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Author Topic: Washout in Polyhedral wing HLG  (Read 647 times)
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Sailaway
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« on: August 25, 2008, 05:07:30 PM »

Hi. My first time here. When cleaning out my basement I found a very old plan of an HLG called Yellow Bird 13. I believe it was a old british Mercury model. So that has got me interested back in building it after couple of decades.

It is quite straight forward. A 1/16 tilt of stab, skew the wing , 1/64 negative incidence on the stab , wing mounted at Zero degrees, CG at almost 3/4 or more from the back and 1/16 Washin on the inboard left wing section which is polyhedral. They also recommend to thread the LE of the entire wing to cat as a turbulator I guess.

But, they do not talk about any washout on the outer panels either side of the wing on the plan. Should i do it any ways? What would be the best way to do it ? Steam in the washout ,cut the dihedral offset ( not parallel to wing center line) or just sand off the botom section of the tips ?

Appreciate any input .

I also found a pod and boom type Mercury model HLG called Sailaway that I had no luck with when i first bought it. The wing here is mounted in center not on top of the fuse , swept back , plain dihedral, but looks great. Perhaps try that a little later. Feels good with nostalgia.

Thanks in advance.
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Ployd
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« Reply #1 on: August 25, 2008, 11:13:15 PM »

Hi Sailaway

Going by your descriptions you will find that the Yellow Bird 13 (centre page lift out) and Sailaway HLG designs were both published in Aeromodeller magazine around the late 60's (I have the plans for them) and was never associated with Mercury Models. That said, wing tip washout was never an issue with either model and I would not add it but, if you so desire just steam in a small amount, no more than 1/16" on the Yellow Bird only. The Sailaway because of its swept wing does not need washout (inherent stability).

Ployd in OZ
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JetPlaneFlyer
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« Reply #2 on: August 26, 2008, 02:22:51 AM »

The Sailaway because of its swept wing does not need washout (inherent stability).

From my own experience (not with HLG I have to say) and from a theoretical viewpoint swept wings actually have greater need for washout than straight ones.
Washout is usually put there to prevent tip stall (it reduced the angle of attack of the tips so that the wing root stalls first) : http://en.wikipedia.org/wiki/Washout_(aviation). Swept wings are far more prone to tipstall than straight wings so they actually can benifit greatly from washout. It's not there to add 'stability' in the conventional sence, like dihedral does.
On a parallel chord straight wings washout also gives a better spanwise lift distribution which increases efficiency, this is even more true of a swept wing because without washout a swept wing has a very bad lift distribution (doing most of it's lifting at the tips).

Washout can also be used on tailless swept wings to give stability in pitch but unless your model is talless that's probably not relevant..

Steve
« Last Edit: August 26, 2008, 10:37:26 AM by Forum Staff » Logged
JetPlaneFlyer
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« Reply #3 on: August 26, 2008, 04:55:26 AM »

Oops... the link I posted earlier had a mistake in it... try this one: http://en.wikipedia.org/wiki/Washout_(aviation)


Note from Ratz: It's a weird link for sure. The last character ")" won't stick. It's odd to see parentheses used to begin with. Simple solution, if you click the link, you'll need to add the ) at the end when you arrive at the incorrect page.
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Sailaway
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« Reply #4 on: August 26, 2008, 03:06:12 PM »

Thanks for the quick responses. Appreciate that.

Plyod, I will not put in any Tip washout on Yellow Bird 13 as per your experience. I will see how it behaves and go from there by steaming in some. I should have initial trimming flights this weekends and will report in as to how it goes. Regarding the turbulator string that they suggest on plans at LE, a dental floss should do ? correct diameter but not heavy at all ?

The Sailaway will be my next project so I will wait.Plyod, do you still have the center pull out full size plans for sailawy ? if so can you tell me the root chord length and where the CG is located ? That would really be great. My plans are not full size , I am just drawing my own based on what I have in the inset of the build instructions. There it shows CG way at the back but no measurement.

Jetplane flyer thanks for the link. Very interesting. Let me check out how the YB and then Sailplane works out and see what happens in small HLG's.

PS ; I thought it was a Mercury model as I had purchased Gnome right about then . It was hiden in the same box. My bad making an assumption.
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