[NSRCA-discussion] Electric props vs. standard pattern

[Bob Kane]

Nice analysis, thanks for sharing.

 Bob Kane
getterflash at yahoo.com




________________________________
From: Dave <DaveL322 at comcast.net>
To: General pattern discussion <nsrca-discussion at lists.nsrca.org>
Sent: Tuesday, September 8, 2009 1:06:03 AM
Subject: Re: [NSRCA-discussion] Electric props vs. standard pattern

 
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
 
 
 

________________________________
 
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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