Logo
Builders' Plan Gallery  |  Hip Pocket Web Site  |  Contact Forum Admin  |  Contact Global Moderator
August 07, 2020, 05:37:21 AM *
Welcome, Guest. Please login or register.
Did you miss your activation email?

Login with email, password and session length
 
Home Help Search Login Register
Pages: 1 2 [3]   Go Down
Print
Author Topic: Plastic Bird  (Read 2401 times)
0 Members and 1 Guest are viewing this topic.
piecost
Gold Member
*****

Kudos: 10
Offline Offline

United Kingdom United Kingdom

Posts: 499

Topic starter


Ignore
« Reply #50 on: May 21, 2020, 08:02:40 PM »

I finally went flying in the evening an enjoyed very still air bedore the weather was due to break. As predicted; I broke both my new wings and the original wing survived. I am not upset since the failures point towards improvements to be made on succesive models.

The homemade carbon tube fuselages were a success. When fitted to my first wing (with the original shape tail) the increased stiffness increased the tail effectiveness  which allowed the CG to be moved back to a similar location to the Lazy Bird and Yashinskiy models.

I had sanded the flaps on my first wing and reduced their stiffness so they flexed as intended. This increased the launch height. Javlin and discus launchs worked Ok. I next swapped for a fuselage with a Yashinskiy tail and was supprised that the model was still in glide trim. However discus launches reasulted in a barrel roll to the left. This was similar behaviour to that when I installed a Y tail without dihedral. Perhaps the 20 deg wing dihedral requires more than 10 degrees on the tailplane to prevent this ocurring. I don't inderstand how this works.

The Plastic Bird 3 model showed much promise despite the glass cloth delaminating from the foam prior to even flying for the first time. The wingtips felt far too fragile, but did not break. Javlin launches sometimes got to a good height but transition was unreliable. However, the glide was lovely and floaty. The flaps seemed to work well.  Unfortunately; it would not turn, even with lots of rudder. I started to bend one flat down more to acheive a turn but broke the wing before making much progress. The RH upper spar delaminated just outboard of the dihedral brace and balsa shear web. I was kind of expecting a failure here,  but on the bottom spar cap, not the top (as per my second model when discus launched).I regretted not extending the shear web further outboard but this may not have helped. I think that employed a twice as thin, but twice as wide spar cap may have contributed to the failure. I theorise that the stiffness of the cap itself is important in resiting it peeling off the foam. The thinner but wider cap has less stiffness and is more likely to try and peel of. But it had more gluing area than a thicker and narrower spar; so who knows?

Plastic Bird 4 broke after only a few.launches with the LH wing bending down with a crack on launch. The bottom spar cap peeled a layer of foam and buckled. This was again at a stress concentration; just outboard of where the cap went from 2 adjacent pieces of carbon to 1 and where the balsa shear web terminated. I am not sure if more carbon would be better than extending the balsa shear web further outboard. This wing lacked the glass cloth leading edge covering. I don't think that this caused a problem with the wing twisting so will leave it off the next wing. Especially since I had such problems getting the glass to stick to the foam.

My conclusions are that;

The homemade carbon sock fuselages are a big improvement over fishing rod tips. They are stiffer and the increased tail effectiveness allows a more aft CG with resulting saving in nose weight.

My Yashinskiy shaped tails made in foam and carbon were robust enough (so far) and of similar wieght to using 5 lb/ft3 balsa.

The yashinskiy tail combined with a wing with 20 degrees of dihedral cannot discuss launch without barrel rolling to the left.

My new wings employing the tapered flat plate sections were an improvement over my earier attempts at aerofoil shaped wings. The better quality of hotwiring allowed a more precise control over flap thickness and stiffness.

Both new wings seemed to have stiff and strong enough dihedral braces. Perhaps I can save more weight by omitting a layer of carbon from the brace on future wings.

The wing without glass cloth broke before I could establish if it was too flexible in twist. But i will omit the glass on the next wings since it saves much weight and hassel in manufacture.

Both new wings failed with the spar caps buckling and peeling the foam. Increasing thw spanwise extent of the shear web may help and be lighter than employing more carbon.

Perhaps i will make some new wings in a batch with more extensive shear webs. I may inset some Rohacell instead of balsa extending much further outboard. I could see if the thinner carbon cap sticks better to this than the blue foam.

If I omit the glass cloth on the leading edge then I can employ the elegant Yashinskiy curved leading edge.

I have so many ideas to try that I am risking trying too many at once and not knowing which ones work!

Attached files Thumbnail(s):
Re: Plastic Bird
Re: Plastic Bird
Logged
OZPAF
Palladium Member
********

Kudos: 186
Online Online

Australia Australia

Posts: 5,711



Ignore
« Reply #51 on: May 22, 2020, 05:52:55 AM »

I haven't digested all of this interesting report Piecost but have a couple of comments re the shear web and the spar buckling.

Definitely the risk of buckling of the caps reduces as the caps are made thicker and this will require more support from the shear webs.

Instead of using balsa for your shear webs it may be better to use say even  light glass cloth at 45 deg. I have done this in a 20" RC foamie which was fully aerobatic and never looked like breaking the wing spar. My shear web of 3 oz cloth was not much thicker than paper and it was place in a knife cut between the sanded out cap positions and the carbon laid in either side of the web which had been trimmed to be just below the level of the wing surface. It doesn't need much glue on the shear web to hold it in place.

This would not be all that much heavier and I feel would be a lot stronger.

Good luck with your plastic bird.

John
Logged
piecost
Gold Member
*****

Kudos: 10
Offline Offline

United Kingdom United Kingdom

Posts: 499

Topic starter


Ignore
« Reply #52 on: May 22, 2020, 10:10:47 AM »

Thanks for the tip John. Am I correct in thinking that the spar web protruded into the spar pocket and the caps butted against it? Were the caps wet layup or pultruded? I will keep this suggestion in mind, but thought of some ideas last night when I couldn't sleep.

Unlike my 2nd wings failure where the foam crushed; both new models failed by the caps peeling from the foam. Actually the foam itself peeled away rather than the glue failing.

I may experiment with a full span 1/32 verrical grain balsa web. This may not give more resistance to the cap peeling off, but the increased ristance to crushing may be good enough. Indecated by the wings failing immediately next to where the web terminated.

I will also try a Rohcell web the full width of the caps. This should resist cruahing and the material should not fail in peel, I hope. For the latter the foam leading and trailing edges will be butted against the spar assembly. I figured a way of sanding the web to get the spar caps flush with the foam wing profile.

Tim
Logged
OZPAF
Palladium Member
********

Kudos: 186
Online Online

Australia Australia

Posts: 5,711



Ignore
« Reply #53 on: May 24, 2020, 06:27:47 AM »

Quote
Am I correct in thinking that the spar web protruded into the spar pocket and the caps butted against it? Were the caps wet layup or pultruded?

The shear web did run into the cap cutouts in the foam and thus bonded to the carbon filaments when they were laid up wet in the groove. I split a 12k tow - 1/2 on top and the other on the bottom. I used to lay the tows down on the sticky side of a length of masking tape(secured at each end with more tape). The tows were then wet out, excess resin blotted of with paper, and then still stuck to the tape - transferred to the cap cutouts and pressed flat.

I sanded in the cap cutouts which were in the form of a flat triangle with the max depth at the central shear web. From memory I allowed 1mm2  area for 12k  - so in this case I had 0.5mm2 top and bottom.

It may be a little heavier - but I think it is a better way to go with foam as you are not relying on a foam carbon joint for buckling support of the cap - the triangular shape provides that and the foam supports the shear web in buckling and this requires very little if any glue.



John
Logged
piecost
Gold Member
*****

Kudos: 10
Offline Offline

United Kingdom United Kingdom

Posts: 499

Topic starter


Ignore
« Reply #54 on: May 27, 2020, 06:32:42 AM »

I tested a fifth wing employing a 1/32" shear web to 80% span with 0.008" thick spar caps tapering from 5mm to 1mm. After a few javelin launches the RH lower cap delaminated with a crack at about one third span. The web had possibly stopped the foam being crushed but had not stopped the spar cap from pealing away.

The other problem with the RH wing was that the flap trailing edge was too thin and became wavy. In places it went down to 0.23mm and could not hold a straight trailing edge. The LH wing was about 0.44mm down to 0.32mm at the tip and was fine. So, I need to be more careful in future to keep the trailing edge to about 0.45mm thick. The good news was that the upper and lower dihedral braces did not fail. They consisted of 4 laminations of 100g/m2 unidirectional carbon, tapering from 5mm at the centreline to 0 at 40mm span.

I am allready constructing a new wing employing a Rohcell spar web the full width of the spar caps to see if that helps resist the failure in peel. This employs the thinner carbon. If this fails then I may return to adding thin glass cloth over the leading edge as it must help resist the cap buckling.
Attached files Thumbnail(s):
Re: Plastic Bird
Logged
piecost
Gold Member
*****

Kudos: 10
Offline Offline

United Kingdom United Kingdom

Posts: 499

Topic starter


Ignore
« Reply #55 on: May 27, 2020, 09:41:37 AM »

To correct my above post; the dihedral braces were 3 laminations. I am glad that I did not spend time sanding the leading edge on this wing as it is now scrap! Once I have found a concept that is strong and light I'll start spending time in sanding and finishing the wings to a higher standard. 
Logged
piecost
Gold Member
*****

Kudos: 10
Offline Offline

United Kingdom United Kingdom

Posts: 499

Topic starter


Ignore
« Reply #56 on: June 02, 2020, 03:14:17 PM »

Photo of all my wings; the first is on the left and the last on the right. The 3 new wings on the right have Rohcell spar webs. Hopefully this will prevent the carbon peeling away as it buckles and will allow the carbon to use more of its theoretic material properties. Wing 6 and 7 are identical except the former employs 0.004" precured uni carbon epoxied onto the Rohcell and wing 7 is 100g/m2 uni cloth wet laid up onto the Rohcell. On both wings the carbon and Rohcell spar assemblies were constructed first then butt joined to the blue foam. It was intended for the carbon to be flush with the outside of the foam, but this was not possible in practice. Some parts protrude, some are flush and others are sunken.

Wing 8 has a 50% increase in carbon width (15mm) at the root incase the others are still too weak. The Rohcell was glued into the foam wing, sanded flush and the carbon laid on top. This was much easier to do. These last 3 wings employ upper and lower doublers at the root consisting of 2 layers of 100g/mw uni cloth, again wet laid up.

I am awaiting a calm evening to test them.
Attached files Thumbnail(s):
Re: Plastic Bird
Logged
piecost
Gold Member
*****

Kudos: 10
Offline Offline

United Kingdom United Kingdom

Posts: 499

Topic starter


Ignore
« Reply #57 on: June 08, 2020, 05:14:43 PM »

I managed a few test flights of my 3 new wings in rather breazy conditions. I was happy in that I demonstrated that the spars employing Rohcell webs were strong enough (so far). The dihedral braces employing 2 laminations of 100g/m2 unidrectiinal cloth also held up.

However, now that I potentially solved the strength issue, I need to concentrate on the flaps. The hotwiring process induces a natural camber into the foam. However I use the same jig to cut both wing halfs and invert one side. This causes the RH wing to have its flap camber deflecting the wrong way. Bending it by hand over a cardboard tube tends to crease the foam.  This is not good enough. I am going to try a heat gun to see if I can induce a bend into the foam. I don't expect that it will work.
Logged
piecost
Gold Member
*****

Kudos: 10
Offline Offline

United Kingdom United Kingdom

Posts: 499

Topic starter


Ignore
« Reply #58 on: June 27, 2020, 06:07:28 PM »

I had a quick flying session with my three latest wings. Annoyingly I forgot a scrap of sandpaper and could not sand the pylon to adjust the wing setting so had to employ ugly tape trim tabs on the tailplane. I didn't have time to do a careful trimming session but unwittingly had an aft CG and added bluetac to get the model to transition at all reliably. I was javlin launching and the flaps seemed too stiff to allow much height gain. My model with the  number 6 wing statistics were:
Mass 9.1g
CG 53mm aft of root leading edge

This is still behind the 48mm CG on the reference model and I will move further forward to improve reliability.

I still have not solved the camber on the right hand flaps, where the curl induced by hotwiring bends the wrong way.

Even though the heat gun showed promise in making the foam mallable I had not solved the problem. Holding the heatgun in a vice and manipulating the foam by hand prooved too difficult to get an even heating and curved shape without local creases or over heated parts and a shrivelled, wavy traiIing edge. So I  made a vacuum box with the target contoured shape. This worked better in preventing the thin trailing edge shrivelling up. But, caused dimples where the vacuum holes were located. It did not give enough camber so I will contour it with twice the requirefld curvature to allow for springback. More holes may reduce the dimpling and hold the trailing edge better.

The simpler answer would be to adjust the hotwire jig to make each wingblank handed or to make a second jig to avoid adjusting them. I want to only use those options as a last resort.

There is a Youtube clip or Mr Yashiblnskiy bending his flaps over a counter edge to form the shape. My foam is thinner than his but seems reluctant to take a set without creasing. He seems to drum sand his wings after the foam is glued to the balsa. Perhaps hotwiring gives a skin making the foam reluctant to bend. But this is not obviously the case.

The number 8 wing broke with a crack on a javlin launch. The left hand lower spar buckled and delaminated 95mm from the centreline. I was very disapointed since this wing was supposedly overbuilt with 15mm wide spar caps at the root, compared to 10mm on wings 6 and 7. The failure ocurred at a rough spot on the lower foam where the hotwire dragged. I don't know whether this is a coincidence or the cause. The spar was glued over a Rohcell insert flush with the blue foam. The insert was narrower than the cap; being inset 1mm infront and behind the carbon, so an imperfection in the blue foam should not have affected the more important carbon to Rohcell bond.

This wing was much simpler to build than numbers 6 and 7 where the caps and Rohcell where built as seperate assmblies before gluing the bluefoam onto it. I hope that gluing the Rohcell into the blue foam first then adding the caps is not a weaker method.

So, my plan is to continue flying wings 6 and 7, to find out if they are strong enough. To adjust then CG and wing incodence to achieve a reliable transition. To also figure out how to get the right hand flaps bent correctly.

I attach photos of the broken wing 8 and the vacuum box.
Attached files Thumbnail(s):
Re: Plastic Bird
Re: Plastic Bird
Logged
piecost
Gold Member
*****

Kudos: 10
Offline Offline

United Kingdom United Kingdom

Posts: 499

Topic starter


Ignore
« Reply #59 on: June 27, 2020, 06:08:47 PM »

The broken wing
Attached files Thumbnail(s):
Re: Plastic Bird
Logged
Pages: 1 2 [3]   Go Up
Print
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.21 | SMF © 2015, Simple Machines Valid XHTML 1.0! Valid CSS!