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Question about Trailing Edges

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Aerodynamics Discuss the physics of flight revolving around the aerodynamics and design of aircraft.

Question about Trailing Edges

Old 03-27-2008, 08:20 PM
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JKEpps
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Default Question about Trailing Edges

I believe I read somewhere that it's better to have a trailing edge end with some thickness and a squared off edge, rather than a knife edge. It apparently causes enough "disturbance" to allow the upper and lower airflows to rejoin, whereas a knife edge doesn't promote the rejoining.

Is this an urban legend or useful information?

Thanks

-Jorden
Old 03-27-2008, 09:03 PM
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da Rock
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Default RE: Question about Trailing Edges

It's good to have a tapered surface back to well defined TE. The taper reduces the weight of the surface as a major benefit. The narrowing reduces the width of the TE. Thick trailing edges of any shape are less than optimum for drag. The TE width doesn't have to be sharp but benefits drag and flutter resistance when narrow. What you do need is a well defined one, and having a bit of thickness helps create and maintain the definition. Having a narrow, well defined, uniform TE helps reduce flutter tendencies. Flutter is after all a fluctuation. And having a somewhat narrow place for the upper and lower air to meet that isn't sloppy round or vaguely round removes the room for fluctuations to develop.

So the best would be a taper to something like 1/16" thickness. The taper will then remove an appreciable percentage of wood. Lighter surfaces are less apt to flutter. And the edge will be small enough to not create a big bluff ending so won't cause excess drag. Or give flutter any room to build. But the edge will be thick enough to resist hangar rash.

As for any aerodynamic magic, don't look for tricks. The urban legend stuff comes out in the explanations for all the other shapes.
Old 03-27-2008, 10:22 PM
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seanreit
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Default RE: Question about Trailing Edges

Rock, this conversation comes up at my house at times. Would you suggest the same 1/16" for the jets? I'm currently bringing it down to about 1/8" on the control surfaces, and a little less (maybe a 1/16 on the rudder).

I sandwich 1/64 3 ply at the trailing edge and sheet 1/16 balsa over that, glass etc.
Old 03-27-2008, 10:52 PM
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Default RE: Question about Trailing Edges

I think the mystery is when you have "real world" construction techniques that don't allow for super sharp shapes then what is the best shape to have. Rounded or squared off. For this dulled down trailing edge the optimum is the squared off shape. Rounded shapes tend to promote flutter by the air cyclicly holding onto first one side and then the other. Like a flag in the wind. A squared shape forms a little turbulent bubble in the wake of the blunt edge and the air flows around that small turbulent "wire" in a far more predictable and consistent manner.

Seanreit, for jets where speed is the goal you should be aiming at reducing your trailing edges down far more than that. I'd suggest something I used on some sailplane parts that never ended up seeing the air but the method worked out well. Use your 1/64 ply on only one side with balsa on the other. Ideally you would make up some trailing edge edging stock that has the ply on one side and chord wise balsa on the other. This edge cap would then glue to a thicker and normal spanwise trailing edge to form a small extension that can then be sanded down really fine right to the point where you're feathering out the 1/64 ply by a little bit. The chordwise balsa on top that builds up this edging will help provide a lot of stiffness. When you finish glass over all the works the glass and resin will bond it together and add further stiffness. If you can do try to let the glass cloth run out past the edge of the ply by a 1/16 inch. Then feather THAT out for a super sharp edge that should be decently strong. Just dub it off a hair so you can't cut yourself on it.

If you do that your surfaces will have the least drag possible and far less drag than 1/8 flat edges.
Old 03-28-2008, 05:55 AM
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da Rock
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Default RE: Question about Trailing Edges


ORIGINAL: seanreit

Rock, this conversation comes up at my house at times. Would you suggest the same 1/16" for the jets? I'm currently bringing it down to about 1/8" on the control surfaces, and a little less (maybe a 1/16 on the rudder).

I sandwich 1/64 3 ply at the trailing edge and sheet 1/16 balsa over that, glass etc.

I would suggest whatever each modeler decides is right for his building skills versus his hopes for the airplane's performance.

I built a 3m glider some years back that had a sheeted wing. I sandwiched the top sheeting to the bottom sheeting with a ribbon of Carbon Fiber in between. Tapered that TE to be almost sharp, to the thickness of the CF. That glider is awesome.

I left the TE of the elevator on my ARF Tiger60 fat and round. It flies ok. Haven't had any problems with flutter or anything. Well, the model has almost no ability to snap but that's probably because that elevator is so wimpy. And truth is, I'm about to double it's chord as we speak. And absolutely plan to taper the whole thing to the TE.

Most of our daily flyer models will perform adequately with almost any thickness TEs. It's why this subject has so many different sides.
Old 03-28-2008, 11:28 AM
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Tall Paul
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Default RE: Question about Trailing Edges

The model's purpose dictates the shape.
Fast and slippery, needs to be tapered.
Yank and bank, machs nichts!
Weight hanging off the hinges, without adequate support from the servo connection, will flutter regardless of the shape.
Old 04-03-2008, 07:46 PM
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Default RE: Question about Trailing Edges


Ever since I switched all planes to PULL - PULL with fishing line snugged up, There is no hint of flutter.

Most flutters are due to flexing pushrods in 1 direction. When they are pushing there is a real possibility that the wire can flex if not supported very well by the guide tube in the pushing direction.
In a highly loaded jet I would do PULL - PULL with the lightest, SS, braided, fishing leader.
You can put a short spring in the down wire to create tension during final crimping of the lines.

Pull - Pull also prevents servos from striping their gears for NO reason.
Flutter is not always heard when flying alone or with other planes.

Rich

Rich
Old 06-03-2008, 02:43 PM
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Default RE: Question about Trailing Edges

if i remember correctly a thick TE reduces chances of flutter... look at the Bell x15,s Trailing edges, especially the rudder, its huge
Old 06-03-2008, 02:55 PM
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Tall Paul
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Default RE: Question about Trailing Edges

Models don't fly at Mach 5.
For a model jet, the thinnest trailing edge is best.
Many gliders go for really thin trailing edges also.
Indoor 3D planes have blunt leading and trailing edges.
.
"The model's purpose dictates the shape.
Fast and slippery, needs to be tapered.
Yank and bank, machs nichts!
Weight hanging off the hinges, without adequate support from the servo connection, will flutter regardless of the shape."
Old 06-03-2008, 07:11 PM
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Default RE: Question about Trailing Edges

I build my models with as thin a T.E. as the material I'm using will allow without splintering, and within the design parameters of the full-scale I'm modelling (no razor sharp edges on a WWI, etc). The thinner the edge, the less drag. Flutter prevention can be addressed other ways, like no play in the control linkages, sealing the hinge gaps, adaequate stiffness of the control surface, and proper shaping of the LEADING edge of the control surface, where induced turbulence due to the change in surface (wide hinge gap) and disrupted boundary flow can induce flutter over the control surface.

Hangar rash can be a problem, beating up thin T.E.'s. I harden the edges of my control surfaces with thin Ca after careful sanding to the final thickness. It doesn't guarantee no damage, but what would otherwise be a definite ding and dent just becomes a small tear in the cover, or other more easily repaired boo-boo.

There's some nice stuff becoming more and more popular for skinning material (or maybe I learned about it after everybody else )
G-10 fiberglass in assorted thicknesses. I only wish you could get it in corrugated panels for certain scale aileron applications. You can really get thin T.E.'s and maintain a good strength to the control surface.
Old 06-03-2008, 11:30 PM
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HighPlains
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Default RE: Question about Trailing Edges

I don't think that ultra thin trailing edges buys you very much in terms of drag reduction. The air has gone completely turbulent long before you even get to the hinge line, let alone the trailing edge.
Old 06-04-2008, 05:07 AM
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da Rock
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Default RE: Question about Trailing Edges


ORIGINAL: JKEpps

I believe I read somewhere that it's better to have a trailing edge end with some thickness and a squared off edge, rather than a knife edge. It apparently causes enough "disturbance" to allow the upper and lower airflows to rejoin, whereas a knife edge doesn't promote the rejoining.

Is this an urban legend or useful information?

Thanks

-Jorden

Interesting idea, but the air above and below is going to rejoin no matter what. Our models simply do not have the energy and speed to create pure vacuum in their wakes. And with the air already turbulated, it's going to become a turbulent combination, not suddenly become laminar.

The argument for thick trailing edges being a cure for flutter really runs headon into the fact that flutter is a fluctuating phenomenon, and with a wider TE there actually is more room for the wake to fluctuate. And more room gives more leverage and more momentum to the airflow.

The thick TE on some designs is often more the result of the designer wishing to reduce the angle the surface moves away from the airflow than it is a desire to have a larger blunt TE.
Old 06-04-2008, 09:24 PM
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Default RE: Question about Trailing Edges


ORIGINAL: HighPlains

I don't think that ultra thin trailing edges buys you very much in terms of drag reduction. The air has gone completely turbulent long before you even get to the hinge line, let alone the trailing edge.

Tell that to Selig et al after they saw results supporting sharp TE's in their wind tunnel work.

Turbulent separation bubbles and a turbulent boundry layer are two animals of different stripes. A turbulent boundry layer is extremely thin and any sort of obvious bluntness to the TE is going to be bigger than the layer. Also the turbulent boundry layer is moving with the airflow and if it falls off into a blunt area it'll form a typical bubble that adds drag.

Granted only the most fussy of sailplane fliers or extreme speed model fliers is going to see the difference but when you're talking about that last fraction of a % to gain first place it counts.
Old 06-04-2008, 11:21 PM
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HighPlains
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Default RE: Question about Trailing Edges

I never raced in a wind tunnel. I did hold the national record in 428 Quickie however. While I find the first third to front half of the airfoil critical, the rest has little effect other than trim.

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