RPM Adjustment

[A64Pilot]

5 hours ago, PT20J said:

horsepower = torque/5252 x rpm  (5252 is a conversion factor for English units of measurement in hp, lb-ft, rpm)

If the BMEP (torque) is constant, then horsepower varies linearly as rpm. 

I’m familiar with the HP formula, and every single dyno chart that’s not been doctored will have HP and Tq converging and crossing at 5252 RPM. On motors that exceed that RPM it’s one way to see if dyno chart has been doctored, quite a few have

But here is the thing, most engines begin to lose torque as RPM increases, eventually reaching a point where HP decreases with higher RPM cause the tq falls off so quickly. 

Are our motors near that? I don’t think so, and I base that on helicopter motors that often turn much higher RPM than we do in order to get more HP, an airplane’s RPM is often a noise or prop efficiency limit. A j model with its 74” prop standard day turning 2700 RPM prop’s tip speed is only .74 Mach, for comparison a Thrush T-34 with a 106” prop turning 2200 is .90 Mach, Noisy yes, but we have a lot of room RPM wise on our prop, Thrush not so much. Note I don’t know our hubs centripetal force limit which is often what determines a props structural RPM limit.

So long story short I theorize our engines have more power in them with increasing RPM, that we aren’t on the slope of torque reduction, and the prop isn’t anywhere near its Mach efficiency limit either

Pure irrelevant speculation because TCDS limits us to 2700 RPM. But fun to speculate.

[takair]

4 hours ago, A64Pilot said:

I’m familiar with the HP formula, and every single dyno chart that’s not been doctored will have HP and Tq converging and crossing at 5252 RPM. On motors that exceed that RPM it’s one way to see if dyno chart has been doctored, quite a few have

But here is the thing, most engines begin to lose torque as RPM increases, eventually reaching a point where HP decreases with higher RPM cause the tq falls off so quickly. 

Are our motors near that? I don’t think so, and I base that on helicopter motors that often turn much higher RPM than we do in order to get more HP, an airplane’s RPM is often a noise or prop efficiency limit. A j model with its 74” prop standard day turning 2700 RPM prop’s tip speed is only .74 Mach, for comparison a Thrush T-34 with a 106” prop turning 2200 is .90 Mach, Noisy yes, but we have a lot of room RPM wise on our prop, Thrush not so much. Note I don’t know our hubs centripetal force limit which is often what determines a props structural RPM limit.

So long story short I theorize our engines have more power in them with increasing RPM, that we aren’t on the slope of torque reduction, and the prop isn’t anywhere near its Mach efficiency limit either

Pure irrelevant speculation because TCDS limits us to 2700 RPM. But fun to speculate.

I think the RPMs they run at Reno validate the idea that there is more HP to be had, but the TBO…..if they make it there….might be as low as a race weekend.  Also, the accessories are also a limiting factor, nylon gears in mags, vacuum pump vanes, alternators.  I’m also not sure how the harmonic balancers and prop harmonics respond to RPM outside of their design limits?

[A64Pilot]

9 hours ago, takair said:

I think the RPMs they run at Reno validate the idea that there is more HP to be had, but the TBO…..if they make it there….might be as low as a race weekend.  Also, the accessories are also a limiting factor, nylon gears in mags, vacuum pump vanes, alternators.  I’m also not sure how the harmonic balancers and prop harmonics respond to RPM outside of their design limits?

No idea, if I were to do such a thing and I won’t as I believe there is no upside, but I’d stay below the helicopter RPM limits. But 50 RPM I wouldn’t get upset with, I’d just turn the prop out 1/2 turn or whatever gave me full RPM.

I have no idea about Reno, never been, one guy here crews for a Formula 1 plane, if I have the name correct, it uses an O-200 engine. I assume they have dyno’d them and know the RPM where power is max, I know they have several props and may run different ones from one day to the next.

One day I ought to go to Reno

[Pinecone]

14 hours ago, A64Pilot said:

I’m familiar with the HP formula, and every single dyno chart that’s not been doctored will have HP and Tq converging and crossing at 5252 RPM. On motors that exceed that RPM it’s one way to see if dyno chart has been doctored, quite a few have

But here is the thing, most engines begin to lose torque as RPM increases, eventually reaching a point where HP decreases with higher RPM cause the tq falls off so quickly. 

Are our motors near that? I don’t think so, and I base that on helicopter motors that often turn much higher RPM than we do in order to get more HP, an airplane’s RPM is often a noise or prop efficiency limit. A j model with its 74” prop standard day turning 2700 RPM prop’s tip speed is only .74 Mach, for comparison a Thrush T-34 with a 106” prop turning 2200 is .90 Mach, Noisy yes, but we have a lot of room RPM wise on our prop, Thrush not so much. Note I don’t know our hubs centripetal force limit which is often what determines a props structural RPM limit.

So long story short I theorize our engines have more power in them with increasing RPM, that we aren’t on the slope of torque reduction, and the prop isn’t anywhere near its Mach efficiency limit either

Pure irrelevant speculation because TCDS limits us to 2700 RPM. But fun to speculate.

Agreed

My gut feeling is, at the low RPM of aircraft engines, we will not see significant torque reductions with reasonable RPM increases.  I know the serious aerobatic people run their engines a very high (for aircraft) RPM.  3000 - 3400

Take a Lycoming 180 HP (at 2700 rpm) and run it at 3400 RPM and you are around 226 HP (assume no torque reduction).

Yes, a lot of TCDS RPM limits are to limit HP.  My TSIO-360-SB is limited to 2600 RPM.  The -MB is 2700 RPM, and the engines are identical internally.  And same prop.

[PT20J]

The horsepower formula is simply a definition. In physics terms, the energy released in the cylinders does work on the pistons which the crankshaft converts to rotary motion. Work done by a rotating machine is torque. Work per unit time is power. 

As the torque of an Internal combustion engine varies with rpm, so must power. However, over a small range of rpm, it is a reasonable assumption that torque may be considered constant and thus power varies as rpm.

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[Pinecone]

Power varies with torque and RPM.  By the definition of HP.

Torque is affected by the amount of air the engine can get into the cylinders at the given RPM.  One reason torque falls off as RPM increases.

The torque peak is generally the point of the lowest BSFC.  That is, the least amount of fuel burned per HP generated.

And some engines do no vary torque by much over the entire RPM range.

 

 

[jaylw314]

6 minutes ago, Pinecone said:

Power varies with torque and RPM.  By the definition of HP.

Torque is affected by the amount of air the engine can get into the cylinders at the given RPM.  One reason torque falls off as RPM increases.

The torque peak is generally the point of the lowest BSFC.  That is, the least amount of fuel burned per HP generated.

And some engines do no vary torque by much over the entire RPM range.

Power varies directly with RPM when rich of peak.  Power is entirely limited by the amount of fuel that can be burned per second, and that, in turn, is limited by airflow, which is directly proportional to RPM.

So, in an ideal motor that turns fuel into power, double the RPM, double the power.

Torque is proportional to Power divided by speed.  IOTW, in that same ideal motor, the torque should be a constant and unchanging throughout the RPM range.

Torque is not flat in real life simply because efficiency drops as RPM's increase.  In addition, typical dyno testing to measure peak torque and power curves is usually dynamic, with acceleration and deceleration, so it's not quite the same concept

[AH-1 Cobra Pilot]

2 hours ago, PT20J said:

Work done by a rotating machine is torque. Work per unit time is power.

In order to avoid confusion:  This is not correct, even though the units may be the same.  Work is a force applied over a distance, while torque is a force applied at a distance.  A force/torque can also be applied over a rotational distance to yield work.

Also, I think you might try to compare the torque/rpm curves of an O-300 engine with a GO-300 engine.  The GO-300 will turn much faster to deliver a similar prop rpm, so the chart may indicate more rapid torque decline towards the higher rpm.

Edited October 6, 2022 by AH-1 Cobra Pilot

[Pinecone]

2 hours ago, jaylw314 said:

  In addition, typical dyno testing to measure peak torque and power curves is usually dynamic, with acceleration and deceleration, so it's not quite the same concept

 

A real dyno measures torque at a static RPM.  But typically you need to have the engine out and and mounted to the dyno.  A typical one basically has the engine drive a pump and restricting the outflow to provide the load.

If you want to dyno you car, they use a chassis dyno, that current ones use acceleration of a known weight cylinder.   Using F=MA, then converting the force to a torque taking into account rolling radius of the wheel/tire, and gear ratios.

[jaylw314]

1 hour ago, Pinecone said:

A real dyno measures torque at a static RPM.  But typically you need to have the engine out and and mounted to the dyno.  A typical one basically has the engine drive a pump and restricting the outflow to provide the load.

If you want to dyno you car, they use a chassis dyno, that current ones use acceleration of a known weight cylinder.   Using F=MA, then converting the force to a torque taking into account rolling radius of the wheel/tire, and gear ratios.

Neat, I've never seen a real dyno then, obviously  I figured they must exist, though!

[Pinecone]

https://en.wikipedia.org/wiki/Dynamometer

Scroll down to real engine dynos.

 

https://powertestdyno.com/products/engine-dynamometers/

https://superflow.com/products/engine-dynamometers/

 

 

 

[PT20J]

7 hours ago, AH-1 Cobra Pilot said:

In order to avoid confusion:  This is not correct, even though the units may be the same.  Work is a force applied over a distance, while torque is a force applied at a distance.  A force/torque can also be applied over a rotational distance to yield work.

This is, of course, correct. I oversimplified trying to make a point. In order to convert torque to work, the torque must be applied through an angle of rotation. The dimensionless term 2pi rad/rev is included in the 5252 multiplier in the earlier cited equation.