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Old Jan 10, 2015, 04:23 AM
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Elevator Profile selection

I am designing a Pilatus PC-6 Porter.

For the wing i have chosen to go for a Clark-Y with flaps. I need a large area of flat-surface due to the build limitations.

the original airfoil for the elevator is double-curved.

Right now I see 3 options:
  • Use a Clark-Y on the elevator and install it upside down
  • Use a flat profile with LE and TE shaped to look right
  • One of you guys knows a good profile for this use

The scale is 1:10 resulting in about 1.5m wingspan. Weight will be something around 2-3kg, i hope..

Thank you for your advise
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Old Jan 10, 2015, 09:58 AM
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When you say "elevator" do you mean the whole tailplane/horizontal stab ? The elevator is just the moving part.

Either way I'd probably use a flat surface with just some shaping of the LE and probably the elevator tapered to the TE (but I only do semi-scale models not serious scale). Converting it to a cambered airfoil like Clark-Y (either way up) is not a good idea.

Steve
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Old Jan 10, 2015, 03:35 PM
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You are over thinking it.

At the size you're building if you want it to look more scale then use the scale airfoil. If making it easy to build and simpler is the goal for a "stand way off and squint" style of scale model then use the flat sheet surfaces and sand the edges to something suitable.

The upside down Clark Y would simply look odd. So I'd skip on that.
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Old Jan 11, 2015, 02:04 PM
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Also, if usig a Clark Y wing section, DON'T make the bottom 'flat' with regards to the fuselage center line and tailplane. This will give you far too much wing incidence. The Clark Y is quite close to a semi-symmetrical section.
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Old Jan 11, 2015, 03:51 PM
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thanks for the hints...

Is there any proven conservative wing incident angle?

I know that the plane might be relatively heavy, this is why i have chosen a PC-6 and clark Y with slotted flaps to provide good lift. Usually i build my planes with zero degree installation angle on wing an aileron. but this does not mean anything, since most of my planes have flat wings with shaped LE and TE.

thanks
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Old Jan 12, 2015, 03:53 PM
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It all depends on if you're using a proper and true shaped Clark Y or a typical and commonly mis named flat bottom with curved top shape. Too may folks see a flat bottom airfoil and call it a "Clark Y" when it's actually something shaped around a shoe's outsole line and then given a rounded nose and flat bottom. Avoid that sort of mistake.

A proper Clark Y will have roughly a +2.5° angle of incidence when the flat bottom is level. If you set the stabilizer angle so it's parallel with the Clark Y's flat bottom then you will have a decalage angle between the wing section and the tail of 2.5°. And for a somewhat heavy and general fly around model that's actually a pretty good starting point. From there you will find that depending on where you set your CG that you may need a little positive or negative elevator trim for level cruise power flying.

If the plan is to fly faster all the time or to work the CG back further for less pitch stability in flight then you could set the tail to level and the wing so that the flat bottom is 1 to 1.5° nose down. That way you'll need less permanent down elevator trim all the time.
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Old Jan 13, 2015, 12:29 PM
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thanks for the confirmation. so i install the wing and elevator parallel to the fuselage and hope for the best.

I have use a "pseudo" Clark-Y with a flat bottom from 20% to 100%.
Attached you´ll find the profile i have found somewhere in the internet...
If i measure the angle from LE to TE and the bottom, it´s about 2.1°

I have chosen that one due to the fact that i plan to print the entire airframe... i hope.. And here a flat bottom helps a lot.

"General flying" is a good description. I will do that.

I always liked the PC-6 and now i (maybe) have the chance to build one with all the features (slotted flaps,...) that i never build due to too much work involved...
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Old Jan 13, 2015, 02:13 PM
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That looks like a lovely shape. After all the Clark Y wasn't the only flat bottom airfoil around and anything that looks about right is going to perform just fine. And yours is well within the thickness and camber values of what works along with having a nice looking TLAR nose shape.

The built in 2.1° of angle means that you'll do just fine to make the stabilizer parallel with the flat bottom for general sport flying. Any resulting elevator trim you end up requiring due to the final CG placement should be minor to the point of not being noticeable.

Best of luck with your PC6 design and build.
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