[NSRCA-discussion] Electric props vs. standard pattern

[Dave]

Dave,

 

The ideal prop airfoil shape is compromised to provide the needed structural
integrity to withstand aerodynamic, gyroscopic, and impulse loads (from
firing of combustion engines, and even pulses from electric motors).  Ideal
in this context meaning most thrust and/or speed for least amount of input
power.  Because the impulse loads from an electric motor are lower, and
electrics are generally run at lower RPM (primarily to take advantage of
larger more efficient props), it is possible to design a prop that is closer
to the ideal shape and lighter while still having the needed structural
integrity.

 

Several years ago, I played with a very high RPM outrunner (295 KV) and
checked many different props, including some E vs glow prop comparisons -

16x10E - 8400 RPM, 79 amps

16x10 - 8800 RPM, 69 amps

Performance in the air was very similar, but the E prop was much lighter
(about 65% the weight of the glow prop).  I preferred the glow prop because
it afforded the same performance with cooler motor, ESC, and lipo temps
(less mah discharged for an equal flight).

 

I've run high RPM (7500 - 8000) outrunners in pattern quite a bit, and had
always found the glow props to be better - in terms of lower input watts
needed and fewer mah consumed for equal flights...and I had a theory
why...but never "proved" it to myself until this past year.  I laminated a
number of E props of various sizes for various motors/RPMs with CF cloth,
and found on the higher RPM setups, the E props with CF would turn more RPM
and draw less amps..and in the air they indeed performed better than a stock
E prop or a similar sized glow prop.  With respect to pattern sized
motors/power...I saw very little improvement at <6500 RPM, but noticeable
improvement >7000 RPM.

 

The vast majority of electrics being used in pattern are <6500 RPM (and the
unload in the air is a reduction in amps, but very little increase in RPM),
while the majority of glow stuff is running between 7400 and 8400 (and can
unload to ~+10% RPM in the air)...so the difference in 6500 and 8700
(unloaded) is pretty substantial when you start looking at prop structure
needed to eliminate flexing.

 

Regards,

 

Dave Lockhart

DaveL322 at comcast.net

 

 

 

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From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of Ken Velez
Sent: Monday, September 07, 2009 11:41 PM
To: 'General pattern discussion'
Subject: Re: [NSRCA-discussion] Electric props vs. standard pattern

 

Hi Dave,

 

It seems to me that the electrics are not turning comparable numbers. Most
setups are turning in the low to mid 6000 and only 1 or 2 set ups are in the
7000 and none in the 8000 that I know of. This is based on what I have
observed only; so there could be a setup turning higher numbers; and on a
10S battery. If you go to more cells = more Volts = more RPM's also more
Amps. FAI is caped to 10 cells not sure about AMA. My Hacker A-60 is on the
mid 7000 and we were using the 18.1X10-11-12 glow props; while it flies very
nice with these props the plane is very slow for top classes and or wind. So
right now I'm using an E prop; huge difference. It's also my understanding
that the E props are more efficient than the regular glow props not 100%
sure but pretty certain about it but you are correct about the stiffer is
more efficient when comparing 2 identical props lets say one plastic and one
carbon but not equal when comparing different airfoils. An APC 20X10 E will
be more efficient than a 20 X10 Glow and if you can make the APC 20X10E in
Carbon it will be more efficient than the original 20X10E etc. Just my
observations. 

 

Ken

 

  _____  

From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of Dave Burton
Sent: Monday, September 07, 2009 11:04 PM
To: 'General pattern discussion'
Subject: Re: [NSRCA-discussion] Electric props vs. standard pattern

 

Why are we using the Electric style APC props on electric powered pattern
planes rather than the standard pattern props? We are turning the electric
motors at comparable RPMs to a glow engine. The standard pattern props are
stiffer and should be more efficient it seems to me. I can understand a
little weight savings if needed but is that the only reason for using the
E-props? What am I missing? Anyone compared similar E and standard props in
the same sizes? What results did you find? 
Dave Burton

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