Why 1.3 Vs for approach?

[IGh]

Certification req' changed from '38 to 1941

Why 1.3Vso?

For questions relating to such epistemology of Aircraft Performance guidelines in certification, generally one can go to Joop Wagenmakers’ book, and papers.

But, for the initial appearance of the specific “1.3Vso” in Airworthiness Regulations, this wording predates the Turbojet revolution and predates swept wing aircraft. This specific “1.3Vso” was included the CAR 4b, also included in the earlier CAR 4a (1950), and during the 1940’s while using CAR Part 4.

Before that, certification was conducted to earlier standards defined by CAR Part 4, dated May 1938. Nothing about “1.3 Vso” appears in the 1938 wording, but you can see two mentions of speeds, one using a lower limit of “20 per cent in excess of stalling speed”, and mentioning a second high limit of “ at a speed not in excess of 140 per cent of the maximum permissible landing speed ” [see paragraph 04.704 from 1938 describing “balance”].

CAR Part 04 (1938), “Airplane Airworthiness”, dated May 31st, 1938

paragraph 04.700(b) defines the max landing speed (70 mph);

Paragraph 04.71 “Modified performance requirements for airline carriers”.

04.713 "In no case shall the provisional weight exceed a value corresponding to a landing speed of 5 miles per hour in excess of that specified in [paragraph] 4.700, a take-off distance of 1,500 feet in the case of landplanes, or a take-off time of 60 seconds in the case of seaplanes…."

In the 1938 version of CAR 4, the SPEED to derive Landing DISTANCE was complicated:

04.730 Flare and landing. Under the most critical center of gravity ... fully throttled, propellers ... low pitch and with high lift devices ... the horizontal distance to come to a full stop ... from the start of a normal flare at 50, 100, and 150 ft ... the optimum speed corresponding to each flare ...

CAR Part 4, “Airplane Airworthiness”, Change dated April 1st, 1941, first offers alternate wording relating to Vso, but the wording differs slightly:
Paragraph 04.7503

“Landing. The horizontal distance required to land and come to a complete stop from a point at a height 50 feet above the landing surface, subject to the following conditions:
(a) … steady gliding approach shall be maintained with a true indicated flight path airspeed not less than 130 percent of stalling speed …” [that from Apr’41].

The change in wording (to include “1.3 Vso”) occurs with the next change in Nov’ 1943, see paragraph 04.7533-T(a) which specifically cites that “1.3 Vso” speed at 50’ for use in determining Land Distance.

To understand the “why” of the change, consider this time period just prior to the onset of the “1.3Vso” in CAR Part 4, & consider flight test projects of 1938 and 1939. At Boeing, these projects were the B314 and then the B307 Stratoliner .

FF of B314 (flying boat) was on 8Jun38; Eddie Allen, and Earl Ferguson as pilots and Bill Lundguiest acting as F/E. Designed for Pan Am, which was already operating Martin M-130 "China Clippers" and Sikorsky S-40 and S-42 seaplanes.

Boeing Model 307 "Stratoliner" first flight on 31Dec38. TWA started service on 8Jul40 with a 14:09 time from LGA to Burbank, with stops.

Perhaps those 1938 Regulations for certification were too limiting to advance beyond the B307, because of the fixed maximum “distance” for Takeoff, and fixed limit “speed” to derive landing distance, and a climb time to 300 feet (but not a gradient nor distance, only TIME to height).

From B307 flight test notes you can see some examples of performance, the first two tests were taxi-testing, then FF:

First Flight away from a runway was done on TEST # THREE of 31Dec38: Crew aboard: Allen, Ferguson, West, Barr, Gaylord. T/O Gwt = 32,000 Lbs. Planned test for directional stability on ground. Time from brake release to airborne was 9 seconds, flight then continued due to limited runway remaining. Attempted to retract Landing Gear, but the Tail Wheel failed to retract. Pilot's comments from his report, "Good rolling moment due to yaw."

Test # FOUR done 4Jan39: Taxi testing focused on brake pressures and temperature. Flt time = 1hr 48 minutes; logged engine time as 3 hrs. Crew was Allen, Barr, Ferguson, West, Cram, Jewett, Anderson. At T/O Gwt of 37,000 Lbs, T/O distance was 800 feet into a light SSE wind of 3 mph; after tracking 2300 feet along and above runway the height was 100 feet, using 1100 hp…. [flight test reports are in Boeing Archives].

Mainly, it seems that other reg's of 1938 boxed-in any further development; and so the multiples (130% &ct) of a new ship's Stall Speed helped get newer/faster aircraft certified, even if the new ships flew approach faster than the 75 mph certification-limit of 1938.
= = = = = = = =

The official explanation for the CAR [Performance Section] revision of 1941 appears in CAM 04T, three years after the revised CAR (and the onset of war).

CAM 04, Nov’41, the “Introductory Note” , flatly tells the reader to wait for a future explanation about these 1941 new changes to CAR’s Performance section:

“this manual contains material which is intended to interpret and explain the airplane airworthiness requirements specified in Part 04 of the Civil Air Regulations … This edition of CAM 04 contains material pertaining to CAR 04.0 through 04.4. The remaining sections of CAR 04 will be covered by future additions….

In the 1944 Manual, the explanation appears, in various paragraphs.

http://ntl1.specialcollection.net/sc...&site=dot_cams

CIVIL AERONAUTICS MANUAL 04-T,
TRANSPORT CATEGORY REQUIREMENTS, NOVEMBER 1, 1944

04.753-T - Required Performance and Performance Determination

04.7530-T - Stalling Speed Requirements

The limitations which are imposed by the requirements of this section upon the stalling speeds have been dictated primarily by the effect of the speeds at which it is necessary that the airplane be operated during an approach and landing under adverse weather conditions, upon the safety of that operation. All of the airplanes which had been used in civil operation had, at the time these regulations were written, been designed to comply with a maximum landing speed requirement which had in no case exceeded 70 MPH where passengers were to be carried in the airplane. ... it was considered unwise to abandon ... because the alternative limitations are not absolute ... therefore, possible by the installation of a sufficient amount of power ... and by ... runways long enough, to design an airplane having stalling speeds far in excess of those with which we have been familiar.

04.7533-T - Landing Determination

"... minimum approach speed of 1.3 Vso ... is intended to provide a reasonable margin above the stalling speed."