|Jan 05, 2013, 02:08 PM|
Joined Jan 2013
help required ! motor specs..
hi there , wish everyone a very haapy new year !
i have few questions and i would be more than thankful for anyone who clarifies it to me.
how to calculate the thrust required at hand launch ? i need an equation for this...
how to calculate the thrust required at cruise ?
how to calculate motor static thrust and how to match props with it ?
what does wattage power have to do with performance of the plane ?
what is the optimum wing loading for carbon fiber wing spars ?
In a LiPo battery , how does the variation in voltage and current rating (mAh) affect the performance of a motor? suppose if we use 2s and 3 s .. does 3 s simply mean more flight time and more current means more power ?
what is the relation b/w esc ampere rating and motor ?
duct fans vs outrunner with large props ? which provide more static thrust ?
sorry for the long post, but i would be really greatly for any info on this..
thanks in advance
|Jan 05, 2013, 03:15 PM|
First off I need to say that I hate math and don't keep the equations sitting around. So others will be along shortly to supply those where they fit.
It's tough to calculate such things since there are so many variables. And even when optimum theoretical shapes are considered the builder's execution of the shapes can be varied enough that the calculations become flawed. But generally if you make the wings glider like enough and keep the fuselage as small as possible and tear drop like the drag is fairly low.
For electric models that are aerodynamically clean like a sailplane with an electric motor 7 to 8 watts per lb of power to maintain a cruise is going to be difficult but not unrealistic. Meanwhile regular sport models and many blockly looking UAV style models that think they are efficient but really are not are up around 13 watts per lb to maintain a cruise.
Static thrust is only useable as an indicator of performance over a very select range of applications. The two key areas being 3D fun fly models that prop hang and fly around in a post stalled nose up attitude a lot and the large props used on electric powered sailplanes or slow flying long duration models. Beyond that the results from static testing become meaningless to what actually goes on during moderate to high speed flight.
When you select a prop to go with the motor you have to realize that the motor cannot turn as big a prop with a higher cell count pack. For example if a motor is able to turn a 10x6 prop on a 2S pack and be right at the safe power limit if you put a 3S pack on the motor that same prop will result in drawing too much power and burn out the motor. To hold the total watts at the motor constant you need to switch to a smaller diameter or pitch or both to reduce the load so the motor can only draw the same wattage as on the 2S pack.
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