Author: Richard

Pull-through Strength of Composite Laminates

Recently I got pulled into some problem solving on a part 23 aircraft program. It is great to suddenly get thrown into a new group of people and try and unravel all of the technical and personal knots. These were a great bunch of engineers and it was a pleasure to spend an afternoon with them. No names of course – but you know who you are…

One of the tasks was to come up with a pull through strength method for composite laminates (secondary structure) that was acceptable to the DER without having to do any testing to prove it. There is some data in this reference  (NASA-TM-87603, 1985), but it is all specific and there is no general approach or conclusion that has been drawn from the data. I am reluctant to try and derive a general analytical rule based on my interpretation of someone else’s 30-year-old data set that I do not have full knowledge of. So I turned to the internet and started to pan for digital gold….

I was very surprised when this nugget of a reference “Pull Through Failure of Bolted Composite Joints” appeared. This thesis does exactly what I was looking for. This paper derives a ‘general’ rule. The rule is expressed as follows:

Where:

The “Mat” factor is my own addition, a factor of 1.0 should be used for carbon fiber laminate and I recommend a factor of 0.75 for fiberglass laminate. As always for mechanically fastened joints in composite components the laminate should be as close to quasi-isotropic as possible.

I have compared this method to the public domain and proprietary data that I have and it looks to be generally conservative by about 15-20%. So I recommend this approach for initial sizing for primary and secondary structure.

We have created a spreadsheet for this method here: AA-SM-101-025 and this method will be included and explained in the upcoming Second Edition of our free textbook.

Enjoy!

 

 

Using Microsoft Excel as a Analysis and Report Tool – Post 1

First Post – Introduction

In writing our free textbook I had to make what I consider to be a huge personal and professional concession. I had to use Microsoft Word. I started out writing the book in Excel but the page formatting and section numbering and referencing issues were too great.

Word is great writing resumes, letters and engineering textbooks. For engineering reports, it has several significant drawbacks. These problems include:

  1. Word tries to do too much and can end up creating very large unstable files that get ‘corrupted’. i.e. Word ends up doing something that even Word rejects and you end up losing data and time.
  2. Word does not let you keep calculations ‘live’. This results in report updates for new loading, geometry or materials taking a significant amount of time as every numerical value has to be updated

We still use Word for writing reports. When a client has an internal reporting system that flows down to us and we must use Word the whole team just has to knuckle down and live with the inefficiency and frustration.

To be fair, Microsoft Excel is a spreadsheet tool that was initially optimized for accounting. Excel has significant limitations:

  1. Excel is not WYSIWYG and looks different at different screen magnifications and needs to be tailored for each different printer it may be printed out on
  2. Excel has limitations on how subscript and superscript characters are processed (more on this later)
  3. Excel is not a word processor or a graphics tool.

To use Microsoft Excel as an efficient technical reporting tool you have to adopt a code of ‘best practice’. Our ‘best practice’ is informed by my experience and the experience of other senior level technical people I have worked with.

Why not use Mathcad? Mathcad is a great tool but it has several drawbacks:

  1. It is expensive
  2. It is not universally used
  3. Mathcad is great a presenting math. My experience of work created and presented in Mathcad is that the user gets carried away with the beautiful mathematics and you end up with a report that consists of page after page of mystifying math with few diagrams and little commentary.

Having listed the main reasons why we do not use any other package and the negative points about Excel let me list the positive aspects.

  1. Excel is a universally used – almost everyone has an Excel license. Analysis files are easily shared and edited.
  2. Excel is the most stable of the Microsoft Office suite of programs.
  3. Excel is a general tool – it does many things moderately well

Reports written in Word tend to have a lot of prose and not enough math. Report written in Mathcad tend to have a lot of math and not enough prose. Reports written in Excel tend to naturally strike a balance between prose and math because it is equally good (and bad) at both.

Excel can also be used as a FE output database for storage and processing. We also use Excel to create input loads files for Finite Element models in the correct .bdf or .dat format.

We also use Excel to create simple engineering drawings and for creating commercial logos and graphics for most of our company needs.

Excel mirrors good practical engineering traits:

Jack of all trades and master of none, But oft times better than a master of one

 Important Terms:

Workbook – a discrete Excel file, usually has .xlsx extension

Worksheet – a sheet tab within a workbook, a workbook can contain hundreds of individual worksheets.

Cell – an individual referenceable item of data within a worksheet, cells are shown as a grid of rectangles on the worksheet.

Print Area – the area of the worksheet that is printed.

Continued in the next post……..

All Simple Beam Analysis Spreadsheets Uploaded!

As we work towards the second edition of our textbook we are completing and uploading the accompanying spreadsheets. We have finally completed the simple beam analysis section of the book and the 33 spreadsheets that will accompany that chapter in the book are now written and uploaded (We will leave multi-span beams and curved beams to the third edition).

These spreadsheets all plot the shear force, bending moment and deflection. All the spreadsheets use the XL-Viking add-in to display the math but the answers are correct with or without the add-in.

Diamond Aircraft – Sale or Investment?

I love Diamond Aircraft – the company and the product. I used to work for Diamond Canada and there are great people working at all levels of the company. They are led by Christian Dries, a very reclusive, very wealthy and very generous individual who created and driven the company for over 20 years. He has created one of the largest general aviation aircraft companies on the planet from nothing.

I received a press release today that confirmed the rumors I have been hearing for weeks. Diamond have sold 60% of Diamond Canada and the rights to the DA40 and the DA62 to Wanfeng Aviation, a Chinese-funded entity.

This is being announced as an investment, I am sure it is that, but it is also a sale. Diamond Austria have retained ownership of the DA42 (the cash cow) and the DA20. The shuttered D-Jet program has been sold to Wanfeng as well.

I am sure this is good news. Chinese-backed investors have made critical investments in the US general aviation sector and this has proven to be very positive – thus far. Cirrus. Mooney, and Continental motors are still in operation in part because of the generosity of Chinese backers.

So – what does this mean for Diamond Canada? According to the media release (attached to this post) the manufacturing of the DA40 and DA62 will transition to Canada by the end of 2017 and they will continue to manufacture the DA20 and DA42. Diamond have also been successful in bringing in work from outside the company from Northrop Grumman and the Dornier Seaplane Co.

My assessment is that this will be a positive change for Diamond Canada and we wish them all the best for the future.

Diamond Canada Investment Announcement 20 12 2016 final

Textbook Update – Second Edition

In my spare time I have been working on the next edition of the textbook. I was aware that the first edition was ‘incomplete’. In essence, every edition will be incomplete as there is always something more we can add. The First Edition was less comprehensive than I had hoped, but we had to release something at some point and the First Edition was a compromise. The Second Edition will be a compromise as well, but it will be a compromise I am much happier with.

In the Second Edition, we will cover some more subjects:

Section properties: In this section, we define the first moment of area of the shape as well as the typical section properties. The first moment of area is used to calculate the actual elastic shear distribution in a cross section and is also used to define the plastic bending section shape factor used in the Cozzone method.

Beam Analysis: We will include the definition of shear force, bending moment and deflection for typical beam configurations and there will be a spreadsheet link for each method. This has taken a long time to implement but I believe it will be very useful.

Stress  Analysis: The section that I regretted most not including in the original edition was a section on basic stress tensors and material failure criteria. This section is aimed at isotropic materials (the equivalent plane strain failure criteria is already defined in the composite section of the first edition). This will come with links to spreadsheets for all of the methods.

Using Excel: I have been using Microsoft Excel to write stress reports for two decades and it is my platform of choice. Like the textbook, it is a compromise. But we have developed a way of working with Excel that we think makes it the best choice for most analyses and report writing. In the second edition of the book I have tried to explain the golden rules we use when using Excel as a report writing tool.  This is the philosophy that we apply to all of the standard analysis sheets.

General updates and improvements: As always we are beholden to the generosity of the engineering community and you have provided valuable feedback to help us catch errors and make improvements to the First Edition. I have lost count of the number of tweaks we have made to the existing content but we are somewhere in the hundreds.

I regret that I can’t find more time to work on the book – it is a source of interest and enjoyment for me and I am looking forward to getting the Second Edition done so I can start to work on the new content and methods for the Third Edition.

If there is anything you would like to see included that is not in the first edition or mentioned above please let me know.

Debt Recovery and the Family Owned Business

We are a family owned business – most small businesses are. We work for larger corporations. Some of our clients are also small businesses and are family owned as well. Simple stuff. Well, it is simple until one of your clients start to owe you money.

Like all businesses we are understanding of clients having short term cash flow issues, you have to roll with the ups and downs of being a small fish in a big pond in the same way as other corporations do. The development programs we work on are usually for startup companies with investors who sometimes use their cash as a means to reward and penalize project progress or delay. It is tough for a lot of our clients and we always do our best to be as understanding as possible.

If a debt becomes delinquent what can you do to recover it? As a small business, the amounts you are owed are generally small amounts. How small – well – certainly less than $250,000. In my mind that number is important as, for us, it is the amount of money that it is not worth going to court over. The cost, both direct and indirect, make restitution through the legal system ineffective for recovering amounts of this value or less.

When a larger corporation owes you less than $250,000 what can you do to recover it? Well, first we consider the nature of the debt.

  1. Did the client express any dissatisfaction with our service and is non-payment an aspect of a wider contractual issue?
  2. Has the client paid other people in the same period and not paid you?
  3. Is the client keeping open communication with you or are they forcing you to chase them for news of payment?
  4. How long has the money been owed?

Some time ago (in a different life) we used to own rental properties. Our experience of debt collection is that point 3. above is pivotal moment. Once a debtor stops voluntarily keeping you informed of their plan to pay you back and giving you regular updates it is time to start to worry. It is problematic that you have to commit more of your time chasing your debtors for updates of their ability to pay you.

We do not charge late payment fees or charge interest on late payments. We are as understanding and patient as we can be – up to a point.

We have reached this point with one of our clients. They meet all of our criteria for delinquent debt – they have expressed no dissatisfaction with our work and we do not have a wider contractual issue to settle, they have been paying other suppliers, I am forced to chase them for updates and news of their plans to pay us and the money has been owed for a long time.

To be honest, they have paid some partial payments along the way, but there is still a significant amount outstanding and our ability to continue to supply them credit is also limited by our own financial obligations. During this time members of staff in out office has received calls from recruiters on behalf of the client.

Should we use the power of public knowledge to encourage the client to treat us as they have treated other suppliers who have got paid during the period that they have owed us money?

Would going public with this information harm us?

Filing with a court to recover the debt would also make it public, would using social media be any different?

Let us know what you think.

 

 

 

Stratos Aircraft – First Flight Successfully Completed!

On the 21 November 2016 the Stratos 714 took to the sky. We are proud to have played a part in this program and give our congratulations to CEO Michael Lemaire, CTO Carsten Sundin and the rest of the team. I am the structures manager for the program (with more than able assistance from the in-house Stratos engineers Martin Neumuller and Michael Dagenais and the rest of the engineering and build team) and Abbott Aerospace has carried out all of the structures analysis for the aircraft, headed up by myself and Nirav Shukla – with the exception of the landing gear analysis which was done by our good friends at Endeavor Analysis.

One of the many positive aspects of working on prototype programs and start-up companies is that you get the honor of playing a part in making history – you get the chance to play a part in putting something unique into the sky for the very first time. The Stratos 714 was devised as a response to the lower performance single engined VLJs proposed about a decade ago – the D-Jet, the Cirrus Jet and the Piper Jet – and some others. Stratos saw little advantage in developing a single jet-engined aircraft that has lower performance than an equivalent propeller aircraft. They stratos-first-flightdecided to go high and fast, therefore the Stratos 714 has a service ceiling of 41000ft and will cruise at 415KTAS. It also looks great – thanks to Gordon Robinson the chief aerodynamicist and scotch whiskey connoisseur (that’s a long story).

Stratos brings to the table a history and experience of developing and working with kit aircraft. As a senior engineer from the certified aircraft business I felt some trepidation at first – how could ‘home builders’ create and fly a high-performance jet? I look back on my early anxiety and I feel foolish now. Everyone on the project has built a personal aircraft apart from me (if you are going to build an aircraft this turns out to be useful experience – who knew?) and everyone is a qualified pilot apart from me – the ‘real’ engineer. Hmmmmm.

Everyone on the project is also multi-disciplined. Puts me in mind of the full saying: Jack of all trades and master of none – but oft times better than a master of one. The Stratos project proves the point.

Next steps for the company is to gather flight test data and prove out the performance and handling predictions. Then Stratos have to decide what to do next and they have to find a way avoid the VLJ single graveyard – the only bright spot being the recent release from the certification long-term care unit, the Cirrus jet – and even that ‘success story’ is many times over the original budget and many years late.

unnamed-2There is no blueprint for success for VLJ singles getting to market. The upside to this is that the market is not crowded and there is clearly room for competition to the Cirrus jet. Stratos have a plan to get there. I am looking forward to a time when pilots and operators trade in their Cirrus SF-50s to upgrade to a Stratos.

For now, congratulations are due to the Stratos team and we are very happy to have played a small part in helping them make history.

Boom Aero – A New Hope

I was privileged to attend the roll-out of the Boom Aero technology demonstrator on Tuesday night in Denver. To give complete disclosure, we had some input early in the program just as Blake was getting started and we did some very preliminary layouts for the production version. In no way can we claim credit for any of the real progress made on the program, we were just lucky to be involved in a small way early on. All credit is due to Blake and his current team.

Apologies for the quality of the picture for this article – I had my kindle fire in my pocket and had to make do with what I had to hand. I should have remembered my boy scout motto…..

I really believe in the Boom operating model for the aircraft in service. I think they have that just right. I think the aircraft (the final production configuration) looks great and the technology demonstrator looks fabulous.

I have personally worked for aircraft startups since 2001 and Abbott Aerospace has done 95% of our business on multiple aircraft startup programs since 2008. We have seen the good and the bad and no project has a monopoly of either.

Harking back to previous posts on this subject (here and here), there are numerous financial pitfalls that the aircraft startup company faces. I believe that Boom has a market and their product is very well tuned to meet that market – they have defined their product and have a commercial argument better than the competition. Boom’s problem is how to get there without getting themselves and their investors into serious financial problems.

There are several relevant data points in terms of development costs – the Boeing 787 cost between $15Bn and $30Bn (depending on your source material) to develop and bring into volume production. The G650 took around $1Bn (it was probably more, but that is the published figure). It is worth noting that the G650 traded on a lot of the previous aircraft technology from Gulfstream while the 787 was a wholly new composite venture for Boeing.

However, both Boeing and Gulfstream had an established an organization, infrastructure and engineering and process/quality systems that they could use. They also had existing facilities and engineering teams not to mention global MRO and spares and repairs capability.

Setting up the organization from scratch, going supersonic, convincing the FAA (and overseas certification agencies) that you should be allowed to operate your aircraft, steering the company and the aircraft through the first few months or years of inevitable teething problems in service are all eye-wateringly expensive.

A startup company has to develop all of these aspects from the ground up and these are often the most underestimated aspects of the complete development. You must cover not only the engineering product but the means to manufacture it per the type certificate and keep your customers happy once the aircraft is in service.

The startup company has to develop (and pay for) all of these things. In the case of Boom considering their overall development cost, it would be better to purchase an existing suitable company. Lear, for example, are ripe for the picking and they would acquire floor space, an entire set of company engineering and build processes, an engineering team and a global spares and repairs operation. It would be much more economic to purchase an existing operation and adapt it to meet the needs of your program than it would be to invest in setting up all of these things.

Boom has a good chance of success. Their product has a market, they have some very good people involved and they have a self-confessed ‘zealot’ leading the charge. My fear is that they will not temper their blue-sky thinking with enough earthy pragmatism. But then again what do I know? In the end failure only shows us how to do it wrong. I sincerely hope Blake, his investors and his team are all successful in their aspirations and usher in a new golden age of both aircraft development and air travel. We could do with both.

More Beam Analysis – Spreadsheets and Sources

This is a companion post to the first beam analysis post here.  These methods again reference the excellent  NASA-TM-X-73305 Structures Manual and cover the rest of the cantilever conditions and start on the single span beams:

AA-SM-026-009 Free Engineering Spreadsheets: Beam Analysis – Cantilever – Moment at Free End

AA-SM-026-010 Free Engineering Spreadsheets: Beam Analysis – Cantilever – Moment Anywhere

AA-SM-026-011 Free Engineering Spreadsheets: Beam Analysis – Simply Supported Both Ends – Point Load Mid Span

AA-SM-026-012 Free Engineering Spreadsheets: Beam Analysis – Simply Supported Both Ends – Point Load Anywhere

AA-SM-026-013 Free Engineering Spreadsheets: Beam Analysis – Simply Supported Both Ends – UDL along Entire Length

AA-SM-026-014 Free Engineering Spreadsheets: Beam Analysis – Simply Supported Both Ends – UDL Part Span

AA-SM-026-015 Free Engineering Spreadsheets: Beam Analysis – Simply Supported Both Ends – Triangle Load Whole Span

AA-SM-026-016 Free Engineering Spreadsheets: Beam Analysis – Simply Supported Both Ends – Triangle Load Peak at Center

AA-SM-026-017 Free Engineering Spreadsheets: Beam Analysis – Simply Supported Both Ends – Triangle Load Zero at Center

AA-SM-026-018 Free Engineering Spreadsheets: Beam Analysis – Simply Supported Both Ends – Moment at End

AA-SM-026-019 Free Engineering Spreadsheets: Beam Analysis – Simply Supported Both Ends – Moment Anywhere