A place for Pushy Cat builders to hang out

[Colin Leighfield]

Dave, if the slight bow is span-wise, as long as it's reasonably even both sides, I'd ignore it. Also, if the slight twist is even both sides and gives you a bit of wash-out, I'd ignore that too, it might even be a good thing! If it's wash-in or the opposite on each side though, I'd be looking to work it out, perhaps even pin them absolutely flat, re-dope them on the top-surfaceand leave them like it for a day or so. Once the dope is surface dry, perhaps even park a brick on each one.

You might find that sliding the tail-booms on helps to flatten them out, particularly if they are a good tight sliding fit. A bit of patience and fiddling usually gets you somewhere with problems like this. Whenever I'm applying dope and tissue to sheet balsa wings and tails, I also put two or three coats of sanding sealer on first, sanded smooth. It's very quick and easy and seals the wood so that the dope doesn't penetrate the grain. I find that helps to prevent warping like this.

[Erfolg]

Colin

I was unaware that the Vampire had been fitted with a larger engine.

My comments were or are of a general nature, that a problem with engine upgrades particularly in the 50s is the fuel consumption generally goes up. This in principal poses some issues, for the same range extra tankage is required. The obvious solution is internally, which either is accomplished by taking something out or making the fuz bigger, generally longer, as military nor civilian aircraft seldom have unused space. The trouble with this approach, the weight invariably goes up, for the whole flight, and cause performance issues, although the fineness ratio, would improve, which could be useful on a relatively tubby Vampire. The other approach is to provide drop tanks, which means additional consumables, some issues with the plumbing and sealing. Although without the tanks, the performance is improved.

It is interesting (at least to me) that the UK Phantom, was not as fast in a straight line as the standard USA aircraft, although the engines provided more power. I guess other aspects relying on power over aerodynamics were better, such as climb rate.

[brfc7]

Dave, I was a bit unsure about how strong my wings would be and if they would warp but once I glued the tail booms and the fuz on the wings are very solid

baz

Edited By brfc7 on 21/12/2013 12:39:24

[Colin Leighfield]

Erfolg.

You're absolutely right about the fuel consumption issue. if you look at the Venom, it normally wore tip tanks which were not usually jettisoned because they were found to improve the aerodynamics and it was faster with them than without them. If you look at pictures of the late model FB4, you will see it often carrying under-wing drop tanks as well.

There's no doubt that the Venom all round was a lot more efficient than the Vampire and the main contribution was the thinner wing. The swept back leading edge was for cg purposes with the heavier engine rather than aerodynamic, as was the moderately swept wing on the Me262, which also had nothing to do with aerodynamics, as most people seem to believe.

The RR Spey engines in the UK Phantoms was really a result of political shenanigans by the lousy Wilson govt. to demonstrate a commitment to UK industry after massacring most other worthwhile defence projects. The large by-pass turbo-fans didn't suit the aerodynamics of the plane and although still very good, it was actually slower than the lower powered US original. It was still an excellent kite though, I understand.

[fred clarke]

Dave

re your wings. I would echo Colin's thoughts - it all depends in which direction and how much. Although I am a great fan of the all sheet wing they can be problematic if you go the tissue-dope route. Certainly, using sanding sealer first does help.

I have flown a small all sheet model that had about 1/4" wash-in on one wing. The first 30 secs of the first flight were rather troublesome not helped by poor tx mixing (it was tail-less) on my part. However, once trimmed it look very strange on the ground with the amount of control deflection needed to balance out the warp; however, flew fine!

Is it time to cut your losses and build another wing and cover in film?

Also - re the Phantom. The loss of speed of the RAF to US version was often explained as 'That's what happens when you Spey a bitch, she goes fat in the a**e and slows down!'

fred

[Colin Leighfield]

I love that one Fred, never heard it before!

[Erfolg]

I guess this idea is of little or any use now with respect to the warped sheet wing.

In the past it was common to at least to have wing tips which were made from balsa where the grain was at 90 degrees to the main panel wing grain. Not having the plan to hand (filed away) I do not know if the PC plan calls for this?

There was another technique which although I am sure works, must weaken the wing without some compensating local spars and that was inserted keys/pieces of balsa, which again run roughly in this case at 90 degrees to the wing, these would be located at mid span, to supplement the tip blocks.

In all cases when covering with tissue and dope, they used to hold the item down flat on a board until the dope had dried. That was all in the last century

I did consider covering my wing with Glass cloth and WBV (Ronseal Hardglaze). In the end I decided against it. Mainly because I wonder about my wings performance, and wanted the option to easily chop it about if necessary. Hoping that it will be fine, although I seem to be a little over cautious.

It may interested you that my tailplane has warped/bent, particularly in the middle across the width, providing a cambered section. In some ways I think I would have prefred the camber the other way. The reason being that the pitching moment from most aerofoils at some AoA is a pitching down. With luck the section on the tailplane could provide a balancing force. Although I do have doubts if the idea actually works. I have seen in the long, distant past, a few FF models rigged this way. In my opinion they seemed to perform no better than the conventional set up.

[Dave Miller]

The warping resulted in both a bow and a twist with the twist giving an increased angle of attack at the tips. It’s not a great deal; when the undersides are placed together there is a gap of about 3mm at the middle of the leading edge extending to the tips although the trailing edge and roots touch all the way around, but then they are small panels.

The balsa tips do have their grain running cord-wise and I used a firmer grade of wood for them and I did give a couple of coats of sanding sealer before applying the tissue.

Anyway I have left them firmly pinned down on Clingfilm whilst the Aeropoxy cures. It will be interesting to see how much weight that process adds as the panels weighed about 60grams each before glassing. Given the time that it took to shape the solid wood version I may revert to built-up panels if the glassing is unsuccessful.

[Erfolg]

Dave

I do sympathise with your situation.

If your issues are cured I guess will be dependent on a number of factors.

I once was a avid user of epoxy skinning. The results can be most satisfactory. But there was a downside with hindsight. The process takes some time, great care was needed to avoid weight build up. My last efforts with epoxy was with a non finished model boat, using "Z" Epoxy Laminating grade. I was fortunate, again with hindsight, my wife was tolerant, not as tolerant some +20 years later, in that I would bag, suck the skins down into the polystyrene blanks, all left in the Dining Room, for days. Yes it works, but the time, the messing, OMG

Then, I found WBV, or it found me, having some (a remnant) given me by a "floorer". When I read in Radio World (no, I did not swear, honest, it was him) that a propriety brand of water based finish for modellers, was actually a trade Ronseal product, repackaged, into small amounts, the original being sold in 5 litre barrels at some price like +£60. I thought, hmm, may be, yes, I could try this stuff on a model. Since then I have tried a variety of WBV products, they all seem much of a much ness.

The results (with remnants) were as good in finish as Epoxy, it dried in less than 1 hour, I could wash the brushes out in soap and water, there was no real smell, it wetted out the cloth more effectively than epoxy, removing the tendency for the glass to float on the varnish. The weight build up seems much easier to control. Although I no longer look for the super finish of my youth, seeing models as expendable, in that I will crash them, and do not like crying over all the hours lost.

You may prefer to go the Poly C route, although I like the lower cost Ronseal route, for the future, it is so much easier than epoxy, which ever way you look at the method

[Colin Leighfield]

Dave, the only other thing I would add is that when I'm doing this, I always thin the dope down at least 50% with cellulose thinners. You can of course use non-shrinking dope as well. With my PC I want to keep the build simple and I'm going to use Solarfilm I've already got, although I'm not normally a huge fan of it.

I still haven't tried the Poly-C/Ronseal/Ezekote route, I must have a go one day!

[Dave Miller]

As Erfolg says Colin Poly C and the water based floor sealers are all of a muchness and much quicker and easier to clean up after use than epoxy skinning, but I have found warping problems when using it on balsa structures.

My dope is well thinned as I always tip the cellulose thinners I use to clean my brush with into the dope tin after use.

[Colin Leighfield]

Dave, I've just finished planing and sanding my wing panels and bearing in mind that they started at only 1/4", by the time I'd finished I'm not surprised that they might warp easily.

I used dope and tissue very successfully on the Tucano and had no problems at all, but the wing is thicker, 3/8" from memory and maybe the way Nigel designs it with balsa spars glued in between the panels might break up any stress being transmitted across the wood grain? I don't know. Good luck with it anyway.

I've got to cut the booms and tail bits next, so it should go together soon. I'm going to coat it with Balsaloc before I put the film on and I might take earlier advice and do it before I stick it all together.

[Dave Miller]

Posted by Colin Leighfield on 21/12/2013 18:30:31:

Dave, I've just finished planing and sanding my wing panels and bearing in mind that they started at only 1/4", by the time I'd finished I'm not surprised that they might warp easily.

I used dope and tissue very successfully on the Tucano and had no problems at all, but the wing is thicker, 3/8" from memory and maybe the way Nigel designs it with balsa spars glued in between the panels might break up any stress being transmitted across the wood grain? I don't know. Good luck with it anyway.

I've got to cut the booms and tail bits next, so it should go together soon. I'm going to coat it with Balsaloc before I put the film on and I might take earlier advice and do it before I stick it all together.

I also plan to cover the booms before I finally fit them, that's after a trial fit or three of course.

[Dave Miller]

OK an up-date.

I unpinned the panels this morning and can report that the twist has gone but a slight bow remains; that's the good news.

Not so good is the not unexpected fact that the weight of each panel has increased by 20 grams to 82 grams with a little more resin required on a couple of patches but the weight of that will probably be off-set by a good rub down.

I shall try making a built up panel later to compare the weight and rigidity with the solid units.

[Colin Leighfield]

Sounds as if you're going to be ok Dave. These things probably aren't that critical any way on models like this. I see it as a fairly casual quick build anyway and making a built up panel sounds like hard work I'd rather put into another plane!

[Dave Miller]

Posted by Colin Leighfield on 22/12/2013 10:44:33:

Sounds as if you're going to be ok Dave. These things probably aren't that critical any way on models like this. I see it as a fairly casual quick build anyway and making a built up panel sounds like hard work I'd rather put into another plane!

I agree that it's not a critical item but as it's not exactly hard work I am making a panel, just takes a little time; anyway I'm now interested in a direct comparison of the two construction methods.

[Colin Leighfield]

I managed to get a couple of hours in and completed the kit of parts. You can see the little changes that make it look quite different. I've extended the tail booms forward so they also act as wing fences as well. Also I've dog-toothed the leading edge and will run a wing fence across each wing to line up with that. I've slimmed the booms down a bit, left off the 3/16x1/8 spruce "skids" because I don't think they're necessary and fitted instead 3/16 square spruce stiffeners inside the booms, glued directly to the underside of the wing as per the photo. I've re-shaped the fins a little to make them look a bit more "De-Havilland" and extended the tailplane slightly outside the fins, similar to the Venom. The reason I've not fitted the shaped wing tips is because I'm going to fit tip-tanks. All simple things to do but at the end of it I will hopefully have a Pushy Cat that looks like my imagined "Super Venom"! Hopefully it will still fly like a Pushy Cat should.

[Dave Miller]

Posted by Colin Leighfield on 22/12/2013 19:13:28:

I managed to get a couple of hours in and completed the kit of parts. You can see the little changes that make it look quite different. I've extended the tail booms forward so they also act as wing fences as well. Also I've dog-toothed the leading edge and will run a wing fence across each wing to line up with that. I've slimmed the booms down a bit, left off the 3/16x1/8 spruce "skids" because I don't think they're necessary and fitted instead 3/16 square spruce stiffeners inside the booms, glued directly to the underside of the wing as per the photo. I've re-shaped the fins a little to make them look a bit more "De-Havilland" and extended the tailplane slightly outside the fins, similar to the Venom. The reason I've not fitted the shaped wing tips is because I'm going to fit tip-tanks. All simple things to do but at the end of it I will hopefully have a Pushy Cat that looks like my imagined "Super Venom"! Hopefully it will still fly like a Pushy Cat should.

It's looking good, I hope you are going to fit a pilot in the office.

[Colin Leighfield]

Definitely Dave. Nothing is glued together in the photo and the canopy is just resting on the fuselage. I'm going to put in a pilot with ejector seat, nothing very ambitious but enough to look the part when it's airborne (hopefully)!

[Dave Miller]

As promised I made a built-up wing panel yesterday and took some pictures to show the construction. I used some 4mm carbon fibre tube for spars as I didn't have anything more suitable in my scrap box, and it's the contents of that which are being used to make this model.

First the bottom wing skin was made up from a single sheet of 1.5mm 4"x36" balsa.

Then the carbon spars were glued in place.

Followed by the 4mm deep centre ribs cut from 3mm sheet.

And then the front tapered ribs and the rear aileron re-enforcement sheet.

After drying the rear strip was planed to its tapering section and the top sheet was added and the whole assembly left pinned down to dry overnight.

After the edges are trimmed I shall add the tip block and compare the rigidity and weight to the glassed panels.

Edited By Dave Miller on 23/12/2013 09:30:44

[Dave Miller]

As a follow up the completed trial panel is more rigid spanwise but can be twisted with about the same force and weighs 55 grams, so a potential 2oz saving on the overall weight or about 8% of the flying weight. Timewise I think that the built up construction is an hour or so quicker per panel since I found the planing and sanding of the solid construction quite time consuming and its also no where near as dusty.

Edited By Dave Miller on 23/12/2013 11:17:48

[Colin Leighfield]

That should stiffen it up Dave. It will be interesting to see the weight comparison. One reason I extended the tail booms across the wing chord and plan to fit full chord fences at the dog-tooth points is to stiffen the wing chord-wise and keep it flat.

With the Tucano and Seafang I've used carbon-fibre tube as leading edges to give a good entry profile and prevent ding damage, but didn't bother with the PC, thinking I'd just see how it goes. However, I have got some CF rod about 1mm diameter and I might cyano that along the leading edges before I apply the Solarfilm, to give some protection.

[Erfolg]

I totally agree Colin, the nose entry is so important for an effective wing. Yet their is a temptation to make a knife entry, when the wing is thin and you are left to your own judgement, with razor plane and sanding block. A carbon tube LE, should pretty much guarantee a consistent entry and encourage a stagnation point movement in a more predictable manner.

It is with some disappointment that i did not weigh my wing for comparison. I guess when we are all finished, weighing with servos and perhaps with a finished wing, could reveal the full extent of the weight increase. Ranging from a built up wing, a skinned wing and the as drawn wing. The absolute weights are not important, ball parks weights being good enough.

[Dave Miller]

The only problem with using carbon rod or tube for a leading edge (something I have been tempted to do in the past) is that of having a constant radius on a tapering wing but then again I am being really pedantic with regard to something like a Pushy Cat where it would make no difference.

I have planned mine down to 3mm and then rounded them off by sanding but left the trailing edge 3mm thick and square.

[Colin Leighfield]

Erfolg, Dave, I'm increasingly convinced that in these sizes the only really important thing is the wing being the same size and shape on either side of the fuselage, accepting of course that any warping is equal on both sides!

My Hughes XF11 is a 55" span twin and after my total surprise with how well the Fizza flew with a flat sheet wing, I thought I'd try it on the XF11 in spite of the larger size, because I originally saw it as a test mule for a larger model and wanted it to be a "quick build". To save weight (I thought), I actually did something similar to what Dave has done with his PC wing, although instead of CF I did it with 1/4" square and 3/32" skins top and bottom, rounding off the leading edge and sanding the trailing edge down to the usual taper. It flew remarkably well and hopefully will again soon when I've finished renovating it.

The only unusual thing I noticed was that with the cg in the the typical 25% mean chord position, the usual 3 degree incidence on the wing was clearly too much and it needed down trim on the elevator. It's fascinating to see that both the Tucano and PC are set up zero/zero and yet fly brilliantly, you wonder just how the flat plate wing actually works and it begs all kinds of questions about conventional aerofoil sections!

With about half of the wing chord being flat, you can see elements of both laminar flow and supercritical characteristics in this wing style. I won't get too carried away about that, but the way you seem to get stable lift from flat out down to walking pace does look though as if you are actually getting very good laminar flow across a wide speed range. The other noteworthy point that BEB picked up on in an earlier thread conversation about this was the way the stall appeared very suddenly if you flared very sharply on finals. I can vouch for that with my Tucano as well, if you give a sharp up input at low throttle, there seems to be a point where induced drag skyrockets and lift dramatically reduces at the same time. The plane does nothing nasty, it just slows down sharply nose up, gently drops the nose, picks up speed and flies on without losing much height. It looks as if when you get airflow separation, it happens suddenly and maybe that is a distinct feature of this wing.

So I'm looking forward to seeing how my PC goes, also the Seafang with the same wing principle. Sorry about being so boring, but I'm really intrigued by all of this!

Edited By Colin Leighfield on 23/12/2013 17:10:43