Hangar Flying

Hangar Flying => Hangar Flying => Topic started by: rmartell on June 15, 2005, 01:44:42 PM

Title: attaching winglets on vez with kevlar?
Post by: rmartell on June 15, 2005, 01:44:42 PM

I am attaching the winglets and have a question?  
I am using kevlar bid,  can I sub the uni fiberglass with the kevlar bid?
If not,  Is it ok to use carbon fiber uni with the Kevlar bid?
thanks :?:  :?:

Title: Re: attaching winglets on vez with kevlar?
Post by: Dan Patch on June 15, 2005, 04:48:48 PM

Assuming that your question is 'for real' and not just to provoke a heated discussion,  it cannot be answered "yes" or "no" without an in-depth, engineering structural/aerodynamic/aeroelastic analysis.  I would submit that few on this forum (myself included, even with an advanced engineering degree) have the competence to do this properly; and that the degree of effort required would be more than sufficient to preclude anyone from doing this 'just for fun' and then attaching their name to the resulting conclusion.  

There have been reams of words written about the implications of substituting different composite materials, altering layup schedules, etc., etc. and I'm not going to re-plough that ground other than to suggest that this is super-serious business that is far outside the realm of eye-ball engineering, 'rules-of-thumb', shortcuts, etc... And for attaching the winglets no less, arguably about the most complex and critical structural element in the entire airframe!    :roll:

The super-short answer to your question is: Neither substitution (Kevlar or graphite) is acceptable because the resulting structure will have properties significantly (much?) different (maybe better, maybe worse, but for sure, different) than those intended for what is a proven design.  I'll leave the long answer(s) to those who would want to delve into issues of load distribution paths in materials with dissimilar elastic modulii, the compressive performance of kevlar, graphite tensile failure modes, etc. etc.  

Two alternative questions I would pose would be, "You haven't really substituted Kevlar for glass in a major structural layup already, have you?"

And, "If so, why would you do such a thing?"

Title: attaching winglets on vez with kevlar?
Post by: Anonymous on June 15, 2005, 06:03:51 PM

No,  I havent used it yet,  will look into it though.  Kevlar was not avail when the vez was designed.  Newer and better products are avail. I will consult with some engineers and pose the question.  Maybe they can provide further insite.  Will wait and see.

Title: attaching winglets on vez with kevlar?
Post by: JonC on June 15, 2005, 07:06:41 PM

I don't believe there have been any reported issues with winglet seperation in properly built LE or VE aircraft. I could understand maybe putting some kevlar on the under side of the nose or maybe on the lower winglets incase you decide to graze one on the runway, but I fail to see the reasoning behind using kevlar to attach the winglet, when the whole winglet and wing are made of fibreglass. If your wing stays in one piece, then the winglet should stay attached quite well with the plans fibreglass, so why spend the extra money to use kevlar?

Title: attaching winglets on vez with kevlar?
Post by: Glider pilot on June 15, 2005, 10:20:09 PM

Are you trying to become a test pilot? Even with proper engineering done for such a substitution and you find out the engineers made a bad calculation, you may be a test pilot with no winglets.  Hopefully the winglets  don't come off, but you will have to wonder if they are going to hold on every time you take to the air.  There is very little to gain and a lot to loose by subsituting materials for the tried and tru. Stick to the plans and you will fly sooner, safer and with peace of mind.

Title: attaching winglets on a vez with kevlar
Post by: Joe Person on June 15, 2005, 10:34:13 PM

Engineer's (airframe & mech systems engineer) opinion:

Don't ever make a material substitution just because a material seems/sounds "better".  Kevlar is no longer used to the extent it was in the 70's and 80's (in airframes) because of some of it's less-than-desirable properties.  It is hydrophyllic, and moisture ingression was a big problem (and this with advanced, high-temp epoxies with autoclave-cures).  

Carbon fiber sounds dandy, and the physical properties and structural performance seem grand.  Show me those physical properties using our construction techniques (They don't exist when used in the manner we build airframe structires in our garages).  Hand-layup properties (strength-to-weight) of S-Glass are damn-close to hand-layed carbon, and S-Glass is cheaper.  And no, it is not necessary to use S-glass for your winglet attach either.

As Dan Patch mentioned, mixing different composites (with differing modulii & other physical properties) is nowhere near a simple "sum of the parts" exercise.  Avoid such practice for any primary structure, period.

There is absolutely NO need to look at material substitutions in the Rutan airframes.  I am a reasonably-experienced engineer in the airplane biz, and I have no desire at all to charge out and try to make improvements - I don't have the time, money etc to support the engineering effort (which would include all sorts of physical tests to boot, to qualify my re-engineering).  

Keep it safe - do it right (good workmanship and attention to detail) and stick with the plans.  You will be much happier, and dollars ahead, in the end.

Title: attaching winglets on vez with kevlar?
Post by: Tom on June 18, 2005, 01:33:13 AM

Its soo nice to see people that understand stress analysis and "e" (not to mention moment of inertia!)

Burt is a Brilliant man! that wanted to see his airplanes fly>> NOT MAKE MONEY!

Burts plans are solid, and unless you have flown into space lately, dont question them..


Tom