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Archer III Vno illegally low?

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Flow

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Hi folks does this make sense?

As defined by TCDS certification basis:

CAR 3 effective May 15, 1956, including Amendments 3-2 and 3-4; paragraphs 3.304 and 3.705 of Amendment 3-7 effective May 3, 1962; FAR 23.207, 23.221, 23.955 and 23.959 as amended by Amendment 23-7 effective September 14, 1969; FAR 23.1557(c)(1) as amended by Amendment 23-18 effective May 2, 1977; and FAR 23.1327 and 23.1547 as amended by Amendment 23-20 effective September 1, 1977. FAR 36, Appendix G, Amendment 36-16 for the PA-28-181 (Archer III), S/N 2890206 through 2890231, 2843001 and up, and 2881001 and up.)


Vno Definition:

Per CAR 3 (5-15-1956)

§ 3.740 Maximum structural cruising speed (Vno).

This operating limitation shall be:

(a) Not greater than Vc chosen in accordance with § 3.184.
(b) Not greater than 0.89 times Vne established under § 3.739.
(c) Not less than the minimum Vc permitted in § 3.184.


§ 3.184 Design air speeds. The design air speeds shall be chosen by the designer except that they shall not be less than the following values:

Vc (design cruising speed)

= 38 √ W/S (Normal and Utility)

And further provided, That the required minimum value need be no greater than 0.9 Vh actually obtained at sea level.



I know it doesn't strictly apply to 3.184 but from AC 23-19A - Airframe Guide for Certification of Part 23 Airplanes p.26-27

46. Why would I want to define VC as equal to 0.9 VH? Use this definition if you are designing an airplane that is capable of a sustained speed (VC) higher than that obtained by using the wing loading (W/S) formula.


Note that for CAR 3 / CFR 23 aircraft gust penetration envelops are 30fps at Vc and 15fps at Vd. For comparison, transport category aircraft require 66fps Vb, 50fps Vc and 25fps Vd.

Turbulence defined as:
Light – 5fps - 20fps
Moderate - 20fps - 35fps
Severe - 35fps - 50fps
Extreme - > 50fps

§ 3.185
upload_2020-5-9_20-17-18.png


So Vno must not be greater than the Vc chosen nor less than the Vc min permitted.

Vc min permitted must be at least = 38 √ W/S = 147MPH EAS for the Archer III’s 15lbs/sqft wing loading. It is however only 140MPH CAS which calls into question some kind of OEM FAA deal possible via weight/wing area growth from the earlier Cherokee’s which share the 140MPH CAS Vno…? Or question what is it about the airframe that required a Vno less than legal Vc min? Or is this just a regulatory expedience? Maybe a carryover from early spar failures in the first tapper wing PA-28s but unlikely...

This is the only GA aircraft I am aware of with this limitation and I have not been able to get a sensible answer on it.

Vno in the Archer III is: 125KIAS (121KCAS) Shared by all of the four cylinder PA-28 series other than the short wing Arrows.
 

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