gear ratio help

[ggraci]

Just an understang please. I have looked at charts for different gear ratio's with a prop size and the battery voltage/amps. The spreadsheet I have looked at ranges from letter "A" (5.86:1) through "E" (10.7:1). The only thing I can understand from this is that battery run time seems to increase, the higher up the alphabet you go (all things remaining equal, ie same prop) Does this mean the plane goes slower as well? That's all common sense is telling me. More than that I simply do not have enough info.

What do the numbers represent (5.86:1)?

I'm refering to this link:
http://www.balsapr.com/catalog/motors/1208436Thrust.asp

Thanks,
Gino.

[Dave Robelen]

Hello Gino,
The chart you are reffering to has a few more numbers that will help clear things up. First, the number (5.86-1, etc.) is the ratio of the gears that determine how many times the motor shaft rotates for one turn of the propeller. along with the increased battery run time, there are columns that give the thrust, and current. Using the same propeller, the thrust will decrease, and the run time will increase for the same battery. A starting point is to note the figures for the propeller, gear ratio, and voltage you have at present. Normally, folks increase the ratio to be able to run larger propellers for increased thrust. For example, I have been using a 10.7-1 gear unit with a 12-8 propeller to acheive increased thrust with a modest current increase and a small drop in flying time. This would be a huge propeller for the Moth, personally, I would stay with the 5.86 "A" unit, and perhaps a 9-7 prop, or a 10-4.7. Experimenting in flight with several propellers is very instructive to get the best results for your style of flying.
Regards, Dave

[ggraci]

thanks Dave. However, I still have a dunce cap on my head, forgive me. Does IPS "A" 5.86:1 mean that the prop turns 5.86 times for every 1 turn of the motor shaft?
The IPS "B" 7:1 means 7 prop turns for every 1 turn of the motor shaft? There is a thrust "oz" column and a thrust "gram" column. If I want more thrust, which of these two columns would I look at? Does it matter?

Last question on this chart: The "Amps" column? This is how many amps will be drawn from the motor at full power? Since I have 7.4V/1300mAH, I should look at the last column right? Close enough?

Propeller: Again, what does the number represent? I know they are measurements, but what measurements? What is a 9-7 prop?

Thanks again,
Gino.

[Dave Robelen]

Hello Gino,
My abbreviated replies get me in trouble from time-time. The numerical ratio refers to the motor spinning faster than the prop by the number stated. In the "A" gearbox, the motor is turning 5.6 times faster than the prop.
You can use either column for thrust, grams or ounces. They should be the same if you divide the gram value by 28.4. As you increase the gear ratio for a given prop/motor combination, the motor will spin faster, the prop slower, and the current will be reduced along with the thrust.
Assuming the Tiger Moth, if the performance is satisfactory with the 5.86 gearing, moving to a higher ratio will require a propeller change to get back to the same level of thrust that you had originally. A side issue is that as the gear ratio increases, the diameter of the large gear also increases. You would likely find that some of the higher ratio drives would not fit inside the cowling of the Moth.
The current value describes the amps the motor is drawing from the battery,, which directly impacts the run time. Another reason for considering the current is the lifespan of the motor. As the current rises, the motor life gets shorter. This is most apparent with the brush type motors, but the brushless motors also have limits where the heat involved running at high currents reduces the potency of the motor magnets, and thus the power of the motor. Meanwhile, the various controllers will also have some maximum safe current, as well as the batteries. Typically the safe battery current for the lipo style is expressed as a "C" value. If your 1300 mah battery is rated at say, 5C, the maximum safe current would be 6.5 A. In typical applications, the lower currents mean longer run times, and longer life for that expensive battery.
The modeler has to choose the performance level that is satisfactory, combined with the flight time desired, and match the system components to achieve these objectives. Typically, moving to more thrust for extra maneuvering ability means more current, shorrter flights, and shorter equipment life. The opposite is true as you select for a cruising type flight with lower current required.
I probably have really muddied the water now.
Regards, Dave

[ggraci]

Dave, you have not muddied the water. That was a great explanation. I think it is finally making sense to me now. I guess the reason for the gearing is to slow-down the propeller, thus making it a "slow park flyer"? If the ration were reversed or even one-to-one, it would no longer be a "park flyer" right?

My last question on numbers: Do you know what the 9-7 prop actually means? The "9" and the "7"?

[Dave Robelen]

Hello Gino,
I am glad that this is helping. Actually, the reason for the gearing is to match the propeller size to the motor and airplane. Typically, a GWS (or similar brushless) motor must turn relatively fast RPM's to make it's best power. This would lead to mounting a small propeller on the motor shaft. If the model were a small, fast flyer, that would be appropriate. In the case of the typical "Park Flyer" it is intended that the models fly slower, which leads to larger, lighter models. For an efficient propeller match, gearing is used to let the motor run at best speed, while the larger prop turns slower. The one exception would be the "outrunner" brushless motors, which have substantially more torque at lower speeds, and are able to turn the larger props directly.
The numbers on the prop describe the diameter and theoretical pich. Typically the diameter is listed first, usually in inches in our market, then the pitch. In the case of the 9-7 mentioned, the 7 refers to the distance in inches that the propeller would move forward in one rotation in there were no slippage. Obviously there is slippage, total if you are restraining the model from movement, and less in flight. The industry has settled on this reference system to be able to compare blades from various sources. For that matter, the full scale folks use the same concept.
Regards, Dave

[ggraci]

Thanks Dave. Very informative. Hey how much do you know about Lipoly batteries?

http://forums.radiocontrolzone.com/showthread.php?t=212834

[Dave Robelen]

Hello Gino,
I principally fly micro models with electric power, so pretty much all of my experience has been with using single cells. From observation, following directions that are furnished with the packs and chargers can keep you out of trouble. The site www.fmadirect.com has some very good basic information on the subject.
Regards, Dave