PA-28-181 Vno below Vc minimum design speed? NOT SOLVED
The PA-28-181 Archer III does not seem to be compliant with Vc min regs, it looks be 7MPH short. Other PA-28s appear OK but not the Archer III:
Vc min for 15lbs/sq.ft wing loading = 147mph EAS PA-28-181 Vno = 140mph CAS What I am missing? From the PA-28-181 Archer III TCDS p. 21,37 Airspeed Limits: Never exceed 171 mph (148 knots) CAS Maximum structural cruising 140 mph (121 knots) CAS Maneuvering @ 2550 lbs. 128 mph (111 knots) CAS Maneuvering @ 1634 lbs. 102 mph ( 89 knots) CAS Flaps Extended 115 mph (100 knots) CAS Certification Basis: PA-28-181: 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. 3.184 states: § 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 (NU) = 42 √ W/S (A) except that for values of W/S greater than 20, the above numerical multiplying factors shall be decreased linearly with W/S to a value of 33 at W/S=100: And further provided, That the required minimum value need be no greater than 0.9 Vh actually obtained at sea level. Vno is defined thus: § 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 Additional data for the PA-28-181 Archer III: W/S = 2550lbs/170sq.ft=15lbs/sq.ft From 3.184 above the following applies: Vc (design cruising speed) = 38 √ W/S (NU) 38x √ 15 = 147.17 mph Vc = 147.17 mph Vno = Vc However Vno as per TCDS = 140 mph CAS Not sure what is going on here? Wondering what is actually meant by: And further provided, That the required minimum value need be no greater than 0.9 Vh actually obtained at sea level. V-n diagram from 3.185 for reference if required. https://cimg5.ibsrv.net/gimg/pprune....e82c183929.png V-n CAR 3.185 |
For comparison:
182S under 23.335, pretty close: Max Structural Cruising 140 Knots IAS (138 Knots CAS) Wing loading 17.8lbs/sq.ft 33 √ 17.8 = 139kt Beechcraft F33A under CAR 3.184, well over: Maximum structural cruising 190 mph (165 knots CAS) Wing loading 18.8lbs/sq.ft 38 √ 18.8 = 164mph Note the C182(S&T) (CFR 23.335) are certified in knots rather than mph CFR 23.335 (1) https://rgl.faa.gov/Regulatory_and_G...ElemFormat=gif(in knots) may not be less than - (i) https://rgl.faa.gov/Regulatory_and_G...ElemFormat=gif(for normal and utility category airplanes) |
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From the EASA PA-28-181 TCDS you linked: 6. Requirements elected to comply: None 8. Exemption: None Hence: And further provided, That the required minimum value need be no greater than 0.9 Vh actually obtained at sea level. |
From a very knowledgeable friend. Great to bottom it out!
Begin with 38 √ W/S (NU). Then calculate 0.9 Vh actually obtained at sea level (Actually, it appears that " Vh actually obtained at sea level" would be a flight test value, the "calculation" relates to multiplying the (Test) Vh by 0.9. If the 0.9 Vh is less than 38 √ W/S (NU), then (and only then) use 0.9 Vh. While it is not applicable, you may find a clearer wording in early versions of Part 23. You might also review AC23-8, "Flight Test Guide for Part 23 Airplanes". The beauty of an AC is it will often give guidance on how to determine the correct answer. There would also be more stuff in the NPRM associated with that specific rule - easy if you know where to click on the FAA website/Regulatory and Guidance Library; there is often a link at the bottom of each section, where the background to each Amendment is discussed. Hope this helps. If further discussion is needed, let me know...but we're probably heading into territory that involves a lot of references, like weight growth/wing area increase/power increase as the PA-28 evolved. Superficially, it is unlikely that cruise speed would be greater than 0.9 Vh, so why bother meeting a more stringent requirement - the 38 √ W/S (NU) would appear to be an empirical value that corresponds to a typical Cl max for airfoils of the CAR 3 era, along with the assumption of a "typical" sharp edged gust. Later codes include a gust alleviation factor, as the science developed. |
Haaaaang on.. 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. Vc min for 15lbs/sq.ft wing loading = 147mph CAS PA-28-181 Vno = 140mph CAS § 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 |
This is still bugging me.
The Dakota also has a Vno less than Vc min design limitations. MAUW=3000 Wing Area=170sqft Wing Loading=17.65lbs/sqft Vc design minimum = 38 √17.65 = 159.63 MPH EAS ≈ 138.72 KEAS Published Vno = 135 KCAS It seems a number of the heavier Cherokees appear to have lower than legal Vnos. The Warrior III for eg. Piper PA-28-161 Warrior III (2440lbs With installation of Piper Kit 88050 STC#: SA00397NY/170sqft) Vc min = 125 KEAS Vno = 122 KCAS With out the paper MAUW increase of the post 1983 Warrior III models Vc min = 122 KEAS ie. spot on. Has Piper done some sort of deal with the FAA here to allow increases in gross weight for the Cherokees without complying with the Vno requirements? For reference here are some other CAR 3 / FAR Part 23 aircraft: All with Vno ≥ Vc min design speed. Piper Comanche PA-24-180 (2550lbs/178sqft) Vc min = 125 KEAS Vno = 139 KCAS Piper Comanche PA-24-250 (2900lbs/178sqft) Vc min = 133 KEAS Vno = 156 KCAS Piper Comanche PA-24-400 (3600lbs/178sqft) Vc min = 149 KEAS Vno = 182 KCAS Cessna 172R (2450lbs/174sqft) Vc min = 124 KEAS Vno = 126 KCAS Cessna P210R(4100lbs/174sqft) Vc min = 162 KEAS Vno = 167 KCAS Beechcraft C23 Sundowner (2450lbs/146sqft) Vc min = 135.3 KEAS Vno = 135.5 KCAS Beachcraft S35 Bonanza (3300lbs/181sqft) Vc min = 141 KEAS Vno = 165 KTIAS ≈ KCAS Mooney M20J (2740 lbs. - S/ N 24-0001 thru 24-3200, 24-3202 thru 24-3217) 2900 lbs. - S/N 24-3201, 24-3218 and ON and S/N 24-1686 thru 24-3200, 24-3202 thru 24-3217 when c/w MAC dwg. No. 940071 and insertion of applicable AFM Supplement into the appropriate AFM/174.8sqft) Vc min = 131 KEAS (2740lbs) or 135 KEAS (2900lbs) Vno = 152 KIAS or 174 KIAS for Serial No’s. 24-0171 and on, (and 24-0002 through 24-00170 if S.B. M20-198 is complied with). Mooney M20M (3368lbs/174.8sqft) Vc min = 145 KEAS Vno = 174 KIAS |
The Vc takes the compressibility correction. Where Vno is only corrected for position error based on what is shown you can't make a direct comparison...that I believe is the reason.
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Originally Posted by AC103
(Post 10323156)
Not sure what is going on here? Wondering what is actually meant by:
Surely 38 √ W/S is the minimum compliance standard while faster aircraft need not prove 30ft/s gust protection at any more that 0.9Vh but must still adhere to 38 √ W/S as the minimum standard. I have used it myself for one or two projects. |
Do you have the guidance reference for that one djpil?
All I can find is FAR23's which suggests the opposite: I assume 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. |
Originally Posted by AC103
(Post 10788596)
Do you have the guidance reference for that one djpil?
It goes on to state “with the following understanding” that Vc is less than or equal to 0.9Vh. Again, I don't see that the words in FAR are consistent with that. But who am I to argue with the FAA and their current AC. I am unable to find the first issue of that AC online. The predecessor to that AC is AC 23-3 of 1985 and there is nought in there about Vc. Interesting that the text of CAR 3 is different: “And further provided, That the required minimum value need be no greater than 0.9 Vh actually obtained at sea level.” Quite clear to me. |
I should add that FAR 23 uses similar wordings elsewhere.
"Va may not be less than" ... but goes on to state that "Va need not exceed the value of Vc used in design." Check out the notes on Va in that same AC. "The positive limit maneuvering load factor n may not be less than" ... calculate a number ... "except that n need not be more than 3.8." |
I agree, "And further provided" has an "AND" operator. It looks like the FAA changed their mind or realized that they hadn't been clear and didn't want to invite a whole load of paper MAUW increases.
Please do share any designs you know of that have Vcmin below 38 √ W/S. That would be a very interesting list. I have not managed to find any yet besides the later Cherokees. Don't get me started on design vs operational Va... now that is a proper mess. |
Originally Posted by AC103
(Post 10789443)
I agree, "And further provided" has an "AND" operator. It looks like the FAA changed their mind or realized that they hadn't been clear and didn't want to invite a whole load of paper MAUW increases.
Please do share any designs you know of that have Vcmin below 38 √ W/S. That would be a very interesting list. I have not managed to find any yet besides the later Cherokees. Don't get me started on design vs operational Va... now that is a proper mess. |
How funny, it looks like MAUW could be a Kiwism. https://www.aviation.govt.nz/assets/...ht_Balance.pdf
I can give plenty of examples of types with Vc outside the range given as gospel by that AC |
Originally Posted by AC103
(Post 10789582)
How funny, it looks like MAUW could be a Kiwism. https://www.aviation.govt.nz/assets/...ht_Balance.pdf
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Rarely can a question on here have been researched in such detail before being asked. It took me quite a while to get my head around the detail but was not unwelcome as a distraction from the DIY and 'exercise' groundhog day that is lockdown. The bottom line is I think djpil is correct. If the Vno is not less than 90% of VH then it complies.
Vno is defined thus: § 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 Surely 38 √ W/S is the minimum compliance standard while faster aircraft need not prove 30ft/s gust protection at any more that 0.9Vh but must still adhere to 38 √ W/S as the minimum standard. |
I think you talking about an ‘aeroplane’.. Not a car.. Why on earth did you start this discussion with MPH?
The wind is knots, the airspeed indicators are in knots.. The industry uses and knows about knots.. .. Can we start again? |
Originally Posted by advent
(Post 10789912)
I think you talking about an ‘aeroplane’.. Not a car.. Why on earth did you start this discussion with MPH?
The wind is knots, the airspeed indicators are in knots.. The industry uses and knows about knots.. .. Can we start again? |
Originally Posted by 733driver
(Post 10790130)
MPH was quite common in general aviation some decades ago.
Yes indeed alleviated gust values do decrease with increasing speed... |
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