|Sep 23, 2005, 12:53 PM|
Lake Stevens Wa
Joined Feb 2005
.20 or .40 Size Glow Engines comparable to Brushless.
Stupid Question of the Month (Maybe Year!).
What if any brushless motors are comparable to a .20 size Glow motor, and likewise a .40 size motor?
I have no experience with glow motors when I got into this (3 Years ago) all I did was Slope or Sailplanes. I am not questioning weather or not to go with glow power to stick strictly with brushless motors.
All of the aircraft I plan on building ar 1/12 Scale (35-60 inch wing spans) or sport Planes, and I am not really interested in 3d Flying.
|Sep 23, 2005, 02:20 PM|
unfortunately its not a straight forward thing..
theres some kind of strange algorythm that they use to figure it out.. it has something to do with the absolute value of 453932575475 devided by the diameter of the average sand grain times the square root of the kool aid man.
|Sep 23, 2005, 02:57 PM|
comparing ic to electric
some brushless motors have a rating that is a direct reference to ic (15, 20, 40)
but there is a way to work it out, 1bhp is around 740 watts
my OS32sx has a stated bhp of 1.1, this means that it produces around 815 watts
electric power systems are measured in watts also (voltage * amps) = watts
so to get the same sort of power you would need something like 20volts at 41amps
but it's not that simple
i used to fly my 44" mustang on electric with several different setups, now its Ic and its not a lot faster
axi 2820/10 and 10 cp1700 cells pulling 35amps with a 11*6 prop
so (10*1.2)*35=420watt and around 8800rpm
os 32sx that should produce 815watts (1.1bhp) on a 11*5 prop, i've not measured the rpm though
on paper you would say that the ic setup is nearly twice as powerfull but it was seriously quick with the axi setup and would climb vertically untill the battery voltage dropped
although the Ic plane is a little faster i don't think its twice as powerfull, maybe my os isn't producing the 1.1bhp that is quoted
the axi setup has a great amount of torque so if you opened the throttle quickly the plane would excellerate faster that the ic up to a certain speed but then the ic would over take it again
so to recap
you can do a calculation (1bhp=740watts) but its not spot on
|Sep 23, 2005, 09:23 PM|
The starting assumption is not quite correct. Although an OS 32SX is listed at 1.1 bhp, this value has no practical meaning - it's at something like 16,000 - 17,000 rpm and we never (almost) prop our engines/planes so the engine will turn these rpm. Another factor to be considered is that propellers are more effective at lower rpm. An OS FS-70SII is also rated at 1.1 bhp. If you use the FS-70SII in a 5.0 - 5.5 lb. plane as let's say H9 Twist (I quote this plane and engine because I have that engine in that plane), you'll be able to hover it around 2/3 throttle with an APC 14x4W prop. Not possible with an OS 32SX in that same plane even at WOT. Nevertheless the 32SX WILL FLY the plane, but I've never seen such an engine/plane combo. In electrics, the watts-per-pound (oz) ratio is what matters (as I understand it). One can (and must) choose the proper motor/ESC/battery pack/plane/propeller of an electric plane before actually assembling the plane. Improper component selection will lead to unsatisfactory results and might require replacement of 2 or more expensive components (motor, ESC, battery pack). With fuel planes, changing the prop on the field is most of the time enough for gaining the desired result. In worst case scenario a different engine (more powerful in most cases) will provide the desired result.
|Sep 24, 2005, 01:49 AM|
Belfast / Dublin
Joined May 2004
just remember that BHP figures on glow engines are really only indicative of useful power.
BHP = torque x RPM
so top BHP figures are measured at top RPM, but as Ivan says - when do we fly at 16000 RPM? Never, or practically never.
So the BHP rating is optimistic to say the least - though the manufacturer is not actually lying, the BHP figure needs some interpreting - that is to say that the useful power of this glow motor will be considerably less than 1.1 BHP ie. in reality you are only ever going to access just over half of those stated horsepowers/watts.
Often electric setups produce a lot of torque (turning power), so they can turn big efficient props and produce lots of thrust. Remember, thrust is largely a product of diameter - so if you don't need huge pitch speed, then you also don't need massive RPM.
|Sep 26, 2005, 04:08 PM|
Joined Feb 2002
Hobby Lobby in their ads in RC mags talks about this and goes through calculations similar to the above. They claim the 4000 series AXI motors (which of course they carry) are equivalent to glo. I agree.
From my own experience, I believe any motor/prop/battery combination that delivers 30-35 amps at 15 V (4 lithium cells in parallel) is roughly the equavlent of a .40 glo engine.
|Sep 27, 2005, 08:39 PM|
Joined Aug 2005
One thing that is misunderstood (the Hobby Lobby Adds for an example) is that the battery current times the battery voltage equals the INPUT watts to the system - not power at the prop
The actual power at the motor shaft equals the battery voltage times the battery current multiplied by the efficiency of the controller multiplied times the efficiency of the motor - and also times the efficiency of the gearbox if you have one - this is quite a bit less than the voltage time the current
An example is that if you have 10 volts at 10 amps you have 100 watts input to the system. The controller might have and efficiency of .95 and an outrunner might have an efficiency of .83 - therefore you would get
10 x 10 =100 x .95 x .83 = 78.8 watts at the shaft - considerably less than 100 watts
If yoiu have a gearbox you get even less at the shaft - outrunners are less efficient - but it gets made up by not having a gearbox.
The difference between the 100 watts in and the 78.8 watts out is heat generated in the controller, the motor winding losses, the motor leads and the heat generated by the gearbox.
The heat generated in the battery is caused by the internal resistance of the battery - which closely related to the no load battery voltage minus the loaded battery voltage times the battery current or
(V no load - V loaded) X current
Knowing the loaded and unloaded voltage at a given current - you can calculate the internal resistance of the battery
Current squared x resistance = power
So if you LIPO had a voltage of 8.4 volt with no load and a terminal voltage of 7.8 volts at 10 amps - then the power disapated in the battery would be
8.4-7.8 = .6 x 10 amps = 6 watts and the internal resistance of the battery would be = I squared x R = 6 watts
10 x 10 x R = 6
R = 6/100 0hms or .06 ohms - the lower the internal resistance - the higher the discharge C that can be tolerated.
In the IC engine you would get a shaft output rating at maximum RPM
Most large electric motors are rated at the shaft
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