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Old Dec 01, 2012, 05:21 AM
I don't like your altitude
Stupot46's Avatar
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Question
'flat' or airfoil shaped fin

I'm in the process of designing a plank.I've read through a lot of relevant threads,and found that most use a flat plate fin.The odd one or two which use a foil shape say that it will reduce drag.
The span is 1500mm,set up similar to the one in the pic(which is a great design IMHO,and performs well.
Do you think there will be any significant benefit to using a foil shape;if so any suggestions for a profile.
Regards Stuart
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Old Dec 01, 2012, 07:31 AM
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Make it thick enough to be stiff enough to resist the likely flight loads .
after that - it is not all that important
a gentle clamshell shape with highpoint at about midpointis structurally good
structural integrity is more important than the shape.
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Old Dec 01, 2012, 09:32 AM
An itch?. Scratch build.
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Richard, am I right in saying, (thinking), that for a flat plate surface, whether fin, tail, or wing, a radiused leading edge is good, but not the trailing edge.

The trailing edge should be tapered or left square edged.

I remember someone else telling me this, (or reading it), but cannot remember the general reason for not radiusing the T.E.
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Old Dec 01, 2012, 09:50 AM
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It depends - but yes- leaving the TE blunt is really of no disadvantage as it tends to add drag at that point - Many pattern (F3A designs use extremely wide blunt rudders for the same reason
I never understood efforts to reduce drag at the rudder
I always thought the best flying stuff had a drag profile where drag increased ,the further aft you went.
It may also tend to stick the air more firmly along the fin/rudder
a good thing ---
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Old Dec 01, 2012, 12:33 PM
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An airfoiled fin will have lower drag, can get rid of the control hysteresis that flat plate surfaces can have, and can be thicker for better structure although that usually isn't too much of an issue on a glider. A sharp TE will have lower drag, and on a sailplane that matters.

Dr. Drela designed a whole series of good airfoils for different Re on tail surfaces, his HT series:

http://www.charlesriverrc.org/articl...t-airfoils.htm

The SD8020 was designed for tail surfaces and also works very well, with no hysteresis even at pretty low Re. It works well thinned as well.

Blunt trailing edges can reduce the control hysteresis that a flat a flat plate fin or stab can have. Wider trailing edges like use on some pattern airplanes work like T-Gurney flaps, and increase effectiveness of the rudder. The T-flap actually is more effective at raising the control surface effectiveness, and with lower drag, than the more elegant looking widened trailing edge design that was developed and patented by one of the US aerospace companies. The Gurney flap design was public domain, so they couldn't use that! These all increase the drag, which may not matter for a power airplane but is important for a glider.

Drag at the trailing edge of the rudder is a poor way to add directional stability. The moment arm will be quite short for reasonable yaw angles. Even a low AR surface will likely have a L/D of 5, so you can get easily 20 or 30 times the yaw stability through lift of the fin with a typical tail arm, than you would for a given drag increment.

Kevin
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Old Dec 01, 2012, 01:30 PM
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Quote:
Originally Posted by kcaldwel View Post
Drag at the trailing edge of the rudder is a poor way to add directional stability.
Kevin
It works very well
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Old Dec 01, 2012, 01:39 PM
An itch?. Scratch build.
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I suppose the ultimate 'square cut' trailing edge was on the North American X-15 vertical stabs.
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Old Dec 01, 2012, 02:01 PM
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Quote:
Originally Posted by eflightray View Post
I suppose the ultimate 'square cut' trailing edge was on the North American X-15 vertical stabs.
Hypersonic flight has quite different rules than low Re model tails.

Kevin
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Old Dec 01, 2012, 03:49 PM
I don't like your altitude
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Drag at the trailing edge of the rudder is a poor way to add directional stability. The moment arm will be quite short for reasonable yaw angles. Even a low AR surface will likely have a L/D of 5, so you can get easily 20 or 30 times the yaw stability through lift of the fin with a typical tail arm, than you would for a given drag increment.

Kevin[/QUOTE]

This was basically the reasoning put forward in one of the threads I read.I know it's not going to make an enormous difference to performance,but it won't be much extra work,and will certainly look better
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Old Dec 01, 2012, 03:51 PM
I don't like your altitude
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These are three of the foils from the link.The 12 seems to be the one nearest to what I'm aiming at.
Stuart
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Old Dec 01, 2012, 04:05 PM
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Originally Posted by Stupot46 View Post
These are three of the foils from the link.The 12 seems to be the one nearest to what I'm aiming at.
Stuart
I wouldn't worry about it - Looks like you have plenty of area to get the job done -
Apparantly some of the readers here never fly any precision aerobatic powered stuff - and on these it is helpful to move the drag aft without changing (increasing) the size of the surface or changing moment arms .
wagging tails on some of the big aerobatic designs is a real issue and adding a plate or even a piece of thick tubing showed very quickly how this simple device could eliminate the issue
As long as yer choice of shapes is stiff and light - you are home free.
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Old Dec 01, 2012, 06:42 PM
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Selig/Donovan tested thckened trailing edges at Re 150k and up, on 3 airfoils. The results are in Soartech 8. The trailing edges were thickened to 0.086" to 0.101" from fine edges, over varying distances back. The minimum drag of all three airfoils increased by over 10%.

"Test on three airfoils (DAE51, E374, and SD6080) showed that thick trailing edges produce measurable drag penalties. In order to achieve maximum performance, at least at the higher Rn's, it is necessary to have the thinnest possible trailing edges."

Of course the contribution of the fin TE to the overall airplane drag will be small. The effect over an entire airplane, with the wing, stab and fin would be significant.

Kevin
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Old Dec 01, 2012, 10:40 PM
I DS slower than I build!
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Of course a foiled tail would be a bit better, but some very fast DS planks such as the 234 mph JW have flat plate tails. This illistrates what Kevin was saying about the tail contributing a small portion of the total drag.
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Old Dec 02, 2012, 01:40 AM
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Quote:
Originally Posted by kcaldwel View Post
Selig/Donovan tested thckened trailing edges at Re 150k and up, on 3 airfoils. The results are in Soartech 8. The trailing edges were thickened to 0.086" to 0.101" from fine edges, over varying distances back. The minimum drag of all three airfoils increased by over 10%.

"Test on three airfoils (DAE51, E374, and SD6080) showed that thick trailing edges produce measurable drag penalties. In order to achieve maximum performance, at least at the higher Rn's, it is necessary to have the thinnest possible trailing edges."

Of course the contribution of the fin TE to the overall airplane drag will be small. The effect over an entire airplane, with the wing, stab and fin would be significant.

Kevin
.
Which is the whole idea... add drag at the back end.
Maximum performance has many definitions, depending on what the purpose of the plane is. Extreme duration... get that drag low with a goodly taper.
Manuverability at -all- airspeeds, with no regard for going as fast as possible, the square cut rudder is the rudder to have.
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Old Dec 02, 2012, 03:57 AM
I don't like your altitude
Stupot46's Avatar
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Interesting discussion.I think it was nmasters(Norm)who posted a link to a paper saying that a square cut,or even concave( that would take some doing!)trailing edge performed better than a knife sharp one.I'll seeif I can find the post.
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