Are our wings over-engineered?

[kc]

Our our wings over engineered? Most plans in the 48 to 60 inch span range seem to have a D box LE sheeting, capstrips and webs to the spars. No doubt they all work well.

However I noticed that some designs don't use webs & omit the LE sheeting and capstrips. Good example is the 60 inch span Ugly Stik which uses 1/2 x 1/4 spars set vertically top and bottom. It does have an extra 3/16 spar between the LE and main spar. Very simple wing design which should be much quicker to build than a D box wing with LE sheeting, webs and capstrips. Very thick wing section of course. Now the Ugly Stik has been built since 1966 and is a most popular design so if there were a weakness then it should be known by now! Anyone had a problem with an Ugly Stik built to the plan?

Several other designs use similar construction - Bandito, Bingo by Doc Mathews also Bandit by Mike Lovell ( AMI Sept 1999 ) and others.

So do we need D Box construction for models with thick wing sections?

 

Edited By kc on 02/05/2020 19:11:44

[Paul Marsh]

Better over-engineered than under-engineered!

Having see wings fail, I think the former is better than the latter!

[Richard Acland]

Quite agree. Give me over engineered any day. I am currently building a Stolp Starlet from plans. The plans show no shear webs between ribs and no cap strips. Fear not, I shall be fitting them.

[Peter Miller]

While the Ugly Stick wing may be quite strong enough and obviously works well, I can see that that structure could warp and distort easily.

Warps and twists are not really a good idea

[Geoff S]

I've just about completed the build of the 46" wings for the Das Liddle Stik I started last week. I built it more or less to the plans except I adjusted the main spar angles slightly so that the interspar webs glued properly to both top and bottom spars as well as the ribs. There are no cap strips and no l/e sheeting. The wing is quite stiff in bend as it should be with 1/4" square spruce main spars and l/e and a pair of 1/8" spruce spars further forward but without a full 'D' section it will certainly twist quite easily. I've tried it.

If I build another then I think I would eschew the 1/8" spars and sheet the l/e back to the main spars as well as fit cap strips. Most of the ribs are 3/32" (2mm) and could be 1/16 if cap strips were fitted and there's be no need for the sub ribs which support the film at the l/e. I doubt it would be any heavier, as easy to build and considerably stiffer in twist.

Geoff

PS I witnessed Doug Rigby's BD2 folding its wings at one of the shows a few years back because he hadn't fitted an adequate spar.  It was quite specatcular

 

Edited By Geoff S on 02/05/2020 20:42:17

[Chris Walby]

KC, what do you mean by over engineered?

Is the wing meant to last one flight, 10, 100, 1000 or 10,000?

Flown by a professional who can do repetitive perfect landings or bounced in by a numpty (includes me!)?

IMO some of my most liked models I have flown are quite rugged, perhaps because I have only been in the hobby five years.

Its interesting looking at say a BH mosquito wing, there is hardly any balsa in it (has a D section LE, but its very rigid. I think they made them using jigs, but the average model builder would/couldn't do that so the designer adds a bit more in due to the building technique.

I have a good true story about being careful regarding thinking things are too strong because they never failed, but its off topic. Could be persuaded.

[J D 8]

Thing is full size aircraft have a gee limiter on board, the pilot. Where as models are often pulling high levels that would render a pilot unconscious.

Come on Chris spin the story, we need entertainment these day's

John.

[john stones 1 - Moderator]

No, mine are just right, never had a failure and therefore, never put anyone at risk.

[Paul Marsh]

Years ago I was a spotter for someone at a LMA show. the model was a large Ultimate, but he only flew level and the wings folded, smashed the engine and was a write-off. the wings were strong, but flawed, and as you can see what happened.

Edited By Paul Marsh on 02/05/2020 22:24:13

[John Robertson 3]

Chris Walby & JD8: "...I have a good true story about being careful regarding thinking things are too strong because they never failed, but its off topic. Could be persuaded...". Go On, Go On, Go On. Please.

[Jeffrey Cottrell 2]

Hi kc, timely question.
I am firmly in the over engineered court.
Just to illustrate. I am currently putting together a T30 from the Dancing Wings kit.
The spars on that are laser cut from the softest balsa I have ever come across. To add insult to injury, to fit in the box, these spars are supplied in two pieces. The illustration indicates they should be butt jointed together.
So, spars the consistency of wet noodles, with a weak butt joint in the middle.
I really don't think so.
Quite why, I don't know but they then use plywood for the spar webs.
Ho hum.
I see wholesale changes in my future.
Cheers
Jeff

[Brian Cooper]

Nope, my wings are never over-engineered. . . If they are still intact at the end of a flight, then they are adequately engineered.

[Nigel R]

That ugly stik structure seems to me to suggest a doped nylon covering which would increase strength somewhat.

Anyway. As a general rule i concur with Brian.

I would be more concerned with making sure the controls are engineered well enough to resist flutter.

[fly boy3]

Come on Chris, let's have the story. You have intrigued me. I agree with JD8, we need entertainment ( if that it what the story is about) during these dark days. LoL

[fly boy3]

How many of us builders have an engineering background. For many, many years I followed the designers plan, and built exactly as required. The thought of adding extra strength (and weight) never occurred to me at least. Lately though. I must say I have been adding a gusset here and there where I think there may be a weakness etc. Have not had a structural failure at all. But I have seen a few wing failures due to pushing the limits off a lightly built plane when flying,many times. Cheers

[Peter Miller]

Just to make you wonder. My design Dancing Girl, a typical 50" span aerobatic model with a standard D box leading age and balsa spars and webs etc.

She flew like a dream and went like a rocket with a Wankle engine fitted and had a G meter fitted.

I did a long vertical dive to terminal velocity and then hauled in full up elevator on high rates.

The model pulled out safely

The G reading was 24.8 G which meant that for a moment that model weighed 95 lbs

Thanks but I will stick to D box wings with sheeting and spar webs.

[J D 8]

In the early 80's I flew a microlight that while in the hands of others suffered structural failure resulting in a number of deaths before the type was grounded.

One of the first things I noted with mine was it was over powered with its 440cc motor. I gave it the beans in level flight only once and when the airspeed read 80mph I rightly got scared and pulled the power off. Things like the tail were flexing a lot.

The wing looked firm though. I had seen a test one weighted to destruction, and it took a lot before it collapsed.

Later tests showed the problem was in the fore and aft plane, with an airflow of some 90mph over the wing the rear spar would fail in compression and the wing would fold back.

John.

[Brian Cooper]

We (rightly) focus our attention on the integrity of the wings, but let's not overlook the tailplanes.

These items come in for a lot of stress too and can fail if not adequately engineered.

B.C.

[Peter Christy]

My very first RC model was a Veron Mini-Robot. It had a single (deep) spar, and no LE sheeting. The wings folded on the second flight due to a combination of the sequential escapement skipping, and pilot inexperience (I didn't recognise what had happened soon enough!).

When I built a replica a couple of years back, for the 50th anniversary of that flight, I used spruce rather than balsa for the spar. It still looks and feels flimsy, and I take great care to avoid spiral dives - which makes limiting height tricky! (No elevators!)

So no, I don't believe our wings are over engineered. You never know when you are going to need every ounce of strength they have!

--

Pete

[Chris Walby]

Relevant yes, off topic definitely and apologies to kc for the post.

I used to ride competitively (as an amateur) methanol burning 4 stroke motorcycles in the 350cc and 500cc classes. The problem with the 500cc class was that professionals joined in when the prize money picked up which made the 500 class almost impossible to do well at.

Rules are limited with only capacity being the absolute rule other than that anything more or less goes (no nitro!).

My father was trained as a mechanical instrument engineer, had built vintage car racing engines and turned his full focus to our hobby, me ride and dad weald the spanners.

Success has two main factors, rider ability and performance/reliability of the engine, one was in my control, the other my father’s area of expertise. The advantage we had with the engines was extensive time, interest and skill could be brought to bear on the engine development side to a point where the 350 was fast, very fast and reliable.

I was about 10st and some of the tracks I raced on would become quite slick which hampered the 500cc powered bikes (power is nothing without control – or the ability to get traction). In the unlimited class (250, 350 & 500 all in together) my weapon of choice was the 350 and I could beat 500 powered machines with more skill full riders on board, but it was pressed very hard.

To that point, we had been running with no major engine failures for a few years and in a chat with dad I said if we had not broken anything then we were not that close to the edge of failure (youth talking!). Dad went away and quietly thought up a plan with the performance/racing engine company he had links with for the next step.

The result was an engine built for the British Championships and yes it was fast, unfortunately the rider could not do the engine justice and the rest of the season continued until the 350 expired flat out mid straight in an unlimited class race.

Initial investigation by rotating the crankshaft by hand revealed no resistance and a tick-tonk sound meaning it was serious and best left for the bench.

The strip down showed the con rod had failed, the piston bent the valves which had wrecked the guides, damaged the seats and marked the head. The rod had damaged the liner and most likely damaged other bits downstairs.

Dad’s conversation with the performance engine company guru was something like oh dear sorry to hear etc. Their question How many had it done then? Dad’s answer about 7, guru oh it should have been okay for 7 races…Dad, no we have done 7 meetings (about 6 to 12 races per meeting) before it let go.

[Chris Walby]

Lesson…Performance comes at a price, the problem most people want the performance but are not prepared to pay the price. Even if its recognised there is a limitation people will just keep pushing it.

As we are in a non competitive sport then the driver for absolute performance is not relevant, but flying performance, reliability and cost are drivers. I don’t know as I don’t have the skill to design, but I would expect they have to consider a wide range of build quality (that they have no control over) and if one claps wings people won’t say it was a pants build, just a poor design.

PS – back to balsa and my current model has the explicit warning not to power dive flat out, full power can be used in all other aerobatic moves…I have been warned, but how many ignore that and do it anyway?

PPS regarding the Ugly Stick, full power dive and a full high rates yank on the elevator repeatedly. What do you think will happen, but is that what the designer intended it to do?

Hope you enjoyed the tale and with a 350 JAP I went on to be pre75 British Champion for three years so just needed a class slow enough for the rider and fast enough for the engine!

[Cuban8]

I've witnessed several models suffer a catastrophic failure when they've been built from old plans - the builders using a mix of trad and modern techniques and then perhaps fitting a modern engine that could easily be several times more powerful than what was originally intended. Modern versions of lightweight boyhood classics can be prone to in-flight break up without careful thought and planning - free flight designs that are put through the mill with R/C and bigger engines.

I recall a lovely scale model built from a well known designer's plan but finished in glass cloth and epoxy and then sprayed with two pack. The plan would have called for a much lighter tissue/dope covering. The plan also called for a non-schnuerle 60 two stroke (late 60s-early 1970s) but was fitted with a modern 120 fourstroke. The wings retained the balsa spars and minimal webbing. The model, a Spitfire of 70" wingspan flew well but soon suffered a wing failure when exiting a loop. The designer's target weight is around 10 lbs, but when built with 'modern' methods and accessories the airframe wound up at 14lbs!

More mass and more power hence much higher performance for the original design to cope with led to disaster!

My friend managed to salvage the fuselage and built another and much strengthened wing in a very short time, and the model continues to perform well today. A case of being appropriately engineered rather than over engineered perhaps?

[Simon Chaddock]

My version of the question is - Are wings engineered to make the best use of the materials that they use?

Model planes have to have a degree of handling & crash resistance which complicates things as it puts on 'non flying' loads but in simplistic terms if a wing is always likely to fail at the same point and in the same way then it could be argued that the rest of it is over engineered and is thus over weight.

The benefit of applying sound structural principles, particularly in areas of high stress, has a direct impact on overall performance.

"To improve performance - add lightness" as Colin Chapman used to say.

[Peter Christy]

Posted by Simon Chaddock on 03/05/2020 10:18:42:

"To improve performance - add lightness" as Colin Chapman used to say.

Which is why Lotus came to stand for Loads Of Trouble, Usually Serious!

You can take lightness too far!

--

Pete

[Paul Marsh]

I found this Wing spar a few days ago, think it was for an ARTF, but I obviously didn't use it, alarm bells ring and don't know what model it was from, but remembered I made a new improved one.

Why lightning holes on this part, what would it save, nothing!