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1:5 Scale Yakovlev Yak-3

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1:5 Scale Yakovlev Yak-3

Old 10-12-2002, 01:49 PM
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Pelle Gris
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Default 1:5 Scale Yakovlev Yak-3

Hi All...

Im in the process of drawing up plans for a 1:5 scale Yak-3 and need some help on the wing design.
The airfoil used on the full size is a Clark YH series, and from looking at the airfoil it seems to me that it should perform quite OK on a model. It is quite similar to a normal Clark Y except that the T.E. curves slightly upwards.
From measuring the 3-view it looks to me like the root airfoil is about 14% and the tip about 8-10%.
The incidences/angles on the original is close to 0-0-0 (wing 0 - tailplane 0 - engine 0).
I plan on introducing 2-3 wash-out in the wing.
My main concern is that given the 0 incidence at the root, combined with wash-out will make the wingtip give negative lift(downforce???)
Does this seem like an OK setup on a model?
Any other advice is most welcome...

Additional specs:
Wingspan: 72.5in
Wing Area: 961sq.in
Length: 67.5in
Weight: 15-17lb (hopefully)
Engine: 35cc Webra Bully on glow
Old 10-12-2002, 07:07 PM
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Default Washout

Hi,

In order to avoid making too much of a twist in the wing, may I suggest using a different airfoil towards the tip. This can be a flat rib section (Like the Clark Y) or else a semi-symmetrical airfoil.
I reckon they have the same effect as having washout.

Regards

Reuben Saliba
Malta (Europe)
Old 10-12-2002, 07:36 PM
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Default 1:5 Scale Yakovlev Yak-3

Depending on the drawings you're working from it's quite possible that the draftsman didn't show any incidence even though there is some there. The other possibility is that you're making the usual mistake of assuming the flat lower surface is the center line of the airfoil. Just checking..... I've seen so many cases where the designer used a Clark Y style of airfoil and then said in the write up that he used 0-0 decalage. Of course this isn't the case and the lower surface is actually about 3 or so degrees different from the actual center line of the airfoil.

If that's not the case then I think you'd be wise to design in a 3 or so degree decalage angle. I like to put positive in the wing itself as it has the effect of automatically giving the same degree of downthrust to the thrust line. From there you can either add a degree or two of negative in the stab to increase the decalage or just live with a small amount of permanent up trim in the elevator as needed. Usually our models end up flying with a slightly rear located CofG compared to full size anyway so we don't often need as much decalage.
Old 10-14-2002, 07:10 PM
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Default 1:5 Scale Yakovlev Yak-3

I am finishing up my fourth Yak-9 I have designed, all have used the Clark YH with 2 deg. washout and are great fliers they can be flown very slowly and refuse to drop a wing, like the post by BMatthews with the bottom of the airfoil flat it's actually about 3deg. Look for my Yak-9 to be in laser cut kit form soon.
Old 10-15-2002, 09:59 AM
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Default 1:5 Scale Yakovlev Yak-3

flailing... am I understanding you right... that the root incidence of your Yak-9 is 3*??? And by having 2 washout the tip incidence would be 1???

The problem is that unless I change the fuse outline a lot Im not able to mount the wing with anything alse but 0 (Chordline... not underside)

Is there a chance of a picture of your -9?
And how about construction/size/weight/engine...
Old 10-15-2002, 10:54 AM
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Default 1:5 Scale Yakovlev Yak-3

The drawings I used for my plans is from a Russian book titled The First Yak, I think I don't read cryllic but a Russian friend said that was the title, the drawings show the bottom surface of the airfoil basically flat at 0deg. and there very good drawings 6 or 7 really nice sheets, like I said this is my fourth one and the book is the best documentation I have seen yet, so yes 3 deg @root and 1deg @tip. The version I am building now is 62" span all balsa/ply, with a 91 4cycle, looks like all up weight will be about 9 pounds, 6 channels. I can't post pictures yet as it's in the kitting stages by a well known manufacturer, but stay tuned you will see it here on RCU.
Old 10-15-2002, 11:08 AM
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Default 1:5 Scale Yakovlev Yak-3

-I can't post pictures yet as it's in the kitting stages by a well known manufacturer, but stay tuned you will see it here on RCU.-

Looking forward to that... And thanks for the quick reply
Old 10-15-2002, 11:08 AM
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Default 1:5 Scale Yakovlev Yak-3

I suspect your documentation is incorrect, if you mounted the Clark YH at 0 deg. MAC that would translate into about -3 deg.
I could scan the drawings I have but I don't know if I will be able to send them as the file size will be larger than my server will handle, but I will try.
Here are 2 sites you may already know about one has the airfoil info for the Yak and the other has som pretty good drawings.
http://amber.aae.uiuc.edu/~m-selig/
http://www.airwar.ru/other/draw/yak9.html
Old 10-15-2002, 03:11 PM
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Default 1:5 Scale Yakovlev Yak-3

Just a correction - Dr. Selig's Applied Aerodynamics Research Group's website moved. We decided to close the web server on amber. The site is now located at:
http://www.aae.uiuc.edu/m-selig/

Even better, if you want to put it in your favorites use this address:
http://www.uiuc.edu/ph/www/m-selig/
That one will always point to the current website.

Regards,
Bennie
Old 10-15-2002, 03:19 PM
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Default 1:5 Scale Yakovlev Yak-3

I wondered what happened, it worked last week
Old 10-15-2002, 06:36 PM
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Default 1:5 Scale Yakovlev Yak-3

Hi again...

After checking my 3-Views of both Yak-1/-3/-7 and -9 it looks to me like all of these planes has close to 0 incidence on the wing. And a few hours ago I found a webpage which details the restoration/modification of a Yak-11 to -3 standard. This page also stated the incedence as being 0.
URL: http://meltingpot.fortunecity.com/sp...289/yak-3.html

So based on this I think I will proceed drawing my plans with 0 incidence... My Compufoil demo stated the zero-lift AoA of a Clark YH as being -2.11, so if I limit the washout to 2 I think that it should be OK.

I have tried to attach a part of the 3-View Im using to draw the plans... hope it works...
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Old 10-15-2002, 06:52 PM
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Default 1:5 Scale Yakovlev Yak-3

I know just enough about aerodynamics to be dangerous, however the incidence settings I used have worked very well for me. Maybe someone with more experience will jump in here.
Old 10-15-2002, 06:56 PM
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Default 1:5 Scale Yakovlev Yak-3

By the way my last Yak-9 was 80" span with a Moki 1.8, man I miss that plane it was lost when it met midair with a trainer flying across the pattern.
Old 10-15-2002, 07:57 PM
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Default 1:5 Scale Yakovlev Yak-3

Just one more question if you dont mind... What kind of retracts were you using on that 80in -9?
As far as I can see on the -3 plan the retract angle is somewhere around 85.

BTW. I think I will build a small simple version of the -3 to test the flight characteristics before I start the 1:5 scale version. I have done the same with a Hawker Typhoon Im also making plans for. It also has a wierd setup with the wing and engine at 0 and the tailplane at -0.5. Flew quite well though...
Old 10-15-2002, 07:58 PM
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Default 1:5 Scale Yakovlev Yak-3

Tinus

Don't let this "downward lift" on the tips concern you too much. It will be happening only at high speed (when the lift coefficient and therefore AoA is low). There it doesn't matter - you loose a littlebit in efficiency due to a less than optimal lift distribution, but because the CL is so low the induced drag is small anyway. Other than a slight loss in efficiency (very small), this downward lift on the tips has no other negative effects.

At lower speeds, your airfoil will be at a positive angle of attack so the effect of the washout will be to unload the tips, which is exactly what you want at high lift coefficients.

So, once again, don't let this concern you - go ahead and add the washout. 2 or 3 degrees should be fine.

BTW, I assume the book you are using as a reference is " " (Pervej Yak) by Gennady Petrov? I heard it is a great book - have to get a copy for myself as I love these Soviet warbirds!

Regards,
Bennie
Old 10-15-2002, 10:34 PM
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Default 1:5 Scale Yakovlev Yak-3

Century Jet retracts I had to use 90 deg. retracts and tilt the mounts 5 deg to get the wheels to retract fully into the wells so the doors would close flush. It is a great book even if you can't understand a word of it.

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