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Old Jan 05, 2013, 01:08 PM
Registered User
Joined Jan 2013
3 Posts
Help!
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
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Old Jan 05, 2013, 02:15 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
11,352 Posts
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.

Quote:
how to calculate the thrust required at hand launch ? i need an equation for this...
The equation(s) would be the same as for the model in a steady state climb. There simply isn't anything "special" about a takeoff or the moment of release from a proper hand launch. When the model leaves the ground or your hand it must be at some flying speed which is above the stall. If it isn't then it's more good luck than good planning if a crash doesn't occur as it accelerates through this dangerous situation. As such the thrust needed to fly away safely is going to be set by the amount needed for level flight along with whatever extra is used to develop some rate of climb. In other words the same equations that pertain to normal cruise flying and climb rate vs power apply at the moment contact is lost between your hand or the ground.

Quote:
how to calculate the thrust required at cruise ?
The thrust needed at cruise is the amount needed to overcome the total airframe drag at the speed being considered. The total drag is a summation of the induced drag related to lift and any parasitic drag of the airframe. The induced drag can be lowered by selecting a more efficient plaform for the wing such as the high aspect ratio wings used on sailplanes and some long distance UAV's. It ends up being tempered by the practicalities of assembly and transport. And on models there's some consideration given to not letting the Reynold's number of the wing become too low and resulting in the airfoil performance being lowered.

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.

Quote:
how to calculate motor static thrust and how to match props with it ?
Simply you can't. You need to test the motor and prop combination. There are databases such as Motocalc that have enough data that they can give you a darn good idea of what a given motor and prop will do but the results are based on some original testing done my many folks over a lot of years and are used to extrapolate the results.

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.

Quote:
what does wattage power have to do with performance of the plane ?
The wattage used by the motor at any given point in a flight determines how fast the model will fly or how fast it can gain altitude. There's a "rule of thumb" around for how much wattage is needed for a desired climb rate for any excess power more than that used for the cruise. But it's more a guide than a highly accurate "scientific" sort of equation. Again the actual performance is simply so dependent on the parameters of the design being considered.

Quote:
what is the optimum wing loading for carbon fiber wing spars ?
The question makes no sense. Or at least you simply are not wording it correctly. The load that a carbon spar ASSEMBLY will tolerate is based on the size and style of carbon fiber used and HOW it is used. Generally the carbon is used as top and bottom caps of an "I" beam style build. As such the rules related to structural I beams apply and the dimensions of the actual wing spar design need to be known before anything else can be figured out. You don't simply toss some carbon into the wing and expect great things. It needs to be used appropriately to perform the desired function.

Quote:
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 ?
2S and 3S refers to the number of cells in the pack. And by extension the pack's output voltage. These same packs have a "C" rating as well. What it means is that the packs can be discharged at that many times the normal one hour discharge capacity rating. So a 25C pack that has a capcity of 1500 mA-Hr can be safely discharged at 25 x 1500 = 37500 mA or 37.5 Amps.

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.

Quote:
what is the relation b/w esc ampere rating and motor ?
To ensure that the ESC does not overheat it's fairly customary to select an ESC which is rated for about 1.3 to 1.5 times the maximum expected load current. You still need to ensure that the ESC is in a spot where it has at least some minimal airflow around it.

Quote:
duct fans vs outrunner with large props ? which provide more static thrust ?
Ducted fans and small fast turning props are great for going fast. But they accelerate from slow speeds poorly and do not climb efficiently. Large props on small slower turning motors, when matched to the motor correctly, climb well and are efficient at slow cruise speeds but do not fly fast at all. This is why electric powered sailplanes generally have larger folding props.
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