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Author Topic: Chrysalis F3-RES, DJAerotech I'll be improving it until it doesn't work  (Read 2993 times)
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USch
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« Reply #25 on: December 05, 2017, 05:12:06 PM »

I really don't have the faintest idea how the wing(?) construction is conceived, but for sure a balsa strip with a carbon strip on only one side is not a good idea. Carbon is more or less insensible to heat and moisture, balsa is the exact contrary even with the smallest change in temp. or humidity. So no wonder your part-spar-longherone or whatever it is bowed.

If it is a long time member of your model (and not a mounting help during construction) you may consider gluing another carbon strip to the underside. This after having slid in the slots the ribs or whatever the slots are for.

Urs
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« Reply #26 on: December 05, 2017, 07:44:50 PM »

I really don't have the faintest idea how the wing(?) construction is conceived, but for sure a balsa strip with a carbon strip on only one side is not a good idea. Carbon is more or less insensible to heat and moisture, balsa is the exact contrary even with the smallest change in temp. or humidity. So no wonder your part-spar-longherone or whatever it is bowed.

If it is a long time member of your model (and not a mounting help during construction) you may consider gluing another carbon strip to the underside. This after having slid in the slots the ribs or whatever the slots are for.

Urs
Urs,
Thank you.

The wing when built has a set of carbon spars, both on the top and on the bottom of the balsa assembly aid /shear web.
This warp really is only a nuisance during the build process. If pinned on the board straight and the top carbon added the stiff carbon will hold the assembly straight, and after a short time the shear web would have taken a set (straight). But as I was able to get the assembly straight even this is not an issue, never really was.

I do have concerns that the ribs might be to thick in the shear web slot. I'll fit the ribs for a close slip fit.
https://www.rcgroups.com/forums/showthread.php?894535-Blade-Type-Spar-Assembly-Aid-Problems-with

Again the designer of the kit conquers with my assessment, that is the warp was much ado about nothing.

All the best,
Konrad
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« Reply #27 on: December 06, 2017, 04:32:43 PM »

Gotta chance to fly a Chrysalis F3-RES this morning. I really need to get moving on this one! I did have some issue with the one I was flying. It looks like the CofG is far too far forward as shown on the plans. The builder of the model I was flying had placed the CofG on the forward side of the recommended location. It was difficult to keep the ship out of flutter on the high start (I don't know if the high-start was F3-RES legal) with the nose being so heavy. But once it popped off the line, the ship was defiantly too nose heavy.

The print shows the CofG at 75mm to 95mm. I put some foam in front of the battery to move it aft, flew it again but with the CofG at about 87 to 90 mm aft. Flew much much better. I was able to keep the nose pointed up (slow) on the high-start. I really liked the feel of the ship with the CofG at this location. With time and some adjustment to the tow hook 95mm might be optimum. I think JDAerotech needs to add an addendum to their web site stating that the CofG range needs to be narrowed to 85-95mm. This should help keep some of the first launches out of the flutter speed zone!

You need to keep the angle of the fuselage at about 75° to 85° on the high-start to about 1/2 to 3/4 of the way up. Keep the speed DOWN on the high start! In glide I had no problems with flutter.

For what it's worth I think the guy covered his ship in So-lite. I'm thinking I'll cover mine in R.A. Micro light of Oracover light. I'll also make sure I've added the glue fillets mentioned in the manual!

All the best,
Konrad
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« Reply #28 on: December 06, 2017, 07:02:56 PM »

Got a response for Don Stackhouse (the designer) he indicates that the center of gravity is to be placed at 87mm. It is good to see that my field testing matches his recommendation rather closely. 87mm verses 90mm heck my finger tips are wider than the 3mm discrepancy!
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« Reply #29 on: December 07, 2017, 12:09:59 PM »

Well, that flight I made yesterday has viscerally shown that Don Stackhouse has the structure close to optimized! Shocked That is to say for a performance ship there is little room for improvement. Those test flight proved that my hypothesis I first proposed was false. Undecided  Now, Don did tell me that if I wanted, the tip plate could be thinned to make a web but to keep about 3/8 inch wide edge at full thickness. He didn’t think the weight saving was worth the effort. As luck would have it I have a Fein triangle sander that made quick work of this.  Again from that flight experience I will be using whatever weight credit thinning the tip gives me and adding it back in gussets at the base of the wing tip panel.
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« Reply #30 on: December 07, 2017, 02:00:28 PM »

I just learned that Don Stackhouse's son is fighting for his life in a battle  against cancer. Yet through out all this Don is still giving exemplary one on one customer support. My deepest felt sympathies for him and his family.

Peace and love,
Konrad
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« Reply #31 on: December 07, 2017, 07:29:05 PM »

Now we come to the real reason I bought the DJAerotech Chrysalis F3-RES. That is the designer is using the allotted amount of carbon to near maximum efficiency.  The carbon beams are as far away as possible and he has placed almost twice as much in the top spar as he did in the lower spar. This uses the stiffness of the carbon most efficiently resisting the buckling loads seen in high “G’s” (launch phase) flight.

I don’t understand why there is a bow in the structure, but this preloads the upper carbon. This must be addressed while bonding the carbon to the balsa shear web. Normally I hate to use CA glue to bond Carbon, as CA usually has a bond strength of less than 1KPSI when bonding most materials.  As a glue it is rather weak. But I did not want to hold this preload as I waited for an epoxy adhesive to cure. So I used medium thick CA in the outer wing panels to bond the top spar to the shear webs.

I will be using epoxy to bond the two layers of carbon on the upper spar of the center section. Truth be told CA is more than adequate as the balsa will fail long before the CA joint will.  

I don't like carbon tubes as they often are not placed to use the carbon to its full advantage. Nor is much of the carbon properly resisting the bending loads.
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« Reply #32 on: December 07, 2017, 07:34:30 PM »

I just learned that Don Stackhouse's son is fighting for his life in a battle  against cancer. Yet through out all this Don is still giving exemplary one on one customer support. My deepest felt sympathies for him and his family.

Peace and love,
Konrad

Don had posted on RCG on 17 November about his son, and that the recent biopsy was probably not cancer (after completing a 2nd round of treatment due to a recurrence).

He hasn't posted anything more recently than that.

I'm guessing you're having an email dialogue with Don (?) ... I hope his son's cancer hasn't returned. That would really suck.

Sad

Regards-
Dave
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« Reply #33 on: December 07, 2017, 07:49:43 PM »

I just learned that Don Stackhouse's son is fighting for his life in a battle  against cancer. Yet through out all this Don is still giving exemplary one on one customer support. My deepest felt sympathies for him and his family.

Peace and love,
Konrad

Don had posted on RCG on 17 November about his son, and that the recent biopsy was probably not cancer (after completing a 2nd round of treatment due to a recurrence).

He hasn't posted anything more recently than that.

I'm guessing you're having an email dialogue with Don (?) ... I hope his son's cancer hasn't returned. That would really suck.

Sad

Regards-
Dave
As I don't waste my time (much) on RCG I wasn't aware of Don's son's health issue. Don never metioned it in our technical corispondence. And yes, Don and I have been in rather constant contact about this model.

This is a good time to mention that one of the benefits of dealing with these cottage industry design and cutting houses is that you often have direct access to the design data. I find this design data invaluable when starting to change the design criteria. Be it a turbine engine or these toy airplanes. I know DJAerotech has not made enough profit on the one sale of this kit to justify the time Don has spent looking over (mercilessly crushing  Shocked ) my design proposals

I hope his son is doing much better. It looks like it will be a hard road ahead of him. But he sound to be up to the task. I wish him and his family all the best.

Konrad
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« Reply #34 on: December 08, 2017, 01:35:57 PM »

Now we’re going to get into the controversial black art of wing tips.   Roll Eyes I went into this a bit here. Remember that their functionality is very dependent on how you are looking at them with respect to the coefficient of lift.
http://www.hippocketaeronautics.com/hpa_forum/index.php?topic=22000.msg206425#msg206425

From the last tip rib to the flat plate I think the designed airfoil is lost. It is just a reality of the building method.  Undecided I like to think that the function of the wing tip is to lessen the energy in the vortex and to shed this vortex with as little impact on the lift generating portions of the wing.

With the philosophical aspects out of the way lets look at what happened to the wing tip as I built them.
To lessen the energy at the tips I like to add wash out. (In some designs I’ve been known to put the last rib in the wing upside down.)

Looking closely at the manual you can see that the back of the tip plate is placed solidly on the building board. This puts the tip plate at a positive angle of incidence (adds wash in). I’m in the habit of placing tip plates level or at a bit of a negative angle of incidence. So I aligned the tip plate even with rear  top of the Trailing Edge (TE) stock. Well, this created a problem, that as you can see in the as removed from the building board photo, that is blending the trailing edge to the tip plate would result in an added flap action. Just what I didn’t want!

My solution was to add on some left over TE stock backwards. That is the thin part of the stock is placed forward on the original TE stock. I winds up with a rectangular TE at the very tip of the wing tip. This allows me to extend the top of the tip plate datum all the way to the aft end of the TE. Actually the datum is above the tip plate a bit as the TE stock has some dimension on the thin side. (See attached photo.) This will allow me to sand out that “flap” at the very tip. And now I’m going cop out and say what we often read in the manuals of old wood models that said something like: “Sand away what doesn’t look like a wing tip”! Wink

I was hoping that the laminated stock might result in added strength with the grain going in different directions. But the fact is that the very tip is only one piece of wood and its grain is in the wrong direction.  To gain some strength back I saturated this area with this CA. This doesn’t add any more weight as I normally saturate the TE tip to try to add some day to day durability to my models. I just don’t like that the grain is not helping with durability.
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Re: Chrysalis F3-RES, DJAerotech I'll be improving it until it doesn't work
Re: Chrysalis F3-RES, DJAerotech I'll be improving it until it doesn't work
Re: Chrysalis F3-RES, DJAerotech I'll be improving it until it doesn't work
Re: Chrysalis F3-RES, DJAerotech I'll be improving it until it doesn't work
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« Reply #35 on: December 08, 2017, 04:10:29 PM »

I want to make it clear that the wing tip mod was not a planed deviation from print. It came about from an oh crap, this is what the wing tip looks like when I lifted the panel off the plans.

Now I’d like to ask the HIP hive mind how do you keep from contaminating the fine balsa work with carbon dust? And once contaminated how do you clean the balsa?

TIA,
Konrad
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« Reply #36 on: December 09, 2017, 05:59:33 PM »

Ran into a minor issue this morning. That is that the aft curve (spline) of rib “B” is a little proud.  This results in an airfoil that looks a lot like Einstein's cat back airfoil, just kidding!!!  Grin But this low spot (0.15mm) is not what we want in our airfoil. I had to delaminate the 1/64 inch top sheeting at the 10-48 location indicated in the photo. Using some 220 grot sand paper between the rib and the sheeting, I sanded the aft part of the rib approximately 0.2mm, The straight edge now only make contact at one point as it moves across the top curve. Both wings were showing this error. I think JDAerotech may want to adjust the aft part of the spline for rib “B” to address this. I know 0.006 to .008” is really small when working with wood.

I think I also see the adjustment in the spline, that is used to adjust for covering sag. That is the apex of the curve is placed ahead of the spar (looks to be at the 9" Hobbico spot on the ruler). When sanding the leading and trailing edges be aware of this. The sag comp. is real.
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« Reply #37 on: December 11, 2017, 06:11:31 PM »

Well, Don got back to me on the "low spot" in the rib profile. He said that was by design. And that it was the key feature of his anti sag wing design. That is the spar sits in the valley of both compensation curves, the forward and the aft.  He said DON'T sand down these humps to meet the spar, or words to that effect!

I wish Don and Joe would put more information in the manual about their unique designs.
When I built my wings the spar was a bit low which resulted in the ribs giving a castellation effect. I knew that this would result in points sticking up through the covering.  So to make a nice smooth framework for the covering I used a sanding block to make a smooth transition between the rib and the spar. This is a mistake that I think anybody with any building experience could far too easily make. I think there should be a notice of the intended rib profile in the manual. I will now try to build up the sanded off bumps with some Ambroid cement. I think I only damaged about 8 ribs. 
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« Reply #38 on: December 12, 2017, 02:10:52 PM »

A little bit more about the sag compensation. The compensation is not trying to get the airfoil correct for the whole wing surface. But rather correct or much closer to the design airfoil over 85% or more of the wings cross section. Unlike conventional open bay designs, where the wing's profile might be as designed over only 15%. With the result being that 85% of the conventional open bay designs looking like the cat back airfoil.

Something I'd like to see is the ribs have a vent hole. This is a real concern with the So-lite and like branded films. When shrinking the covering the air in the bay expands blowing up the film like a ballon. But when cooled the film sags as the cool air is no longer keeping the bay pressurized. Re heating to try to shrink doesn't help much as the hot air was the cause of the problem during the first attempt to shrink the covering.

The designer wants the ribs attached to the covering as a means to supply the much needed lateral rib support, this means the trapped air is even more of a problem trying to get the correct airfoil.

Note that on this wing there is no need to drill the 1mm vent holes on the diagonal stiffeners. As the diagonals are not attached to the covering.
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« Reply #39 on: December 12, 2017, 06:21:37 PM »

It often pains me when those that know more than I agree with me. Angry Don is now of the opinion that I have indeed “Improved the model to the point that I’ve ruined it”. And from Don's perspective as an aerodynamicist I would have to agree. Changing the airfoil no matter wether it was intentional or by mistake often negates much of the design effort put into the aerodynamic balance (lift verses drag).

From this point on I don't think I can call this model a DJAerotech design as the major design element has been changed.

I’m now trying to recover from my error of sanding down the two sag compensation humps on the ribs. I’m using a thick celluloid glue (Duco Cement) to build up the humps. Once the glue dries I will try to sand the humps to match the undamaged ribs. I’m sure I can get the profile close to the original as cut rib profile within 0.15mm (0.006).

Anybody know how well Duco cement will hold up against the heat of the iron during shrinking of the covering?

Don't know what happened to Ambroid glue. It surely isn't what I remember from my yewt! It isn't even Yellow!
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« Reply #40 on: December 13, 2017, 09:00:49 AM »

Don't know what happened to Ambroid glue. It surely isn't what I remember from my yewt! It isn't even Yellow!

I think Ambroid was discontinued ... of course I'm fairly certain that immediately prior to it's demise, the formulation had been tinkered with for "safety" or somesuch.

On a related note, I can't find Duco cement anywhere either. Very irritating.

With that said, IIRC there is a gent on this forum who has his own formulation of Ambroid and nitrate dope (based on full-scale aviation cellulose products). It's been awhile, though, so perhaps I'm mis-remembering.

-Dave
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« Reply #41 on: December 13, 2017, 09:07:30 AM »

Don't know what happened to Ambroid glue. It surely isn't what I remember from my yewt! It isn't even Yellow!

I think Ambroid was discontinued ... of course I'm fairly certain that immediately prior to it's demise, the formulation had been tinkered with for "safety" or somesuch.

On a related note, I can't find Duco cement anywhere either. Very irritating.

With that said, IIRC there is a gent on this forum who has his own formulation of Ambroid and nitrate dope (based on full-scale aviation cellulose products). It's been awhile, though, so perhaps I'm mis-remembering.

-Dave

That would be unfortunate. I think my tubes of Amdroid were purchased less than a year ago.
I find Duco Cement at my local Ace Hardware store. And this is in a city (San Francisco) that is actively trying to destroy products with VOC compounds,
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« Reply #42 on: December 13, 2017, 11:09:08 AM »

Now we come to the real reason I bought the DJAerotech Chrysalis F3-RES. That is the designer is using the allotted amount of carbon to near maximum efficiency.  The carbon beams are as far away as possible and he has placed almost twice as much in the top spar as he did in the lower spar. This uses the stiffness of the carbon most efficiently resisting the buckling loads seen in high “G’s” (launch phase) flight.

I don’t understand why there is a bow in the structure, but this preloads the upper carbon. This must be addressed while bonding the carbon to the balsa shear web. Normally I hate to use CA glue to bond Carbon, as CA usually has a bond strength of less than 1KPSI when bonding most materials.  As a glue it is rather weak. But I did not want to hold this preload as I waited for an epoxy adhesive to cure. So I used medium thick CA in the outer wing panels to bond the top spar to the shear webs.

I will be using epoxy to bond the two layers of carbon on the upper spar of the center section. Truth be told CA is more than adequate as the balsa will fail long before the CA joint will.  

I don't like carbon tubes as they often are not placed to use the carbon to its full advantage. Nor is much of the carbon properly resisting the bending loads.
When I said I don’t understand why there is a bow in the spar structure, I wasn’t questioning the design logic. I was admitting my ignorance as to the reason the feature was there, as it was obviously by design. I was concerned that this bow preloaded the carbon and that this preloading made glueing down the top carbon spar to the shear web a bit challenging.

Don got back to me on this even though I didn’t ask for an explanation of this design feature. Again I see this as great customer support.
He said that the Chrysalis-Lite F3-RES  2 meter wing is an outgrowth of the 1.5 meter Chrysalis, not the 2 meter Chrysalis. So, to allow these 1.5 meter wing sections some volume to accept the wing joiner the outer wing panel was thickened only at the part line and one bay.

Again this makes since, at the Reynolds numbers these wings work in, it is best to keep the airfoil thin. That is why the rest of the wing panel was not thickened. Aerodynamically this would be counter productive and there is no need from a structural perspective to have thicker outer wing panels. Now from an anticipated load perspective the center section added to the 1.5 meter wing to bring the span up to 2 meter  needs to be thickened. By my measurements at the spar not at the sag compensation humps the Chrysalis-Lite F3-RES is using nice thin airfoils. The center section airfoil is only 7.47% thick. And after the wing joiners the outer wing panels goes down to 6.1 % thick.  These are real nice numbers and even more remarkable for a built up balsa wing!

To help show this I’m showing how thick the wing is of my only other open bay 2 meter sailplane. This is a composite wing you will note it is 16% thicker at 8.7% (8.7 / 7.47). Ok. that composite wing has to carry 1.5 Kg of mass. And the Chrysalis-Lite F3-RES is hopefully only carrying 0.4 Kg of mass.

I’m a firm believer that any wing profile that is larger than 9% has the added dimension added out of concern for structure, drag or fuel load.
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« Reply #43 on: December 13, 2017, 11:52:51 AM »

All ribs, other than those at the part line, are now sporting the spar valley. Some ribs still need another helping Duco Cement to get near what I think was the as cut profile. The blue tape was my attempt to protect the undamaged parts of the rib profile from my sanding of the Duco Cement filler.

Note; At the part line there is no covering sag as the 1/64 plywood is there.
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« Reply #44 on: December 18, 2017, 11:32:01 AM »

I have to say I really like working with small cottage firms like DJAerotech. It looks like with each production run they are improving the product, take a look at the Leading Edge (LE) of the stabilizer. As drawn the LE has a step feature on the inside to index the ribs. As cut my kit has a weight saving feature of scallops added, to remove weight.

This proactive approach does have a down side. That is not all documentation align with each other. For example the planes show that the first dihedral joint is to be raised to 2.375”. But the manual states to raise it to 2.500”.
I went with the 2.5” dihedral as this is a competition ship and the added turning power will be much appreciated when trying to hit the spot on landing.

I really like how the diagonals have stiffened the wing. I have to say that they really lock the wing. In one of the photos I’m trying to show how little washout there is, but it is locked! I think this is critical the wings need to be held at the desired rigging prior to gluing in the diagonals. To that end I build off of glass as my true flat datum. The problem with glass is it doesn’t take to pinning very well! Roll Eyes So I use weights to hold the wing as I glue in the diagonals. Again I need to stress there is little chance of using the covering to induce wanted twists in the section of wing that is braced with the diagonals!

In the Trailing Edge (TE) photo you can see that the lower wing’s TE stock is about 2.5 times thicker, at the very edge, than the upper wing's TE. This variation is as a result of manufacturing tolerances in the ripping of the TE stock. I believe that DJAerotech rips their own TE stock to meet their airfoil’s requirements. This isn’t much of an issue as the amount of material needed to be removed to make a match set in minimal. But do protect the ribs with tape so as to not damage them should the sanding bar touch them.

I trick Don told me to see if the sag compensation is working is to “candle” the surface. That is place a straight edge above the wing and look at its reflection in the covering. There should be no kinks or discontinuity in the reflexed line. The more of the wing that shows a smooth reflection the better. I will use this trick to see how well I’ve recovered from my error sanding the sag compensation humps off of a few ribs I damaged.
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Re: Chrysalis F3-RES, DJAerotech I'll be improving it until it doesn't work
Re: Chrysalis F3-RES, DJAerotech I'll be improving it until it doesn't work
Re: Chrysalis F3-RES, DJAerotech I'll be improving it until it doesn't work
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Cut it twice and it's still too short!
Konrad
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« Reply #45 on: December 20, 2017, 12:39:50 PM »

In an earlier post I mentioned that the leading edge, trailing edge and upper spar make up a tripod like structure trying to control flutter. These three elements are tied together by the wing tip plate. Unlike many designs where the wing tip is an add on feature. In the DJAerotech Chrysalis wing the tip plate is key, tying these three elements together to work to control flutter. This is why Don said NOT to remove tip plate, as I had first envisioned.

Don did say that parts of the tip plate could be thinned to make a web. I did this thinning of the tip plate and have decided to use the weight saving credit to add what I hope is some durability to my wing.

With my landing style (read ground loops) I tend to stress the wing tips. I have found with other structures that the bowed leading edge often breaks loose. I think a lot of this is because CA glue often has issues with the oils in hardwoods (pine). Also there really isn’t enough glue area to support the bowed LE. (I know the covering will offer a lot of support.) To help the wing withstand my landings I added this gusset at the base of the wing tip panel. I also added a second gusset to support the dihedral rib.

Note that I scalloped the gusset to give a concave look. This is to minimize any stress risers from the change in material cross section. I don’t think wood is a notch sensitive material. But as I’m sure there is a lot of movement (load changes) at this joint, I thought it best to allow these forces to spread along the glue line. Also the gusset grain is going across the joints. Note; that the brown gussets I drew in the first post actually has a stress riser, in that the as drawn feature is convex. 

That one launch into flutter has scared me! Let me say that the structure as designed is more than adequate in meeting the design criteria imposed by the competition class rules. In high performance structures there is little left over structure that is not needed to get into the winner’s circle.

It pains me to say this, but I know I’m not a good enough pilot to fly at the bleeding edge of failure and not cross it! To help my wing survive my day to day flying I’ve added another diagonal to stiffen the dihedral joint. The aim of this diagonal is to keep the dihedral glue joints of the leading edge, trailing edge and spars rigid as the tip panel flexes. This should add to the cycle life of the wing.

I think I’ve actually added a bit of weight. But as I took some mass out of the wing tip plate that was way out at the tip (moment arm) and put it near the dihedral joint it shouldn’t effect the wing’s ability the signal lift. I also moved the mass from behind the spar to ahead of the spar which should aid in controlling flutter.
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Re: Chrysalis F3-RES, DJAerotech I'll be improving it until it doesn't work
Re: Chrysalis F3-RES, DJAerotech I'll be improving it until it doesn't work
« Last Edit: December 20, 2017, 01:31:33 PM by Konrad » Logged

Cut it twice and it's still too short!
mike
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« Reply #46 on: December 20, 2017, 05:55:57 PM »

If flutter is an issue, an increase in torsional stiffness would help.  Plastic films add very little torsional stiffness.  Why not use doped tissue on the wing?
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Konrad
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« Reply #47 on: December 20, 2017, 06:12:17 PM »

If flutter is an issue, an increase in torsional stiffness would help.  Plastic films add very little torsional stiffness.  Why not use doped tissue on the wing?
Plastic film is a very broad family. Many films offer more stiffness than others. Doped tissue in this application often is not advantageous as it soften up with moister with the loss of any advantage it might offer when dry and tight. Tissue over mylar has some merits but often isn't worth the effort. I agree that the iron on film (polyester ?) from the firm Solarfilm, does not add much torsional advantage.

I need to be clear that as designed the structure will resist flutter in the flight envelope it was designed to operate. See attached video with prototype model covered the Solarfilm's So-lite.
http://www.djaerotech.com/news-updates/chrysalislite-launching-beyond-res-regulations/

The added diagonal adds just that, torsional stiffness to the core structure of the last panel around the dihedral joints. Hopefully relieving some stress from the glue joints should I fly the structure into flutter.
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Cut it twice and it's still too short!
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« Reply #48 on: December 21, 2017, 04:24:32 AM »

Thanks for the video link.  The model featured had a 'heavier-looking white' covering over the 'd-box' area.  What was it?
It was in the right place to help with torsional stiffness.

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lincoln
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« Reply #49 on: December 21, 2017, 08:35:59 AM »

Looks like a fun project. If I was a prolific builder, I might get my current backlog out of the way while this kit is still in production. But I'm not.

It's hard to come up with changes that actually improve things for a design like this. I think anything that does will probably increase the work needed or the expense. For instance, there may be areas where you can use lighter than expected wood that tests out as stiffer than usual, since there's lots of variation between pieces of wood, even at the same density. Or, sheet tail surfaces can be tapered to be thinner out at the tips. That might allow more thickness, and therefore lighter balsa, near the root. Trailing edges could be made from punky wood with something on the outside to make them stiffer than the stock t.e. at the same weight. Some of the places where Don uses basswood, you could probably use thicker but lighter balsa, IF you selected the balsa carefully and changed to the right thickness. I imagine, for parts like that, selecting the right balsa drives kit manufacturers crazy. Anyway, I think the stuff I've thought of above has very minor advantages for the amount of effort. Plus, if incorporated in the kit, they'd require more of builders, thus making for a lot more calls to DJ Aerotech. The thing is that you have to avoid mentioning these things on RC Groups or Don will jump in and spend time commenting on them anyway. (Don, you have my permission, if you feel you really MUST respond, to leave it at: "Lincoln, your post is all wrong." But it's beter if you don't respond, since I've said it for you, and you have more important things to get to.) Those who want to do things like I've mentioned above may be better off scratch building a Drela design, with mods of course. I think Mark Drela is sufficiently busy with the double bubble airliner that it's unlikely he'll comment on the mods. Meanwhile, those of us who can't help writing too much...

pull strings:
As long as the string has significant tension, the stiffness is that of the string plus the spring. Once the string goes slack, then the stiffness is only that of the spring. An advantage is that it takes the backlash out. I'm not necessarily advocating springs and strings, but I thought that ought to be clarified.

spar caps:
If I was building one of these, and I was a total weight weenie, I might sand the ends of the spar caps to a taper, rather than allow an abrupt transition. Slight weight loss, plus a reduction in stress concentration. OTOH, the thicker part of the spar probably extends well past where it's really needed so that the stress concentration doesn't matter much. Where the joiner is, one could taper the inner ends a bit, but then you might need to wrap the spar in that area.

low spot:
I wonder if it would be worth putting baking parchment on the depressed area to keep the covering from sticking there? Or just a strip of covering with the stickum on the up side, if you could figure out a way to make it stay put!

venting:
The low spot, where the covering isn't sticking, ought to provide more than enough venting, though of course a 1mm hole in the right place won't be a problem.

cellulose glue:
I wonder if Fab Tac (sticks covering down on full scale airplanes) or Sig Stix It would be useful as glue if Duco becomes unavailable? Both are much sticker than the nitrate dope I've seen. Also, I have some nitrate dope for full scale that's stickier than Sig's nitrate. Sig still shows Sig-ment on their web site.

Duco and heat:
Well, if the glue is vulnerable to heat, you can always glue on thin, soft balsa where you would have had the glue. Should sand easier. Duco or Ambroid are probably still the right sort of glue to use for sticking down the balsa.

candling the surface:
Maybe you can cover temporarily with Saran Wrap or something to do this, so you can go back and fix things easily.
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