# Decisions based on props

Can anyone guide us all through a theoretical decision making process for selection of engine or motor based on prop or blade size and weight to be thrust or lifted (Quad)?  You might wake up tomorrow and think to yourself.."self..I want to build a quad that has 24" props.  How much stuff can I fly around the backyard chasing squirrels?"

Maybe this very important new discussion will draw out some guidelines for design reference material instead of guessing or experimenting with wasted money.

I found a RC Prop Chart with optimum and ranges for prop sizes on IC engines:

http://www.coastalplanes.com/tools/propchart.htm

Problem is that the average modeler, who wants to try and put together a new idea, doesn't have a place where he/she can go and get a one stop guide for making smart decisions.  The common questions are what weight will it be?  Fixed wing, Rotory , Dual prop VTOL or Quad Rotor...etc.?  How long do I want it in the air?

The best references are those that the users here have tried and used with success.

One great example is:   http://www.ecalc.ch/    but limted to electric motors and batteries.

And PropCalc:  http://www.drivecalc.de/PropCalc/index.html

In relation to lifting, hovering and flying 25 lbs...what does Thrust (N) and P (W) mean? Power in Watts?  How many watts do I need to lift 20lbs?

Most of the standard commercially available blades and props found on sites like HobbyKing etc. come in specific size and pitch.  Not enough info to run the calculations required for PropCalc.

Maybe someone has a chart with blade and prop sizes (two and three blades), pitches and optimum RPM with estimated lift/thrust potentials.

What are considerations must a modeler think about in the design of a internal combustion engine power train?   Again weight is first (thrust/lift) and then prop size and pitch.  Would a 100cc four stroke give the two props in a VTOL 50cc worth of power each?  How would you calculate power losses and come up with a capabilities comparison between different options? The recommended prop size for a 100cc motor is 26" - 28" at 6000 - 6500 RPM.  Could I run two 24" props on a VTOL with a single 100cc motor at 4500 RPM?   A chart showing prop sizes with necessary RPM to achieve different lift/thrust weights would be nice.

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### Replies to This Discussion

anyone?  right now im trying to figure out what motors, escs, battery and props to buy and the whole ordeal is a bit confusing. Im looking at motors, and im trying to find some with low kv ~ 800 thinking that lower rpm should provide higher torque, which will allow me to use a bigger prop and therefore generate more lift. that is just my logic but i havent been able to find a sure method to figure out exactly what i want like Will was saying above. Is there a way to calculate the thrust a motor will provide from  the kv and the prop size/pitch?   Some websites list the recommended model weight a certain motor is rated for, say 1lb. if im building a quadcopter and use 4 of those motors, does that mean the quadcopter can lift 4lbs?

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