Time to climb, try number two

Wouldn't it be nice to know your airplane's real climb performance before the need for this information, such as a stand of tall trees or a mountain range, is staring you in the face?

Ever wonder where that climb performance table in your airplane's pilot operating handbook (POH) comes from? If your airplane doesn't match these published climb rates, maybe you assumed it was your fault. Perhaps you've even pondered techno-philosophical issues such as "What if I change propellers?" or "How will removing the wheel pants affect climb performance?" or "How much will climb rate suffer if I fly a little faster for better engine cooling?"

Good questions.

Unfortunately, you won't find the answers in the POHs of small general aviation airplanes. The good news is you can find out the answers yourself. Finding your airplane's best climb rate and the airspeed at which it occurs is as simple as flying a series of climbs using different airspeeds and measuring the climb rates you achieve at each airspeed.

Why would you think about doing such a thing? How about accuracy for a reason? Wouldn't it be nice to know your airplane's real climb performance before the need for this information, such as a stand of tall trees or a mountain range, is staring you in the face?

Theory

Flying straight and level requires a certain amount of power. Your airplane's power requirement depends on its airspeed, and it should be obvious that to fly faster at a level altitude, you need more power than you do at a slower airspeed. As slow flight teaches, it also takes more power to fly level at just above stall speed.

Figure 1 shows the relationship between the Power Available (Pa) from the engine and propeller and the Power Required (Pr) by the airplane. The Pa curve represents the power that the engine/propeller combination produces at full throttle. If a lesser throttle setting is used, the Pa curve looks the same, but it's lower on the plot.

The Pr curve shows the power the airplane needs for level flight at different airspeeds. Set the throttle to produce more power than that, and the airplane will accelerate if you maintain your altitude - or climb if you maintain your airspeed. Excess power is the difference between the Pa and Pr curves. Your airplane's maximum excess power airspeed is neither the maximum Pa nor minimum Pr airspeed. The speed where the maximum excess power exists is the speed that yields the airplane's maximum rate of climb, and it's called VY.

You don't have to know anything about the power curve to figure your airplane's climb performance. You can see, however, that an airplane has less excess power at speeds slower and faster than VY. Your climb test data should reflect this characteristic.

You can use any of several flight test techniques to find your airplane's climb performance. We'll use a modified check-climb technique. It's pretty straightforward. You climb at a constant indicated airspeed while you time how long it takes to climb through several altitude blocks, i.e. 1,250 feet, 1,750 feet, 3,250 feet, 3,750 feet, etc. You keep climbing and timing until you reach the highest altitude for which you want to know the airplane's climb performance. You repeat the test using different airspeeds each time. From the data you gather, you can calculate which airspeed produces the best climb rate and what that rate is for each altitude block.

There are a few factors you must take into account, but don't let them intimidate you. The basic test technique is simply a series of constant airspeed climbs, something every pilot learns to do. We'll discuss these factors and their effects later, but for now let's get right to the procedure.

Test Procedure

Load the airplane to represent the climb performance conditions you want. This may be maximum gross weight, half fuel with only the pilot aboard, or your typical flight lesson - full fuel and your flight instructor. Whatever loading you use, compute the aircraft's weight and balance and save the form.

Take off and, when you're ready to begin the test, set the altimeter to 29.92. This lets you record pressure altitude during the test, which you use with the outside air temperature (OAT) to determine density altitude. Your finished climb performance charts will be in density altitude. This way, you can use them anytime by knowing the density altitude. Otherwise your charts would be useful only on days when the barometric and temperature conditions match the test day exactly. Record the current altimeter setting so you can reset the altimeter after your test.

Fly several hundred feet below your first altitude block. Use good judgment. Your lowest block should be at least 1,000 feet above reasonably flat and obstruction-free terrain. Establish the climb speed in level flight. Advance the throttle while raising the nose to maintain the test airspeed until you are stabilized in a climb at full power at the test airspeed. Trim the airplane.

If you aren't stabilized as you climb through the bottom of the first altitude block, reduce power, descend, and set it up again. You may want to start at a lower altitude to give yourself more time to stabilize the airplane on the test condition before you reach the bottom of the first block.

With the plane stabilized, note the time or start a stopwatch as you pass through the bottom of the first block. Hold the airspeed constant through the test block and note the time or stop the stopwatch as you pass the top of the block. Record the time, altitude block, and any other data you think useful. Figure 2 shows a sample data card, which also serves as a worksheet for numbers you calculate after the flight. Continue climbing and repeat the procedure for every altitude block up to the highest block of interest.

Data Quality

The old computer adage - garbage in/garbage out - applies to all data. If you have any doubt about the quality of your data, repeat the test. Here are a few guidelines:

Airspeed control - Some of these results are very sensitive to airspeed variations. Traditional flight test parameters limit airspeed excursions to a maximum of one knot. This may sound unrealistic, but any pilot can do it with a little practice and a diligent trim effort.

Smoothness counts - Keeping your airspeed plus or minus one knot at the expense of large or abrupt flight control deflections will contaminate your data. Every time you move a flight control, you change the airplane's drag. The bigger and faster the control surface moves, the greater the drag change. Some small adjustments are expected, but make them smoothly.

Turbulence - To avoid thermal turbulence fly your test early in the morning or just before dusk. In a small airplane it only takes one bump to invalidate your airspeed or control deflection tolerance. Avoid flying near rapidly changing terrain that may produce thermal variations or up/down drafts. You shouldn't be anywhere near clouds.

Pitch attitude reference - As good as some attitude indicators are, their indications are generally too coarse for the fine airspeed control you need. Perform the test on a clear day with a distinct horizon, and use the horizon to maintain the proper pitch attitude for the climb. A grease pencil mark on the windscreen or side window may help you detect and correct tiny pitch changes before they affect the airspeed.

Straight climb - Perform the entire test on a reasonably constant heading. Substantial turns, even between altitude blocks, will affect fuel consumption/airplane weight and can introduce wind gradient effects.

Height of altitude block - Accurately timing your climb through a 100-foot altitude block will be difficult in an airplane climbing at 2,000 feet per minute (fpm). On the other hand, maintaining the climb speed tolerances through a 1,000-foot block in an airplane ascending at 300 fpm will be difficult also.

Choose block heights that make sense for your airplane. Make the block tall enough so a one or two second timing error won't make a substantial difference in the average rate of climb through the block. Make the block short enough to avoid any appreciable difference in climb rate or true airspeed between the bottom and top of the block. It's okay to have taller blocks at lower altitudes, where the climb rate is better, and smaller blocks at the higher altitudes.

Take a break - You don't need to maintain the precise test tolerances between the altitude blocks. You're not recording any data, so relax until you approach the bottom of the next block. But don't let the airspeed stray too far from the target speed, or you'll just have to work that much harder to re-establish the stabilized condition.

Don't trust the VSI - Feel free to record the vertical speed indicator reading within the test block, but use it only for correlation with your timed data. Most VSIs are just too inaccurate for this test.

Subjective assessment - Even if you've flown the test profile within the limits, you may want to make a qualitative comment about the test. You'll know whether you really nailed it right on the airspeed, with perfect timing and the pitch attitude set in granite - or whether you pushed the limits with airspeed going from one knot fast to one knot slow four times during the block, constantly searching for the exact pitch attitude, etc. Making such a note can help explain a wayward data point later.

After you complete your series of tests through the altitude blocks, relax. Make a 180-degree turn and descend. On the way down, fly level at the mid-point of each test block long enough for the OAT to stabilize, then record it. Some OAT gauges are too slow to react to the rapidly changing temperature during the climb, particularly if you're flying a high performance machine with an eye-watering climb rate.

You'll probably want to land at this point and review your data. Before you land, don't forget to reset your altimeter to the current barometric pressure. You now have one airspeed mapped. Re-load the airplane to the same condition it was before this flight, and repeat the climb test at a different airspeed. Re-load again and fly another different airspeed. Continue the process until you fly every airspeed of interest. Because you base your tests on density altitude, you don't have to fly all your tests on the same day.

Data Reduction

When you have a handful of data cards brimming with numbers, it's time to transform them into something you can use. Data reduction is a bit time-consuming but it's not difficult, and you don't need any math skills beyond those necessary to pass a private pilot knowledge test. The goal is to determine which airspeed yields the best climb rate, what that climb rate is, and how VY and climb rate vary with density altitude.

You'll reduce one set of data (one airspeed) at a time, and this data should be on one data card. Compute the height of each altitude block by subtracting the block's bottom altitude from its top altitude. Calculate the rate of climb (ROC) through each block by dividing the block height by the time it took to climb through it. If the block is in feet and the time is in seconds, multiply the result by 60 to get an answer in feet per minute.

Altitude blocks are necessary for timing, but they're cumbersome for flight planning. It's easier to use the block's mid-point altitude. This is consistent with using the average ROC through the block, although we know the airplane climbs marginally faster at the bottom and marginally slower at the top of the block. If the blocks are excessively large you will see an appreciable difference in ROC between bottom and top, so choose your block heights accordingly.

You already know the pressure altitude and OAT for each altitude block mid-point. Using a density altitude chart or flight computer, compute the density altitude (DA) for the midpoint of each altitude block.

It's time to draw, so get a sheet of graph paper. Plot ROC versus DA as shown in Figure 3a, then connect the dots. Repeat the data reduction for every airspeed tested. When you're finished, you should have a plot resembling Figure 3b.

When you're finished you know that airspeed V3 gives you the fastest ROC among the airspeeds you tested, but you don't know whether a speed between V2 and V3 or between V4 and V3 gives you an even better ROC. To find this intermediate speed, plot ROC versus airspeed for a particular DA. Draw a vertical line up from the DA axis in Figure 3b through the lines of different airspeeds. Draw horizontal lines from the intersection of the vertical line you just drew and each airspeed line it intersected to the ROC axis (Figure 3c). Now you can read the ROC for each airspeed at this DA.

On a clean sheet of graph paper plot ROC versus airspeed (Figure 4a), and include the corresponding values from Figure 3c. Fair a curve through these points. You can now see the maximum ROC occurs at the top of the curve at a speed between V2 and V3. Repeat this cross-plot procedure for several density altitudes, i.e. several vertical lines on the ROC versus DA plot. Your plot of ROC versus airspeed should now look like Figure 4b. The peaks of each curve represent the maximum ROC and VY for each density altitude.

Two more steps and you're finished. Figure 4b is nice, but it only shows VY and the corresponding ROC for the density altitudes plotted. To create a plot of VY for all density altitudes, you'll have to do another cross-plot. Draw vertical lines from the peaks of the DA curves down to the Airspeed axis of Figure 4b. Where the vertical lines cross the Airspeed axis is the VY for each DA. Use these VY/DA data pairs to create a new plot of VY versus DA (Figure 5a). Fair a line through the data points.

The final step is just labeling another vertical axis on Figure 5a. The dotted line connecting the peaks of the curves in Figure 5 shows the relationship between VY and ROC. Draw a series of vertical lines from the Airspeed axis to this dotted line. Then draw horizontal lines from the intersection of the vertical lines you just drew and the dotted line connecting the peaks to the ROC axis. Notice that for every VY there is only one ROC associated with it regardless of density altitude. Draw another vertical axis on the right side of Figure 5a so it looks like Figure 5b, and just annotate the ROCs associated with the VY directly across from it.

Now you have a handy, single-source plot that depicts your airplane's VY and ROC for a range of density altitudes. No longer will you have to look up tables, make percentage calculations for non-standard temperatures, or wonder about the applicability of POH figures to your tired/old/modified airplane.

These data are also useful for variations in climb schedules that POHs rarely address. Figure 5, for example, shows the ROC penalty you'll pay if you choose to perform a cruise-climb at a speed faster than VY for better engine cooling or outside view over the nose.

Ready to Do It Again?

Figure 5b applies to the airplane's weight, external configuration, and center of gravity during the tests. You can repeat the tests for a variety of these conditions to create a more complete reference. Because the final plot will have the same axes, you can simply add the curves pertaining to different weights, etc., but don't forget to label which is which.

If you'd rather spend your flying time some other way, you can test and plot the extremes and interpolate as needed. For example, your plot can contain a curve for maximum gross weight and minimum expected weight. All other weight conditions will fall somewhere between. This should be enough information for a conservative flight planning approach.

You can do the same thing for different configurations with a clean airplane curve and a gear-down, full-flaps airplane. Center of gravity location matters in theory, because it affects the airplane's trim drag. In reality, the effect on VY is probably small enough to ignore for the light general aviation airplanes most of us fly.

This procedure omits a few traditional data corrections. Altimeters are calibrated for standard day temperature gradients. If the air temperature is 20?F warmer or colder than standard on the day you fly your test, your timed climb rates could be as much as four percent lower or higher respectively than the airplane's actual climb rate.

Besides the temperature effects on engine power that we already compensated for in our density altitude calculations, there is another effect. This correction amounts to less than five fpm for most general aviation airplanes and is neglected. Finally, let's look at wind. A wind gradient (change in wind speed as the airplane climbs can affect the airplane's pitot/static instrument indications. To help avoid this complication, perform your climb tests perpendicular to the wind.

What if you go through all this and find your airplane's climb performance is exactly what the POH claims? You won't have wasted your time because the test technique motivates you to hone your piloting skills. You'll learn flight test techniques that can be used in any airplane. You'll see that you don't need a doctorate in rocket science to do performance flight testing. You'll know your airplane better. Most importantly, you'll have confidence in the operator's handbook climb performance tables.

The final point to emphasize is safety. The tests described necessitate a diligent instrument scan and potentially prolonged nose-high pitch attitudes. Both affect your ability to see and avoid other aircraft. Be careful. Don't perform these tests on a gorgeous Saturday morning near a busy airport. Having someone aboard to help share the visual responsibility is a good idea. Think about lowering the nose or performing a belly check between altitude blocks. Keep an eye on your engine. Low speed climbs at full power tax the engine and inhibit cooling. Remember to fly the airplane first - collect data second.