Testing Fuel Flow, Airspeed, Endurance, and Range

I had N2468Z, my Bede BD-4C airplane, up for 1.7 hours on April 6. This flight was designed to test fuel flow and accurately measure airspeed so that I could calculate the airplane’s endurance and range, and calibrate the airspeed indicator. I flew up the Mississippi River past Quincy, IL and back. The flight went fine, cruising above the scattered white puff-ball clouds on an otherwise blue sky day.

Updated April 13: I checked the EFIS and found that the K-factor for the fuel flow sensor was set to 7,000 instead of 18,000. I applied a correction of 7/18 to the EFIS fuel flow measurements and updated this blog post.

Updated October 6: Corrected captions on ground speed graphs.

Before the flight, I calibrated the altimeter. It had been reading about 90 feet too high, 530 feet in my hangar instead of the actual field elevation of 437 feet. With a few taps on the setup screen of my MGL iEFIS, I removed this error.

Flight Test Card

I have mentioned that I make a flight test card for every flight. I print this and have it on my knee board for ready reference. I embolden the key words, which makes it easier for me to find the important bits of information during the flight. For this flight, the test plan was:

1. Use RIGHT fuel tank for initial and final parts of flight.
2. Climb to 4500 on northwest course
3. Set power to 75% power: 2400 RPM, 24.4” MP [manifold pressure] and lean mixture
4. Switch to LEFT fuel tank
5. Fly 30 minutes
6. Switch to RIGHT fuel tank
7. Collect airspeed calibration data:
   1. Fly heading 000 for 2 minutes
   2. Fly heading 120 for 2 minutes
   3. Fly heading 240 for 2 minutes
8. Climb to 5500 on southeast course
9. Set power to 65%: 2400 RPM, 21.6” MP and lean mixture
10. Fly 15 minutes
11. Set power to 55%: 2100 RPM and lean mixture
12. Fly 15 minutes
13. Return to airport
14. Refill LEFT tank and note amount of fuel added

This gave me several bits of information.

By flying for exactly 30 minutes off of the left fuel tank, and then refilling it after the flight, I learned exactly how much fuel the airplane burns at 75% power in 30 minutes and, by simple arithmetic, how many gallons per hour it burns. I had corresponding data from the EFIS and I could check the actual fuel burn against the fuel flow that the EFIS recorded. The EFIS also recorded airspeed.

By flying 15 minutes at 65% power and 55% power, I got corresponding data for lower, more fuel efficient, power settings.

The airspeed calibration data allowed me to check the accuracy of the airspeed indicator.

After I landed, I filled the left fuel tank with 6.1 gallons of avgas, for a fuel flow rate of 12.2 gallons per hour. That is just what I had hoped to get from my 200 HP Lycoming IO-360 engine running at 75% power.

Summary of Results

When all was said and done, I learned a lot from this flight:

75% Power

• Airspeed: 133 knots
• Fuel flow: 11.6 GPH
• Endurance: 4.0 hours
• Range: 525 NM
• Range with 1 hour reserve: 394 NM (St. Louis to Minneapolis in 3 hours)

65% Power

• Airspeed: 125 knots
• Fuel flow: 8.4 GPH
• Endurance: 5.5 hours
• Range: 684 NM
• Range with 1 hour reserve: 560 NM (St. Louis to Houston in 4.5 hours)

55% Power

No valid data.

Airspeed Indicator Error

Insignificant (1.1 knots)

Other Stuff

I confirmed that the EGT sensor for cylinder #1 is not working.

I tried a thinner seat cushion. That helped with the lack of headroom and the seat was still comfortable. Good news!

Fuel Flow Analysis

After landing, I extracted the flight data from the EFIS and graphed it. Here is an overview of the flight. (You can click on any of these graphs to enlarge them.) You can see that I flew the outbound segment of the flight at 4500 feet and then climbed to 5500 for the return. You can see the airspeed drop in the second segment, when I reduced power from 75% to 65%, and then the airspeed dropped again, when I further reduced power to 55%. Finally, you can see the airspeed increased as I went “downhill” while descending to land at the airport.

Here is an overview of engine RPM and manifold pressure and fuel flow rates. The combination of altitude + RPM + manifold pressure determines power; that is why I wrote down specific altitudes, RPM, and manifold pressure values for each of the three tests.

After setting the RPM and manifold pressure, you adjust the mixture (air to fuel ratio) for best efficiency. There is no sense in burning more gas than necessary, especially when avgas at my home field costs about $4.25 per gallon.

The green line on the graph below is fuel flow. From the measured 6.1 gallons that I added to the left tank after the flight, I would expect the fuel flow during the first half hour of this flight to be 12.2 instead of the 30 that the graph shows. This tells me that I need to change the K-factor of the fuel flow sensor in my EFIS setup screen.

The 75% power segment of the flight is from 15 to 45 minutes.

The 65% power segment is from 55 to 70 minutes. You can see that I reduced manifold pressure, but not RPM, during this segment, and the fuel flow dropped accordingly.

The 55% power segment is from 75 to 90 minutes. You can see that I reduced RPM, but not manifold pressure, from the settings in the 65% power segment. The fuel flow did not change significantly, though, which indicates that I did not properly lean the mixture. The engine was burning much more gas than necessary and my test yielded no useful data during this segment.

Here is an enlargement of the 75% power portion of the flight. I think that it is kind of interesting to see how the mechanical propeller governor does hold the RPM perfectly steady; it varies by 30-40 RPM. It is the sort of variance that I would not even notice in the airplane but shows right up when I graph the data. I think it is OK but will check with the manufacturer to confirm.

Here is an enlargement of the 65% power segment of the flight.

Airspeed Indicator Calibration

To calibrate the airspeed indicator, I configured the airplane for cruise in level flight. Using my GPS, I then measured the ground speed while flying on headings of 000 degrees, 120 degrees, and 240 degrees. While flying these three segments, I did not vary the altitude, the RPM, or the manifold pressure. By averaging these together, I got my actual average airspeed. I then compared it to my the speed from the airspeed indicator.

Here are the graphs for flights on the three heading.

I calculated an average ground speed for each leg and then averaged those three values together to get the average speed of 132 for the entire test.

Ground Speed @ 000 degrees = 145.57803796748894
Ground Speed @ 120 degrees = 124.60515658747298
Ground Speed @ 240 degrees = 126.99368665891345

Average Speed = 132.39229373795845

I performed similar calculations to get the average airspeed for the test and it came out to 133.5 knots. Total error was just 1.1 knots, small enough that I consider the airspeed indicator to be “perfectly” accurate.

Endurance and Range

Here are my calculations for endurance and range. Note that range is in nautical miles. Add 15% to get statute miles.

Endurance and Range at 75% Power

usable_fuel = 46
gallons_per_hour = 11.589352050565791
airspeed = 132.65814670655223
Endurance = 3.9691606398093917 
Range =  526.541494457707 
Range with 1 hour reserve =  393.88334775115476

Endurance and Range at 65% Power

Although the fuel flow from the EFIS did not have the right K-factor, the ratio of the values it reported at 75% and 65% power are accurate. So I can use that ratio (17.6 / 22.4) to calculate the gallons per hour at 65% power.

gallons per hour = 8.371649907044901
airspeed = 124.5666266944043
Endurance = 5.494735268526952
Range =  684.4606369791743
Range with 1 hour reserve =  559.8940102847699