Adding lightness, how far do you go?

[Chris Walby]

I normally rely on the designer to decide where to put ribs, bulkheads spars and all the rest together in the right place and then follow that unless something needs changing or installing that is an alternative e.g. electrifying my Grumpy Tiger Cub) which was done before and I didn't take any wood out as it was going to end up on target weight anyway.

 

Now I have noticed that some of the forum members do lighten things and in this case the design I will be building brings the model out on the heavy side. Its fully sheeted and I intend to use thinner sheet as it will be glassed anyway.

 

This brings me on to the wings and in particular the ribs and the question, just how many holes can I get away with?

 

The holes in rib 25 are 14mm and 8mm respectively for the removable wing tips as its a 80 inch model

The two rectangular slots are for the original ply wing spars hence I don't think there is an issue with strength  

 

My line of thinking is that its going to have to look like swiss cheese to make any real difference and I am prepared to go to that extreme for a couple of reasons:

1. The mark 1 version was over weight and had a high wing loading that didn't help 
2. With the model completely sheeted and glassed it proved to be a tough bird even when we poked it out of a tree

 

This leads on to what's the best way to make hole in ply and balsa ribs?

 

Details of the project will be forthcoming in due course ?

 

[Ron Gray]

The ribs in my Dane RC Leprechaun had quite a bit of 'meat' taken out of them, here's a shot from their build instructions

 

 

Regarding cutting holes in ribs, I've used sharpened (very sharp) tubes to do the job before.

[leccyflyer]

The best way to make holes in balsa ribs IMO is with a set of Brian Gaskin's Softbore tools. If you don;t have those, you can make your own from sharpened lengths of brass (or steel( tube, cut some simple teeth into the sharpened edge with a file to give them more "bite".

For ply ribs, in a srack, you could clamp them together and use a Forstner bit, or even a hole saw, if the holes are big enough - these need a sacrificial piece of timber beneath to prevent breakout of the ply into a splintery mess on the underside.

[Nigel R]

If the ribs are balsa I would not bother with holes. Ribs are such a low fraction of the overall weight.

 

[Andy Gates]

I normally build from plan hence the following comments. They may not apply to you as you appear to have a rib pack.

 

I would consider the material used and the likely stresses involved.

 

Ply for ribs with stress - especially where joiners go though the ribs near the roots.

The rest for me would be balsa, no more than 1/8" thick maybe even 1/16" especially if they are to have caps or sheeting over them.

 

I hate light ply - to me it is an oxymoron. If you need strength use proper ply, elsewhere balsa.

If there is going to be stress going into balsa sheet such that it may split it (eg fuselage sides), I normally laminate the balsa. So for 1/8" I would laminate 2 pieces of 1/16" with the laminations slightly skewed - not necessary to do the 90 degree thing.

[Simon Chaddock]

Structurally cutting holes in a sheet with a grain is far from ideal as to achieve a significant weight saving it will have a distinct impact on the strength of a rib. Holes in a play rib are better because of it reduced grain effect,

If you really want a light weight wood rib it has to be of a built up construction where the material is used to make the best use of its properties and the forces concerned.

Light strong and rigid but an awful lot of work!

[Nigel R]

Lite ply has its charms. Its good for moderate stress parts, servo mounts, fuselage formers, reinforcing ribs for things such as u/c mounts. It is also rather cheaper - relatively speaking - than it used to be, given what has happened to the cost of balsa.

 

I also use 4mm diy shop ply for most jobs that I used to use 1/8 birch ply for, as it's much cheaper and seems to work OK. Two layers laminated together makes a decent firewall or u/c bearer.

[Martin McIntosh]

Have a peek at `Sea Fury 77"` which I an now constructing. I have removed all I dare from the balsa ribs which at the end of the day only hold the wing skins apart.

I tried this on a fun fly with foam ribs but the removed parts came to a total of one whole ounce. The above pic from SC shows how a full size wing is made. I once had a part in restoring a Rothmans Stampe wing and the ribs were made like that.

[Nigel R]

Some homebuilt types use ply ribs with holes in, don't they?

[Chris Walby]

Big thanks to all that have contributed as its been very helpful in formulating a plan and suitable method for cutting the holes.

 

As the ribs have already been cut I have missed a trick in at the initial production stage although can weight everything up and see what I will be saving.

 

One last thing, any tips on weight reduction of the fully sheeted fuselage as well as the bulkheads having cut out/holes through them already?

[Simon Chaddock]

Chris

The trick to weight saving is to ask yourself "Does this bit have an appropriate strength & stiffness for the duty it has to undertake and would a change in shape or material reduce its weight for the same performance?"

Sounds simple but to arrive at a valid answer you have to a} know what its required duty is and b) how close to its structural limit it is.

 

In model planes 'strength to weight' is made more complex as they are two distnct load conditions, flying and 'contacting the ground'. The strength requirements tend to be very different so whilst reducing weight may not effect its 'flight' strength it may result in any sort of crash being a complete write off.

 

It is fair to say that removing material will reduce strength. Whether the resulting reduction in strength is worth the benefit of the weight reduction is a very complex subject with more variables involved than you can shake a stick at! 

 

Given this you may have to trust that the original designer knew what he/she was doing for its intended use unless you are sure you can do better to suite your particular requirements.

 

 

  

 

 

[Andy Gates]

With fuselage formers, I make them out of 4 parts rather than sheet, excluding the firewall on IC aircraft.

Shaped top & bottom parts and 2 strips for the sides which overlap onto the top and bottom for strength.

Thickness can normally be reduced as you will be using wood in its strongest direction in each piece. Also no holes to cut out. 

If the formers are big then bracing pieces can be added about 1/2 way up to preserve shape and strength.

 

2 hours ago, Simon Chaddock said:

The strength requirements tend to be very different so whilst reducing weight may not effect its 'flight' strength it may result in any sort of crash being a complete write off.

While I agree with the above in part, a lighter aircraft will carry less inertia into a crash so less damage when it stops quickly. Also a lighter aircraft may be able to fly out of the crash situation easier so it may avoid the ground.

 

I have a 75" wingspan Chipmunk which is built very light and is now in its 15/16th year of flying. Even at quite slow speeds, it can almost turn in its own length without it falling out of the sky - I would not like to try that with a heavier machine.

 

View it not so much as adding lightness, but building it to fly rather than to survive a crash.

[Simon Chaddock]

Andy

I agree entirely about building planes to fly rather than crash but for many a plane that cannot handle say a rough landing or needs extensive repair if it does is considered unsatisfactory. This was why I added '...to suite your own requirements'.

 

I am quite happy to fly a plane (of my own design and build) that not only requires suitable weather conditions but also a perfect landing to stand a chance of returning undamaged.

 

[Martin Harris - Moderator]

I don’t know about trusting designers - so many ARTFs are woefully deficient in the undercarriage mounting department with little thought given to distributing landing loads on anything other than perfect surfaces and good landings.

[Chris Walby]

Thanks for the comments and just putting this into context for the model under discussion.

• Firstly the original plans are from 1976 and don't show much lightning 
• The wings and fuselage are shown as sheeted with 2.5mm of for the time, but seems excessive 
• The original was 70 span and AUW 4.6Kg, really not sure how that was achieved, but my 80 inch version was a lot heavier than that.

My ribs look like this and I think its perfectly possible to get them looking like Ron's example with no detrimental impact on strength because there is an additional pair of wing tubes going through ribs for the removable wing tips 

 

 

 

Off topic, but IMHO sometimes its better to rip the UC mounting plate out than major structural damage to the fuselage.

 

PS - if this model was light I would have serious consideration about taking weight out, but it has proven one thing and that its tough (one gear up landing/crash, falling 30ft out of a tree and a main gear heavy landing that compressed the mains until it tail struck and then ripped the nose UC out at the bulkhead. The only time the wing failed was the final flat spin from 40ft landing on the fuselage and ripping the two ply spars through the wing ribs. TBH I don't need anything that robust, what is needed is a wing loading the right side of 35oz/sq ft ?.

[Martin Harris - Moderator]

As long as the spats don’t take out the wing covering…but good load distribution can absorb a lot of hard landings. 
 

I certainly haven’t been convinced of any real benefits in lightening wing ribs though - how much weight will you actually save?  Where it can make a meaningful difference is at the tail end, where every ounce saved can potentially reduce added nose weight by a factor of 3 or 4 - a win/win situation. 

[Chris Walby]

Agreed with most models, but not this one. No lead anywhere and a C of G that started with the RX battery in the tail and ended up behind the TE. I suspect a combination of long nose with uc, narrow wing and distinctly short rear fuselage to tail contributed to the initial forward C of G (ell in from of the one indicated on the plans).

 

I will be turning my attention to removing weight both fore and aft plus anywhere in between to lose as much as reasonably practicable, but nothing like the ASM Tigercat.

[Nigel R]

10 hours ago, Chris Walby said:

• The wings and fuselage are shown as sheeted with 2.5mm of for the time, but seems excessive 
• The original was 70 span and AUW 4.6Kg

 

Chris

• run a quick calculation on the surface area of the wing, you have an upper and lower skin, without better figures I would guess around 1400sq in of wing skin in total
• now run a calculation on the area of the ribs

Hopefully that will demonstrate the wing skin is the bulk of the wing. Especially if you have a thick skin (3/32) fitted, it could be as much as half the total weight. Back in the day it would be done with contest grade wood (6lb/cuft), and then going to town with sanding of the sheet before you stuck it on. These days, the 'light' sheet is very consistent (well, SLEC are) but often more like 8 or 10lb/cuft - the easy win here is sheeting with 1/16 instead. The next heaviest bits are the other spanwise parts, LE, TE, etc. These can often be thinned down a bit on older designs too.

 

Overall model weight, don't forget no four strokes in 1970s, it would likely be flyweight crossflow two strokes with small, light, rudimentary silencers.

 

Going from 70 to 80 inch, weight increases with the cube, so 8/7 x 8/7 x 8/7 equals approx 150%, so 15lb would be starter for ten as a weight target.

Edited October 4, 2021 by Nigel R

[Martin Harris - Moderator]

Let’s say you really go to town on your lightening and remove the equivalent of a 1/8 x 36 x 4 sheet of balsa - that’s a lot of material…

 

I just weighed one at random and it came out at 1.1 ounces…

[Chris Walby]

Nigel R, Thanks for the information and the point around LE and TE is especially relevant as the scaled up plans increase both in an unnecessary way, might need to have a think as to how I lose weight and retain the shape though.

 

The wing ribs weigh 283g without any holes added which does not sound much, but if you add that to the fuselage and nacelle bulkheads the total (without any sheeting) comes to 961g. Just removing 20% would save 192g which is the weight of the RX battery pack. This coupled using sheeting half the thickness specified will make a difference. I appreciate your comment regarding ribs although its worth bearing in mind that all but three are ply (9) hence forming holes in them will have a positive impact. 

 

I get the impression that its quite a traditional conservative build which coupled with your comments regarding original wood selection would bring it out with an acceptable wig loading. There are a number of other oddities features this model has like 20% less wing area compared with a similar warbird of the same wingspan which all adds up to shaving as much weight as possible during the build to achieve a decent wing loading (for a warbird).

 

PS If I weigh the wood pack (ribs, bulkheads and sheeting) it comes to 2.294 Kg which equates to about 40% of the guide AUW which does not leave much for everything else that needs to go onboard! 

[Martin Harris - Moderator]

2 hours ago, Chris Walby said:

I appreciate your comment regarding ribs although its worth bearing in mind that all but three are ply (9) hence forming holes in them will have a positive impact.

That puts a slightly different perspective on it. Does that mean it uses 9 ply ribs each side, only has 6 ribs to each wing or does the (9) refer to something else?  I realise some ply will be needed for the engine nacelle mountings but is there any scope to substitute balsa for any of the ply?  Composite ribs maybe where the engine loads aren't significant towards the rear?

[Nigel R]

Chris just stick a scrap piece of sheet to the front of each rib. Shape the profile before assembling. Nobody will ever know, it'll be inside the sheeting. 

 

The fact you have a lot of ply ribs is new news. 

 

Can you substitute any for lite ply?

 

Can any be made from balsa with a little local reinforcement?

 

What about ailerons and flaps, I presume at this point they are built up and sheeted? Probably the best we can do there is optimise how much material your chosen hinge method will need?

[Chris Walby]

Finally got around to getting the box of hole saws out (the ones I had when I was an apprentice!) and set to work with adding lightness!

 

Not too fussed about the balsa ribs, but might have a go with the sharp tube technique for experience + a couple of ribs could do with a few more small holes in them.

 

And the results are in ?

 

Total weight saving 15% , with best weight loss of 21% and a couple that didn't tip the scales either way hence recorded a zero.

Okay it only saved 42g overall, but with the thinner sheeting everywhere + the nacelle bulkheads (ply) can be lightened then an overall impact can be made and its worth the effort. 

 

If you want to continue following developments please search Focke Wolf Ta154 Laser build thread.

PS thanks to all that have made positive contributions.