(1) Atmospheric Standards. The purpose of § 23.45(a) is to set the atmospheric standards in which the performance requirements should be met. The air should be smooth with no temperature inversions, mountain waves, and so forth. This is essential to obtaining good data and repeatable results. N on-standard conditions of temperature, pressure, and so forth, can be corrected to standard, but there are no corrections to compensate for poor quality data due to turbulence or poor pilot technique. A thorough knowledge of the limitations of the testing procedures and data reduction methods is essential so that good engineering judgment may be used to determine the acceptability of any tests.
(i) Reciprocating Engine-Powered Airplanes Below 6,000 pounds (2730 kg) Maximum Weight. Performance tests will normally be conducted in non-standard atmospheric conditions, but ideally for accuracy in data reduction and expansion, tests should be conducted in atmospheric conditions as near those of a standard atmosphere as possible. Accounting for winds and non-standard conditions requires testing procedures and data reduction methods that reduce the data to standard atmospheric conditions.
(ii) Reciprocating Engine-Powered Airplanes of More Than 6,000 pounds (2730 kg) Maximum Weight and Turbine-engine Powered Airplanes. Performance tests should be conducted in the range of atmospheric conditions that will show compliance with the selected weight, altitude, and temperature limits. Refer to § 23.53 of this AC for guidance on extrapolation of takeoff data and § 23.75 for extrapolation of landing data.
(2) Standard Atmosphere. The Standard Atmosphere is identical to the International Civil Aviation Organization (ICAO) Standard Atmosphere for altitudes below 65,000 feet.
Appendix 7, figure 1, gives properties of the Standard Atmosphere in an abbreviated format.
(3) Installed Power. The installed propulsive horsepow er/thrust of the test engine(s) may be determined using the applicable method described in appendix 1, based on the power approved during airplane certification. The meth ods in appendix 1 account for installation losses and the power absorbed by accessories and services. Consideration should also be given to the accuracy of the power setting instruments/systems, and the pilot's ability to accurately set the power/thrust.
(4) Propeller Cutoff. In general, if the airplane will be approved with an allowable cutoff for the propeller, then the performance flight testing should be done using the most critical propeller diameter. In most cases, this is expected to be the minimum diameter propeller allowed.
(i) For normal, utility, and acrobatic category airplanes only, a two percent margin was the allowed reduction in propeller diameter. Historically, the two percent margin was selected as being the maximum permissible reduction in diameter of a given propeller that will not noticeably reduce performance. Service history has shown this to be an acceptable margin without additional flight testing.
(5) Flight Procedures. The flight procedures must not be unduly sensitive to less than ideal atmospheric conditions. The atmosphe ric conditions "reasonably expected to be encountered in service" may be different depending on the class of aircraft, but should cover at least the maximum demonstrated crosswind component established in compliance with § 23.233(a).
(6) Flight Test Data. For calibrated engines, test day power is the calibrated test day power. For uncalibrated engines, an acceptable met hod is to assume that the test day power is the upper tolerance chart brake horsepower; however, the upper tolerance chart brake horsepower for reciprocating engines using 14 CFR part 33 tolerances of +5 percent, -0 percent may lead to unrealistic performance. Historically, the two major reciprocating engine manufacturer’s engines have tolerances closer to +2 percent, -0 percent. Refer to appendix 1 for further information. The performance data required by § 23.1587 is dependent on the horsepower assumed for the various temperature and altitude conditions. Refer to appendix 1, which deals both with test data reduction and expansion.
(7) Humidity Correction. Refer to appendix 1.
b. Procedures. Refer to appendix 1.
c. Operation on Unpaved Runways.
(1) Small airplane operations from grass runways. For airplanes less than 6,000 pounds (2730 kg) maximum weight, the factors given below may be quoted in the flight manual, as an alternative to the scheduling of data derived from testing or calculation. It should be noted that these factors are intended to cover the range of airplane types in this category and are necessarily conservative. Manuf acturers are, therefore, encouraged to produce and schedule their own data in accordance with (1) and (2) above to obtain optimized performance for their airplane.
Table 4 - Smooth, Flat Runway Takeoff Landing Dry Grass 1.2 1.2 Wet Grass 1.3 1.6 Note: If the runway is not smooth, the grass is very long or very short, higher factors may be warranted.
Very short grass = golf course length Very long grass = unmowed field or high interstate highway median grass length
(2) Airplanes with 6,000 pounds (2730 kg) maximum weight or more. The following guidance was developed to include high performance multiengine airplanes. The level of detail should be scaled back to that appropriate for the airplane. Simple, single-engine turbine bush airplanes, for example, do not need the level of detail discussed here.
Operations on other than smooth, dry, hard runway surfaces may require specific approval and the scheduling of information on the effect of those surfaces on takeoff and landing distances in the flight manual. Applicants may use the factors for grass runways in paragraph
(1). These factors would, in many cases, result in such conservative distances that operational utility may be compromised. Applicants may propose their own factors if accompanied with supporting data. To obtain approval for take off and landing operations on unpaved runway surfaces, compliance with the following should be shown:
(i) Each type of surface must be defined so that it can be recognized in operations in service including conditions such as day or wet.
(ii) It must be determined that the airplane can be operated on each defined surface without hazard from likely impingement or engine ingestion of any foreign objects that constitute parts of the surface.
(iii) If any special procedures or techniques are found to be necessary, these must also be determined and scheduled.
(iv) The takeoff and landing performance on each defined surface must be determined in accordance with §§ 23.53 and 23.75, as modified in (3) below.
(3) Takeoff and Landing Data. Takeoff and landing data must be determined for each type of unpaved surface for which approval is requested.
(i) The test runways on which the takeoff and landing distance measurements are conducted should be chosen to be representative of the worst characteristics (that is, high rolling friction, low braking friction) of each of the types of runway under consideration.
(ii) In establishing the operation limitations for a particular type of unpaved runway, the runway’s load bearing characteristics, rolling and braking friction, and impingement and ingestion characteristics, should be considered.
e. Wet and Contaminated Runways. Reserved.
2. § 23.49 Stalling Speed.
a. Explanation.
(1) 61 Knot Stall Speed. The 61 knot stalling speed applies to the maximum takeoff weight for which the airplane is to be certificated.
(2) Background. Since many of the regulations pertain to performance, handling qualities, airspeed indicator markings, and other variables that are functions of stall speeds, it is desirable to accomplish the stall speed testing early in the program so the data are available for subsequent testing. Because of this interrelationship between the stall speeds and other critical performance parameters, it is essential that accurate measurement methods and careful piloting techniques be used. Most standard airplane pitot-static systems have not been found to be acceptable for stall speed determination. These tests require the use of properly calibrated instruments and usually require a separate test airspeed system, such as a trailing bomb, a trailing cone, or an acceptable nose or wing boom. The stall speed determinations necessary for marking the airspeed indicator are in terms of indicated airspeed (IAS) corrected for instrument error.
The other stall speeds are in terms of calibrated airspeed (CAS). Thus, a production airspeed system should be available during stall speed measurements to determine stall speeds in terms of IAS.
(3) Stall Definition. Section 23.49(d) requires the V S0 and VS1 speeds to be determined using the procedures specified in § 23.201. Refer to 14 CFR part 1 and § 23.49 for definitions of VS0 and VS1. “Landing configuration” in part 23 is gear down and full flaps.
Section 23.201(b) defines when the airplane can be considered stalled for airplane certification purposes when one of three conditions occurs, whichever occurs first. The conditions are:
(i) Uncontrollable downward pitching motion;
(ii) Downward pitching motion resulting from the activation of a device (for example, stick pusher); or,
(iii) The control reaches the stop.
(a) For those airplanes where the control reaches the stop, VS is considered to be the minimum speed obtained while the control is held against the stop. Elevator limited airplanes may or may not develop a minimum steady flight speed. Refer to figure 1 for a graphic representation of stall speed time histories for various configurations. The time the control is held against the stop for stall speed determination should be a minimum of two seconds and consistent with the time against the stop for stall characteristics testing in § 23.201.
Additionally, for airplanes with a stall barrier system, stick pusher operation has been considered as the stall speed. The term "uncontrollable dow nward pitching motion" is the point at which the pitching motion can no longer be arrested by application of nose-up elevator and is not necessarily the first indication of nose-down pitch.
(4) Reciprocating Engine Throttle Position. For reciprocating e ngine airplanes, the stalling speed is that obtainable with the propellers in the takeoff position and the engines idling with throttles closed. As an alternative to "throttles closed," the regulations allow the use of sufficient power to produce zero propeller thrust at a speed not more than 10 percent above the stalling speed. The regulations do not allow any alternative to the use of "propellers in the takeoff position," nor is any alternative intended except that the use of a feathered propeller in certification stalling speed tests is acceptable only when it has been determined that the resulting stalling speed is conservative (higher). If the stalling speed tests are to be conducted with the propellers delivering zero thrust, a dependable method of determining inflight thrust, such as a propeller slipstream rake, should be available in flight. The practice of establishing zero thrust
r.p.m. by calculation is also acceptable. One calculation method is given in paragraph (5) below.
Analytical corrections may be acceptable if satisfactory accounting is made for the effects of propeller efficiency, slipstream, altitude, and other pertinent variables.
(5) Zero-Thrust r.p.m. Calculation.
(i) Zero-thrust r.p.m. can be calculated by using the propeller manufacturer's propeller coefficient curves. The thrust will be zero when the propeller thrust coefficient is zero for the particular propeller blade angle. Using the propeller coefficient curves, obtain or construct a chart like figure 2, where:
CT = thrust coefficient CP = power coefficient ß = blade angle setting J = advance ratio 16
Figure 1 Stall Speed TRIM SPEED
1.1 Vs:
NOT ELEVATOR:
LIMITED ENTRY- RATE SLOPE TRIM : NOSE
SPEED DOWN PITCH
AIRSPEED ELEVATOR
REACHES ELEVATOR STOP
CONTROL LIMITED* ENTRY- RATE
TRIM SLOPE
SPEED (MINIMUM STEADY
1.1 Vs FLIGHT SPEED)
ARTIFICIAL BARRIER (PUSHER SYSTEM) ENTRY RATE
SLOPE E PUSHER FIRED
TIME - SECONDS 17
(ii) The propeller blade is usually against the low pitch stop position, in the speed range of interest. Knowing the bl ade angle setting, the advance ratio J can be determined to give zero-thrust for the particular propeller under consideration. Knowing the value of J for zerothrust, the propeller r.p.m. for various velocities can be calculated as follows:
Propeller r.p.m. = 101.27 V JD Where: V = airplane true airspeed in knots J = advance ratio D = propeller diameter in feet
(iii) The calculated velocities and propeller r.p.m. for zero-thrust can be plotted as shown in figure 3.
(iv) Another approach is to use the prop rake method. In some cases this may be easier to use and more accurate.
(6) Turbopropeller Thrust. For turbopropeller airplanes, § 23.49(a)(2) requires the propulsive thrust not be greater than zero during stall speed determination or, as an alternative to zero thrust, if idle thrust has no appreciable effect on stall speed, stall speed can be determined with the engines idling. If the ai rplane has a flight idle position, this would be the appropriate throttle position. Flight test experience has s hown that some turbopropeller-powered airplanes may demonstrate a relatively high positive propeller thrust at the stall speed with the engines at flight idle. This thrust condition may yield an un conservative (lower) stall speed; therefore, just as for piston-powered airplanes, some dependable method to determine zero thrust should be available for comparison of zero thrust stall speed and flight idle stall speed or for determination of zero thrust stall speed. Residua l jet thrust should be considered. Comparisons of zero thrust stall speed and flight idle stall speed should be investigated at high and low altitudes. Use of feathered propellers is acceptable if the feathered stall speeds are found to be conservative (higher).
(7) Fixed Shaft Turboprops. Experience on some fixed-sh aft turboprop installations indicates that stall speeds can be evaluated at mid-altitudes and appear to be totally conservative.
However, if stalls are conducted at altitudes of 5,000 feet or below, the stall speed can increase dramatically. This occurs because the propeller drag characteristics are a function of true airspeed, and as true airspeed decreases, the drag goes up substantially and the flow behind the propeller on wing-mounted engines causes premature inboard wing airflow separation.
(i) In addition, if the horizontal tail and the elevator are exposed to the same flow, the elevator power is decreased and tends to compound the problem. It is recommended that stall speeds be re-evaluated at low altitudes on all fixed shaft turboprops to assure that the stall speeds have not increased.
Figure 2 Propeller Coefficients
CP - POWER
COEFFICIENT
J - ADVANCE RATIO CT =.10 CT=.08 CT=.06 CT=.04 CT=.02 CT=0 B=10 O B=15 O B= 20O B=25 O B=30 O Figure 3 Zero Thrust P R O P E L L E R R P M
V-TAS IN KNOTS 19
----
(8) Turbojets and turbofans.
(i) Stall speeds are normally defined with the thrust levers at idle; however, it is necessary to verify by test or analysis that engine idle thrust does not affect stall speeds to an extent that they are appreciably lower than would be experienced at zero thrust. Negative thrust at the stall, which slightly increases stall speeds, is acceptable.
(ii) To determine whether thrust effects on stall speed are significant, at least three stalls should be conducted at one flap setting, with thrust set to approximately the value required to maintain level flight at 1.6 VS in the selected configuration.
(iii) These data may then be extrapolated to a zero thrust condition to eliminate the effects of idle thrust on stall speeds (refer to figure 4). If the difference between idle thrust and zero thrust stall speed is 0.5 knots or less, the effect may be considered insignificant.
Figure 4 Thrust Effect on Stall C L
ENGINE THRUST
STALL CL
THRUST EFFECT ON
STALL CL IDLE THRUST THRUST FOR 1.6 Vs
IN LEVEL FLIGHT ZERO THRUST
b. Procedures.
(1) Instrumentation.
(i) Test Systems. As previously mentioned, the production airspeed system is normally not sufficiently predictable or repeatable at high angles of attack to accurately measure the performance stall speeds of an airplane. However, a production airspeed system should be installed during stall speed tests to define the airspeed indicator markings required by § 23.1545.
(a) The performance stall speed test system utilized in a type certification program should be calibrated to a minimum speed at least as low as the predicted minimum stall speed anticipated on the test airplane. Test systems that have been utilized to accurately define the performance stall speeds include, but are not limited to the following:
1. Boom Systems. Swivel-head, boom-mounted, pitot-static systems with sufficient free-swivel angle to cover the stall angle-of-attack range of the airplane have been found to be acceptable. Some angle-of-attack co mpensated fixed pitot heads have also been found to be acceptable over a wind tunnel defined angle-of-attack range. Another way to make the fixed pitot work for stall speeds is to mount the boom at a negative incidence angle biased for the stall angle of attack. In all wing-mounted boom systems, the boom mounted static source should be at least one chord length ahead of the wing leading edge. On nose-boom mounted systems, it has been generally accepted that the static source should be at least one and one-half fuselage diameters ahead of the nose. All boom systems should be installed in a manner which assures that the boom and boom pitot-static head are structurally sound (both static and dynamic) within the proposed operating range.
2. Pitot-Static Bombs. Pitot-static bombs that are stable through the stall maneuvers have been found to provide acceptable data.
3. Trailing Cones. A trailing cone static source dynamically balanced with a swivel head pitot source, or dynamically balanced with a fixed pitot source of proven accuracy in the stall angle-of-attack range has been acceptable. The stability of the cone should be verified during stall tests and throughout its intended operating range. The length of the cone may need to be adjusted on individual airplane installations to assure cone stability.
(ii) Lag Equalization. All of the systems described in paragraph (i) could involve the use of long lengths of pressure tubing, and the associated pressure lags then occur whenever either speed or altitude, or both, are changed. Probably the most important consideration in these installations (on most small general aviation airplanes) is that the test pitot-static systems should be dynamically balanced. This is easily accomplished experimentally by putting both the total head and static orifices in a common chamber and varying the pressure in the chamber at a rate corresponding to a 2,000 to 3,000 feet-per-minute rate of descent. Various volumes are inserted in the total head line until the airspeed indicator has no tendency to move in either direction from zero during the simulated rate of descent. This method results in approximately the same volume in both systems, and, for the same size tubing, the Reynolds Number of the flow through both lines will be the same. A dynamically balanced airspeed system has equal lag in both the total and static sides. Use of a balanced system si mplifies the interpretation of recorded stall time histories.
(iii) Lag Correction. When a balanced test airsp eed system is used, it is often unnecessary to determine the actual amount of lag present. When such a determination is necessary, a method of accounting for lag errors is described in NASA Reference Publication 1046, "Measurement of Aircraft Speed and Altitude," by W. Gracey, May 1980. A lag correction is unnecessary if it can be shown that the system lag is small enough to be considered insignificant.
F
(iv) Transducer Location. The instrumentation should account for any difference in the installed location of the pressure transducers used in the cone, boom, or bomb and the aircraft system.
(2) Test.
(i) Stall Speed. The actual test should be comme nced with the airplane in the configuration desired and trimmed at approximately 1.5 VS1 or the minimum speed trim, whichever is greater. Th e airplane should be slowed to about 10 knots above the stall, at which time the speed should be reduced at a rate of one knot per second or less, until the stall occurs or the control reaches the stop. Where exact determination of stalling speed is required, entry rate should be varied to bracket one knot per second, and data should be recorded to allow the preparation of time histories similar to those shown in figure 1. The indicated airspeed at the stall should be noted, using the production airspeed system. Both the IAS and the calibrated stall speeds may then be plotted versus entry rate to determine the one knot per second values.
(ii) Bomb. When using a bomb, caution should be used in recovering from the stall so that the bomb is not whipped off the end of the hose.
(iii) Weight and C.G. The stalling speed should be determined at all weight and
c.g. positions defining the corners of the loading envelope to determine the critical condition.
Data should be recorded so that the weight and c.g. at the time of the test can be accurately determined. This can often be done by recording the time of takeoff, time of test, time of landing, and total fuel used during the flight.
(iv) Power and Configuration. The stall should be repeated enough times for each configuration to ensure a consistent speed. If a correction is to be made for zero thrust, then the stall speed and power at several power settings may be recorded for later extrapolation to zero thrust.
(v) Control Stops. The elevator up stop should be set to the minimum allowable deflection. Flap travels should be set to minimum allowable settings.
(3) Data Reduction. The correction involves:
(i) Correction for airspeed error. IAS to CAS (correct for instrument as well as position error) when CAS is required.
(ii) Correction for weight. Multiply the test calibrated stall speed times the square root of the standard weight divided by the test weight.
WS VS = VST WT 22
Where: VS = Stall speed (CAS) VST= Test stall speed (CAS) WS = Standard weight (pounds) WT = Test weight (pounds)
(iii) The correction for weight shown above applies only where the c.g. is not also changing with weight. Where c.g. is ch anging with weight, such as between forward regardless and forward gross, stall speed should account for this. A straight line variation between the measured stall speeds for the two weight and c.g. conditions has been found to be an acceptable method.
3. § 23.51 Takeoff Speeds.
a. Explanation. The primary objective of this section is to determine the normal takeoff speeds for non-weight, altitude, and temperature limited airplanes. For WAT limited airplanes, the objective is to determine the takeoff speed schedules for all takeoff configurations at weight, altitude, and temperature conditions within the operational limits selected by the applicant.
b. Procedures. For normal, utility, and acrobatic category airplanes, the rotation speed, (VR) in terms of calibrated airspeed, must be selected by the applicant. VR is constrained by § 23.51(a), as follows:
(1) For multiengine landplanes. VR must not be less than the greater of 1.05 VMC or 1.10 VS1;
(2) For single-engine landplanes. VR must not be less than VS1; and
(3) For seaplanes and amphibians taking off from water. VR may be any speed that is shown to be safe under all reasonably expected conditions, including turbulence and complete failure of the critical engine.
c. Procedures. For normal, utility, and acrobatic category airplanes, the speed at 50 feet should be determined by:
(1) Multiengine 50-foot Speed. For multiengine airplanes, § 23.51(b)(1) requires the speed at the 50-foot point to be the higher of the following:
(i) A speed that is shown to be safe for continued flight (or land back, if applicable) under all reasonably expected conditions, including turbulence and complete engine failure; or
(ii) 1.1 VMC; or
(iii) 1.2 V S1.