.20 or .40 Size Glow Engines comparable to Brushless. - RC Groups
Sep 23, 2005, 01:53 PM
Registered User

# .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.

Thanks
 Sep 23, 2005, 03:20 PM chicks dig rockers 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. -Joe
Sep 23, 2005, 03:57 PM

# comparing ic to electric

hi

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

electric setup
axi 2820/10 and 10 cp1700 cells pulling 35amps with a 11*6 prop
so (10*1.2)*35=420watt and around 8800rpm

ic
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

atb

kev
 Sep 23, 2005, 10:23 PM S.A.D. member 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, 02:49 AM Registered User 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. big bird
Sep 26, 2005, 01:10 PM
Registered User