PA-28-181 Vno below Vc minimum design speed?

[AC103]

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.

Regs: Design air speeds are defined in CAR 3.184 and CFR 23.335 no amendments to these sections are listed in the certification basis above so 3.184 in CAR 3 effective May 15, 1956 p.12 rather than CFR 23.335 is the defining regulation on design speeds for this type:

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.

Vc is not published in the TCDS but Vno is.
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

So Vno essentially = Vc

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.

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.

V-n diagram from 3.185 for reference if required.


V-n CAR 3.185

[AC103]

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) (in knots) may not be less than -
(i) (for normal and utility category airplanes)

[NutLoose]

https://www.easa.europa.eu/sites/def...03%20final.pdf

[AC103]

Originally Posted by NutLoose

https://www.easa.europa.eu/sites/def...03%20final.pdf

Thanks Nut, but not sure how that explains the how the published Vno is 7mph under Vc min for an aircraft of that wing loading?

From the EASA PA-28-181 TCDS you linked:

6. Requirements elected to comply: None
8. Exemption: None

I wonder if that provision from 3.184 is intended to remove the onus on manufactures to provide gust protection to a speed that the aircraft will realistically never achieve in level flight?
Hence:

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

[AC103]

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.

[AC103]

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.

So The PA-28-181 Archer III does not seem to be compliant with Vc min regs, it looks be 7MPH short. Other PA-28s Cessnas etc. appear OK but not the Archer III:
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

[AC103]

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

[Pugilistic Animus]

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.

[djpil]

Originally Posted by AC103

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.

What it means is that Vcmin reduces to 0.9 Vh if it is applicable and the manufacturer chooses to apply that rule.

I have used it myself for one or two projects.

[AC103]

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.

[djpil]

Originally Posted by AC103

Do you have the guidance reference for that one djpil?

My time was some time back so prior to that AC. It mentions Vcmin and Vcmax but there is nothing in FAR 23 about a Vcmax. I can give plenty of examples of types with Vc outside the range given as gospel by that AC - of course, like me they predate that AC.

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.

[djpil]

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."

[AC103]

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.

[733driver]

Originally Posted by AC103

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.

Sorry, I'm not qualified enough to comment on the actual question here but I was wondering about the term MAUW. I know what it stands for but is that, or was that, the official term in your part of the world? I believe the US still use weights such as MTOW and in Europe we use masses such as MTOM but the term max all up weight just seems so unscientific to me if that makes sense.

[AC103]

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

Please do share if you manage to dig them up!

[733driver]

Originally Posted by AC103

How funny, it looks like MAUW could be a Kiwism. https://www.aviation.govt.nz/assets/...ht_Balance.pdf

Thanks. I think I have seen it in other parts of the English speaking world before but always assumed it was an outdated term. It's possible, and I'm sure others oo here will know, that it was in the past common in the UK but has since been replaced by EASA terminology. I also suspect Australia may still use it.

[oggers]

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

So Vno essentially = Vc

But Vno is not necessarily = Vc. It is allowed to be between design Vc actually chosen (we don't have that figure) and the minimum permitted design Vc. For all we know, 0.9 VH (permitted) could be 139mph (entirely plausible I think) in which case Vno could be as low as that and still be compliant.

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.

If the Vc is reduced then the load resulting from a gust at Vc is reduced. Hence, with the Vno not being permitted to be higher than design Vc, this problem takes care of itself, so to speak.

[advent]

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?

[733driver]

Originally Posted by advent

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?

MPH was quite common in general aviation some decades ago.

[Pugilistic Animus]

Originally Posted by 733driver

MPH was quite common in general aviation some decades ago.

Lots of Aerobatic planes use MPH... Piper Cub too

Yes indeed alleviated gust values do decrease with increasing speed...