Cessna dude

I went to a local hobby store and purchased a electric stick from Great planes. The kit called for a 40 sized engine but the folks behind the counter said that the 20 sized motor will work. Im not schooled properly on the numbers of this new to me hobby, so I may sound incorrect. I noticed that the 20 sized motor length is a little bit longer in profile than the recommended motor, I was wondering if it would badly effect the center of gravity for this bird? It also calls for a 11 inch prop, but the btc guys said the motor has good torque and a 13 inch prop will do. Will it have enough clearance during vx or vy? Its a tail dragger so it might need to do a slight nose down rotation to get airborne for vx or vy. Any input would be greatly welcomed thanx, Cessna dude!

tkametz

I would recommend a Power 32, an 60 amd esc, and a 12 x 8 prop.

ron_van_sommeren

Better ignore glow-equivalencies, e-motors are way more flexible, and can rev at lower rpm, using a more efficient prop. Glow motors max.power is specified at useless rpm i.e. overrated.

Plane mass and type are leading when choosing motor power (in watt):

• Rules of thumb for electric power
  Magic numbers for e-flight - WFF
• Gibbs Guides
  → Articles
  → How to choose an electric power system
• The Ampeer
  → E-book: Everything you wanted to know about electric powered flight
  → Chapter Sizing Power Systems for Electric Airplanes
• How to choose a power system - RCG
• Beginners Guide to Motor and Prop Selection - RCG
• System power - RCG, several messages
• Several design papers
  rcaeronotes.wordpress.com/design-and-analysis-notes

Current is proportional to propdiameter⁴.
Going from 11 to 13 inch, without changing anything else, motorcurrent wants to increase by factor (13//11)⁴ = 1.9, almost double.

General rules for current and power, they give a relative indication:
Motorcurrent is proportional to pitch¹, voltage², Kv³ and diameter⁴.
Power-drawn is proportional to pitch¹, voltage³, Kv³ and diameter⁴.

Without the exponentation
extra current with one or two cells added, simple table

So changes in setup (and lousy Kv specifications!) can have surprisingly considerable/huge effects.
E.g. doubling voltage will four(2²)fold current, doubling Kv will eight(2³)fold current, and doubling prop diameter will sixteen(2⁴)fold current.
Even a small 10% change/difference in Kv will already lead to a 30% difference in current.

Some well-structured reading for rainy/windy days, and some handy e-tools as well. Will save you, and us a lot of questions. Notably the 'what went wrong?' kind of questions Will also prevent you from burning up several controllers and/or motors and/or battery:
E-flight primer and tools

And pleasepleaseplease, do your RC equipment, wallet, ego, battery, controller, motor, house/garage/car a big favour ... get a watt-meter. It will more than pay for itself, will save you at least one fried motor and one fried controller. Will also help you finding the best setup.
Keep battery/-watt-/multi-meter wires short!
too long wires batteryside will kill ESC over time: precautions, solutions & workarounds

Vriendelijke groeten Ron
• Without a watt-meter you are in the dark ... until something starts to glow •
• e-flight calculators • watt-meters • diy motor tips • Cumulus MFC •