnet
And about your first point. You really don't want to fly near the stall speed. If so, any manuever you perform or sudden change in the wind can drop your speed below stall speed, and there you are, stalled and falling out of the air!

Jim
Agreed - flying near the stall speed is a bad thing. Just making the point that if your plane isn't desinged to fly slow, it's not gonna fly slow no matter what you do.
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Old Dec 12, 2007, 11:02 AM
David
dleviner's Avatar
Monrovia, AL, USA
Joined Mar 2001
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Mike... it all depends on what plane you're trying to spec a power system for???

Help us out here - we need to know what plane you are trying to spec a motor/prop/esc/battery combo (i.e., power system) for.

How slow a plane "CAN" fly all boils down to wing loading and the airfoil shape of your wing... so first what type of airfoil is it??? In general is it a sleek hotliner/pylon, fat 3D, pattern/freestyle aerobatic, flying wing, undercambered trainer, beginner flat bottom????? NEXT, find out the weight of your plane minus power system and the wing area. Next spec out the appropriate motor (kv and max watts being important here), prop (size and pitch important here), battery (will it support the current draw the prop imposes and give you sufficient run-time), ESC (will it support the max current draw of the motor/prop and battery type) combo. You choose a motor/prop/battery combo that will give you the thrust needed to pull through vertical maneuvers given the AUW of the plane (this includes the weight of the power system being proposed) and gives enough speed to keep your plane flying without stalling. Remember, differing motor weights and battery weights will significantly change the wing loading and thus stall speed so try not to go overboard on the size of the battery and motor.

Key is to play around with the power system calculators out there and READ!!!!!! Yes, you actually have to do a little "work" here

here is another useful article on the subject.
http://www.rcgroups.com/forums/showthread.php?t=333326




Quote:
Originally Posted by mspicela
Brian, that all makes sense. I want to just focus on the prop size and pitch portion though. What do changing those two variables do? When would I want to increase or decrease size and when do I want to increase or decrease pitch. For instance, Rick suggested that I go with a smaller prop with more pitch when I moved from a plane to my stryker. I assume it was to make it spin faster but cut more air for more thrust .... not sure though.
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Last edited by dleviner; Dec 12, 2007 at 11:11 AM.
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Old Dec 12, 2007, 01:51 PM
Matt Scott
Falcon929's Avatar
United States, AL, Huntsville
Joined Nov 2007
50 Posts
David is right. How fast or slow a plane will or can fly mostly comes from the aerodynamics of the air frame. A specific airframe at a specific weight has a specific stall speed (Vs). Then above that, you need a certain amount of airspeed to maneuver (Vmc). These speeds don't change with thrust. These are built into the airframe at its weight and making the airframe lighter for slower speed can only do so much. You can fly your plane to a nice high altitude and try turning off the motor and glide the plane to get an idea where the plane stalls and how much speed above that you need to safely maneuver. If the Vmc speed seems to fast, then the plane is out of you league. Your skill level is very important to keep in mind whenever you try a new plane. This is an area a pilot must be completely honest with him/herself. The further out of you league a plane is the more you will be fixing it, and if you don't like fixing it, you need to make sure you fly planes well within you skill zone.

Back to the thrust issue, you are talking about changing the propeller which means you are talking about thrust. To decrease thrust you can slow the throttle. If your first step or two of throttle that gets the motor to turn makes the plane fly to fast then you throttle is not calibrated very well. Generally speaking, those first few steps (Lower 15-25%?) of throttle should give you a thrust that that keeps you comfortably above your Vmc generally used for landing maneuvers. Remember you don't have to fly at full throttle all the time. In fact a well configured stable plane should give you a nice cruise at around 50-75% throttle depending on the plane type. I donít believe changing you propeller should not be the first step you take here. Since you already have your plane flying, you need to be testing the flight characteristics of your plane to know what is required to fly it. Iím sure flying the plane at a reduced throttle will give you an idea about how the plane is meant to fly, and how it might fly with a different prop.

Please take somebodyís advice before you dive into anything!!

Regards,
Matt

Reference V speeds:
http://en.wikipedia.org/wiki/V_speeds
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Last edited by Falcon929; Dec 12, 2007 at 02:10 PM. Reason: Fixed incorrect information
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Old Dec 12, 2007, 03:38 PM
When's the next fly-in?
dee-grose's Avatar
Tanner, Alabama
Joined Oct 2003
6,439 Posts
Quote:
Originally Posted by Falcon929
Remember you don't have to fly at full throttle all the time.
Well said, my friend...well said.

That is one of the biggest things I preach. Keep your left thumb out of it some and learn how to fly the plane!

Andy
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Old Dec 12, 2007, 06:13 PM
Fly it like you stole it..
Tram's Avatar
Florence, Al
Joined Oct 2000
29,301 Posts
Hmm.. How to make a plane fly slow...

Lots of drag... Lots of wing.. Lots of drag..

As far as picking a "thrusty" prop, thrust on the prop can be equated with the second number.. It's a rule of thumb, not an exact science.. The smaller the second number, the more thrust you can expect..

12x4 or 12x9... 12x4 is "thrusti-er?"

Jeff
www.CommonSenseRC.com
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Old Dec 12, 2007, 09:21 PM
David
dleviner's Avatar
Monrovia, AL, USA
Joined Mar 2001
1,253 Posts
Jeff don't you mean the first number (size / length)????? The second number is the pitch measured in inches moved through air (at sea level) per revolution, which governs speed.

It is true though that a 12x4 will produce more "thrust" than a 12x9 in a real world scenario... this is due to the fact that the motor is loaded more with the 12x9 trying to turn the prop at the same rpm but having to do more work moving the airframe faster. Because of this loading and the motor losses associated, the motor spins the prop slightly slower thus not producing as much torque.


Thrusty props: 9x3.8, 10x4, 11x5, 12x5 even 7x3.5 and 6x3 for smaller setups
Speed props: 4.1x4.1, 5x4.5, 8x7, 9x7.5, 10x8, 11x10

But keep in mind, you wouldn't put something like a 11x10 on a 400 size pylon plane nor would you put a 7x3 on a freestyle 3D ship with a 38" WS and AUW around 21oz.

You think we have confused Mr. Spiceland enough
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Old Dec 12, 2007, 09:27 PM
Fly it like you stole it..
Tram's Avatar
Florence, Al
Joined Oct 2000
29,301 Posts
Quote:
Originally Posted by dleviner
Jeff don't you mean the first number (size / length)????? The second number is the pitch measured in inches moved through air (at sea level) per revolution, which governs speed.
I almost didn't leave my post as I figured someone would be tempted to call me on it.. You are completely right here and this is how props are named..

1st number - Diameter
2nd number - Pitch

More pitch speed = more speed..
Less pitch speed = less speed..

A 10 pitch will/should move the aircraft through the air faster than something with a 4 pitch speed..

Quote:
Originally Posted by dleviner
It is true though that a 12x4 will produce more "thrust" than a 12x9 in a real world scenario... this is due to the fact that the motor is loaded more with the 12x9 trying to turn the prop at the same rpm but having to do more work moving the airframe faster. Because of this loading and the motor losses associated, the motor spins the prop slightly slower thus not producing as much torque.
This is what I was getting at.. Thus the "it's not an exact science" comment.

If prop "size" in length is not an issue, going down in pitch will generally produce more thrust/less RPM.. However, going up in diameter will also help..

Sooooo.. Yeh, I did mean the 2nd number, but there is more to it, obviously than that..

If you wanna go fast - 12x10..
If you wanna hover - 12.25x3.75..

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Old Dec 12, 2007, 09:35 PM
David
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Monrovia, AL, USA
Joined Mar 2001
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See now... we are crystal clear, aren't we...
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Old Dec 12, 2007, 09:36 PM
Fly it like you stole it..
Tram's Avatar
Florence, Al
Joined Oct 2000
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Clear as mud..
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Old Dec 13, 2007, 06:42 PM
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mspicela's Avatar
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Those last responses were exactly what I was looking for :-) I'd found that from reading one of the links in the earlier posts but those responses validated it.
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