Flight Testing: Rate-of-Climb Performance Charts

 
 
The next step in flight testing is to determine the Rate of Climb graph at Vy (Best Rate speed). We will build on the work previously conducted to determine Vx and Vy speeds. To review, we conducted a saw-tooth climb and descent series at various airspeeds and plotted the data. The spreadsheet makes this easy as much of the calculation and analysis is done automatically, leaving just a bit of interpretation required from the user to "extract" Vx and Vy.

Once these two speeds are determined, a follow-on flight test will gather Rate of Climb (ROC) data while climbing at Vy. The ideal way to collect this data is to load the aircraft to a known weight, select a day that is smooth and stable, and tune the engine so that it is capable of sustaining an extended full power climb. I like to use two weights for my charts: one at a typical solo flight weight of 950 lbs and the other at gross weight. If the atmosphere is windy or bumpy, it will be difficult to hold the climb airspeed precisely and the data will have some scatter. If the engine has a tendency to overheat in extended climb, then you'll have to break up the climb into smaller "chunks" with a level-off and cool down period in between each segment. If you go this route, ensure that you descend approx 500 ft below the next segment to allow the rate of climb to stabilize before resuming the timed portion of the climb. Additionally, lean as needed during your climb to keep the engine happy and producing good power.

Once again, we'll prepare a data sheet in advance and take it along during the flight. The data sheet will list each altitude block "segment" with a space to record outside air temp (OAT) and the elapsed time when crossing that segment. In practice, it might go something like this. Flight is flown at 950 lbs (aircraft, full fuel, and pilot). We determine the fist segment will start at 5000 ft, so the aircraft begins the climb at 4500 ft. Apply full power, lean as needed, pitch to Vy and hold it as accurately as possible (+/- 3 mph is good), then climb steadily until passing 5000 ft. At the exact moment you pass through 5000 ft, start the timer on your stopwatch, and note the OAT. This is your starting point. If you started your stopwatch at the takeoff roll like I do, then note the elapsed time when crossing 5000 ft (say 12 minutes, 20 seconds). Either way is fine. Continue to climb and each time you cross another 500-ft segment record the OAT and elapsed time on the watch. No need to reset the watch each time. It just complicates the process of flying the airplane. We'll plug the data into the spreadsheet and automatically calculate the number of seconds to climb each segment. Continue this routine until you feel you have enough data, you have to stop due to altitude considerations (airspace, clouds, etc), or you're approaching your max altitude, the rate of climb is really slowing down and it's just getting boring at this point. We'll call it 10,000 ft as our stopping point. This will yield 10 segments, each 500-ft high, with elapsed time readings starting at 12:20 (5000 ft) and ending at 26:15 (10000 ft).

This data is then plugged into the spreadsheet. The simple routine at the bottom allows you to plug in the raw elapsed times in and automatically calculate the segment time. Additionally, the spreadsheet used OAT in degrees Celsius, so I have a conversion from F to C as well. These calculations are simply a timesaver. The data doesn't link to anything, so you'll have to manually plug in your data into the top of the sheet. For those spreadsheet-savvy this will be effortless and self evident, but may be confusing for some others. The cells requiring user input are colored, while the rest contain formulas performing automatic calculations.

The spreadsheet converts the data into a Rate of Climb and a Climb Angle for each segment, then plots this data automatically in the charts imbedded in each sheet. The chart fits a linear trendline to the data points and displays the equation for the trendline. You as the user will need to extract the "slope" and "intercept" from these charts, as I'm not savvy enough to have Excel do this automatically. The spreadsheet notes should help in this regard.

The next tab on the spreadsheet takes the equations generated and uses them to generate "cleaned up" graphs that are suitable for including in your Pilot Operating Handbook (POH). This is the bottom-line result of the climb test: a simple chart that shows Vy Climb Rate plotted against Density Altitude. If you repeat the test for a gross weight condition, this line will also appear on the chart. This is probably the single biggest performance chart you'll use in your POH, and it's accurate and specific to your airplane!

You'll notice a chart also included in the sheet that lists a variety of performance figures: Rate of Climb, Time to Climb, Distance flown during the climb, and Fuel Used. All these figures are calculated automatically! This is one side benefit to calculating the "Angle" of climb and not just focusing on the "Rate". We'll use climb angle again when we generate performance table for take off landing.

These performance charts and tables add incredible value to your POH. They are accurate for your airplane, with its myriad of minor variations, and are well worth the time and effort to gather the data. It can be as simple as three or four flights worth of data gathering, but in practice it will probably take more. If conditions aren't quite right and the data isn't real great, fly it again and cross check the results over several tests. You've got 40 hours of flight testing, so put those hours to good use rather than flying circles burning up time.


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Sonex Vy Climb Testing Spreadsheet (Excel)

 

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Updated: 20 Dec 13