Kevin Horton
Well Known Member
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.
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.
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