Old 29th May 2020, 22:23
  #29 (permalink)  
AC103
 
Join Date: Jul 2015
Location: NZ
Posts: 45
Originally Posted by oggers View Post
Ok, so the quote below is what we are talking about from post 21:
The key point in there is where you say "descent profiles now start to look limited by Vno". Would you prefer the recommended descent speed was higher than Vno or where exactly? The structure does comply with the gust penetration requirements. You are reading too much into this. The recommended descent profile is 'nice to have'. It is not a limitation. It is not even an unduly limiting recommendation because it turns out to be one knot faster than a 75% cruise at 4000' DA (according to Fig 5-21 of the POH).
The aircraft is certified. That is the fact. Its flight envelope provides the same safety factor of 1.5 between limit loads and ultimate loads as every other certified aircraft.
The AC is: "not mandatory.....not a regulation.....not the only means of compliance". Other manufacturers have complied with certification by using the other of two alternative and equally valid criteria in the regulations. The reg has since been reduced to something vague by the new FAA part 23, but the original certification basis is still extant in EASA land where:
"(3) VC need not be more than 09 VH at sea level."
This aircraft is fully compliant. There is no "deal with the FAA here to allow increases in gross weight for the Cherokees without complying with the Vno requirements?". because it complies with the pre-existing foreign regs too. You are barking up the wrong tree.
I disagree with pretty much all of that oggers, as previously clarified. Also there is no tree, no-one is barking and reading and writing is all that we are doing here. Please don't drag this thread down there.
My objective when discussing Aviation via an HID is to stimulate deeper understanding and help protect the communities I care about from potentially risky dark corners. There are many of those in this world worth shining a light on, this is one of them. And your objective here?

There is no doubt the later heavier Cherokees are an outlier by a meaningful margin on declared Vno relative to wing loading that does impact operations and safety margins when compared to its peers, and there is also reasonable ground that the designers may have misinterpreted the regs.

If the regs were written as follows there would be no question on the interpretation,
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:
(a) 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:
(b) The required minimum value need be no greater than 0.9 Vh actually obtained at sea level.


But they do not say that. They says this:
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.


This begs the question what are they 'providing' for.
What they were actually providing for is later clarified in AC 23-19A - Airframe Guide for Certification of Part 23 Airplanes p.26-27
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 regs would have been better written as follows:
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,
for an airplane that is capable of a sustained speed (VC) higher than that obtained by using the wing loading (W/S) formula, the required minimum value need be no greater than 0.9 Vh actually obtained at sea level.


This is the interpretation that majority of design teams complied with including Piper teams for their other models.

As an operator this is reassuring as it requires designers to build stronger aircraft with wider envelopes that don't have their descent profiles as tightly boxed in between engine care and Vno as these later Cherokees. This has a significant real world impact on tactical options available during normal operations for dealing with weather, ATC etc.

Again please review the post on the Coleal Interpretation to understand why there are still may ambiguous and just plain off the reservation regs still in black and white today that the FAA do not want us to comply with as written.

If I was to guess I would say that there was some doubt about the longer tapper wings being able to cope with a normal >38 √ W/S based Vc combined with pressure to increase the MTOW for things like air con maybe, especially considering the number spar issues and changes to Va and control deflection limitations that occurred as this wing was introduced.

It would be very nice to hear from a Piper engineer involved though as that is just a guess.

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