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Horsepower/Torque

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Posted (edited)

Opinion question for the gearheads? In an aircraft engine situation, which is more important, horsepower or torque? Such as low horsepower/ high torque or high horsepower/low torque. And no they don't always run hand in hand. I have my opinion and theory but would like to hear others.  Been debated since the invention of the engine. Just off the top of your head, not the official calculations.

Edited by Allen Sutphin

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Posted (edited)

I'll give it a shot here:  First off you cant have one without the other since they are tied to each other but in my simple (and sometimes warped) way of thinking - It's either additional torque or additional rpm that "makes" additional HP.  One (or both) has to "be the multpilier" in the equation.

I think of torque as a big guy slowly (low rpm) powering someone over when armwrestling.  Torque makes the power... more torque wins.

Each engine is obviously different but typically when an engine begins to level off at peak torque then additional rpm can increase HP, even though torque is not changing much with more rpm.  You can see that in dyno charts and in the general equation:

HP= (Torque x RPM) /5252

I think of extra rpm making HP as beating a stronger guy (guy with more torque than you) in armwrestling by gaining speed and using that speed to beat him.  With the extra speed you may have less torque than him but you have more resultant HP (due to inertia/momentum).  Speed (rpm) makes more power and more speed wins.

Side notes:

The rpm side of the equation is where the little 1000cc 4cyl Yamaha gets its power (rpm makes HP).  It would be a pretty modest powerplant if it's rpm were more "normal".  

Boost (turbo) gives an engine more torque at a given rpm, and therefore more HP (torque makes HP). 

Also, most importantly our PSRUs primarily bring prop rpm in range, but also are torque converters...  trading RPM for a huge increase in torque.  So when measured at the prop, torque always is the bigger multiplier (over rpm) in the power equation for what we are doing, and I think that is why most people will say that "torque turns the prop".

 

Edited by Yamma-Fox
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Posted

Nice summary!

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Posted

  It’s time to chose the engine for your Lancair legacy,  do you go for the 750 hp Pratt Whitney pt6 at a measly 130ft lbs torque, or spring for the lycoming 180hp o360 at a whopping 350  ft lbs torque?

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Posted

Rotax rates the 582 as such and such horsepower and torque at certain RPM's but doesn't mention the gearbox, ratio, or prop hub horsepower or torque. Now days most people are fixed on the horsepower number and nothing else. I guess most can grasp the horsepower rating without understanding anything about torque or what it does.

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Posted

I guess I look at HP as king in determining an engines "power" because it is the combined product of torque and rpm.  It gives a better sense of what work the motor can do.

As mentioned above the awesome total (horse)power of a PT6 is all about rpm and very little about torque.  And a steam engine's awesome (horse)power is all about torque and very little about rpm.  IMO you really need to rate each by HP to be fair to one another.

So anyway thats why I think HP is a better "guage" of total power but next to as important is the "power band" which in my mind is best defined by torque graphed against rpm.  A long flat torque curve = a broad powerband.

Two charts below show this.  The Yamaha Apex is an example of a broad powerband, with torque (value read off vertical scale on right) remaining very constant from 6500 to 10,500 rpm.  A 4 thousand rpm power band where consant torque gives HP the opportunity to build from about 95 to 150 (read off vertical scale on left)

The second graph is of a 2 stroke 2cyl liquid Rotax 440.  Two strokes generally build great torque but hold it for a shorter rpm range.  The "rpm length" of that flat area of torque occurs over a shorter range (2 thousand rpm or so).  "Peaky" or "Narrow Power Band" where it is even more important to get your snowmo clutching or prop size and pitch just right to harness the power. 

So IMO HP is the best total power rating and torque curve is the best way to understand how the engine makes that power... and how fussy it might be if "optimal" canditions are not met.

 

sno dt1105_chart 250.gif

mxz44098.jpg

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Posted

For airplanes, HP is the best gauge - the engine will spend most of the time working at or near peak HP, so it doesn't matter how it gets there - High RPM/low torque, or Low RPM/High torque.  Low RPM/High torque means you may not need a gearbox, which can have some advantages, but those engines usually weigh more so the advantage is primarily in simplicity.

If you are pulling train cars, stumps, multiton trailers, or drag racing, torque wins every time.  If you are oval racing, HP is usually the king, unless you don't have enough aero to take the corners at close to full throttle.  Road courses need a balance.

Mark

 

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Posted

Rotax rates the 582 as such and such horsepower and torque at certain RPM's but doesn't mention the gearbox, ratio, or prop hub horsepower or torque. Now days most people are fixed on the horsepower number and nothing else. I guess most can grasp the horsepower rating without understanding anything about torque or what it does.

gearing does not change horsepower.

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Posted

No it does not.  Glad everybody has this torque/horsepower thing figured out. Got the opinions I was expecting.

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Posted

Rotax rates the 582 as such and such horsepower and torque at certain RPM's but doesn't mention the gearbox, ratio, or prop hub horsepower 

 

I guess this part confused me.  I’m guessing  the reason they don’t provide that data is it’s easily computed from the dyno charts they provide.

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Posted

ABECC045-3C95-4671-86D1-690A405D879D.jpeg

83216AFD-928A-4B99-8535-125E12629D98.jpeg

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Posted

It probably isn't easy or cheap to compute but in an aircraft situation, torque and horsepower at the prop is the important numbers. We would probably be surprised to find out how little horsepower it takes to fly a light Avid/Fox.  And Rotax numbers are for a new engine, tuned just right and everything being perfect. Real world experience is a lot less I would guess.

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Posted

Based on my experiences I'd say the manufacturer's numbers are probably pretty darn close for a properly running engine.

I have had the opportunity to run accident aircraft engines at manufacturer facilities in certified test cells . You'd probably be surprised but by and large they put out exactly what the manufacturer says they should even after being subjected to an accident.  One I remember in particular was a IO-520 that spent a week under water before recovery. It had less than  10/80 pre-run compression on all cylinders and yet is started, ran fine and produced full rated power. After the run the compressions were all over 50/80. I don't suspect Rotax's would be much different so would guess they would reach their rated numbers as well provided they are properly maintained.

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Posted

It probably isn't easy or cheap to compute but in an aircraft situation, torque and horsepower at the prop is the important numbers. We would probably be surprised to find out how little horsepower it takes to fly a light Avid/Fox.  And Rotax numbers are for a new engine, tuned just right and everything being perfect. Real world experience is a lot less I would guess.

I think you could figure it out really close if you had an inflight adjustable prop that you could fully load the engine with, I’ll try to explain. As far as I know most dyno charts are presented  at max power, so you will need to fly your aircraft at a predetermined speed with throttle fully advanced. For example I want to know how much hp it takes to fly my 582 powered kitfox at 70mph, level off fully advance the throttle and adjust pitch to attain 70 mph, now take a look at the tach, it indicates 5200 rpm. The 582 chart says at 5200 rpm the 582 can produce about 50 hp.  So with my theoretical example it takes approximately 50 hp to pull my kitfox through the air at 70 mph.

  Most of my background  is with Diesel engines but my experience has been that as an engine accumulates hours performance is hardly affected, I suspect gas motors to be similar as long as the carbs and   Ignition are maintained.

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Posted

Im wondering if I need to stop thinking out loud, I’m starting to poke some holes in my theory...

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Posted (edited)

Im wondering if I need to stop thinking out loud, I’m starting to poke some holes in my theory...

I'd say you are spot on.  

Many people don't understand that dyno charts ARE at full throttle for all data points.  You open it up and let the dyno load it down to different rpms and take torque readings. Then HP is figured backwards through the formula (T x RPM) / 5232 and the curve between the points that the dyno recorded is interpolated.

Common misunderstanding is for someone to think that if they throttle back to 5200rpm (in your example) that they are getting the  50 HP from the chart but the engine is actually putting out far less when throttled back.  

Edited by Yamma-Fox

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