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Author Topic: PZL Wilga for .5cc diesel  (Read 8213 times)
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DHnut
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« Reply #175 on: May 17, 2019, 05:02:28 AM »

Chris,
        I think Ralph has put his finger on the problem. One of our members built the Zaunkonieg powerd with an AMCO .87 and it needed full power and climbed in a stable manner and if it slowed for any reason it nosed up slowed and the nose dropped and it picked up flying speed and resumed the climb. When the engine cut the glide was steep but stable and quite slow. Also Brian Conroy had a Storch theat behaved in a similar manner. I am sure someone can elaborate on that model as it flew at the UK Nats some while ago. Your 12 oz/ sq ft wing loading does not seem unreasonable for a monoplane.
Ricky 
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billdennis747
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« Reply #176 on: May 17, 2019, 05:29:49 AM »

The original blurb with the plan refers to a 'gentle glide'. Looking at the video again, it is a dive, not a high sinking rate. The blurb also mentions more packing needed under the tail so I would agree with Ralph - find a slope and keep packing till you get a stall and then start power trimming again.
Not sure why it would tip stall with the slots taped up - there aren't any warps are there?
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Yak 52
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« Reply #177 on: May 17, 2019, 06:23:27 AM »

Looking at the video again, it is a dive, not a high sinking rate...

I must admit I thought this too Chris, although it's very hard to tell from a few seconds of video of course. I got the impression the model was slightly under elevated, even in the power phase. Giving it a bit more decalage and a smidge of downthrust might translate the power to more of a climb too.

Re taping up the slots - you might try taping the inner 2/3rds of the wing. This might still save some drag but control the tip stall?

It does look great in the air mate!

Jon
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Squirrelnet
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« Reply #178 on: May 17, 2019, 04:40:56 PM »

Thanks all for the thoughts

Strat-o  The slats and flaps are fixed on the Wilga I think

Ricky - The slats and flaps are certainly adding stability, with them taped up it felt like a faster brick in the glide rather than a better one. I think I will pursue the glide trimming first as its clearly not not right .. I wonder if the steep dive is result of not enough downthrust so during the power the phase the model is being held up by the motor at below its gliding speed so when the engine cuts it has to accelerate into its steep glide ?

Bill - I had 3/16 packing under tail to get that 'glide' , I joked the declage was 45 Deg but actually it wasn't far off !!! I think I will try a slope and retrim the glide. I'll try moving the cg back a bit too, its currently about 33% of chord so I can't go too far

Yak 52 - I'll try  a re trim first but that may work

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TimWescott
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« Reply #179 on: May 17, 2019, 08:06:35 PM »

I'll try moving the cg back a bit too, its currently about 33% of chord so I can't go too far

I wouldn't be too sure about that.  It's got a pretty long fuselage, big tail, and a skinny wing.  If you go by this page then you should be able to put it back behind 50% safely (note that I raised my eyebrows at this -- I may have flubbed the math, or mis-measured the Wilga 3-view on Wikipedia).

Given just how far back that thing says you can put the CG, I think I'd make a little balsa chuck glider to experiment with before I trusted my nice scale plane with the results.
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OZPAF
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« Reply #180 on: May 17, 2019, 09:01:02 PM »

I was not able to view your video - no doubt a problem at my end, however reading the comments leads me to the following conclusions.

The slats and flaps do appear to work on what is a fairly heavily loaded model but it is behaving as though it is under elevated on glide and power(needs full power to climb). I suspect that the slats and flaps are producing a heavy nose down pitching moment and as has been mentioned the CG will need to go back to enable the glide to flatten with a reasonable decalage, and also produce a better climb.

Tim's link is handy but I would think that 50% is too much and would think that Ralph's suggestion of using the slope is the best way to check at this point, as the effect of the slat/flaps will be too hard to incorporate and most likely be inaccurate on a small glider unfortunately.

Jon's(Yak52) idea of partial taping the slot is a good idea and could help (will also reduce the pitching moment) but it appears that the model needs the extra lift of the flaps/slots, as was shown by the increase in speed when they were fully taped up.

I wouldn't be surprised if the full size Wilga has large trim changes when using the flaps/slots.

Good luck with this Chris - it's a nice model.

John
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TimWescott
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« Reply #181 on: May 18, 2019, 08:24:46 PM »

Tim's link is handy but I would think that 50% is too much and would think that Ralph's suggestion of using the slope is the best way to check at this point, as the effect of the slat/flaps will be too hard to incorporate and most likely be inaccurate on a small glider unfortunately.

I don't think the slats & flaps are going to make that much difference to stability.  Trim, yes -- stability, no.  Stability is a function of balance, where the surfaces are, and how much the coefficient of lift changes with angle of attack -- and as long as the airfoil isn't stalling, the change in coefficient of lift with angle of attack is pretty constant.  If taping over the slats is changing things, it's because you're changing the trim, not the stability.

I built a chuck glider.  It glided nicely on the first try with the CG well behind the 50% that I was calling out.  The tail moment arm is deceptively long, which, I think, plays with brains that are used to Piper Cubs.

Note that I used the 3-views from Wikipedia, which show a wildly inaccurate wing mounting, so the stab on my chuck glider is also wrong to match.
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OZPAF
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« Reply #182 on: May 18, 2019, 09:59:53 PM »

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I don't think the slats & flaps are going to make that much difference to stability.  Trim, yes -- stability, no.
. Agree - but it is actually trim that was the reason for my suggestion of moving the CG back. I feel that there is a strong possibility that the slats/flaps are causing the airfoil to have a strong nose down pitching moment which the tail is struggling to handle(as per the comments of the large tail packing by Chris) at the current CG position. Thus moving the CG back should help to achieve trim balance, unloading the tail, - subject to retaining sufficient pitch stability.
The behaviour of slats/flaps at model Re's are fairly unpredictable and that's why I thought that Chris was basically stuck with some careful glides on his slope.

John
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« Reply #183 on: May 19, 2019, 10:41:11 AM »

I used a Wilga drawing in the gallery, just for a quick BOTE calculation.  The ratio of stab area to wing area is .21 on this particular drawing.   I calculated a tail volume coefficient of .80.  CG position suggested by these calcs is 44%   
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« Reply #184 on: May 19, 2019, 01:31:51 PM »

Thanks for all the help guys  Grin

I had resigned myself to building new wings but after hearing the discussion here I think it's worth pursuing the slatted and flapped version . It certainly looks good on power

Thanks Tim, I think you are onto something with the CG position as backed up by the others

Moving the CG back is the first thing I will try, it seems the best way forward. Now it's pointed out it behaves like a very nose heavy model as Jon said - it looks under elevated -  but my previous experience has been trimming with a cg much further forward which has thrown me a bit, this model is obviously very different.

 I think the nose down pitching increase from the slats and flaps sounds like the culprit too John. I like Jon's idea of partial covering of the slats. I built a Bowden Meteorite, a post war sports model designed by Col C E Bowden which has tip slats. It has a lovely glide and is remarkably stable it may be worth checking where the CG is on that now I think about it. I will try again with full slats and flaps first though

Before embarking on more glides I thought I better repair the damage to the undercarriage following it's 'nice gentle glide'. The undercarriage design isn't the best I've seen and isn't even that scale with forward wire braces. Anyway all strengthened now and new fairings for the legs, this time with some movement possible in the lower part to absorb some of the 'landing' loads

During trimming last weekend the model did turn right at one point, the rudder is very sensitive and it flew straight into a tree under power!  A combination of tissue over Mylar covering and good old luck meant there was no damage and after a bit branch bending we were able to get it out, I was very surprised it came out unscathed.

Here's some pics of the undercarriage damage - its actualy torn the thread and epoxy binding from the ply former. I would advise a different UC design if anyone does build one, maybe a torsion bar on the main legs ?

And a couple of pics of my old Bowden Meteorite.. with tip slats.. lovely glide and still going strong 7 years later though looking a bit tatty now  Undecided ... and yes the engine really is held on by rubber bands, it works very well


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TimWescott
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« Reply #185 on: May 19, 2019, 05:25:00 PM »

Moving the CG back is the first thing I will try, it seems the best way forward.

Just had to shout this one out because of the (perhaps unintentional) play on words.

Before embarking on more glides I thought I better repair the damage to the undercarriage following it's 'nice gentle glide'. The undercarriage design isn't the best I've seen and isn't even that scale with forward wire braces. Anyway all strengthened now and new fairings for the legs, this time with some movement possible in the lower part to absorb some of the 'landing' loads.

I was going to suggest leaving the undercarriage off for the rough part of test flying, at least if you have grass to fly on -- but, it's too late, and I can understand someone not wanting to go there.
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DHnut
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« Reply #186 on: May 19, 2019, 05:44:32 PM »

It is really interesting how we all get fixated on a particular line of thought and ignore the obvious. We almost have too much information available sometimes. Chris, Ian Lever had a Wilga that did one year at the Nationals and he might be able to give you an indication of his trim settings.
It seems the one area of weakness in design is often the undercarriage, especially when the model gets larger when a little bit of give is often preferable to trying to absorb the energy in the air frame. Even on smaller models I try and work in a torsion bar of some sort. It has reduced damage.
Ricky
 
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OZPAF
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« Reply #187 on: May 19, 2019, 07:45:41 PM »

Fair bit of damage there Chris - what a pain! Very nice Bowden model and I noticed that the Colonel was keen on using lots of shock absorption on his UC's.

The highly cambered  airfoil on the Metorite would have had a large nose down pitching moment and I feel would have required a fairly aft CG for trim so agree it's worth looking at.

Good luck with the repairs and further testing.

John
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Squirrelnet
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« Reply #188 on: May 20, 2019, 02:53:04 AM »

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Ian Lever had a Wilga that did one year at the Nationals and he might be able to give you an indication of his trim settings.

I haven't spoken to Ian directly but quote I have from him is 'It flew like a brick until I tissued over the slats'. This was my plan too or rather build another pair of wings with a cheeky bit of extra chord and span to help the wing loading... if that's allowed for AM designs ?

However as the AM comp is Saturday and there are some idea on here about getting a better glide I thought I'd try that first.

John I will check the Meteorite


Chris
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Yak 52
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« Reply #189 on: May 20, 2019, 05:08:50 AM »

Hi Chris,

I had a closer look at the plan and ran some numbers through Hepcat's calculator. Tail Volume is 0.87 - in other words massive! This is a classic case of a high aspect wing tricking the eye. While the geometry of the aircraft looks fairly normal what matters for pitch stability is the tail moment arm measured in relation to the wing chord. In this case the tail moment arm is 3.5 times the chord. Most scale models are 2-3 times (ish.)

This is great for stability but it does mean that the CG can go back further than you would think. The calcs gave me 51% with a stable static margin of 10%.

The other thing I spotted on the plan is that the wing section is pretty thick at 15%. This will be responsible for quite a lot of the drag issues even without the slats and flaps!

So I think it's worth persevering with the trimming. You seem to have the right amount of power if you can get it a little floatier Smiley


Jon
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« Reply #190 on: May 20, 2019, 08:59:10 AM »

Thanks Jon.

That's very interesting, yes pursuing with slats and flaps at the moment. After reading your post I think I'll reset the CG at 45% and try that with a bag a of plastercene on hand if needed.
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« Reply #191 on: May 20, 2019, 01:00:12 PM »

Interestingly the CG on the Bowden Meteorite is at 38% so about normal for a sport model. The tip slats are small though, just  6" per side on a 48" wing

Finished the Undercarriage repairs, paint is still drying. I just need to add the paper fairing between the leg and the fuselage skin.
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« Reply #192 on: May 21, 2019, 06:09:03 AM »

Hey Chris,
if you're looking to throttle that Redfin, Martin Lambert (the same Lambert from IIFI), in Germany, makes carbs for it and other deezils, along with some nice turbines.  I've seen the jewels, and, tho a bit pricey are works of art.  I've got his contact info someplace but a search for "Lambert turbines" might turn up the data.

Pete
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« Reply #193 on: May 21, 2019, 09:44:03 AM »

Chris, I've got Martin's email address - will PM you.

Jon
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« Reply #194 on: May 21, 2019, 09:58:18 AM »

Thanks both. I think I saw one at Old Warden on an engine, they are certainly beautiful bits of kit.
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billdennis747
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« Reply #195 on: May 23, 2019, 06:55:50 AM »

Just leafing through the Plans Handbook. The Wilga is recommended as 'an ideal first scale subject', so what's all the fuss about?!
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« Reply #196 on: May 23, 2019, 04:47:07 PM »

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The Wilga is recommended as 'an ideal first scale subject'

I hope we haven't lost too many future free flight scale flyers with such careless talk
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DHnut
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« Reply #197 on: May 23, 2019, 04:57:26 PM »

There are a few plans that come into that catagory that I would question their suitability for a beginner. I think you would have to look at some of the newer designs to find appropriate models as some of the older designs have structural flaws. Also the trimming notes are sparce to say the least and in some cases somewhat misleading. Your experience with the Wilga will be very useful and could be the subject of a piece in Aeromodeller together with any mods made to the model. The ebenezer plan was great.
Ricky
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« Reply #198 on: May 24, 2019, 02:01:33 PM »

Thanks Ricky, interesting idea

I certainly wonder how many of the old 1950's FF Scale designs were actually built and flew and how many were designers fantasy.

Right thanks to a gentle slope, some long grass and the excellent advice on here by various people all pointing in the same direction (Huge thanks to all  Grin)  I have reset the the CG to 45% and a 'not bad' glide has resulted.  I tried a few angled launches too and it seems to self correct so looking like there is still a margin for stability as you guys predict.

The undercarriage legs split again, even with just test glides so they now have been wrapped in a layer on LW glasscloth and epoxy resin so hopefully that means they will survive a bit better.

Off to the FF Nationals tomorrow so looking forward to trying again, I've added a gnat's more downthrust too in anticipation of an increased  angle of climb under power ....hopefully my guesses prove right  Undecided

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« Reply #199 on: May 24, 2019, 02:36:01 PM »

I certainly wonder how many of the old 1950's FF Scale designs were actually built and flew and how many were designers fantasy.
I suspect quite a few fell into that category. Many are the stories of models being hung from strings for photos. I built the Stan Perry Flycatcher - an absolute plumber's toolbag. I met a bloke from Stan's club who had built one - also untrimmable - and he said even Stan's never flew.
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