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RAPID Software Manual Official Link? 2
- Thread starter artking
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fyi RAPID is not approved for DTA evaluation of cutouts anymore.
If you want to use it as a guideline or to understand the procedures you could use this:
If you want to use it as a guideline or to understand the procedures you could use this:
FYI, I was a beta tester for the software back in the 90s. Software was only meant for screening of repairs to determine safety risk following aloha. No real value in the software itself and the method has been proven to be unconservative and so no longer accepted for any analysis.
I believe the procedure described by the Chicago ACO paper (Eastin) and/or the Seattle ACO, augmented by Tom Swift's papers, are still the preferred guidelines for skin repairs and antenna penetrations.
If you absolutely must have the RAPID documentation, say for historical purposes, then see the attached files below.
You will need a PC with Windows 95 or 98 to install and run RAPID. Maybe XP will run it with some gentle coaxing.
In the unlikely chance that you are dealing with a structural failure in an aircraft, and you need to use RAPID is because it was the "substantiation" of a repair/penetration of the skin that has now failed, please let me know. I never deleted the old RAPID stuff from my archives, and could dig up more if you need it.
If you absolutely must have the RAPID documentation, say for historical purposes, then see the attached files below.
You will need a PC with Windows 95 or 98 to install and run RAPID. Maybe XP will run it with some gentle coaxing.
In the unlikely chance that you are dealing with a structural failure in an aircraft, and you need to use RAPID is because it was the "substantiation" of a repair/penetration of the skin that has now failed, please let me know. I never deleted the old RAPID stuff from my archives, and could dig up more if you need it.
LiftDivergence
Aerospace
In the enhancements document (AFRL-VA-WP-TR-2000-3030) there is some interesting information including beta factors / FEM verification, and some studies of stress and load transfer effects for skin repairs in close proximity.
There's also some good stuff on simplified loading spectrum development and single-cycle equivalent stress calculation.
So the software itself is past it's time but the backup manual can be a good reference.
Regarding the "preferred" methods for fuselage stress development, the ACO publications are also outdated in a sense.
The FAA provides guidance through it's sponsored course which essentially presents the UDRI method (DOT/FAA/TC-12/17) which incidentally is also what is basically presented in RAPID, and there is more detail there.
The issue with the Chicago ACO method is that it is using Broek's formula for inertial bending stress. But a problem with Broek is that it assumes no stabilizer input is being used to balance the aircraft. The UDRI method back-calculates a stress from ultimate assuming load factors from the CFRs, so it essentially makes a "conservative" assumption that by design, the MoS for the max maneuver/gust is +0.0.
HOWEVER, there are still several cautions with this method, and it should be applied carefully by cognizant analysts:
There's also some good stuff on simplified loading spectrum development and single-cycle equivalent stress calculation.
So the software itself is past it's time but the backup manual can be a good reference.
Regarding the "preferred" methods for fuselage stress development, the ACO publications are also outdated in a sense.
The FAA provides guidance through it's sponsored course which essentially presents the UDRI method (DOT/FAA/TC-12/17) which incidentally is also what is basically presented in RAPID, and there is more detail there.
The issue with the Chicago ACO method is that it is using Broek's formula for inertial bending stress. But a problem with Broek is that it assumes no stabilizer input is being used to balance the aircraft. The UDRI method back-calculates a stress from ultimate assuming load factors from the CFRs, so it essentially makes a "conservative" assumption that by design, the MoS for the max maneuver/gust is +0.0.
HOWEVER, there are still several cautions with this method, and it should be applied carefully by cognizant analysts:
- Relies on some nebulous information for it's joint efficiency factor
- We obviously know there is some allowable damage and structural margin not being accounted for
- Note that their formula gives the maximum stress at any point in the fuselage. Basically in the crown over the wing box. The RAPID document (Section C6.2) also shows how to calculate a stress at any other point along the length of the fuselage by assuming a linear stress distribution, and also accounting for the distance from the neutral axis, and the weight distribution (since it is not uniform). This is where the payload factor comes in.
- Note that in RAPID, this is called "C2" and all they say is that it has an average value of 0.70.
- RAPID applies the payload factor on the entire 1g stress, but it should only act on the stress due to inertia on the payload. The result ends up like [C2_eff = C2_rapid + (1- C2_rapid) * (ratio of structure weight to total weight)]. This ratio should be a bit different for each aircraft and can be determined from the FAA Damage Tolerance Handbook Volume 2. This factor usually ended up around 0.87-0.90.
- RAPID assumes the bending stress is linear (basically the fuselage is a cantilevered Euler-Bernoulli beam). This is probably not true for several reasons - the mass is not homogeneous. The payload factor is not a good way to account for longitudinal differences, but it goes beyond that. The distribution in any cross section is not uniform. Also this method relies on the shear stiffness of the beam being much greater (>>>) than the flexural stiffness. This may not always be true for built-up semi-monologue structure.
- Spectrum effects / spectrum factor will still need to be handle/applied separately
Note, there are multiple recent papers, publications and training courses which demonstrate that the RAPID, Chicago ACO and UDRI approaches do NOT meet current FAA regulations and can in fact produce very unconservative results.
Just because an applicant does not know how to develop (or doesnt want to spend the time and money) the necessary fatigue loads and corresponding spectrum using service load histories, doesnt mean they can short cut the requirements. There is no "quick" solution to be used for all applications on all aircraft. Just do the work, its not difficult and the process is well documdented these days for developing fatigue spectra. There are public reports documenting the service load histories for almost all aircraft models and types.
Good luck.
Just because an applicant does not know how to develop (or doesnt want to spend the time and money) the necessary fatigue loads and corresponding spectrum using service load histories, doesnt mean they can short cut the requirements. There is no "quick" solution to be used for all applications on all aircraft. Just do the work, its not difficult and the process is well documdented these days for developing fatigue spectra. There are public reports documenting the service load histories for almost all aircraft models and types.
Good luck.
@crackman,
Not looking for short-cuts or "quick" solutions. The goal is to provide a substantiation that my regulator will accept. If we don't provide a report with "FAA ACO" in the methodology section, the engineer reviewing our work has to spend more time learning something new and complicated. Taking a new approach with them needs to be justified.
Why do you say that these ACO methods produce unconservative results? Why does FAA-published methodology not meet current FAA regulations? Is this just nit-picking over the recent-ish change to the residual stress calculation, which the Chicago ACO paper pre-dates, or something truly fundamental that challenges the premises of these methods?
Not looking for short-cuts or "quick" solutions. The goal is to provide a substantiation that my regulator will accept. If we don't provide a report with "FAA ACO" in the methodology section, the engineer reviewing our work has to spend more time learning something new and complicated. Taking a new approach with them needs to be justified.
Why do you say that these ACO methods produce unconservative results? Why does FAA-published methodology not meet current FAA regulations? Is this just nit-picking over the recent-ish change to the residual stress calculation, which the Chicago ACO paper pre-dates, or something truly fundamental that challenges the premises of these methods?
Hi Sparweb
What I meant is this. The so called ACO methods are not "approved" methods at all. They are presentations various FAA people have made in the past but are NOT accepted methods for compliance. Many of these FAA people that made these presentations have no background in fatigue loads or spectrum development with little or no understanding of how to develop them or the actual technical requirements. Only the Advisory Circulars provide the details necessary for acceptable means of compliance.
For example, the often referenced Chicago Antenna Doubler presentation is referred to as approved data but I can tell you it is NOT. It does not meet any of the 25.571 requirements. In particular service history fatigue loads and dynamic loads must be accounted for in any method used and this presentation ignores all of this.
If you look to the recent Airworthiness Assurance and Sustainment Conferences, ASIP Conferences as well as the AFGROW Workshop Conferences over the past few years you will find presentations and training courses which demonstrate that these previous "quick fix" approaches in these papers are not only unconservative but do not meet the requirements of 25.571.
Let me know if you need any further info.
Take care.
What I meant is this. The so called ACO methods are not "approved" methods at all. They are presentations various FAA people have made in the past but are NOT accepted methods for compliance. Many of these FAA people that made these presentations have no background in fatigue loads or spectrum development with little or no understanding of how to develop them or the actual technical requirements. Only the Advisory Circulars provide the details necessary for acceptable means of compliance.
For example, the often referenced Chicago Antenna Doubler presentation is referred to as approved data but I can tell you it is NOT. It does not meet any of the 25.571 requirements. In particular service history fatigue loads and dynamic loads must be accounted for in any method used and this presentation ignores all of this.
If you look to the recent Airworthiness Assurance and Sustainment Conferences, ASIP Conferences as well as the AFGROW Workshop Conferences over the past few years you will find presentations and training courses which demonstrate that these previous "quick fix" approaches in these papers are not only unconservative but do not meet the requirements of 25.571.
Let me know if you need any further info.
Take care.
Hi Sparweb
I would add one more thing. The folks who presented these FAA papers provided them as information and not as approved methods or data. All of these were produced as a result of the Aging Safety / Repair Audit effort in an attempt to screen repairs for further analysis. They were never meant to be used as the approved methods for all fatigue spectra as they do not meet the requirements.
Take care
I would add one more thing. The folks who presented these FAA papers provided them as information and not as approved methods or data. All of these were produced as a result of the Aging Safety / Repair Audit effort in an attempt to screen repairs for further analysis. They were never meant to be used as the approved methods for all fatigue spectra as they do not meet the requirements.
Take care
Hi Crackman,
Thanks for the additional comments. I want to look into this more, but that will take more than a minute. Here's just a quick reply of acknowledgement.
I can't go back to edit my earlier comment to include your good and important point that the AC's are the preferred guidance, so hopefully the OP and future readers scroll down here to see this clarification. I seem to have skipped the middle step going from requirement, to acceptable means of compliance, to methods of analysis. Those papers I referenced are better described as "analysis methods", like RAPID was, too. No claim of suitability for any particular aircraft or equipment was made.
Thanks for the additional comments. I want to look into this more, but that will take more than a minute. Here's just a quick reply of acknowledgement.
I can't go back to edit my earlier comment to include your good and important point that the AC's are the preferred guidance, so hopefully the OP and future readers scroll down here to see this clarification. I seem to have skipped the middle step going from requirement, to acceptable means of compliance, to methods of analysis. Those papers I referenced are better described as "analysis methods", like RAPID was, too. No claim of suitability for any particular aircraft or equipment was made.
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