Understanding Performance Flight Testing: Kitplanes and Production Aircraft

By Hubert C. Smith

*Covers lightplane performance flight testing methods, measures, and computer applications
*Includes CD-ROM with sample spreadsheets containing equations to help readers perform their own flight tests
*Describes GPS (Global Positioning System) test method for airspeed calibration and rapid-wind camera method for takeoff performance

• Language: English
• Publisher: McGraw-Hill Education
• Release date: Sep 18, 2001
• ISBN: 9780071662604

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To my mother and father, who made my career possible, and to the little town of Dillsburg, where I grew up

Contents

Acknowledgments

Introduction

1 Effects of Atmosphere

2 General Flight Test Procedures

Methods of Obtaining Data

Gross Weight Determination

Weather for Flight Testing

Use of Equations

Presentation of Data

3 Airspeed Calibration

Airspeed Errors

Test Procedure

GPS Method

True Airspeed

Airspeed Calibration Example (Speed-Course Method)

Airspeed Calibration Example (GPS Method)

4 Stall Speed

Bank Effects on Stall

Test Procedure

Stall Speed Test Example

5 Takeoff Performance

Forces Acting

Test Procedure

Photo Recording Method

Velocity Calculation

Wind Correction

Altitude Correction

Takeoff Distance to Clear Obstructions

Takeoff Test Example

6 Climb Performance

Drag

Power Required

Power Available

Rate of Climb

Test Procedure

Rate-of-Climb Test Example

Time and Distance to Climb

7 Cruise Performance

Power Determination

Cruising Speed Test Procedure

Maximim Speed Test Procedure

Range

Range Test Procedures

Endurance

Cruise Performance Example

8 Descent and Landing Performance

Determining Glide Ratio

Landing

Landing Test Procedure

Glide Distance Example

Landing Distance Example

9 Computer Methods

Airspeed Calibration

Stall Speed

Takeoff Performance

Rate of Climb

Cruise Performance

Glide Performance

Landing Distance

Conclusion

10 Evaluating Modification Effects

Speed

Climb Performance

Stall Speed

Appendix A:Symbols,Conversion Factors, Data Sheet

Appendix B:Performance Data Examples

Bibliography

Index

Acknowledgments

I am very grateful to Capt. William L. Posnett, USN (Ret.), former Navy test pilot, for developing a number of the data reduction methods used in Chapter 9, and for reviewing that chapter. I would like to thank Micro Aerodynamics, Inc. and Sporty’s Pilot Shop for providing pictures, as well as Cessna Aircraft Corp. and The New Piper Aircraft, Inc. for allowing me to publish performance information from their handbooks. I am indebted to Kristie Kalvin for her excellent typing services on this edition. Also, not to be overlooked is Karen Rider, who had the unenviable task of translating my handwritten notes into a typed manuscript for the first edition of this book back in 1982.

Introduction

In the late 1930s, Clarke Gable starred in the movie Test Pilot. He was dashing, daring, adventurous, rugged, and not too terribly bright. In appropriate garb he could equally well have played a cowboy, private eye, explorer, sea captain, or any one of dozens of heroes typical of that era. Of course, the helmet and goggles, the leather jacket and boots, and the white scarf unmistakably identified him as a pilot. In this role he threw airplanes around the sky, put them into screaming dives, and otherwise pushed them to and beyond their design limits, often to the chagrin of his employers. Sometimes he barely escaped by parachute as the airplane literally fell apart around him. Thus was established the Hollywood image of test flying.

Real-life testing (like most subjects exploited by the film industry) is quite different, however. New airplane designs are tested very carefully and methodically. They are flown only by the most skillful and experienced of pilots whose brains usually exceed their brawn. Their judgment must be honed to an even finer edge than their skill, and procedures must be followed to the letter.

In the early stages of flight testing, the airplane is very gently maneuvered to determine its degree of stability and control. Only after its handling qualities have been determined as acceptable is it pushed to test its endurance near design limits. If any problems arise, it goes back to the drawing board (or computer display, nowadays).

Once the airplane is proved to perform safely, it then progresses to another stage of flight testing. In this stage the airplane is carefully tested to see if it meets the performance that was expected of it when it was designed. This procedure is, therefore, appropriately termed, performance flight testing. Aircraft manufacturers employ rather sophisticated and complex equipment to make this determination, particularly in the case of military or other high-performance aircraft. For airplanes on the other end of the performance scale, there are many old techniques that are much simpler and work almost as well. Many of these can be employed by the average pilot with little technical experience beyond that normally required to obtain flight ratings. These procedures are pretty much devoid of thrills, involving maneuvers less hair-raising than those typically required to satisfy a commercial pilot examiner. They do, on the other hand, require some precision flying if fairly accurate results are to be obtained. Such precision would need to be at about the level expected in normal instrument flying.

But why go through a lengthy procedure to flight test an airplane that has already been tested by the factory? There are several answers to this obvious question. First of all, the manufacturer tests a brand new, tightly fitted and carefully rigged airplane. Aircraft deteriorate, aerodynamically, with age. Scratches and dents gradually appear; door, window, and other seals begin to leak; patches are added to repair minor damage; and the continual disassembly and assembly required for inspections cause minor distortion and misalignment of airframe components. None of these flaws means very much in itself, but collectively they can add up to a significant decrease in performance. There is also decreased power output with engine wear. New airplanes also have brand new engines when performance is measured. All of these factors can mean a noticeable difference between the performance of an airplane right off the production line and one 25 years old with 5000 hours on it.

Then, too, not all airplanes are exactly alike, even when new. Surely you have seen ads for used airplanes where the craft was extolled as exceptionally fast for a Skybird II. Some of these claims may not be exaggerations. Certain airplanes just turn out to be slightly better performers than their more normal siblings. By the same token, some also turn out to be a bit more doggy. You never see this fact mentioned in ads, however.

So far, we have been discussing production airplanes. If you are completing a homebuilt, then the necessity for flight testing should be obvious. Even those built to the exact specifications of the designer will vary considerably with construction technique. And what home-builder has not substituted here and there for costly or unavailable materials or parts? In fact, one of the big attractions of homebuilding is the freedom to customize the airplane. You usually have a choice of engines, propellers, wheels, instruments, and other accessories. The temptation to make further modifications is great. Changing from conventional to tricycle landing gear or adding wheel fairings can often be accomplished without completely redoing the original design. But these modifications cause changes in performance. Even the addition of a canopy to an open cockpit can make a difference. Consequently, very few homebuilts of the same basic design are really alike. If you fly one of these, you really should test it to determine its particular performance. Of course, this goes without saying if you are building an original design.

There is one more reason to go through a series of performance flight tests, and that is simply to learn more about the airplane’s performance. Certainly, you can refer to the charts in the operator’s handbook if you fly a fairly average production airplane. But the numbers here take on a deeper meaning when you have run through the tests and verified them for yourself. There are also a number of things revealed that do not show up in the standard performance charts. Few pilots know such things as the rate-of-climb when flying at other than best rate-of-climb speed or the L/D ratio at various airspeeds. Even the maximum L/D ratio is an obscure number to veteran throttle pushers. The variation in stall speed with gross weight or the change in takeoff distance with density altitude will become very apparent as you plot it out yourself. In short, testing your airplane will enable you to understand its performance a lot better and will, consequently, make you a safer pilot.

This book describes a series of tests by which most of the performance information normally required for airplane operation can be obtained. The tests are designed to be carried out by the average light airplane pilot or someone with equal aviation background. No special equipment is required other than a stopwatch and a basic electronic calculator. In some cases, such items as a tape recorder or camera are helpful but not absolutely essential. The standard instruments in the airplane are the primary devices used to obtain the necessary measurements. For many of the tests only an altimeter, an airspeed indictor, and an outside air temperature gauge are required.

The first two chapters deal with background material and general procedures to be followed in conducting the tests and obtaining and reducing flight data. The next chapter explains the calibration of the airspeed indicator so that accurate information on airspeed is available for the subsequent tests. Chapter 4 outlines tests for the determination of stall speed, which is also needed before some of the other tests can be performed. The next four chapters deal with the primary performance areas of takeoff, climb, cruise, and descent and landing, respectively. In addition to describing the flight test procedure for each of these areas, some discussion is included to explain the reasoning behind the procedure and why the airplane performs as it does. This information is presented in a very basic way and does not involve complex mathematics or require any special technical background for understanding of the material.

A more streamlined approach to data reduction and presentation is given in Chapter 9 for those who prefer to work with digital computers. All of the performance items in the previous chapters are covered here. The data are assumed to be collected in the same manner as before, but, instead of performing numerous hand calculations, they are simply entered into spreadsheet programs. The computer then converts the raw data into useful performance information, and plots it out in easily readable charts. Microsoft Excel is utilized for these programs. In the back of this book, a diskette containing the software for all of these programs, including sample spreadsheets and charts for each of the tests, is provided. The final chapter deals with evaluating performance before and after aerodynamic modifications. Quick and easy methods are presented here for those who do not wish to go through extensive tests for complete performance measurement.

An appendix is