Super nerdy technical content warning!!! This post may contain formulas unsuitable for those without high school education. 
Here's the rub. A plane needs to move through the air faster than the stall speed to be able to generate enough lift from the wings to sustain flight. (There are obvious notable exceptions to this like flat plat airfoils that have their own breed of dynamics - to be acknowledged but ignored. There are other uber-nerdy facts and figures using names like Reynold and words like coefficient that are also acknowledged but again suitably ignored). To do that, the propellor needs to generate enough thrust to overcome the drag of the wing and airframe.
As basic as it can be, the stall speed of an aircraft relates to its wing loading which is often expressed as oz/sqft or g/sqdm. It appears that oz/sqin is more popular with online calculators so let's run with it for now.
As a rule of thumb the stall speed in mph is approximately equal to 3.7 times the square root of the wingloading in ounces per square foot.
So then, there are two targets to hit:
1. Try to get the pitch speed of the prop to be above 2.5x the stall speed of the plane. ie. rpm*pitch=inches/minute and rpm*pitch*63 360/3600=miles/hour.
2. Ensure that the thrust of the prop is more than 1/2 the weight of the craft. 3D craft need at least 1.5x the weight of the craft but scale planes can cope with less than the weight of the plane. We often over power our craft in this respect. Prop thrust is really hard to calculate properly but here's a very rough starting point:
Thrust = Pitch X Diameter^3 X RPM^2 X 10^-10 oz.
So: You need a motor to spin the prop of the chosen diameter and pitch fast enough so you don't stall and generating enough thrust to push the craft through the air.
As suggested, the most reliable way to work this out is to ask the guy who's plane looks like yours and flies really nicely what he uses.
Simple aint it?
Here's the rub. A plane needs to move through the air faster than the stall speed to be able to generate enough lift from the wings to sustain flight. (There are obvious notable exceptions to this like flat plat airfoils that have their own breed of dynamics - to be acknowledged but ignored. There are other uber-nerdy facts and figures using names like Reynold and words like coefficient that are also acknowledged but again suitably ignored). To do that, the propellor needs to generate enough thrust to overcome the drag of the wing and airframe.
As basic as it can be, the stall speed of an aircraft relates to its wing loading which is often expressed as oz/sqft or g/sqdm. It appears that oz/sqin is more popular with online calculators so let's run with it for now.
As a rule of thumb the stall speed in mph is approximately equal to 3.7 times the square root of the wingloading in ounces per square foot.
So then, there are two targets to hit:
1. Try to get the pitch speed of the prop to be above 2.5x the stall speed of the plane. ie. rpm*pitch=inches/minute and rpm*pitch*63 360/3600=miles/hour.
2. Ensure that the thrust of the prop is more than 1/2 the weight of the craft. 3D craft need at least 1.5x the weight of the craft but scale planes can cope with less than the weight of the plane. We often over power our craft in this respect. Prop thrust is really hard to calculate properly but here's a very rough starting point:
Thrust = Pitch X Diameter^3 X RPM^2 X 10^-10 oz.
So: You need a motor to spin the prop of the chosen diameter and pitch fast enough so you don't stall and generating enough thrust to push the craft through the air.
As suggested, the most reliable way to work this out is to ask the guy who's plane looks like yours and flies really nicely what he uses.
Simple aint it?
Drew
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Built-up
Balsa: Commercial: Depron:
Great Planes PT-20 (modified) HK Hawk 80 Index 3
Indoor Thingy Skyartec Cessna 182
Katana X lite (in construction) UM SU-26 12"WS Shoestring (Plantraco)
HK Extra 260
and a Fox chuck glider converted to RC
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