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

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

Old 22nd May 2020, 19:36
  #21 (permalink)  
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Arghem... advent, please read post #2! (All of the words and numbers..) Thanks 733driver.

Oggers, kind of you to say and thank you for your input, now what is your response to the later clarification by the FAA in 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.
FYI per the Archer III POH Speed Power chart 5-21 Vh is 133 KTAS = 153 MPH 0.9 = 135.75 MPH so a Vno of 140 MPH CAS is conceivable if we ignore the above AC but personally I would prefer not to ignore a later clarification. Understood from Piper's perspective this is hindsight.

I am still interested in a list of any other Vno < Vcmin designs besides the later/heavier Cherokees. I am sure there will be and it will be an interesting list for sure.

With regards to the problem taking care of its self, this issue is that if an aircraft design increases its MTOW on paper and then artificially reduce their Vno as their structures are not able to comply with the gust penetration requirements this is an indication of a weaker than required air frame which becomes relevant in descent and this is where it gets more interesting.

The Archer III POH specifies a Fuel, Time, Distance to Descend chart based on an engine RPM (2500) and KIAS (122) which puts it an additional 3 knots under Vno, limiting the descent rate to only ~450fpm whilst keeping the engine warm.
This configuration is mentioned again in the Descent subsection of Normal Procedures in reference to carb ice. (The Archer III has a cooler air pick up than previous 4cyl PA-28s) so its descent profiles now start to look limited by the low Vno.
These Fuel, Time, Distance to Descend charts are published for the later Pipers (Seminole, Malibu, Warrior 161s, Archer 181s etc) and not published by the other manufactures singles mentioned above including popular IFR trainers like the DA-40 and DA-42 where these tables would be more expected.
The exception to the above is the later Mooneys. However they configure on a 750fpm descent rate rather than an airspeed likely due to their published Vno being some 20% above Vcmin.

Are there any ex Piper engineers here that might know or do we have any friendlies in their current design team?
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Old 23rd May 2020, 00:43
  #22 (permalink)  
 
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Originally Posted by AC103 View Post
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.
Yes, it seems to me that something has changed and I missed it. Perhaps they have been seeing faster airplanes wrt to their wing loading than earlier designs? I see this from the Light Aircraft Association http://www.lightaircraftassociation....0envelopes.pdf. I like what was quoted from VLA: "It is recommended that Vc should not be less than 0.9 Vh" Perhaps Francis Donaldson could fill in the history and perhaps explain why the AC mangles the text of the rule instead of changing the rule. Of course, we have a fancy new FAR 23 now and I eagerly await a new AC 21-19.

Originally Posted by AC103 View Post
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.
The Aviat Husky series is one that I am very familiar with. The current model A-1C has W/S of 12 so 33 √ W/S = 114 kts but Vc used in design and Vno is 103 kts.

Originally Posted by AC103 View Post
Don't get me started on design vs operational Va... now that is a proper mess.
Sorry but I take more interest in this now, having retired from engineering and instruct aerobatics these days. I like the bit in AC 21-19A about "For airplanes where VA>VS√n, the pilot would have to check the maneuver; otherwise the airplane would exceed the limit load factor." Sometimes my discussions with instructor trainees venture into that area. Perhaps the subject of another thread.
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Old 23rd May 2020, 21:47
  #23 (permalink)  
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Originally Posted by djpil View Post
Perhaps Francis Donaldson could fill in the history and perhaps explain why the AC mangles the text of the rule instead of changing the rule…
…The Aviat Husky series is one that I am very familiar with. The current model A-1C has W/S of 12 so 33 √ W/S = 114 kts but Vc used in design and Vno is 103 kts…
…I like the bit in AC 21-19A about "For airplanes where VA>VS√n, the pilot would have to check the maneuver; otherwise the airplane would exceed the limit load factor."... Perhaps the subject of another thread.
Indeed as you say the VLA also provides the updated guidance on Vc. Adding the context.

You are not constrained to use these minimum values. Indeed, if your aircraft has high performance in terms of a high cruising speed, the value of Vc would be much greater than the minimum. Additionally:

It is recommended that Vc should not be less the 0.9 Vh
Forgive me if I am stating the obvious but as I understand it updating a rule is slow, onerous and expensive in comparison to issuing letters of interpretation https://www.faa.gov/about/office_org...terpretations/ or updating ACs. (NPRM, comments, replies to comments etc.) Here is a good example. The wording of Part 43 Appendix A and AC43-12A that conflict with the FAR 1.1 definition of preventative maintenance have still not been updated 11 years (ATOW) after the Coleal Inerpretation from 2009.

43 Appendix A: On the list of 32 preventive maintenance items.
(c) Preventive maintenance. Preventive maintenance is limited to the following work, provided it does not involve complex assembly operations:
AC43-12A:
If a task or maintenance function does not appear in the list, it is not preventive maintenance.
Coleal Interpretation: (David Coleal from Lear asking if crews are allowed to check the ~200psi tire pressures on a Lear 60 required daily under an Airworthiness Limitation)
https://www.faa.gov/about/office_org/headquarters_offices/agc/practice_areas/regulations/interpretations/data/interps/2009/Coleal%20-%20(2009)%20Legal%20Interpretation.pd

Yes, crews can check the tire pressure under Part 91
AND
Even though the introductory text of subparagraph (c) states that "preventive maintenance is limited to the following work....", in view of the broader definition of preventive maintenance in section1.1, we believe that such limitation is not controlling.
AND
we also believe that the… sentence in Advisory Circular 43-12A, Preventive Maintenance … is overly restrictive
AND
the lists are better viewed as examples of the tasks in each category-they cannot be considered all-inclusive
AND
There are, no doubt, many "simple or minor preservation operations [tasks]" and many" replacement of small standard parts not involving complex assembly operations" performed daily, especially on small general aviation aircraft, that the agency would consider to be preventive maintenance, though they are not included in the 32 listed items. It is our understanding that Flight Standards' Aircraft Maintenance Division is planning to clarify this issue in a future revision to the AC.
So owners/operators have more scope to undertake work of comparable ease, complexity and risk as those on the list and make log book entries in accordance with CFR 43.9 with the date, the description of the work performed and signed with their pilot’s certificate number. So long, of course, as they follow the acceptable methods, techniques and practices out lined in CFR 43.13 eg. they have the relevant AMM, IPC, SB’s and tools at hand and the work is performed to a standard at least equal to its original or properly altered condition (with regard to aerodynamic function, structural strength, resistance to vibration and deterioration, and other qualities affecting airworthiness) and ‘should’ (though not required for pilots even though it is required under Part 66 for mechanics) have been supervised for the first time that they perform the task.

Yet the regs and the AC still say they cannot to this day.



Thanks for the Husky

You are totally on the right track with Va. Yes probably for another thread. It may well have already been flogged here due to the AA587 accident.

I sure would love to talk to a Piper engineer to understand which structure they believe would not be able to withstand loads at proper Vc min for its wing loading.

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