SZD30 Pirat

Let us all watch your new project progress.
Nigel Argall
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Re: SZD30 Pirat

Post by Nigel Argall » 14 Feb 2019, 11:54

Thank you - that is really helpful. By 'three piece wing' you mean something like a Slingsby Skylark with a big centre section? I fly 100% slope so need to take this on board.

Richard Farrer
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Re: SZD30 Pirat

Post by Richard Farrer » 14 Feb 2019, 11:59

The Pirat is quarter scale. There is a plywood tongue at the trailing edge that locates in the fuselage former and at the front two plates, one on the wing, the other in the fuselage. They clamp together with horizontal bolts accessed from canopy. One of the plates is slotted so the bolts can remain in place. Friction between the plates increased by sandpaper. Not keen on this arrangement.
Should I use nylon bolts instead of steel so it is not too rigid?
Thanks for further info Chris.

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chris williams
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Re: SZD30 Pirat

Post by chris williams » 14 Feb 2019, 12:51

In the words of Jack Reacher: 'hope for the best, plan for the worst'. On the Skylark, Super Javelot, and the Eagle, I used two bolts on the centre section, with a hatch for the front bolt in the thickest part of the wing. To alleviate the shock of a hard landing, I used the MPX wing retaining system. This worked quite well, although you constantly find yourself knocking the wings back into place. (As an aside, a bolted on centre section allows the use of a D-connector to speed up the rigging process)
I didn't want to bring this up, because I'm still in therapy, but I made the mistake with my 3.5 scale Duster of taping the outer wing panels to the centre section to assist the MPX retainers. Halfway through a perfectly smooth landing, one of the wings caught on a clump of grass, and resulting rigidity half wrecked the centre section. I have yet to come up with an ideal system for fixing a 3-piece wing to a fuselage: the best you can hope for is that the bolts break off and the wing is surgically removed from the fuselage with as little damage as possible...
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P1030773 copy.jpg

Richard Farrer
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Re: SZD30 Pirat

Post by Richard Farrer » 14 Feb 2019, 21:47

Following Brian Sharp's account of a near loop on first launch of his Pirat I have looked closely at the plan and the longitudinal dihedral, or should it be the decalage, is 5.5 degrees. This is the angular difference between the chord lines of the wing and tailplane. The tailplane is symmetrical so no problem there as the zero lift line is the chord line. The wing section, however, is fat and undercambered, see photo. Can any one help me with the calculation of the zero lift line of the wing? Do I in fact need it?
Brian suggested 1.5 degrees of incidence would be sufficient. If that is the angular difference between chord lines then serious packing up of the tailplane leading edge will be required to avoid the same problem Brian had. Any input would be welcome.
I am now in the process of making an incidence meter using a protractor/clinometer app on my smartphone. I will then be able to check that the actual model conforms to whatever setting is ultimately recommended.
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2019-02-14 21.37.29.jpg
Pirat wing rib

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RobbieB
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Re: SZD30 Pirat

Post by RobbieB » 15 Feb 2019, 00:37

Richard,

The physical appearance of an aerofoil is not always an accurate indication of its camber.

Also, the thickness of an aerofoil and the point of maximum thickness can deceive you into seeing a distorted camber. Any thickness form can be ‘bent’ around a particular camber line.

First impressions of the aerofoil on the Pirat the camber doesn’t appear to me to be particularly excessive and anyway, camber produces a nose down pitching moment under normal conditions of flight and consequently, increased camber...... etc etc.

To find out better what it might actually look like, draw a straight line from the LE through to the TE then, at various points along that line draw lines vertical to that chord line you have just drawn. Draw those lines to the top and bottom of the aerofoil at their respective positions; they don’t have to be equally spaced. On each of those lines mark the mid point between the top and bottom of the aerofoil then draw a line through all those points, that is your mean camber line. Do that and post up another photo showing the camber line.

That said, far more important is that 5.5º of decalage – far, far too much and will result in a model that is not very nice to fly. As Brian has already commented, 1.5 degrees will be so much better. Too many model designers have copied full size rigging angles in the past and down at our scales is not appropriate.

Rectifying this you will need to consider carefully how you are going to go about it. Just cranking up the leading edge of the tailplane is likely to give the model an unnatural ‘sit’ in the air with a pronounced nose down attitude and will just not look right. You might want to consider a re-adjustment of the main wing incidence to correct this. If it was me, I would aim for: rear fuselage top at 0º, tailplane at 0º with the wing aerofoil chord line at +1.5º. If you are too far on with the construction for this to be possible then the tailplane it has to be. Whatever, removing that 5.5 degrees is the important thing. Before you do anything, once you have your incidence meter up and running check what the main wing is set at in relation to the top rear fuselage set at 0º.

…………...and no, you don’t need to know the zero lift angle of the aerofoil for this exercise. Without knowing exactly what that aerofoil is, that would be extremely difficult or impossible to do anyway.

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B Sharp
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Re: SZD30 Pirat

Post by B Sharp » 15 Feb 2019, 08:48

Spot on Robbie. A much better explanation than I could manage.
The problem that I had with my Pirat was the F/G fuselage which had totally flat wing mounting area. As I had no reference, I mounted the wing with the under surface and the trailing edge both in contact with this flat surface. I later built up the flat surface to accommodate the undercamber and LE profiles.
After the first flight I modified the wing seat and packed up the leading edge of the tailplane. I suspect that I still got it a bit wrong as the model, while flying perfectly, always had a slightly nose-down sit in the air.(in these days I didn't have an incidence meter.)
Having said all that, it flew well for many years before being sold on to Ray Jones to use as a trainer in the Clwyd club.
Brian. :)

SP250
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Re: SZD30 Pirat

Post by SP250 » 15 Feb 2019, 10:54

Brian

Ah, so that's where the Pirat came from.
Ray did a mass group of "A" tests once, whilst we were all on our annual week long soaring holiday at Cim Farm on the Lleyn peninsular.
Your Pirat got passed around about 5 people one day for the tests, as it was the easiest to fly of everyones models.
They all wound me up to do it with an ex Vic Steele Skylark 4 though, which I tried to "strain" through a chain link fence on the landing approach - my depth perception is not what it used to be!
Needless to say 5 people got their A tests and one didn't on that day.

The only issue I can see with a 3 piece C/S mounted like the Skylark, is that however you do it make provision for a frangible attachment.
The Skylark, like the full size, had two wire pins engaging in aluminium angle brackets below the wing and locating through two 1/4 ply formers front and rear.
Even the lightest touch of a wingtip on a tuft of grass when landing twisted the whole wing and tried to break out the formers from the fuz sides.
I spent too many hours repairing the sides and formers in the fuz, and landing in heather at Long Mynd or Clwyd did even more damage, so I sold it and will not have another scale glider with that type of wing mounting again.
John M

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RobbieB
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Re: SZD30 Pirat

Post by RobbieB » 15 Feb 2019, 17:44

Surely John, there is a serious design issue there for the attachment system to be so fragile.

My Skylark emulates the full size almost exactly and has had more than its fair share of less than ideal arrivals without any issues relating to the wing attachment. At 5.2 M and the attachment points only 6.5cm apart, that's one heck of a moment arm (this will be the Kiss of Death, of course).

IMG_0044.JPG
IMG_0048_2.JPG

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RobbieB
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Re: SZD30 Pirat

Post by RobbieB » 15 Feb 2019, 17:51

...............and now back on track with the Pirat aerofoil issue:

To demonstrate how misleading the appearance of an aerofoil can be, the two examples below only have 0.5% difference in their camber value.

quabeck.jpg
BRNDBRG5_2.jpg

SP250
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Re: SZD30 Pirat

Post by SP250 » 16 Feb 2019, 16:22

Robbie

The Skylark 4 I had from Vic was almost the same wing fixing arrangenment you show for yours.
There may well have been a stuctural issue in that the angle ali brackets and 1/4 ply formers were too stong for the former to balsa planked fuz joint - hence the formers and planking coming off second best each less than perfect landing (and I've yet to do one of those).

Sorry to clog up the Pirat A of A aerofoil discussion with Skylark 4 stuff.

John M

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