Things that effect the fatigue life of a Sonex, (or any metal aircraft)

This is just a reposting of a thread I put on the Sonex builder's forum. There has been a fair amount of discussion recently about fitting heavier engines, particularly the Corvair, although that would be an unlikely choice in Australia. however, their is a price to pay when you increase the maximum all up weight from that specified by the designer.

 

I can't tell you what the fatigue life of a Sonex is because it depends on so many different factors, but there has been a fair bit of discussion recently about the pros and cons of fitting heavier engines and flying with a greater maximum all up weight so I thought I'd toss in a bit of information about one of the consequences of increasing the weight of your Sonex.

 

 

The above graph shows stress against number of cycles to failure. It's not necessarily for 6061-T6 but it is a typical for S-N curve for aluminium. You'll notice that the bottom scale is logarithmic, not linear. If you pick any particular stress and then add just 10% to it, say by going from 200 MPa to 220 MPa, it results in roughly a halving of the fatigue life.

 

You could argue that the Sonex is so strong that fatigue will never be a problem, and you could be right. I certainly have confidence in my Sonex but there is a weak point, even in those massive main spars, and it's not hard to increase all the stresses by 10%. For example, by increasing the MAUW from 1150 pounds to 1250 pounds, an increase of 8.7%, you are actually increasing all the loads by about 9.6% (the weight of the wings doesn't count as part of the load). If you'd like to read about this in a bit more detail, go to my blog at How Strong is the Sonex? | Peter Anson – Engineering

 

Peter

 

Sonex 894

 

357 hours and going strong

 

 

Interesting! I like the Sonex. Being financially crippled, I am an ultralight enthusiast, but the Sonex seems a strong great flying little plane. They get some less than favorable reviews at a couple blogs here. "Under powered, Sonex guys never fly their planes anywhere," bla bla bla. Not the case! I attend a fly in at Recklaw Texas, and there is a group of 6 Sonex flyers that come here from all over the USA. Even the couple guys flying Monet's aerovee conversion have no complaints about power. As far as structural durability, I am no engineer, and would stay within Monet's guidelines for a power plant. One day I hope to own a Sonex. Possibly with Jabiru's flat 6.

 

 

I am a member of an airplane club where the majority of the members are employees of Lockheed Mfgr in the town of Marietta, Georgia

 

USA. Several of these guys are engineer types and several years ago one of he guys gave a lecture about fatigue and different materials specifically the different common aircraft alloys of aluminum since aluminum was being used to build 5 or 6 different planes in the club. An Important takeaway I remember is an amount of stress called the "endurance limit" ( I hope I remember this

 

correctly ) and at that amount of stress and lower the aluminum incurs zero fatigue buildup. The presenter explained that

 

although it may be engineering overkill .... whenever a large weight penalty is "not" incurred that it is a good idea to design

 

structures with such low stresses that they rarely incur any cumulative fatigue. Hence those parts of structures have such extended

 

projected lifespans ( say 25,000 hours) that it is simply never a worry. I am not familiar with the Sonex but I am guessing that

 

the designer overbuilt to a degree the most critical areas subject to stress and fatigue so that fatigue should never be a

 

worry in any flyers/owners lifetime..

 

 

Overkill, and redundancy are our guardians.

 

As long as it doesn't fly like a manhole cover.

 

 

 

[quote= An Important takeaway I remember is an amount of stress called the "endurance limit" ( I hope I remember this

 

correctly ) and at that amount of stress and lower the aluminum incurs zero fatigue buildup. The presenter explained that

 

although it may be engineering overkill .... whenever a large weight penalty is "not" incurred that it is a good idea to design

 

structures with such low stresses that they rarely incur any cumulative fatigue. ..

 

Not so for aluminium but can be true for mild steel.

 

Composites are good.

 

See here: Fatigue limit - Wikipedia

 

 

Interesting! I like the Sonex. Being financially crippled, I am an ultralight enthusiast, but the Sonex seems a strong great flying little plane. They get some less than favorable reviews at a couple blogs here. "Under powered, Sonex guys never fly their planes anywhere," bla bla bla. Not the case! I attend a fly in at Recklaw Texas, and there is a group of 6 Sonex flyers that come here from all over the USA. Even the couple guys flying Monet's aerovee conversion have no complaints about power. As far as structural durability, I am no engineer, and would stay within Monet's guidelines for a power plant. One day I hope to own a Sonex. Possibly with Jabiru's flat 6.

Hi Charlie, I'd agree with everything you say and have done many trips in my Sonex. See http://www.ansoneng.com/sample-page/sonex-trips/ for reports on some of them. When I started building my Sonex, I too was a little financially crippled because I already had an aircraft and couldn't afford two of them so I built mine from scratch. You can still do that with the Sonex (as well as some Zenith models I think). It took me longer than I had hoped, 6 years, but that just meant the expense was also spread over 6 years. The dribble of money on the Sonex was small enough that it had little effect on the family finances and at the end I had a fantastic little aeroplane. My Sonex has withstood loads in Thermals that would have ripped the wings off my previous aircraft. On the other hand my previous aircraft would have been 50 knots slower when it hit the thermal so maybe it wouldn't have been a problem.

 

I am not familiar with the Sonex but I am guessing thatthe designer overbuilt to a degree the most critical areas subject to stress and fatigue so that fatigue should never be aworry in any flyers/owners lifetime..

You are correct, and I think the Sonex designers certainly aimed for this, but my point was that the consequence of adding weight to the build could be that fatigue life does become a worry. John Monnett is obviously not happy about people changing the MAUW of his design, but this isn't just a Sonex thing. It applies to all metal aircraft.

NTSB finds fatigue failure led to trainer crash | Flight Safety Australia

 

Metal does fatigue. The above accident happened to a fully certified, well maintained, production aircraft. This is actually frightening. The airframe was built in 2007 and had 7900 hours. The left aileron appears to be unpainted. It was a Piper Arrow, so not a basic trainer used by inept ham fisted individuals leainging basic flying skills... The wing departed the airframe in the circuit pattern. No amount of flying skill would save a pilot in this situation Build to plans, future owners of your aircraft will appreciate it, and DO NOT GO OUTSIDE DESIGN LIMITS! Please.

 

 

What is Endurance Limit (Se)? - Definition from CorrosionpediaEndurance limit (Se) is the stress level below which a specimen can withstand cyclic stress indefinitely without exhibiting fatigue failure. ...Endurance limit is also known as fatigue limit.

 

Fatigue Limit | Definition of Fatigue Limit by Merriam-Webster

 

https://www.merriam-webster.com/dictionary/fatigue%20limitDefine fatigue limit: the highest stress that a material can withstand for an infinite number of cycles without

 

breaking —called also endurance limit.

 

 

Reminds me of the Zenith CH650. There is not a hinge per say for the flaps. The hinge is sheet aluminum. The skin on the wing covers the top of the flap. It seems to work fine. The flap I guess doesn't deflect enough to cause any stress on the aluminum sheet.

 

I would think it eventually it would develop a crack or a tear. They have been using this system on the CH650 for as long as I can remember the plane.

 

 

There is not a hinge per say for the flaps

The hingeless design is for the ailerons, not the flaps. Most have moved to hinges as ailerons now have to be balanced. But the hingeless design does work and has been tested for thousands of cycles.

Yup, my bad. It was the ailerons.

 

I got to see a CH650 at Recklaw last year.

 

very nice airplane. Can carry more weight than the Sonex. More room too. But I think the new Sonex B may be catching up.

 

 

I looked at a Sonex under construction and told the builder that a couple of the controls seemed to be non standard practice. He looked up the drawings and showed me that they were as designed. I cannot remember exactly where the control joints were, but they were putting loads into a member that could bend, rather than a double sided member.

 

That plane is flying now, in fact I have flown in it. I just assume that the loads are too small to cause a problem.

 

 

Hi Yenn, in fact all the control bell-cranks in the various Sonex types are single sided. I have never heard of any bolt failures but admit I didn't like the look of them so made all mine double-sided as shown: