Right result, wrong reason. An IC engine creates a power pulse every time it fires while an electric motor creates its power smoothly. That's why the props for IC engines have thicker hubs. Keeps'em from flexing and destroying themselves.

Ounces? You'd have to get a pretty big prop to say that, but the idea is sound.

Errr, no. Props designed for electric motors, which create more torque than 2c IC, are better for electric motors. You just got through saying the IC props weigh more and now you advocate using them? In the same fashion, 4c engines also create more torque than their 2c brothers, so wider, thinner blades are more efficient. Take a close look at the electric slow flyer props and you can see the extreme of this. I don't about "Rev-ups", but Master Airscrew and other major manufacturers still have the dedicated 4c props.

The power and torque needed to turn a prop is exponential to rpm. The torque needed to turn a prop is proportional to the square of the rpm and the power needed to turn a prop is proportional to the cube of the rpm. Thus, to double the rpm of a prop, you need to quadruple the torque delivered to it. Doubling the rpm of a prop also quadruples the thrust BTW. Four times the torque at twice the rpm is eight times the power.
A .15 glow engine will spin a 8x4 about 13,000 to 14,000 rpm. That exact same prop will turn about 7,000 to 8,000 rpm when bolted onto a Cox .049 Texaco glow engine!! An engine with only about 1/3 the displacement.

Not understanding the exponentional nature of prop load has lead to the misconception that 4-strokes have more torque than 2-stroke engines. It takes a lot more torque to turn a 10x6 14,000 rpm than it does to turn a 12x6 8,000 rpm.

I do a lot of dealing with electric motors and believe me, fans smoke more electric motors than nearly any other application because someone decided to change pulley sizes without understanding the exponentional nature of the load.

Good B.L.E.

B.L.E., I don't accept your premise. 2 cycle, 4 cycle, and electric motors do not have the same power curve. It's simply not true. They work differently. There's plenty of real world analogies, but you can find them easily enough if you want to.

Hello!
B.L.E You are correct....some modelers still seem to think 4 -strokes puts out more torque just because they turn less rpm than a equal size 2-strokes do...puzzles me.
Why electric props don't have that fat hub is like ordinary props do is because they don't have to deal with the power pulses that a ignition /glow engine puts out.

This as usual has got way off the trail ,but it does make for some interesting reading .Now i'm told that a 2 stroke engine has more torque than a 4 stroke .I'm sorry but I dont buy that for a sec .If that was the case they would be able to swing as big or bigger props than the 4's can in the same size of engine and turn the same RPM's . Agreed in some cases there close but from what I have seen the 4 will still turn larger props without overloading the engine .

In smaller engines, size-for-size, a 2-stroke will outswing a 4-stroke. For example, a 2-stroke 46 will easily out swing a 4-stroke 46.

When the sizes more-or-less reach ~ 1 cu in, the 4-strokes are about on par, & as the sizes get larger, they begin to have an advantage.

2-strokes are not generally used like that, & don't make their peak power when over-propped, but they can certainly do it if appropriate attention is paid to cooling, lubrication & mixture.

I have a TT 42 GP on a 1/7 scale Bucker Jungmeister, swinging a 12-5 APC @ 11,100 RPM. It's only making ~ 0.6 HP when used that way, but it is right in there with any 40 4-stroke that I am familiar with & it's not even a high-output 2-stroke. A TT 40 Pro will turn the same prop at ~12,700 RPM -- show me a 40 4-stroke that can match that.

4 stroke engines do indeed often have more torque if we compare HP to HP. Power is a function of torque times RPM. That relation is linear. Take two engines that both make 1HP, one is a 2S the other a 4S. Most likely the 2S will make it’s 1HP at a higher RPM than the 4S. Then is should be obvious by inspection that the 4S makes more torque. if it makes the same power, but at a lower RPM. In practice, this is what generally happens. It is this higher torque at lower RPM that can effect prop design. A 4S, because of the higher torque, lower RPM, can turn a high load prop. On the flip side, a 2S with lower torque, but high RPM, prefers a lighter loaded prop. Yet they both can produce the same power.

The comparison of 2S to 4S based solely on displacement is a good “text book” exercise, but really doesn’t apply much in real life. Displacement isn’t important when looking at selecting a prop… torque and RPM are important.

There is some confusion on the nonlinear relation between power and prop RPM. There is also some confusion I think on what power and work is vs torque and RPM. Torque is a force. Power is a force over time which can do work. Ultimately, we are interested in work. It is basically true that you need 4 times the power to spin said prop twice as fast, but this in no way negates the fact that 4S engines tend to produce higher torque at lower RPMs compared to 2S engine. The examples by B.L.E are correct, but I respectfully submit that they don’t prove his conclusion on 4S vs 2S.

Britbrat, a 12-5 APC @ 11,100 RPM I’m showing in Thrust HP to be over 1HP, not the 0.6HP you mention. How did you compute the 0.6HP?

Cheers.

Sort of like a left-handed hammer.

How I did that is by making a mistake in reading my own test data -- I inadvertantly pulled the RPM data from an 11-6 APC on the GP 42 [] & for some reason I couldn't find the test data for the 12-5, so I quoted the output from my old test calcs for an APC 11-7, which has a very similar load to a 12-5, thus giving me ~0.6 HP.

But it gets even more confusing -- I just looked at the prop on the model -- it's an APC 12-4 -- which is why I didn't have data for a 12-5. Sooo, I checked the proper notes & it swings that 12-4 @ 10,500 RPM -- sorry for the confusion. [&:]

However, all is not lost, I double-checked the TT 40 Pro data and that does indeed turn the 12-5 @ 12,700 RPM.

Too many seniour moments these days.