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Old Nov 25, 2004, 08:43 PM
EPP Rules!
aussie's Avatar
Sunshine Coast, Australia.
Joined Oct 2003
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Why planks for EPP DS?

I need some convincing as to why plank-style flying wings (25% chord line is straight) are better suited to high DS speeds compared to swept flying wings (25% chord line is swept back towards wingtips).... I'm more interested in aerodynamic reasoning rather than just comparing models because obviously all the fastest, most popular, EPP DS models are currently planks (or close to it) and most of the swept wings are designed for combat.


Kye.
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Old Nov 26, 2004, 05:13 PM
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what do you think

how does your booby fly is it fast how does this compare ith a plank wing lets say a gulp
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Old Nov 26, 2004, 05:27 PM
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The only thing I can think of is less drag, and the weight is built solely into the flying surfaces IE wing and tail. The EPP redback has gone 100+ from what I hear.

Cody
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Old Nov 26, 2004, 05:55 PM
AustinTatious
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Hurst, Texas, United States
Joined Jul 2003
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I think that the answere you are looking for is this:

Ive read some post that it is believed that as the COP (center of pressure) moves aft due to increasing speed, a Swep flyign wing eventually runs out of elevator.

Ive never seen this happen. Others may disagree.

A plank is also easier to put a good spar into, zagi type planes dont exactly have a reputation for being stiff!

You could put a sonic ( flying wing) together and make it real stout and heavy and you would have a REAL fast ship for DS-ing with.

I think Planks are a better choice because they have much better flyign characteristics IMHO.
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Old Nov 26, 2004, 07:42 PM
EPP Rules!
aussie's Avatar
Sunshine Coast, Australia.
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mmmmm...... nah, still not convinced.

Cody, just for clarification, I'm comparing swept to unswept (plank) flying wings - not flying wings versus conventional layout (fuse with tail).

Austin - If anything, a "shifting COP" would be MORE critical on a plank because a swept wing has plenty more rearward wing area. I personally think reports of "running out of elevator" are due to control linkage slop and/or lack of torsional stiffness in wing and/or lack of torsional stiffness in control surfaces and/or too nose heavy. Stiffness is a good point but I think I've overcome that one. Exactly what do you see as the favourable flight characteristics of a plank? Pitch instability?

PS. As much as I'd like to discuss my Boobies (which, along with the Skuas, actually weren't specifically designed for DS but by many accounts show a lot of potential - oops, I'm discussing!) I'd like to try and keep specific models out of this thread as much as possible.

However, comparing the Sonic (swept wing) and JW (plank wing) may be interesting as I think they share the same airfoil (?). Anyone out there tried DS both? Comparisons? Is stiffness the main issue/difference?

Thanks for firing up your brains a little,

Kye.
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Old Nov 26, 2004, 08:07 PM
AustinTatious
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Ive flown a sonic ( King of the Hill's) and it was awesome!

Quote:
Exactly what do you see as the favourable flight characteristics of a plank?
They seem to turn much tighter and also have way more yaw stability. They fly more like a real airplane than a wing. Plus they never tip stall and start spinning!
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Old Nov 26, 2004, 08:42 PM
EPP Rules!
aussie's Avatar
Sunshine Coast, Australia.
Joined Oct 2003
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Thanks for that Austin... Kind of confirms my suspicions.

I think I'll just need to play around with some more prototypes. While I'm not convinced a plank is inherently better/faster for DS over a swept wing, I think the key may lie in less pitch stability ie. less elevator control input required to turn (pitch axis). Given that much of DS is turning (pitch, not roll) I think minimising drag (incresing energy retention) through turns is very important.

A plank turns (elevator control) via pitching moment of the airfoil (very sensitive) whereas a swept wing in more lift oriented (lift back towards tips relative to forward centre section) and has more built-in aerodynamic stability due to it's sweep which needs to be overcome.

However, I think there may be significant advantages and possiblities for swept wing DS models.... Give me a couple more months and you may see what I mean. Or, you may just see a new plank!

Kye.
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Last edited by aussie; Nov 26, 2004 at 08:46 PM.
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Old Nov 26, 2004, 11:32 PM
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United States, NV, Henderson
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This is straight out of Model Airplane News " Basics of RC Model Aircraft Design".
Tapered wings (planks) with a tip cord 40% of root comes closest to the IDEAL elliptical planform in both induced AOA and induced DRAG.
Delta wings are low aspect ratio "flying wing tips". The wings stall at a high AOA, but with HIGH induced drag and vortex flow.
IMHO the plank is simply a superior planform. Less drag, no stall, and extremely responsive.
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Old Nov 27, 2004, 05:31 AM
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I agree with the above. Low drag, easier to make stiff
with full length spars and no tip stall tendencies, ever.
I've seen some very fast swept wings out there, but
only when they were ballasted up well beyond what's
necessary for an equivalent span plank.

ian
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Old Nov 27, 2004, 07:44 AM
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Punta Gorda, FL
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EPP is not strong or stiff compared to others. Wood is very strong and stiff compared as EPP. Mylar is very strong and stiff compared as EPP. Carbon is very, very strong and very, very stiff compared as EPP. The EPP is best for crashing. EPP is not for aerodynamic reasons. The pure EPP structure could not even DSed with no Mylar, wood or carbon. Why use EPP for aerodynamic reasons?
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Old Nov 27, 2004, 12:09 PM
Striving to Rip the Bring
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When you add sweep you require a higher aoa for the same turn (and decreases maxCl), causing more drag. Swept wings tend to have a lower AR because the sweep gives a lower span.

The only advantage to swept wings is that you do not have to use a heavily refexed plank foil. The washout used in the typical swept wing to then counter the downwards pitching moment reduces high-speed effeciency.

That's why planks are better. Swept wings are better for combat because they are often more durable and recover faster.

--Alex
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Old Dec 05, 2004, 06:40 AM
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Correct me if wrong, but intuitively it seems that the flow over a swept flying wing would tend to be pulled inboard, converging toward the middle of the aircraft, where the flows from each side will be forced to converge, causing vortices and local turbulence. Vortices represent energy wasted (drag) required to spin-up a mass of air. . . . seems there'd be a huge interference drag effect.

Getting back to flying wing efficiency, specifically planks, here's an ambitious how-to project for a computer-stabilized plank utilizing non-reflexed airfoils Albatross
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Old Dec 05, 2004, 11:47 AM
Striving to Rip the Bring
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Quote:
Originally Posted by green66
Correct me if wrong, but intuitively it seems that the flow over a swept flying wing would tend to be pulled inboard, converging toward the middle of the aircraft, where the flows from each side will be forced to converge, causing vortices and local turbulence. Vortices represent energy wasted (drag) required to spin-up a mass of air. . . . seems there'd be a huge interference drag effect.
No, that's wrong. Sweep back pushes air to the tips, and the effect is stronger at the tips. That is why swept wings stall tip first.

--Alex
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Old Dec 05, 2004, 12:58 PM
DS JUNKY
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Joined Jun 2004
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I think the biggest reason for Planks vs Wings is simply twist resistance. Graps a wing tip with each hand on a JW and try and twist the model then compare that to a flying wing. You'll see that the wing has MUCH MUCH more twist slop. This is of course is the limitation of the inherently flimsy nature of EPP. What do you think happens when you break through the sheer at high speed and the whole model twists up? You'll immediately flutter and lose control. Infact this is why the JW is the fastest of the EPP stuff.
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Old Dec 05, 2004, 04:55 PM
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Quote:
Sweep back pushes air to the tips....
If the flow is outboard, it seems there would be a beneficial effect in reducing induced drag by countering the normal inboard flow (on upper surface) due to the tip vortex which is significant on any real wing. There must be some point along the span where the flow changes from outboard to inboard.

With outboard flow, seems there must also be some flow into the middle region to compensate for the outgoing flow, e.g. flow from the underside coming up and around the trailing edge at the middle?
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