Hi, All.

I'm trying to figure out what motor I want for a plane.  I'm referencing the Cox .15 engine props and RPMs for the plane, and was wondering how the old wooden or plastic nitro props compared to electric props.  For instance, the instruction sheet for a TD .15 says it turned an 8"x4" prop at 15,700RPM.  Would an APC 8x4E electric prop have approximately the same thrust as the nitro 8x4 prop if run at the same 15.7K?  I'm just trying to pick the motor KV/Battery voltage combo that will match the stated numbers for the Cox .15 engine.

I know, I've used the search function to see what other people have used for .15 sized planes, but there is still a large variation in motors used, and most seem to be in the overkill category as far as wattage is concerned.  Besides, I'm just wondering about the general characteristics of electric vs. nitro props.

Thanks,
Mark

Please don't try running an APC E prop at that rpm.  Can't remember the recommended max rpm, 12k comes to mind though.  You may want to check on the APC web site.  The e-props are probably more efficient at certain rpm's, but are limited due to the thickness. Can't have everything.

What exactly did you want to do with the motor?  What plane?

Hi Mark,
You can swing a fine pitch prop with a small diameter at that sort of rpm, but it is noisy.......and dangerous if the prop throws a blade. I've been playing with small motors for the last year or so and find that a good .15 equivalent is a motor that weighs around 70 to 75 grams with an APC 8 x 6 E prop. I'm hoping to fly my new test model that is a bit bigger than the Sig Akromaster this week-end and then I'll have some more figures to post. You can use a bigger diameter prop to get the thrust but then prop clearance becomes and issue, so for me 8" is the maximum for practical reasons. I'm using one of RSM's Black Tiger 3530 C motors with a 1400 Kv and this on my bench tests sounds about right. One of my KR Compact timers would also work nicely on the Akromaster as it takes up less space. If you want to use another type of timer then just remember to use an esc with some kind of governor system, as the systems without governors are not a good idea.

There is a lot of good data on the Cobra motors website with all of the prop tests that you can think of for each motor. A good starting place might be the Cobra C2213/18 motor with a 7 x 6 prop on a 3 cell lipo. The Akromaster would most likely fly well with a Cox Tee Dee .09. You could most likely compete with a Goodyear racer with a screaming Tee Dee .15! The old Enya .15 plain bearing engines worked fine on the Akromaster so I would really suggest looking at that particular Cobra motor or something similar. Just keep asking questions here if you're stuck. There are a lot of very experienced guys here that have been playing with e-power since it was introduced and they are all willing to help. Have fun!

Keith R

Thin blades flex more than thick blades.  Electric props have thinner blades than glow props.  When the prop stalls there is a significant increase in loudness as the blades flex, flap, and beat the air.  The edge of the prop disk become distorted visually, which is what should be avoided.  Problem is you need to look somewhat edgewise on the disk to see the effect, and your head should not be there.

Use a servo tester or RC unit to ramp the prop up to the desired RPM.  If it stalls making a whole bunch of racket in static thrust significantly below the target RPM then there is a good chance it will do the same in the air.  If it it stalls and flutters real close (1k-2k) to the target RPM it may be fine.  It would be best if it didn't flutter at all though (it can still stall in static thrust without flutter, which should be fine since this plane is not expected to "prop hang").

Just because it is electric doesn't mean that the prop in the kit is the right prop for YOUR setup (same as with glow).  My RC arf experience says the prop in the kit usually sucks and a good heavy glow prop produces more actual thrust anyways.  I would try the props on your recommended list and see what happens.

I really wish I could find a particular youtube that demonstrates this on the bench, with sound and close up video of the prop disk as it starts fluttering.  This video shows the effect, but not very well.



Phil