I am confused about plane sizes. Not whether they are big or small, but the numeric rating, like 15 size, 40 size, ect. Just what does the number mean?

Stems back to matching the power of the two stroke glow fuel engine to the airplane. The numbers "15" and "40" equate to the engine displacement in cubic inches i.e. a 0.15 cuin engine and a 0.40 cuin engine.

When the four stroke engines first came out, because they only "hit" on every other stroke there was a general opinion that for a conversion from a two stroke to a four stroke you doubled the size of the displacement. For a Kadet LT 40 which flies well on any 0.40 cuin two stroke, you needed a 0.80 four stroke to get the same performance. THIS IS NO LONGER TRUE!!!

Now if you want to switch to electrics, the easiest is just look at the eflite range of motors they name them to the equivalent two stroke power.

MTC YMMV

The number is the recommended glow plug (internal combustion) engine size.

15 = 0.15 cubic inches (2.5cc)
40 = 0.4 cubic inches (6.5 cc)

Best Regards,
=Adrian=

These numbers have no direct correlation to electric motors.  Althought some of the electric motor manufacturers have tried to equate their motors to equivalent glow sized motors so glow pilots going to electric can come over more easily.

Note that for a given size, glow planes tend to be heavier as they need a lot of reinforcement to stand up to the vibration of the motor.  Electric motors don't vibrate like that so we don't have to reinforce for it.

Also glow planes have to be fuel proofed where electrics don't.

When sizing motors for planes, use the watts/poung guide lines as discussed in this e-book.

<div>EVERYTHING YOU WANTED TO KNOW ABOUT ELECTRIC POWERED FLIGHT</div><div>http://www.rcuniverse.com/forum/m_7100376/tm.htm</div><div></div>

One crude, but ballpark useful, conversion factor "rule of thumb" is watts = cubic inches x 2000.... thus a .15 glow equates to about 300W-in electric... a .40 equates to about 800W-in electric.

Thanks guys.

I am a noob and when I see things like 15 or 40 size plane e.g. the E-Flite Apprentice is referred to as 15 size plane and another that was smaller referred to as 40 size, it was confusing. Have a better idea now of what I am looking at. Has to do with that watts per pound thing. The smaller plane mentioned was balsa and heavier than the foam apprentice. Think I got it now.

OBTW - I did read through "EVERYTHING YOU WANTED TO KNOW ABOUT ELECTRIC POWERED FLIGHT" just didn't make the relation between the W/Lb and the sizing.