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Stage Three: Expanding the Flight Envelope
By Tony Bingelis (originally published in EAA Sport Aviation, March 1989)
Sometime after that memorable initial test flight, and before you fly your airplane again, check conditions inside the engine compartment. You can't be too careful at this early operational stage. Remove the cowling and look for fuel and oil leaks, loose clamps, wiring problems, and the security of all installed components. It might, also, be advisable to remove all inspection covers and take a look inside.
Incidentally, you should plan to make a similar inspection of the airplane after it has been flown about 10 hours - just to reassure yourself that everything is O. K.
What Lies Ahead
Your flying for the next 25 to 40 hours will have to be within the limits of your assigned flight test area. This could be very monotonous if you were to merely bore holes in the sky with no particular objective other than flying off the mandatory hours in ever decreasing concentric circles like the mythical "Kiki Bird". But, this need not be.
That initial test flight proved your airplane will fly and that it is reasonably controllable. Now you will have to prove to yourself that it can perform safely under a variety of service conditions.
This means you should now begin to gradually, and carefully, expand its flight envelope. After all, there's still much you don't know about the airplane and a lot of questions need answers.
For example, your initial flight was probably made with only half the fuel capacity and with a minimum payload. But how will the airplane behave with full fuel, and at gross weight . . . and what effect will that have on the CG? Will it remain safely within design limits?
Although you may have been pleased with the controllability and flight characteristics exhibited on that first flight, be realistic and accept that you may yet have to face up to some quirks that are not so good.
At this early stage, it's normal to experience a degree of anxiety and doubt regarding the homebuilt's controllability in the high speed ranges, and most of all regarding its freedom from flutter. These particular evaluations are considered critical and are potentially the most dangerous characteristics to explore.
The only way to get all the answers you want is by working the airplane through a variety of flight conditions while gradually working up to the maximum performance limits you hope to establish for the airplane. This is what is meant by "expanding the flight envelope".
Start your evaluation tests by systematically performing all the ordinary maneuvers normally encountered in flight. We all know what these are. There's nothing complicated about them. They should include at least the following:
- Climb performance tests
- Establish service ceiling and absolute ceiling
- Slow flight maneuvers - Gliding tests - Stall tests - Stability tests - Landing/take-off trials
The following evaluations and ongoing tests can be worked in as you like throughout the entire test period:
- Airspeed calibration tests (as soon as possible)
- Engine cooling evaluation - Fuel consumption calculations - Propeller evaluation - CG loadings - Performance checks
NOTE: The more potentially dangerous test evaluations, such as the following, should be deferred to sometime much later in your test program:
- Structural flight testing - Flight flutter tests - Spin tests - Inverted flight - Aerobatics
Each new maneuver and test you perform will reveal more and more about the airplane. In addition, performing these test maneuvers will help sharpen your skills for handling the new airplane as well.
Repeat tests, if necessary, until you are satisfied with the airplane's responsiveness, and your ability to perform them precisely . . . after all, you're not going anywhere for the next 25 (40) hours, anyway.
Don't slight any of the simple easy-to-do tests because you feel you should concentrate on others you believe to be more important.
Your assessment may be true, but all of them - even the simple ones - are important as they will provide you with the operational data you should know for your airplane.
Here is a sobering thought. Simple or not, you must assume that each test will involve an element of risk . . . or may even be downright dangerous to perform. Always approach a new test with caution, and be prepared for the unexpected.
Like most builders, you will probably opt for a limited number of very conservative tests with no spins or aerobatics intended.
On the other hand, you may be planning to undertake an extensive series of tests pushing the design limits - with each test thoroughly documented in a scientific manner (calibrated instrumentation, development of graphs, tables, etc.). If that idea intrigues you, go for it! After all, that is the kind of fascination this remarkable amateur built program holds for many of us.
More Pertinent Thoughts
Plan to devote the first portion of each flight to the performance of the one or two test elements selected. Don't waste your time. Know, exactly, what you intend to accomplish during that flight before you take-off. Think out how you will do it - and approach each test carefully and cautiously. Complete only the test items scheduled - no more . . .then spend the rest of the time sightseeing or just basking in the pleasure of flight.
To save time, you may find it convenient to perform two or more test evaluations in the same flight.
For example, you know that you will lose considerable altitude in the process of performing the gliding maneuvers. It would, therefore, be logical to begin the first part of that particular flight with a series of climb tests. The altitude gained can then be used to 'pay' for the gliding portion of the planned test flight.
Record all your observed results - instrument readings and other data, on a knee clipboard, or preferably, on a small pocket recorder. Don't trust to memory alone.
Except as previously pointed out, the sequence in which you schedule the various tests need not be accorded any particular priority. So, by all means, schedule them to suit the weather conditions, and your own personal preference.
Tests flown in windy conditions, and when the air is rough, are very inaccurate and, consequently, the conclusions reached will generally be unreliable or useless.
Do not try to undertake too many tests in one flight, but, by all means, allow as much time for each test as you want.
After each flight, debrief yourself. That is, review the things you did wrong (and right). Study the data gathered and try to absorb what you have learned.
Remind yourself, frequently, that you must correct whatever problems may arise during a flight . . . and do it before undertaking the next one.
Such problems as engine malfunction (however slight), a strange noise or unexplained vibration, signs of longitudinal instability, control difficulty, or major trim problems can be serious and MUST be corrected as soon as you detect them.
Let's review a few typical tests in greater detail.
Climb Tests - Determine Your Best Rate of Climb
Use full throttle and check the rate of climb for several different airspeeds. Start at a fairly low altitude soon after leaving the traffic pattern. At full throttle, stabilize your airspeed and begin your timing as you climb through the next thousand foot level. Note how many feet you climb in one minute, in two, three, four and five minutes. Notice how the rate of climb gradually falls off with altitude.
Beginning again at some lower base altitude, try some climbing turns to the left and to the right. Notice the difference in rudder pressures required. Look for any unusual control difficulties.
Try climbing with flaps deployed 10 degrees, and with half flaps. Could you make a go-around with full flaps?
Determine the Best Angle of Climb
Once again, set up a full throttle climb and note your position over the ground as you pass through a selected base altitude. Continue the climb for 200 feet (500 feet for a high performance homebuilt) and again note your position over the ground. Go back and repeat the process at a different airspeed. After checking your measured climb at several different airspeeds you will know what airspeed will get you up to that 200 foot level in the shortest distance covered over the ground. The results (best angle of climb) are only approximate even when obtained in dead calm air. Nevertheless, the information is useful to know should you have to one day decide whether you can clear some hills beyond the end of a short runway.
At some later date, repeat all of the climb tests at full gross weight and compare the results.
Slow Flight Maneuvers
The idea is to become familiar with the trim and attitude changes that take place while you are trying to maintain your altitude at minimum flight speed, with different power settings. Try a few level turns at what you believe to be the minimum controllable speed. Careful, you could stall unexpectedly. Do these maneuvers at a safe altitude. Try a few level turns with and without flaps, and with the landing gear deployed (if your aircraft is a retractable).
In the event of an engine failure, it would be very nice to know what airspeed will give you the minimum gliding angle. These tests, logically, are most effectively performed following your climb tests because you could then use the altitude gained.
Always clear the area around and below you before performing any maneuvers in which altitude will be lost or gained.
Start with plenty of altitude and complete your last practice turn at least 1,000 feet above the ground (AGL). Clear your engine briefly after each 90 degree turn.
If you don't have a rate of climb indicator (VSI), time your descent through different thousand foot levels.
To learn how your airplane behaves in gliding turns, practice a few and note how the rates of descent changes with airspeed and increases in the turns. It is important to keep your gliding turns coordinated. Try doing them at different airspeeds and record your observations.
These gliding turns are essential to practice because you will be duplicating them each time you turn final for landing. Be careful - an excess of uncoordinated rudder input (slip or skid) and excessive back pressure on the stick can cause the airplane to snap over the top, or snap under to an inverted attitude. At traffic altitude, if turning final this can be fatal.
CAUTION: Should this happen, try to continue the roll with aileron input until you are right side up again. Do not pull back on the stick and split-S out . . . you might exceed structural limitations for the aircraft and pull the wings off . . . if you have sufficient altitude remaining.
Determine and record how much altitude is ordinarily lost in making a 90 degree gliding turn, a 180 degree turn and a 360 degree turn. Make similar checks with partial flaps and with full flaps deployed.
You have done these countless times, too. Run through the whole series of stalls. I think most of us realize that the airspeed indicator doesn't read accurately at stalling speed. That fictitious indicated stalling speed of 38 mph or so would more likely be 58 mph or more. However, the readings are relative and you can believe that your gage will indicate the same stalling speed consistently - if the stall is approached at the same rate each time.
Plan to complete your last stall no lower than 1,500 feet AGL.
Except for accelerated stalls and secondary stalls, approach each slowly, while keeping the nose from turning with the rudder. Allow the speed to bleed off until you feel a slight buffet. Note the airspeed and recover with a smooth forward movement of the stick as power is added. Maybe simply removing back pressure from the stick when the stall occurs would be sufficient for your airplane - maybe not.
However, to begin with, do your stalls by the book. Later you can modify your technique to suit yourself.
Look for any unusual behavior in stalls. If one wing has a sharp tendency to drop, try catching it by applying top rudder, and not by instinctively reacting with aileron input.
Engine Cooling Checks
You will, of course, monitor engine temperatures on every flight. However, you should also study and record the effects produced by aggressive mixture control manipulation, changes in power settings and airspeeds.
Prolonged climbs and glides will probably induce drastic changes in your engine temperatures and you should know to what degree. Remember, hot summer free air temperatures can intensify high engine temperature indications . . . often to a critical degree.
This sounds impressive and complicated. It is not. Stability tests are about as simple a series of checks as any you will make. They are, nevertheless, important. An inherent lack of stability in one or more axes could portend a dangerous condition.
Check for stability in all three axes, longitudinal (pitch), lateral (roll), and directional (yaw).
NOTE: You should delay your stability checks until after you have had the opportunity to get the airplane trimmed so that it will fly hands off. This may necessitate the addition of external fixed trim tabs.
Longitudinal (pitch) stability check -Trim the aircraft for level flight. Pull back on the stick until the nose rises and the speed drops off about 20%. Release the stick. The airplane will nose down and gain some speed before it starts to rise again. These oscillations should die out within 3 or 4 cycles - if the airplane is stable in its pitch axis. If not, the aircraft is unstable and you may have a weight and balance problem you didn't know about that needs correcting. If the oscillations continue or increase, it could be an indication of a serious design deficiency.
Lateral (roll) stability check - Trim level and hold a selected heading with the rudder. Move the stick to the maximum left (or right) position and release it. The wing should return to level attitude. Check the opposite side in a like manner. If the amount of roll stays the same or increases, the aircraft is laterally unstable and exhibits what should be considered a dangerous tendency.
Directional (yaw) stability check -Trim for level cruise and remove your feet from rudder pedals. The aircraft should hold the heading. Push on the left rudder and then release the pressure. The nose of the aircraft should weathervane back to the original heading if it is directionally stable. If not, or if it continues to 'hunt', it is directionally unstable. This is generally due to improper rigging, a misaligned vertical stabilizer, or one that has insufficient fin surface area.
Determine that you are getting the desired static rpm at full throttle on the ground as recommended by the engine manufacturer.
Also, your propeller should load the engine sufficiently in level flight so that the engine, at full throttle, will not exceed its redline limit. Similarly, the engine, with your installed propeller, should not exceed its maximum allowable rpm during take-off at full throttle. Unfortunately, some builders find that the two requirements are hard to meet in the same prop.
If you believe you do have a performance problem due to an inadequate propeller, you should seek help from a propeller professional in selecting the correct one for your airplane. On the other hand, your propeller performance may be as good as you can expect. When it comes to propeller efficiency and performance, some people lie a lot.
Structural Flight Testing
Do not test your airplane to higher structural load limits than those you expect to encounter in the type of flying you will be doing.
Perform your high speed evaluations with a forward CG only.
For these tests, try to borrow an accelerometer (G meter) if you don't already have one installed.
Since it is a self-contained unit with no connections required, obtaining the temporary use of one for your structural flight testing makes good sense. You won't need it after the testing is over.
Testing to a modest 3 G limit should be sufficient for most non-aerobatic lightplanes. (Commercial categories are 3.8 Gs and 4.4 Gs for the Utility category, while a 6 G capability is expected for aerobatic aircraft.)
In essence, the tests usually consist of a series of dives and pull-ups to impose the desired G loads. These pull-ups are performed at gradually increased speed increments. Carefully inspect the structure after each major increment of loading for signs of skin wrinkling, deformity or loose wires and fittings.
I believe the safest way to impose flight loads (Gs) gradually, and with the greatest control, is with steep turns -not by making Hollywood style speed dives and pull-ups.
Flight Flutter Testing
Flight flutter testing, like structural flight testing, requires the airplane to be flown at high speeds. This series of tests is the most dangerous. Unfortunately, these tests are essential to the establishment of the aircraft's freedom from flutter up to its red line (Vne or never exceed) speed. This speed is generally established to be about 10% higher than the maximum cruise speed you would expect to fly.
All aircraft will experience flutter at some speed . . . so, keep your goal conservative.
Conduct your flutter tests at high altitude, wear a parachute and be prepared to use it. If flutter develops, its intensity increases so rapidly that structural disintegration can occur before the pilot can react.
Make your first flutter investigation at a low speed (cruise) trimmed for level flight. Excite only one control at a time beginning with the elevator.
Slap the control stick in an aft direction. If there is no indication of stick oscillations, repeat the test at a slightly higher indicated airspeed (about 5 mph). This means that you will have to dive the airplane to pick up the higher speed.
However, do not slap the control until you have caused the nose of the aircraft to begin to rise, and the speed begins to bleed off. If flutter should develop then, the airplane will already be slowing down and the control flutter may be dampened in time.
Test the ailerons and the rudder in the same manner - by striking the stick on the side (or kicking the rudder). Be sure you never slap (excite) any control until the nose is coming up and the airspeed is bleeding off.
Continue the testing cycles for the elevator, ailerons and rudder at increased 5 mph speed increments until you have tested the airplane to the maximum speed you want. Usually this is up to the figure established by the aircraft's designer, or when a speed 10% over the redline (Vne speed you want to placard) has been reached. All surfaces must be free from flutter up to that speed. But be very cautious in attempting to reach that speed. Flutter can destroy the structure in the blink of an eye.
For all practical purposes, spin testing is considered to be unnecessary for the average low performance aircraft.
Do not attempt spin entry and recovery tests without a parachute, and a reasonable provision for exiting the aircraft if you do have to use it. Do not attempt to perform spins with a known aft CG condition.
It is generally believed that any airplane with a reasonable CG will easily recover from a one-turn spin. However, as the number of turns is increased before recovery is attempted, the greater the risk that the spin might become flat and difficult to impossible from which to recover.
The original military spin recovery technique, and the most reliable one, I think, is essentially as follows:
a) To recover - slam in full opposite rudder (against the spin). The stick is still back.
b) Wait a half turn.
c) Briskly shove the stick forward (down elevator). Be alert. As soon as the spin stops, ease off on the elevator forward pressure before you go beyond that normally steep dive to one that is near vertical, or inverted.
d) Pull up from the dive gradually (without pulling off the wings). Add power as the nose comes up through the horizon.
One or two other spin recovery methods have their advocates and may be equally effective for some aircraft designs.
You may prefer to placard your aircraft, "SPINS PROHIBITED".
Inverted Flight and Aerobatics
Inverted flight without an inverted system (fuel and oil) is impossible for any length of time, so why try it?
Aerobatics should not be performed unless your aircraft is designed with this in mind. If your aircraft is capable of doing some aerobatic maneuvers, record those that you have successfully accomplished and you should be able to get your Operating Limitations issued to reflect approval for those maneuvers.
The scope of foregoing tests is quite broad and should prove sufficient for any aircraft regardless of its intended use.
The limits to which any of the tests is carried is strictly up to each individual test pilot. He should, however, always be aware that he must complete each test at a safe altitude.
Test the airplane only to the degree needed, and then placard it so that the limits you safely met and established will not be inadvertently exceeded by yourself and others. This applies, in particular, to the aircraft's "redline" or never exceed speed.