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percent of power calculations

RonB

Member
Good evening

Does anyone know what is required to determine the percent of power a Lycoming IO360 180 HP engine may be producing at a particular setting?

I have a Dynon EMS and it's obviously showing a much higher percent of power than is actually being produced by my engne and the folks at Dynon haven't been able to offer any help.

Thanks

Ron B
 
Good evening

Does anyone know what is required to determine the percent of power a Lycoming IO360 180 HP engine may be producing at a particular setting?

I have a Dynon EMS and it's obviously showing a much higher percent of power than is actually being produced by my engne and the folks at Dynon haven't been able to offer any help.

Thanks

Ron B
Kevin Horton has that one covered. Look down the left side of the page under "Top Links."
 
Power

Good evening

Does anyone know what is required to determine the percent of power a Lycoming IO360 180 HP engine may be producing at a particular setting?

I have a Dynon EMS and it's obviously showing a much higher percent of power than is actually being produced by my engne and the folks at Dynon haven't been able to offer any help.

Thanks

Ron B

Here's what I use in my equations. First, for your engine, determine the MAP that the factory shows on their chart of the power-altitude-rpm graph. That is the MAP that you ratio to with your engine's running MAP. For instance, on my O-235, rated power occurs at 28.4", not 29.92! So if I'm cruising along at 8000' and 22", the first part of the power ratio will be 77.5%. Next, you have to ratio your actual rpm vs rated rpm. So if rated is 2700 and you're at 2500, this part is 92.6%. This asumes that these engines have a fairly flat torque curve over a range of rpm so that power is directly proportional to rpm. So now we have 77.5% (0.775) times 92.6% (.926) which multiplied together gives 71.7%. Then to this I ratio the inlet temperature plus stagnation temperature rise and minus fuel evaporation drop and convert this to absolute temperature and divide this by the sea-level absolute temperature minus fuel evaporation drop, and get the square-root of this. So on this standard day the OAT was 30.5F and at 200 mph the stagnation rise was 7F. I add 30.5+7-24+459.7, 473.2, and divide this by 459.7+59-24, 494.7, giving 0.957, and a square-root of 0.978. Multiply this times the previous 71.7% and get 70.1% power Now without MAP and all of the temperatures, I could have just gotten the density ratio relative to sea-level for 8000' on a standard day which is 0.786. Some smart people found that on average, because of how the engine reacts to the induction temperature's effect on the inlet density and power, that if you raise the density ratio to the 1.135 power that will compensate. 0.786^1.135=0.761. Multiply that by the rpm ratio, 0.926, and get 70.4% power. But that only works for WOT, so if you have the throttle reduced, you need to use the first more-complete calculation with MAP. Nothing to it, right?
 
Here's what I use in my equations. First, for your engine, determine the MAP that the factory shows on their chart of the power-altitude-rpm graph. That is the MAP that you ratio to with your engine's running MAP. For instance, on my O-235, rated power occurs at 28.4", not 29.92! So if I'm cruising along at 8000' and 22", the first part of the power ratio will be 77.5%. Next, you have to ratio your actual rpm vs rated rpm. So if rated is 2700 and you're at 2500, this part is 92.6%. This asumes that these engines have a fairly flat torque curve over a range of rpm so that power is directly proportional to rpm. So now we have 77.5% (0.775) times 92.6% (.926) which multiplied together gives 71.7%. Then to this I ratio the inlet temperature plus stagnation temperature rise and minus fuel evaporation drop and convert this to absolute temperature and divide this by the sea-level absolute temperature minus fuel evaporation drop, and get the square-root of this. So on this standard day the OAT was 30.5F and at 200 mph the stagnation rise was 7F. I add 30.5+7-24+459.7, 473.2, and divide this by 459.7+59-24, 494.7, giving 0.957, and a square-root of 0.978. Multiply this times the previous 71.7% and get 70.1% power Now without MAP and all of the temperatures, I could have just gotten the density ratio relative to sea-level for 8000' on a standard day which is 0.786. Some smart people found that on average, because of how the engine reacts to the induction temperature's effect on the inlet density and power, that if you raise the density ratio to the 1.135 power that will compensate. 0.786^1.135=0.761. Multiply that by the rpm ratio, 0.926, and get 70.4% power. But that only works for WOT, so if you have the throttle reduced, you need to use the first more-complete calculation with MAP. Nothing to it, right?
See Ron, it's simple!
 
I think that exhaust backpressure plays a role too.

If you look at the manufacturer's power charts, it appears that 25" MAP at 8000ft yields more power than 25" MAP at SL. I believe this difference is due to there being less (absolute) exhaust backpressure at altitude.

-DC
 
For cruise at either 'Best Power" (about 150 degrees ROP) or "Best Economy" (peak EGT) mixture settings you can use the "Part Throttle Fuel Consumption" graph in your Lycoming Owners Manual. All you need to use this graph is fuel flow and rpm. Be aware this is for a standard Lycoming with mags. Higher compression, electronic ignition etc will likely result in more HP. I THINK you may be able to also use this graph for LOP operations as Specific Fuel Consumption is fairly constant from around peak to about 70 degrees LOP (see Lycomings graph; "Representative Effect of Leaning on ..........."

Fin
9A

EDIT. There are a few different Part Throttle Fuel Consumption graphs in the Owners Manual. I am referring to the appropriate one for your engine model that has "Mixture Control - Manual to Best Economy or Best Power as Indicated"
 
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I think that exhaust backpressure plays a role too.

If you look at the manufacturer's power charts, it appears that 25" MAP at 8000ft yields more power than 25" MAP at SL. I believe this difference is due to there being less (absolute) exhaust backpressure at altitude.

-DC

That must be a supercharged engine to get 25" at 8000' where standard pressure is 22.22". 25" at sea-level would cause some power loss due to pressure drop across the throttle valve, whereas 25" at 8000' would be due to a MAP boost and less TV drop. 'Just wundrin'
 
If your Dynon is far off, then there must be some error in your fuel flow transducer calibration, the OAT reading, or the MAP pressure reading or you are using something other than stock compression ratio? Do you have the HP set correctly for your engine?

Good evening

Does anyone know what is required to determine the percent of power a Lycoming IO360 180 HP engine may be producing at a particular setting?

I have a Dynon EMS and it's obviously showing a much higher percent of power than is actually being produced by my engne and the folks at Dynon haven't been able to offer any help.

Thanks

Ron B
 
The engine red line parameter setting in the Dynon will also affect the percent power readout. Dynon uses the red line setting as the RPM at which the engine generates 100% power at sea level. My engine is a O320 which has a red line of 2700 RPM BUT my prop red lines at 2600. So, to get all the warnings from the Dynon when I get close or hit the 2600 RPM prop red line, I have configured the engine red line RPM in the Dynon for 2600 RPM. This affects the engine percent power readout by about 2%. When the engine is running at 75% power the Dynon reads 77%. 65% engine power reads about 67%. Not a big deal but if you set your Dynon red line to something other than engine max power RPM, your percent power readout will be affected.
 
Here's a power setting table I got from Lycoming:

Well.... I couldn't link it to this post so I'll try to email it to you. It's in a word document file.
 
That must be a supercharged engine to get 25" at 8000' where standard pressure is 22.22". 25" at sea-level would cause some power loss due to pressure drop across the throttle valve, whereas 25" at 8000' would be due to a MAP boost and less TV drop. 'Just wundrin'

Elippse,

You caught me! I wasn't looking at a chart when I wrote that post.

As penance, here is an excerpt from an O-540-A power chart:

power.png


The highlighted line shows power vs. altitude for a setting of about 22.3"/2350 rpm.

You'll notice that the rated power is 168HP at 1000', but increases to 181HP at 6000' when the the MAP/RPM are held constant. I believe that this difference is due to the reduced exhaust backpressure at altitude, but there may be other factors as well.

The full page (PDF) is here for reference.

Blue skies,
David
 
Power

Hi, David!
First, thanks for the 540 power curve; that makes a great addition to my files! But keep in mind, that in order to drop the MAP to 22.3" at 1000', where standard pressure is 28.86", requires a large pressure drop across the throttle valve which gives pumping losses. That was the lesson Lindbergh showed to the P-38 pilots in the Pacific in WWII - keep the throttle wide open and reduce propeller rpm. I'll have to go back to Taylor to see what power loss occurs from Pin to Pout. I'm planning on using carb heat at altitude to reduce rpm and increase engine efficiency on trips. Even though increasing induction temperature reduces induction density directly, an engine's efficiency increases with the square-root of the temperature, which is why the power correction factor in the box on the upper left of the complete graph, which is not shown on your post, shows the HP vs square-root of the absolute temperature ratio, which is what I wrote about on my original response. 'Let you know what Taylor has on the pressure ratio.
 
Taylor on pumping loss

David; I did a very brief and not at all comprehensive research of CF Taylor, Vol.1, 2nd Ed., Revised, on pumping losses. If I'm interpreting Fig. 9-22 correctly, the major contributor to power loss is due to throttling of the inlet.
 
Theory vs. Charts

Early in this thread, Paul Lipps said, in part, this:
---------------------------------------------------------------
"Here's what I use in my equations. First, for your engine, determine the MAP that the factory shows on their chart of the power-altitude-rpm graph. That is the MAP that you ratio to with your engine's running MAP. For instance, on my O-235, rated power occurs at 28.4", not 29.92! So if I'm cruising along at 8000' and 22", the first part of the power ratio will be 77.5%. Next, you have to ratio your actual rpm vs rated rpm. So if rated is 2700 and you're at 2500, this part is 92.6%. This asumes that these engines have a fairly flat torque curve over a range of rpm so that power is directly proportional to rpm. So now we have 77.5% (0.775) times 92.6% (.926) which multiplied together gives 71.7%. "
-------------------------
The rest was about correction for temperature and the correction was small.

This makes perfect sense to me. Assuming a flat torque line, the HP should be proportional to the amount of fuel-air mixture burned per unit of time. RPM and density therefore should give a correct answer as per Paul's calculations. So, do they?

Ever curious, I took a look at the power chart for the Superior Vantage IO-360 (very similar to my XP IO-360. I tested the theory for the sea-level, best power condition on an Excel spreadsheet. The lines are pretty flat.
Here is the chart for you to double check my work.

Oops, the chart does not agree with the theory. For example:
1. Paul uses 22" by 2500 RPM and gets around 71.7%. The chart says 59.4%. Error = 20.7%
2. I calculated all the MP's in one inch intervals from 18" to 29" for 2700 RPM. Paul's method gives a difference from the chart between 1.0 and 1.32, increasing as MP declines, perfect at 29" which is where the engine is rated.
3. I calculated all the RPM's in 100 rpm increments for 18" MP. The error was from 1.32 to 1.41

I was really hoping to solve this problem because you can't get prop efficiency right without correct horsepower data. I have seen other formulas and an early version of a spreadsheet from Kevin Horton. I'm still looking for the magic bullet. I certainly don't have the answer.
 
Power curves

WOW! 32% to 41% error! I must have really goofed this time! I often do that when writing a posting without having set down my ideas and calculations beforehand! But somehow I'm not seeing where I went wrong on my initial analysis. Because my original posting was about an engine that produced rated power at 2700 rpm, and had a sea-level WOT MAP of 28.4", I got out my Lycoming curve #13381 on the 160 HP O-320-D, from Lycoming specification #2283-G, and did the called-for graphing of the lines at 8000', 22" MAP, and 2500 rpm. And without correcting for the manifold temperature difference, I got 115 HP, which is 71.9% power. That seems to be pretty close to the 71.7% I originally calculated! Did I draw my lines wrong on the graph? There must be something that I am missing. Will someone please enlighten me?
 
It's a puzzle

WOW! 32% to 41% error! I must have really goofed this time! I often do that when writing a posting without having set down my ideas and calculations beforehand! But somehow I'm not seeing where I went wrong on my initial analysis. Because my original posting was about an engine that produced rated power at 2700 rpm, and had a sea-level WOT MAP of 28.4", I got out my Lycoming curve #13381 on the 160 HP O-320-D, from Lycoming specification #2283-G, and did the called-for graphing of the lines at 8000', 22" MAP, and 2500 rpm. And without correcting for the manifold temperature difference, I got 115 HP, which is 71.9% power. That seems to be pretty close to the 71.7% I originally calculated! Did I draw my lines wrong on the graph? There must be something that I am missing. Will someone please enlighten me?
Maybe I'm in error. Your idea seemed correct to me. Try the link and see if I'm reading the Superior chart correctly. I have a hard time imagining that Superior and Lycoming could be much different. I can put the XLS on the webserver, too, if it will help. RSVP.

Remember, though, that I used the sea level chart and it is commonly held that you get more HP at higher altitudes. The Superior chart, if I'm reading it right, gives 107 HP for the IO360 (180 HP) at sea level, 22" x 2500 RPM.
The 360 should give about 12.5% more HP for the same RPM and MP, right? So if 107 is right on my chart, it would be 95 HP for a 320. I'd love to find out where we are actually differing.

BTW, the Superior is rated at 29", not 28.4"
. If I am reading the right Lycoming chart, then sea level performance, 22 x 2500 is about 102 HP which reduces our difference to 7 HP (102 - 95), some of which may be the difference in the MP for rated power. 22 is a greater percentage of 28.4 than of 29 by enough to account for about 2 HP. That takes the difference down to 5 HP.

Here's the problem, though: The spreadsheet says that the Ellipse formula is most accurate at the highest power settings and the least accurate at the lowest ones. The example we are discussing is at the top end where errors would be smallest.

If a theory seems good and works in one test then the theory is supported but not proven. However, if it fails in a second, valid but different test, then it is not supported and, at a minimum, needs revision. That's where I am for this idea at this time.

I wish I had even a theory to explain why %RPM x %MP does not equal %PWR. Here is one possibility. The relationship between air density and air pressure is not linear. The table I have handy goes from sea level to 15000' but it shows that while pressure% changes from 100 to 56.43, the density% changes from 100 to 62.92. This is a 11.5% difference from linear. The trouble is, I think this goes in the wrong direction to explain the problem! Either way, though, it impinges on the theory.
 
Power

Howard, I see now where you are coming from; you want a formula which will give engine power over the full range of operation, not just at WOT. WOT is what I always use in my equations because I design a propeller for the maximum power, rpm, and speed at a given density altituder, and that is what I thought was what people want. But I can see where those who have a CS prop might want to see what their power is at various combinations of MAP and rpm. If their avionics are giving them percentage of power, they must have a formula with the elements of Ps, OAT, rpm, MAP, and density. On the engine power graphs put out by the engine makers, the right-hand graph shows power from rpm and MAP vs altitude. whereas the left-hand page shows power at sea-level conditions from rpm vs MAP. On this one is shown the effect of operating the engine at less than WOT where the effect of throttling the engine and thus reducing its MAP vs sea-level pressure is shown. This shows the HP loss that occurs from the pumping loss across the throttle valve. I'm going to see if I can work up an all-encompassing equation that will give the power with the differential pressure across the TV. I have a curve-fit presently for my O-235 with four coefficients that I made years ago that matches this left side, and I'll see if I can refine that to work with all engines. It goes: HP = (rpm X a + b)(MAP + c) + d where a=1.213E-3, b=2.287, c=4.91, and d=-71.05
 
Well, I think I may have come up with a reasonably simple and fairly accurate method of determining engine power from rpm and MAP. I did these calculations using engine data sheets from an O-235 L2C, an O-320B, and an O-320D. Keep in mind that these engine power graphs are not exact, they are based on estimates from various tests. Look at the lines on the graph from an angle and on some data sheets most of the lines are parallel and some converge at a distant point, and some even have some lines parallel and one or two others angled! Go figure! But they never measured the engine horsepower in flight at all of these altitudes and at all of these rpm and MAP values. They were based on engineering estimates. The sea-level numbers, of course, were probably obtained from tests on engines in a test cell operated at these various rpm and MAP, and then corrected to sea-level MAP and temperature. Do you really think they waited for a 59F day at 29.9213" pressure, or had a sealed test cell where they maintained the temperature and pressure inside the cell to a very high accuracy? No, they used correction factors with the measured torque, rpm, pressure, and temperature just as do all of the engine makers with their dynos except the aircraft engine data was probably reduced with slide rules whereas all of the new dynos do it with built-in computers. As far as the formula for power, here's what you do: first subtract the MAP from the engine's sea-level MAP at rated power, then multiply this remainder by an engine-related constant. Subtract this new value from the SLMAP and divide this by the SLMAP. Multiply this by the actual rpm and divide by the rated rpm. Multiply this by 100 to get percentage of power. You can further refine this for inlet temperature as given previously. For my O-235, SLMAP is 28.4", rated rpm is 2800, and the constant is approximately 1.4. This constant can be calculated for each engine from the data sheet. This probably isn't the untarnished silver bullet (Lone Ranger's?) that Howard was looking for, but it probably comes very close, maybe as close as the original engineering estimates!
 
Thanks, I'll try that!

Paul, we may be the last two guys on earth willing to go that deeply into the subject, but I will, for certain, test this with the charts for my Superior 360. More to come. Thanks.

btw - this must apply only to "best power" mixture. And LOP works a whole different way as Walter Atkinson has posted on this forum before.
 
Most of the WW2 engines were tested on a dyno and altitude cells as it was clear that altitude affected power output through reduced exhaust back pressure. You can add that to your list of variables of rpm, MAP, IAT, humidity and AFR, not to mention throttle angle (pumping loss variations and mixture distribution changes in O-XXX engines), ignition timing differences and exhaust system differences.

With enough assumptions made, inaccuracies invariably creep in. While a tidy math model would be nice, there is no way to verify the theory without the two items listed above. Interesting discussion nevertheless.:)

I believe I read a long time ago that Lycoming did test some of their large engines in altitude cells, not sure about the atmo fours. Does anyone know for sure?
 
Better, Much Better

Here is a spreadsheet which implements Paul's formula for the Superior IO-360+ (180 HP).

Here is the chart for that engine.

The low and mid power error is in the low 4% range. The higher power error is less than 1%. Pretty good. I arbitrarily set the constant for the 75% value.

There is no temperature nor altitude correction built into this one. The user can insert columns or rows for other intervals of MAP or RPM and copy the formula from adjacent cells.

Let's see how it works on some other engines.
 
Well, I think I may have come up with a reasonably simple and fairly accurate method of determining engine power from rpm and MAP. I did these calculations using engine data sheets from an O-235 L2C, an O-320B, and an O-320D. Keep in mind that these engine power graphs are not exact, they are based on estimates from various tests. Look at the lines on the graph from an angle and on some data sheets most of the lines are parallel and some converge at a distant point, and some even have some lines parallel and one or two others angled! Go figure! But they never measured the engine horsepower in flight at all of these altitudes and at all of these rpm and MAP values. They were based on engineering estimates. The sea-level numbers, of course, were probably obtained from tests on engines in a test cell operated at these various rpm and MAP, and then corrected to sea-level MAP and temperature. Do you really think they waited for a 59F day at 29.9213" pressure, or had a sealed test cell where they maintained the temperature and pressure inside the cell to a very high accuracy? No, they used correction factors with the measured torque, rpm, pressure, and temperature just as do all of the engine makers with their dynos except the aircraft engine data was probably reduced with slide rules whereas all of the new dynos do it with built-in computers. As far as the formula for power, here's what you do: first subtract the MAP from the engine's sea-level MAP at rated power, then multiply this remainder by an engine-related constant. Subtract this new value from the SLMAP and divide this by the SLMAP. Multiply this by the actual rpm and divide by the rated rpm. Multiply this by 100 to get percentage of power. You can further refine this for inlet temperature as given previously. For my O-235, SLMAP is 28.4", rated rpm is 2800, and the constant is approximately 1.4. This constant can be calculated for each engine from the data sheet. This probably isn't the untarnished silver bullet (Lone Ranger's?) that Howard was looking for, but it probably comes very close, maybe as close as the original engineering estimates!
Keep in mind that the relationship between rpm and power is not necessarily linear. For example, the angle-valve Lycomings have a tuned induction system that helps them make more power than the angle valve engines, but this is only working well at high rpm. The advantage of the tuned induction system whithers away as the rpm decreases, and somewhere around 2300 rpm the two engines make the same power. At lower rpm the parallel valve engine is actually more powerful.

For example, power at 28.6" MP at sea level, std temp:
rpm IO360A O360A
2100 147.1 152.7
2200 154.6 161.1
2300 165.6 166.4
2400 174.8 171.7
2500 183.2 175.9
2600 192.6 180.1
2700 200.0 183.1


At 24" MP at sea level, std temp:
rpm IO360A O360A
2100 117.3 120.1
2200 123.4 126.7
2300 132.1 131.2
2400 139.2 135.7
2500 146.2 139.0
2600 153.5 142.3
2700 160.1 145.3
 
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In Kevin's example the IO has torque peak at around 2600/2700 rpm while the O peaks at 2200 rpm. Torque peak coincides with maximum volumetric efficiency. A linear hp relationship only exists where the VE is the same at all rpms and frictional and pumping losses are linear or where VE increases offset other losses- which they rarely do.

The torque curve is remarkably flat on these engines, varying only about 6% in the case of the IO across the rev range. The IO hp numbers here indicate a straight linear change with rpm while the O numbers do not. It may be safe to surmise that on engines which have the hp and torque peaks at nearly the same rpm (and up high in the rev range), a near linear hp progression with rpm within a narrow rpm band is possible. On engines with torque peak occurring lower in the rev range as with the O example, it is not possible to have a linear progression with rpm due to higher frictional losses and a simultaneous loss in VE as rpm rises.

As far as humidity effects go, very high humidity in high ambient temperature conditions can reduce power output by almost 4% compared to a less than standard day with no humidity. Very few places have 0% humidity but it is easy to have a 1.5-2% error in normal flying conditions without taking humidity effects into consideration. Dynos always take dry and wet bulb readings to correct hp for humidity.

So again, this is why engines are validated on dynos. There are just so many variables to consider that a simple math model with limited information is probably not too accurate, especially when an existing engine is used as a baseline and changes have been made to various things like induction, exhaust and ignition. The more assumptions that are made, the less accurate the model generally. Any model which does not take temperature, humidity and altitude into effect is more useful for theoretical discussions as worst scenario cases could result in errors of easily 10%.

Where more information is available like camshaft timing, cylinder head airflow, bore to stroke ratios, connecting rod length, induction and exhaust specs etc. fairly accurate math models are available today to predict hp from a given engine configuration in the racing world. A low cost example of this type of software is here: http://www.quarterjr.com/engine_jr.htm There are many other more expensive and complex programs available.

A fascinating subject...
 
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Power calcs

As pointed out by Ross and others, the power an engine produces is such a tangled web of so many components that the only method I'm aware of to know what your power is in flight is to have a torque sensor on the propeller shaft and use this with rpm to calculate power. What I tried to present was a short-hand method to arrive at a power estimate that you could use to compare with the number that the came out of the avionic's power estimate. Notice the operative word: ESTIMATE! Maybe we could get Saber to integrate a wireless torque strain-gage in their prop extension and use that, along with rpm and a simple micro-processor to give us true horsepower. Now all we would need is a thrust sensor in the extension too, and that, with TAS to compute thrust power, would tell us prop efficiency! Ok, guys! Here's a business opportunity!
 
As pointed out by Ross and others, the power an engine produces is such a tangled web of so many components that the only method I'm aware of to know what your power is in flight is to have a torque sensor on the propeller shaft and use this with rpm to calculate power. What I tried to present was a short-hand method to arrive at a power estimate that you could use to compare with the number that the came out of the avionic's power estimate. Notice the operative word: ESTIMATE! Maybe we could get Saber to integrate a wireless torque strain-gage in their prop extension and use that, along with rpm and a simple micro-processor to give us true horsepower. Now all we would need is a thrust sensor in the extension too, and that, with TAS to compute thrust power, would tell us prop efficiency! Ok, guys! Here's a business opportunity!

I like Paul's idea of actual in flight torque measurement combined with some additional information- this would allow you to develop an accurate math model for that particular engine. That would be a most interesting project!:cool:
 
Project for the day

I don't have any idea how to put a torque sensor on the airplane in flight, but here is an idea for validating at least one or a few points on the curves. I have not worked out the practical aspects yet. Who will try this?

1. Build some L shaped chocks big enough for next step.
2. Put aircraft scales on the chocks.
3. Put the airplane on the scales. The chocks are now holding the airplane on the scales because they extend vertically enough.
4. Tie back the airplane using the mains high on the legs.
5. Record the weights on the three scales.
6. For various RPM's, static, record the RPM and the MAP as well as temp, pressure altitude and any other variable you can name. Record the weights for each data point. For CS props, try with combinations of MAP and RPM.
7. At any time, measure the distances from the center of the plane to the center of the contact patch of the tire.
8. Using the arm you measured and the change in weights on the mains (did weight on tail/nose change too?) compute the torque.
9. Compute the BHP from the torque and the RPM for each data point.
10. Compare to the manufacturer's chart.
 
I don't have any idea how to put a torque sensor on the airplane in flight, but here is an idea for validating at least one or a few points on the curves. I have not worked out the practical aspects yet. Who will try this?

I wrote about this method several years ago and it was published somewhere. We tried this on my friend's gyro and got fairly good results. About 2 years ago I had a friend read the scales while I managed the throttle on my plane; the results weren't too good as any slight variation in wind caused enough jiggling of the scales that they couldn't be read reliably. Digital scales were even worse than bathroom scales because the numbers jumped about so much that there was no way to even get an eyeball average of what they displayed! We did get a good WOT thrust number from the spring scales we connected from the main gear legs to a car nearby with 1/8" cables. There are two distance measurements that must be taken into account. The first is the radial distance from the crank centerline to the tire contact patch, and the second is the horizontal distance from one tire to the other. The force normal (perpendicular) to the radius, the torque, must be calculated from the angle that the radii make with the horizontal distance, resolving the force on the scale, and then this used with the radial distance to estimate the torque, then that used with rpm to get the horsepower. I have a formula and drawing somewhere illustrating this method. I think I called this the "poor man's dyno!". BTW; it might be possible to use the RFID technology to wire-less transmit the rotating torque and thrust data.
 
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Thanks

I wrote about this method several years ago and it was published somewhere. We tried this on my friend's gyro and got fairly good results. About 2 years ago I had a friend read the scales while I managed the throttle on my plane; the results weren't too good as any slight variation in wind caused enough jiggling of the scales that they couldn't be read reliably. Digital scales were even worse than bathroom scales because the numbers jumped about so much that there was no way to even get an eyeball average of what they displayed! We did get a good WOT thrust number from the spring scales we connected from the main gear legs to a car nearby with 1/8" cables. There are two distance measurements that must be taken into account. The first is the radial distance from the crank centerline to the tire contact patch, and the second is the horizontal distance from one tire to the other. The force normal (perpendicular) to the radius, the torque, must be calculated from the angle that the radii make with the horizontal distance, resolving the force on the scale, and then this used with the radial distance to estimate the torque, then that used with rpm to get the horsepower. I have a formula and drawing somewhere illustrating this method. I think I called this the "poor man's dyno!". BTW; it might be possible to use the RFID technology to wire-less transmit the rotating torque and thrust data.
I don't recall that write-up, but I guess great minds do think alike (said with an exaggerated sense of self-worth). I do recall in an EAA article a method of testing the engine on a test stand with a long arm and a scale. On-aircraft is a little trickier. Just a note that static thrust is pretty much meaningless, but the torque measurement, if well done, will give a sound basis for BHP. Good point about needing to draw the triangle to get the torque measurement correct.

It is worth noting that the results of such a measurement can be compared to the the manufacturer's curves so that they can be viewed realistically. Some have noted that there may be a "loss" factor for accessories, etc. when comparing reality to test cell. In other words, my 180 HP engine may not really put 180 BHP to the prop. We also don't know that all engine curve test cell procedures are equal; it is likely they are not.

The variables noted by rv6ejguy above affect the undependable relationship of the curves vs. the reality, but we are still trying to get a formula that matches what the manufacturer says. Both valid, but entirely different issues.
 
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