Power from fuel flow - looking for data

The classical engine power charts provide power as a function of rpm, MP and altitude, but they are only valid if the mixture is set for best power. Many people like to cruise with some other mixture setting, so there is interest in a means to determine power either richer or leaner than best power mixture.

I've got an old Lycoming document that describes a method to determine power during cruise performance testing, using fuel flow as the main input. It isn't usable real-time in flight, but it could be used in conjunction with flight testing to produce power setting tables for a handful of combinations of rpm, MP, altitude and fuel flow. I've played around with this power calculation method a bit, trying to figure out if it produced consistent, credible results. If it does, I'll document it in a Kitplanes article and produce a spreadsheet to use it. If it doesn't provide consistent results, then I'll drop it. I'll give an overview of this old Lycoming method in the next message in this thread.

The big question I need to answer is does this method produce a consistent power no matter what mixture is used. I will answer that question by plotting speed vs calculated power, with points at the same rpm, MP and altitude, but at several different fuel flows. I've got a bit of data from my RV-8, but it will be some time before I can get any more, and I am not completely satisfied with the stability of my fuel flow indication, which makes is difficult to draw solid conclusions from my data. Also, I really need data from more than one aircraft, with more than one pilot, to see whether this method is useable in the real world.

I'm looking for a few intrepid RVators to help me test this fuel flow from power method. I need data from several aircraft with a Lycoming or Lycoming clone engine. The compression ratio must 6.75, 7.0, 7.2, 7.3, 8.0, 8.5, 8.7, 9.0 or 10.0 (one of the steps in the method is to look up iSFC, and the chart only has data for compression ratios that Lycoming sold). The engine displacement must be 235, 320, 360, 480, 540 or 720 (sorry, this document predates the 390s by several decades, and I assume the 290 was out of production when the document was produced). The aircraft must have a constant speed prop and a fuel flow system that gives current fuel flow and fuel remaining. I'm interested in data from engines with two mags, one mag + one EI, and two EI.

The test procedure is as follows:

Before flight, ensure you know the aircraft weight by accounting for all the stuff in the cockpit and baggage areas. If the item wasn't on the aircraft when the weight and balance was done, either remove it or figure out how much it weighs and account for in the gross weight. Fill the fuel tanks, and record the calculated gross weight with full fuel, all occupants, etc.

Find a test area with very smooth air, and no vertical air motion. No mountain waves, etc. Record the pressure altitude (i.e. with altimeter set to 29.92) and OAT. I don't care what altitude you use, as long as the air is smooth and has no vertical motion.

Set the desired rpm and MP, and don't change them for the duration of the test. Record the rpm.

Slowly adjust the mixture to find peak EGT, and record the fuel flow at peak EGT. Note: the test method assumes that all cylinders peak at the same fuel flow, but the real world doesn't work like that. Ideally you would record the fuel flow when each cylinder peaked (i.e. record four or six fuel flow values). Or, if they all peak at about the same fuel flow, give me an eyeball average of the fuel flow at peak EGT for all cylinders. Don't send just the fuel flow when the first cylinder peaks.

When looking for fuel flow at peak EGT, be very aware of how quickly or slowly your EGT system responds to changes in mixture. If your EGT system is slow to respond, you'll need to be very patient when adjusting the mixture, to let the EGT stabilize after each change. Otherwise it is quite possible to record a too low fuel flow at peak EGT.

Without changing rpm, MP or altitude, record level flight IAS vs fuel flow for a wide range of fuel flow values, both ROP and LOP. The wider the range of fuel flows the better, as long as the engine is running smoothly, with no misfiring. At each fuel flow, wait long enough for the IAS to stabilize, which may take several minutes. Record fuel flow, fuel remaining, and IAS.

For extra brownie points, you could repeat the above at one or more different conditions. I.e. change one or more of the altitude, rpm or MP. Data from several different flights is OK, as long as the aircraft CG remains pretty much the same - i.e. if you have an RV-4 or -8, don't do some flights with a passenger, and some without, as that will affect the relationship between power and speed.

Send me the following data:

Engine model
Engine compression ratio
Type of ignition system
Prop model (I don't need a detailed model number - I just want to confirm it is a constant speed prop)
Aircraft gross weight on the day of the test with full fuel.

For each altitude, rpm and MP that you have data, send me:
Altitude
OAT
RPM
Fuel flow at peak EGT for each cylinder

Then, for each mixture setting, send:
fuel flow
fuel remaining
rpm
altitude
OAT
IAS
Remarks - I am particularly interested in the stability of your fuel flow indication. I.e., with constant rpm, MP, altitude, mixture control, how much does the fuel flow indication vary up and down? Knowing this will help me interpret any noise in the results - i.e. is the noise due to issues with the method to calculate engine power, or is it possibly due to uncertainty in the fuel flow data).

I created a test card in Excel (and also PDF format) - Updated test cards uploaded on 25 May 2009 at 1715 EDT. I also created fixed pitch prop test cards in Excel (and also PDF format).

Caution - it isn't smart to run at peak EGT or lean of peak at too high a power. Use your best judgement on what power settings to use for these tests. Be nice to your engine.

The method to determine power from fuel flow is fairly interesting. It starts off with the fuel flow at peak EGT. Lycoming claims that the fuel flow at peak power is 1.178 times the fuel flow at peak EGT. You multiply the fuel flow at peak EGT to get the fuel flow at peak power. You take the compression ratio, and look up the indicated specific fuel consumption (iSFC) at best power mixture. Multiply the calculated fuel flow at peak power times the iSFC, to get the indicated horsepower (ihp) if the mixture had been set to best power. Indicated power is the power produced by the pressures in the cylinder, before the loss due to friction.

The method provides curves that show how the power varies with fuel flow both rich and lean of best power mixture. You take the ratio of actual fuel flow to calculated fuel flow at best power, and go into these curves to determine the indicated power at the actual fuel flow. Next, you go into another set of curves that give you the friction power as a function of rpm, engine displacement and whether it is a geared or ungeared engine. Subtract the friction power from the ihp to get bhp - the power that is available at the crankshaft to spin the prop.

The concept behind this method looks sound. But I don't know if the actual curves are correct. I also don't know how well it will work with engines with electronic ignition, as that will affect the iSFC somewhat.

The curves of friction power are interesting. They highlight the point that if you want to produce a certain amount of power, you are better off doing it at low rpm and high MP than at high rpm and low MP. For example, with an O-360 or IO-360, 22 hp are lost to friction to turn the engine at 2300 rpm. If we spin the engine at 2500 rpm, the friction power is 25.9 hp. So, if the power we want can be made at either 2300 or 2500 rpm, if we choose 2300 rpm we'll get 3.9 more horsepower for the same fuel flow than if we used 2500 rpm. Or, if we kept the power the same, the fuel flow at 2300 rpm would be about 0.25 gal/h less than the fuel flow at 2500 rpm.

Kevin,
I think Scott, Larry and I can probably accommodate you with some very comparative data( See oil cooler thread). We like having a mission.

Fuel flow, power and true airspeed

Kevin,

Man, you are going after data big time on this subject. Typical engineering effort. My take on it is much simpler.

I have always use fuel flow relative to a given TAS for flight planning purposes in these airplanes. It's the only thing that makes consistent sense to me. It takes a few data gathering flights to build a simple chart based on these factors. With the Subby, this information was developed up to 12,000' and included a still air MPG factor. That made it possible to pick an efficiency level appropriate for the flight, set the fuel flow and let it roll.

Consistent leaning technique is a given. That was not an issue with Subby but is with Lycoming. I used the fuel flow method of setting power in a previous Lycoming powered airplane with a fixed pitch prop and found it to be quite accurate for flight planning purposes.

David-aviator said:

Kevin,

Man, you are going after data big time on this subject. Typical engineering effort. My take on it is much simpler.

I have always use fuel flow relative to a given TAS for flight planning purposes in these airplanes. It's the only thing that makes consistent sense to me. It takes a few data gathering flights to build a simple chart based on these factors. With the Subby, this information was developed up to 12,000' and included a still air MPG factor. That made it possible to pick an efficiency level appropriate for the flight, set the fuel flow and let it roll.

Consistent leaning technique is a given. That was not an issue with Subby but is with Lycoming. I used the fuel flow method of setting power in a previous Lycoming powered airplane with a fixed pitch prop and found it to be quite accurate for flight planning purposes.

Click to expand...

I agree that if you have established a very consistent leaning technique, then looking at TAS vs fuel flow as the prime cruise performance test data is a good option. You don't need to do a huge amount of testing, as there are ways to gather TAS vs fuel flow data at one weight, altitude and temperature, and correct that data to predict performance at other weights, altitudes and temperatures. I'll eventually submit an article to Kitplanes that covers this.

But, there many good reasons why people are interested in knowing engine power, so I am looking at this old Lycoming method to see whether it will provide a useful answer to that question.

I like your plan Kevin - if you get enough data, you should see interesting results! One thing that you might watch out for though is the variation in fuel flow measurements based on where/how people have mounted their fuel flow 'ducers. I routinely see a small, periodic variation in FF during cruise - a period of about 10 seconds, variation of about 0.3 gph. So folks will have to patiently do a little "eyeball averaging" before writing down a data point.

Oh yes - smooth air, consistent atmosphere....reminds me of the Shuttle Crosswind DTO (Detailed Test Objective) we defined back in 1988. We wanted a few data points at a couple of different crosswind conditions. Because "we get what we get" on our few landings each year, we are still looking for a few data points....

Paul

Ironflight said:

One thing that you might watch out for though is the variation in fuel flow measurements based on where/how people have mounted their fuel flow 'ducers. I routinely see a small, periodic variation in FF during cruise - a period of about 10 seconds, variation of about 0.3 gph. So folks will have to patiently do a little "eyeball averaging" before writing down a data point.

Click to expand...

Yeah, the quality of the fuel flow data is one of my big concerns too. I wonder how much of that 0.3 gph variation is due to the float valve in the carb modulating the fuel flow, and how much of it is due to inaccuracies in the fuel flow system itself. My fuel flow indication moves around a lot more than I would like, and it may limit the usefulness of this approach in my aircraft. The one thing that we do have in our favour is that the calculation is much less sensitive to variations in fuel flow at peak EGT than it is to variations in fuel flow at the test point. If the recorded fuel flow at peak EGT is too high, because the fuel flow system was reading high, that causes the calculated power at mixture for best power to be high. But, it also gives the impression that you are further LOP than you really are, so that means there is a bigger decrement from this too high calculated power at mixture for best power. In other words there are two effects that cancel each other out by about 80%. It is important to get a fairly accurate power at the steady state test point, but there we have the luxury of being able to spend a few minutes to get a good average reading.

Another potential issue is how quickly EGT indications respond when adjusting the mixture to find the fuel flow for peak EGT. My EGT indications are fairly sluggish - perhaps my EGT probes are further from the exhaust port than optimum. So I need to really take my time when looking for the fuel flow for peak EGT.

My third big concern is the effect of electronic ignition. The data that was used to produce this test method would have been from engines with two mags. It may tend to calculate a too low power for engines with EI at lower power settings. If I get enough data from aircraft with EI, perhaps there may be a trend that will suggest an empirical increment to the iSFC curves at low power conditions.

I'm looking forward to seeing what the data tells us.

Fuel Flow Variation: Not just the carb float..

Kevin Horton said:

Yeah, the quality of the fuel flow data is one of my big concerns too. I wonder how much of that 0.3 gph variation is due to the float valve in the carb modulating the fuel flow, and how much of it is due to inaccuracies in the fuel flow system itself. My fuel flow indication moves around a lot more than I would like,...

Click to expand...

Kevin, mine is muti-port FI and it does the same thing. I probably have the same sensor that Paul has since we both have GRT's. I have the best location (on the floor forward of the FI pump, aft of the firewall) that I can from the point of view of straight runs coming and going, but in that location the mechanical pump is sucking gas through it, not pushing it through. Anyhow, I think that the carb float is a good explanation, but not completely sufficient.

For those with EFIS's, it may be possible to use the gallons-used field that GRT and likely others provide. That and a stopwatch might solve the accuracy problem. I've not tried it yet, but I might, someday. I'm disqualified from your study with my FP prop.

I just took a look at my "decode" XLS from recording my GRT readings in flight. It has a very precise flight timer and a fuel flow reading. A little work with Excel (or OpenOffice) can improve your data considerably if your testers use GRT or equivalent. Averaging looks feasible.

Earlier in this thread I said that I only wanted data from aircraft with CS props, as the method is designed around a constant rpm. But, I have an idea of a possible way to extend this method for FP props. I'll have to crank out a new test card, and a slight change to the test procedure to gather rpm data at each fuel flow. Stand by a day or two and I'll open the doors to FP planes.

Have data to send you.

Kevin - Check PM.

Kevin, nice work in progress!

A couple things - it does seem that those with flow transducers before the fuel pump wander around a bit. I have mine after the fuel servo, and is usually will only flicker between tenths of a gallon/hr. Maybe those reporting data to you can also give you an idea of tank to tank reliability of the reported fuel burn vs actual.

I would think the variation in mixtures between cylinders will be difficult to handle - the few carb'd planes here that we have taken data for have variations of 1 to 2 gph between first and last peaks. It is more like four engines connected together. Perhaps you have a method for dealing with that.

Good luck - we will follow this one with interest!

Prop efficiency

might be another variable you want to look at. I'd at least get information on the prop used so you can evaluate if this is an issue...

This flight testing stuff reminds me of a vehicle dynamics class I took at UofI way back. Real hardware and real data. Fun stuff!

frazitl said:

might be another variable you want to look at. I'd at least get information on the prop used so you can evaluate if this is an issue...

Click to expand...

Yeah, prop efficiency is definitely a variable. It is a very difficult variable to deal with though, as it is difficult to get prop efficiency maps from the manufacturers, and even if you get them you need to take them with a grain of salt, as they are theoretical predictions, which assume a generic engine cowling shape. According to Les Doud from Hartzell, the RV cowling shape is favourable compared to the generic one they assume. And even if you could get prop efficiency maps, they are a real PITA to deal with.

Fortunately, over the small range of powers and speeds that I am looking at for each test, the prop efficiency won't change much at all. So I assume it is constant. The error that this introduces is certainly much smaller than the errors in IAS and fuel flow.

Initial Results Look Good

I've got the first two data sets. Thanks Webb.

These first results are very encouraging. I plotted calculated power vs IAS, and the ROP and LOP test points match up very nicely. If I assume that each IAS could have up to one kt of error (which is really quite a small error, if you consider the small variations you see in IAS even in smooth air), all test points fall within 1 hp of a line of perfect fit.

That suggests that this method might actually work, and it also suggests that our intrepid RV test pilot did a pretty good job gathering the data. Congrats to Webb.

I'll hold off posting the data plots until I get his permission. He may not want Bob Axsom to know just how fast his RV is.

Fixed Pitch Prop Test Cards

I created fixed pitch prop test cards in Excel (and also PDF format). The difference with fixed pitch prop is that the rpm will change when you change the mixture. So, we need to also record rpm at each mixture, and I need to attempt to deal with the impact on the power calculation. I think I have a way to deal with that complication, but we'll see how well it works once I have some data to analyze.

I suspect the data from aircraft with fixed pitch props will have more noise than data from aircraft with constant speed props. That is because the rpm will vary during the small IAS variations you get even in the best attempt at level flight. These rpm variations cause the power to vary, which makes the IAS variations even worse. The use of an autopilot with altitude hold, if available, will help immensely.

People with constant speed props can get test cards in Excel (and also PDF format).

Data Plots - Webb Willmott

Webb Willmott sent me data from 2400 rpm, full throttle, at 8,500 and 10,500 ft.

The first two plots are IAS * calculated power vs IAS^4. In a perfect world, with perfect data, zero error in the airspeed system, constant prop efficiency, and a perfect way to calculate power, all points would fall on a straight line. The real world is not perfect, so we see some noise in the data. But the ROP and LOP points line up nicely, which suggests that this method of calculating power may be able to account for different mixture settings.



The next two plots are calculated power vs IAS. You can see that if we assume any of the points could have an error of one kt (which is a very small error, given the normal variations in speed) that all points fall within one hp of the fit line. Very encouraging.


Thanks Webb.

Updated Fixed Pitch Test Cards

A sharp-eyed RVer pointed out that I had an error in the fixed pitch prop test cards test card (PDF format). I had built the fixed pitch one starting from the constant speed card, and hadn't changed the text that talked about keeping rpm constant when you changed the mixture. I knew that was impossible to do, but forgot to change the text.

Now I have changed the procedure description to say to keep the throttle position constant while you change the mixture. The updated cards are at the above links. Thanks Howard.

People with constant speed props can get test cards in Excel (and also PDF format).

Updated Cards - Added MP

I updated all the test cards, yet again, as I realized it would be useful to also record manifold pressure (if the aircraft has an MP gauge). That will provide an answer to the inevitable question of "how does the power calculated by this method compare to that from the Lycoming power charts?"

See earlier posts for links to the test cards.

First set of Fixed Pitch Data

I got some more power from fuel flow data today - this data was from Hevansrv7a's RV-7A with "Superior IO-360 Plus (roller lifters) 180 HP", two mags and a Catto three blade fixed pitch prop. He got me four different fuel flows at about 2500 rpm at 7600 ft - one pretty close to peak power, and one at peak EGT and two more LOP. I ran the data through the Lycoming power from fuel flow method, with a hypothetical addition to the method to make it work for fixed pitch prop. The plots certainly look good. The variation of speed vs calculated power looks very good, which suggests that this power calculation method provides powers that vary correctly with fuel flow.

The basic method as described by Lycoming assumes the rpm is the same with mixture for peak EGT as it is at the fuel flow of interest. Thus the method is only workable for a constant speed prop, as the rpm will vary with fuel flow if you have a fixed pitch prop. I created a hypothetical extension to Lycoming's method, that assumes that fuel flow will vary linearly with rpm for small changes of rpm at a constant MP, altitude and mixture control position. So, for example, if we have 2500 rpm and 10 gph at peak EGT, and 9 gph and 2400 rpm at the fuel flow of interest, I assume that if you could keep rpm constant, the fuel flow at peak EGT at 2400 rpm would be = 10 * 2400/2500 = 9.6 gph. Given that this calculation method is much less sensitive to errors in fuel flow at peak EGT as it is to fuel flows at the test condition, I think this approach will work for fixed pitch props.

It is too early to declare victory yet, but the first two results are encouraging. Note: I don't yet have a very good idea as to whether this method accurately calculates power. At the moment my investigation has focused on whether it provides a calculated power that varies correctly as the mixture varies. The question of whether it provides the correct power value is a more difficult one to answer. I hope to eventually have enough data to be able to compare the power values from the classical Lycoming power charts. But even that will not provide a definitive answer, as there are so few of our engines actually match a configuration that was delivered by Lycoming, so it is hard to find a good power chart to use for every engine.

Thanks Howard.



Keep the data coming guys.

% Power while LOP

Back in my Cirrus days we used a formula to calculate % Power which is directly proportional to Fuel Flow while LOP.

Formula: FF X 14.95 = HP, HP/Max HP = % Power

7.7gpm x 14.95 = 115.12HP, Assuming a 180HP IO-360, 115.12/180 = 64% Power. Sound about right?

I use that formula for an 310HP IO-550 but I believe the constant is constant for our internal combustions engines but, only while running LOP. Walter Atkins might chime in to verify.

Gerry

ghatch said:

Back in my Cirrus days we used a formula to calculate % Power which is directly proportional to Fuel Flow while LOP.

Formula: FF X 14.95 = HP, HP/Max HP = % Power

7.7gpm x 14.95 = 115.12HP, Assuming a 180HP IO-360, 115.12/180 = 64% Power. Sound about right?

I use that formula for an 310HP IO-550 but I believe the constant is constant for our internal combustions engines but, only while running LOP. Walter Atkins might chime in to verify.

Gerry

Click to expand...

Might be a Misprint

Yes, it should be gph... sorry

ghatch said:

Back in my Cirrus days we used a formula to calculate % Power which is directly proportional to Fuel Flow while LOP.

Formula: FF X 14.95 = HP, HP/Max HP = % Power

7.7gpm x 14.95 = 115.12HP, Assuming a 180HP IO-360, 115.12/180 = 64% Power. Sound about right?

I use that formula for an 310HP IO-550 but I believe the constant is constant for our internal combustions engines but, only while running LOP. Walter Atkins might chime in to verify.

Click to expand...

This is essentially what this old Lycoming method is doing, but it tweaks things to account for compression ratio (higher compression is more efficient, so you get more power from the same fuel flow) and rpm (less power lost to friction at low rpm, so more power from the same fuel flow). It also accounts for how far LOP you are - their data shows the efficiency dropping off if you go too far LOP. And, there is data to cover ROP too.

Kevin Horton said:

I've got an old Lycoming document that describes a method to determine power during cruise performance testing, using fuel flow as the main input. It isn't usable real-time in flight, but it could be used in conjunction with flight testing to produce power setting tables for a handful of combinations of rpm, MP, altitude and fuel flow. I've played around with this power calculation method a bit, trying to figure out if it produced consistent, credible results. If it does, I'll document it in a Kitplanes article and produce a spreadsheet to use it. If it doesn't provide consistent results, then I'll drop it. I'll give an overview of this old Lycoming method in the next message in this thread.

The big question I need to answer is does this method produce a consistent power no matter what mixture is used. I will answer that question by plotting speed vs calculated power, with points at the same rpm, MP and altitude, but at several different fuel flows. I've got a bit of data from my RV-8, but it will be some time before I can get any more, and I am not completely satisfied with the stability of my fuel flow indication, which makes is difficult to draw solid conclusions from my data. Also, I really need data from more than one aircraft, with more than one pilot, to see whether this method is useable in the real world.

Click to expand...

Kevin,

Where did this analysis end up? Ever do that Kitplanes Article?

Noah said:

Kevin,

Where did this analysis end up? Ever do that Kitplanes Article?

Click to expand...

I got enough data to satisfy myself that the method is valid. I started drafting a Kitplanes article, but never finished it. It is in the queue of a list of flight test articles I intend to write. I just need to get off TDC and crank them out. I'm hoping to take a few days off in the fall and get started.

Looking forward to it, Kevin!

Kevin Horton said:

It is in the queue of a list of flight test articles I intend to write. I just need to get off TDC and crank them out.

Click to expand...

Yes. You Do.