Floating wing joiners, in a modern RC glider (how do these function?)

[Konrad]

I've been away from high performance gliders, more or less since the turn of the millennium. After a short stint working the ISR (2018) I was reinfected with the slope bug! Looking at what is out there I've noticed many designs do not hold the wing rigid to the fuselage. These designs generally use a large wing joiner that bridges the fuselage. The only interaction between the wing and the fuselage is by the means of small pins. I have grave concerns that these small pins can deal with the flight loads and landing loads. These concerns are born out by the fact that every used glider I've inspected (prior to purchase) has shown stress crack or worse around the rectangular opening that allows the wing joiner to float through the fuselage.

In this thread I'd like to discuss the reasoning and implementation of this design in an effort to understand its design features and constraint.

I'll start with a few photos showing legacy designs. Later I'll draw up some cartoons in an effort to discuss the features in detail. (These drawing often take me a few hours each to draw).

I'll start with the 1975 Hobie Hawk. This was a standard class 2 channel glider that used an aluminum rod that passed through the fiberglass part of the fuselage. This through passage was a metal tube that was well gusseted to the inside of the fiberglass part of the fuselage. The fuselage tube and aluminum rod were close fitting effectively resulting in a solid connection with a lot of surface area to distribute the flying and landing loads. There was also a small rod (pin) in the front of the wing to hold the wing and fuselage in proper relationship to each other. This rod transmitted rotational forces back and forth between the wing and the stabilizer. The fiber glass boom and heavy plastic front of the fuselage did a good job of keeping the wing in check during landings.

In 1989 we have a Graupner Chile hot liner. This is a 2.2 meter model where most of the mass in in the fuselage. She is carrying 16 round cells and uses an Astro FAI 25 for power. The wing to fuselage interface is much like the 1975 Hobie Hawk except that the wing joiner is a much stiffer carbon rod. This keeps the wing in check preventing them from wanting to sweep forward on abrupt landings. This allows for a lighter fuselage layup compared with the Hobie Hawk. The fuselage can still whip an bit and stress the area near the leading edge of wing on the fuselage (see grazing in the paint). A solution to this is the use of a a solid spreader bar to carry the compressive loads from one wing to the other through the fuselage. (See photo from Multiplex manual).

In 1996 we have a Michel Clavier 3.7 m Cumulus electro. This is again very similar in that there is virtually a solid connection between the wing and the fuselage. This time we remove the carry through tube in the fuselage to allow ether side of the fuselage some freedom to flex independently, from the other side of the fuselage. This adds a great deal of durability to the fuselage. Note the huge gusseted bosses around the through hole. You might also notice that the alignment pin is a long through rod. This was done to avoid the damage you might see in the Chili photo from when the fuselage flexed allowing the pins to pop out of the fuselage.

In 2003 we have an FVK Brisk III 2.8m with a rectangular wing joiner. This is real nice as it puts a lot of the carbon from the wing joiner in the correct position to control wing flex fore and aft, that has proven to be so detrimental to the durability of our models during landings. The Brisk III still used a slip fit (zero clearance) through fuselage joiner. What is innovative about this one is that it is fuselage through hole (joiner box) is 0.7mm oversize when looking at the part of the joiner that fits the wings. The center of the joiner has an enlarged pad to bring the joiner back to size with zero clearance with the joiner box. The joiner box is reinforced with huge amounts of carbon tow. Again like the Cumulus there are two independent bosses on which the joiners works against. This is also the first joiner I'm showing where the joiner has a dihedral kink in the center. This model is also sporting 2 per wing 4mm x 30mm steel pins to align the wings with the fuselage. These along with the rectangular joiner transmit the forces between the stab and the wings.

So far all these models have a solid (zero clearance) connection between the fuselage and the wing joiner. This has resulted in a very crisp response between the stab and the wings.

I now have a new glider in my stable, this is a Samba Mach 2, (2012). This model uses a floating joiner (or spar). The wing joiner looks to be a strong straight rectangular carbon type. But what is new to me is that in the fuselage there is clearance all around the joiner during normal operations. It looks as if the sole function of the joiner is to keep the bending moments of the wing under control. The joiner is not used to tie the wings to the fuselage and stabilizer. All loads between the wing and the fuselage are carried through 4 small 2mm aluminum pins. I have to be honest, that with my current understanding of how this system works I'm not comfortable subjecting 4 small 2mm aluminum pins against the flight and landing loads of a 1.5kg high performance glider!


All the best,
Konrad

 

[Konrad]

What adds to the apprehension is that I just subjected my Mach 2 model to some simulated loads, similar to a dutch roll. Even without the inertia that ballast would add, the wing separated by 2 mm far too easily. With ballast there would be even more forces acting against the flight surfaces. In fear that tape used to seal the wing to fuselage part line will have to be thought and spec'd as a structural component used to keep the wing against the fuselage to control span wise movement of the wings along the joiner. ( Graupner used a fork and blade to control that on the Chili).

I now see that it looks like the RedShift guys are using electrical tap around the wing shoulder joint. Is this more than a seal?

I've heard that some pilots feel that the through spar is less responsive, dampened, sluggish. This isn't my experience , but I wonder it the issue in't with the "Free Floating" through spar (joiner).

Some links to similare subjects.
https://forum.alofthobbies.com/index.php?threads/manufacturing-design-flaws.365/
https://forum.alofthobbies.com/index.php?threads/redshift-emerges.13/page-3

All the best,
Konrad

 

[Doc James Hammond]

Cross fuselage braces are a really good idea front and back.

This is something I usually do, and always forget to mention so thanks a lot for mentioning them here, Konrad.

What I do at the back is use a piece of plywood with a V-Cut out of it to strongly brace at the wing but taper the reinforcement off as it goes back from the wing roots.

On the Gremlin, which has a fairly large roomy fuselage compared to my other models, the rear brace is pre-installed.

Gremlin also has a round wing joiner, but its light and agile so is quite unlikely to be dumped at joiner breaking speed, and on that little model the joiner can be replaces with metal for ballast, or filled with metal slugs, so its quite convenient.

Cheers,

Doc.

 

[Konrad]

Thank's Doc,

Yep, all shoulder and mid-wing ships should have some kind of anti-crush feature. The spreader bars (keep the fuselage sides spread apart) you show are simple and effective! I like that they also are sporting a load spreading feature, fish mouthed ends. All doublers should have some kind off load spreader. An abrupt change will cause load concentrators. This results in areas that will fail far too early than otherwise desired. We touched on this a bit in reference to flexible verses rigid structures. The jaws of the fish mouth allows the structure a long area to gently flex before reaching the solid portion of the doubler.

I'm showing that your Models (Schwing) has this thought out feature, load spreaders. I'm also showing that I use this even on less costly models like the Elf. I'm also showing that even high end OEM's often over look this basic of mechanical principles.

Now it isn't alway possible to install such straight forward spreaders. There often are servos, linkages, ballast tubes and maybe flight batteries that get in the way. I like to add stiff carbon hoops to my models to help distribute the crushing loads to both sides of the fuselage and reinforce the fuselage seam. How many time have you all seen a fuselage split on the top between the canopy (slip on nose) and the wing's LE. There is a reason (logic) for this!

All the best,
Konrad

 

[Konrad]

It was mentioned elsewhere that the flex of the wing and joiner might lock the wing and fuselage into a single system under high G loads. Again I don't see this. Leaving for now how does one get the wing into a higher coefficient of lift (flaps of elevator input). It was hypothesized that the wings would clamp onto the top of the fuselage as the wings flexed locking the wings to the fuelage. And that this would necessitate a stiff fuselage top if one didn't want the crush the fuselage top seam.

Now what I've seen, and and I hope have demonstrated, is that the wing will move on the "alignment pins" until they hit the fuselage. This point of contact will become the pivot point on which the additional loads will pry the wings apart. After a few loading and unloading event it is very possible that the wings will pull the alignment pins away from the fuselage. I think you can see this in that 2mm gap I showed earlier, and this was just from some low energy dutch rolling.

Unless there is some kind of retention feature, (Graupner blade and fork, Multiplex (Top Model) socket locks or tape) the wings will move out board as the loads are cycled.
https://alofthobbies.com/multiplex-multi-lock-uni-set.html
https://alofthobbies.com/top-model-wing-locks.html

Now landing loads such as touching a wing tip or even more dramatic stops will introduce fuselage crushing loads. (see earliers posts).

All the best,
Konrad

 

[Doc James Hammond]

Thank’s Doc,

Yep, all shoulder and mid-wing ships should have some kind of anti-crush feature. The spreader bars (keep the fuselage sides spread apart) you show are simple and effective! I like that they also are sporting a load spreading feature, fish mouthed ends. All doublers should have some kind off load spreader. An abrupt change will cause load concentrators. This results in areas that will fail far too early than otherwise desired. We touched on this a bit in reference to flexible verses rigid structures. The jaws of the fish mouth allows the structure a long area to gently flex before reaching the solid portion of the doubler.

I’m showing that your Models (Schwing) has this thought out feature, load spreaders. I’m also showing that I use this even on less costly models like the Elf. I’m also showing that even high end OEM’s often over look this basic of mechanical principles.

Now it isn’t alway possible to install such straight forward spreaders. There often are servos, linkages, ballast tubes and maybe flight batteries that get in the way. I like to add stiff carbon hoops to my models to help distribute the crushing loads to both sides of the fuselage and reinforce the fuselage seam. How many time have you all seen a fuselage split on the top between the canopy (slip on nose) and the wing's LE. There is a reason (logic) for this!

All the best,
Konrad


View attachment 1180
View attachment 1174View attachment 1175View attachment 1176

It's also a really good idea to try to make every internal or external corner of the radio tray or cross braces with radii and not 90 degree sharp cuts. The bigger the radii the better, but its possible even on servo pockets.

Look what happened to the DeHavilland Comet. (I used to work for them a million years ago)

Now we are getting somewhere!

Doc.

 

[Doc James Hammond]

It was mentioned elsewhere that the flex of the wing and joiner might lock the wing and fuselage into a single system under high G loads. Again I don’t see this. Leaving for now how does one get the wing into a higher coefficient of lift (flaps of elevator input). It was hypothesized that the wings would clamp onto the top of the fuselage as the wings flexed locking the wings to the fuelage. And that this would necessitate a stiff fuselage top if one didn’t want the crush the fuselage top seam.

Now what I’ve seen, and and I hope have demonstrated, is that the wing will move on the “alignment pins” until they hit the fuselage. This point of contact will become the pivot point on which the additional loads will pry the wings apart. After a few loading and unloading event it is very possible that the wings will pull the alignment pins away from the fuselage. I think you can see this in that 2mm gap I showed earlier, and this was just from some low energy dutch rolling.

Unless there is some kind of retention feature, (Graupner blade and fork, Multiplex (Top Model) socket locks or tape) the wings will move out board as the loads are cycled.
https://alofthobbies.com/multiplex-multi-lock-uni-set.html
https://alofthobbies.com/top-model-wing-locks.html

Now landing loads such as touching a wing tip or even more dramatic stops will introduce fuselage crushing loads. (see earliers posts).

All the best,
Konrad
View attachment 1177

Thats what the electrical tape is for, Konrad.

Holds the wings in so they don't move during flight stress, but its also a kind of mechanical "fuse" which will break if over strained.

Actually you might be surprised how much force it takes to break a 6" length of electrical tape in a sideways motion - but it often happens which proves the system works.

Doc.

Redshift nicely taped below:

 

[Konrad]

It's also a really good idea to try to make every internal or external corner of the radio tray or cross braces with radii and not 90 degree sharp cuts. The bigger the radii the better, but its possible even on servo pockets.

Look what happened to the DeHavilland Comet. (I used to work for them a million years ago)

Now we are getting somewhere!

Doc.

Now we are moving into my wheel house, pressure vessels. Not sure I follow how low and high cycle fatigue or cycle life limits pertain to the load path of the four alignment pins. But I'll try to keep an open mind on the subject.

The De Havilland DH 106 Comet, who would have thought that such an elegant design would spend most of its service life as the homely Nimrod

Its been 40 years since I read any engineering reports on the inflight break up of the DH 106 Comets. Now the square windows were a contributor to the failure, but they really were a minor player. The main failure was inadequate stringer support.

Now this was a lesson for me in that marketing hype rarely is based on sound facts. The B.O.A.C (British government) want to send a clear message to the flying public that they found the issues and that it was easy to see that they had fixed it. That is why so much attention was given to the square window. This was so after the reintroduction anybody could see that the new jet was safe, it had elliptical windows.

I didn't see this kind of marketing hype in my working life time until the Concord, Air France Ft 4590. Most of the public thinks the Concord crashed as a result of the fire. But the final analysis for the loss of life was a omission of a 1.6mm spacer on the port landing gear truck. This shim was to keep the truck pointed straight should any of the tires blow. With the Concord pulling to port she ran over the runway light FODing the engine which the flight engineer instinctively shut down. At gross weight and with less the 3/4 power the Concord could not fly. She lost air speed and rolled over. There was little or no evidence that the fire had destroyed any of the flight control system.

But prior to reentry into service there was a huge push to install shock resistant self sealing tanks. That quartering head on view from that passenger in the 747 cemented in the publics mind that the Concord crashed because of the fire. It crashed because the flight engineer shut down the FODed engine after it hit the runway light. Had the landing gear tuck stayed pointed straight Ft 4590 would not have hit the runway light and with all 4 engine producing trust she would have flown out of the danger area and into the thinner air where the fire would have blown out. As it was, with the shut down engine Ft 4590 would have crashed even without the fire. Yes there are a few more mitigating circumstances.

Back to gliders, what I hope to learn is what are the load paths for this free floating wing joiner system? And how does one size the pins for the yield and ultimate load conditions?

 

[Konrad]

Thats what the electrical tape is for, Konrad.

Holds the wings in so they don't move during flight stress, but its also a kind of mechanical "fuse" which will break if over strained.

Actually you might be surprised how much force it takes to break a 6" length of electrical tape in a sideways motion - but it often happens which proves the system works.

Doc.

Do you provide instruction that the use of the tape is critical to the safe functioning of the ship? Do you spec the type and amount of tape needed?

I know I've received none of this information with my Sanda Mach 2 model

Actually I'm not surprised at how low the tensile strength is for electrical tape. Yes, designed failure points (mechanical fuse) are often critical to the safe operation of many systems.

Now that you and others have said that the pins are part of the primary load path I'll have to give these pins, Area A in my drawing, a closer look when inspecting used gliders. I had thought of them as alignment pins only

My club is going to have an R/C auction in a few weeks. This will give me an opportunity to inspect a large population of used gliders. I hope to see a few used free floating joiner system.

What kind of wear tolerance would one think is safe for the area "A" feature in my drawing? Do you have over sized pins available should the holes become too large and /or elongated. Put another way how does one inspect and maintain this kind of system?

All the best,
Konrad

Disclaimer:
I'm not trying to put Doc on the spot. This system and its constraints are not unique to Aeroic. Actually I'm very thankful that Doc is trying to address my concerns with the system. So while I am asking Doc these question and may not be fully satisfied with the answers. I'm ecstatic that he is attempting to address the question. This goes a long long way towards me placing Aeroic near the top of my list for my next ship.

IT REALLY IS ALL ABOUT CUSTOMER SERVICE!

 

[Doc James Hammond]

Now we are moving into my wheel house, pressure vessels. Not sure I follow how low and high cycle fatigue or cycle life limits pertain to the load path of the four alignment pins. But I’ll try to keep an open mind on the subject.
The de Havilland DH 106 Comet, who would have thought that such an elegant design would spend most of its service life as the homely Nimrod

Now its been 40 years since I read any engineering reports on the inflight break up of the DH 106 Comets. Now the square windows where a contributor to the failure, but they really were a minor player. The main failure was inadequate stringer support. Now this was a lesson for me in that marketing hype rarely is based on sound facts. The B.O.A.C (British government) want to send a clear message to the flying public that they found the issues and that it was easy to see that they had fixed it. That is why so much attention was given to the square window. This was so after the reintroduction anybody could see that the new jet was safe, it had elliptical windows.

I didn’t see this kind of marketing hype in my working life time until the Concord, Air France Ft 4590. Most of the public thinks the Concord crashed as a result of the fire. But the final analysis for the loss of life was a omission of a 1.6mm spacer on the port landing gear truck. This shim was to keep the truck pointed straight should any of the tires blow. With the Concord pulling to port she ran over the runway light FODing the engine which the flight engineer instinctively shut down. At gross weight and with less the 3/4 power the Concord could not fly. She lost air speed and rolled over. There was little or no evidence that the fire had destroyed any of the flight control system.

But prior to reentry into service there was a huge push to install shock resistant self sealing tanks. That quartering head on view from that passenger in the 747 cemented in the publics mind that the Concord crashed because of the fire. It crashed because the flight engineering shut down the FODed engine after it hit the runway light. Had the landing gear tuck stayed pointed straight Ft 4590 would not have hit the runway light and with all 4 engine producing trust she would have flown out of the danger area and into the thinner air where the fire would have blown out. As it was, with the shut down engine Ft 4590 would have crashed even without the fire. Yes there are a few more mitigating circumstances.

Back to gliders, what I hope to learn is what are the load paths for this free floating wing joiner system? And how does one size the pins for the yield and ultimate load conditions?

Hi Konrad thanks for the history - I honestly don't think you have enough to do

OK back to the wheelhouse:

Pins:

Normally I don't count the pins as making any contribution other than alignment, but when you think of the cross sectional volume of Duralumin (which I use) Its quite substantial, and unlike the wing joiner, the pins are not really floating.

Wing joiner:

What I do to get a rough working idea is to take the wing area then calculate the possible pressure/bending forces overall, cut the result in half, (For half the wing) and then concentrate it all at a single point (The wing root).

Then I (roughly) calculate the cross section of HM carbon needed to resist this forces. It's surprisingly small, but that was with no margin of error - which by the way was a mistake I made with the Typhoon design.

Then I doubled the sectional area needed, but I still did not like the size, and since I fully intended to use the same joiner for a few models of differing spans (Stormbird, Stormbird Corsa, Schwing, Schwing Corsa) I doubled it again.

With that kind of cross section the joiner does not actually need much length as long as the wing joiner box is adequately strong, but its always best to distribute the loads as much as possible so the length is also probably at least double that it actually needs.

All this belt and braces treatment seems to work as I have never seen or heard of my wing joiners breaking in any crash ever.

Spar:

For the spar itself its a bit more difficult to calculate as I use a Sine Wave configuration with the carbon in several plies at different orientations. So in that case I resorted to physical tests, more than calculation to find. happy medium.

The sine wave frequency was an amazing piece of luck because it turns out that corrugated roofing material comes in several frequencies, most of which are close to the ideal for the spar configuration. I don't remember the exact figure but I think it was somewhere around 60mm peak to peak for a 2 to 3M wing was ideal. As long as the wavelength is not too long (Not enough compression strength) or too short (Not enough torsional strength) its not SUPER critical.

It was soon apparent that the sine wave using HM UD carbon at 180, 90 and 45 degrees ply combinations or layers at 60mm frequency was really strong when suitably sandwiched between two HM UD caps.

But it has other really cool advantages too in that:

a) you can lay up a vast sheet of spar material then just cut the tapered spars as you need them.

b) you can not only vary the ply orientations, you can vary the amount of plies ie: spar thickness, and therefore strength/weight which is easy to taper to the tip direction.

c) In the y/z axis the spar is flexible to a certain extent when it has no spar caps, so it gives the added advantage of being able to bend along the wing thickness high point. It does not have to be straight or angled like a traditional spar, so it an go end to end of the wing of needed.

Overall as I had hoped, it had a fraction of the weight of the "traditional" block balsa/foam/carbon types.
But the real breakthrough apart for the weight etc was the torsional stiffness, which because of the effective width had about 4 times the torsional stiffness potential.

So thats what I have right now, unless we go into the other parts of the wing structure - but here both my views and my methods are controversial so its probably not a good idea!

I don't know how other designers do it, but I tend to do all the sums first, and then try to make something that looks good around the number framework.

Cheers,

Doc.

 

[Doc James Hammond]

Do you provide instruction that the use of the tape is critical to the safe functioning of the ship? Do you spec the type and amount of tape needed?

I know I've received none of this information with my Sanda Mach 2 model

Actually I'm not surprised at how low the tensile strength is for electrical tape. Yes, designed failure points (mechanical fuse) are often critical to the safe operation of many systems.

Now that you and others have said that the pins are part of the primary load path I'll have to give these pins, Area A in my drawing, a closer look when inspecting used gliders. I had thought of them as alignment pins only

My club is going to have an R/C auction in a few weeks. This will give me an opportunity to inspect a large population of used gliders. I hope to see a few used free floating joiner system.

What kind of wear tolerance would one think is safe for the area "A" feature in my drawing? Do you have over sized pins available should the holes become too large and /or elongated. Put another way how does one inspect and maintain this kind of system?

All the best,
Konrad

Disclaimer:
I'm not trying to put Doc on the spot. This system and its constraints are not unique to Aeroic. Actually I'm very thankful that Doc is trying to address my concerns with the system. So while I am asking Doc these question and may not be fully satisfied with the answers. I'm ecstatic that he is attempting to address the question. This goes a long long way towards me placing Aeroic near the top of my list for my next ship.

IT REALLY IS ALL ABOUT CUSTOMER SERVICE!

Thanks for the caveat and the plug Konrad.

To answer your questions:

Do you provide instruction that the use of the tape is critical to the safe functioning of the ship? Do you spec the type and amount of tape needed?

No I don't and part of the reason for that is that the different countries I sell to, have different tape. However I have seen some really nasty dirty sticky messes from time to time when people try to use sellotape or something like that - and I don't recommend that!

What I will say is that the USA has REALLY HIGH QUALITY electrical tape which seems to break at just the right tension, also stretches nicely around curves, and you can even get it in nice colours for your baby (and your plane)

I know I've received none of this information with my Sanda Mach 2 model

Actually I'm not surprised at how low the tensile strength is for electrical tape. Yes, designed failure points (mechanical fuse) are often critical to the safe operation of many systems.

I think the adhesive "Stickiness" of the tape has a big effect on its usefulness too. And as to the tensile strength, it's a mechanical fuse after all.

Now that you and others have said that the pins are part of the primary load path I'll have to give these pins, Area A in my drawing, a closer look when inspecting used gliders. I had thought of them as alignment pins only

No, actually I don't consider them a primary or even a secondary load path at all Konrad.
Years ago I used to use Carbon fibre pins before we had carbon fibre wing joiners, because I liked the pins to break and be easy to replace rather than bend and get stuck.

My club is going to have an R/C auction in a few weeks. This will give me an opportunity to inspect a large population of used gliders. I hope to see a few used free floating joiner system.

Anything that is over 2.75M and is competitive will use this system.

What kind of wear tolerance would one think is safe for the area "A" feature in my drawing? Do you have over sized pins available should the holes become too large and /or elongated. Put another way how does one inspect and maintain this kind of system?

No - simply not needed.

If the pin is the correct size, a nice close fit in the hole, a bit of Vaseline or candle wax will keep the pin and the hole clean and tidy. There is very little wear unless the pin starts off the wrong size, to loose or too tight.

Cheers,

Doc.

 

[jvaliensi]

A couple notes:
1.Since the wings and fuselage are 'floating on the spar the crushing action cannot happen - the wings will move out on the spar with increased loads, even the tape will stretch a bit.
2. How much does a box spar deflect under load? It is an easy static test to do. All one needs is a load and a dial indicator.
3. Twisting of the wing - how much force is developed to twist the wing in flight? How much can wing a spar resist? The floating connects work against us here because of the slop. I'm guessing the the aerodynamic forces are much less than the G force loading from turning and looping.
4. I'd say cracking a the fuselage, of a sailplane with 4-pins and a box spar is more likely due to rough landings, where impacts forces can exceed 300G's.

Personally, I'd like a sailplane that could survive my landings!

 

[Konrad]

How is the end user to know that tape (regardless of quality) is part of the retention feature of the design? I've historically only taped my wings out of aerodynamic concerns, high pressure air bleeding over to the low pressure side.

Coming for the glow powered (oily slimmers) side of the hobby, taped seals were troublesome and were to be avoided. In FAI classes (F3D Pylon) if you lost part of the aircraft the flight was disqualified. If I lost a piece of tape used to seal a pressure cowl I got a goose egg for that round!

3M tape whenever possible!
Yep, whenever I wrote a requirement that needed tape, usually part of a manufacturing process, I'd not only call out the AMS spec but I'd also restrict the purchase to a 3M part numbers (if I felt strongly about it). Purchasing departments often hated that I would not allow lower quality materials.

In my 40 years of R/C flying I can only recall having a wing fail under high G loads twice. Most of my in flight failures can be traced to flutter and other issues (failed props etc.)
Even this mis-manufactured joiner doesn't give me too much concern.
https://forum.alofthobbies.com/index.php?threads/sanda-models-mach-2-carbon-set-up.354/

Now I do have a concerns with the use of aluminum adjacent to carbon.* Since carbon and aluminum are at the extreme ends of the galvanic scale I have great concern that the aluminum pins will corrode in short order from galvanic corrosion. As I tend to keep my models for decades this is a real concern.

As jvaliensi has noted there are a lot of other forces on the airframe other than the bending moment from the wings. The torsional loads working against the vertical fin and inertia of the fuselage are surprisingly high. I have some aerobatic gliders where the stabilizer and wing can twist in the air about 30° relative to each other. This wrecks havoc with the elevator timing! Ok, these are not great aerobatic gliders!

300 G's might be a bit too extreme a case. But my main concerns are with the landing loads putting so much force on that "A" feature. Particularly with fully ballasted ships. Unless the wings are trying to leave the fuselage perpendicular to it, I see a lot of shear forces on those pins. You can see this in that the alignment holes in used gliders are often oblong and even chipped towards the front.
This is a concern even with the classic zero clearance 20mm round joiner as we often we damage around the fuselage to wing joint even in "Normal" landings.

To jvaliens comments (1&2) I don't see the fuselage floating on the spar. Rather I see it interacting with the wing through the alignment pins. To test the elastic response of materials or a systems often takes a bit more than this.

To get a visceral understanding of the loads the wing to fuselage junction sees, I like to have guys fly a model that uses rubber banded on wings. Place a few pieces of paper between the wing and the fuselage. Then fly your aerobatic routine. Remove the rubber bands one at a time and re-fly until you loose one of those pieces of paper.** Most will be surprised at how much force is required to keep the wings solidly held down to the fuselage.

* This is a real concern with full size aircraft! You should see the herculean efforts taken to keep aluminum and carbon structures separated.
** For this test to be meaningful it is assumed your lands are smooth. If not then the pieces of paper should be near the landing site.

All the best,
Konrad

 

[Doc James Hammond]

A couple notes:
1.Since the wings and fuselage are 'floating on the spar the crushing action cannot happen - the wings will move out on the spar with increased loads, even the tape will stretch a bit.

No much, I have seen it maybe two or three times only after thousands of flights.

2. How much does a box spar deflect under load? It is an easy static test to do. All one needs is a load and a dial indicator.

Not much as far as I know and since I have never, ever had one fail - it's not a test that is urgent. I normally design my models with a +1 safety factor so when I do the calculations and it needs a certain amount of reinforcement, I double it.

3. Twisting of the wing - how much force is developed to twist the wing in flight? How much can wing a spar resist? The floating connects work against us here because of the slop. I'm guessing the the aerodynamic forces are much less than the G force loading from turning and looping.

Far far less. By far the biggest forces are due to compression from manoeuvres. That part of the reason why the fuselage can float quite happily - very little force is exerted in flight.

4. I'd say cracking a the fuselage, of a sailplane with 4-pins and a box spar is more likely due to rough landings, where impacts forces can exceed 300G's.

Yes, true.

Personally, I'd like a sailplane that could survive my landings!

Me too, problem is, Inconel is really heavy!

Hi James - a couple of answers above:

Doc.

 

[Doc James Hammond]

How is the end user to know that tape (regardless of quality) is part of the retention feature of the design? I’ve historically only taped my wings out of aerodynamic concerns, high pressure air bleeding over to the low pressure side.

Bleeding is no problem as the model is changing attitude so often in flight that its only there for seconds, if that. Unless of course you fly for very long periods in only one direction.

As to the end user knowing about the use of tape, frankly if he has flown moulded models for any length of time he will know, because everyone, thousands and thousands of us do it.

I was using it in 1975 when I made my first moulded plane.

It's like reminding the end user that pulling back on the stick will make the model go up - or worse.

Coming for the glow powered (oily slimmers) side of the hobby, taped seals were troublesome and were to be avoided. In FAI classes (F3D Pylon) if you lost part of the aircraft the flight was disqualified. If I lost a piece of tape used to seal a pressure cowl I got a goose egg for that round!

Happily, few people put glow engines on the front of gliders.

3M tape whenever possible!
Yep, whenever I wrote a requirement that needed tape, usually part of a manufacturing process, I’d not only call out the AMS spec but I’d also restrict the purchase to a 3M part numbers (if I felt strongly about it). Purchasing departments often hated that I would not allow lower quality materials.

In my 40 years of R/C flying I can only recall having a wing fail under high G loads twice. Most of my in flight failures can be traced to flutter and other issues (failed props etc.)
Even this mis-manufactured joiner doesn’t give me too much concern.
https://forum.alofthobbies.com/index.php?threads/sanda-models-mach-2-carbon-set-up.354/

Well done! It seems you don't do such DS.

Now I do have a concerns with the use of aluminum adjacent to carbon.* Since carbon and aluminum are at the extreme ends of the galvanic scale I have great concern that the aluminum pins will corrode in short order from galvanic corrosion. As I tend to keep my models for decades this is a real concern.

Never seen it. But I do use vaseline.

As jvaliensi has noted there are a lot of other forces on the airframe other than the bending moment from the wings. The torsional loads working against the vertical fin and inertia of the fuselage are surprisingly high. I have some aerobatic gliders where the stabilizer and wing can twist in the air about 30° relative to each other. This wrecks havoc with the elevator timing! Ok, these are not great aerobatic gliders!

I'd suggest getting some new planes!

300 G’s might be a bit too extreme a case. But my main concerns are with the landing loads putting so much force on that “A” feature. Particularly with fully ballasted ships. Unless the wings are trying to leave the fuselage perpendicular to it, I see a lot of shear forces on those pins. You can see this in that the alignment holes in used gliders are often oblong and even chipped towards the front.
This is a concern even with the classic zero clearance 20mm round joiner as we often we damage around the fuselage to wing joint even in “Normal” landings.

I think this problem, with a well designed and constructed model is down to normal wear and tear to be honest. As I mentioned in James reply above, Inconel is heavy.

To jvaliens comments (1&2) I don't see the fuselage floating on the spar. Rather I see it interacting with the wing through the alignment pins. To test the elastic response of materials or a systems often takes a bit more than this.

Both are true.

To get a visceral understanding of the loads the wing to fuselage junction sees, I like to have guys fly a model that uses rubber banded on wings. Place a few pieces of paper between the wing and the fuselage. Then fly your aerobatic routine. Remove the rubber bands one at a time and re-fly until you loose one of those pieces of paper.** Most will be surprised at how much force is required to keep the wings solidly held down to the fuselage.

Not too easy with my planes.

* This is a real concern with full size aircraft! You should see the herculean efforts taken to keep aluminum and carbon structures separated.

Never seen it. I keep my batteries in the fuselage - not new ones in the wing attachments.

** For this test to be meaningful it is assumed your lands are smooth. If not then the pieces of paper should be near the landing site.

Difficult, as I said.

All the best,
Konrad

View attachment 1183

Hi Konrad, some answers inside your message above.

Doc.

 

[RalfH]

Really interesting discussion. I’m a bit confused by the floating wing joiner. How is the weight of the fuselage supported? It can’t just be through the alignment pins, as the name suggests their purpose is to keep things aligned, but they shouldn’t carry the load.

My take is that floating means a somewhat loose fit, but that under load the spar settles against the top/bottom of the joiner box or tube. Or am I missing something?

Ralf

 

[Doc James Hammond]

Really interesting discussion. I’m a bit confused by the floating wing joiner. How is the weight of the fuselage supported? It can’t just be through the alignment pins, as the name suggests their purpose is to keep things aligned, but they shouldn’t carry the load.

My take is that floating means a somewhat loose fit, but that under load the spar settles against the top/bottom of the joiner box or tube. Or am I missing something?

Ralf

Hi Ralf, I can't speak for other designers, but in my case you are exactly right.

I make the wing joiner passage in my fuselage large enough to allow the joiner to pass through, but no larger - which is why Konrad saw one that was a bit too small once at the Aloft shop.

Generally the pins together are enough to support the fuselage loads, but obviously if there is more loading than they could handle then the actual wing joiner will come into play. Thats why my holes are close.

Good call!

Doc.

 

[Konrad]

Hi Ralf, I can't speak for other designers, but in my case you are exactly right.

I make the wing joiner passage in my fuselage large enough to allow the joiner to pass through, but no larger - which is why Konrad saw one that was a bit too small once at the Aloft shop.

Generally the pins together are enough to support the fuselage loads, but obviously if there is more loading than they could handle then the actual wing joiner will come into play. Thats why my holes are close.

Good call!

Doc.

Nothing is obvious.
In your Redshift thread I read that the joiner just passed through the fuselage, there was no joiner box (contact area) upon which the joiner interacts with the fuselage during normal loading.
https://forum.alofthobbies.com/index.php?threads/redshift-emerges.13/page-3#post-4228
https://forum.alofthobbies.com/index.php?threads/redshift-emerges.13/page-3#post-4233

So it is more than just a rectangular hole!
So there is direct contact with the fuselage and the wing joiner? See the fuselage and joiner here
https://alofthobbies.com/schwing-88-carbon.html

To be clear your design uses a joiner that is close fitting (around 0.1mm clearance) with the fuselage through hole, and that this hole has enough contact area to carry the anticipated landing and air loads above those which the pins can handle.

This joiner under load is held captive by the fuselage. This looks to be a classic wing joiner and fuselage attachment system.

This then brings your system to be one that would subject the fuselage joiner system to the same cracking we see in other designs with their stress risers.

THIS IS NOT WHAT I THINK OF AS A FREE FLOATING SPAR/JOINER SYSTEM!

Ralf,
I think your question has lifted the veil of ignorance from my eyes. There was no free floating joiner/spar. There was/is contact between the joiner and fuselage in the direction of load.

It was very puzzling to me how the alignment pins were to transmit the loads from the fuselage to the joiner/ wing spar through the wing root (ribs). But now that we have learned that the fuselage under load is in direct contact with the joiner all is clear.

Now in this thread post #7 you can what is usually though of as a free floating spar (joiner). There is over 1mm of clearance between the joiner and the pass through joiner box.
https://forum.alofthobbies.com/index.php?threads/sanda-models-mach-2-carbon-set-up.354/

Thanks!

All the best,
Konrad

 

[Doc James Hammond]

Hi Konrad.

The joiner is actually free to move through the wing and fuselage except of course that the wing joiner boxes have ends that stop it from doing so, and a nifty little bit of tape to stop the wings and fuselage from moving too far apart.

BUT it is a free floating joiner system.

The spar cannot possibly be "floating".

If you go to the shop, pull out one of my planes and take a really good look at it, you will end all of your gaps in understanding, and save me an awful lot of time. Everything so far discussed is readily apparent just by looking at the physical examples.

Cheers,

Doc.

 

[Konrad]

Hi Konrad.

The joiner is actually free to move through the wing and fuselage except of course that the wing joiner boxes have ends that stop it from doing so, and a nifty little bit of tape to stop the wings and fuselage from moving too far apart.

BUT it is a free floating joiner system.

The spar cannot possibly be "floating".

If you go to the shop, pull out one of my planes and take a really good look at it, you will end all of your gaps in understanding, and save me an awful lot of time. Everything so far discussed is readily apparent just by looking at the physical examples.

Cheers,

Doc.

"BUT it is a free floating joiner system."-Doc

If you are defining free floating as not bonded to the structure then I'd agree. But I would not call a spar (joiner) that is held captive and only allowed to move freely within the 0.10mm slip fit clearance a free floating system.

This slip fit through spar has been used in our models since the beginning of time. I show that even the great old Hobie Hawk has this system.

The joiner is part of the spar, but all parts of the spar are not joiners.
So the joiner that bridges the wings through the fuselage in designs that are free floating are floating spars.
The center part of the spar (joiner) usually see the heaviest bending loads

"If you go to the shop, pull out one of my planes and take a really good look at it, you will end all of your gaps in understanding, and save me an awful lot of time. Everything so far discussed is readily apparent just by looking at the physical examples."-Doc
This is far from the truth!

Far too often what I think is self explanatory often winds up not being so! So the RedShift has the same joiner design as the Schwing 88? I can see that, but don't know that!

All the best,
Konrad