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Old 08-27-2012, 08:20 AM   #1
npowell28
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Default Swapping Airfoils on published plans

I'm still very new to this game having only been flying for about 2 years. I've built a fair few from plans and have just finished building my second scratch built from balsa and foam. I've also been buying RCME and Quiet and Electric to get the free plans. There are some fantastic looking plans in there such as the FW 190 which Id love to build but the airfoil, which is semi-symmetrical, scares me. You see just about everything Ive built so far has had flat bottom foils such as the Clark Y. Obviously it gives great lift and also slows the plane down I find. Looking at the FW 190 plan got me thinking. Could I change the published foil shape for a Clark Y. Would this have a dramatic effect on the way it flys or would it actually make it a bit easier

Or do I need to just get out of my comfort zone a bit???

Any advice would be great.

Cheers
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Old 08-27-2012, 09:04 AM   #2
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2 years ? I think its time to "get out of comfort zone" as you put it.

A semi-symet is not the beast you may think ... it's actually a clean aerofoil and flies well.

If you swap the FW90 over to Clark Y ... it will fly, no doubt, but - this is just thoughts going through my mind - you will have to redo it's neutral wing incidence, it will tend to balloon up as the lift increases if you don't as a Semi-Symet has a slight increased AoA compared to Clark Y. Other atributes will suffer such as inverted etc. as now the model will be a right way up flyer.

Go for the original and enjoy yourself ... seriously.

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Old 08-27-2012, 12:33 PM   #3
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+1 on Nigel's comment, just build it as per plan.
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Old 08-27-2012, 01:31 PM   #4
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Build to the plan.

You lose nothing going from Clark Y to a semi symmetrical airfoil and potentially gain a lot. If you want to fly slowly, build light.

Twice I've built the same airplane and switched from a Clark Y to a semi symmetrical airfoil (NACA 2412). There weren't huge differences but in both cases the NACA 2412 wing flew better. Aerobatics are easier, especially inverted flight. Less drag meant I could fly with less power and therefore have longer flight times on the same battery. The only downside, if you could call it that, was I'd have to bleed off air speed well before touch down on approach or I would over shoot my landings.

Many plans were written for Clark Y because the flat bottom makes it easier to build (and draw) a traditional balsa wing. Making the flat portion of the airfoil parallel to the datum line happens to be pretty close to the optimum incidence angle for a Clark Y which makes designing easier too.

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Old 08-27-2012, 01:50 PM   #5
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Just for the record ... my Text Book on the shelf behind me has 24 pages of Semi-Symet aerofoil data sheets ... NACA 2412 is only one of many ...

For the OP ... the term is really a poor wording .. as it means the underside has curvature but significantly less pronounced than the top surface, often as approx. midway from flat Clark Y to a full symetrical form.
In fact flat underwing is not always Clark Y ... as there are various forms of flat under as well. The main difference being where the point of maximum wing depth occurs ...

Just thought I'd throw the curved ball into the ring !!

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Old 08-27-2012, 03:13 PM   #6
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And FWIW the Clark-Y isnt really flat bottom. The front 25% or so of the airfoil is curved, only the rear part is flat.

And while I'm splitting hairs and picking up on Nigel's observation....'Semi-symmetrical' is really not a proper term for airfoils and would not be recognised by anyone outwith the modelling community. An object is either symmetrical or it is not... 'Semi symmetrical' is nonesense.

Steve
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Old 08-27-2012, 04:07 PM   #7
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Originally Posted by JetPlaneFlyer View Post
And FWIW the Clark-Y isnt really flat bottom. The front 25% or so of the airfoil is curved, only the rear part is flat.

And while I'm splitting hairs and picking up on Nigel's observation....'Semi-symmetrical' is really not a proper term for airfoils and would not be recognised by anyone outwith the modelling community. An object is either symmetrical or it is not... 'Semi symmetrical' is nonesense.

Steve
Ta ... I half expected someone to argue with me !!

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Old 08-27-2012, 04:37 PM   #8
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Originally Posted by solentlife View Post
Ta ... I half expected someone to argue with me !!

Nigel
I think I'm agreeing with you
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Old 08-27-2012, 05:09 PM   #9
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Originally Posted by JetPlaneFlyer View Post
I think I'm agreeing with you
Be careful ... it may just become a habit !!

Cheers ...

Nigel

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Old 08-29-2012, 05:38 AM   #10
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Default Airfoils can make a huge difference.

Many years ago there was a modeler who build Don McGovern's Custom Privateer, a 114" WS flying boat, from plans he had purchased. He decided to "update" the plane as it had been designed in the 50's. He changed the flat bottom airfoil to a "modern" full symmetrical airfoil, the plane would not lift off the water.

He then contacted Don for his advice. Don congratulated him on his new BOAT, then explained that the wing was shaped as it was for a reason. The Custom Privateer would never tilt back far enough on water to give an angle of attack necessary to get the new full symmetrical airfoil to provide lift.

I am leaving the airfoil the way McGovern designed on my Custom Privateer,

Dave R, Proud PGR rider.
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Old 08-29-2012, 01:56 PM   #11
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You have to trust that the designer wouldn't have gone through the trouble of publishing the plan if the original plane didn't fly well.

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Old 08-29-2012, 02:15 PM   #12
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When it comes to fiddling with airfoils although I may tweak slightly I won't normally change the airfoil class. I tend to think that if I'm going to mess about that drastically with the plan I might as well design the whole thing myself .

Basically if you're not going to trust the designer to know what he was doing then don't build one of his models .

BTW the advantages of talking about semi-symmetrical airfoils are that a) almost everyone knows what you mean and b) there's no other concise term you can use that says the same thing. The disadvantage is only that you'll have to explain yourself if you ever end up discussing such things with a professional aerodynamicist who is not used to models (but that's never happened to me and I doubt it ever will ).

Steve
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Old 09-06-2012, 12:48 PM   #13
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Thanks for the advice guys. It was more about trying to adapt the model to make it a bit easier to fly whilst i'm learning still. Whilst i have long term plans, as i'm sure everyone does, i want to shallow the learning curve a bit. Cheers guys. Advice taken on board.
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