fabric on ribs with huge spacing, doable?

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leviterande

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Im deisgning and testing various smaller prototypes towards a blended wing/lifting body concept, kind of like the facetmobile.

I have a question about fabric covering .

The concept involves big spacing between the ribs/members. Few members are around 3.5m apart between. I.e.a round 3.5meters of empty space is planned to be covered with fabric. the fabric after heating will be in tension. All frame members are made to withstand all the stress forces.

The question is if the fabric will be sagging too much? I calculated that on a 3.5m long fabric carrying a distributed lift force load of 40kg a tension of around 135kg is required to sag around 10cm(which I think is good for the experiment purposes). Do I still need to "bridge" the two members to decrease sagging as seen in the picture(brown bridge)
I know the best way to find the answer is to carry on the experiment but because a lot of experiments are being carried and a lot of people are experienced with covering aircraft I wondered if anyone could perhaps give some feedback whether this is doable or absolutely not? All support/notes are appreciated.

Thank you
/Levi
 

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BBerson

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It doesn't "sag" on flat panels. It actually bulges out slightly. Is that a problem?
Most of the air lift force is near the leading edge. So different at each area.
 

stanislavz

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Just build it from corrugated carbon fibre panels. Done by your self, or bought ready made. How much area/weight you need ?
 

Riggerrob

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Think about how much fabric will flex and in which direction.
For example, most bottom skins will be blown upwards, while top skins will be sucked upwards.
However, this will all change when you approach zero angle of attack Then some skins will flutter.
This flutter can fatigue skins where they attach to solid structural members.
This is one of the reasons why Facetmobile has flexible fabric skins. Fabric is flexible to resist fatigue damage and is light enough to fly behind a small Rotax engine.
You could build a Facetmobile with rigid skins, but it would be far too heavy to fly behind a small Rotax.

May I suggest an alternative?
Look at ram-air parachutes. Even small scoops - in the leading edge - admit enough air to pressurize both skins, reducing fluttering.
 

Dana

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You need to consider what wing ribs do: Besides giving the airfoil shape to the fabric, they transfer the air loads on the fabric to the aircraft structure.
 

leviterande

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It doesn't "sag" on flat panels. It actually bulges out slightly. Is that a problem?
Most of the air lift force is near the leading edge. So different at each area.
Thank you! I was concerned more about the lower bottom area where the acting net lifting force is mechanically going to push the entire body up. (as in the typical wing load sandbagtests videos where the tests actually tests two main things: the spar -which is no problem for me- and the skin which is a concern if it becomes too saggy )

Just build it from corrugated carbon fibre panels. Done by your self said:
Thanks! really interesting idea and I thought about it in the beginning! :). It turned out that the typical fabric is the lightest(??). But perhaps newer materials can change that?

Riggerrob! Thank you! really helpful information, specially about the facetmobile. I didnt know about that detail! So it had flexible fabric and it was fine I guess? For me, having a flexible fabric isnt a problem per se. I am just wondering if it is okay if the fabric is flexible (as long as the frame is strong ofcourse) I mean hanggliders are prime examples that it should be no problem. So perhaps you answered it right there that it was okay and I maybe over analyse things(as I always do)


Hi Dana, thanks! yes the frame is able to handle the loads that are coming from the sailcloth that are coming from the aerodynamic loads:). My question is whether the flexing sagging/bending of the stretched fabric, can create problems ? Apparently it is ok and completely normal as seen from Riggerrobs post?

hmm Havent learned the quote system :)
 

bmcj

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Look at your rib strength. The ribs transfer the lift loads to the spars. Fewer ribs means more load per rib. Can they handle it?
 

proppastie

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non-certified fabric is cheap, do not have to finish it for a test, make a sample panel of your largest triangle ....out of cheap wood even....get a couple of cameras. for top and bottom, and run it down the road on top of your car. ....see how much it moves and if it will be a problem with your airfoil/design.....Or just copy the rib/support spacing on a like weight/speed fabric covered wing.
 

stanislavz

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Thank you! I was concerned more about the lower bottom area where the acting net lifting force is mechanically going to push the entire body up. (as in the typical wing load sandbagtests videos where the tests actually tests two main things: the spar -which is no problem for me- and the skin which is a concern if it becomes too saggy )
Its all depends on how much area your need.
 

wsimpso1

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A few big comments:

Load from air passing over airfoils and carried by fabric can be computed from V/v charts for airfoils as found in TOWS and other data on foils. The analyst can take the existing charts for certain Cl, and interpolate for other Cl, then determine local V of air passing over the foil. Note that over most of the foil, the moving air outside vs the stationary air inside the foil results in net inflation effects. There is some area of net positive pressure from the outside on the envelope near the leading edge, particularly at high alpha. In any event the air load distribution on the fabric can be estimated. When this is done, you will find that at low Cl, low alpha conditions, much of the fabric is being pulled away from the rest of the structures. Differences between the pulls up and down are the lift of the foil. In slow airplanes, this effect can be rather small, but these forces go up with airspeed, regardless of lift being made. Fast airplanes can have far more inflation loading than lift.

The local loads on the fabric add to the static tension in the fabric and those loads are collected by the structures around the periphery of each fabric panel. Some of the load may go directly into a spar type structure, and some finds it way to the spars by first going through the ribs. Ribs carry substantial loads and must be carefully evaluated as beams and/or as trusses, both for their internal loads and for the loads transmitted to adjacent structures.

The fabric sees substantial loads. Membrane analysis methods are used to determine stresses in the tautened fabric used in such craft. That is why we glue and tape, then either rib stitch or use big pop rivets or clips, then put a tape over that along each rib.

The combination of these effects is why most airplane wings, both metal skinned and fabric skinned have small rib spacing at the root and increase spacing towards the tips, as airload is lower toward the tips, in a distribution that appears elliptical in almost all wing shapes.

Let's do the "does anyone else think this is OK?" test. If the designer of this ship is planning to use fabric panels (as defined by fabric area or linear distances) that are significantly larger than other airplanes of similar speed and wing loading, I will suggest that the panel sizes be re-evaluated very thoroughly.

I have analytical methods for doing this for the stiff composite wing panels I design in, but fabric covered airplanes are not my area, so I can not direct you immediately to a suitable reference. The innovator's risk does come at the cost of doing the research and then make the structure adequate to the task at hand. I suggest that the inventor do some research and find suitable analytical or empirical guidance on the topic.

One other issue, when you check out all the stuff attaching fabric with conventional spacings to the ribs, you may think it is terribly overbuilt. Probably it is about right with a FOS so it has some durability. Moreover, I suspect that here is a bunch of institutional memory in the fabric attachment methods and rib spacings we see.

Good luck,

Billski
 
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Mavigogun

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I was concerned more about the lower bottom area where the acting net lifting force is mechanically going to push the entire body up.
Close examination of the Facetmobile in flight, as picture above, shows clear upward billowing of the top sail material. Barnaby reported sail scrubbing sounds at high angles of attack; leading edge vortices accelerate inboard airflow, reducing pressure. Reviewing circulation patterns of very low aspect ratio deltas provides indication of atypical placement of low pressure relative to conventional wings.

I believe the Facetmobile's sail was stretched over the frame, then the sail capped over the frame sections by aluminum strips, which were then covered with sail tape- a frame-sail-strip-tape sandwich.

Build a quarter scale model. Cover it in bits of string to indicate airflow. Mount a camera. Fly it. Post the video here.
 
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Doran Jaffas

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Im deisgning and testing various smaller prototypes towards a blended wing/lifting body concept, kind of like the facetmobile.

I have a question about fabric covering .

The concept involves big spacing between the ribs/members. Few members are around 3.5m apart between. I.e.a round 3.5meters of empty space is planned to be covered with fabric. the fabric after heating will be in tension. All frame members are made to withstand all the stress forces.

The question is if the fabric will be sagging too much? I calculated that on a 3.5m long fabric carrying a distributed lift force load of 40kg a tension of around 135kg is required to sag around 10cm(which I think is good for the experiment purposes). Do I still need to "bridge" the two members to decrease sagging as seen in the picture(brown bridge)
I know the best way to find the answer is to carry on the experiment but because a lot of experiments are being carried and a lot of people are experienced with covering aircraft I wondered if anyone could perhaps give some feedback whether this is doable or absolutely not? All support/notes are appreciated.

Thank you
/Levi
Large spacing between ribs can/will result in flexing both directions.
 

wsimpso1

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Large spacing between ribs can/will result in flexing both directions.
Everything flexes some amount. Deflection of plates supported around edges goes with the short dimension of the plate to the 4th power while the long dimension is also in there. Roark's page 502.

Diaphragm stresses are discussed in Roark's starting page 448. Deflection are still a function of the panel size to the 4th power.

So, double the panel dimensions, edge size to hold the panel increases 2x , forces go up 4x, and deflections increase 16 x. So, set panel sizes carefully.

Billski
 

Doran Jaffas

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A few big comments:

Load from air passing over airfoils and carried by fabric can be computed from V/v charts for airfoils as found in TOWS and other data on foils. The analyst can take the existing charts for certain Cl, and interpolate for other Cl, then determine local V of air passing over the foil. Note that over most of the foil, the moving air outside vs the stationary air inside the foil results in net inflation effects. There is some area of net positive pressure from the outside on the envelope near the leading edge, particularly at high alpha. In any event the air load distribution on the fabric can be estimated. When this is done, you will find that at low Cl, low alpha conditions, much of the fabric is being pulled away from the rest of the structures. Differences between the pulls up and down are the lift of the foil. In slow airplanes, this effect can be rather small, but these forces go up with airspeed, regardless of lift being made. Fast airplanes can have far more inflation loading than lift.

The local loads on the fabric add to the static tension in the fabric and those loads are collected by the structures around the periphery of each fabric panel. Some of the load may go directly into a spar type structure, and some finds it way to the spars by first going through the ribs. Ribs carry substantial loads and must be carefully evaluated as beams and/or as trusses, both for their internal loads and for the loads transmitted to adjacent structures.

The fabric sees substantial loads. Membrane analysis methods are used to determine stresses in the tautened fabric used in such craft. That is why we glue and tape, then either rib stitch or use big pop rivets or clips, then put a tape over that along each rib.

The combination of these effects is why most airplane wings, both metal skinned and fabric skinned have small rib spacing at the root and increase spacing towards the tips, as airload is lower toward the tips, in a distribution that appears elliptical in almost all wing shapes.

Let's do the "does anyone else think this is OK?" test. If the designer of this ship is planning to use fabric panels (as defined by fabric area or linear distances) that are significantly larger than other airplanes of similar speed and wing loading, I will suggest that the panel sizes be re-evaluated very thoroughly.

I have analytical methods for doing this for the stiff composite wing panels I design in, but fabric covered airplanes are not my area, so I can not direct you immediately to a suitable reference. The innovator's risk does come at the cost of doing the research and then make the structure adequate to the task at hand. I suggest that the inventor do some research and find suitable analytical or empirical guidance on the topic.

One other issue, when you check out all the stuff attaching fabric with conventional spacings to the ribs, you may think it is terribly overbuilt. Probably it is about right with a FOS so it has some durability. Moreover, I suspect that here is a bunch of institutional memory in the fabric attachment methods and rib spacings we see.

Good luck,

Billski
I would be VERY concerned about larger than proven spacing between ribs. As you noted...it can depend on variables in speed, wing loading etc. but if one has ever seen the upper skin on a low wing sheet aluminum aircraft pulse under even a normal load it is something I would not want to experiment with in flight even with a lot of research having been done unless a BRS system was in place. The last thing , or one of the last things the pilot wants to worry about is the wings airworthiness in flight.
Though Steve Wittman did not have the same issue as being talked about here, the tragic results could be the same. Why chance it.
The QUESTION I HAVE IS THIS...WHY DO YOU WANT LARGER THAN PROVEN SPACING? I am very curious.
 
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leviterande

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Thank you so much guys for your input. You are a gem of a community 🙌

It is amazing how much research & development and design is contained in an aircraft project. Design is like 95% of the aircraft. Building it is a childplay. People who know me ask me where is the project, where is the aircraft hehe. Its a tedious process of researching, simulations, calculations, testing in the garage and outdoors and repeat.

The frame is made to carry all loads. It is the fabric covering the large areas, and its "sagging/flexing" around a maximum of a calculated 10cm that made me wonder weather it could cause any problems. I found no easy way to verify or debunk my concern and thus why I am here


Hypothetically speaking:
So the framemembers are strong to handle any loads, the fabric is attached and good. The fabric -of the lower crafts surface-now under load is bulging/deflecting up 10cm around the center of the area of the 3.5m long triangular face.
The question is how does this affect forward flight.

I will do a simple static test and load a fabric up but I am not sure I will get more data than what I roughly calculated, i.e. around a peak deflection of 10cm
I can make an RC model but it wont reflect any "surprising"scaling effects in a full scale model.

Reason for big surface spacing between members
Simply put to save weight and make the craft as light as possible.
One of the weight saving features is to have a somewhat "different"construction technique with all the "outer trailing/leading edge members" being very strong carrying the entire load. Basically the craft when fully heatshrunk wont need any other members than the outer trailing/leading edges and the inner hub. The fabric is tensioned between them. Its a bicycle wheel type construction. Thus any addition of members inside the craftframe is only going to serve as a "sail sagging decreasing agent" and not like a typical member.



Fabric "rigidity " requriement is affected:
The craft is meant to have a lower wing loading than typical ultralights
The design cruise speeds are slow like around 45mph

Thanks
 

proppastie

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"sail sagging decreasing agent"
simple 1/2x1/2x.025 aluminum angle fluted one leg to follow the airfoil shape might be enough to stop the sagging and probably will not add very much weight...you might need a bending brake,.... round aluminum bent tube also will work....I like the angle...... lots of UL and Hang glider use tube.
 

leviterande

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Indeed that was the first original plan, it is very tempting to have a circular planform as shown :): the superiority of the low weight structure is not matched by any other planform or construction method.

However midway during my research I realized that while the bike wheel is the superior low weight structure, the aerodynamic qualities were inferior compared to a similar sized delta. The aspect ratio in a delta is better. Furthermore the delta can be transported easily on roads due to lower total for-aft length than en equal area saucer. Furthermore and Most importantly was the fact that a delta has already all of the components inbuilt in one and the same body: the tail surfaces is basically the outer tips on the delta: sitting far back and to the outer back of the main body! But a saucer would need a separate "high tail" in order to have the same stability. You really dont want to have the tail surfaces inbedded in the flow stream directly behind the main wing/bodyof a Low AR aircraft. The ideal tailbody for any aircraft is to be far back and to the "sides" as confirmed by my many RC model testing. the delta perfectly does that! A small washout and the delta pretty decent!

You want the tail surfaces separate to the sides ideally(completely free of the main wing wash). in an ordinary airplane the tail is so far back to work efficiently. in a tailless saucer adding the tail surfaces on the trailing edges wont be as effective as moving the tail surfaces a little bit outwards to sides like the Zimmerman flapjack.

Adding a separate tail is adding weight. Still it is a highly interesting alternative!


proppastie
Pretty valid idea! I have been thinking about that too. I may do that in the end. The bridge piece however should now be able to handle not only the fabric tension but also the vertical lifting load which means additional truss structure should be added to reinforce this member in the vertical lift direction.
 

stanislavz

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Indeed that was the first original plan, it is very tempting to have a circular planform as shown :): the superiority of the low weight structure is not matched by any other planform or construction method
I miss spelled - not umbrella, but parasol.

Just afaik - but for any flying wing, you need forward swept wing for stability anyway... So delta in my opinion is quite unstable. But - it was only afaik.
 
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