Thread: Biscuits V gussetts for strength.

This will help.

Al :eek:

Originally Posted by Deskpilot

G'day all, my name is Doug Mansfield and I'm about to start designing and building an experiemntal, recreational aircraft as a retirement project. Whilst researching tools I came across your site and see that you have all, well maybe most, of the answers. My question is: Would an inlaid 'biscuit' be stronger than an outside gusset in a butted T joint, bearing in mind that the frame members have to be slotted. I imagine the 'biscuit' to be approx 100mm in diameter(if such a sized circular saw exists) and inset on the centre line frame members. Approx 75 mm will be inserted into the upright frame member.I'm not sure of the frame dimensions at this time but the insert size can be ajusted accordingly. The main members will be Spruce or similar ozzie wood and the ply wood of aircraft grade. Under the 'experimental' rules, one is allowed to try anything, AT THEIR OWN RISK. Whilst this method will incure additional work, it will make for a much 'cleaner' joint. What do you think?
Regards, Doug.

G'day Doug, I take it you won't be the pilot?

Given the stresses inflight I suggest that you may be in for some trouble if you are building an experimental aircraft and are not an aeronautical (structures) engineer. Selection of a given joint is dependant on where the joint is to be used, directional loads, thickness and type of material etc. Unless these details are known it would be hazardous for anyone to suggest what to use. I did some work for a friend who built a Mustang scale replica and we did not vary from the engineering drawings without a lot of investigation using websites and, finally, getting approval from the designer. The guy who built the aircraft was an aero eng and was still reluctant to jump in and make mods because of their overall affect on the airframe.

I strongly suggest you go to aircraft design sites and only use information from there as, although the answers here may be well intentioned and correct for normal woodwork and furniture design, an unsuitable joint may cause a structural failure in an aircraft.

I wouldn't use biscuits, they are an over rated joinery method which is only popular as they're easy to do.
If the design says gussets that's what I'd stick with.

If the design says gussets that's what I'd stick with.

I only use biscuits as an alignment aid when joining, as I consider that the glue I use is probably going to provide more than enough holding power.

Hi Deskpilot, please stick with external gussets. The gusset isn't only holding the joint together, it is also reducing the bending moment by a factor of hundreds to one. Biscuits are ok for furniture etc where they are able to restrict shear forces but the bending moments are accounted for by the design. ALSO, I don't know too many folk whose lives are dependent upon their furniture!
Fletty

give yourself a tick from me Oz, I've got some spreadin' to do!

cheers,

P

Originally Posted by bitingmidge

give yourself a tick from me Oz, I've got some spreadin' to do!

cheers,

P

Wish I could....

Al

Gentlemen, Thanks for your very prompt responses. Unfortunately, I think some of you were too eager to post and didn't fully understand the question before you. I'll expand on that in a moment, but firstly let me answer some of your critisms.
Groggy, Actually, I will be the pilot. I took on the user name of Deskpilot many years ago and have stuck with even thoungh I am now a qualified pilot. I still fly my desk occassionally (flight sims). You assume that I'm not an aeronautical enginneer, but seeing as the building of wooden aircraft isn't taught nowadays, (all aircraft are either metal or plastic), I might well be qualified but need the quidance of people who regularly wook with wood. Actually I'm a metrologist (nothing to do with the weather) and if you don't know what that is, look it up. I'm very familiar with the WAR system as used in scale Mustangs and will basing my design on that system. However, If we didn't look for improvments in design, we'd still be flying Wright Bros aircraft, driving Ford Model T's and using dowels instead of biscuits.
Echnidna, Himsol. I agree with you in that biscuits, as you know them, are only suitable for alignment and probably have less shear strength than the 'old fashioned' dowel.
Fletty. You've almost hit the nail on its head. It's the bending moment that I'm interested in. Let's go back to the question, but I'll put it in another way. Same T joint, same gusset(spelt correctly this time) but now lets put that gusset down through the center of the T rather than on front or back of it. Same left to right strength and rigidity, but I think much more back and forth rigidity. Think in terms of swinging on the down piece of the T. As the plywood gusset (read biscuit) is now glued on both sides rather than only one as in the normal concept, it must now have a much better bend resistance in the backward/forward plane, hence a stronger joint. The unknown at the present is whether slotting the frame members will significantly affect their strength. My 'biscuit' would be made of aircraft grade plywood and circular instead of trunkated triangle in shape for easier slotting of the T joint. I'm not sure that I can put it any clearer.
Care to give it another go Gentlemen! Cheers, Doug

Well you could always make up some test joints and load them up to destruction. You would probably need to do a few of each type to be statistically accurate

Don't know if the name Hazelton means anything to you but my family's got a pretty good reputation as far as aviation goes. My father's not only a pilot, but a flight engineer, a qualified ground engineer - and a pretty good carpenter. Having said all that, I'm not sure I'd want to go flying with him in anything 'experimental'...

I don't think anyone here was trying to insult either your flying skills or engineering talents, but the fact you're looking in the Woodwork Forum for structural advice on an aeronautic engineering question calls for caution on the part of the advice given. There are plenty of people still building and restoring aircraft with wood, and there are certainly forums out there better suited to answering your questions.

The Vampire was not only England's first jet to see standard military service, but was constucted mainly of wood. I'm quite certain it was designed to handle more stress than what you've got planned. On top of that, the Poms are notorious for over-engineering everything! It's been 50 years since they were flying around, but still quite a leap from the Model T...

I would suggest you enter your idea into a drawing program before showing it to people who've either seen the idea already, or are more familiar with the pros and cons of such a design under the exact stresses to which it will potentially be subjected.

Damien

The joint may be stronger and more rigid, but this might have the nasty effect of transferring stress to other parts of the structure that aren't designed to take it.

Slotting out the member and then glueing in your gussett should be as strong as the original timber (however it might not have the strength in the right place or posess the same fatigue qualities).

The tests conducted by West System on their epoxy adhesive could be interesting reading for you.

g'day again Deskpilot,
I've restored a number of wooden aircraft wings in an earlier life and have noted that the timber usually fails/rots/splits before the properly made joints do. The double external gusset is theoretically and, in my experience, practically stronger than any other joint I've restored and has the added advantage of giving a surface on the aerofoil shape to which the wing skins/fabric can be fixed.
I think I understand your proposal to cut into the 'T' but how would you do that for a through joint (ie an 'X') and I'd be worried about the stress concentrations cause by the ends of the slots and the fact that these could start splits right down the centre of the structures.
Fletty