We like HP but LOVE torque

[N941WR]

Torque is what really makes our planes fly but for some reason we always talk in terms of HP.

Using the formula Torque = (HP*5252)/RPM, I calculated the following for the most popular Lycoming engines:

Engine ....HP.......RPM.......FT-LBS
O-235.....108........2700........210
O-290.....135........2600........273
O-320.....150........2700........292
O-320.....160........2700........311
O-340.....170........2700........331
O-360.....180........2700........350
IO-360....200........2700........389
IO-390....215........2700........418
IO-540....260........2700........506

[hevansrv7a]

Technically correct but since RPM was all the same, they will be in exact proportion to HP. Anyhow, it is HP and not torque that makes the airplane go. Torque is a force (foot-pounds) and HP is "the ability to do work" (33,000 foot-pounds per minute). You could have an engine with 1 ft-lb of torque, a really good PSRU and tons of RPM's and you'd still get the HP and thus you could still fly as fast. Conversely, you could have twice the torque and half the revs and get the same result - think monster marine diesels for example. Lastly, a jet engine's torque is not relevant, but its (shaft) HP is very relevant.

[ericwolf]

Originally Posted by hevansrv7a

Technically correct but since RPM was all the same, they will be in exact proportion to HP. Anyhow, it is HP and not torque that makes the airplane go. Torque is a force (foot-pounds) and HP is "the ability to do work" (33,000 foot-pounds per minute). You could have an engine with 1 ft-lb of torque, a really good PSRU and tons of RPM's and you'd still get the HP and thus you could still fly as fast. Conversely, you could have twice the torque and half the revs and get the same result - think monster marine diesels for example. Lastly, a jet engine's torque is not relevant, but its (shaft) HP is very relevant.

Exactly. HP is a unit of power, or force applied over a unit of time. Torque is not an independent and elusive property as some people seem to treat it.

[RVbySDI]

Originally Posted by ericwolf

Exactly. HP is a unit of power, or force applied over a unit of time. Torque is not an independent and elusive property as some people seem to treat it.

Well, fellas this is an interesting discussion about HP and torque that has always been intriguing to me.

I am not an engineer so my concepts/questions are not based on any formal studies of power and motion but I do have some thoughts on all this. Of course anyone who wants to set me straight on any of my thinking that may be in error please feel free to do so.

I have always considered torque as the measure of ability of a spinning object to exert force, or work. In the case of a spinning flywheel on an internal combustion engine (ICE) the torque will be the amount of force the flywheel can produce. So my question is this. If torque is not as important as HP then why should we care whether we load down an engine or not?

I say we have to pay attention to torque because there is a limit to how much force the xxx HP rated engine can produce if the spinning flywheel meets up with an amount of force equal to or greater than the spinning flywheel's ability to "FORCE" movement (torque). If there is enough resistive power against the spinning force (torque) of the engine the engine will no longer be able to produce movement. It will load up and if resistive force continues to exceed the engine's ability to overcome that force the engine will stop completely, sometimes with disastrous results. This "unit of force" is the important part of the HP equation that we have to pay attention to when evaluating our engine's ability to provide "moving force" for our airplanes, or for that matter, anything that we wish to move with that engine.

You can experiment with this principal by using a small house fan. The electric motor has xxx amount of HP that allows it to produce xxx amount of torque (An interesting side note here is that electric motor HP numbers do not equate to the same when compared to ICE HP. One of the primary reasons for this is that electric motors can produce a higher amount of torque with less RPM than can an ICE.). This HP (and subsequent torque) allows the fan to spin the fan blades. With no load on the motor, it will spin away happily at a high RPM. However, if you take your hand and hold one of the fan blades then turn on the motor (Be careful if you actually try this. No need for injuries just to prove a point.) your resistance to the spinning blades will load up the motor. This action is now preventing that electric motor from producing work. The motor does not have enough "POWER" to overcome the resistive force of holding the fan blade. It lacks the proper amount of TORQUE to produce work. Regardless of how fast the motor can rev up, if it does not have enough torque to overcome the resistive force it will no longer produce measurable work.

Now that is my understanding of HP and torque. If I am totally off base feel free to set me straight.

[rv7boy]

My real world example is the modification I made to my little 50cc Honda C110 motorcycle in 1963. I had the Honda dealer install a 1/2" stroker kit, which was essentially an offset crank pin.

The result was a little Honda that would accelerate rapidly with lots of low RPM torque, especially when pulling hills, but the downside was that it didn't rev as high. Top speed suffered even with different drive sprocket combinations. I became unhappy with the stroked engine and actually had the Honda dealer put it back to stock configuration.

What does this have to do with our Lycomings? They are designed with long strokes compared to the cylinder bores and thus respond very well to RPM changes. They've often been called Tractor Engines, but the engineers who designed Lycomings and Continental aircraft engines chose the bore and stroke for much the same reasons as the engineers who designed tractor engines. (As an aside, I believe Lycoming in the early years provided air cooled engines to the tractor manufacturers but I don't have a source to quote here.) Aircraft engines are designed for a narrow RPM range within which they are expected to respond to load changes. That's why both the Torque and HP curves are important when considering any engine's performance.

That's my real-life experience and my paradigm based on it. You may disagree, but "that's the way I see it."

[RV6_flyer]

Where did this formula for torque come from?

It does not match anything that I have ever seen.

http://en.wikipedia.org/wiki/Torque

[N941WR]

Originally Posted by RV6_flyer

Where did this formula for torque come from?

It does not match anything that I have ever seen.

http://en.wikipedia.org/wiki/Torque

The formula is on the wikipedia page you referenced, all you have to do is scroll down.

Power AKA HP = (Torque x RPM)/5252

Thus...

Torque = (HP x 5252)/RPM

[pczar3]

Power = work / time and work = force * distance. An amusing thing about the definition is if there is no movement there is no work and therefore no power. Torque = force * distance (sound familiar?) What really comes into play is the horsepower and torque "curves". No engine has a flat horsepower or torque output. I should add that power is approximately (torque * rpm)/5252
Nice thread, it let me get back to my physics roots.
Paul

[ericwolf]

Originally Posted by RVbySDI

If torque is not as important as HP then why should we care whether we load down an engine or not?

I guess that it depends on what you mean by loading down an engine. As you apply more load to an engine at full throttle, the RPM will decrease and you will not be able to produce as much power. An example being in too high of a gear in a car or a cruise fixed pitch prop during takeoff.

The IO-360 on my -8A produced 180HP at 2700RPM, which means that the torque at this condition is 350 ft-lbs. It is not an independent property. There is no way that the torque could be anything other than that in this condition. My car is rated at 170HP at ~5500 RPM (don't remember exact RPM). The torque produced at this condition is 162 ft-lbs. Notice that to get to the same horsepower, my car engine had to spin up to twice the RPM as my IO-360. Why? It only has 153 cubic inches of displacement vs. the 360 ci of the IO-360 (and some other design factors).

[Ted RV8]

So now I'll jump in.

Without torque there is no horsepower.

HP = rpm x T(torque)/5252(constant)


Torque is a raw power measurement and horsepower is how far you can carry that power.

Horsepower is what most people want to hear. Really in aircraft I agree that torque is a better indicator.

OK now I'll really step out here! This is why most of your auto conversions don't match the power of standard aircraft engines and need to run a reduction unit to get their rated horsepower at a propeller speed. Once this is done the typically are burning more fuel.

As to the Honda 50 post. If you increased the stroke by .500 inch you probably ran out of volumetric efficiancy of the cylinder head. The engine couldn't achieve the same high RPM it was capable of before the increased stroke. It no longer had the breathing capacity for the increased displacement. This was probably a good thing as your piston speed was so high from that much of a stroke increase. The engine would have self distructed before you reached your old rev limit when it was a 50cc.

Let the flogging begin.

Ted