I read somewhere a formula for tourqe of a motor.
It said that ideal rpm would be at 80% of the high end of the rpm range.
Example My DL-50, 80% of 9,000 is 7,200rpm. ( this seems right ).
Is this a good formula?

I am tring to figure what props fall in my tourqe range of a few different motors.
The Fuji BT43EI-2 high rpm is 11,000, 80% of that is 8800rpm.
The manual says an APC 20 X 8 get 8000rpm. Model Airplane News test reveiw on the engine tested the APC 20 X 8 @ 7750rpm. The author says the tourqe sweet spot is between 6800 - 7100rpm.
So this formula doesn't seem right.
Do you think that is right?
Fuji just must have there engine high end rpm rated way to high.

Another.

CRRC Pro GF26I, 9500 max, 80% is 7600rpm. ( this seems right ).

Originally Posted by 2robinhood

I read somewhere a formula for tourqe of a motor.
It said that ideal rpm would be at 80% of the high end of the rpm range.
Example My DL-50, 80% of 9,000 is 7,200rpm. ( this seems right ).
Is this a good formula?

I am tring to figure what props fall in my tourqe range of a few different motors.
The Fuji BT43EI-2 high rpm is 11,000, 80% of that is 8800rpm.
The manual says an APC 20 X 8 get 8000rpm. Model Airplane News test reveiw on the engine tested the APC 20 X 8 @ 7750rpm. The author says the tourqe sweet spot is between 6800 - 7100rpm.
So this formula doesn't seem right.
Do you think that is right?
Fuji just must have there engine high end rpm rated way to high.

Another.

CRRC Pro GF26I, 9500 max, 80% is 7600rpm. ( this seems right ).

It depends on what you are trying to do.

With 2-strokes, and propellors, you basically want to turn the prop as fast as you can without ripping it, because when it rips it loses efficiency. Most props gain 10% or so in RPM over ground RPM numbers when they unload.

Say, your 28" prop rips at 7000 rpm. . you'd want a ground rpm of about 63-6400 for max efficiency in the air. the same holds true for smaller props. . if a 23" prop rips at 8500 rpm, you'd want a ground rpm of about 7600 for best efficiency. If your engine can turn the prop at those speeds. . great. If not, you need to prop it accordingly for best efficiency.

Now, as for engine Torque? All torque is, is the amount of twisting force the engine can exert. Horsepower is the amount of twisting force, correlated against the rpm at which that force occurs. There is a crossover point at 5250 rpm where torque and horsepower are exactly the same number. People in RC constantly misuse the term "Torque" because they have absolutely no idea what they are really trying to say.

When someone says "Torque". .ask. . "At what rpm?" 1 lbs-ft of torque at 10,500 rpm is about 2 hP. . 1 lbs-ft of torque at 1000 rpm is about 2/10 hP. They are both one lbs-ft of torque, but at 10,000 rpm you are doing 10 times the amount of work.

Now. . would you rather turn a 30" prop 1,000 rpm. . or a 15" prop at 10,000 rpm? both are being driven by 1 lbs-ft of torque, but the 15" prop will haul a 7 lbs plane straight up at 30 mph, while the 30" prop is only good for going 5 mph straight and level on a very low drag airframe.

So, "torque". . forget about it. Just prop the engine to turn about 7500 or so, in the case of a 50cc class engine, and then adjust the characteristics of your propellor (by changing props) until it flies the plane the way you want it to. And, leave all this "Torque talk" to the 3W guys. They're the ones who got everyone using the terminology incorrectly, and out of its proper context, in the first place.

This maybe a stupid question, But, What is prop ripping?
The prop tips going super sonic?

Originally Posted by 2robinhood

This maybe a stupid question, But, What is prop ripping?
The prop tips going super sonic?

It's almost that. ACtually it's the prop cavitating and ripping a hole in the air, and then the air collapses into this hole, making noise. Looking at your Fuji, I'd follow the original recommendations about prop size and type, to start with, then experiment from there. It's really hard to rip a 20" prop with a 43cc gas engine.

not only the 3W guys, but also BME and Mintor guys will tell you what kind of stump pullers those engines are.

So, "torque". . forget about it. Just prop the engine to turn about 7500 or so, in the case of a 50cc class engine, and then adjust the characteristics of your propellor (by changing props) until it flies the plane the way you want it to. And, leave all this "Torque talk" to the 3W guys. They're the ones who got everyone using the terminology incorrectly, and out of its proper context, in the first place.[/QUOTE]

some engines have more torque than others, it is not a confusing argument nor a 3W monopoly neither, it is real. You will be able to feel it after trying several engines. People that are stick to only one engine producer ( the one with low compresion) will tell you that is crap. But it is not, actually: it is very noticiable for the experienced pilot. Just my humble opinion, really dont want to start nothing. Peace.

3W engines certainly have more compression, and with the proper prop and tuning, will pull with more force than your comparable motor build in Tucson. That also means more heat is produced and tuning is very critical.

I've had both brands, all the way from 50cc to 170cc, the 3W's will fly a heavy plane a lot better, and the others are the most reliable, dependable motors on the planet..... for me.

Your mileage may vary....

Thread start date 12-11-2007
.oO(wonder if he's still looking for the answer)

Aren't we all?