Van's Air Force
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We like HP but LOVE torque
- Thread starter N941WR
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This is the exact operating point that you see with some "Motor Power" rating for home tools. The worst being vacuum cleaners. They make claims of up to 6 HP for a tool that plugs into 110 VAC outlet, absolute nonsense.
What they are doing is what you described. They are locking the rotor in a vice turning on power and measuring the current to the motor. They then use this to calculate "Power". The motor is doing No Work because it is not turning. However it is consuming power from the grid. It has become a big Heater, NOT a motor. In the case of the electric motor it can only Heat for a fraction of a second before the breaker goes.
You can get the same thing in a car with an automatic transmission. Hold the brakes on, or with a very heavy load the engine is turning but the car is not moving, so no useful work is happening. The transmission heats up very quickly and can be destroyed in short order if this continues.
You can have torque without doing useful work, as described before with the breaker bar. ( I prefer the term a fellow mechanic told me of, Torque Improver) However for a motor to produce power there has to be movement or rotation of the output shaft, otherwise energy may be consumed but no work is done.
Bob Parry
William Slaughter
Well Known Member
I've long thought that this unfortunate circumstance is the source of much confusion on this subject. If torque were routinely expressed in "twisterinos" rather than ft-lb or lb-ft, it's likely that far fewer people would be driving off into the intellectual ditch on this subject. The principles of the force/work/power hierarchy have been well defined for hundreds of years, yet here we are. Those still in doubt should stop by the library on the way home and pick up a basic physics textbook. As Dan succinctly put it, "Good grief!"
ergie63
Active Member
Force the Issue...
It takes a force to make something move. Horsepower is measure what that force accomplished. The classic example is a jet aircraft. engines roaring at full throttle, brakes locked and the aircraft is not moving means that there is zero horsepower (ignoring accessory PTOs). Once the brakes are released and the aircraft starts moving (doing work) then horsepower can be calculated. At that point, who cares?
Back to the propellers. The propeller is acting on air in the same manner as a jet engine. Since f = ma, and since for every action there is an equal and opposite reaction ma = ma. The propeller pushes on the air changing its velocity and the air pushes back on the propeller changing its velocity in equal proportion which we perceive as, and engineer to be, forward motion. The fan on a jet engine is a glorified (or enhanced, or specialized perhaps) propeller.
f = ma is the ultimate goal of the power source in our aircraft. It just so happens that our power source is a reciprocating engine for which it is easy to measure its power capability. Further, horsepower is a "bottom line" sort of number that makes comparisons between various engines easy. Replace a 160hp O-320 with an 180hp O-360 and you know that your aircraft will go faster.
Beware! The bottom line nature of the horsepower can get one into trouble. There are dozens of knock-on effects of any substitution, which is why Van's warns against the above example in RV-9s.
A force is a force of course of course,
And no one can work without a force,
Unless of course that force can move a course.
Go right to the source and get a force,
It'll help you get work done that you'll endorse,
Be always on a steady course,
Talk to Issac Newton!
It takes a force to make something move. Horsepower is measure what that force accomplished. The classic example is a jet aircraft. engines roaring at full throttle, brakes locked and the aircraft is not moving means that there is zero horsepower (ignoring accessory PTOs). Once the brakes are released and the aircraft starts moving (doing work) then horsepower can be calculated. At that point, who cares?
Back to the propellers. The propeller is acting on air in the same manner as a jet engine. Since f = ma, and since for every action there is an equal and opposite reaction ma = ma. The propeller pushes on the air changing its velocity and the air pushes back on the propeller changing its velocity in equal proportion which we perceive as, and engineer to be, forward motion. The fan on a jet engine is a glorified (or enhanced, or specialized perhaps) propeller.
f = ma is the ultimate goal of the power source in our aircraft. It just so happens that our power source is a reciprocating engine for which it is easy to measure its power capability. Further, horsepower is a "bottom line" sort of number that makes comparisons between various engines easy. Replace a 160hp O-320 with an 180hp O-360 and you know that your aircraft will go faster.
Beware! The bottom line nature of the horsepower can get one into trouble. There are dozens of knock-on effects of any substitution, which is why Van's warns against the above example in RV-9s.
A force is a force of course of course,
And no one can work without a force,
Unless of course that force can move a course.
Go right to the source and get a force,
It'll help you get work done that you'll endorse,
Be always on a steady course,
Talk to Issac Newton!
To further muddy the waters (like we need that at this point!
) this statement is only true if you are looking at the aircraft as a whole. The engine itself is indeed producing power - which can be measured as the volume of air that is accelerated rearward at a particular velocity. This is work by definition, produced by the engine. That work then produces a force by Newtonian reaction which is applied to the aircraft, and we call thrust. The aircraft will react to that force with an efficiency of 0% as long as the brakes are locked. The net FORCE created by the engines WORK does zero WORK on the aircraft as long as the brakes are locked, since no movement occurs. This is true of propellor-driven aircraft as well as jet or rocket power.My high school physics professor would have had a field day with this discussion...
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ergie63
Active Member
You're quite right... ...and the problem may be with your high school teacher. One classic way to teach the concept of work is to have a student lean against the wall...
"Are you doing work?" The teacher asks.
The student says, "Yes."
"No you're not. The wall didn't move. Try again." The teacher says. So the student replants his feet and leans harder into the wall.
"Are you doing work?" The teacher asks.
The student hesitates, "Uh, it uh feels like it."
"No you're not. The wall didn't move." The teacher says,
Then an unusually bright student like one of us pipes up, "Yes he did do work, because he moved his hands, feet and arms from one spot to another and his vocal cords vibrated when he spoke all of which is work."
"Yes, but it was not useful work." comes the retort, shifting the discussion from physics to philosophy and thus thoroughly confusing the average student.
* * *
I was trying to refute some of the earlier assertions that it takes horsepower to make something move. It does not, only a force can make movement. When the throttle is moved, what is adjusted is the amount of force generated by the new level of fuel/air consumed. Horsepower is just a calculation that gives us an idea of what we can achieve with that force. In fact, that why its horses in the first place. It was a sales pitch that a customer could relate to back when horses were common and mechanical engines not. Watt's wrong with that?
I'm just bummed nobody caught on to the theme from Mr. Ed (a horse with a whole lot of power on the show). Can we invite the performing arts teacher too?
"Are you doing work?" The teacher asks.
The student says, "Yes."
"No you're not. The wall didn't move. Try again." The teacher says. So the student replants his feet and leans harder into the wall.
"Are you doing work?" The teacher asks.
The student hesitates, "Uh, it uh feels like it."
"No you're not. The wall didn't move." The teacher says,
Then an unusually bright student like one of us pipes up, "Yes he did do work, because he moved his hands, feet and arms from one spot to another and his vocal cords vibrated when he spoke all of which is work."
"Yes, but it was not useful work." comes the retort, shifting the discussion from physics to philosophy and thus thoroughly confusing the average student.
* * *
I was trying to refute some of the earlier assertions that it takes horsepower to make something move. It does not, only a force can make movement. When the throttle is moved, what is adjusted is the amount of force generated by the new level of fuel/air consumed. Horsepower is just a calculation that gives us an idea of what we can achieve with that force. In fact, that why its horses in the first place. It was a sales pitch that a customer could relate to back when horses were common and mechanical engines not. Watt's wrong with that?
I'm just bummed nobody caught on to the theme from Mr. Ed (a horse with a whole lot of power on the show). Can we invite the performing arts teacher too?
skylor
Well Known Member
force vs. power...
Long time lurker and RV-8 builder here. I couldn't resist jumping into this discussion, especially after seeing the following incorrect assumption:
This is actually backwards, sort of...
Yes, force is one component required to make something move, but you can not propel something without power.
Repeat after me: "Force without power can not provide propulsion."
Force is a component of power, but power IS NOT a component of force. As several others have pointed out, it's possible to apply a lot force with our bare hands and arms (in the form of torque) with a long wrench, but we simply can't achieve much work within a small amount of time (power) this way, relative to an engine.
Here's an example that I like to use when discussing torque vs. power:
Imagine that we have a car on a level surface (a drag strip, if you like), except the engine has been removed. In place of the engine, we simply have a wrench attached to the input shaft of the transmission. Now we know that (as others have pointed out) we can apply a significant amount of torque (relative to an engine) with a reasonably long wrench, say 250 ft-lbs. But let's see how fast we can make that car move by turning the wrench. I bet two people turning the wrench together couldn't get a 3000 lb car to go 15 mph, while an automobile engine can do this quite easily while putting out less torque, too. What are we missing? Power! So, is force (torque) required to make something move? Yes. Is force the only thing required to make something move? No!
Force without power is useless!
Skylor
RV-8 Under Construction
(almost done!)
Long time lurker and RV-8 builder here. I couldn't resist jumping into this discussion, especially after seeing the following incorrect assumption:
This is actually backwards, sort of...
Yes, force is one component required to make something move, but you can not propel something without power.
Repeat after me: "Force without power can not provide propulsion."
Force is a component of power, but power IS NOT a component of force. As several others have pointed out, it's possible to apply a lot force with our bare hands and arms (in the form of torque) with a long wrench, but we simply can't achieve much work within a small amount of time (power) this way, relative to an engine.
Here's an example that I like to use when discussing torque vs. power:
Imagine that we have a car on a level surface (a drag strip, if you like), except the engine has been removed. In place of the engine, we simply have a wrench attached to the input shaft of the transmission. Now we know that (as others have pointed out) we can apply a significant amount of torque (relative to an engine) with a reasonably long wrench, say 250 ft-lbs. But let's see how fast we can make that car move by turning the wrench. I bet two people turning the wrench together couldn't get a 3000 lb car to go 15 mph, while an automobile engine can do this quite easily while putting out less torque, too. What are we missing? Power! So, is force (torque) required to make something move? Yes. Is force the only thing required to make something move? No!
Force without power is useless!
Skylor
RV-8 Under Construction
(almost done!)
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Mike S
Senior Curmudgeon
Nothing wrong, James.
Since the term horsepower is inclusive of the concept of force, this would seem to be a rather obtuse semantic argument.
I suppose we could rate engines by peak BMEP (a measurement of force). Imagine that diesel engine owners surprise when a high revving gas engine with a lower BMEP passes him on the straightaway. If only the diesel could generate that higher BMEP as often per minute as the gas engine.
Point being that horsepower is the more useful and descriptive metric of an engine's utility, although certainly not the only one.
Of course, the supreme metric is the graphical hp (or torque) curve. This describes everything you want to know, in one eyeful. The peak amount, the area under the curve, the rpm range, it's all there. In fact, it's so useful that an entire industry has sprung up just to facilitate generating this data.
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Garage Guy
Well Known Member
History lesson
So Aristotle thought: The natural state of things is to be at rest, and it takes a force to make them move. And Aristotle was very smart and so people believed him for a long time.
Newton thought something else: It is certainly possible for things to move without any forces acting on them, but only in a straight line and at constant speed. It takes a force to make something accelerate.
Then Einstein thought something else again, and I hope someday to have an RV that is fast enough so that makes a difference.
--Paul
So Aristotle thought: The natural state of things is to be at rest, and it takes a force to make them move. And Aristotle was very smart and so people believed him for a long time.
Newton thought something else: It is certainly possible for things to move without any forces acting on them, but only in a straight line and at constant speed. It takes a force to make something accelerate.
Then Einstein thought something else again, and I hope someday to have an RV that is fast enough so that makes a difference
.--Paul
rzbill
Well Known Member
rzbill
Well Known Member
Can't add the weight.
I can't stand the thought of relatavistic mass increase making my bird heavier.
A difference relative to what inertial reference frame?
I can't stand the thought of relatavistic mass increase making my bird heavier.
I can't stand the thought of relatavistic mass increase making my bird heavier.
On the plus side, just think of the increased energy density in the fuel mass!
n5lp
fugio ergo sum
Pretty good thread
When this thread raised its ugly head, once again, I shuddered. I won't do the search, but I remember the guy who's A&P told him about torque and gearing and how you just needed a small engine and the power could be multiplied by gearing. Think perpetual motion!
Really, I think this one turned out rather well. The informed people informed politely and I think lots of people learned things.
Horsepower matters!
When this thread raised its ugly head, once again, I shuddered. I won't do the search, but I remember the guy who's A&P told him about torque and gearing and how you just needed a small engine and the power could be multiplied by gearing. Think perpetual motion!
Really, I think this one turned out rather well. The informed people informed politely and I think lots of people learned things.
Horsepower matters!
With apologies to the Bard, "There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy.
To produce 90hp @ 4000rpm the engine is producing about 118 lb/ft of torque, @ 4000rpm. That is a very believable value, given that the engine is operating twice as fast as it's torque peak. It is typical for an IC engine's torque to fall off in the upper rpm range of the engine.
You didn't think that the 149 lb/ft of torque was maintained forever, did you?
A major goal of IC engine design is to have the torque peak as early as possible in the rpm band, and maintain that torque as late as possible. For several reasons related to simple physics, this is a difficult task.
Thanks everyone for proving that the formula is not correct. The formula may be correct for one and only one RPM as you point out but cannot be used for all RPM and HP values.
skylor
Well Known Member
Horsepower Formula
Gary,
Exactly what leads you to believe the horsepower formula is incorrect? You're disagreeing with an awful lot of engineering textbooks and theory with that statement and I'm trying to understand exactly what leads you to that conclusion.
Automobile manuals typically give two values for engine performance; Maximum horsepower and maximum torque. These two peak values typically occur at different engine speeds, which is the reason you see peak torque specified at one engine speed and peak horsepower specified at another. Please note that in the examples you gave the engine speed is always specified for the given torque or horsepower value. That is the speed that each of those values occur, and nowhere else. If you can find somewhere a plot or table of horsepower and torque vs. engine speed, the formula horsepower = (torque*rpm)/5252 will hold true at any given engine speed. Incidentally, you will also find that horsepower and torque are ALWAYS equal at 5252 RPM (the plots cross each other at that point), at least for engines capable of running that speed. Stated another way, horsepower and torque at any specific speed (RPM) are directly related by the above formula, but if you compare horsepower at one speed to torque at another the formula will not work (and of course if it did, it would violate the definition of a mathmatical function).
Skylor
Gary,
Exactly what leads you to believe the horsepower formula is incorrect? You're disagreeing with an awful lot of engineering textbooks and theory with that statement and I'm trying to understand exactly what leads you to that conclusion.
Automobile manuals typically give two values for engine performance; Maximum horsepower and maximum torque. These two peak values typically occur at different engine speeds, which is the reason you see peak torque specified at one engine speed and peak horsepower specified at another. Please note that in the examples you gave the engine speed is always specified for the given torque or horsepower value. That is the speed that each of those values occur, and nowhere else. If you can find somewhere a plot or table of horsepower and torque vs. engine speed, the formula horsepower = (torque*rpm)/5252 will hold true at any given engine speed. Incidentally, you will also find that horsepower and torque are ALWAYS equal at 5252 RPM (the plots cross each other at that point), at least for engines capable of running that speed. Stated another way, horsepower and torque at any specific speed (RPM) are directly related by the above formula, but if you compare horsepower at one speed to torque at another the formula will not work (and of course if it did, it would violate the definition of a mathmatical function).
Skylor
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Dave Cole
Well Known Member
Excellent summary
Ted, I think you have perfectly summarized this entire thread.
With apologies to Charles Babbage: "I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a statement".
Ted, I think you have perfectly summarized this entire thread.
Exactly correct. Note that when you see an engine advertisement exclaiming torque and horsepower ratings, you will always see the rpm for that particular claimed value - and you will NEVER see the other value for that same rpm. For example - they will claim peak torque is XXX ft-lbs at 1900 rpm, but will not state the horsepower produced at that rpm. Likewise, peak horsepower will be stated flashily as "XXX HP (at 4200 rpm)!" but no torque value will be given for that rpm. This is marketing at it's best - giving the best of the values that people have been trained to look for, while ignoring the rest of the picture. The only meaningful measurement of an engines performance against another engine would come from an examination of the entire torque-rpm curve, and the BSFC-rpm curve. All else is flash and glitter for the marketing folks.
This would be similar to saying "My airplane can pull 6 G's!", "My airplane can haul 780 pounds of useful load!" and "My airplane can stall at 42mph!" All those statements may be valid, but they will not occur at the same time - gauranteed. That's marketing.
Torque and power are inextricably tied to each other - and the horsepower/torque formula is that tie that binds. It is correct, and has been in use for over a hundred years by an awful lot of PhD's - if there was a flaw in a formula as basic as that, it would have been discovered and corrected long since.
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rv7boy
Forum Peruser
I was going to post my thoughts about my previous post being misunderstood (the torque on my Honda 50 with the stroker kit was noticeably improved and different...and my decision to go back to stock wasn't due to lack of horsepower due to "volumetric efficiency being reduced" because it had plenty of power...and how sometimes feeling a machine operate is so much more educational than all the equations in the world)...but decided instead to politely bow out of this discussion by saying...
May the FORCE (or TORQUE) be with you!!!
Don
BSME, MSME, PE (I know what volumetric efficiency of an IC engine means so don't try to explain it to me! )
May the FORCE (or TORQUE) be with you!!!
Don
BSME, MSME, PE (I know what volumetric efficiency of an IC engine means so don't try to explain it to me!
)
ergie63
Active Member
Correct! "Make something move" is colloquial. Acceleration upon the example's brake release was implied. In practice if you don't continuously apply a force to replace energy dissipated, say from drag, you will feel an acceleration as you slow down. Which is why it took so long before someone like Newton realized that forces external to objects slow them down as opposed to slowing down being an intrinsic property of objects. We fly at constant speed so long as the force known as thrust is equal to the force known as drag. Power, within a relevant range of values, gives us an idea of what that speed might be in our aircraft.
Garage Guy
Well Known Member
example hp and torque curves
Just trying to help out here...!
--Paul
http://www.team-integra.net/sections/articles/showArticle.asp?ArticleID=467
Just trying to help out here...!
--Paul
Darylat8750ft
Member
Ross mentioned his motorcycle and trucks. Where the trucks and tractors are designed to run normal rpm above torque peak so as they are "loaded up" they have more torque, racing motorcycles are designed to operate above torque peak so when you spin your drive wheel exiting a corner the torque decreases as the wheel speeds up to help you regain traction. (Hopefully before you test your leathers. )
I have spent hours leaning on a wall, applying force, but accomplishing no work.
Daryl
)I have spent hours leaning on a wall, applying force, but accomplishing no work.
Daryl
Any torque you want!
An O-200 developing 100 HP at 2750 rpm has 191 lb-ft / torque, right? So if I take a 1/4 HP motor that develops 0.75 lb-ft of torque at 1750 rpm, and gear it down 255:1, I'll get 191 lb-ft of torque! See! HP doesn't matter! Just have a long enough extension cord or a battery and we can solve the world's energy problems! Algore would be proud! 'Course, it would only turn the prop at 6.9 rpm, but at least the LE wouldn't deteriorate so much in the rain! There! How's that for thinking outside the box!
An O-200 developing 100 HP at 2750 rpm has 191 lb-ft / torque, right? So if I take a 1/4 HP motor that develops 0.75 lb-ft of torque at 1750 rpm, and gear it down 255:1, I'll get 191 lb-ft of torque! See! HP doesn't matter! Just have a long enough extension cord or a battery and we can solve the world's energy problems! Algore would be proud! 'Course, it would only turn the prop at 6.9 rpm, but at least the LE wouldn't deteriorate so much in the rain! There! How's that for thinking outside the box!
pierre smith
Well Known Member
My little Ford 4000 tractor....
...with only 35 horsepower pulls my 3800 lb. empty Air Tractor in and out of my hangar so easily with idle power, it's gonna make me think of you every time I jump on it, Paul..
Horsepower multiplication, I believe it is!
Best,
...with only 35 horsepower pulls my 3800 lb. empty Air Tractor in and out of my hangar so easily with idle power, it's gonna make me think of you every time I jump on it, Paul..
Horsepower multiplication, I believe it is!
Best,
LAMPSguy
Well Known Member
I'm not that great with numbers...
...so I think about it like this. torque=force. Force=mass * acceleration.
Horsepower is how fast you can go.
Torque is how quick you can get there.
They both have their place, depending on what you want out of a particular piece of gear.
...so I think about it like this. torque=force. Force=mass * acceleration.
Horsepower is how fast you can go.
Torque is how quick you can get there.
They both have their place, depending on what you want out of a particular piece of gear.
pierre smith
Well Known Member
FWIW...
...the PT-6 in my Air Tractor turns 37,500 RPM on takeoff, yup, thirty seven thousand, five hundred, but the prop is driven by a double planetary gearbox that reduces it by 17 times for a redline of 2200.
So the normally rated 680 SHP engine developes 1633 lbs of torque during all this.
Best,
...the PT-6 in my Air Tractor turns 37,500 RPM on takeoff, yup, thirty seven thousand, five hundred, but the prop is driven by a double planetary gearbox that reduces it by 17 times for a redline of 2200.
So the normally rated 680 SHP engine developes 1633 lbs of torque during all this.
Best,
Air Tractor engine
So Pierre! What you're saying is that your engine has a rating of 680 SHP at 37,500 rpm and 96 lb-ft of torque on its little turbine blade? That's only half of the O-200 torque, and yet you say it still gets that big Air Tractor off the ground? Will wonders never cease! You're basically saying that engine must have come to us from some alternate universe where our horsepower and torque laws don't apply! C'mon, Pierre! Quit pulling our legs! You've been spending too many late night hours listening to George Noory's-Art Bell's Coast-to-Coast-AM. But come to think of it, 37500X96/5252 or 2200X1633/5252 = 680! I knew it! There's magic in numbers. That 5252 comes from 550X60/(2Xpi), and everyone knows that the value "pi" is a magical number, and was first was discovered in the writings of Merlin the Magician! Now it all makes sense!
So Pierre! What you're saying is that your engine has a rating of 680 SHP at 37,500 rpm and 96 lb-ft of torque on its little turbine blade? That's only half of the O-200 torque, and yet you say it still gets that big Air Tractor off the ground? Will wonders never cease! You're basically saying that engine must have come to us from some alternate universe where our horsepower and torque laws don't apply! C'mon, Pierre! Quit pulling our legs! You've been spending too many late night hours listening to George Noory's-Art Bell's Coast-to-Coast-AM. But come to think of it, 37500X96/5252 or 2200X1633/5252 = 680! I knew it! There's magic in numbers. That 5252 comes from 550X60/(2Xpi), and everyone knows that the value "pi" is a magical number, and was first was discovered in the writings of Merlin the Magician! Now it all makes sense!
I came across this site when trying to find the definition of HP and Torque. It explains the concepts better than I could.
http://www.epi-eng.com/piston_engine_technology/power_and_torque.htm
But I would like to emphasize a point made in the article.
1. TORQUE is MEASURED. HORSEPOWER is CALCULATED. An engine dynamometer is a device for measuring the output of an engine. A dynamometer loads the engine with a brake which is restrained from rotating by a load cell. The load cell measures force (lbs), the moment arm distance is known and fixed (ft), and the rpm is read on another instrument. The output torque is Force x Distance (lb-ft). NOTE the units. Torque is NOT measured in ft-lbs in spite of colloquial usage. Work is measured in ft-lbs which may be the source of confusion, but if you keep the units straight the confusion is eliminated. Power is work per unit time. Horsepower is a unit of power, originally devised by James Watt to compare the power of his steam engines to the power of horses, and is defined as 33,000 ft-lbs/minute. So to create a graph of engine output, a dynamometer is used to measure the torque at wide open throttle at various RPM increments from idle to maximum rated rpm. Then the horsepower is calculated from the torque and rpm readings and plotted. Usually the graph is made with two vertical scales, one for torque the other for horsepower with a common horizontal scale for rpm. So HORSEPOWER is CALCULATED from MEASURED TORQUE and RPM.
Now in the case of our airplanes, what we need to make the airplane move is THRUST, or the force exerted by the propeller rotating through the air. A propeller has a power absorption curve that is proportional to the rpm^3. So a propeller that absorbs 200 hp at 2500 rpm would absorb the following at various rpms
RPM - Horsepower absorbed
0 - 0
500 - 2
1000 - 12
1500 - 43
2000 - 102
2500 - 200
Now we see that the propeller must be matched to the engine so that the propeller absorbs the full engine power at the rated rpm. Note how fast the absorption curve is rising. This is why the rpm doesn't increase much if you increase the engine power by some means.
Finally, the propeller, by absorbing the power of the engine and accelerating the air through which it is moving, converts the engine power to thrust (less efficiency losses) which acts on the crankshaft thrust bearing, through the crankcase, motor mount and into the airframe move the airplane through the air.
So for me, the answer to airplane performance is that engine horsepower is calculated from the torque measured on a dynamometer, and in order to convert the engine power to thrust it must be matched to a propeller that absorbs the engine's maximum power at rated rpms.
http://www.epi-eng.com/piston_engine_technology/power_and_torque.htm
But I would like to emphasize a point made in the article.
1. TORQUE is MEASURED. HORSEPOWER is CALCULATED. An engine dynamometer is a device for measuring the output of an engine. A dynamometer loads the engine with a brake which is restrained from rotating by a load cell. The load cell measures force (lbs), the moment arm distance is known and fixed (ft), and the rpm is read on another instrument. The output torque is Force x Distance (lb-ft). NOTE the units. Torque is NOT measured in ft-lbs in spite of colloquial usage. Work is measured in ft-lbs which may be the source of confusion, but if you keep the units straight the confusion is eliminated. Power is work per unit time. Horsepower is a unit of power, originally devised by James Watt to compare the power of his steam engines to the power of horses, and is defined as 33,000 ft-lbs/minute. So to create a graph of engine output, a dynamometer is used to measure the torque at wide open throttle at various RPM increments from idle to maximum rated rpm. Then the horsepower is calculated from the torque and rpm readings and plotted. Usually the graph is made with two vertical scales, one for torque the other for horsepower with a common horizontal scale for rpm. So HORSEPOWER is CALCULATED from MEASURED TORQUE and RPM.
Now in the case of our airplanes, what we need to make the airplane move is THRUST, or the force exerted by the propeller rotating through the air. A propeller has a power absorption curve that is proportional to the rpm^3. So a propeller that absorbs 200 hp at 2500 rpm would absorb the following at various rpms
RPM - Horsepower absorbed
0 - 0
500 - 2
1000 - 12
1500 - 43
2000 - 102
2500 - 200
Now we see that the propeller must be matched to the engine so that the propeller absorbs the full engine power at the rated rpm. Note how fast the absorption curve is rising. This is why the rpm doesn't increase much if you increase the engine power by some means.
Finally, the propeller, by absorbing the power of the engine and accelerating the air through which it is moving, converts the engine power to thrust (less efficiency losses) which acts on the crankshaft thrust bearing, through the crankcase, motor mount and into the airframe move the airplane through the air.
So for me, the answer to airplane performance is that engine horsepower is calculated from the torque measured on a dynamometer, and in order to convert the engine power to thrust it must be matched to a propeller that absorbs the engine's maximum power at rated rpms.
pierre smith
Well Known Member
Actually, Paul..
...the torque is measured against the outer planet gear in the gearbox. That gear has teeth at an angle that are housed in matching teeth in the case. As the torque increases, the gear rotates slightly, moving aft and pushes a piston, displacing oil and fed to the torque gauge in the cockpit, so you are in fact, reading the torque at that reduction planet gear, and not anywhere in the power section.
The "little" turbine wheel you mention is the Compressor turbine..CT wheel, and is 8" in diameter and bathed in flames all day long and I sat in it today for 7 hours and my % RPM was around 96% for the day..or 36,000 on that wheel....one reason it costs near $20,000 and the engine has 8900 TT and never majored!!
Best,
...the torque is measured against the outer planet gear in the gearbox. That gear has teeth at an angle that are housed in matching teeth in the case. As the torque increases, the gear rotates slightly, moving aft and pushes a piston, displacing oil and fed to the torque gauge in the cockpit, so you are in fact, reading the torque at that reduction planet gear, and not anywhere in the power section.
The "little" turbine wheel you mention is the Compressor turbine..CT wheel, and is 8" in diameter and bathed in flames all day long and I sat in it today for 7 hours and my % RPM was around 96% for the day..or 36,000 on that wheel....one reason it costs near $20,000
and the engine has 8900 TT and never majored!!Best,
10' arm
I'm going to attach a 10' arm to my crankshaft and pull on it with 100 lb of force. That that will give me 1000 lb-ft of torque, which is a lot more than my O-235's 234 lb-ft, over four times as much! 'Wonder if I'll be able to go four times as fast, or only cube-root of 1000/234 or 1.62 times as fast. 'Course, at my age, I may tire out quickly and not go very far!
I'm going to attach a 10' arm to my crankshaft and pull on it with 100 lb of force. That that will give me 1000 lb-ft of torque, which is a lot more than my O-235's 234 lb-ft, over four times as much! 'Wonder if I'll be able to go four times as fast, or only cube-root of 1000/234 or 1.62 times as fast. 'Course, at my age, I may tire out quickly and not go very far!
Thinking!
After much thought and consideration, I've come to the conclusion that torque and horsepower don't really matter a hill of bean's worth. It's rpm that really counts! When I get down low and turn up 2950 rpm I can coax 213 mph out of my beast. But the Reno biplanes go one better, and can easily hit 3500 rpm to give them 252 mph around the course, to say nothing of the IF1s who hit 4500rpm! But, at least on this forum, the winner hands down is Pierre Smith's Air Tractor whose engine turns 37,500 rpm! See! it's really rpm that tells the story. Without it, you go nowhere!
After much thought and consideration, I've come to the conclusion that torque and horsepower don't really matter a hill of bean's worth. It's rpm that really counts! When I get down low and turn up 2950 rpm I can coax 213 mph out of my beast. But the Reno biplanes go one better, and can easily hit 3500 rpm to give them 252 mph around the course, to say nothing of the IF1s who hit 4500rpm! But, at least on this forum, the winner hands down is Pierre Smith's Air Tractor whose engine turns 37,500 rpm! See! it's really rpm that tells the story. Without it, you go nowhere!
pierre smith
Well Known Member
Well, yes and no, Paul....
...because the RPMs serve a purpose. We're getting close to comparing apples to cumquats because the racers all have direct-drive engines and I have a gearbox. While the directly measured torque of my power producing engine component is small, the torque multiplication of the gearbox, as in my Ford 4000 tractor, then becomes serious. Think P-51 or Spitfire!
A good comparison would be a Peterbilt, hauling a 40,000 lb. load, uphill, on 300 HP. My kit-built 427 Shelby Cobra had around 475 HP but couldn't pull what the Peterbilt does. Not with the standard four speed gearbox, anyway.
Best...gee...this is fun
...because the RPMs serve a purpose. We're getting close to comparing apples to cumquats because the racers all have direct-drive engines and I have a gearbox. While the directly measured torque of my power producing engine component is small, the torque multiplication of the gearbox, as in my Ford 4000 tractor, then becomes serious. Think P-51 or Spitfire!
A good comparison would be a Peterbilt, hauling a 40,000 lb. load, uphill, on 300 HP. My kit-built 427 Shelby Cobra had around 475 HP but couldn't pull what the Peterbilt does. Not with the standard four speed gearbox, anyway.
Best...gee...this is fun
Thinking #2
In my previous post about rpm, I was trying to do what my mentor RL says about illustrating absurdity by being absurd. First off, what's wrong with HP being a calculated value? Think about it, torque = pounds of force times a distance in feet. Multiplication implies a calculation, non? So, too, rpm, revolutions per (divide) minute. Any quantity that contains two different terms is arrived at by some arithmetic process, a calculation. Now, of course, we have sensors and tools that use two different functions to indicate one of these multi-term quantities. For instance, a lever-type torque wrench is based on the deflection of a calibrated lever with a known compliance from a force applied at a particular distance. The mechanical tachometer and the mechanical speedometer, which really measures drive-line rpm, made use of a rotating magnet of known magnetic force inside a cup of highly conductive metal such as aluminium or copper to generate a magnetic field in it by an eddy current, which then tried to follow the rotation but was prevented from doing so by a calibrated spring, so that the deflection was proportional to the rate of rotation.
But if you tell me the horsepower of any engine I can give you the rpm-torque pair resulting from any gear-box attached to the engine. You cannot do that with just rpm or torque. By themselves they mean absolutely nothing, for as I illustrated in a previous post, I can get the same torque from an O-200 or a 1/4 horse electric motor. Give me the horsepower of an engine in a plane and the characteristics of the plane and propeller and I can tell you how fast the airplane will fly at any altitude. I'd like to see you do the same with just a torque number. Since horsepower equals force times distance over time, I dont need torque to give me horsepower; I can do the same lifting or pushing a load. If I raise 220 lb 2.5' in 1 sec I've generated 1 HP. See, no torque or rpm! If I push a 55 lb.sliding load across a floor at 10 ft/sec Ive generated 1 HP. Again, no torque, no rpm! How about this statement instead; rpm and torque are nice things to know, but I LOVE any horsepower I can get!
In my previous post about rpm, I was trying to do what my mentor RL says about illustrating absurdity by being absurd. First off, what's wrong with HP being a calculated value? Think about it, torque = pounds of force times a distance in feet. Multiplication implies a calculation, non? So, too, rpm, revolutions per (divide) minute. Any quantity that contains two different terms is arrived at by some arithmetic process, a calculation. Now, of course, we have sensors and tools that use two different functions to indicate one of these multi-term quantities. For instance, a lever-type torque wrench is based on the deflection of a calibrated lever with a known compliance from a force applied at a particular distance. The mechanical tachometer and the mechanical speedometer, which really measures drive-line rpm, made use of a rotating magnet of known magnetic force inside a cup of highly conductive metal such as aluminium or copper to generate a magnetic field in it by an eddy current, which then tried to follow the rotation but was prevented from doing so by a calibrated spring, so that the deflection was proportional to the rate of rotation.
But if you tell me the horsepower of any engine I can give you the rpm-torque pair resulting from any gear-box attached to the engine. You cannot do that with just rpm or torque. By themselves they mean absolutely nothing, for as I illustrated in a previous post, I can get the same torque from an O-200 or a 1/4 horse electric motor. Give me the horsepower of an engine in a plane and the characteristics of the plane and propeller and I can tell you how fast the airplane will fly at any altitude. I'd like to see you do the same with just a torque number. Since horsepower equals force times distance over time, I dont need torque to give me horsepower; I can do the same lifting or pushing a load. If I raise 220 lb 2.5' in 1 sec I've generated 1 HP. See, no torque or rpm! If I push a 55 lb.sliding load across a floor at 10 ft/sec Ive generated 1 HP. Again, no torque, no rpm! How about this statement instead; rpm and torque are nice things to know, but I LOVE any horsepower I can get!
Love horsepower too
Paul,
As I read this thread from the beginning, it seemed to me that several previous posters think torque and horsepower were different things, as if you had to choose between them. Especially the comments related to the "diesel truck up a mountain" justification of torque vs. horsepower. In my comments I was trying to show the relationship between the two, the fact that you can't have one without the other, and especially, when it comes to engines, the fact that the horsepower of the engine is not measured directly. So yes, you are right, we have to measure force, know the moment arm length and rpm to be able to plot a torque curve. And then we have to multiply those values by a constant to calculate the horspower.
And I hope the gearbox examples, especially Pierre's PT-6, make it clear that you can get any torque value you want out of any engine with the correct gear ratio. Of course the horsepower of the engine cannot be increased by the gearbox, in fact, it is decreased by the frictional losses (heat and noise) inside the gearbox.
Paul,
As I read this thread from the beginning, it seemed to me that several previous posters think torque and horsepower were different things, as if you had to choose between them. Especially the comments related to the "diesel truck up a mountain" justification of torque vs. horsepower. In my comments I was trying to show the relationship between the two, the fact that you can't have one without the other, and especially, when it comes to engines, the fact that the horsepower of the engine is not measured directly. So yes, you are right, we have to measure force, know the moment arm length and rpm to be able to plot a torque curve. And then we have to multiply those values by a constant to calculate the horspower.
And I hope the gearbox examples, especially Pierre's PT-6, make it clear that you can get any torque value you want out of any engine with the correct gear ratio. Of course the horsepower of the engine cannot be increased by the gearbox, in fact, it is decreased by the frictional losses (heat and noise) inside the gearbox.
L.Adamson
Well Known Member
Of course you can get the torque value with gear ratios. Yet the indicated road "speed" up that mountain road may vastly vary. When it comes to diesel trucks, rated torque does mean a lot.
And, as it's very apparent, torque must be the reason our Lycomings are getting off the runway faster than the Subies. The Subie just has to wind up to those high RPM's, while the Lyc doesn't. And the six cylinder Subie even has a higher horsepower rating.
L.Adamson --- RV6A
Nope
It's not apparent to me. Torque at rated rpm is the same at the prop hub on a 180hp Lycoming or 180hp Subaru/ Chevy/ Honda at the end of the gearbox. Transient response from idle to rated full prop rpm if you ram the throttle open will be pretty much the same. My Sube with just the 4 lb. flywheel and gearbox on would go from idle to 5000 rpm in less than .75 seconds, just like a bike- zing, zing.
I can easily be off the ground in under 10 seconds in my Subaru from a standing start without ramming the throttle open quickly. About the same as any Lycoming powered RV on YouTube.
You will notice that all aviation engines are rated by hp, not torque. As Paul states, hp encompasses rpm which is vitally important in any comparison involving work, like accelerating a vehicle or counteracting drag on an aircraft.
If torque was the deciding factor, Lycoming powered RVs would actually be faster than Randy Crothers' Subaru STI powered 7A or Robert Paisley's 7. (350 vs. 250 lb. ft.)
It's not apparent to me. Torque at rated rpm is the same at the prop hub on a 180hp Lycoming or 180hp Subaru/ Chevy/ Honda at the end of the gearbox. Transient response from idle to rated full prop rpm if you ram the throttle open will be pretty much the same. My Sube with just the 4 lb. flywheel and gearbox on would go from idle to 5000 rpm in less than .75 seconds, just like a bike- zing, zing.
I can easily be off the ground in under 10 seconds in my Subaru from a standing start without ramming the throttle open quickly. About the same as any Lycoming powered RV on YouTube.
You will notice that all aviation engines are rated by hp, not torque. As Paul states, hp encompasses rpm which is vitally important in any comparison involving work, like accelerating a vehicle or counteracting drag on an aircraft.
If torque was the deciding factor, Lycoming powered RVs would actually be faster than Randy Crothers' Subaru STI powered 7A or Robert Paisley's 7. (350 vs. 250 lb. ft.)
Peoplepower?
Let's forget engines for a moment. Consider instead one of those things they exercise horses with where they walk around in a circle turning what amounts to a big crank. Now suppose you tried it yourself and found that you could push it around with x pounds of force, at a linear speed of y mph. Your power output is proportional to xy (in whatever units you may like), but the torque is dependent on how far out from the axis you happen to be. If you were to move twice as far outward from the axis, the RPM would be cut in half, the torque would double, but your power output would stay the same. Of course this all assumes that the rig has some type of mechanism that soaks up whatever you dish out. Finally, as was pointed out earlier, the power output is the same whether you're going in circles or pushing the rig in a straight line.
Let's forget engines for a moment. Consider instead one of those things they exercise horses with where they walk around in a circle turning what amounts to a big crank. Now suppose you tried it yourself and found that you could push it around with x pounds of force, at a linear speed of y mph. Your power output is proportional to xy (in whatever units you may like), but the torque is dependent on how far out from the axis you happen to be. If you were to move twice as far outward from the axis, the RPM would be cut in half, the torque would double, but your power output would stay the same. Of course this all assumes that the rig has some type of mechanism that soaks up whatever you dish out. Finally, as was pointed out earlier, the power output is the same whether you're going in circles or pushing the rig in a straight line.
Yes, I think this one has been flogged until all that is left is the hoofs!
- Status
